Phosphasilenes transient

The first evidence for a phosphasilene was reported in 1979.19 Thermolysis of 1,2-phosphasiletane 1 at 100°C gave the transient silylidenephos-phane Me2Si=PPh 2, which undergoes head-to-tail [2 + 21-cyclodimerization, as well as an insertion reaction into the Si—P bond in 1, leading to 3 and 4 (Scheme 1). 

Scheme 2. Trapping reactions of the transient phosphasilene /Bu2Si=P(Si/Bu3) (6), leading to 8, 9,10, and 11.

A transient phosphasilene 827, which was generated from 1,2-phosphasiletane 826, was first reported in 1979 (equation 286)380. Just recently, this cycloreversion reaction was used to produce the phosphasilene in the gas phase and to prove its existence by high resolution mass spectrometry and photoelectron spectroscopy381. Higher temperatures are needed for the decomposition of the diphosphadisiletane 828 to generate 829. The first ionization potentials are listed (equation 287). 

The most common route to phosphasilenes is by salt elimination from lithium(fluorosilyl)phosphanylides. Examples of transient to moderately stable species are t-Bu2Si=PBu-t 830, which could be observed by 31P NMR spectroscopy382, and t-Bu2Si=P(SiBu-Z3) 831 whose existence could only be proved by trapping reactions383. 

The attempt to stabilize transients in the formation reaction of phosphasilenes by transition metal centres directly bound to the precursors was not successful392. The metal substituent was either cleaved or behaved like a normal substituent, as is shown for the synthesis of phosphasilene 842 from 841 (equation 291). The NMR parameters for 842... 

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