123.202 lecture 8 - alkenes
DESCRIPTION
123.202 alkenesTRANSCRIPT
©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
©tintin44@flickr
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