[democanarchis]

Hey all, just reading a paper by Knochel (dx.doi.org/10.1002/asia.200700099) and am interested in using his procedure to make a boronic ester from a MgCl.LiCl grignard species, however I am having a bit of difficulty working my head around this step:



So, the grignard reagent is reacted with the electrophilic triisopropyl borate, standard stuff, but then neopentyl glycol is added, stirred at rt for 12 h and it transesterifys? What is going on here? 6mmol of B(OiPr)₃ is used vs 6.25mmol of neopentyl glycol, so its barely in excess, I would have though hardly enough to drive equilibrium to the neopentyl ester. There isn't any heating or use of a dean stark or anything like that, so what is happening to what I would have thought is the relatively stable isopropyl boronic ester?

I'm only interested in the boronic acid so was planning to hydrolyse after the isopropyl borate addition + stirring overnight. Are neopentyl boronate esters much more stable or reactive? If not, why does he not just hydrolyse instead of converting to the neopentyl ester?

Maybe its just a friday evening and my brain is mushy, but ideas anyone?

[Doc Oc]

If I remember correctly the cyclic boronic ester is MUCH more stable.  Also, I think the isopropyl borate is more volatile and pyrophoric whereas the neopentyl ester is not.

Keep in mind that although you're only using 6.25 equiv of neopentyl glycol that it has two alcohol groups, so effectively there's 12.5 mmol of competing alcohol (and once one end of it condenses the intramolecular cyclization should be much more facile).

[g-bones]

A few things here.  you are correct that nu attack on the triisopropoxyboronate delivers the first isopropoxy boronic ester. The transesterification is driven by a few things.  One is the thorpe-ingold effect (http://en.wikipedia.org/wiki/Thorpe–Ingold_effect) in which that gem-dimethyl aids in cyclization once one of the alcohols replaces a isopropoxy.  furthermore, when you write out the equation for the final esterification you notice you are moving toward a more entropically favored state (2 goes to 3).  the neopentyl boronic ester should be more stable than the ispopropoxy (which would likely hydrolyze to the boronic acid pretty readily) but it is definitely not the most stable boronic ester, especially compared to a pinacol boronic ester.  good luck hope this helps.