Photolysis of pyrazolenines

These compounds upon irradiation in the (n, n ) region give rise to cyclo-propenes via carbenoid decomposition of a vinyl diazoalkane intermediate , viz. 

Pyrazolenine esters under similar conditions give allene and 1,3-diene esters by an ionic mechanism involving both inter and intramolecular processes, viz. 

A similar heterolytic cleavage reaction has been considered for diazoacetate esters to explain their alcoholic exchange reaction .  

---

Considerable effort has been devoted to studying the photolysis of pyrazolenines since it still constitutes the best way to obtain cyclopropenes. An important publication by Gloss... 

The photolysis of pyrazolenine derivatives 113 yields mainly allene 114 and 1,3-diene derivatives 115. According to Day and Whiting... 

Products of the Photolysis of Pyrazolenine Esters in the presence of External Acid... 

Finally photolysis of substituted pyrazolenines 140 in ether is reported (118) to yield allene 142 and diene 143 as products via vinyl cation 141. 

The neutralizing effect of hydrogen ions upon ion pairs was originally suggested by Day and Whiting (1967) in connection with the photolysis of certain types of pyrazolenine esters. While the reactions of pyrazole-nines are not as well imderstood as the rearrangement of nitrosoamides or the reaction of diazoalkanes with acids, their mechanism is difficult to... 

When the reaction is carried out in ether solution, it was found that, whereas addition of acetic acid to a solution of pyrazolenine propionate led to a mixture of propionate and acetate esters in the products, the addition of large amounts of the more strongly nucleophilic methanol gave only a barely detectable fraction of methyl ethers. Such behaviour closely parallels that observed by White in the rearrangement of nitrosoamides and may be accounted for in terms of a similar mechanism, if it is assumed that the initial photolysis of the pyrazolenine gives a diazoalkane. 

Vinylcarbene, generated by photolysis of substituted pyrazolenine, affords a mixture of the cyclopropene derivative and vinyloxirane 200 via insertion into an a-hydroxy group [66JCS(C)1719]. Phenyloxiranes... 

---