Triphenylphosphonium cyclopentadienylide

Triphenylphosphonium cyclopentadienylide (12.109) reacts at the 2-position with diazonium ions and with diazocyclopentadiene (12.110), due to the aromatic character of the five-membered ring, as shown in the mesomeric structures 12.109 a and 12.109b (Ramirez and Levy, 1957, 1958a, 1958b). 

Electrophilic substitution has been studied extensively for triphenylphosphonium cyclopentadienylide. Reaction takes place preferentially at the 2-(5-)position this has been rationalised both in terms of electron transfer theory [158] and by the fact that attack at the 2-positions provides a linearly conjugated o-complex whereas attack at the 3-position gives a cross-conjugated o-complex [159] ... 

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]. 

This is shown, for example, by dipole moment measurements on a series of tetraphenyl-cyclopentadienylides. The moments for the diphenylsulphonium, triphenylphosphonium and triphenylarsonium derivatives are, respectively, 6.69, 7.75 and 8.32 Similarly triphenylarsonium fluorenylide was shown to be more polar than its triphenylphosphonium analogue. ... 

H-N.m.r.spectra also indicate greater polarity in triphenyl-arsonium cyclopentadienylide compared to its triphenylphosphonium analogue [150], In the former there is evidence for greater delocalisation of electrons and equalisation of the ring bond lengths. 

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