Jo Sweeney (firstname.lastname@example.org)
Mon, 19 Jun 95 15:24:20 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,
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