Thanks for the reply
Yes that is the structure of the Grignard reagent.
I'm thinking that the alkyne is itself susceptible to nucleophilic attack from the Grignard reagent perhaps after an initial reaction with A to form a compound containing both an alkyne and an alkene. Although this does not agree with the nmr.
However, there is nothing there to stabilize the negative charge that would result.
I should note that I was given some nmr data of B(70 percent yield)
1.53 (3H, dd)
2.13(6H, s)
3.50(1H, s)
4.43-4.80(2H, m)
7.19(5H , m)
Are you perhaps suggesting that the alkyne can be deprotonated by the Grignard reagent?