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 °C generates [(CO)5M(CH2Cl2)]. Subsequent sequential reaction of [(CO)5M(CH2Cl2)] with terminal alkynes, HCCR (R=COOMe, Ph, p-Tol), and diazoalkanes, R1(R2)CN2 (R1=Me, Et, Ph; R2=Me, Et) affords 3H-pyrazole complexes, [(CO)5M–(R1)(R2)]. These complexes are derived from addition of diazoalkanes to alkyne complex intermediates, [(CO)5M(HCCR)]. Their vinylidene tautomers, [(CO)5MCC(R)H], present in solution in equilibrium with the alkyne complexes, do not react. Likewise, isolable disubstituted vinylidene complexes, [(CO)5MCC(R‘)R*], do not add diazoalkanes.