Cyclopentadienone hydrazone

The barrier to rotation of the cyclopentadienone hydrazone 37 is reported to be less than 11 kcal/mol (71). Introduction of a formyl group into the 2-position of the cyclopentadienylidene ring raised the barrier to 11.8 kcal/mol (71). 

Diazocyclopentadienes react with methyl lithium or phenyl lithium to form azo-substituted cyclopentadienide salts [56,57,74]. Protonation of these salts gives rise to cyclopentadienone hydrazones e.g. 

This phosphinazine is thermally stable [71], It is converted into cyclopentadienone hydrazone on acid hydrolysis [73] Diazocyclo-pentadiene also couples with dialkyl phosphites [59], A variety of substituted dlazocyclopentadienes also react with triphenylphosphine to.give phosphinazines [63,82] but if there are alkyl or aryl substituents at both the 2- and 5-positions no phosphinazine is formed [82]. 

Cyclopentadimylidene (2). 2 was very easily obtained by photolysis or thermolysis of the corresponding diazo-cyclopentadienes (72). These were prepared by straightforward procedures (diazo-group transfer, dehydrogenation of hydrazones, or Bamford-Stevens reaction of tosyl-hydrazones) 26-80) from either cyclopentadienes (77) or substituted cyclopentadienones (73). 

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