C[BOND]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(LMe)Me] (; Cp*=η5-C5Me5, LMe=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 LMe. Complex is found to activate the C[BOND]H bonds of furan, thiophene, and benzene, giving rise to aryl complexes, [Cp*Fe(LMe)(aryl)] (aryl=2-furyl (), 2-thienyl (), phenyl ()). The C[BOND]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 (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 with 2 equiv of HBpin results in the quantitative formation of 2-boryl-furan and the borohydride complex [Cp*Fe(LMe)(H2Bpin)] (). Heating a solution of in the presence of tert-butylethylene led to the formation of an alkyl complex [Cp*Fe(LMe)CH2CH2tBu] (), which was found to cleave the C[BOND]H bond of furan to produce . On the basis of these results, a possible catalytic cycle is proposed.