Phosphaalkenes Diels-Alder reactions

The hitherto unknown l-aza-3-phosphabenzenes are obtained via a regiospecific Diels-Alder reaction with inverse electron demand of 2-trifluormethyl-4-methyl-6//-l,3-oxazin-6-one with phosphaalkynes (81) or phosphaalkenes [Eq. (31)] (82). 

When the cyclobutadienecarboxylate 21 [35] stabilized by the presence of bulky substituents, is allowed to react with the phosphaalkynes 9a-d, the 2-phospha-Dewar-benzenes are obtained in quantitative yields [36] (Scheme 6-5). For steric reasons the [4 + 2]-cycloaddition takes place at Cl and C4 and is highly selective with regard to the dienophile orientation. In the cases of the phosphaalkynes 9a-c, products 22a-c dominate over the isomers 24a-c by a ratio of at least 93 7. Within the detection limits of P-NMR spectroscopy, the Diels-Alder reaction 21 + 9d furnishes 22 d exclusively. The structure of 22 d was confirmed by X-ray crystallography [36]. The phosphaalkene moieties of the phospha-Dewar-benzenes 22a-d, which are utilized in the subsequent reactions, can be recognized, among others, by their low-field P-NMR signals (5 = 312-315). 

Finally, several imines give products of an aldol-t)fpe condensation rather than Diels-Alder reaction when reacted with (1) in the presence of Lewis acids. Phosphaalkenes react with (1) to give ultimately the aromatic phosphinine and dimethyl azodi-carboxylate affords the expected [4 + 2] adduct in 83% yield. ... 

Cycloaddition reactions of two-co-ordinate phosphorus compounds described this year includes the [2 + 2] cycloaddition of an iminoborane to aminoimino-phosphines to give 1,3,2,4-diazaphosphaboretidines (125). In an attempt to prepare hydrazino-bis-phosphaalkenes (126) as new candidates for hetero-Cope rearrangements, the tricyclic compounds (127) were obtained. The result is explained by a Diels-Alder rearrangement of (126), as shown. 

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