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

Section 9.7 ( ) Acid-Catalyzed Hydration Section 10.3 ( ) Oxymercuration / Reduction

Larger than H, Hg bridges near the middle, preventing cation rearrangement. Subsequent reduction by BH 4 - completes Markovnikov hydration without rearrangement.

Section 10.2b ( ) Addition of Cl 2, Br 2 (more bridging)

electrophile LUMO (  *) Ethylene HOMO (  ) bonding antibonding + - HOMO (p) HOMO-2 (where the  electrons went) Chlorine Two Cl-C Bonds? “oxidizing agent” removes e - from alkene poor overlap

LUMO (  *) nucleophile HOMO (  *) Ethylene + - HOMO-1 Chlorine :

LUMO - HOMO-1 HOMO + -

LUMO + - H2CH2C CH 2 :Cl Cl Cl - H2CH2C CH 2 Cl + H2CH2C CH 2 Cl “Electrophilic” Addition of Cl 2 to an alkene is both electrophilic and nucleophilic simultaneously.

Section 10.2b ( ) Halohydrins H2CH2C CH 2 :Cl Cl Cl - H2CH2C CH 2 Cl + H-O H H2CH2C CH 2 Cl O-H H + + H : :

Bromonium Regiochemistry p. 420 How do we know the ion is bridged? “Halohydrin” shows Markovnikov Regiochemistry (like oxymercuration)

49:48-54:24 Section 7.9C ( ) Stereochemistry

Bromonium Stereochemistry p. 419 H 2 O attacks  * C-Br from backside. (Unbridged C + would have been attacked from both sides.) Anti Addition

Other “Simultaneous” Reagents Cl 2 C: (Carbene) R 2 BH (Hydroboration) CH 2 I 2 Zn/Cu (Carbenoid) O 3 (Ozonolysis) H-metal (Catalytic Hydrogenation) R-metal (Metathesis, Polymerization) RC (Epoxidation) OOHOOH 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 X 2 C:X3CX3C X 3 C HO-C(CH 3 ) 3 K+K+ ( X = Cl,Br ) X Reaction of CH n Cl 4-n with HO - CHCl 3 CH 2 Cl 2 CHCl 3 CCl 4 fastslowv.slowfast Reaction of CH n Cl 4-n with PhS - fast slowv. slow base nucleophile

C C H H H H LUMO HOMO C Cl Most reactions in this lecture use analogous LUMOs and HOMOs to mix with the  HOMO and  * LUMO of H 2 C=CH 2, respectively, so as to form two bonds simultaneously. But both pairs are orthogonal! + +

Transition State Motion for F 2 C: + H 2 C=CH 2 shrinking rotating stretching

LUMO F 2 C: H 2 C=CH 2 Transition State F 2 C: + H 2 C=CH 2 HOMO LUMO

How do you know both bonds form at once?

Sections ( ) Hydroboration Oxidation

Transition State Motion for BH 3 + H 2 C=CHCH 3 shrinking stretching rotating

HOMO Transition State BH 3 + propene BH 3 C3H6C3H6 HOMO LUMO + 3 LUMO

Rearrangement after Oxidation of BR 3 CH 3 R2BR2B O HO

Subsequent CH 3 BH 2 -O-OH rearrangement is like S N 2 Cl -  * C-Br CH 3 R2BR2B O HO  * O-O  B-C

Subsequent CH 3 BH 2 -O-OH rearrangement is like S N 2

Transition State HOMOs very similar Transition State LUMOs very similar

Why do Hydroboration/Oxidation if 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-electron transfer (SET) e Metal + R X Metal R-M X +

The next three slides suggest a plausible, but incorrect, two-step mechanism for addition of ICH 2 ZnI to H 2 C=CH 2

ClZnCH 3 Model for I-Zn-CH 2 I 4s Zn LUMO 4p Zn LUMO + 1 bent for transition state LUMO` 4sp n Zn HOMO

Model for I-Zn-CH 2 I LUMO`  Zn-C HOMO

Model for I-Zn-CH 2 I ClZnCH 3 CH 2 ClZn CH 3 IZn CH 2 I HOMO  Zn-C LUMO  C-I ZnI 2 CH 2 “S N 2” If it were the diiodide instead of the model… But these two transition states were just guessed, not calculated quantum mechanically…

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