Anti Markav question

[bakeit1]

Hey all, i'm learning about alkene mechanisms and had a general question regarding AMA. Can something that usually only does MA have an exception and do AMA? If so, how, thanks.

[big314mp]

When adding water across a double bond, the hydroboration-oxidation reaction will give you anti markovnikov addition.

 

http://en.wikipedia.org/wiki/Hydroboration-oxidation_reaction

[bakeit1]

Ok, however, on a test I just took there was a reaction with an alkene and HBR where the HBR did AMA which ALWAYs is MA. You had to explain why the AMA of HBR was more stable, how's this possible?

[big314mp]

What was the alkene?

 

And was there a peroxide present?

[bakeit1]

I forgot the alkene, but yah I believe peroxide was present.

[big314mp]

With HBr + Alkene, the reaction usually follows markovnikov's rule. This is due because MA favors the more stable carbocation.

 

Now when a peroxide is added, the mechanism changes. The peroxide acts as the initiation step for a radical chain reaction:

 

http://en.wikipedia.org/wiki/Free_radical_addition

[bakeit1]

So other than in the presence of peroxide, is this the only time HBR would bond via AMA to an alkene in terms of stability? Thanks btw.

[big314mp]

Yeah, its the only one I can think of off the top of my head (read: I haven't looked too hard). Its one of those textbook example things that you just memorize.