Ring Formation and Alkenes

I was consulting Wade and I came across this problem and solution:

While I agree with the solution manual, I can't help but wonder if my proposed mechanism is correct.

I had the water attack the hydrogens attached to the alkene. I knew that alkenes are slightly electron-withdrawing and this makes the hydrogens an appealing target for a base.

Once the hydrogens were cleaved, I had the remaining pair of electrons attack the carbocation. Is this a valid pathway? Valid as in plausible? Would you accept this mechanism on a test?

The main problem I see with this mechanism is that yield is exceedingly low as sp2 carbons are not especially acidic, and water is a poor base. On the other hand the original problem does specify that this product is minor. But then, it appears that I chose the more minor of two minor pathways.

• Not a definitive answer, but one problem I see with that mechanism is that it entails a concerted, bimolecular process, which should really be much less probable than any given unimolecular process of comparable energy and leading to the same product. – Greg E. Aug 3 '14 at 17:02
• I'm also skeptical that the required configuration of molecular orbitals in the transition state is reasonable. I'm imagining your mechanism would require the electrons of the base flowing into a $\sigma^{*}_{CH}$ antibonding orbital, and I think this would really require the base (and the electrofugal proton) to be roughly coplanar with and on the same side as the newly forming $\ce{C-C}$ bond if it were all to happen concertedly. I could certainly be mistaken, but that doesn't seem at all plausible to me. – Greg E. Aug 3 '14 at 17:39
• No, this is a serious mistake, look at the pKa of alkenes. Don't take it in a bad way, it's part of the learning process. What is the main product of the reaction? A pyran or just dehydration? – K_P Aug 3 '14 at 20:55
• Acidity of the alkene was mentioned in my OP – Dissenter Aug 3 '14 at 21:41