Heteroallenes, cycloaddition

Heptagold heteronuclear cluster compounds X-ray crystallography, 39 371-373 Heptasulfur imide, 2 161-164 Herbicide-binding protein, 33 224 Heteroallenes, cycloaddition to iminoboianes, 31 161... 

Bestmann established a useful method for the preparation of thietanes by cycloaddition of heteroallenes with phosphaallene ylides 88 or phosphacu-mulene ylides (89). °° Because of the strong nucleophilic character of the sulfur atom, 1,4-cyclization via the intermediate 90 takes place. The polarization of the C=S bond may exert a stabilizing effect on the positive charge... 

It was shown earlier <1998J(P1)1755> that heteroallene salts with A, A -dialkylcarbodiimides undergo cycloaddition to furnish l,3,4,5-tetrasubstituted-4,5-dihydrotetrazolium salts (cf. Section 6.07.9.3), which on heating in acetonitrile eliminate an alkene to afford 1,3,5-trisubstituted tetrazolium salts. 

Heteroallenes (e.g. caibon disulfide, carbon dioxide, carbon oxysulfide, isocyanates, isothiocyanates, ketenes, ketenimines) and vinylidenephosphoranes can form polar intermediates from which two isomeric products may result in a 1,4-cycloaddition (equation 113). The direction of the ring closure depends decisively upon the nucleophilic character of Z compared with that of Y. 

Sometimes [4 + 2] cycloaddition of the dipolar 1 1 intermediate with a second molecule of heteroal-lene proceeds faster than the 1,4-cyclization, thus leading to six-membered rings (equation 115). Obviously this type of reaction especially occurs if the lifetime of the dipolar intermediate is sufficiently long owing to stabilization of the positive charge. While phenyliminovinylidenetriphenylphosphorane reacts with heteroallenes only in the molar ratio 1 1 the corresponding oxo, thioxo and diethoxy compounds can react both in 1 1 and 1 2 ratio. 

The [2 + 2] cycloaddition and the [4 + 2] addition of vinylidenephosphorane with heteroallenes have been applied to the synthesis of a great variety of four- and six-membered heterocyclic compounds carrying an exocyclic ylide function. - ... 

The formal acetals of ketene 2, e.g. (diethoxyvinyhdene)triphenylpho-sphorane 4, are highly reactive and, hke 2, add all sorts of H-X compoimds such as carboxylic acids, alcohols, amines, 1,3-dicarbonyls etc. across their C=C bond. Heterocycles result from cycloadditions of these yhdes to heteroallenes [17]. 

Scheme 1 gives a postulated reaction pathway for all these metathesis like reactions. The heteroallenes or the heteroalkenes are supposed to react in a 2 + 2 cycloaddition reaction with Cl3(dme)WCtBu to form metalla cyclobutene derivatives. Electrocyclic ringopening reactions of these (not isolated) metallacycles yield tungsten imido or 0X0 complexes with sigma bonded vinyl, iminyl, ketenyl or keteniminyl ligands. 

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