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27.24.3 Product Subclass 3: Phosphacumulene Ylides

DOI: 10.1055/sos-SD-027-00847

Schobert, R.; Gordon, G. J.Science of Synthesis, (2004271042.

Phospha(hetero)cumulene ylides 137, phosphavinylidene ylides 138, and phosphaallene ylides 139 feature unique electronic and structural properties and exhibit a chemistry which is quite distinct from that of ylides bearing three substituents on the ylidic α-carbon atom.[‌259‌] The molecular structures as obtained from X-ray diffraction analyses of a single crystal of the ylides 137 (X=NPh),[‌551‌] 137 (X=O),[‌552‌] and 137 (X=S)[‌553‌] suggest electronic structures described best by resonance forms 137A and 137B and by a hybridization of the α-carbon atom ranging from sp2 to sp depending on the nature of the substituent X (Scheme 49).[‌52‌,‌186‌,‌259‌,‌551‌‌561‌] In a series of these phosphacumulenes, the angles PCαCβ increase and the bond lengths CαCβ decrease when going from ylide 137 (X=NPh), through 137 (X=O), to 137 (X=S), reflecting the order of electronegativities and the declining tendency of the group X to participate in double bonding. A similar trend is observed for their reactivity toward electrophiles, which diminishes from ylide 137 (X=NPh) to 137 (X=S). Phosphavinylidene ylides 138, which are generally more reactive than the phosphaheterocumulene analogues, invariably possess structures with PCαCβ angles close to 120° and similar to typical sp2-hybridized alkenyl systems; an alkyne component analogous to that present in the form 137B is not possible here. Ylides 138 exist as mixtures of Z- and E-isomers rapidly interconverting at temperatures as low as 70°C. Phosphacumulene ylides 137 and 139 exhibit only nucleophilic properties, adding electrophiles in a variety of ways and undergoing cycloadditions both at the polar PCα and at the Cα=Cβ bond. Wittig alkenation, which is so characteristic of common trivalent ylides, is sluggish and rarely observed with phosphaheterocumulene ylides 137, but has been frequently reported for ylides of types 138 and 139. The phosphacumulene ylides do not undergo any electrophilic reactions typical of dipolar ketenes and ketenimines (including dimerization) and are thus more stable. For example, solid samples of the phosphacumulene ylides 137 (X=O, S) can be stored for months and handled under ambient conditions without decomposition. This stability is due to the presence of the additional electron pair on the α-carbon atom leading to an orthogonal set of two π4-electron systems spread over three atoms (Scheme 49). Such compounds are isoelectronic with carbon dioxide, isocyanates, and carbodiimides and so lack the electrophilicity of ketenes featuring a dipolar π4π2 system. Table 4 summarizes structural and other physical properties of selected phosphacumulene ylides.[‌51‌‌53‌,‌55‌,‌186‌,‌551‌‌558‌,‌561‌]

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