-
STRUCTURE AND REACTIVITY OF POLYCYCLICCROSS-CONJUGATED
ir-ELECTRON SYSTEMS
KLAUS HAFNER
Institute of Organic Chemistry, Technische Hochschule
Darmstadt.Germany
ABSTRACT
In contrast to the well-known monocyclic conjugated systems with
(4n + 2)and 4nit-electrons, non-benzenoid polycyclic conjugated
it-electron systemwhich should not obey Hückel's rule contain the
element of cross-conjugation.In bicyclic and linear annelated
polycyclic conjugated systems this cross-conjugation is not
associated with branching of the it-electron system, but thiswill
however occur in pericondensed tn- and polycyclic compounds.
Theparticipation of the element of cross-conjugation in the
it-electron systems ofsuch polycycles should affect their
properties and result in characteristicdifferences in bonding
character and reactivity compared with monocyclicconjugated
compounds with the same number of it-electrons.
To obtain experimental support for these theoretical
predictions, severalnon-benzenoid bicyclic as well as tn- and
tetracyclic pericondensed it-electronsystems were synthesized and
studied with respect to the magnetic criteriaof aromaticity and the
connection between structure and reactivity. Thesuccessful
preparation of most of the described hydrocarbons has centred on
a
single basic and rational synthetic principle.
INTRODUCTIONIn the last few decades, the chemistry of aromatic
compounds has occupied
the interest of chemists to an increasing extent. The synthetic
accessibility ofnumerous new cyclically conjugated it-electron
systems and their theoreticalunderstanding has given rise to
vigorous development of an interesting fieldof organic chemistry.
Owing to the advancement of quantum chemistry theconcept of the
aromatic sextet1 of electrons has been deprived of its leadingrole
and has been replaced by the postulate that planar monocyclic
con-jugated systems with a closed shell configuration of(4n +
2)it-electrons shouldin general possess special electronic
stability2. Besides the well knowncyclopentadienyl anion, the
tropylium cation and the related tropone andtropolones which
support the electron sextet3, the successful synthesesof
cyclopropenylium cations4, of the equally stable negatively
chargedlOir-electron systems—cyclononatetraenyl anion5 and
cyclooctatetraenyldianion6—as well as of the annulenes7 and bridged
annulenes8 have con-firmed these views. In studying these
cyclically conjugated it-electronsystems the more or less arbitrary
nature of a differentiation between aro-matic and non-aromatic
compounds based solely on criteria of special types
153
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KLAUS HAFNER
of reactivity and kinetic stability must not be overlooked. More
recentlya thermodynamic criterion has been applied whereby cyclic
conjugatedsystems are considered aromatic if cyclic delocalization
of ic-electrons makesa notable negative contribution to their heat
of formation. Furthermoresuch aromatic' molecules are capable of
sustaining a strong induced diamag-netic ring current in the
presence of an externally applied magnetic field.This property
manifests itself in the n.m.r. spectra9 or magnetic
susceptibili-ties'° of such molecules. As an extension of this
definition of aromaticity,Breslow1' predicted the precisely
opposite phenomenon, namely destabiliza-tion by it-electron
delocalization. He termed this phenomenon 'antiaromati-city
signifying that in such molecules resonance would lead to an
increasein energy. In accord with this it has recently been
possible to show that inseveral cyclic 4n n-electron systems a
paramagnetic ring current is induced7" 2•
In the past five years several monocyclic conjugated systems
with (4n + 2)and 4n n-electrons have been most successfully studied
from this point ofview7'8" . But nonbenzenoid polycyclic conjugated
n-electron systemswhich should not obey HUckel's rule are also well
worth consideration. Incontrast to monocyclic molecules these
systems contain the element of cross-conjugation. In bicyclic and
linear annelated polycyclic conjugated systemsthis
cross-conjugation is not associated with branching of the
it-electronsystem, but this will, however, occur in pericondensed
tn- and poly-cycliccompounds'4. The participation of the element of
cross-conjugation in then-electron systems of such polycycles
should affect their properties and resultin characteristic
differences in bonding character and reactivity comparedwith
monocyclic conjugated compounds with the same number of
n-elec-trons. It was, therefore, of interest to study such
molecules in respect of thethermodynamic and magnetic criteria of
aromaticity, so learning moreabout the connection between structure
and it-electron delocalization on theone hand and the ability of
polycyclic systems to sustain an induced dia-magnetic or
paramagnetic ring current on the other.
Prompted by the desire to answer these questions we have
synthesizedseveral polycyclic nonbenzenoid conjugated compounds.
Systems composedof 5- and 7-membered rings seemed to be most
promising because thesecontain the cross-conjugated patterns of
double bonds as found in the wellknown penta- and hepta-fulvenes3.
Thus, azulene (I), for example, combinesthese two cross-conjugated
systems, the result being a resonance stabilizedbicyclic iOn
electron system, a formal combination of the two charged
(a) (I) (b)
\+RHR \
(III) (II)
154
-
POLYCYCLIC CROSS-CONJUGATED it-ELECTRON SYSTEMS
sextets of the cyclopentadienyl anion (Ia) and the tropylium
cation (Ib).In accord with such considerations electrophilic
reagents react with azulenein a way reminiscent of their behaviour
towards fulvenes and the cyclo-pentadienyl anion. In this way
attack takes place at the 5-membered ringand is concomitant with
the formation of a tropylium cation (11)15. Nucleo-philes on the
other hand, as might be expected from their behaviour
towardsheptafulvenes and tropylium cations, react with the
positively polarized7-membered moiety of azulene with formation of
a cyclopentadienyl anion
CH
{R2 lBxe —HX
/+ HX/c'cH
+ R2N—(CH=CH)—CHO(cH2
1120
çCH __C(CH*)
(CH -(c' \\ -R2NHCH),'H r!.4R2
NR2HII ci-!(c \ -R2HCH)—.-
//(CH
(CH('c-' \\CH).
(C*CH\CH)
(111)16. A lack of reactivity towards dienopbiles17 indicates a
pronouncedperipheral ic-electron delocalization in azulene. The
n.m.r. spectrum ofazulene (Figure 1) showing the proton signals in
the region characteristicfor benzenoid compounds confirms the
assumption and indicates stronginduced diamagnetic ring currents in
the 5- and 7-membered rings1 8•
The successful preparation of numerous nonbenzenoid polycyclic
con-
H5C6
+ H3CCH=CHCH(IV)
OCH_CH*
Bj—H20
(V)
H5C6CH3 H5C6CH3
(VI)
C6H5NH
CH3
(1)
155
-
KLAUS HAFNER
jugated ic-electron systems has centred on a single basic and
rationalsynthetic principle, the substitution of mono- or
poly-cyclic systems by anacid amide or its vinylogues via
condensation or electrophilic substitutionto give an enamine or
immonium salt, subsequent intramolecular cycliza-tion and final
n-elimination.
