Organic syntheses via transition metal complexes, XCl1. Dihydropyridinyl carbene complexes by [4 + 2] annulation of alkenyl- or aryl imines to (1-alkynyl) carbene complexes of chromium and tungsten

Article abstract of DOI:10.1016/S0022-328X(97)00526-3

Reaction of (1-alkynyl)carbene complexes (CO)₅M=C(OEt)C≡CPh 1a,b (M = W, Cr) with alkenyl imine PhCH=CH-CH=N(i-Pr) 2 affords dihydropyridinyl carbene complexes 3 (by an overall 4 + 2 cycloaddition) and zwitterionic 1-azonia-5H-cycloheptatrien-3-yl carbonylmetalates 4 (by an overall 4 + 3 cycloaddition). The product ratio 3:4 is strongly influenced by steric demand of the metal unit as well as of the substrate. Thus, compounds 3 (generated by 4-addition of the imino function to the C≡C bond of 1a) are major products in case of M = W, whilst compounds 4 (generated by 2-addition of the imino function to 1b) are major products in case of M = Cr. Reaction of sterically congested imines, e.g. phenanthrene-9-carboximine 5, affords [4 + 2] cycloadducts, e.g. 6a,b as the only detectable products. Compounds 6 undergo hydrolysis on chromatography on silica gel and give a carbaldehyde 7. Crystal structures are reported of compounds 3a and 6a.