Chemistry Reference
In-Depth Information
5.7. RECENT PROGRESS OF OTHER CARBON - HETEROATOM
BOND FORMATION
The purpose of this section is to highlight the recent exciting advances in asymmetric
carbon-heteroatom bond formation such as C-Si, C-B, and C-P bonds.
5.7.1. Diboration, Disilation, and Silaboration of Unactivated Olefi ns
Organoboranes and organosilanes are versatile reagents for organic synthesis. Recently,
catalytic enantioselective methods for the diboration, disilation, and silaboration of
unsaturated substrates have been developed [73]. In 1997, Ito, Suginome, and others
reported the impressive levels of asymmetric induction in the intramolecular disilation
with chiral isocyanide ligands [74]. Recently, Morken and others reported highly enan-
tioselective diboration reaction of unactivated olefi ns [75a]. In this study, the complex,
derived from Rh(I) salt and (
S
) - Quinap
188
, was found to be a highly effective catalyst
for the diboration of alkenes with B
2
(cat)
2
(bis(catecholato)diboron) [75]. Oxidation of
two C-B bonds in diboration product
185
provided chiral 1,2-diol
186
. Additionally, the
net enantioselective carbohydroxylation of alkene
184
giving product
187
was achieved
by tandem diboration/Suzuki cross-coupling/oxidation reaction (Scheme 5.56) [75b].
Fernandez and others also studied the chiral Ag-NHC complexes for diboration and the
effect of ligands in Rh - catalyzed diboration [76] .
OH
H
2
O
2
/NaOH
OH
p
-Tol
Me
Me
B(cat)
(nbd)Rh(acac)
186
(74%, 96% ee)
B(cat)
(
S
)-Quinap
188
p
-Tol
p
-Tol
Me
Me
Me
Me
B
2
(cat)
2
184
THF
185
OH
N
dppfPdCl
2
p
-Tol
N
N
Me
Me
OO
B
Ph
2
P
187
(58%, 96% ee)
Br
B
O
O
B
2
(cat)
2
then H
2
O
2
/NaOH
(
S
)-Quinap
188
Scheme 5.56.
Suginome and others reported the asymmetric silaboration of symmetrically substi-
tuted
meso
- methylenecyclopropanes via carbon - carbon bond cleavage (Scheme 5.57 )
[77] . With MePh
2
SiB(pin) as the silylboron reagent and chiral catalyst derived from
Pd(dba)
2
and chiral ligand
191
, methylenecyclopropane
189
was converted to the ring-
opened product
190
in 95% yield and 91% ee. The stereocontrol in silaboration of
unactivated dienes was also studied by Gerdin and Moberg [78].
