Borane, electrophilicity reaction with alkynes

The search for potential BNCT drugs has re-awakened interest in the organic chemistry of the 12-vertex polyhedral boranes.1 The most widely studied chemistry is of o-carborane, where two carbon atoms are available for a number of organic reactions.2 Most substituted carboranes are prepared either by interaction of alkynes with decaborane,3 cross-coupling of Li, Na, Mg-carboranes with electrophiles 4 or by the organic reactions on the side chains (Scheme 1). ... 

They react with terminal alkynes by electrophilic addition of the empty p-orbital to the unsubstituted end of the triple bond 83. The intermediate would then be the more substituted vinyl cation 84. It is easier to draw this mechanism with R2BH than with the full structure for 9-BBN. The intermediate 84 is not fully formed before hydride transfer begins so that the reaction is semi-concerted and the transition state is something like 86. The result is a regioselective and stereospecific cis hydroboration of the triple bond to give the A-vinyl borane 85. The intermediate 84 is quite like the radical intermediate in hydrostannylation but the difference is that hydrogen transfer is intramolecular and stereospecific in hydroboration. 

Here is a simple example in the field of prostaglandin synthesis where 9-BBN was used on a protected optically active propargyl alcohol.12 The starting material is identical to the alkyne 78 that we reacted with Bu3SnH above and the result is the same - cis hydrometallation with the metal atom at the terminus. However that was a thermodynamically controlled stereoselective radical chain reaction while this is a kinetically controlled stereospecific electrophilic addition to give the vinyl borane -87. 

Examples of electrophilic addition of secondary phosphines to alkenes or alkynes were described. [114, 124, 125, 135]. Glueck [124-126] reported enantioselective tandem reaction of alkylated/arylation of primary phosphines catalyzed by platinum complex, proceeding with formation of chiral phosphaace-naphthenes. Palladium-catalyzed hydrophosphination of alkynes 219 tmder kinetic resolution conditions gave access to 1,1-disubstituted vinylphosphine boranes 220. However, despite screening several chiral ligands, temperatures, and solvents, the... 

There have been significant discoveries of methods that enable the enantioselective addition of an alkyne to an aldehyde or a ketone [182]. The resulting chiral propargyl alcohols are amenable to a wide variety of subsequent structural modifications and function as useful, versatile chemical building blocks. In 1994, Corey reported the enantioselective addition reactions of boryl acetylides such as 292, prepared from the corresponding stannyl acetylenes (e.g., 291) in the presence of the oxazaborolidine 293 as the chiral catalyst (Scheme 2.36) [183]. Both aliphatic and aromatic aldehydes were demonstrated to participate in these addition reactions, which proceeded in high yields and with impressive enantioselectivity. The proposed transition state model 295 is believed to involve dual activation both of the nucleophile (acetylide) and of the electrophile (aldehyde). The model bears a resemblance to the constructs previously proposed for alkylzinc addition reactions (Noyori, 153) and borane reductions (Corey. 188). 

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