For the syntheses of polycycles with 5-membered rings
cyclopentadieneproved to be a suitable starting material. In the
case of the synthesis ofazulene'9 the reaction scheme consists of
base catalysed condensation ofcyclopentadiene (IV) with
5-(N-methylanilino)2,4-pentadienal (V) to givethe fulvene (VI) and
subsequent intramolecular electrophilic substitutioninvolving and
all-cis transition state with elimination of N-methylaniline.
Thedriving force for the amine elimination is the gain in
delocalization energyof the bicyclic lOit-electron system. That
this reaction is so facile is under-standable in terms of Baker's
'rigid group principle'20. Several new bi- andpoly-cyclic systems
are accessible by appliçation of similar reaction schemes.A few
representative examples are discussed below.
PENTALENE AND HEPTALENEJust as azulene (I) can be considered as
a cs-bridged and therefore planar
system related to cyclodecapentaene, so may pentalene (VII) and
heptalene(VIII) be viewed respectively as planar cr-bridged
cyclo-octatetraene orcyclododecahexaene. Unlike azulene (I) the two
latter compounds lackresonance stabilization. They are 4n
ic-electron systems and therefore, ifthe Hückel rule is applicable,
non- or even anti-aromatic.
CO CO(VII) (VIII)
Dauben's2' elegant synthesis of heptalene (VIII) has shown it to
be therm-ally unstable. The highly reactive bicyclic l2ir-electron
system is obtainedonly at temperatures below —70°C. The n.m.r.
spectrum of heptalene (VIII)(Figure 1) shows signals between 3.8
and 5.2 r indicating either a smallparamagnetic ring current or,
more probably and, in accord with the resultsof quantum chemical
calculations22'2 3, a lack of ic-electron delocalization.
As for heptalene (VIII), theory23 also predicts thermal
instability and highreactivity for pentalene (VII). The few well
known derivatives of pentalene,such as dibenzo[ae]pentalene (IX)24
or hexaphenylpentalene (X)25, do not
H5C6 C6H5
EZ1IjIII1I H5C6——C6H5(IX) H5C6 C6H5
(X)
provide the best information about the bicyclic 8ic-electron
system. In thefirst case two of the double bonds are part of
benzenoid systems whilst inthe second case the inductive and
mesomeric effects of the phenyl groupscannot be neglected. The
synthesis of pentalene as such has so far proved tobe elusive.
156
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POLYCYCLIC CROSS-CONJUGATED ic-ELECTRON SYSTEMS
In view of the theoretical predictions it is not surprising that
attemptedsynthesis of pentalene (VII) in a way analogous to that
used for azulene (I),i.e. by ring closure of
6-(2'-dimethylaminovinyl-l')-fulvene (XI), was notsuccessful.
Although amine elimination takes place even at low temperatureonly
ill-defined tarry products could be isolated26.
7
Figure 1. N.m.r. spectra of azulene (I) and heptalene (VIII) (H.
J. Dauben and D. J. Bertelli21) incarbon tetrachioride.
Attempted synthesis of simple stabilized pentalene derivatives
provedto be more informative. Just as pentafulvene is stabilized by
electron-donating groups at the exocyclic carbon atom27, so might
pentalene—abicyclic fulvene—be stabilized by such groups in the 1-
or 3-position.6-Dimethylaminofulvene (XII)28 is resonance
stabilized by the contribution
3.8—4.5
cc4.7-5.2
TMS
1.6—3.2 ccTMS
I I I1 2 3 4 5 6 7 8 9 10
0(IV)
H2
+
.CHOHC
CH
CH3)2
-H20
JF(CH3)2—(CH3)2NH
N(CH3)2I-J(XI)
157
w(VII)
-
KLAUS HAFNER
of a dipolar 67t-electron canonical form. Similar stabilization
was to beexpected for 1-aminopentalenes.
N(cH3)2
(XII)
N(CH3)
In an attempt to verify this supposition sodium
cyclopentadienide wascondensed at —20°C with the salt (XIV)
obtained by reaction of N, N,N',N'-tetramethyl-3-aminocrotonamide
(XIII) with triethyloxonium fluoroborate.The resulting fulvene
(XV.) rapidly lost dimethylamine even below 0°C anda yellow
crystalline thermally unstable product was isclated in 20 per
centyield. The constitution of this product was shown to be that of
the tautomer
CH3CH,N(CH3)2
(CH3)2N
(XIII)
N(cH3)2
(XVI)CH3
R(CH
(XVII)
0÷{(c2H5)3orBF4e
—(C2H5)20
N(cH3)2
H3C N(CH3)2
CH3CH..,N(CH3)2I i( BF4e
(cH3),ri 02H5(XIV)
+ JNa —C2H,OH— —NaBF4
(CH3)2f H3
oc f\J (XV)
(XVII) of the desired pentalene derivative (XVI)29. This
unexpected resultpoints once again to the thermodynamic instability
of the bicyclic andpossibly antiaromatic 8ir-electron system. In a
similar fashion intramolecularcyclization even at 20°C of the
6-methyl-6-(2'-methyl-2'-dimethylamino-vinyl-1')-fulvene (XIX),
obtained by reaction of sodium cyclopentadienidewith the immonium
salt (XVIII) leads to 25 per cent of
1,3-dimethyl-3-dimethylamino-2,3-dihydropentalene (XX) together
with 15 per cent of thetautomer (XXII) of the desired
1,3-dimethylpentalene (XXI)30. These resultscontrast with the
corresponding but successful preparation of azulenederivatives
where the products are aromatic lOir-electron systems.
It was therefore apparent that one amino substituent does not
confersufficient stability upon the bicyclic octatetraene system.
