From 2-phosphino-2H-phosphirene to 1-phosphino-1H-phosphirene, 1 lambda(5),2 lambda(3)-diphosphete, and 1,2-dihydro-1 lambda(3),2 lambda(3)-diphosphete: an experimental and theoretical study

The [bis(diisopropylamino)-phosphino](trimethylsilyl)carbene 4 reacts clean ly with tert-butylphosphaalkyne 2 to give 2-phosphino-2H-phosphirene 5, whi ch was spectroscopically characterized. Heterocycle 5 is thermally unstable and quantitatively rearranges after 3 h at room temperature into the 1 lam bda(5),2 lambda(3)-diphosphete 3. Irradiation of 5 at room temperature (lam bda = 254 nm) with a Rayonnet photochemical reactor produces 1-phosphino-1H -phosphirene 6 (10%), 1,2-dihydrodiphosphete 7 (3%), and diphosphete 3 (87% ). Irradiation of 3 with a high pressure mercury lamp at lambda = 254 nm af fords the dihydrodiphosphete 7, which was isolated in 69% yield. Calculatio ns carried out on heterocycles 3 and 5-7 emphasize the crucial effect the a mino substituents have on the stability of lambda(5)-phosphacyclobutadiene derivatives, and show that the intrinsic difference in the thermodynamic st ability between 1H and 2H-phosphirenes is rather small (68 kJ mol(-1)).