Dipolarophiles phosphaalkenes

Phosphaalkenes that possess a A, a -phosphorus atom can be isolated when appropriately substituted (151). These systems exhibit a much more expressed dipolarophilic than dienophihc reactivity, probably as a consequence of the polarity of the P=C bond. The [3 + 2] cycloaddition of diazo compounds with phosphaalkenes 87 leads to 4,5-dihydro-3//-l,2,4-diazaphospholes 88 (Scheme 8.21) that are not always isolated. Quite often, ehmination of molecular nitrogen occurs during the cycloaddition at or below 20 °C. In other cases, N2 extrusion is achieved at... 

In contrast to the P=C bond of phosphaalkenes (Section 8.2.2), double bonds between phosphorus and a heteroatom have not been used much as dipolarophiles. Most of the studies reported so far were devoted to the reactivity of the (X )P=N bond of iminophosphanes. Amino(iminophosphanes) react with diazoalkanes to form 4,5-dihydro-3//-l,2,3,4-triazaphospholes or, by N2 loss from the latter, to imino(alkylidene)-X -phosphoranes (5,238). With P-halogeno-(arylimino)phos-phanes 174 and the appropriate diazo compounds, 3//-l,2,3,4-triazaphospholes 175 (167) and 176 (239) (Scheme 8.40) were obtained as the major products after cycloaddition and eliminative aromatization. 

The distinct dipolarophilic reactivity of the phosphaalkenes can be demonstrated, among others, with the l-chloro-2-phenyl-2-trimeth-ylsilyl-l-phosphaethene. It reacts at once with diazoalkanes even at room temperature in a [3 + 2] cycloaddition. The expected primary products of addition aromatize, spontaneously eliminating trimeth-ylchlorosilane, to the l//-l,2,4-diazaphospholes, showing the triply... 

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