REACTIONS OF COMPLEX LIGANDS .74. ALKYNE CARBENE CHROMIUM CHELATES - SYNTHESIS, STRUCTURE AND CARBENE DIMERIZATION

An alkyne-carbene chromium chelate 2, a model complex for the first in termediate of the benzannulation reaction, is generated by low-tempera ture photodecarbonylation of l[(tert-butylethynylphenyl)methoxycarbene ]chromium 1. X-ray structure determinations and C-13 NMR studies on bo th compounds indicate only a weak coordination of the alkyne both in t he solid state and in solution. Whereas thermal dimerization of the ca rbene ligand is directed by less bulky alkyne substituents to give chr ysenes, pentacarbonyl complex 1 affords a mixture of stilbene diastere omers. (C) 1997 Elsevier Science S.A.