We therefore
158
-
CH3 CH3 CH3 CH3
(CH3)2N=C ,-e CH -(CH3)2H N(CH3)2
BF4—NaBF4
(XXI) CH3
XIX)
sought a method for the further enhancement of the thermodynamic
stabilityand turned our attention to a synthesis of a
1.,3-bis(amino)pentalene.Accordingly 3-(dimethylamino)-
1(2H)-pentalenone (XXV) was prepared bycondensation of sodium
cyclopentadienide with the alkylation product(XXIII) from
N,N,N',N'-tetramethyl-malonamide and triethyloxoniumfluoroborate
and following cyclization of the aminofulvene derivative(XXIV) in
boiling xylene. The bicyclic fulvene (XXV) proved to be a
suitableprecursor for the 1,3-bis(dimethylamino)pentalene
(XXIX)31
The conversion of derivatives of bicyclo[3,3,O]octanes to
pentalenes hasfrequently been studied but without success. Of
significance in this context
01/(CH3)2N— C\
CH2/(CH3)2N—C\
+ [(c2H5)30]WBF4e
-(C2H5)20
0C2H5
(CH)2N-c" BF4CH2
(CH3)2 N— C\\0 (XXIII)
+ Na —NaBF4—C2H5OH
(CH3)2
H2(xxv)
140°C
-(CH)2NH
159
F(CH3)2{CH2\:J /\\(CH3)2N 0
(XXIV)
POLYCYCLIC CROSS-CONJUGATED it-ELECTRON SYSTEMS
Na +(XVIII)
—(CH3)2iH
120°C
H3H3C (CH3°2
(XX)
CH3
K' I )H2\Lm/
(XXII) 'H2
-
KLAUS I-IAFNER
e0 0
0 00(XXVI)
(CH) fi(CH)
e32(XXV) (XX VII)
is an experiment by Dauben32: namely the reaction of the
bicyclo[3,3,0]-octadiendione (XXVI) with strong bases. The expected
enolization of thediketone was not observed; an enol was not
detected even in traces. Con-version of the twofold
c,13-unsaturated carbonyl system to the bicycloocta-tetraene is not
energetically favourable. As expected,
3-(dimethylamino)-1(2H)pentalenone (XXV) shows a different
behaviour. The yellowishcompound can be converted with bases such
as potassium tert-butoxide orGrignard reagents at 20°C in ether to
a blue thermally not too stable material(XXVII), the ultra-violet
spectrum of which resembles that of hexaphenyl-pentalene (X) and
which regenerates the starting material with protic acids.The
n.m.r. spectrum confirmed the assumption that the enolate of (XXV)
ispresent. Unfortunately attempted isolation of the enolate met
with no moresuccess than did its conversion to an enol ether31.
An access to simple, stable pentalenes was, however, opened when
thepentalenone derivative (XXV) was reacted with dimethylamine in
thepresence of perchioric acid. The stable crystalline symmetrical
immonium
N(CH) (CH )2
H
2
+[(CH,)2NH2]CIO4 f\ H2 -H20 \L./ c104e0 N(CH3)2(XXVIII)
(XXV)-HC1O4//
//+HCiO4
N(CH3)2 (CH3)2 N(CFI3)2
CH3)2- CH3)2 -
CH32(XXIX)
160
-
POLYCYCLIC CROSS-CONJUGATED it-ELECTRON SYSTEMS
salt (XXVIII) is formed. The deprotonation to
1,3-bis(dimethylamino)-pentalene (XXIX) occurs easily with strong
bases in aprotic solvents, i.e.with isopropylmagnesium chloride in
ether. The substituted pentalene isobtained as deep blue crystals,
soluble with a blue colour in polar aproticsolvents, stable to
heating to 120°C or for some time towards oxygen,sublimable at
115°C in high vacuum and melting with decomposition at163°C. In
protic solvents the compound is quickly converted to violet
andstrongly fluorescent materials of unknown constitution whilst
with mineralacids in aprotic solvents the pentalene derivative
(XXIX) reacts as an enamineregenerating the immonium salt
(XXVIII)3'.
Figure 2. Ultra-violet spectra of
1,3-bis(dimethylamino)pentalene (XXIX) in methylene chlorideand of
hexaphenylpentalene (X) in dioxane (E. LeGoff25); vertical rulings
show the predicted
absorption maxima for (XXIX).
The ultra-violet spectrum (Figure 2) of the pentalene derivative
(XXIX)resembles that of hexaphenylpentalene (X) and agrees
satisfactorily withSCF-calculations33. The calculated values for
ic-electron densities and bondorders are well consistent with
enamine character and require a highelectron density at C-2, the
centre of the trimethinecyanine group. The sameconclusion must be
drawn from the n.m.r. spectrum (Figure 3) where, inaddition to the
olefinic A2B-spectrum of the unsubstituted 5-memberedring, the
doublet for the two protons at C-4 and C-6 and the triplet for
theproton at C-S with a coupling constant of 3.4 Hz, one observes
12 methylprotons as a singlet at 6.92 and the C-2 proton as a high
field singlet at 7.20 t,This latter value is characteristic for
3-protons of enamines. It follows that
161
4— V
200 300 1.00 500 600 700 800 nm
-
KLAUS HAFNER
the n.m.r. spectrum of this bicyclic compound offers no
indication of anysizeable ring current.
It is apparent that the amino groups in the 1- and 3-positions
of (XXIX)exert a strong influence on the bicyclic it-electron
system which may beaccompanied by a significant stability increase.
The reactivity of the moleculeis essentially that of the enamine
function. This is reflected not only in the
7.20
IMS
Figure 3. N.m.r. spectrum of 1,3-bis(dimethylamino)pentalene
(XXIX) in hexadeuteroacetone.
above mentioned reversible protonation but also in
1,2-cycloaddition withdimethyl acetylenedicarboxylate which takes
place even below 0°C. Theexpected tricyclic adduct (XXX) is not
isolatable due to its rapid valenceisomerization to the red azulene
derivative (XXXI)3 '. Such a tendency toundergo cycloaddition to
activated acetylenes has been similarly observed,
P(cH) N(CH3)23 2
KIII1COOCH,(CH3)2 (CH3)2N COOCH
(XXIX)(XXX)
CH3)2I,-'' COOCH3OOCH3
t(CH3)2
(XXXI)
162
N(CH3)2
N(CH3)2
6.92
/.31
0 1I I
2 3 4 5 6 7 8 9 10T
-
POLYCYCLIC CROSS-CONJUGATED it-ELECTRON SYSTEMS
albeit at elevated temperatures, for hexaphenylpentalene (X)25.
The inherentinstability of the pentalenoid it-electron system is
once again demonstratedby the reaction of (XXIX) with active
methylene groups. Thus followingattack at C-i elimination of
dimethylamine is in the sense required to producecompound (XXXII)
rather than to regenerate the energetically less favouredpentalene
(XXXIII)34. It may be seen from the foregoing examples that
theformation of the less stabilized bicyclic 8it-electron system is
not favoured.This is surely due to resonance destabilization which
is in turn obviouslymoderated or perhaps even cancelled by
electron-donating substituents.
f4(CH3)
H2(CH3)2N' CH
A possibly promising route to pentalene itself or to simple
alkyl or arylderivatives involves
6-(2'-dialkylamino-vinyl-l')-fulvenes (XXXIV). Theseare easily
prepared by condensation of co-amino-acroleins or
J3-dialkyl-aminovinyl ketones with cyclopentadiene and undergo
intramolecularcyclization in boiling piperidine. This cyclization
proceeding by Michaeladdition is followed by isornerization and the
resonance stabilized 1-dialkyl-amino-2,3-dihydropentalenes (XXXV)
are obtained in 50 to 70 per centyield. Reduction with lithium
aluminium hydride or alternatively with alkylor aryl lithium and
subsequent amine elimination yields the hithertounknown
1,2-dihydropentalene (R' = H) or its 3-alkyl or -aryl
derivatives
(XXXIV)
2
(XXXV)
163
(CH3)2
N(CH3)2
X+ H2CY
(XXIX)
(XXXII) (XXXIII)
-
KLAUS HAFNER
NR2
(XXXV)
+L1R'
or LiAIH4
R R2
IH2Li H2
+ H20
tJ5H2(XXXVI)
H2
—R2NH
H1\ R2
(JIH2
(XXXVI) respectively30' 35• The attractive possibility of
dehydrogenation ofthese hydrocarbons is currently under
investigation.
