Addition of diazoalkanes to alkyne(pentacarbonyl)-chromium and -tungsten -: formation of 3H-pyrazole complexes

Photolysis of [M(CO)(6)] (M = Cr, W) in CH2Cl2 at -78 degreesC generates [(CO)(5)M(CH2Cl2)]. Subsequent sequential reaction of [(CO)(5)M(CH2Cl2)] with terminal alkynes, HCequivalent toCR (R = COOMe, Ph, p-Tol), and diazoalkanes, R-1(R-2)CN2 (R-1 = Me, Et, Ph; R, = Me, Et) affords 3H-pyrazole complexes, [(CO)(5)M-N=N-C(R)=C(H)-C(R-1)(R 2)]. These complexes are derived from addition of diazoalkanes to alkyne complex intermediates, [(CO)(5)M(HCequivalent toCR)]. Their vinylidene tautomers, [(CO)(5)M=C=(R)H], present in solution in equilibrium with the alkyne complexes, do not react. Likewise, isolable disubstituted vinylidene complexes, [(CO)(5)M=C=C=(R')R*], do not add diazoalkanes. (C) 2002 Elsevier Science B.V. All rights reserved.