Re: 23: The 1st mechanism
Jo Sweeney (email@example.com)
Mon, 19 Jun 1995 14:54:33 +0000
Dear Drs. De Sousa and Gypser,
Thank you for your comments.
1.(de Sousa). We occasionally see 1-2% of the regioisomer, but under no
circumstances can we persuade it to become the predominant one. If we use 18(O)
water, I'd expect the label to end up in the acetate rather
than the hydroxyl; we presume there is a neighbouring group effect by
which the acetate carbonyl lone pair attacks the intermediate iodonium
ion. Water then attacks the oxolonium ion, leaving what was the carbonyl
oxygen attached to the cyclohexane rather than the ester. Thus, I'd
predict the label to end up in the carbonyl of the ester. We really
should do this experiment!!
2.(Gypser) Good question and a simple answer-there is a linear
deceleration in rate with decrease in temperature. In other words, we do
not enhance the formation of any regioisomeric compounds (in the case of
acetoxyiodination) and the hydroxyselenenylation reaction just stops at
low temperature. I think the inductive deactivation of the alkenic bond
by the acetoxy group is strong enough to make the reactions so slow as
to be unreliant upon temperature control to provide stereoselctivity.
Having said that, we have not performed a detailed temperature analysis,
so I might be grossly oversimplifying the reaction mechanism!!
Thank you both for taking the time to comment.
Dr. J.B. Sweeney,
School of Chemistry,
University of Bristol,
Tel. +44 117 928 8165
FACS. +44 117 929 8611
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