Low stereoselectivity in methylacetylene and cyclopropene reductions by nitrogenase

The stereochemistry of reductions catalyzed by nitrogenase in 2H2O has been investigated by using allene, methylacetylene, and cyclopropene as substrates. Deuterium labeling patterns in the reduction products were determined by mass spectroscopy, infrared spectroscopy, 2H-decoupled 220-MHz 1H NMR, and 1H-decoupled 30.7-MHz 2H NMR. Reduction of allene gave pure [2,3-2H2]propene. Reduction of methyl acetylene gave a 1.8:1.0 mixture of [cis- and [trans-1,2-2H2]propene. (Similar reduction of acetylene reportedly gave virtually all [cis-1,2-2H2]ethylene.) Reduction of cyclopropene gave [cis-1,2-2H2]cyclopropane and a mixture of [2H2]propenes. The major propene 2H2 isomers formed were [trans-1,3-2H2]-propene (approximately 2), [cis-1,3-2H2]propene (approximately 1) and [2,3-2H2]propene (approximately 1). Cyclopropene appears to be unique as a nitrogenase substrate in that it simultaneously undergoes parallel reductions, one of which proceeds with high stereoselectivity while the other proceeds with low stereoselectivity. The weakly selective stereochemistry observed in these reductions is not consistent with a completely concerted dual proton-dual electron transfer mechanism. The results provide a basis to probe stereochemical effects in nitrogenase and in biomimetic model systems.