catalyst, it is worth mentioning that Pt, Co, and Ru catalysts showed no activity for
this reaction. The hydrosilylation of unsymmetrical internal alkynes may afford four
different vinylsilanes, the β-(E), β-(Z), α-(E), and α-(Z). Remarkably, this methodology allows the selective preparation of the α-(Z) isomer, i.e., the reaction is α
regio- and syn stereo-selective. A broad variety of thioalkynes with several synthetically relevant hydrosilanes were tested, leading to yields up to 89% and selectivities
β/α and Z/E up to >30/1.
A mechanism that explains the selectivity of the reaction was proposed and
substantiated by DFT studies (Scheme 20). Chalk-Harrod-type mechanisms were
discarded based on the high energy barrier of the processes. The postulated mechanism consists of the formation of an unsaturated Ir-hydride species by reaction with
two equivalents of the silane. This species is able to coordinate the alkyne, which
evolves to give the more stable S-chelated intermediate. Finally, oxidative addition
of the Si–H bond leads to formation of an Ir(III) intermediate, which upon reductive
elimination affords the vinylsilane and regenerates the Ir-H active species.
Complex [IrH(κ [9]-hqca)(COE)] (46) (COE ¼ cyclooctene; hqca ¼ 8oxidoquinoline-2-carboxylate), an iridium(III) hydride complex featuring an
ONO-pincer ligand, proved an efficient catalyst for the hydrosilylation of a variety
of aromatic and aliphatic terminal alkynes (Fig. 18). The reaction was selective
toward the β-(Z)-vinylsilane in general, but bulky substituents, such as t-butyl, led to
preferential formation of the β-(E) isomer. It is noteworthy that, after consumption of
(EtO) 3 SiH
[ICl(COD)] 2 (2 mol%)
R
1
SR
2
R
1
SR
2
a -(Z)-vinylsilane
Si(OEt) 3
H
CH 2 Cl 2 , rt, 24 h
Scheme 19 Selective
hydrosilylation of internal
thioalkynes catalyzed by [Ir
(μ-Cl)(COD)] 2
1/2 [IrCl(COD)] 2
Si H
H
Ir
Si Cl
+
Si H
Me
SMe
Ir
H
Si
Me
Me
S
H
H
Ir
Me
S
Me
H
H
Ir
Me
MeS
Me
MeS
H
Si
Scheme 20 Catalytic cycle
proposed for the selective
formation of the α-(Z )vinylsilane
248
M. Iglesias and L. A. Oro
this reaction. The hydrosilylation of unsymmetrical internal alkynes may afford four
different vinylsilanes, the β-(E), β-(Z), α-(E), and α-(Z). Remarkably, this methodology allows the selective preparation of the α-(Z) isomer, i.e., the reaction is α
regio- and syn stereo-selective. A broad variety of thioalkynes with several synthetically relevant hydrosilanes were tested, leading to yields up to 89% and selectivities
β/α and Z/E up to >30/1.
A mechanism that explains the selectivity of the reaction was proposed and
substantiated by DFT studies (Scheme 20). Chalk-Harrod-type mechanisms were
discarded based on the high energy barrier of the processes. The postulated mechanism consists of the formation of an unsaturated Ir-hydride species by reaction with
two equivalents of the silane. This species is able to coordinate the alkyne, which
evolves to give the more stable S-chelated intermediate. Finally, oxidative addition
of the Si–H bond leads to formation of an Ir(III) intermediate, which upon reductive
elimination affords the vinylsilane and regenerates the Ir-H active species.
Complex [IrH(κ [9]-hqca)(COE)] (46) (COE ¼ cyclooctene; hqca ¼ 8oxidoquinoline-2-carboxylate), an iridium(III) hydride complex featuring an
ONO-pincer ligand, proved an efficient catalyst for the hydrosilylation of a variety
of aromatic and aliphatic terminal alkynes (Fig. 18). The reaction was selective
toward the β-(Z)-vinylsilane in general, but bulky substituents, such as t-butyl, led to
preferential formation of the β-(E) isomer. It is noteworthy that, after consumption of
(EtO) 3 SiH
[ICl(COD)] 2 (2 mol%)
R
1
SR
2
R
1
SR
2
a -(Z)-vinylsilane
Si(OEt) 3
H
CH 2 Cl 2 , rt, 24 h
Scheme 19 Selective
hydrosilylation of internal
thioalkynes catalyzed by [Ir
(μ-Cl)(COD)] 2
1/2 [IrCl(COD)] 2
Si H
H
Ir
Si Cl
+
Si H
Me
SMe
Ir
H
Si
Me
Me
S
H
H
Ir
Me
S
Me
H
H
Ir
Me
MeS
Me
MeS
H
Si
Scheme 20 Catalytic cycle
proposed for the selective
formation of the α-(Z )vinylsilane
248
M. Iglesias and L. A. Oro