Chemistry 125: Lecture 49 February 10, 2010 Electrophilic Addition to Alkenes with Nucleophilic Participation This For copyright notice see final page.

Chemistry 125: Lecture 49February 10, 2010

Electrophilic Addition to Alkenes with

Nucleophilic Participation This

For copyright notice see final page of this file

Section 9.7 (380-384) Acid-Catalyzed Hydration

Section 10.3 (421-423) Oxymercuration / Reduction

Larger than H,Hg bridges near

the middle, preventing cation rearrangement.

Subsequent reduction

by BH4- completes

Markovnikov hydration without rearrangement.

Section 10.2b (414-421) Addition of Cl2, Br2

(more bridging)

electrophileLUMO (*)

Ethylene

HOMO ()

bonding

antibonding

+

-HOMO (p)

HOMO-2(where the electrons went)

Chlorine

Two Cl-CBonds?

“oxidizing agent”

removes e- from alkene

poor overlap

LUMO (*)

nucleophileHOMO (*)

Ethylene

+

-

HOMO-1

Chlorine

:

LUMO

-

HOMO-1HOMO

+

-

LUMO

+

-

H2C CH2

:Cl

ClCl-

Cl-

H2C CH2

Cl+ H2C CH2

Cl

Cl

“Electrophilic” Addition of Cl2 to an alkene is both electrophilic

and nucleophilicsimultaneously.

Section 10.2b (417-420) Halohydrins

H2C CH2

:Cl

ClCl-

H2C CH2

Cl+

H-OH

H2C CH2

Cl

O-HH

+

+O-HH

: :

Bromonium Regiochemistry p. 420How do we know the ion is bridged?

“Halohydrin”shows

MarkovnikovRegiochemistry

(like oxymercuration)

49:48-54:24

Section 7.9C (308-311)

Stereochemistry

Bromonium Stereochemistry p. 419

H2O attacks *C-Br from backside.

(Unbridged C+ would have been attacked from both sides.)

Anti Addition

Other “Simultaneous” ReagentsCl2C: (Carbene)

R2BH (Hydroboration)

CH2I2 Zn/Cu (Carbenoid)

O3 (Ozonolysis)

H-metal (Catalytic Hydrogenation)

R-metal (Metathesis, Polymerization)

RC (Epoxidation)OOH

O

Jack Hine, Ph.D. (1950)

“about as earthy a research project as could be appropriate for a postdoctoral”

-Elimination

Section 10.4d (431-2)

Carbenes

X2C:X3CX3C H O-C(CH3)3

K+

( X = Cl,Br )

X

Reaction of CHnCl4-n with HO-

CHCl3 CH2Cl2 CHCl3 CCl4

fast slow v.slowfast

Reaction of CHnCl4-n with PhS- fast slow v. slowbase

nucleophile

C

C

HH

HH

LUMO

HOMO••CClCl

Most reactions in this lecture use analogous LUMOs and HOMOs to mix with the HOMO and * LUMO of H2C=CH2,

respectively, so as to form two bonds simultaneously.

But both pairs are orthogonal!

+

+

Transition State Motion for F2C: + H2C=CH2

shrinkingshrinking

rotating

stretching

••

LUMO

F2C:

H2C=CH2

Transition State F2C: + H2C=CH2

HOMO

HOMO

LUMO

How do you know bothbonds form at once?

Sections 9.10-9.11 (390-401)

HydroborationOxidation

Transition State Motion for BH3 + H2C=CHCH3

shrinking shrinking

stretching

stretching

rotating

HOMO

Transition StateBH3 + propene

BH3

C3H6

HOMO

LUMO + 3

LUMO

Rearrangementafter

Oxidation of BR3

CH3R2B

OHO

Subsequent CH3BH2-O-OH rearrangement is like SN2

Cl-

*C-Br

CH3R2B

OHO*O-O

B-C

Subsequent CH3BH2-O-OH rearrangement is like SN2

Subsequent CH3BH2-O-OH rearrangement is like SN2

Transition State

HOMOs

very similar

Transition State

LUMOs

very similar

Why do Hydroboration/Oxidationif it just adds H and OH to C=C?

Product is syn (H,OH from same face of C=C)

and anti-Markovnikov (less substituted at C-OH)!

(Contrast with acid-catalyzed hydration)

Simmons-Smith“Carbenoid”

Metal R-X Metal+

R-X

single-electrontransfer(SET)

e

Metal+

R X Metal

R-M X +

The next three slides suggest a plausible, but incorrect, two-step mechanism for addition of ICH2ZnI to H2C=CH2

Cl Zn CH3

Model forI-Zn-CH2I

4sZn

LUMO 4pZnLUMO + 1

bent for transition stateLUMO` 4spnZn

HOMO

Model forI-Zn-CH2I

LUMO`

Zn-CHOMO

Model forI-Zn-CH2I

Cl Zn CH3

CH2 CH2

Cl Zn CH3I Zn CH2 I

HOMOZn-C

LUMO

C-I

ZnI2CH2

“SN2”

If it were the diiodide instead of the model…

But these two transition states were just guessed, not calculated quantum mechanically…

End of Lecture 49Feb. 10, 2010

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