Boronic esters, alkynyl reagents

Chiral boronic esters react with organolithium reagents to form diorganylalkoxyboranes (borinic esters). Subsequent reaction with the anion of dichloromethyl methyl ether then yields chiral ketones by rearrangement of both of the groups on boron (Scheme 42). No racemization is observed in this sequence and alkyl-, aryl- or alkynyl-lithium reagents can be used. 

Boron reagents add to conjugated carbonyl compounds. Alkynyl borate esters (p. 815) give conjugate addition in the presence of boron trifluoride ethe-... 

However, the application of these classical procedures for 1-alkenylboronic acid or ester synthesis may suffer from the formation of small amounts of the opposite stereoisomers, or from bis-alkenylation leading to the boronic acid derivatives. Also, formation of trialkylboranes may occur. A recent useful variant utilizes organolithium reagents and triisopropyl borate, followed by acidification with HCl to give directly alkyl-, aryl-, 1-alkynyl-, and 1-alkenylboronic esters in high yields, often over 90% (Scheme 2-6) [27]. Triisopropyl borate was shown to be the best of the available alkyl borates to avoid multiple alkylation of the borates. 

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