SYNTHESIS AND REACTIVITIES OF PYRROLYLIMIDO COMPLEXES OF MOLYBDENUM AND TUNGSTEN - FORMATION OF PYRROLE AND N-AMINOPYRROLE FROM MOLECULAR NITROGEN

Hydrazido(2-) complexes trans-[MX(NNH2)(dppe)(2)](+) (M = Mo, W; X = F, Cl; dppe = Ph(2)PCH(2)CH(2)PPh(2)) and cis,mer-[WX(2)(NNH2)(PMe(2)Ph)(3)] (X = Cl, Br), which are readily derived from trans-[M(N-2)(2)(dppe)(2)] (1) and cis-[W(N-2)(2)(PMe(2)Ph)(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-[WX(2)(NNCH=CHCH=CH)(PMe(2)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 beta-position to give the corresponding beta-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 a-position. This beta-regioselectivity is in sharp contrast to the a-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 [MH(4)(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. beta-Heptylpyrrole was also prepared by starting from 3c(+) (M = W, X = Cl) by the beta-selective heptanoylation followed by the reduction with LiAlH4. 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.