C-H bond activation/borylation of furans and thiophenes catalyzed by a half-sandwich iron N-heterocyclic carbene complex.

A coordinatively unsaturated iron-methyl complex having an N-heterocyclic carbene ligand, [Cp*Fe(L(Me))Me] (1; Cp*=eta(5)-C(5)Me(5), L(Me)=1,3,4,5-tetramethyl-imidazol-2-ylidene), is synthesized from the reaction of [Cp*Fe(TMEDA)Cl] (TMEDA=N,N,N',N'-tetramethylethylenediamine) with methyllithium and L(Me). Complex 1 is found to activate the C-H bonds of furan, thiophene, and benzene, giving rise to aryl complexes, [Cp*Fe(L(Me))(aryl)] (aryl=2-furyl (2), 2-thienyl (3), phenyl (4)). The C-H bond cleavage reactions are applied to the dehydrogenative coupling of furans or thiophenes with pinacolborane (HBpin) in the presence of tert-butylethylene and a catalytic amount of 1 (10 mol% to HBpin). The borylation of the furan/thiophene or 2-substituted furans/thiophenes occurs exclusively at the 2- or 5-positions, respectively, whereas that of 3-substituted furans/thiophenes takes place mainly at the 5-position and gives a mixture of regioisomers. Treatment of 2 with 2 equiv of HBpin results in the quantitative formation of 2-boryl-furan and the borohydride complex [Cp*Fe(L(Me))(H(2)Bpin)] (5). Heating a solution of 5 in the presence of tert-butylethylene led to the formation of an alkyl complex [Cp*Fe(L(Me))CH(2)CH(2)tBu] (6), which was found to cleave the C-H bond of furan to produce 2. On the basis of these results, a possible catalytic cycle is proposed.