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.