Diphosphenes synthesis

Closely related with the synthesis of ylides from carbenes is the use of ylides as carbene transfer reagents (CTR), that is processes in which the ylide is cleaved homolytically, liberating the nucleophile and the carbene, which could remain both coordinated to the metal or not (Scheme 10). Diphosphirane (34) can be obtained from the diphosphene by reaction with sulfur ylide Me2S(0)=CH2, which behave as a carrier of the CH2 unit [95]. Recent work of Milstein et al. shows that sulfur ylides decompose in the presence of Rh derivatives with vacant coordination sites affording Rh(l)-carbene complexes [96, 97]. Complexes (35-37) can be obtained from... 

Synthesis, structure, and reactivity of cyclic diphosphenes 92CRV1839. 

The synthesis of diphosphaseleniranes 18 by treating diphosphenes with selenium has already been discussed in <1996CHEC-II(1)457>. The method has been extended to asymmetric compounds <1995HAC33>, as shown in Scheme 14. The reaction proceeds at 60 °C in benzene when pyridine is used as a catalyst. The compound 18a was isolated by column chromatography as a stable colorless crystalline solid in 37% yield, although 18b and 18e were not stable enough to permit isolation by the method which was employed for 18a. 

The presence of bulky substituents in diphosphenes has a stabilizing effect, and generally a large excess of reactant is necessary. All these diphosphiranes are thermally stable (2a-s). The synthesis of methylene diphosphirane (2a) under sonication by transposition of the Simmons-Smith reaction to diphosphene which, like the synthesis of the ew-dihalogenodiphosphiranes (2d-e), uses inexpensive KOH pellets (instead of Bu OK or BunLi) with a short reaction time has been described <91TL5965>. 

During the synthesis of diphosphiranes from the symmetrical diphosphene by cyclopropanation, whatever the nature of the carbenoids used (diazo derivatives, carbenes or halogenocarbenes), in all cases the formation as byproducts of the phosphaalkenes and the unstable phosphinidene was observed (5-20%). So, for isopropylidene carbene, the formation of the phosphacumulene, more stable than the parent diphosphirane can reach 30% <91TL3687>. 

The iminophosphine (134) with 1-alkynes gave various products of 1,1- or 1,2-additions. A new synthesis of diphosphenes (135) from aryldichlorophosphines and a tungsten complex has been described the tungsten complex catalysed an exchange between two diphosphenes to give the unsymmetric diphosphene. A ferrocenyl-substituted diphosphene (136) has been prepared which seems to represent a borderline case with respect to dimerisation thus the dimer (137) readily dissociates to (136) upon heating in xylene. 

In fact, my group was not quick enough and not really lucky. Becker [8] and Yoshifuji [9] succeeded in the synthesis of the phosphaalkyne 5 and diphosphene 6, respectively, in 1981, a few months before we obtained the same compounds in Toulouse. However, in the case of the diphosphene 6, we were able to prove that we were not so late compared to Yoshifuji (now, one of my best friends), since a few months after his paper appeared, we published [10] the correct NMR chemical shift for 6 (+493 ppm, instead of -59 ppm, reported by Yoshifuji). 

The planar P4 ring can act as a 12-electron donor as in (8.255e) which is obtained from W(CO)6 and white phosphorus in THF [22]. Some four-membered rings, for example, (8.256) may exist in equilibrium with diphosphenes. Another synthesis is (8.257). 

Equation 11. Synthesis of bis[tris(trimethylsilyl)silyl]diphosphene. 

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