CYCLOPENT[cd}AZULENEIt should be emphasized that isolatable
compounds which may be for-
mulated as pentalenoid have been known for several years. Thus
the.cyclopent[cd]azulene system (XXXIX) can be viewed as a
pentalene deriva-tive with a pen-fused cycloheptatriene ring. Here
the pentalene moiety isagain stabilized by an electron donor, the
cycloheptatriene ring exhibitingits tendency towards the
6ir-electron cation. The preparation of this yellowl2it-electron
hydrocarbon is related to the basic principle of azulene
syn-thesis. The aldimmonium salt (XXXVIII), easily prepared by
Vilsmeier
CH3
(XXX VII)
4-(CH3)2N-CHO+Poc13—HPO2CI2
CH3
(T/L_CH3
e HC\ CH3ci N(CH3)2(XXXVIII)
lB t_HCI
CH
HA H2t(CH3)2
CH3
(XXXIX)
-(CH3)2NH
164
-
POLYCYCLIC CROSS-CONJUGATED it-ELECTRON SYSTEMS
formylation of 4,6,8-trimethylazulene (XXXVII), undergoes
base-catalysedintramolecular cyclization involving one of the
activated methyl groups atthe 7-membered ring thus giving 65 per
cent of the tricyclic system (XXXIX)36.
In good agreement with theoretical predictions37 based on the
SCF-method the x-ray analysis38 of the 2-phenyl derivative of
(XXXIX) shows avirtually planar structure and nearly constant bond
lengths in the 7-memberedring. The 5-membered rings show
alternation of bond length in the range1.465 to 1.356 A (Figure 4).
The n.m.r. spectrum of (XXXIX) (Figure 5) isquite compatible with
these results, does not bear comparison with the
Figure 4. Experimental and calculated bond lengths of
2-phenyl-5,7-dimethyl-cyclopent[ccl]azulene.
isomeric acenaphthylene and is, therefore, clearly not that
which would beexpected for an ethylene-bridged azulene. It shows
resonances for the7-membered ring protons and for the methyl
protons at the same positionsas observed for the comparable
4,6,8-trimethylazulene (XXXVII). However,the two AB-systems of the
four 5-membered ring protons appear at higherfield than do the
corresponding azulene protons. This indicates that the7-membered
but not the 5-membered rings of (XXXIX) may be comparedwith the
corresponding ring of azulene. Thus the larger ring of (XXXIX)
iscapable of sustaining an induced diamagnetic ring current whilst
in the twosmaller rings this current is much reduced, perhaps even
to be replaced by aslight paramagnetic effect39. In this light the
hydrocarbon seems to representa superposition of the aromatic
azulene and the non- or even anti-aromaticpentalene.
This dualism of behaviour of the hydrocarbon (XXXIX) is mirrored
byits chemical properties. Thus on the one hand attack by
electrophules at the5-membered rings40 is reminiscent of azulenes
whilst on the other handfacile 1,2-cycloaddition to one of the
5-membered ring double bonds is asexpected for a pentalene
derivative and moreover regenerates the azulenoid
165
r9
Experimental*Calculated (SCF calculation with vartat ion)
-
CH3
CH3
2 3-3 0
CH3
CH3
I I I1 2 3 4 5 6
Figure 5. N.m.r. spectra of 5,7-dimethyl-cyclopent[cd]azulene
(XXXIX) and 4,6$-trimethyl-azulene (XXX VII) in carbon
tetrachioride.
KLAUS HAFNER
7.2—7.5
+ N2CFICOOC2H.5/Cu
-N2
CH
(lL\_CH3
(XXXIX)
CH3
(XL)
CH3
H2 CH3
(XLII) (XLI)
166
-
POLYCYCLIC CROSS-CONJUGATED it-ELECTRON SYSTEMS
it-electron system. It is not surprising that the tricyclic
hydrocarbon (XXXIX)reacts with ethoxycarbonylcarbene—generated by
copper catalysed thermo-lysis of ethyl diazotate—to give the
cyclopropane derivative (XL) bycycloaddition to the 1,2-bond41. The
product (XLII) isolated is, however,a derivative of the hitherto
rather inaccessible 2H-benz[cd]azulene42 whichwarrants interest as
an isomer of phenalene. Compound (XLII) probablyarises by
spontaneous valence isomerization of (XL) to the
cross-conjugatedtricyclic compound (XLI) which then undergoes a
hydrogen shift.
CH3
CH3A
CH3
(XXXIX)
CH3
CH3
CH3OOCCH300C
25°C +CH3OOC—CC----COOCH3
CH3
ç1)_CH3
jCOOCH3COOCH3
B
CH
CH3
CH3OOCCH300C
+
(XLIII) (XLIV)
140°C 140°C
CH3
ç_CH3
COOCH3
(XLV) (XLVI)
Furthermore the cyclopent[cd]azulene (XXXIX) combines with
dimethylacetylenedicarboxylate at 25°C in the 1,2- or 3,4-position
to regenerate theazulenoid it-electron system and to form the blue
cyclobutene derivatives(XLIII) and (XLIV) in the ratio 4:1. The
high bond order of the 1,2- and3,4-linkages in (XXXIX) and the
polarization consistent with participationof the resonance
structures (XXXIXA and B) permit the suggestion that thereaction of
(XXXIX) with electron-deficient alkenes36 and alkynes are two-
167
-
KLAUS HAFNER
step cycloadditions proceeding via resonance stabilized dipolar
inter-mediates. Thermal valence isomerization of the adducts in
boiling xyleneleads to the green aceheptylenes (XLV) and (XLVI) in
yields of about 90 percent43. In these tricyclic systems the
unstable heptalene is fused in a pen-position with a
cyclopentadiene ring which, in accord with its enhancedstability in
the anionic form, acts as an electron acceptor and stabilizes
theheptalene moiety.
