Phosphaalkenes dipolarophilic reactivity

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... 

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. 

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See also in sourсe #XX -- [ ]

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