Chemistry Reference
In-Depth Information
Table 11.1 Cross-coupling of racemic allylic boranes.
55,a
R
2
R
2
BPin
Ph
Ph
PhI
+
R
2
R
1
R
1
R
1
R
3
R
3
Pd/PPh
3
/Ag
2
O
R
3
g
-product
a
-product
g : a
a
E : Z
b
Yield (%)
c
Entry
Compound
BPin
1
97 : 3
99 : 1
65
Ph
H
3
C
(
E
)-60
CH
3
BPin
2
92 : 8
99 : 1
70
Ph
(
Z
)-61
CH
3
BPin
3
92 : 8
99 : 1
40
H
3
C
Ph
62
BPin
28
d
4
39 : 61
86 : 14
Ph
Ph
(
E
)-63
BPin
n.d.
e
5
8 : 92
58
Ph
C
4
H
9
(
E
)-64
BPin
n.d.
e
6
19 : 81
53
Ph
C
6
H
13
(
E
)-65
a
Conditions: PhI (1 equiv.), Pd(dba)
2
(5%), PPh
3
(10%), Ag
2
O (1.5 equiv.), DME (0.1 M), 90 1C, 16 h.
b
g : a ratio determined by GC and/or
1
HNMRspectroscopyandE : Z ratio given for the major isomer.
c
Isolated yield of major isomer unless noted otherwise.
d
Yield of a mixture of regioisomers.
e
Not determined.
98 : 2 ratio with the corresponding a-product (Scheme 11.7). When the
isomeric substrate 75 was employed, the same major product was obtained,
but the selectivity was significantly lower. This implies that intermediate II
formed by transmetallation via allylic transposition is more prone to
equilibration via p-allyl-Pd intermediates in cases where this results in
re-establishment of stable styrene-type units.
55
Aggarwal's group has also demonstrated that propargylic boronic esters
also react with high yield and perfect enantiospecificity under the conditions
initially developed for benzylic systems, even in the case when the boron
substituent is at a fully substituted tertiary carbon atom [eqn (11.18)].
56
This
method leads to a very valuable synthetic route for the preparation of chiral
tetrasubstituted allenes.
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