From: David O'Dell (dko$##$ou.edu)
Date: Thu May 13 1999 - 14:49:01 EDT
Jack Sullivan wrote:
> I'm interested in getting some direction on references or experience in
> hydroborating 1,2-disubstituted alkynes with reagents such as 9-BBN or
> catecholborane in order to produce vinyl boronic acids. For discussion,
> the substiturent on C-1 would be an aryl group while that on C-2 could
> be either alkyl or aryl.
>
> I assume from the references that I have that the addition would prefer
> the least hindered carbon atom of the alkyne. With C-2 being alkyl I
> would imagine this would yield predominantly C-2 addition. If both
> substituents are aryl, I would also assume that a mixture of C-1 & C-2
> boronic acids would result & that these should be separable by either
> crystallization or chromatography.
>
> As a final point, I would also presume that the resulting vinyl boronic
> acids would have the more stable trans configuration.
> --
> Jack Sullivan
>
If you are interested in boranes, try looking under H.C. Brown (he's the
man in borane chemistry) I think I've read that boranes add to alkenes and
alkynes in a syn manner. The mechanism is semi-concerted probably the
borane adds first, the stability of the carbocation or carbocation like
character of the alpha carbon in the intermediate must govern the postion
of the addition, hence the anti-Markovnikov product is formed
predominately. Its fairly easy to get either the cis or trans isomers
though, I think the procedures invovle iodine and base, look in Organic
Synthesis volume? I can't remember. If you are still interested I'll dig
up an extensive list of references for you, just let me know.
cheers,
David
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