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#1 2009-01-28 02:22:27


Alkene reductions, hydroboration, oxymercuration, and ozonolysis

Posted on organic chemistry message board

The problems are paradoxical. If this is for an introduction to organic chemistry, then they are quite complicated. I don't know what might have been discussed in the class that would provide clues to how I might answer the stereochemical aspects of the problems. Experience has taught me that selectivity issues are not something to be concerned about in a predictive way. There have been many examples in which predictions have been wrong. That is experienced chemists cannot correctly predict the products before doing the reactions. Secondly, virtually all selectivity problems have a complementary strategy to give the other isomer.

The other factor within these problems is that some are pretty straight forward and predicting the products is easy. Then in the others, I rationally predict mixtures of products. I don't know if that is part of the class strategy or not.

a) and b) are straight forward.
c) I could argue either way on this reaction. Is the methoxy going to be an electron donor or an electron withdrawing element in this reaction? I don't know that the preference for an intermediate with a charge on the tert-carbon will be reversed, though I could imagine that as well.
d) I predict a mixture
e) I presume a big excess of reductant is present. The fastest reaction will be cleavage of the iodide. Then deprotonate of the HOCH2R. This could eliminate to the acetylide or it could give a trans reduction of the alkyne.
f) This tricycle is going to be puckered. I am guessing the mercury attacks from teh outside. Therefore the OH will come from the inside. Because the lactone is in a five membered ring and borohydride can reduce strained esters, I don't know if the reaction conditions (or the student knowldege level) means a reduction of the lactone to a diol or the lactone is meant to be preserved.
g) This also looks like a mixture will result. I don't see how I could predict one isomer over the other.


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