Organic Syntheses via Transition Metal Complexes, 90:¹ Novel Route to Cyclopentadienes from (1-Alkynyl)carbene Complexes of Chromium and Tungsten via 2-Amino Metalla-1,3,5-hexatrienes

1,5-Diamino cyclopentadienes 9a-c are obtained by thermally induced ring-closure of 2-amino-1-metalla-1,3,5-hexatrienes (CO)₅M=C(NMe₂)-CH=C(Ph)-C[=C(HNR)Me]COMe (Z)-7 in 50 - 58% yields. The reaction involves intermediate formation of novel type 1-metalla-cyclopentadienes 8, which were characterized by X-ray structure analyses. Starting compounds (Z)-7 are easily available from (1-alkynyl)carbene complexes (CO)₅M=C(OEt)C≡CPh 1a,b (M = Cr, W) in two steps either via formation of 2-ethoxy-1-metalla-1,3,5-hexatrienes (Z)-3b (addition of enamines to 1 and subsequent aminolysis) or of 2H-pyran-2-ylidene complexes 5 (condensation of 1 with enolizable carbonyl compounds and subsequent aminolysis). Thus, the reaction leading from 1 to 9 involves an overall [3+2] cycloaddition, in which the (1-alkynyl)carbene complex functions as a C3 component, and the enamine, and the carbonyl compound, respectively, as a C2 component.

cyclopentadienes 1,5-diamino - 1-Metalla-1,3,5-hexatrienes - cyclization - enamines - (1-Alkynyl)carbene complexes - Pyranylidene complexes - Chromium - Tungsten