"Front Matter". In: Organosilanes in Radical Chemistry - Index of

Carbon–Carbon Double Bonds 95
However, poor to moderate yields were obtained for the reaction of Et3SiH at
60 8C [28]. Thiol catalysis is more effective for the addition of arylsilanes than
trialkylsilanes, presumably because the hydrogen abstraction by thiyl radical is
more rapid from aromatic silanes (Reactions 5.10 and 5.11) [29,30]. This methodology
has also been extended to enantioselective hydrosilylation either using 1,2asymmetric
induction [29] or optically active thiols [30,31]. Reaction (5.10) shows
the hydrosilylation of a-chiral (E)-and (Z)-alkenes 9 with Ph2SiH2. The reaction
of (E)-alkene could occur via a Felkin–Ahn transition state similar to 8 (see
above), whereas an allylic-strain transition state 10 might explain the 1,2-stereoinduction
with (Z)-alkene, since the silyl radical attacks preferentially from the
face without significant steric hindrance from the neighbouring dioxolane ring.
Reaction (5.11) shows the hydrosilylation of methylenelactone 11 with a few
silicon hydrides and thioglucose tetraacetate 12 as the catalyst. Both yields and
enantiomeric purities increase with the degree of phenyl substitution at silicon.
Thiols have also been shown to catalyse the addition of (TMS) 3SiH to alkenes [31].
O
O
9
O
11
Ph2SiH2 t-C12H25SH AIBN, 70 �C
CO2Et (E)-9
(Z)-9
O
O
H
O
Ph 2 HSi
Ph 2 HSi
O
10
H
+
CO 2 Et
CO 2 R
O
Ph 2 HSi
O
69%, syn:anti = 70:30
75%, syn:anti = 91:9
AcO
AcO
O
AcO
SH
+
O
R3SiH 12
t-BuONNOBu-t, 60 �C
R3Si *
O O
PhMe 2SiH
Ph 2MeSiH
Ph 3 SiH
(TMS) 3SiH
OAc
52%, ee 23%
65%, ee 32%
72%, ee 50%
92%, ee 47%
CO 2 Et
(5.10)
(5.11)
Higher enantioselectivities were generally found when b-mannose thiol 13
was used as hydrogen donor. The extra bulkiness provided by the gem-bdiphenyl
groups in alkene 14 compared to alkene 11 is indicated to be responsible
for the high enantiomeric purity observed (Reaction 5.12) [31].