Lithium 1-alkynylboronates

The reactivity profiles of the boronate complexes are also diverse.43 For example, the lithium methyl-trialkylboronates (75) are inert, but the more reactive copper(I) methyltrialkylboronates (76) afford conjugate adducts with acrylonitrile and ethyl acrylate (Scheme 16).44 In contrast, the lithium alkynylboronates (77) are alkylated by powerful acceptors, such as alkylideneacetoacetates, alkylidene-malonates and a-nitroethylene, to afford the intermediate vinylboranes (78) to (80), which on oxidation (peracids) or protonolysis yield the corresponding ketones or alkenes, respectively (Scheme 17).45a Similarly, titanium tetrachloride-catalyzed alkynylboronate (77) additions to methyl vinyl ketone afford 1,5-diketones (81).4Sb Mechanistically, the alkynylboronate additions proceed by initial 3-attack of the electrophile and simultaneous alkyl migration from boron to the a-carbon. 

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). 

Lithium 1-alkynylboronates are attacked by many electrophiles at the position P to the boron atom. The subsequent rearrangement gives a variety of functionalized 1-alkenylboronanes (eq (28)). The stereochemistry can be either " or Z, or a mixture of the two [15]. The reaction with chloro(diethyl)borane or chloro(trimethyl)silane and -stanane stereoselectively produces c /5-bimetallic compounds [41]. 

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]. 

The synthesis of alkynyltrifluoroborate salts begins with the generation of lithium acetylides at low temperature followed the addition of a trialkylborate (Scheme 6.53) [104]. Once the intermediate alkynylboronate was generated, aqueous KHF was added to generate the target compounds. The resulting alkynyltrifluoroborates were isolated in moderate to excellent yields. Both alkyl- and arylalkynes were well tolerated by this chemistry. 

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