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