In 1972, Kumada's group and Corriu's group independently reported cross-coupling reaction of Grignard reagents with aryl and alkenyl halides catalyzed by nickel(II) halides. 1 The catalytic cycle, which involves oxidative addition, transmetalation, and reductive elimination steps, has become a prototype of a more practical Pdcatalyzed cross-coupling reaction. These reactions proceed smoothly using a variety of organometallic reagents containing B, Mg, Li, Sn, Al, and Zn as the metal connecting to alkyl, alkenyl, aryl, alkynyl, allyl, and benzyl groups as the organic part. 2 As for the coupling partner, however, the scope is generally limited to aryl and alkenyl moieties. The use of alkyl halides, triflates, or tosylates usually gives unsatisfactory results due mainly to the slow oxidative addition to transition metal catalysts and the facile -elimination from the alkylmetal intermediates. Thus, the alkyl-alkyl crosscoupling reaction catalyzed by transition metal complexes has remained as an interesting and challenging theme to be solved in this field. 3-7 Recently, we have developed regioselective monoand dialkylation of alkenes or dienes with alkyl halides or tosylates using titanocene 8 or zirconocene 9 catalysts. During the course of our study on transition metal catalyzed alkylation reactions, we have found that Ni catalyzes the cross-coupling reaction of alkyl chlorides, bromides, and tosylates with Grignard reagents in the presence of a 1,3-butadiene as an additive (eq 1).For example, a reaction of n-decyl bromide with n-butylmagnesium chloride (1.3 equiv) in the presence of isoprene (1.0 equiv) and NiCl 2 (0.03 equiv) at 25°C for 3 h gave tetradecane in 92% yield along with trace amounts of decane (<1%) and decenes (2%) ( Table 1, entry 1). In the absence of isoprene, tetradecane was obtained in only 2% yield and significant amounts of decane and decenes were formed (entry 2). The use of Ni(acac) 2 and Ni(COD) 2 also afforded tetradecane in high yields (entries 3 and 4). When nickel complexes bearing phosphine ligands, such as NiCl 2 (PPh 3 ) 2 and NiCl 2 (dppp), were used, tetradecane was obtained only in 45% and 22% yields, respectively. Under similar conditions, FeCl 3 and CoCl 2 (dppe) were ineffective, and PdCl 2 gave a moderate yield of tetradecane (entry 5). Next, we examined the effect of additives which are essential to promote the present coupling reaction. Unsubstituted 1,3-butadiene shows by far the highest activity for this cross-coupling reaction (entry 6). 2,3-Dimethyl-1,3-butadiene, COD, alkynes, and alkenes are far less effective under the same conditions (entries 7-10).Optimization of the reaction conditions using 1,3-butadiene revealed that use of only 1 mol % of NiCl 2 and 10 mol % of 1,3-butadiene (0.07 M in THF, 10 equiv to Ni catalyst) based on the halides at 0°C afforded coupling products quantitatively in the reaction of primary bromides with primary alkyl Grignard reagents (Table 2, entries 1 and 2). Interestingly, the bromo substituent on the aryl ring remained intact in this r...
