Thermolysis of 3-Alkyl-4-phenyl-2-oxetanones and Related Compounds

Article abstract of DOI:10.1021/jo01300a017

Thermolysis of cis- and trans-3-methyl-4-phenyl-2-oxetanones (1c and 1t), cis- and trans-3-isopropyl-4-phenyl-2-oxetanones (2c and 2t), and cis- and trans-3-tert-butyl-4-phenyl-2-oxetanones (3c and 3t) resulted in quantitative formation of the corresponding styrene which kept stereochemical uniformity.The strict stereospecificity suggests the concerted nature of the fragmentation.The rates of reaction of these 2-oxetanones and related compounds, including 3,3-dimethyl-4-phenyl-2-oxetanone (4), 3,3-dimethyl-4,4-diphenyl-2-oxetanone (5), 4,4-dimethyl-3-phenyl-2-oxetanone (6), and cis- and trans-3,4-diphenyl-2-oxetanones (7c and 7t), in decane and/or dodecane were studied.The reactivity of 4-phenyl-2-oxetanone (8) was estimated from that of p-nitro derivative 9.The relative rates at 150 deg C were 1c/2c/3c/1t/2t/3t/4/5/6/7c/7t/8, 1.0:0.31:0.028:11:4.3:2.1:0.29:6.9:3.8:(1.7):(13):(17).The least substituted 8 exhibited the highest reactivity.In 3,4-disubstituted derivatives, the trans isomer reacted more rapidly than the corresponding cis isomer without exception.For 1-3, it is shown that the bulkier the alkyl group, the slower the fragmentation.Since significant positive charge development has been previously shown to occur only at C-4, but not at C-3, the reactivity sequence cannot be attributed to the electronic effect of the alkyl group.On the other hand, if an assumption that the important factor is the increased steric strain at the concerted 2_s + 2_a transition state is accepted, the results can be explained reasonably.Indeed, log k₁ values of 1c, 2c, 3c, and 8 correlated quite linearly with the steric parameter E_s or ν.The relative reactivities of 4-7 are also discussed.