Hydroboration on Limonene

Question

I saw that the hydroboration of Limonene (1-Methyl-4-(1-methylethenyl)-cyclohexene) does only happen on the methylethenyl group. Up to now, I wasn't able however to find out why. Generally speaking, a hydroboration can happen on any double bond, so why doesn't it react – at least a bit – with the cyclohexene?

I could argue, that the stereochemistry of the cyclohexene is not as favorable as the one of the methylethenyl group, but I don't see this being the case. On the cyclohexene the methyl group is in the way, where as on the methylethenyl group it is the cyclohexene.

Answer 1

The borane reagent in question, disiamylborane, is one of the many bulky hydroborating agents which preferentially add to less hindered double bonds.

In this case, the 1,1-disubstituted double bond outside the ring is more prone to hydroboration than the trisubstituted double bond within the ring, simply due to sterics.

There are many more examples of hydroborations with this reagent which are dictated by sterics: for further examples see its corresponding entry in the Encyclopedia of Reagents for Organic Synthesis. In all cases, disiamylborane is selective for terminal alkenes (i.e. either monosubstituted alkenes, or 1,1-disubstituted alkenes).