what do you mean by "d,lhydrobenzoin"?

The mostly likely reason for the meso product is due to the intramolecular hydrogen bonding while in the "cis" formation.

I think the explanation is a little more subtle than the hydrogen bond.  I would imagine that hydrogen bond is worth a lot less than the difference between the meso and DL forms, but I don't have the numbers to back that up.  Also you could have H-bonding in the DL form and avoid the steric interactions between the two phenyl groups when the H-bond is in effect.

I would propose that after the first ketone is reduced the alkoxide that forms coordinated to the boron.  This in turn can chelate to the adjacent carbonyl oxygen, locking the conformation.  Then the addition of the second H- is blocked from attacking the opposite face of the first attack because of the phenyl ring. The second attack thus comes from the same side as the first.

If you build a model this explanation should make a lot more sense.

Here's the structure

http://www.kanto.com.tw/japanese/catalysis/2img/pic17.gif

The steric effects aren't so great, and there are countless stable compounds with steric interaction of greater degrees.  Note that when referring to such reactions we are talking about the relative amounts of the products formed, despite the stability of one over the other; the species is not absolute, more transient, there is an interchange of the species formed.

Why are you searching for an absolute explanation...most explanations of chemical phenomena pertain to various factors and the hydrogen bonding is certainly a major factor, certainly one that can add significant favorability to the mesocompound.

I'm not an expert in boron chemistry, nevertheless, I suggest you point out some research to back it up, there are probably some "subtle" factors you have not considered in the formation of your suggested boron adduct, despite boron being a lewis acid.

owever, hydride delivery from a borohydride reducing agent is an irreversible process so the lower energy transition state is what dominates.

Note that borohydride is somewhat of a catalyst, are you aware of what a catalyst does (borohydride is strong reagent)?  This would indicate a REVERSIBLE reaction.

Once again, the hydrogen bonding is more than a significant factor... to say that "because of steric interactions, hydrogen bonding becomes difficult" is ridiculous.  Once again, the steric interactions are NOT so influential in this case, I don't know why you are so obsessed about the steric interaction as to completely rule out the hydrogen bonding.

You seem to be ad hocking completely, I have come to the conclusion that you have no idea what you are talking about but just wish to carry out the argument for the sake of reputation.

I'll carry on with the discussion if you provide some solid, specific, and coherent evidence with your arguments.  

Why don't we settle this, ask your professor or research the internet to invalidate my hydrogen bonding argument.  It is frequently the case that intramolecular hydrogen bonding results in greatly increased stability.  My suggestion relates to an frequently established explanation.  You seem to be making up your own "subtle" and incoherent arguments, you need to rely on best and established explanations (such as you find frequently in a org chem book) and not posit your own vague theories.  Note that we are NOT researchers arguing about a complex theory, the truth regarding this topic is out there and I'm sure that it is very, very, very simple.

Sodium borohydride is not a catalyst in this case.  It is consumed in the reaction.  Check you definition of catalyst.

You should read into what I write more carefully.

Why is meso-hydrobenzoin obtained from the above procedure rather than more stable d,lhydrobenzoin or mixtures of meso and d,l ?

This quote is a commentary from the original person who had asked the question...it implicates the questioner's suspicion, his/her original query to begin with.  In this case, I'm guessing that she is wrong, she/he has underestimated the stability of the meso compound.

Sodium borohydride is not a catalyst in this case.  It is consumed in the reaction.  Check you definition of catalyst.

I apologize, I said "somewhat" of a catalyst (strong reagent)" and I wished to emphasize the latter; my doubts about your assertion of this being a irreversible reaction.

Hydrogen bonding is such a small effect, it will not decide stereochemistry. Chelation of the boron group is going to direct the formation of the meso compound.

What?  By now Mitch, I'm sure you've heard of  cases where the equilibrium is actually shifted due to the hydrogen bonding of the product and even small effect ionic interactions regarding acid base reactions where the conjugate base is stabilized due to these latter interactions.  I've heard of countless examples where the equilibrium is shifted due to intramolecular hydrogen bonding, I'm surprised that both of you have not brought this in to case to begin with.

Perhaps you can search the internet to find some detailed explanation on your boron intermediate theory...if it is true as you say, than I'm more than positive that there will be lectures, articles, and even personal websites which one can easily find.  I'm more open minded, just find it.  None of you seem to be interested in finding the conclusion.

But both the meso and racemic forms can have a hydrogen bond.

Really?  Are you referring to INTRAmolecular hydrogen bonds.

Higher in energy?  Shouldn't the more stable compound be lower in energy?