ORGANIC SYNTHESES VIA TRANSITION-METAL COMPLEXES .55. 5-AZA-1-METALLA-1,3,5-TRIENES OF CHROMIUM AND TUNGSTEN, AND THEIR CYCLIZATION TO 2H-PYRROLE COMPLEXES

5-Aza-1-metalla-1,3,5-trienes 5 and 6 [L(n)M = C - C = C - N = C, a: L(n)M = Cr(CO)5, b: W(CO)5] are obtained in two steps from aminocarbene complexes 1 [L(n)M = C(NH2)Ph, a: L(n)M = Cr(CO)5, b: W(CO)5]. The first step involves the condensation of 1 with benzaldehyde (2) in the presence of TiCl4/Et3N to give 3-aza-1-metalla-1,3-dienes 3 (L(n)M = C - N = C). 3 adds to the 1-aminoalkyne 4 to give 5 and 6 by a chain extension of two carbon atoms. The trans isomers 5 are formed as the major products and are stable both in the solid state and in solution. The cis isomers 6 undergo a novel-type cyclization to give 2H-pyrrole complexes 8. Triene 5b was characterized by an X-ray analysis.