Of 3H-pyrazoles

The photochemical study of 3H-pyrazoles was carried out in the search for a route to cyclopropenyl tertiary alcohols. Irradiation of 63a in dry dichloromethane at 300 nm and at room temperature for 0.5 h led to the exclusive formation of the gem-dimethylcyclopropene 65 (Scheme 17). The formation of cyclopropene 65 arises from the loss of N2 and cycUzation of the vinylcarbene intermediate (III). 

The analogous photochemical reaction (300 nm) of 3H-pyrazole 64 in dichloromethane at room temperature led to cyclopropene derivative 66, possessing a a-hydroxy group, isolated in 70% yield (Scheme 18). 

The extension of this reaction to include the conversion of 3H-pyrazoles into cyclopropenes is established.86 3,3,5-Trimethyl-3/i-pyrazole (100 R = H), for example, on photolysis in pentane solution gives 1,3,3-trimethylcyclopropene (101), and an intermediate diazo-alkene (102) has been characterized. The proposed conversion of the diazoalkene into the cyclopropene (101) via a vinylcarbene has a... 

No sensitization experiments to generate the excited triplet states of 3H-pyrazoles have been reported so far. On the other hand the triplet multiplicity of the intermediates has been detected by ESR. 

The most common source of a,j5-unsaturated carbenes for cyclopropene synthesis is from the decomposition of 3H-pyrazoles which are, in turn, available from the addition of a diazo compound to an alkyne. On photolysis these compounds suffer ring cleavage to an a>diazoalkene prior to nitrogen loss and carbene formation. In this way cyclopropene 27 is available from either 25 or 26 by way of common diazo and carbene intermediates. ... 

Subjection of 3H-pyrazoles to thermolysis often results in cyclopropene formation but competing reactions, the subtleties of which are not yet understood, interfere (equations 14 and IS). It is therefore the photochemical reaction which has received most... 

The analogous conversion of 3H-pyrazoles into cyclo-propene derivatives is known to proceed via cyclization of a transient vinyl carbene. The cyclopropene (33), for example, has... 

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