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