alkyne or alkene, which act as hydrogen acceptors. One of the first examples of a
selective catalytic systems was the reaction of ethylene or 1-hexene with Et 3 SiH
using [IrH 2 (SiEt 3 )(COD)L] (L ¼ PPh 3 or AsPh 3 ) as catalyst. The dehydrogenative
silylation products were obtained in relatively good selectivities. Only 20% and 8%
hydrosilylation products were obtained for ethylene and 1-hexene, respectively, with
L ¼ AsPh 3 [131]. When [Ir(μ-OMe)(COD)] 2 + 2 L systems were used as catalysts,
[IrH 2 (SiR 3 )(COD)L] species were formed by the oxidative addition of Et 3 SiH to the
[Ir(OMe)(COD)L] intermediates [132].
Excellent selectivities toward the β-(Z)-vinylsilane were obtained by
dehydrogenative silylation of a broad range of terminal alkenes using norbornene
derivatives as hydrogen acceptors. The catalyst was generated “in situ” from [Ir
(μ-OMe)(COD)] 2 and dtbpy (4,4-di-tert-butyl-2,2-bipyridine). The best results were
obtained at 40
C, in tetrahydrofurane, employing 3 equivalents of 2-norbornene and
Et 3 SiH. Under these conditions, this methodology gives rise to Z/E ratios up to 10/1
and almost quantitative yields in 2 h. Moreover, the presence of a variety of
functional groups – namely, ketones, ketals, amides, esters, alcohols, halides, epoxides, and silanes – was evaluated, showing no interference [135].
Hartwig et al. expanded this methodology to the use of more synthetically
interesting silanes, namely, (TMSO) 2 MeSiH [136]. In this study, several 1,10phenanthroline (phen) derivatives were tested as ligands with [Ir(μ-OMe)(COD)] 2 ,
[Ir(μ-OH)(COE) 2 ] 2 , or [Ir(μ-Cl)(COE) 2 ] 2 , showing that the Z/E diastereoselectivity
may be controlled by tuning the substituents at the phenanthroline scaffold. The
mechanistic studies presented in this work suggest the formation of the silylhydrido-Ir(III) active species upon oxidative addition of the Si–H bond. Subsequently, alkene coordination takes place, followed by a syn insertion of the alkene
into the Ir–Si bond. Rotation around the single C–C bond of the silylalkyl intermediate and subsequent β-hydrogen elimination yields the β-(Z )-vinylsilane. Finally,
hydrogenation of NBE (norbornene) to NBA (norbornane) and the ensuing activation of the silane regenerate the active species (Scheme 23).
Ir(III) complex 47 is able to catalyze a rare case of tandem isomerizationdehydrogenative silylation of alkenes; conversely, the related Rh 48 complex catalyzes very selectively the tandem isomerization/hydrosilylation of internal alkenes to
afford linear alkylsilanes (Fig. 19) [137]. For example, 1-octene, trans-2-octene,
trans-3-octene, and trans-4-octene can be converted by reaction with Et 3 SiH, into a
Z/E mixture of the allylic silane, i.e., triethyl(oct-2-en-1-yl)silane, in conversions
that range from 67 to 73%.
An interesting application of the dehydrogenative silylation of terminal alkenes is
the functionalization of monounsaturated fatty acids from vegetable oils, which may
be employed as renewable feedstock. In this regard, the system [Ir(μ-OMe)(COD)] 2 /
bipy acts as an efficient catalyst for the dehydrogenative silylation of methyl oleate
with Et 3 SiH to give mixtures of β-(Z )- and β-(E)-vinylsilanes in the presence of
NBE [138].
