Chemistry Reference
In-Depth Information
Pd(dba)
2
Ligand
191
Toluene
H
SiMePh
2
B(pin)
+
MePh
2
Si
B(pin)
H
189
190
(95%, 91% ee)
PAr
2
O
MePh
2
Si
B
Ar =
O
MePh
2
Si
B(pin)
Ligand
191
Scheme 5.57.
Suginome and others studied the Pd-catalyzed silaboration of terminal allenes [79].
The reaction proceeded with high regioselectivities, affording the allylsilanes bearing a
boryl group at
- position. At fi rst, Suginome, Murakami, and others reported that a high
level of diastereoselectivity was induced from both a chiral ligand and a chiral auxiliary
on silylborone reagents [79a]. More recently, the catalytic asymmetric silaboration using
achiral silylborone reagent Me
2
PhSiB(pin) in the presence of a catalyst, derived from
Pd(dba)
2
and the chiral monodentate phosphine ligand
191
, was reported (Scheme 5.58)
[79b] . When employing mono - substituted allene
192
, good enantioselectivity was
observed to give the allylsilane
193
in 90% yield and 91% ee.
β
S
iMePh
2
Pd(dba)
2
Ligand
191
Toluene
+
MePh
2
Si
B(pin)
B(pin)
193
(90%, 91% ee)
192
Scheme 5.58.
Catalytic asymmetric biboration of terminal allenes was studied by Morken and
others [80]. The reaction of allene
194
with B
2
(pin)
2
in the presence of a catalyst, derived
from Pd(dba)
2
and a chiral phosphoramidite ligand
196
, gave 1,2-bis(boronate) ester
195
in high enantioselectivity (Scheme 5.59) [80b]. Mechanistic studies were also under-
taken. The catalytic cycle proceeds by a mechanism involving rate-determining oxidative
addition of the diboron to Pd, which is followed by the transfer of both boron groups
to the unsaturated substrate via a π-allyl complex. Synthetic utility of this reaction was
demonstrated by the subsequent transformations of 1,2-bis(boronate) ester products
into homoallylic alcohols and β - amidoketones [80c,d] .
