a Standard Reaction Conditions: 1a (130.1 mg, 1.0 mmol, 1.0 equiv.), 2a (130.0 mg, 1.2 mmol, 1.2 equiv.), Ni(DME)Cl2 (2.2 mg, 0.01 mmol, 1.0 mol%), t -BuOK (4.5 mg, 0.02 mmol, 2.0 mol%), THF (3.0 mL), -30 ºC, 6 h.b Conversions of 1a and yields of3aa were determined by GC analysis (internal standard, 1,2,4,5-tetramethylbenzene). c r.r. was referred to as the ratio of 3aa and4aa , which were determined by 1H NMR analysis. d l:b was referred to as the ratio of 3aa and 5aa , which were determined by1H NMR analysis. e Isolated yield.
Based on previous experience,10 low temperatures are usually favorable for the electrophilic activation processes. Hence, we began the investigation under -30 ˚C using Ni(DME)Cl2(1.0 mol%) as precatalyst and t -BuOK (2.0 mol%) as activator (Table 1). Surprisingly, the reaction of (E )-buta-1,3-dien-1-ylbenzene (1a ) and phenylsilane (2a ) could give the desired 3,4-anti -Markovnikov product3aa with excellent yield (90% GC yield) in THF after 6 hours (entry 1), and the regioselectivity (r.r. ) andanti -Markovnikov/Markovnikov selectivity (l:b ) of the hydrosilylation were both preeminent (>99:1). Through a further nickel catalysts evaluation, Ni(DME)Cl2 still was the best choice. Changing anion or coordinating molecule led to decreases in conversion (entry 2) or yield (entry 3). Other available Ni(II) salt caused lower regioselectivity (entries 4-6). For the activators, the weaker bases delivered low results because of their relatively weak activation to silane (entries 8, 9) or low solubility (entry 10). Moreover, cations
Scheme 2 Substrate scope for 3,4-anti -Markovnikov hydrosilylation of conjugated dienesa
a General Reaction Conditions: 1 (1.0 mmol, 1.0 equiv.), 2 (1.2 mmol, 1.2 equiv.), Ni(DME)Cl2 (2.2 mg, 0.01 mmol, 1.0 mol%), t -BuOK (4.5 mg, 0.02 mmol, 2.0 mol%), THF (3.0 mL), -30 ºC, 6 h. The yields are isolated yields. The value of r.r. and l:b were determined by 1H NMR analysis. b The reaction time was reduced to 0.5 h. c Under 25 ºC.
in activator had important effect on reaction due to their different association modes in organic solution,11t -BuONa and t -BuOLi gave very poor conversions but good selectivity (entries 11, 12). Solvents screening showed that coordinating solvents were beneficial for this catalytic hydrosilylation, the reactions proceeded normally in these solvents, such as DME and CH3CN, (entries 14, 15), but not worked in CH2Cl2 at all (entry 16). Additionally, temperature also has a significant impact on the reaction results. The even lower temperature directly led to the deactivation of the reaction, possibly due to the inability of nickel(II) to be reduced (entry 17). While the reaction under room temperature delivered obvious secondary hydrosilylation product 6aa (entry 18). Furthermore, several controlled experiments were also conducted. In the absence of catalysts or activators, the reactions could not proceed normally (entries 7, 13). Eventually, the optimum reaction conditions for 3,4-anti -Markovnikov hydrosilylation of conjugated dienes are determined that 1.0 mol% of Ni(DME)Cl2 as the catalyst, 2.0 mol% of t -BuOK as the activator in THF at -30oC within 6 h.
With the optimized conditions in hand, we set out to explore the generalities of substrates (Scheme 2). Under these conditions, various homoallylic silanes were obtained in moderate to good yields and high regioselectivities via 3,4-anti -Markovnikov hydrosilylation. Especially for the electron-rich aromatic conjugated dienes, the corresponding homoallylic silanes were always generated in excellent yields (>85%) and regioselectivities (up to 99:1), where various electron-donating groups, such as methoxy (3ba -3da ), methylthio (3ea ), dimethylamino (3fa ), naphthyl (3ga , 3ha ) and ferrocenyl (3ia ) groups were tolerated well. Meanwhile, the heteroaromatic conjugated dienes can react with 2a to give the homoallylic silanes with high regioselectivities but sharply reduced yields, because of the generation of polymerized by-products. When we shorten the reaction time to 0.5 h, 3ja and 3ka could also be obtained in good yields. Unfortunately, the reactions of electron-deficient aromatic conjugated dienes gave the poor results, because of the overly substantial coordination between the strong electron-deficient conjugated dienes and the nickel center.12 The conjugated dienes with the strong electron-withdrawing substituents, like trifluoromethyl (3la ), cyano (3ma ) and nitro (3na ) groups, were unable to be successfully converted. While raising the temperature could improve the conversion of 3l , the regioselectivity was lost. With the weak electron-withdrawing substituents, like phenyl (3oa ) and halogen (3pa-3ra ) groups, the conjugated dienes can be successfully converted into homoallylic silanes in good yields with slightly declining regioselectivities. Typically, for the aliphatic conjugated dienes, the reaction of 3s delivered a mixture of regioisomers (3sa ). Furthermore, types of primary silanes were tested. Phenylsilanes with different electrical properties could react ordinarily and produce the corresponding homoallylic silanes, like3ab and 3ac . While aliphatic primary silanes were unactive, no conversion of 1a in the reaction with 2dwas observed.
To the best of our knowledge, there is no successful example of nickel-catalyzed 3,4-Markovnikov hydrosilylation of conjugated dienes. Fortunately, in our attempt to improve the regioselectivity of the reaction with aliphatic conjugated dienes by introducing external ligands, we found that the addition of monophosphine ligands flipped the selectivity of the reaction to 3,4-Markovnikov addition when the temperature was raised from -30 oC to room temperature. (See Supporting Information). Hence, we decided to optimize the conditions for 3,4-Markovnikov addition (Table 2).Si-
Table 2 Ligand effects on 3,4-Markovnikov hydrosilylationa