He H Li Be B C N O F Ne Na K Mg Ca Al Si S P Cl Kr Sn Br I At Xe Rn Fr Rb Sr Cs Ba Ra Ar Sc Y Ti Zr Hf V Nb Ta Cr Mo W Mn Tc Ru Re Fe Os Co Rh Ir Ni Pd Pt Cu Ag Au Zn Cd Hg Ga In Tl Ge Pb As Sb Bi Se Te Po La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Lanthanides in Organic Synthesis Heathcock / MacMillan Seminar Feb. 22, 2000 Tristan Lambert I. Properties of the lanthanides II. Lanthanide metals III. Divalent lanthanides IV. Trivalent lanthanides V. Tetravalent lanthanides VI. Enantioselective processes Reviews: Molander Chem. Rev. 1992, 92, 29 Imamoto, L a n t h a n i d e s i n O r g a n i c S y n t h e s i s, 1994. Oxidation States of the Lanthanides Most stable oxidation state of the lanthanides is +3 For dipositive lanthanides Sm 2+ (f 6 , nearly half - filled), Eu 2+ (f 7 , half-filled), and Yb 2+ (f 14 , filled) are known with relative stability in H 2 O being Ce 4+ (f 0 ) is the only tetrapositive lanthanide stable in water La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 5d 1 6s 2 4f 1 5d 1 6s 2 4f 3 6s 2 4f 4 6s 2 4f 6 6s 2 4f 5 6s 2 4f 7 6s 2 4f 9 6s 2 4f 10 6s 2 4f 11 6s 2 4f 12 6s 2 4f 13 6s 2 4f 14 6s 2 4f 7 5d 1 6s 2 4f 14 5d 1 6s 2 Ionization Energies for Lanthanides 0 1000 2000 3000 4000 5000 6000 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Th Yb Lu KJ/mol 4+ 3+ 2+ 1+ Ionization energies reflect relative energies of the 4f, 5d, 6s orbitals 6s electrons are removed first, hence first two ionization energies for all lanthanides are essentially the same Third ionization usually results from removal of an electron from the 5d orbital Fourth ionization energy reflects successive electron occupation of 4f orbitals Eu 2+ >> Yb 2+ >> Sm 2+ Rf Db Sg Bh Hs Mt
12
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HeH
Li Be B C N O F Ne
Na
K
Mg
Ca
Al Si SP Cl
Kr
Sn
Br
I
At
Xe
Rn
Fr
Rb Sr
Cs Ba
Ra
Ar
Sc
Y
Ti
Zr
Hf
V
Nb
Ta
Cr
Mo
W
Mn
Tc Ru
Re
Fe
Os
Co
Rh
Ir
Ni
Pd
Pt
Cu
Ag
Au
Zn
Cd
Hg
Ga
In
Tl
Ge
Pb
As
Sb
Bi
Se
Te
Po
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr
Lanthanides in Organic SynthesisHeathcock / MacMillan Seminar
Feb. 22, 2000
Tristan Lambert
I. Properties of the lanthanidesII. Lanthanide metalsIII. Divalent lanthanidesIV. Trivalent lanthanidesV. Tetravalent lanthanidesVI. Enantioselective processes
Reviews: Molander Chem. Rev. 1992, 92, 29
Imamoto, Lanthanides in Organic Synthesis, 1994.
Oxidation States of the Lanthanides
Most stable oxidation state of the lanthanides is +3
For dipositive lanthanides Sm2+ (f6, nearly half-filled), Eu2+ (f7, half-filled), and
Yb2+ (f14, filled) are known with relative stability in H2O being
Ce4+ (f0) is the only tetrapositive lanthanide stable in water
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
5d16s2
4f15d16s2
4f36s2
4f46s2
4f66s2
4f56s2
4f76s2
4f96s2
4f106s2
4f116s2
4f126s2
4f136s2
4f146s2
4f75d16s2
4f145d16s2
Ionization Energies for Lanthanides
0
1000
2000
3000
4000
5000
6000
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Th Yb Lu
KJ/
mol
4+
3+
2+
1+
Ionization energies reflect relative energies of the 4f, 5d, 6s
orbitals
6s electrons are removed first, hence first two ionization
energies for all lanthanides are essentially the same
Third ionization usually results from removal of an electron
from the 5d orbital
Fourth ionization energy reflects successive electron
occupation of 4f orbitals
Eu2+ >> Yb2+ >> Sm2+
Rf Db Sg Bh Hs Mt
OH Me OH
Sm(Hg), ICH2Cl
H
H
O
R
R1 R2
CH2
SmI
H
I
Me3C Me
OH
CH2I2
Sm
Me3C Me
OH
Me
OH
CH2I2
Sm
Me
OHMe3C Me3C
OH OHH
H
Me
Lanthanide Metals: Sm0
Cyclopropanation of Allylic Alcohols
THF, -78 oC to rt
98%
d.r. >200:1
d.r. >200:1
Exo: Endo
Sm / CH3CHI2
Et2Zn / CH3CHI2
5:1
1.6:1
Using samarium, only olefin with allylic hydroxyl group is cyclopropanated
Samarium offers enhanced diastereoselectivity over conventional reagents
99%
99%
EvansChem. Rev. 1993, 93, 1307.
Molander J. Org. Chem. 1989, 54, 3525.
Lanthanide Metals: Reductions
Dissolving Metal Reductions Using Yb
OMe
OMe
OMe
O1. Yb, NH3-THF-tBuOH
2. aq. NH4OH
80%
C3H7 C3H7
Yb, NH3-THF-tBuOH
85%
C3H7C3H7
Similar to Birch reduction with alkali metal, however avoids strongly basic hydroxide upon workup
White J. Org. Chem. 1978, 43, 4555.
Hydrogenation Using Lanthanide Alloys
S
CHOLaNi5H6
THF-MeOH
93%
S
OH
CHO LaNi5H6
THF-MeOH
95%
OH
Reduces alkynes, alkenes, aldehydes, ketones
nitriles, imines, and nitro compounds
Not poisoned by amino or halogen containing
compounds
Imamoto J. Org. Chem. 1987, 52, 5695.
O
MeO
MeO
OCH2OMe
H
SmI2
O
OCH2OMe
H
O
Me
OH
HMeMe
H
O
Me
H
HMeMe
HSmI2
C5H11 H
H COCH3O SmI2
HO
CH2COCH3
C5H11
H
Divalent Lanthanides: Samarium IodideReduction of !-Substituted Ketones and Esters
THF-MeOH, -78 oC
92%
THF-tBuOH, 25 oC
87%
Tetrahedron Lett. 1991,32, 6583
White J. Am. Chem. Soc. 1987,109, 4424
!-Halo, sulfoxides, sulfones, and !-oxygenated ketones are reduced
Primary iodides, esters, and ketones are unaffectedJ. Org. Chem. 1986,51, 1135
THF-MeOH, -90 oC
94% J. Org. Chem. 1986,51, 2596
Reduction of optically active epoxy ketones gives "-hydroxy ketones without loss of optical purity