ACEHEPTYLENE
Whilst several substituted aceheptylenes are accessible by a
route36'44analogous to that used for the preparation of
cyclopent[cd]azulenes(XXXIX), the same is not true for the parent
hydrocarbon (LI). This wasprepared starting from 4-methylazulene
(XLVII) which reacts with sodiumN-methylanilide to give the sodium
azuleniate (XLVIII)15. Further treat-ment with the immonium salt
(XLIX) yields the azulene derivative (L). Inanalogy to the
synthesis of azulene (I) the dienamine (L) undergoes
thermalintramolecular cyclization with loss of N-methylaniline to
give 35 per centof the aceheptylene (LI)45.
The picture presented by x-ray analysis and n.m.r. studies of
aceheptylene(LI) is similar to that of cyclopent[cd]azulene (XXXIX)
being one of a super-position of the nonbenzenoid structures of
azulene and of heptalene. Inthe ground state, therefore, the
molecule should neither be represented as anonbenzenoid analogue of
pleiadiene—i.e. as a 1,8-diene bridged azulene—nor as a
1,10-ethylene bridged heptalene. The x-ray analysis46 of the
3,5,8.10-tetramethyl derivative of (LI) (Figure 6) shows in accord
with SCF-calcula-lions37 a symmetrical planar structure and bond
alternance in the 7-mem-bered rings. Signifying a diamagnetic ring
current in the 5-membered ring
H,C\_+
C6H, NaW L) /CH2 I
(XLVIII) CH2(XL VII)
H5C5N_ ,,CH3+ ,N—CH=CH—CH=NN
H,C C6H5
(XLIX) dO?
CH,NH. —NaC1O4
H,C6
H3C
H,c(N\ /CHcH—cH
(LI) (L)
168
e
-
POLYCYCLIC CROSS-CONJUGATED m-ELECTRON SYSTEMS
1.393
Figure 6. Experimental and calculated bond lengths of
3,5,8,1-tetramethy1aceheptylene.
the n.m.r. spectrum of (LI) (Figure 7) shows resonance for the
two 5-mem-bered ring protons as a low field doublet. In contrast it
is probable that the7-membered rings sustain a weak paramagnetic
ring current for the signalsdue to the 7-membered ring protons
appear at higher field than observedfor corresponding azulenic
protons39. The n.m.r. spectrum of 3,5-dimethyl-aceheptylene (LII)
(Figure 7) adds further support to this suggestion, themethyl
protons showing signals above 8 t and thereby contrasting
withmethyl proton resonance for analogous azulenes. One might
justifiablyimagine aceheptylene as a superposition of two azulene
and one heptaleneunit such that the diamagnetic ring current of the
5-membered ring mayweaken the intensity of the paramagnetic ring
current of the heptalenemoiety. We may expect it to combine the
chemical properties of azulene (I)and heptalene(VIII).
The aceheptylene system indeed reacts with electrophiles with
substitu-tion in the 5-membered ring. However, it is surprising
that substitution takesplace preferentially in the 4- and
6-positions, that is to say in the 7-membered
CH3
CH(LII)
+C(CN)2
169
— CH3(CN)
(CN)2
CH3
(LIII)
E x per me nt o
*C1 Ltd (SCF calculation)
-
KLAUS HAFNER
8.0- 8.3
TMS
Figure 7. N.m.r. spectra of aceheptylene (LI),
3,5-dimethylaceheptylene (LII) and 4,6,8-tn-methylazulene (XXXVII)
in carbon tetrachioride.
ring44'47—49. This is probably a reflection of kinetic versus
thermodynamiccontrol. On the other hand the hydrocarbon reacts with
dienophiles in aDiels—Alder fashion involving one of the formal
7-membered ring dieneunits (LII —÷ LIII)36'44.
PENTALENO{6,6a,1,2-del]HEPTALENEWe have seen from the properties
of cyclopent[cd]azulene (XXXIX) and
of aceheptylene (LI) that in pen-condensed tricyclic systems of
two 5-andone 7-membered ring or of two 7- and one 5-membered ring
we have com-bined both aromaticity and non- or even
anti-aromaticity. It follows that byappropriate combination of
these structural elements it should be possible
170
hO- 5.0
7.2
CH3
c?-cH3CH3
7.5
UJ1. 5 6 7 8 9 10
-
POLYCYCLIC CROSS-CONJUGATED it-ELECTRON SYSTEMS
to construct either highly aromatic or alternatively rather
antiaromaticmolecules. These, as will be seen from the following
two examples, seem tobe realizable possibilities.
Addition of a second 7-membered ring in the pen-positions of the
azuleneportion of cyclopent[cd]azulene (XXXIXor alternatively of a
second
iTh
(XXXIX) /5-membered ring to aceheptylene (LI) gives
pentaleno[6,6a,1,2-def]heptalene(LV). This nonbenzenoid isomer of
pyrene is formally not only a combina-tion of two azulenes but also
one of pentalene and heptalene. Quantumchemical
calculations33'39'5° would favour the first of these two
combina-tions as the more meaningful image; all four rings should
sustain induceddiamagnetic ring current and the l6ir-electron
system should have highelectronic stability51.
In order to add substance to this suggestion we determined to
synthesizethe tetracyclic system (LV). To this end our basic
synthetic principle couldonce again be applied. In this way either
the aceheptylene derivative (LI V)36can be converted directly to
the trimethyl derivative of (LV)47 or alterna-tively the easily
accessible 4,6-dimethyl-1,8-cyclopentenoazulene (LVI)could be
subjected to modified Vilsmeier reaction with
3-(N-methylanilino)-acrolein and phosphorous oxychioride to yield
the aldimmonium salt(LVII) This on treatment with bases undergoes
ring closure with theneighbouring activated methyl group.
Spontaneous elimination of N-methyl-aniline then yields the
hydrocarbon (LVIII) which is transformed in 65per
N(CH3)2
H3CçCH3cio?
H3C2/J(3 3 FI3C Cl-I3
(LIV)
171
(LV)
(LI)
-
1 2 3 4 5 6 7 8 9 10
FigureS. N.m.r. spectra of
5-methy1penta1eno[66a,1,2-def]hepta1ene (LIX) in
deuterochioroformand of 46,8-trimethy1azu1ene (XXX VII) in carbon
tetrachioride.
172
(LVI)
(LVII)
lB
H5C_+ N—CH==CH--CHO
H3C+POCI-HPO2CI2
—H2
CH3
HCI
H5C6NNH
H3C7
(LIX) (LVIII)
73
1.6 -3,5
TMS
7.2
-CH3CH3
2.8
2.2
TMS
-
POLYCYCLIC CROSS-CONJUGATED it-ELECTRON SYSTEMS
cent yield to (LIX)52 on treatment with ch)oranil in benzene at
20°C. The easeof this dehydrogenation is noteworthy and indicative
of the thermodynamicstability of the product.
