Diphenylethyne, reduction with

Hydroboration of alkenes in non-ethereal solvent has been reported using diborane generated in situ from a quaternary ammonium borohydride and bromoethane (see Section 11.5). Almost quantitative yields of the alcohols are reported [e.g. 1 ]. As an alternative to the haloalkane, trimethylsilyl chloride has also been used in conjunction with the ammonium borohydride [2]. Reduction of the alkene to the alkane also occurs as a side reaction (<20%) and diphenylethyne is converted into 1,2-diphenylethanol (70%), via the intermediate /ra 5-stilbene. 

Photolysis of triphenyl-l,2,3-triazine 17z in diethylamine (DEA) and triethylamine (TEA) as solvents gives diphenylethyne as the main product and the diazocine 73, its valence isomer l,2,4,5,6,8-hexaphenyl-3,7-diazabicy-clo[4.2.0]octa-2,4,7-triene 74, and a product of reductive ring contraction of 17z, namely the 3,4,5-triphenylpyrazole 75, as minor products (Equation 10). To generate 75, the amine has to act as a sacrificial donor on the excited triazine and it is proposed that, after a series of consecutive electron and proton transfers (totalling four reduction equivalents), contraction of the six- to the five-membered ring occurs with release of ammonia <2002TF11>. 

Reactions with alkynes may lead to the formation of cyclized products. The reaction of iodobenzenes with two equivalents of an alkyne has been shown to give naphthalene derivatives in the presence of cobalt catalyst with a manganese reduc-tant. The process, shown in Scheme 15, is thought to involve oxidative addition of the aryliodide to cobalt followed by double alkyne insertion. The cobalt-catalysed annulation step probably involves an pathway. The cyclopentadienyl-rhodium-catalysed annulation of benzoic acids with alkynes has been used to form isocoumarin derivatives, such as (126). The process is thought to involve cyclorhodation at the ortho-position of a rhodium benzoate intermediate, followed by alkyne insertion to form a seven-membered rhodacycle and reductive elimination The silver-catalysed annulations of diphenylphosphine oxides with alkynes proceed in the absence of rhodium. Benzophosphole oxides such as (127), formed with diphenylethyne, are produced. Here, the proposed mechanism involves homolytic cleavage of the phosphorus-hydrogen bond to give a radical which can add to the alkyne and subsequently cyclize. ... 

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