Thioether-facilitated iridium-catalyzed hydrosilylation of steric 1,1-disubstituted olefins
One facile and efficient strategy for the hydrosilylation of steric 1,1disubstituted terminal alkenes is demonstrated. Investigations on substrate scope and control experiments revealed the necessity of thioether in promoting this process under a simple iridium catalysis system. This convenient and feasible method is expected to be useful in the synthesis of sulfur-containing organosilicon polymers with different side-chains.
Introduction<!>Results and discussion<!>Conclusions<!>Conflicts of interest
<p>Catalytic hydrosilylation of unsaturated bonds has evolved into one powerful tool in creating organosilicon compounds for multitudinous uses. 1 A substantial proportion of this rapidly developing field is the hydrosilylation of 1-substituted alkenes, which has been well developed and widely used in building macromolecular organosilicon compounds of great industrial importance. 2 For instance, various novel polycarbosilanes have been accessed through diverse hydrosilylation polymerization strategies (a, Scheme 1). 3 Besides the numerous alterations in polymer backbones, the introduction of different side chains is expected to have potential beneficial impact on the properties of these intriguing materials. 4 Polymerization based on hydrosilylation of 1,1,-disubsituted alkenes would be one convenient and direct method to achieve this, through which functional substitutions could be anchored next to the silicon center (b, Scheme 1). However, as the steric hindrance in the alkene substrate usually retards the hydrosilylation process, efficient methods toward hydrosilylation of bulky 1,1disubstituted alkenes are far less developed, resulting in the rare reports on corresponding applications in material science. 5 Scheme 1 Hydrosilylation of alkenes.</p><p>In the transition-metal catalyzed functionalizations of unactivated alkenes, the efficiency and selectivity of these processes could be improved by introducing directing groups into the alkene substrates. 6 Utilization of thioether as directing group is of great interest, since thioethers are important substructures in diverse natural products, pharmaceuticals, polymers and functional materials, and easily accessible from a number of synthetic pathways. 7 However, due to the poisonous character of sulfides in deactivating transition metal catalysts and the easy C-S bond cleavage under these circumstances, 8 successful usages of thioether to promote and direct reaction processes are still limited. 9 In the area of alkene hydrosilylations, Van Vranken et al revealed the necessity of thioether in accelerating hydrosilylation of olefins under platinum catalysis. 10 However, only one example of hydrosilylating 1,1disubstituted alkene substrate was presented in their work, and allylic sulfides were demonstrated to react sluggishly under this hydrosilylation condition. Shimada reported the efficient Please do not adjust margins Please do not adjust margins iridium-catalyzed hydrosilylation of various sulfur-containing olefins, but with no investigation on 1,1-disubstituted alkenes. 11 Meanwhile, we disclosed the generality of iridium catalysis in hydrosilylating unactivated 1-substituted alkenes, and likewise probed its efficiency towards sterically hindered disubstituted terminal alkenes. 12 The substrate involving thioether group showed its specificity in affording hydrosilylation product. Encouraged by the promising applications of this type of structure and corresponding synthetic methods in manufacturing new interesting materials, as well as our findings that the interaction between a heteroatom (O, N, S) and iridium catalyst can effectively enhance diverse hydrosilylation processes, 13 here we showed our investigations on the thioether-facilitated hydrosilylation of steric 1,1-disubstituted alkenes under a simple iridium catalytic system (c, Scheme 1). A variety of substrates, including allylic and homoallylic sulfides, were hydrosilylated with high efficiency and selectivity. Control experiments revealed the necessity of thioether in promoting this process. We began our research by using sulfur-containing 1,1disubstituted alkene 1a as the model substrate and PhMe2SiH as the silyl source (Table 1). A few simple commercial iridium catalysts were first screened with DCM as the solvent under room temperature, during which a 89% NMR yield of desired product was observed with the utilization of [Ir(COD)Cl]2 (entries 1-4, Table 1). The yield was optimized to 95% by using 1,4-dioxane instead of DCM (entries 6, Table 1). 1 H NMR control experiment showed this process completed within 2 hours, with observation of trace amount of by-products (Supplementary Figure 1). Considering that both the steric effect of the hydrosilanes and the different electronic property of the Si-H bond of them might influence the hydrosilylation process, 13,14 we next examined the hydrosilane scopei. Decreased yields were obtained with the utilization of Ph2MeSiH, Ph3SiH, BnMe2SiH and Et3SiH (entries 8-11, Table 1). The by-product from alkene hydrogenation was confirmed as the major through GC/MS and 1 H NMR analysis (Supplementary Figure 2). Usage of (EtO)3SiH gave no desired product as well (entry 12, Table 1). The difference is that besides a little partial consumption via side reactions, most of the alkene substrate remained unchanged in this case (Supplementary Figure 3). A variety of powerful non-precious-metal catalytic systems have been developed recently for hydrosilylation of 1,1-disubstituted alkenes, including the valuable asymmetric hydrosilylation protocols. 