Pyridinium Cyclopentadienylid

Pyridinium salts are generally polarographically reducible in alkaline solution and dimerized dihydropyridine derivatives have been postulated as products.119-120 A special compound of this type is pyridinium cyclopentadienylide (120),121 which gives both an anodic and a cathodic one-electron wave in alkaline solution and in acid solution only a cathodic wave (Scheme 14). 

Perhaps the most stable ylids are pyridinium cyclopentadienylid 86> 121) and pyridinium dicyanomethylid 100>. 

A similar sequence has been used to prepare substituted pyridinium cyclopentadienylides [119-121], and trimethylammonium [122], tri-phenylphosphonium [125,124] tri-n-propylphosphonium [125], triphenyl-arsonium [126] and dimethylsulphonium [127,128] cyclopentadienylides. An interesting product, which is both an ylide and a salt was prepared by the reaction of tris(dimethylamino)phosphine with... 

As might be expected from the foregoing,.pyridinium cyclopentadienylide has a much larger dipole moment (13-5 D) [151,152] than triphenylphosphonium cyclopentadienylide (7 0 D) [124]. Also as expected the arson um ylide (XII, X = AsPhj) (8 32 D) has a higher dipole moment than its phosphonium analogue (XII, X = PPh3) (7.75 D) [153]. 

In this way pyridinium [60,102,103], phosphonium [52,104,105], arsonium [105,106], stibonium [107], bismuthonium [108], sulphonium [109-113], selenonium [114] and telluronium [115] cyclopentadienylides have been prepared. Evidence has been provided by thermo-gravimetric analysis experiments that these reactions do proceed via the carbene [116]. 

Pyridinium and ammonium cyclopentadienylides have been prepared from ferrocenes [146,147] ... 

Lithium cyclopentadienylide with benzyl pyridinium chloride gives i the pyridone methide (85) (see below and p. 328). 

Like its lithium analogue, sodium cyclopentadienylide gives, with quaternary pyridinium salts, pyridone methides of the type (85)The same and related reagents yield analogous products by reaction with 2- and 4-bromo-l-alkylpyridinium salts , with 4-methoxy- or 4-phenoxy-1-alkylpyridinium salts . ... 

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