Formation of Cyclopropenes

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). 

R = R1 = Ph) in very high yield by photolysis, the pyrazole is formed in only 3 % yield from diphenylethyne.50) The formation of cyclopropenes from pyrazoles apparently occurs via the corresponding diazo-compound. Thus photolysis of (53, R = R1 = Ph) at between 330 and 410 nm has been shown to lead to the diazo-compound (56) which on further photolysis or pyrolysis is converted to (54)50). Indeed, photolysis of vinyldiazoalkanes provides a good route to cyclopropenes thus photolysis of E- (57) leads to 3-alkoxycarbonyl-3-cyanocyclopropene51), while (58) leads either to the corresponding cyclopropene or to reversal to the pyrazole5Z). 

This chapter will cover eliminations of HX, X2 and XSiR3 (X = halogen) which lead to the formation of cyclopropenes, bicyclobutanes and propellanes and aspects of other elimi-... 

Again, the calculation reproduced almost exactly the experimental results. Moreover, both calculation and experiment accounted for almost 100% of the cyclopropyl group, the possible formation of cyclopropene being completely excluded. 

It is well known that transition metal complexes catalyze the decomposition of diazoacetates in the presence of acetylenes. Thus addition of 63 to the acetylenic species 76 and 77 resulted in loss of nitrogen and formation of the stable C-metallated cyclopropene compounds 78 and 79. For the first time loss of nitrogen in the formation of cyclopropene rings is initiated by the presence of the zirconium metal fragment. 

Examples for the formation of cyclopropenes by dehydrohalogenation of monohalocyclopro-panes arc given in Table 1. 

The formation of cyclopropene (1) by a retro-Diels-Alder reaction occurred with low efficiency, and the product polymerized under the reaction conditions. -... 

Other applications of diazo compounds in this manner are shown for the formation of cyclopropenes 2, while reactions in which pyrazoles are converted into cyclopropenes without the isolation of a diazo compound are given in Section 3.2.1.1. 

Irradiation of l,3-di-to7-butylprop-2-yn-l-oneand 2,3-dimcthylbut-2-ene in benzene saturated with water resulted in efficient formation of cyclopropene 3. ... 

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