[Lo.Lee.Ta.]

I was working on this synthesis problem: http://highered.mheducation.com/sites/0073375624/student_view0/chapter9/synthesis_problem_3-3.html

(S)-2-methyl-3-pentanol ----?------>  (S)-3-azido-2-methylpentane

I thought I could convert the alcohol into an alkyl halide directly using PBr3, pyridine.
But the answer said I should instead convert the alcohol to an alkyl tosylate, and THEN the alkyl halide, which then gets reacted with NaN₃.

They said something like the alkyl tosylate intermediate is good for getting just one stereoisomer.
But I thought alcohol + PBr3 + pyridine always gives you the SN2 product, which is just one stereoisomer...

So why do they say this...?
Thank you!!!

[discodermolide]

Check the stereochemistry here. That should give you a hint.
The fact that you convert to an alkyl halide then displace with azide should also tell you something.

[TheUnassuming]

I thought PBr3 would react with the alcohol, kicking out a bromide which would in turn attack the carbon to give inversion of stereochemistry?  Is there concern about the free'ed bromide attacking already formed product to form the epimer?

[orgopete]

Baring any additional information given or error in the assignment of stereochemistry of the azide, I agree with your possible route.

[discodermolide]

You are assuming that the PBr₃ mechanism is concerted?


Trying to understand why the answer was as posted!

[orgopete]

(S)-2-methyl-3-pentanol ----?------>  (S)-3-azido-2-methylpentane

... the answer said I should instead convert the alcohol to an alkyl tosylate, and THEN the alkyl halide, which then gets reacted with NaN₃.

This should give an (S)-tosylate and then a (R)-azide. An error or some chemistry I don't understand?

[discodermolide]

I think you mis-read: .. tosylate to alkyl halide to azide.

[orgopete]

Indeed I misread it.