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Stereochemistry of olefin formation in the pyrolysis of 3-carbomethoxy pyrazolines Wu, Weh-Sai


The thermal decomposition of cis- and trans-3-methyl-4-ethyl-3-carbo-methoxy-Δ¹-pyrazoline (cis- implies that the methyl and ethyl groups are cis) and trans-3-methyl-4-ethyl-3-carbomethoxy- Δ¹-pyrazoline gave a mixture which contained cyclopropane products, cis- and trans-1-methyl-2-ethyl-1-carbo-methoxycyclopropane; α, β-unsaturated ester products, methyl cis- and trans-2,3-dimethyl-2-pentenoate (cis and trans refer the two methyl groups); and the β, у-unsaturated ester, methyl 2-methyl-3-ethyl-3-butenoate. The α,β-unsaturated esters are formed stereospecifically since cis-3-methyl-4-ethyl-3-carbomethoxy-Δ¹-pyrazoline gave only methyl trans-2,3-dimethyl-2-pentenoate (56% in the liquid phase and 28% in the vapor phase). Similarly in the pyrolysis of trans-3-methyl-4-ethyl-3-carbomethoxy- Δ¹-pyra-zoline methyl cis-2,3-dimethyl-2-pentenoate (13% in the liquid phase and 6% in the vapor phase) was obtained while only a trace of methyl trans-2,3-dimethyl-2-pentenoate was found in both the liquid and vapor phase pyrolysis. The cyclopropane products formed from pyrolysis of cis- and trans-3-methyl-4-ethyl-3-carbomethoxy- Δ¹-pyrzaoline have shown some degree of stereo-specificity with a predominance of the isomer having the same stereochemistry as the starting pyrazoline being obtained. The results of the above experiments suggest that the mechanism of thermal pyrolysis of pyrazolines requires that the nitrogen leaves from the same side as the ethyl group, i.e. trans to the hydrogen which is migrating. The photolysis of cis- and trans-3-methyl-4-ethyl-3-carbomethoxy- Δ¹-pyrazoline has been found to be stereospecific in cyclopropane formation with the absence of isomeric olefin products. A small amount of the olefin corresponding to loss of CH₂N₂ was also found having the same stereochemistry as the pyrazolone. Since the products from photolysis are different from that of pyrolysis, a modified mechanism is required. Insufficient evidence is available to clearly define that mechanism.

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