Vinylphosphoranes

Phenylcyclopent[c]azcpine (33a) and 6,7-fused cyclopentazepines 33b-d are formed in moderate yields in a one-pot, two-stage process involving initial condensation of triphenyl-[(l-phenylvinyl)imino]phosphoranes 32 with 6-(dimethylamino)fulvene-2-carbaldehyde (30), followed by an intramolecular aza-Wittig reaction of the iminophosphorane with the pendant aldehyde function.5 The method fails with the unsubstituted vinylphosphorane 32 (R1 = R2 = H). 

Acid halides react with vinylphosphoranes (56) to afford isolable N-acylaminophosphonium salts (57), which are hydrolyzed by alkali to N-vinylamides (58). By treatment with triethylamine and phenol the halogen in 57 can be nucleophilically exchanged for phenolate. Thiophenol, secondary amines, and hydrazones can be employed instead of phenol this leads to diverse 1-hetero-substituted 2-aza-1,3-dienes 59 (Scheme 31) (90TL3497). 

Scheme 43 shows the details of the different steps involved in the equilibrium. The nucleophilic attack of the P(III) derivative on the acetylenic bond yields a 1,3-dipole which, after a fast protonation, frees aZ ion. If the subsequent addition of this ion occurs on the P atom (reaction a), a P(V) phosphorane is formed, but the addition of Z on the ethylenic C atom (reaction b) results in the formation of an ylide. Both of these reactions occur under kinetic control and, in both cases, X is always an OR group from the initial acetylene dicarboxylic ester. When the acetylenic compound is a diketone and X is an alkyl or aryl moiety, the C=0 group is much more electrophilic and the attack by the Z ion produces an alcoholate (reaction c), a new intermediate which can cyclize on to the P+ to form a phosphorane, or attack the a-C atom to form an ylide as in Scheme 42. Hence, reactions a and c can coexist, and are strongly dependent on the nature of the trapping reagent and of the P compound, but reaction b is blocked, whatever the reagent. This is well illustrated by the reaction of the 2-methoxytetramethylphospholane 147 on diben-zoylacetylene in the presence of methanol as trapping reagent. The proportions of the vinylphosphorane 157 and spirophosphorane 158 formed (Figure 24) are 13% and 84%, respectively.

Following on from a preliminary report <9lT(.6727>, Nitta and co-workers have published details of the formation of a number of 5-azaazulenes by the reaction of cyclopentadienyl aldehydes with vinylphosphoranes, the simplest product being the phenyl derivative, 3-phenylcyclohepta[c]azepine <95JCS(Pl )iooi>. 

Similarly, a detailed study of the reaction of tetraphenylphosphonium bromide with vinyl- and isopropenyllithium gave evidence of the intermediacy of tetraphenyl-vinylphosphorane derivatives 24a 38). 

The perfluorinated vinylphosphoranes were not isolated, but nevertheless are valuable intermediates in the selective synthesis of alkenes having fluoromethylene groups. Hexafluoropropene (5) was converted into (A )-l, 2,3,3,3-pentafluoroprop-l-ene (9) via the phosphorane 8 in very good yield. 

Table 3. Hydrolysis of Vinylphosphoranes to Alkenes Having Fluoromelhylene Groups ...

Vinylphosphorane Ratio ElZr Yield " (%) of Fluorometltylene Alkene Ratio (EjZY... 

Vinylphosphorane Ralio (EiZf Yielda b c (%) of Fluoromethylene Alkene Ratio (.EiZY... 

The compound 61 has two possible reactions with ketones, one is the Peterson reaction to give the vinylphosphorane and the other is the Wittig reaction to give the vinylsilane. In practice the Peterson route dominates, indicating that in this case loss of OSiMe3 occurs faster than loss of 0 = PPh3278,279 (equation 109). If the a-silylcarbanion also has... 

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