The n.m.r. spectrum (Figure 8) of (LIX) is just as might be
predicted ontheoretical grounds39. The chemical shifts of the
multiplets for the nine ringprotons lie in the range 1.6 to 3.5 r
thus pointing to a high diamagnetic ring
Figure 9. Experimental and calculated bond lengths of
5-methylpentaleno[6,6a,1,2-def]heptalene(LIX).
current in all four rings. Furthermore the dipole moment of 1.5
D is incom-patible with charge localization in the ground state. It
is, however, surprisingthat the x-ray analysis53 reveals a not
completely planar structure with aslight propeller-like distortion
of the heptalene moiety and a correspondinglyslight deviation
towards pyramidal geometry about the two central C-atoms.These
distortions must be too small to influence the ir-electron system
to anysignificant extent. Contrary to SCF-calculations33 which
predict a uniformbond alternance throughout the molecule, the x-ray
analysis shows azulene-like bond order in rings A and C (Figure 9)
but relatively pronounced bondalternance for the remainder of the
molecule. The hydrocarbon would seemto be composed therefore of one
azulene portion which is conjugated with aa triene unit (LIXa). The
chemical behaviour of the hydrocarbon (LIX), e.g.
173
1.3821.378
E xperimentat*C lid (SCF co1cua±ion with variafion)
-
KLAUS HAFNER
CR3
(LIXa)
towards catalytic hydrogenation, dienophiles or electrophiles,
closelyresembles that ofazulene52.
AZULENO[8,8a,1,2-del]HEPTALENEWhilst
pentaleno[6,6a,1,2-def]heptalene (LV) shows a degree of n-elec-
tronic stabilization, the same should not be expected for the
18n-electronazuleno[8,8a,1,2-def]heptalene (LXVIII). Here the
azulene unit is combinedwith heptalene, three 7-membered rings
sharing one 5-membered ring.Correspondingly and in spite of its
belonging to the (4n + 2) it-electronseries this molecule should
possess only small it-electronic stability. More orless intensive
paramagnetic ring currents in the 7-membered rings
shouldcounterbalance a diamagnetic ring current in the 5-membered
ring andcause a relative upfield shift of the proton of the
5-membered ring39. So as toobtain experimental support for these
theoretical predictions we syn-thesized this hydrocarbon.
\/(LXViii)
At first sight the synthesis of (LXVIII) by annelation of a
third 7-memberedring to aceheptylene (LI) appeared to be a simple
matter, but unfortunatelydid not in fact prove to be realizable
owing to the exclusive substitution ofthis hydrocarbon by
3-(N-methylanilino)-propenal in the presence ofphosphorous
oxychioride in the 7-membered rings. Thus, for
example,3,5,8,10-tetramethylaceheptylene yields the propenals (LX)
and (LXI)which are transformed in the presence of base to the benzo
derivatives(LXII) and (LXIIfl47.
An appropriate aldimmonium salt (LXVII) could nonetheless be
obtainedin a slightly roundabout fashion. The 7,10-dihydro
derivative (LXV) of the3-methylaceheptylene (LXIV), which is
prepared from the latter by partialreduction with lithium aluminium
hydride, reacts with electrophiles as theazulene derivative which
it is, exclusively in the 3-position referring to azulene.Thus the
dihydro immonium salt (LXVI) could easily be obtained and thisin
turn dehydrogenated by chioranil to regenerate the aceheptylene
structure
174
(LI)
-
POLYCYCLIC CROSS-CONJUGATED it-ELECTRON SYSTEMS
H5C6(1) + N—CH=CH—-CHO
+P0c13
(2) OHe/H20
H3C CH3
+
3 HC T CH3HCCH
(LXI) CHO20— 20
H3C I '7 \T 'i' \J H3C J''LCH3H3C CH3 \\ I /-_-, -(
(LXII) H3C I \\ll(LXIII) N—'in (LXVII). Base-catalysed ring
closure with the activated methyl groupyields seven per cent of the
desired hydrocarbon (LXVIII), a brownishyellow, thermally labile
crystalline substance which is fluorescent in solu-tion47'54. The
hydrocarbon (LXVIII), like the otlr previously discussedpolycyclic
systems, forms an isolatable complex with trinitrobenzene but
ismoreover distinguished by its remarkably pronounced basicity In
this way(LXIX) is soluble with violet colour even in 2N sulphuric
acid from which itcan be regenerated unchanged on dilution with
water. The constitution ofthe conjugate acid (LXX) was inferred
from its n.m.r. spectrum54.
The ultra-violet (Figure 10) and n.m.r. spectra (Figure 11) of
hydrocarbon(LXVIII) and its methylated derivative (LXIX) accord
with quantumchemical predictions39'55 and show most unusual
features. Thus in contrastto the ultra-violet spectrum of the
pentaleno[6,6a,1,2-def]heptalene deriva-tive (LIX) (Figure 10)
which shows the longest absorption maximum at569 nm with further
broad absorption extending up to approximately800 nm and unusual
fine structure, the longest absorption of
azuleno-[8,8a,1,2-def]heptalene (LX VIII) is found in the near
infra-red. An absorptionmaximum at 1075 nm extends with shoulders
of small extinction up to the
175
P.A.C.—28/3-—D
H
(LX)
-
KLAUS HAFNER
CH3(1) LJAJH4
(2) H20
(LXIV)(LXV)
(LXVII)
H5C6+ )—CH==CH_CH0H,C+ Pod3— HPO,C1,
CH5
_H5C6NRH
-H2
Hd1e\H3C H,
B (LXVI)
(LX VIII)
region of 1500 to 1700 nm, a most unusual longwave absorption
for a hydro-carbon.
The n.m.r. spectrum reveals a similar surprise. The singlet for
the protonof the 5-membered ring which at 5.5 r appears at much
higher field than theprotons of the 5-membered ring of azulenes and
tricyclic hydrocarbonsindicates a small or even nonexistent
diamagnetic ring current in this partof the molecule. On the other
hand the proton signals of the 7-memberedrings between 5.6 and 8.4
r—a region normally reserved for the protons of
CH3
(LXX)
176
(LXIX)
-
20 15 10 5x103cm1
200 300 /400 500 600 700 800 900 1000 1 200 1 /400 1 600 1
800A
2000nm
Fogire 10. Ultra-violet spectra of
5-methylpentaleno[6,6a,l,2-def]heptalene (LIX) and
azuleno-[8,8a,I,2-def]heptalene (LXVIII) in n-hexane.
9.68
_JJJL-CH3
CH3
7.2
7.5 TMS
Figure 11. N.m.r. spectra of azuEeno[88a,1,2-def]heptalene
(LXVIII) and 1 1-methyl-azuleno-[8,8a,1,2-def]heptalene (LXIX) in
carbon disuiphide and of 4,6,8-trimethylazulene (XXXVII) in
carbon tetrachloride.
