[willfitz]

I'm trying to deduce the mechanism for the borane reduction of a carboxylic acid to an alcohol. I have it down to where three acetates are attached to the boron and a hydride from another borane attacks the carobonyl carbon and pushes a negative charge onto the carbonyl oxygen. My question is whether the negative charge falls back to push off the O=B--- at this point, or whether the next hydride attacks first and pushes off the O=B this way. I'll attack two pictures to illustrate the two options (The arrows aren't supposed to be radical arrows).

[Dan]

The first one looks much better. Participation of an O lone pair in these sorts of intermediates is almost invoked - example: acetal formation/hydrolysis.

[willfitz]

The first one looks much better. Participation of an O lone pair in these sorts of intermediates is almost invoked - example: acetal formation/hydrolysis.

That is what I thought, but I was not sure if the borane being attached was critical to the hydride being able to attack the carbon.