The ability of 2-phosphabicyclo[2.2.2]oct-5-ene 2-oxides to undergo fragmentation of the bridging P-moiety

New 2-phosphabicyclo[2.2.2]octene 2-oxides (2 and 3) were synthesized by th e Diels-Alder reaction of 1,2-dihydrophosphinine oxides (1) and dienophiles , such as N-phenyl maleimide and maleic anhydride. The X-ray structure of o ne of the products (2Ab) suggests that the phosphabicyclooctenes have a les s strained framework than the phosphabicyclooctadienes described earlier. A s a consequence of this, also confirmed by thermal examinations and semiemp irical calculations, thermal fragmentation of the phosphabicyclooctenes req uires more forcing conditions, than that of the bicyclooctadienes. The meth ylenephosphine oxide (10) ejected could be utilized in the phosphorylation of hydroquinone in moderate yield. Mass spectral fragmentation of cycloaddu cts 2 and 3 under electron-impact conditions seems to be in agreement with the preparative experiences. (C) 1999 Elsevier Science S.A. All rights rese rved.