©DaveKav@flickralkenes IILECTURE EIGHT

PREVIOUSLY...

H Hhydrogenation

H XX

Hstarted addition (hydration etc)

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regiochemistry

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Markovnikov Addition

the rich get richer

Markovnikov Addition

the rich get richer

H

H

HH H

H

H

H

➀ ➀

➁➂

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H X

H

H

HX

stabilitycation

least stablecationprimary

➍oxymercuration

i. Hg(OAc)2,H2Oii. NaBH4 HO

H

MarkovnikovAddition

mechanism

step 1

HgOAcAcO

HgOAc

OAc

HgOAc

OAc

mechanismstep 2

HgOAc

OH

H

SN2-like OHgOAc

H

H

OAc

HOHgOAc

anti

mechanismstep 3

HOHgOAc

NaBH4 HOH

do NOT

you read on...

NOTbelieve everything

➎hydroboration

of BH3

addition

BH3 HBH2

simplificationgross

BH3 is

actually

HB

H HH

BHH

or

XB

HHH

R R

complex of

Lewis Base

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HH

BH

group 13

δ+δ–

HBH2

organoboranesare unstable

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HH

BH

group 13

δ+δ–

typical reaction

i. BH3ii. H2O2/ NaOH H

OH

MarkovnikovAddition

anti

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HH

BH

group 13

δ+δ–

δ+δ–>

areelectron donating

alkyl groups

HBH2 H

BH2

δ+

δ–

δ+

δ–

matchpolarities

step 1 hydroboration

HBH2

H2O2NaOH H

OH

oxidation

retention1,2-shift

step 2

hydroboration

H BH2 BH2H

H

H2O2NaOH

OHH

H

δ+

δ–δ+

δ–

Additionsyn

H BH2 BH2H

H

H2O2NaOH

OHH

H

hydroboration

Additionsyn

H OH

H OH

i. BH3ii. H2O2NaOH

stereospecificAdditionsyn

concertedbond formation

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anti-TBO

O

OH

H

H

colombiasin A

TBSO

OMeOMe

OMeH

i. BH3•THF;aq. NaOH, H2O2ii. PCC

(79% overall)TBSO

OMeOMe

OMeH

O

example