2- phospha-Dewar-benzenes

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). 

The addition of the Lewis base (DM SO) to this betaine gave the tri-phospha-Dewar benzene derivatives via an unstable diphosphirene ring. 

A study of the reactivity of the carbonyl-functional phospha-alkenes (305) has also been reported. The triphospha-Dewar-benzene (306) has been shown to undergo cycloaddition reactions with alkynes to form the triphosphabishomo-prismane system (307). Cycloaddition of t-butylphospha-ethyne to the phosphatriafulvene (308) results in the formation of a single isomer of the diphosphaisobenzene (309), having an allene system within the ring. This... 

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