Phosphene intermediates

As a proven technique for detecting reactive intermediates, flash pyrolysis seems to be the method of choice for direct detection or isolation of phosphenes. The result of thermal decomposition of (a-diazobenzyl)diphenylphosphine oxide (7) was nevertheless disappointing, since only triphenylmethane (75), fluorene (14), and benzophenone (75) but not the desired methyleneoxophosphorane 9 could be isolated 12). 

Trapping reactions of benzoylmethyleneoxophosphorane 39 a with carbonyl compounds dispel any remaining doubts as to the existence of acylated phosphenes. Unlike the diphenylmethyleneoxophosphorane 9, whose P/C double bond participates in cycloadditions, compound 39 a acts as a hetero-1,3-diene and undergoes [4 + 2]-cycloaddition with aldehydes and ketones 10 I7,35> it may again be assumed that the reaction is a two-step process involving 55 as intermediate. 

Reaction (13.311) is believed to proceed via a non-aromatic ring intermediate, while in Equation 13.312 a secondary phosphine is transformed into a phosphene derivative [26,27]. 

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