Alkylation reactions with organometallic reagents. III. The effect of oxygen on alkylation reactions with cobalt(I) compounds

Methylmagnesium bromide reacts with diphenylacetylene in the presence of cobalt(I) and cobalt(II) complexes. Oxygen is shown to have a significant effect on the reaction. Under argon the alkyne is consumed either by formation of cyclisation products or by addition across the triple bond to yield stilbene derivatives and 1,2-diphenylpropene (by mono-alkylation of the triple bond). Under oxygen the consumption of the alkyne through condensation tends to be blocked while the yield of addition reactions to the triple bond increases and includes the formation of 2,3-diphenylbut-2-ene (by dialkylation of the alkyne). The same effect to a smaller extent is observed when CH₃Co(Ph₃P)₃ reacts with PhCCPh. An attempt to rationalize the oxygen effect is presented.