Rings and Cages from Phosphaalkenes

Basically, three-membered phosphorus heterocycles should result from [2+l]-cycloadditions involving phosphaalkenes and carbenes, carbenoids, 

At — 30°C, the thermolabile metallophosphaalkene 3 undergoes formal dimerization with extrusion of C3H4 to afford the dinuclear complex 4. This compound features a /x-phosphiranyl ligand, which formally is generated by the [2+1] cycloaddition of the carbene [ C(H)C2H5] (or a complex thereof) to the P=C bond of the precursor, as in Eq. (2).6 

The first 2/f-l-aza-2-phosphirene complexes 8a,b result from the action of amino-carbene complexes 5a,b with ClP=C(SiMe3)2 in the presence of triethylamine. A rationale for the products 8a,b invokes the initial formation of carbene complex-functionalized phosphaalkenes 6a,b, which undergo 1,3-hydrogen shifts and isomerization of the transient iminophosphanes 7a,b. Intermediates, however, are not detectable see Eq. (3).7 

Oxaphosphiranes are interesting as they are analogs of the well-known oxiranes (epoxides). The direct epoxidation of free phosphaalkenes most likely would be complicated or thwarted by oxygen attack at phosphorus. An elegant route to oxaphosphirane complexes 10a,b makes use of the quantitative oxydation of Tj -phosphaalkene complexes 9a,b with m-chloro-perbenzoic acid in THF see Eq. (4).8 

The course of this reaction is sensitively controlled by the nature of the substituents at the cyclopentadienyl ring, as well as at the phosphorus and the methylene carbon atoms. Thus, warming toluene solutions of the 1-ferriodiphosphapropenes 14a or 14b leads to the formation of the 1,2- 

---

Transition-Metal Assisted Syntheses of Rings and Cages from Phosphaalkenes and Phosphaalkynes... 

---