177
cepCH3
4.0
3.0
2.0 —
CH3
f I I
1.0 -
-
KLAUS HAFNER
saturated carbons—indicate strong paramagnetic ring currents in
the three7-membered rings. An unusually high shielding of the ring
protons and ofthe methyl group not observed so far in such systems
is the result This isillustrated by the position of the signal for
the methyl protons of (LXIX) at9.68 t, a region where proton
signals of methyl groups at saturated carbonsare normally found.
These significant peculiarities in the ultra-violet andn.m.r.
spectra of the tetracyclic hydrocarbon (LXVIII) demonstrate
itsextraordinary position in the series of the polycyclic
conjugated non-benzenoid systems known so far.
An investigation of the reactivity of this compound together
with a moredetailed quantum chemical and an x-ray analysis will
hopefully yield furtherinteresting information about the
interrelations between bond structureand reactivity in nonbenzenoid
polycyclic ic-electron systems. Meanwhileit would be of much
interest to investigate the properties of a pentacycliccompound
related to (LXVIII) by inclusion of a further pen-fused
7-mem-beredring. Synthetic routes are being explored.
CONCLUSIONTo conclude, one can say that polycyclic nonbenzenoid
ic-electron
systems differ, at times considerably, in their bonding
structure and chemicalproperties from the well known monocyclic
conjugated compounds with thesame number of it-electrons. The more
cross-conjugated elements participatein the conjugation the more
noticeable are these differences. The Hückelrule, reliable for
monocyclic it-electron systems seems to be valid to someextent for
bicyclic conjugated compounds and evidently also for tn-
andtetra-cyclic linearly fused it-electron systems. It has no
validity for pen-condensed tn- and tetra-cyclic compounds with
cross-conjugated structuralelements. Just as the reactivity of
azulene can be understood as a combinationof cyclopentadienyl anion
and tropylium cation, the chemical properties,in part even the
physical properties, of the discussed tn- and
tetra-cyclicpen-condensed hydrocarbons may be interpreted
qualitatively as a super-position of the bicyclic structures of
pentalene, heptalene and azulene.
ACKNOWLEDGEMENTSIt is a pleasure to acknowledge gratefully the
cooperation of my associates
F. Bauer, K. R. Bangert, R. Fleischer, W. Friebe, K. Fritz, E.
Goedecke,G. Hafner-Schneider, D. Jung, R. Kaiser, H. J. Lindner, U.
MUller-Westerhoff,V. Orfanos, W. Rieper and J. Schneider whose
efforts, persistence and abilityare largely responsible for the
results described in this paper.
Finally I would like to thank the 'Deutsche
Forschungsgemeinschaft'and the 'Fonds der Chemischen Industrie' for
generous support.
REFERENCES1 j• w•Armit and R. Robinson, J. Chern. Soc. 127, 1604
(1925).2 E. HUckel, Z. Phys. 70, 204 (1931); Grundzüge der Theorie
ungesattigterund aromatischer
Verbindungen, Verlag Chemie: Berlin (1938).D. M. G. Lloyd.
Carbocyclie Non-Benzenoid Aromatic Compounds, Elsevier: London
(1966).
178
-
POLYCYCLIC CROSS-CONJUGATED it-ELECTRON SYSTEMS
' A. W. Krebs, Angew. Chem. 77, 10 (1965); Angew. Chem.
internat. Edit. 4, 10(1965);R. Breslow, J. T. Groves and G. Ryan,
J. Amer. Chem. Soc. 89, 5048 (1967).T. J. Katz and P. 1. Garratt.
J. Amer. Chem. Soc. 85, 2852 (1963); 86, 5194 (1964);E. A.
LaLancette and R. E. Benson, J. Amer. Chem. Soc. 85, 2853 (1963);
87, 1941 (1965).
6 T. 1. Katz, J. Amer. Chem. Soc. 82, 3784 (1960).F. Sondheimer,
J. C. Calder, J. A. Elix, Y. Gaoni, P. J. Garratt, K. Grohmann, G.
Di Maio,J. Mayer, M. V. Sargent and R. Wolovsky in Aromaticity,
Spec. Pub!. No. 21, pp 75—107.Chemical Society: London (1967);G. W.
Brown and F. Sondheimer. J. Amer. Chem. Soc. 91, 760(1969);J.
Griffiths and F. Sondheimer, J. Amer. Chem. Soc. 91, 7518 (1969)
and earlier papers.
8 E. Vogel in Aromaticity, Spec. Pubi. No. 21, pp 113—147,
Chemical Society London (1967);E. Vogel, H. Haberland and J. Ick,
Angew. Chem. 82, 514 (1970); Angew Chem. Internat.Edit., 9,
517(1970), and earlier papers;R. H. Mitchell and V. Boekelheide, J.
Amer. Chem. Soc. 92, 3510(1970) and earlier papers.J. A. Elvidge
and L. M. Jackman, J. Chem. Soc. 859(1961);L. M. Jackman, F.
Sondheimer, Y. Amiel, D. A. Ben-Efraim, Y. Gaoni, R. Wolovsky andA.
A. Bothner-By, J. Amer. Chem. Soc. 84,4307(1962);J. M. Gaidis and
R. West, J. Chem. Phys. 46, 1218 (1967);F. Baer, H. Kuhn and W.
Regel, Z. Naturforschung, 22a, 103 (1967).
10 H. J. Dauben Jr, J. D. Wilson and J. L. Laity, J. Amer. Chem.
Soc. 91, 1991 (1969).' R. Breslow, Chem. & Eng. News, 43, No.
26. p90 (1965);M. J. S. Dewar, Advanc. Chem. Phys. 8, 121 (1965).'
K. G. Untch and J. A. Pople, J. Amer. Chem. Soc. 88,4811(1966);H.
P. Figeys, Chem. Commun. 495 (1967);G. Schröder and J, F. M. 0th.
Tetrahedron Letters. 4082 (1966);J. F. M. 0th and J. M. Gilles,
Tetrahedron Letters, 6259(1968);J. F. M. 0th, G. Anthoin and J. M.
Gilles, Tetrahedron Letters, 6265 (1968).
13 R. reslow, J. Brown and J. J. Gajewski, J. Amer. Chem. Soc.
89,4383 (1967);R. Breslow and M. Douek, J. Amer. Cheni. Soc.
90,2698(1968);R. Breslow, Angew. Chem. 80, 573 (1968); Angew. Chem.
Internat. Edit. 7, 565 (1968).
14 K. Hafner, Angew. Chem. '75, 1041 (1963); Angew. Chem.
Internat. Edit. 3, 165 (1964); Z.Chemie, 8,74(1968).K. Hafner, A.
