[Pace2014]

I'm trying to figure out the mechanism for a regioselective reductive opening of  a benzylidene acetal protecting group (my attempt attached). The first part I'm alright with: according to the litarature,  in anonpolar solvent, the more electrophilic Lewis acid (Bu2BOTf) complexes to the most nucleophilic oxygen (O6) and an oxocarbenium ion is formed.

BH3.THF then delivers hydride to form the product - this is the bit I'm unsure about. How does BH3 act as a hydride source? It looks more like a Lewis acid than anything to me. I'm pretty sure the way I've drawn it is wrong anyway. Can you draw the mechanism in the same way as you would for, say, NaBH4? Or is it far more complicated, something more along the lines of this?

Also, where's the proton coming from to generate the hydroxyl? The only reagents used were the sugar, Bu2BOtf and BH3.TH3. Reaction was carried out in DCM, was quenched with TEA then purified with toluene/EtOAc as the eluent. I'm stuggling to  see where the H+ is coming from.

Any help appreciated, thanks.

[orgopete]

First of all, I agree with your skepticism as to the role that BH3 should play. I think the hydride donor must be a borate species of some sort. It seems reasonable that boron can vary between an octet and sextet. This is how I'd be inclined to write this mechanism. Cleavage of the acetal with the dibenzylborotriflate will give a borate. I anticipate this could release the triflate. I'd use this triflate to form a triflylborohydride. I'd use this as the reducing agent. I'm further guessing this is not a low temp reaction (and the starting material was dibenzyl).

[pgk]

First of all, boron hydride or borane (BH3) is a gas and has the dimer form of diborane (B2H6). Formation of Lewis salt with THF helps storage and handling, in THF solutions. For simplicity reasons, the chemical formula BH3, is widely used in reaction mechanisms.
So, forget the presence of THF and the electrophilic character of borane. During reaction, BH3 delivers hydrides, exactly like NaBH4.
By another point of view, ring opening of benzaldehyde acetal is a reduction (why?)
Imaging that you had pure benzaldhedyde or benzaldehyde hemiacetal, what would be the reduction mechanism?
Ring opening of benzaldehyde acetal, occurs by a similar mechanism, as above.
Usually, BH3 reduction products are purified by aqueous extraction that is a protons source.
Important: Avoid carrying out hydride reactions in chlorinated solvents. This leads to low yields and it is dangerous.