1-Octyne hydroboration

The flask is now cooled in an ice-salt bath and 22 g (0.20 mole) of 1-octyne in 20 ml of diglyme is added rapidly (but the temperature is kept at 0-10°). Gradual warming to room temperature completes the hydroboration. 

Hydroboration is not restricted to alkenes alkynes also react well to give vinyl boranes. These may be used directly in synthesis or oxidized to the corresponding enol, which immediately tautomerizes to the aldehyde. An example of this transformation is the conversion of 1-octyne into octanal by hydroboration with disiamylborane and oxidation with sodium perborate under very mild conditions. 

Hydroboration/oxidation can also be used to form the ketone from 4-octyne. 

Unpublished results. The cataKtic hydroboration of 1-octyne with 7 in benzene in the presence of Pd(PPh,)4 (2 mol n) proceeds at room temperature to give a 4 96 mixture of Uoctenylborate favoring the terminal boron addition product. 

Hydroboration of terminal alkynes, e.g. 1 -hexyne, 1 -octyne or cyclohexylacetylene, with a dialkylborane, such as bis(l, 2-dimethylpropyl)borane, followed by copper(I)cyanide and copper(II) acetate in HMPA containing a trace of water, gives isomerically pure ( )-l-cyanoalk-l-enes (equation 29)133. Successive treatment of 1-bromo-l-alkynes with dialkylboranes and sodium methoxide results in the borinate esters 208, which are converted into ( )-alkenes of greater than 99% isomeric purity by protonolysis. The action of alkaline hydrogen peroxide on the borinates produces ketones (equation 30)134. 

The inability of such photolyzate solutions to hydroborate 1-octyne rules out the presence of neutral boron hydrides. However, the detection of HD, upon the work-up with DOAc, and the formation of undeuterated toluene, upon treatment of the photolyzate with benzyl chloride and deuterolytic work-up, clearly support the presence of borohydrides, such as 45 (6). Finally, evidence supporting the generation of sodium diphenylborate(I) (46) or a similar product was obtained by conducting the photolysis in the presence of diphenylacetylene. Since monomeric 46 is formally isoelectronic with a carbene, adducts like 48... 

Scheme 15.33 Hydroboration of 1-octyne with B2(pin)2 in the presence of a stoichiometric amount of copper chlorides.

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