Why does hydroboration gives the anti-Markovnikov product?

Answer 1

Well, it doesn't. Not really. Or maybe you've made a slight simplification of what Markownikov addition really implies.

If you consider that Markownikov addition results in the more negative part of an asymmetrical adduct attaching itself to the more highly substituted carbon of the alkene, hydrogen is more electronegative than boron, so in that sense the hydrogen is on the more substituted carbon. It's easier to then displace the boron than the hydrogen, so it *seems* like anti Markownikov addition has taken place. Usually, you're dealing with HCl or similar, where Cl is the more negative part of the molecule so with isobutene, the major product under these conditions would be t-butyl chloride. If you can add HCl via hydroboration, then i-butyl chloride is the predominant regioisomer.

Answer 2

A tri borate ester is formed from which alkyls groups separate with retention of configuration.