Enol borinates hydrolysis

Homolytic cleavage of the carbon-boron bond of a trialkylborane can be promoted by oxygen. The so-formed alkyl radical can be used in synthesis (for radical reactions, see Section 4.1). Indeed, triethylborane in air can be used to generate radicals from precursors such as alkyl iodides or selenides. Hydroboration followed by addition of an a,(3-unsaturated aldehyde or ketone leads to transfer of an alkyl group from the boron atom via an alkyl radical intermediate. The reaction takes place by addition of the alkyl radical to the conjugated system to form an enol borinate, hydrolysis of which gives the aldehyde or ketone product (5.37). 

Reaction with trialkylboranes. A trialkylborane (1), readily available by hydro-boration of an alkene, undergoes fast 1,4-addition to methyl vinyl ketone to give an enol borinate ester (2), which on hydrolysis affords a methyl ketone (3).24... 

A trialkylborane. for example (3) resulting from reaction of hexene-1 (1) with diborane (2. generated from NaBH4-l- BFa). undergoes remarkably fast 1,4-addition to acrolein to produce the enol borinate (4). which on hydrolysis gives the aldehyde (5).1 The Brown group found it more convenient to add water to a solution of the... 

B-l-Alkynyl-9-BBN, easily and quantitatively prepared by the reaction of boron trifluoride-diethyletherate with the corresponding lithium methyl alkynyldial-kylborinate [12], undergoes a smooth 1,4-addition in pentane at room temperature to methyl vinyl ketone (MVK) and ketones capable of adopting cisoid conformation [13]. The reaction proceeds through srx-membered cyclic transition state to afford the enol borinate intermediate, which on hydrolysis afford y,6-acetylenic ketones, in high yields (Scheme 7.8). 

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