SYNTHESIS, PHOTOCHEMICAL BEHAVIOR AND CIS-TRANS ISOMERIZATION OF LPHENYL)-2-(2,4,6-TRI-TERT-BUTYLPHENYL)DIPHOSPHENE

Irradiation of lphenyl)-2-(2,4,6-tri-tert-butylphenyl)diphosphene (1a) yields its thermally unstable cis-isomer 1b, the diastereoisomeric cy clisation products 4a and 4b and the cleavage product 5. A mechanism f or the formation of these products is proposed. The activation paramet ers for the thermal isomerisation reaction 1b --> 1a are experimentall y determined as DELTAH(daggerdagger) = 29.5 +/- 1.4 kcal mol-1 and DEL TAS(daggerdagger) = 38 +/- 6 cal mol-1 K-1 (1 kcal = 4.18 kJ); DELTAH( daggerdagger) is the first experimentally determined measure for the s trength of the pi-component of a phosphorus-phosphorus double bond.