Background and Originality Content
Homoallylic silanes and allylic
silanes are versatile organosilicon compounds that play important roles
in organic synthesis, polymerization, and functional
materials.1 As the demand for organosilicon materials
increases, developing efficient and pragmatic approaches to produce
homoallylic silanes and allylic silanes with higher purity and
functionality is crucial. The transition-metal-catalyzed hydrosilylation
of conjugated dienes has emerged as one of the mainstream methods for
providing alkenyl-substituted silanes,2 including
diversified allylic silanes from various available conjugated
dienes.3 However, controlling the regioselectivity in
the hydrosilylation of conjugated dienes has always been a serious issue
for chemists. This is mainly due to two aspects (Scheme 1a): firstly,
conjugated dienes tend to form bidentate coordination with the
transition metal, resulting in the absolute advantage of
1,4-hydrosilylation;4 secondly, due to the relatively
stable π-allyl metal intermediates, Markovnikov selectivity is dominant
in 3,4-hydrosilylation.5 Therefore, efficiently
controlling the regioselectivity of the hydrosilylation of conjugated
dienes to synthesize homoallylic silanes is a great challenge.
In past decades, several different types of multidentate ligands (or
multiple ligands) have been employed to afford the
3,4-anti -Markovnikov selectivity in the hydrosilylation of
conjuga-
Scheme 1 Challenges and strategies for regioselective
hydrosilylation of conjugated dienes
ted dienes (Scheme 1b). These ligands can preferentially occupy the
multiple coordination sites of the metal center to force conjugated
dienes to react in a monodentate coordination mode and avoid the
formation of π-allyl metal intermediates, thus facilitating the
occurrence of 3,4-anti -Markovnikov hydrosilylation. In 2014,
Ritter group successfully achieved Pt-catalyzed
3,4-anti -Markovnikov hydrosilylation of 1,3-butadiene with an
alkyl phosphine ligand.6 Compared with noble metals,
earth-abundant metals, such as iron and cobalt, have smaller atomic
radius, less coordination number, and stable oxidation state, showing
better adaptability to the ligand-control strategy. Co-tridentate-ligand
complexes reported by Fout and RajanBabu, respectively, were active for
the 3,4-anti -Markovnikov hydrosilylation of conjugated dienes
within limited ranges.7 Thomas and Chen groups
reported Fe-catalyzed selective 3,4-anti -Markovnikov
hydrosilylations of isoprene using tridentate pyridine diamine as
ligand.4m, 5a Most recently, Zhu’s group reported a
Fe-TPI (tridentate phenanthroline imine) system to complete the
3,4-anti -Markovnikov hydrosilylation of various conjugated
dienes.8 Although the ligand-control strategies have
been successfully applied, the introduction of elaborate multidentate
ligands has caused high cost and reduced catalytic efficiency, immensely
limiting its application in large-scale production. Therefore, it is
necessary and urgent to explore alternative routes to solve this problem
preferably.
Electrophilic activation hydrosilylation is a highly efficient
silylation process catalyzed by Lewis acidic
transition-metals,9 our group have successfully
accomplished an exclusively selective hydrosilylation of olefins with
primary silanes via electrophilic activation process
recently.10 In our assumption (Scheme 1c), the absence
of migration insertion process of C=C bond to active metal−hydride bond
in the electrophilic activation hydrosilylation can originally avoid the
selectivity caused by coordination mode differences. Meanwhile, the
activated Si−H bond tends to react directly with the electron-rich
terminal alkene moiety to deliver the desired
3,4-anti -Markovnikov hydrosilylation product. Herein, we have
developed a ligand-free strategy based on the Nickel/t -BuOK
system to successfully achieve highly efficient
3,4-anti -Markovnikov hydrosilylation of aromatic conjugated
dienes with excellent regioselectivity and broad substrate scope.
Additionally, with the assistance of elementary monophosphine ligand, we
can easily tune the selectivity toward 3,4-Markovnikov hydrosilylation
(Scheme 1c).
Results and Discussion
Table 1 Reaction optimization for 3,4-anti -Markovnikov
hydrosilylationa