Synthesis and reactivities of pyrrolylimido complexes of molybdenum and tungsten: Formation of pyrrole and N-aminopyrrole from molecular nitrogen

Article abstract of DOI:10.1021/ja00154a019

Hydrazido(2-) complexes trans-[MX(NNH₂)(dppe)₂]⁺ (M = Mo, W; X = F, Cl; dppe = Ph₂PCH₂CH₂-PPh₂) and cis,mer-[WX₂(NNH₂)(PMe₂Ph)₃] (X = Cl, Br), which are readily derived from trans-[M(N₂)₂(dppe)₂] (1) and cis-[W(N₂)₂(PMe₂Ph)₄] by protonation, condensed with 2,5-dimethoxytetrahydrofuran to afford pyrrolylimido complexes of the type trans-[MX(NNCH=CHCH=CH)(dppe)₂]⁺ (3⁺) and cis,mer-[WX₂(NNCH=CHCH=CH)(PMe₂-Ph)₃] (6), respectively. Their structures were characterized spectroscopically and further confirmed by X-ray diffraction study. Electrophilic substitution reactions at the pyrrole ring in complexes 3⁺ occurred selectively at the β-position to give the corresponding β-substituted pyrrolylimido complexes trans-[MX(NNCH=C(E)CH=CH)(dppe)₂]⁺ (E = Br, CN, SO₃^-, COR), although only chlorination of 3⁺ with N-chlorosuccinimide in THF took place predominantly at the α-position. This β-regioselectivity is in sharp contrast to the α-regioselectivity of free pyrrole and is probably caused by the steric effect of the dppe ligands. Complexes 3⁺ were readily reduced under ambient conditions with LiAlH₄ to liberate pyrrole and N-aminopyrrole in high yields. Further, the tetrahydrido complexes [MH₄(dppe)₂], which can be converted back into the original dinitrogen complexes 1, were recovered in moderate yields after the reduction. This accomplishes the synthetic cycle for pyrrole and N-aminopyrrole starting from the dinitrogen complexes 1. β-Heptylpyrrole was also prepared by starting from 3c⁺ (M = W, X = Cl) by the β-selective heptanoylation followed by the reduction with LiAlFL;. On the other hand, reduction of 6b (X = Br) with LiAlH₄ predominantly produced pyrrole, whereas treatment of 6b with KOH/EtOH liberated N-aminopyrrole in a high yield.