Borinic acid vinyl esters

Carbalkoxyhorinic acid esters.—Borinic acid vinyl esters. Ethyl acetoacetate in n-pentane added dropwise to diphenylboron chloride in the same solvent 2-ethoxycarbonyl-l-methylvinyl diphenylborinate. Y 99%.—The product is chelated and quite stable. F. e. s. W. Gerrard, M. F. Lappert, and R. Shaffer-man, Soc. 1958, 3648. 

Non-fluxional vinylic boranes do not react with carbonyl compounds. Nevertheless, the vinylic borane 29 reacts with acetone (2 h under reflux) yielding the Z-isomer of the homoallylic borinic ester 30b. The cw-configuration of the reaction product 30b corresponds to the following sequence of transformations (Scheme 2.11). The [1,7]-H shift in the vinylic borane 29 gives the allylic Z, Z-isomer 28d which immediately reacts with acetone before the equilibrium among the allylic isomers is established. On the other hand, cyclopentanone reacts directly with 29 under mild conditions yielding Z, Z-1,3,5-heptatriene 31 and borinic ester 32 (Scheme 2.12). Apparently 29 reacts with the enol form of cyclopentanone, and a direct splitting of the B-Q ,2 bond takes place. Similar reaction of 29 with acetic acid was used for the preparative synthesis of previously unknown hydrocarbon 31 (Scheme 2.12) [35]. 

Soderquist and coworkers [17] have reported the synthesis of stable c/s-vinyl-OBBDs. TheB-methoxy-9-BBN on selective oxidation with anhydrous trimeth-ylamine-iV-oxide (TMANO) (85%, CHClj, 0°C) affords the corresponding borinic ester. The borinic ester on alkynylation, followed by demethoxylation gives the stable alkynylborinate. The hydroboration of alkynylborination with dicyclohexylborane (Chx BH) affords cleanly the 1 1 gem-diboryl adduct This is selectively protiodeborylated with acetic acid at 0 °C and gives the corresponding cis-B-vinylborinate. The reaction sequence is outlined in Scheme 20.5 [17]. 

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