Rational Synthesis and Mutual Conversion of Bis-N-heterocyclic Diphosphanes and Secondary N-Heterocyclic Phosphanes

Symmetrical N-heterocyclic 1,1,3,3-tetrahydro-2,2-bi-1,3,2-diazaphospholes and 2,2-bi-1,3,2-diazaphospholidines are prepared by time-saving, sequential one-pot syntheses starting from 1,4-diazabutadienes or N-alkyl or N-aryl-substituted ethane-1,2-diamines. This method offers high selectivity and minimizes the loss of products owing to unwanted hydrolysis, and thus grants high product yields. In some cases, secondary phosphanes were formed together with or instead of diphosphanes. This reaction is explained by a follow-up process involving homolytic fission of diphosphanes to give phosphanyl radicals, which then react with ammonium salts to give a mixture of secondary phosphanes and chlorophosphanes. Even if its synthetic scope is as yet limited, this approach seems promising in offering superior selectivity and higher yields than common synthetic protocols that rely on the use of complex hydrides as reducing agents. In addition to the reductive conversion of diphosphanes into secondary phosphanes, a reverse reaction under exposure of the reactants to light is also reported.