Phosphirenes, silyl

Similarly, the (phosphino)(silyl)carbene 2a reacts at -30°C with a slight excess of the tert-butylphosphaalkyne cleanly affording the 2-phosphino-2//-phosphirene 34.53 The reaction leading to 34 is strictly analogous to that observed on reacting the transient dichlorocarbene with the tert-butyl-phosphaalkyne, in which the 2//-phosphirene 36 was obtained.54 The three-membered heterocycle 34 appeared to be rather unstable and rearranged, quantitatively, to afford the lA5,2A3-diphosphete 35 after 3 h at room temperature.55 Once again, these results as a whole indicate that a concerted [1 + 2]-cycloaddition process is involved in the formation of the 2//-phosph-irene 34. 

Silyl substituted lf/-phosphirene 40 undergoes photochemical ring expansion to the 1,2-dihydro-1,2-phosphasilete 41 with cleavage of a silicon-silicon bond <99MI1581>. 

Reactions related to cyclopropanation can also be carried out with (phosphino) (silyl)carbenes (I). For example, benzaldehyde reacts with la at 0 °C leading to the corresponding epoxide, again as only one diastereomer. Even more striking are the reactions with benzonitrile and tert-butylphosphaalkyne that lead initially to azirine and phosphirene. Both three-membered heterocycles subsequently undergo ring expansion reactions affording azaphosphete and diphosphete, respectively (Scheme 8.18). This reaction is a new route for the synthesis of heterocyclobutadienes, and this demontrates the usefulness of (phosphino)(silyl)carbenes (I) for the synthesis of novel species. 

Photochemically induced ring expansion of silyl- or germyl-substituted 1/7-phosphirene proceeds selectively to furnish l,2-dihydro-l,2-phosphasilete and l,2-dihydro-l,2-phosphagermete, respectively (Equation 39) <1999CEJ1581>. 

Photolysis of the silyl-substituted phosphirene 19 proceeds selectively with cleavage of silicon-silicon bond and ring expansion to furnish the l,2-dihydro-l,2-phophasilete 20 (Equation 21) <1999CEJ1581>. The corresponding l,2-dihydrol,2-phophagermete is prepared analogously. 

Free as well as coordinated IFf-phosphirenes A [3] andB [4], respectively, can be prepared by trapping of transient phosphinidenes [5] with alkynes. By extension of this synthetic route, Niecke et al. prepared the first lff-diphosphirene l Pa [6] by reaction of the chloro(silyl)phosphine 1 with the phosphaalkyne 2 (Scheme 1). Some years later,Mathey et al. [7a] and Streubel et al. [7b] independently reported analogous reactions in the coordination sphere of transition... 

The condensation of a carbene with a P=C triple bond theoretically affords a 2//-phosphirene. This was indeed shown to be the case by Regitz and co-workers in the synthesis of the only characterized 2//-phosphirene (4) (Scheme 26) <87AG(E)1257>. Some time later, Regitz showed that the use of a chlorocarbene precursor leads to the formation of 1-chlorophosphirenes via a formal [1,3] shift of chlorine in the initial 2//-phosphirenes (Equation (67)) <89AG(E)225,91CB1207). Chloro-diazirines can be replaced by 1,2-dichlorocyclopropenes <91SL433> in this kind of chemistry (Equation (68)). Finally, phosphaalkynes too can be replaced by silylated chlorophosphaalkenes (Scheme 27) <89TL3951>. 

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