Organophosphorus compounds, 139 On the behavior of 5,8-bis(trimethylsilyl)cycloocta-1,3,6-triene in cycloaddition reactions and subsequent chemistry

Phosphaalkynes 2 and electron-deficient alkynes 11 readily react in a [4 2] cycloaddition process with 5,8-bis(trimethylsilyl)cycloocta-1,3,6-triene (8) in its bicyclic form 10 to furnish regioselectively the tricyclodecadi enes 12 and 13, respectively. The phosphorus-containing compounds 12 exhibi t structural features which make them suitable for homo-Diels-Alder reactio ns with electron-deficient acetylenes. A single crystal structure analysis of the homo-Diels-Alder adduct 14b confirmed the structure and relative con figuration of the phophatricyclodecadienes 12. In solution the tricyclodeca dienes 13 are prone to facile cycloreversion yielding the phthalic esters 1 5 and the cyclobutene 16. The latter is rapidly converted into the correspo nding 1,3-butadiene 18, which can be trapped in a Diels-Alder/phospha-ene/D iels-Alder tandem reaction sequence by phosphaalkyne 2a. The phosphatricycl odecadiene 12 is thermally more stable; loss of cyclobutene 16 only occurs under FVP conditions to afford the lambda(3)-phosphinine 22.