15 The cobalt/isocyanide system reported by Nagashima aroused our interest due to its simple condition and high catalytic activity toward -methylstyrene derivatives. 16 However, treatment of the sulfur-containing alkene 1a with the Co(OAc)2/CNAd catalyst gave no desired hydrosilylation product, which might be caused by the deactivation of cobalt catalyst in the presence of sulfur unit (entries 13 & 14, Table 1).</p><!><p>Scheme 2 Scope of 1,1-disubstituted alkenes. Reaction conditions: 1 (0.50 mmol, 1.0 eq.), 2 (0.75 mmol, 1.5 eq.), [Ir(COD)Cl]2 (0.01 mmol, 2 mol %) in 1,4-dioxane (2.0 mL, 0.25 M) at room temperature for 4 h under N2. Isolated yields. With the establishment of the simple and mild condition, we next sought to investigate its generality towards diverse sulfurinvolving 1,1-disubstituted alkenes (Scheme 2). The reactions of (hetero)aryl allyl sulfides bearing different functional groups were first carried out to probe the efficiency of this catalytic system, all of which went smoothly to afford corresponding hydrosilylated products in good to excellent yields (3a to 3f). Alkyl allyl sulfides could likewise participate in this process, providing the desired products 3j-3k with high efficiency. This Please do not adjust margins Please do not adjust margins protocol is applicable to homoallylic sulfides as well, with the observation of slightly decreased yields (3l to 3n). Besides the methyl group at the  position of alkenes, ethyl and phenyl groups are likewise well tolerated, which allows for the various side-chain decorations in potential polymerization applications (3o to 3r). To reveal the importance of sulfur unit in facilitating this hydrosilylation process, several 1,1-disubstituted alkenes bearing no thioether group were tested under this condition. GC/MS and 1 H NMR analysis (Supplementary Figures 4 to 6) gave no evidence for the formation of hydrosilylation adducts (3s to 3u). Intriguingly, the mixture of 1u with Et3SiH was also stable in the presence of [Ir(COD)Cl]2 (Supplementary Figure 7), which might be due to that the prior coordination of 1u with iridium catalyst prevents its interaction with Et3SiH. To probe the potential application of this protocol in polymer science, we then tested its usability in dihydrosilylation reactions (Scheme 3). In the presence of PhMe2SiH, the dialkenyl-substituted sulfide 1v was converted into dihydrosilylated product 3v in 79% isolated yield. Furthermore, dihydro-substituted silane 2b proved to be effective in modifying and connecting sulfur-containing olefin units (3ab, 3cb and 3ib). These positive results prompted us to investigate the efficiency of this method in hydrosilylation polymerizations (Scheme 4a). 1 H NMR result evidenced the quantitative conversion of monomers (Scheme 4b). GPC analysis illustrated the successful formation of polymer P1 (Mn = 3063 Da, Mw = 5951 Da, PDI = 1.94, Scheme 4c).</p><p>A possible mechanism for this process is proposed on the basis of previous reports and our observations (Scheme 5). 11,12,17 The formation of Ir-H is supported by the observation of hydride signal at -14.5 ppm in the 1 H NMR control experiment study (Supplementary Figure 9A). Existence of sulfur is supposed to benefit the coordination of steric 1,1-disubstituted alkene with iridium center and the stability of possible intermediates C and D. The chemical shift vibrations of protons from 1a shown in Supplementary Figure 10 could in some degree identify the interaction between sulfur and iridium. Usage of (EtO)3SiH instead of PhMe2SiH could likewise promote the formation of intermediate A and C (A and B, Supplementary Figure 11). However, most of the possible proton signals for intermediate C remained unchanged with the addition of two more equivalent of (EtO)3SiH, and no information of desired hydrosilylation product was observed (Supplementary Figure 11C). The different electronic property of the hydrosilanes might be one major factor in leading to these distinctions.</p><!><p>In summary, we demonstrated one simple and mild strategy for the hydrosilylation of steric sulfur-involving 1,1-disubstituted alkenes. The thioether group proves to be critical in facilitating this iridium-catalyzed process. This protocol was successfully utilized for the polymerization between disilane and diene. A plausible mechanism involving Ir-H species was proposed. Further application of this protocol toward more hydrosilylation polymerizations and modifications is in progress. This journal is © The Royal Society of Chemistry 20xx</p><p>Please do not adjust margins</p><!><p>There are no conflicts to declare.</p>