SYNTHESIS, STRUCTURE AND ISOMERIZATION OF FLUORINE-CONTAINING 1,2-DIHYDRO-1,3-DIPHOSPHETES

Reaction of perfluoro-2-phosphapropene 1 with phosphaalkynes of the ty pe RC=P (2) smoothly yields the formal [2 + 2] cycloaddition products, 1,2-dihydro-1,3-diphosphetes RC=P- -CF2- -PCF3 (R = Me2N (3a), Et2N ( 3b), (Pr2N)-Pr-i (3c), Bu-i (3d)). Instead of the not isolable aminoph osphaalkynes Me2NC=P and Et2NC=P, the precursors HP=C(F)NR2 are succes sfully used as synthetic equivalents. The results of a single-crystal X-ray analysis of 3a almost exactly agree with the structural data of an ab initio calculation. The elongated sp(2) CP distance in 3a (exp. 173.7(2) pm, calc. 171.4 pm) is due to the interaction of the Me2N lon e pair with the PC (p-p)pi bond. Surprisingly, the sterically unshield ed derivatives 3a and 3b at 25 degrees C undergo an interesting rearra ngement to the thermodynamically more stable 1,2-dihydro-1,2-diphosphe tes RC=C(F)- -P(CF3)- -PF (R = Me2N (6a), Et2N (6b)) via a 1,2-fluorin e shift, an electrocyclic ring opening to a 1,4-diphosphabutadiene int ermediate and an intramolecular [2 + 2] cycloaddition. This mechanism is supported by high level ab initio calculations.