High selectivities toward the dehydrogenative silylation of several terminal
alkynes were achieved using a mixture Ir 4 (CO) 12 /PPh 3 as catalyst at 100
C, with
hydrosilanes. The use of 2.3 equivalents of the terminal alkyne, which acts as
Iridium-Catalyzed Silylation
251
selective catalytic systems was the reaction of ethylene or 1-hexene with Et 3 SiH
using [IrH 2 (SiEt 3 )(COD)L] (L ¼ PPh 3 or AsPh 3 ) as catalyst. The dehydrogenative
silylation products were obtained in relatively good selectivities. Only 20% and 8%
hydrosilylation products were obtained for ethylene and 1-hexene, respectively, with
L ¼ AsPh 3 [131]. When [Ir(μ-OMe)(COD)] 2 + 2 L systems were used as catalysts,
[IrH 2 (SiR 3 )(COD)L] species were formed by the oxidative addition of Et 3 SiH to the
[Ir(OMe)(COD)L] intermediates [132].
Excellent selectivities toward the β-(Z)-vinylsilane were obtained by
dehydrogenative silylation of a broad range of terminal alkenes using norbornene
derivatives as hydrogen acceptors. The catalyst was generated “in situ” from [Ir
(μ-OMe)(COD)] 2 and dtbpy (4,4-di-tert-butyl-2,2-bipyridine). The best results were
obtained at 40
C, in tetrahydrofurane, employing 3 equivalents of 2-norbornene and
Et 3 SiH. Under these conditions, this methodology gives rise to Z/E ratios up to 10/1
and almost quantitative yields in 2 h. Moreover, the presence of a variety of
functional groups – namely, ketones, ketals, amides, esters, alcohols, halides, epoxides, and silanes – was evaluated, showing no interference [135].
Hartwig et al. expanded this methodology to the use of more synthetically
interesting silanes, namely, (TMSO) 2 MeSiH [136]. In this study, several 1,10phenanthroline (phen) derivatives were tested as ligands with [Ir(μ-OMe)(COD)] 2 ,
[Ir(μ-OH)(COE) 2 ] 2 , or [Ir(μ-Cl)(COE) 2 ] 2 , showing that the Z/E diastereoselectivity
may be controlled by tuning the substituents at the phenanthroline scaffold. The
mechanistic studies presented in this work suggest the formation of the silylhydrido-Ir(III) active species upon oxidative addition of the Si–H bond. Subsequently, alkene coordination takes place, followed by a syn insertion of the alkene
into the Ir–Si bond. Rotation around the single C–C bond of the silylalkyl intermediate and subsequent β-hydrogen elimination yields the β-(Z )-vinylsilane. Finally,
hydrogenation of NBE (norbornene) to NBA (norbornane) and the ensuing activation of the silane regenerate the active species (Scheme 23).
Ir(III) complex 47 is able to catalyze a rare case of tandem isomerizationdehydrogenative silylation of alkenes; conversely, the related Rh 48 complex catalyzes very selectively the tandem isomerization/hydrosilylation of internal alkenes to
afford linear alkylsilanes (Fig. 19) [137]. For example, 1-octene, trans-2-octene,
trans-3-octene, and trans-4-octene can be converted by reaction with Et 3 SiH, into a
Z/E mixture of the allylic silane, i.e., triethyl(oct-2-en-1-yl)silane, in conversions
that range from 67 to 73%.
An interesting application of the dehydrogenative silylation of terminal alkenes is
the functionalization of monounsaturated fatty acids from vegetable oils, which may
be employed as renewable feedstock. In this regard, the system [Ir(μ-OMe)(COD)] 2 /
bipy acts as an efficient catalyst for the dehydrogenative silylation of methyl oleate
with Et 3 SiH to give mixtures of β-(Z )- and β-(E)-vinylsilanes in the presence of
NBE [138].
High selectivities toward the dehydrogenative silylation of several terminal
alkynes were achieved using a mixture Ir 4 (CO) 12 /PPh 3 as catalyst at 100
C, with
hydrosilanes. The use of 2.3 equivalents of the terminal alkyne, which acts as
Iridium-Catalyzed Silylation
251