Pinacol alkenylboronic esters

Preparation of alkenylzinc reagents by boron-zinc transmetalation from stable organoboronic esters and dialkylzinc is hampered by the instability of the evolved alkenylzinc mixtures of the two reagents turned black and the reaction of the evolved alkenylzinc on remaining pinacol alkenylboronic ester was speculated on the basis of the structure of by-products. Conversely, this boron to zinc exchange can lead to good results if the evolved alkenylzinc is trapped in situ by an electrophile. The reaction of an alkenylboronic ester... 

Scheme 3.24 Comparison between (A) pinacol, (B) hexylene glycol and (C) neopentyl glycol alkenylboronic esters in the dimethylzinc-promoted alke-nylation of nitrones. (Adapted with permission from ref. Ill 2010 Elsevier B.V.)...

Free radical additions to alkenylboronic esters provided the first access to (a-haloalkyl)boronic esters [8]. Intramolecular cyclizations involving radical additions to alkenylboronic esters have been reported and, also, a (l-iodo-S-hexenyl)boronic ester cyclizes to the cyclopentyl derivative under radical conditions [83]. Reaction of pina-col (l-iodopentyl)boronate with tributyltin hydride and butyl vinyl ether yielded the addition product pinacol (l-butoxy-3-heptyl)boronate (71%), but addition of the bory-lalkyl radical to methyl acrylate was inefficient and yielded mainly the simple reduction product, pinacol pentylboronate [84]. 

A dichromium derivative has been prepared from pinacol (dichloromethyl)-boronate (163), anhydrous chromous chloride, and lithium iodide in THF at 25 °C [90]. With various aldehydes, RCHO, this reagent adds to the carbonyl carbon to form trans-l-alkenylboronic esters, RCH=CH-B(02C2Me4). For most examples yields were 84-91%, E Z ratios >95 5. This reaction was used recently to convert aldehyde 162 into alkenylboronic ester 164, an intermediate used for a Suzuki-Miyaura coupling in the asymmetric total synthesis of quinine and quinidine (Scheme 8.39) [91]. In the modified procedure, the chromium reagent was generated from 163 in the presence of the aldehyde substrate. 

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