Treatment of a mixture of epoxide and styrene with trimethylsilylmethylmagnesium bromide in the presence of a cobalt-phosphine complex afforded homocinnamyl alcohol in good yield. The reaction should begin with ring opening of the epoxide by the action of magnesium bromide which leads to the formation of a magnesium 2-bromoethoxide derivative and not with a direct single electron transfer from a cobalt complex to the epoxide.