Alkynylboronic preparation

Allenylboranes can be prepared by treatment of propargylic acetates with butyl-lithium and a trialkylborane [19]. The reaction proceeds by initial formation of an alkynylboronate followed by migration of an alkyl group from boron to carbon (Eq. 9.17). 

This reaction has been successfully applied to cyclopentenone annelation onto cycloalkenes. By modifying Danheiser s protocol [15], alkynyl-substituted bi-cyclo[n.l.0]alkanol derivatives are prepared by the reaction of gem-lithio-bromocyclopropanes 27 with alkynylborons, as shown in Scheme 10. 

In addition to their use as precursors for alkynylboron, tin or zinc compounds, alkynylmagnesium reagents show a moderate reactivity toward the coupling with haloarenes and haloalkenes. They are often commercially available, or easy to prepare. Their main drawback is their low chemoselectivity and high nucleophilicity, which implies incompatibilities with functional groups such as nitro and carbonyl. 

A wide range of isoxazole-4-boronates and some 5-boronates can be prepared by dipolar cycloaddition of nitrile oxides with alkynylboronates."""... 

An alternative, less general procedure for the preparation of alkynyl(phenyl)iodonium triflates in moderate yields employs the reaction of alkynylsilanes [493], alkynylstannanes [494], or alkynylboronates [297] with Zefirov s reagent, 2PhI0-Tf20. This method is also applicable to the synthesis of the parent ethynyl(phenyl)iodonium triflate [495]. 

The cyclic dialkoxyborane 4,4,6-trimethyl-l,3,2-dioxaborinane (HBhg), the vinylboronic ester 2-ethenyl-4,4,6-trimethyl-l,3,2-dioxaborinane (vinylBhg) and the alkynylboronic ester 2-ethynyl-4,4,6-trimethyl-l,3,2-dioxaborinane were first reported in 1966-67 by Woods and coworkers. The authors stressed the high stability of HBhg towards disproportionation, as compared to the parents 1,3,2-dioxaborinane and 1,3,2-dioxaborolane, and its potential for the hydroboration of alkynes. In addition, the first reported o-metal-boron bond (rhodium) was prepared from HBhg. ... 

Like their aryl and alkenyl counterparts, alkynylboronic adds can be made by displacement of magnesium or lithium acetylides with borate esters. For example, Mat-teson and Peacock have described the preparation of dibutyl acetyleneboronate from ethynylmagnesium bromide and trimethyl borate [294]. The C-B hnkage is stable in neutral or acidic hydrolytic solvents but readily hydrolyzes in basic media such as aqueous sodium bicarbonate. Brown and co-workers eventually applied their organo-lithium route to boronic esters to the particular case of alkynylboronic esters, and in this way provided a fairly general access to this class of compounds [295]. 

A novel class of quinone boronic esters has been prepared via a Dotz annulation of Fischer carbene complexes with alkynylboronates. The origin of the high regioselec-tivity is discussed in terms of steric and electronic effects. Additionally, these compounds undergo Pd-catalyzed coupling reactions to give substituted quinones and hy-droquinones (Scheme 9.49) [102]. 

The reaction has been extended to alkynylboron dihalides (Scheme 23.74). This new chemistry provides a simple ronte to internal alkynes that are otherwise not readily prepared. 

While it is impossible to anticipate every issue that will arise during the preparation of alkynylboronates and related compounds, the following guide will provide some suggestions in order to circumvent many common issues. 

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