Stephan and C. Bernhard, Liebigs Ann. Chem. 650,42(1961);K. Hafner,
H. Peister and J. Schneider, Liebigs Ann. Chem. 650, 62 (1961).
16 K. Hafner and H. Weldes, Liebigs Ann. Chem. 606,90(1957);K.
Hafner, C. Bernhard and R. MUller, Liebigs Ann. Chem. 650, 35
(1961);K. Hafner, H. Peister and H. Patzelt, Liebigs Ann. Chem.
650, 80 (1961).
17 w Treibs, Naturwissenschaften, 47, 156(1960);K. Hafner and K.
L. Moritz, Liebigs Ann. Chein. 650,92(1961).
18 W. U. Schneider, H. J. Bernstein and J. A. Pople, J. Amer.
Chem. Soc. 80,3497(1958);D. Meuche, B. B. Molloy, D. H. Reid and E.
Heilbronner, Helv. Chim. Acta, 46,2483 (1963).
19 K. Hafner, Liebigs Ann. Chem. 606, 79 (1957).20 W. Baker,/.
Chem. Soc. 200,201,209, 1114,1118(1951);
1443,1447,1452,2991,3163(1952);
Jndustr. Chim. Beige, 17, 633 (1952).21 H. J. Dauben Jr and D.
J. Bertelli, J. Amer. Chem. Soc. 83, 4659 (1961).22 T. Nakajima and
S. Katagiri, Molec. Phys. 7, 149 (1963);
0. Chalvet, R. Daudel and J. J. Kaufman, J. Phys. Chem. 68, 490
(1964).23 P. C. den Boer, D. H. W. den Boer, C. A. Coulson and T.
H. Goodwin, Tetrahedron, 19, 2163
(1963);T. Nakajima, Y. Yaguchi, R. Kaeriyama andY. Nemoto, Bull.
Chem. Soc. Japan, 37,272(1964);R. Zahradnik and J. Michi, Coil.
Czech. Chem. Commun. 30, 3173 (1965);M. J. S. Dewar in Aromacity,
Spec. Pub!. No.21, pp 177—215, Chemical Society: London (1967).
24 C. T. Blood and R. P. Linstead, J. Chem. Soc. 2255 and 2263
(1952);C. C. Chuen and S. W. Fenton, J. Org. Chem. 23, 1538
(1958).
25 E. LeGoff, J. Amer. Chem. Soc. 84, 3975 (1962).26 K. Hafner,
K. H. VUpel and W. Bauer, unpublished results.27 K. Hafner, K. H.
Häfner, C. KOnig, M. Kreuder, G. Ploll, G. Schulz, E. Sturm and K.
H.
Vopel, Angew. Chein. 75, 35 (1963); Angew. Chem. Internat. Edit.
2, 123 (1963);K. Hafner, G. Schulz and K. Wagner, Liebigs Ann.
Chem. 678, 39 (1964).
179
-
KLAUS HAFNER
28 K. Hafner, K. H. Vopel, G. PloD and C. König, Liebigs Ann.
Chem. 661, 52 (1963).29 K. Hafner and F. Goedecke, unpublished
results.30 K. Hafner and R. Kaiser, Angew. Chem. 82, 877 (1970);
Angew. Chem. Jnternat. Edit. 9,
892(1970).' K. Hafner, K. F. Bangert and V. Orfanos, Angew.
Chem. 79, 414 (1967); Angew. Chem.Internat. Edit. 6, 451
(1967).
32 H. J. Dauben, Jr., S. H. K. Jiang and V. R. Ben, Hua Hsüeh
Hsüeh Pao, 23, 411 (1957); Chem.Abstr. 52, 16309h (1958).U.
MilIler-Westerhoff; S. Dãhne; personal communications.K. Hafner and
V. Orfanos, unpublished results.F. Sturm and K. Hafner, Angew.
Chem. 76, 862 (1964); Angew. Chem. Internat. Edit. 3,
749(1964).
36 K. Hafner and J. Schneider, Liebigs Ann. Chem. 624, 37
(1959);K. Hafner and K. F. Bangert, Liebigs Ann. Chem. 650, 98
(1961).' N. K. DasGupta and M. A. Au, Theor. Chim. Acta, 4, 101
(1966);P. Hochmann, R. Zahradnik and V. Krasnièka, Coil. Czech.
Chem. Commun. 33, 3478 (1968);U. Muller-Westerhoff, personal
communication.
38 H. J. Lindner, J. Chem. Soc. (B), 907 (1970).D. Jung,
Tetrahedron, 25, 129 (1969).
40 K. Hafner and it. Fleischer, unpublished results.41 K. Hafner
and W. Rieper, Angew. Chem. 82, 218 (1970); Angew. Chem. Internat.
Edit. 9, 248
(1970).42 K. Hafner and H. Schaum, Angew. Chem. 75, 90 (1963);
Angew. C/tern. Internat. Edit. 2, 95
(1963);V. Boekeiheide and C. D. Smith, J. Amer. Chem. Soc. 88,
3950 (1966).' K. Hafner and it. Fleischer, Angew. Chem. 82, 217
(1970); Angew. Chem. Internat. Edit. 9, 247(1970).'' K. Hafner and
G. Schneider, Liebigs Ann. C/tern. 672, 194 (1964).'1 K. Hafner, U.
Hafner-Schneider and F. Bauer, unpublished results.
46 E.Carstensen-Oeser and 0. Habermehl, Angew. Chem. 80,
564(1968); Angew. Chem. Internat.Edit. 7, 543 (1968);R. Qasba, F.
Brandl, W. Hoppe and R. Huber, Acta Crist. B25, 1198 (1969).' K.
Hafrer, W. Friebe, 0. Hafner-Schneider and F. Bauer, unpublished
results.
48 F. Haselbach, Tetrahedron Letters, 1543 (1970)."Electrophilic
substitution of azulenes takes place in the 7-membered ring only
when thepreferred 1- and 3-positions are blocked [K. Hafner and K.
L. Moritz, Liebigs Ann. Chem.656, 40 (1962)].° R. Zahradnik, Angew.
C/tern. 77, 1097 (1965); Angew. Chetn. Internat. Edit. 4, 1039
(1965)." For diocyclopenta [ef—kl]heptalene, isomeric with (LV),
similar prediction proved to bejusti-fied (A. G. Anderson Jr, A. A.
MacDonald and A. F. Montnna, J. Amer. Chem. Soc. 90,
2993(1968)).
52 K. Hafner, R. Fleischer and K. Fritz, Angew. Chem. 77, 42
(1965); Angew. C/tern. Internal.Edit. 4, 69 (1965).H. J. Lindner,
Chem. Ber. 102, 2456 (1969).K. Hafner, 0. Hafner-Schneider and F.
Bauer, Angew. Chem. 80, 801 (1968); Angew. C/tern.Internat. Edit.
7, 808 (1968)." R. Zahradnik and U. MUller—Westerhoff, personal
communication.
180