92573-95-0Relevant academic research and scientific papers
Substituent effects on rates of formation of methoxy-substituted carbonyl ylides from 2-aryl-2-methoxy- an 5-aryl-2-methoxy-Δ3-1,3,4-oxadiazolines
Bekhazi, Michel,Smith, Peter J.,Warkentin, John
, p. 1646 - 1652 (2007/10/02)
2-aryl-2-methoxy-5,5-dimethyl-Δ3-1,3,4-oxadiazolines (4) and 5-aryl-2-methoxy-2,5-dimethyl-Δ3-1,3,4-oxadiazolines (5) were synthetized.Compounds 4 decompose in solution with first order kinetics.Rate constants are correlated with Hammett substituent constants (?-) with p(49.2 deg C) = 0.74 and 0.89 for CCl4 and CD3OD, respectively.The final products from 4 indicate that thermolysis involves the cleavage of both C-N bonds, to form N2 and initially, a carbonyl ylide.Compounds 5, which were obtained as mixtures of cis/trans isomers containing several impurities, and which therefore gave poorer kinetic data, decomposed in CDCl3 solution with p(45 deg C) = 1.1.Carbonyl ylide intermediates, similar to those from the closelyrelated compounds 4, were assumed on the basis of analogy and on the basis of partial identification of products.The effects of para substituents in the aryl groups of 4 and 5 show that the transition states have greater electron density at C-2 of 4 and at C-5 of 5 than do the starting materials.In spite of the increase in electron density at C-2 of 4, the transition state must be less polar, overall, than the ground state because rate constants for thermolysis of 4 in methanol are smaller than those for CCl4 solvent.A plausible explantation for the substituent effects and solvent effects in that the loss of N2 is concerted, with a transition state resembling more closely a spin paired 1,3-diradical than a 1,3-dipole.Alternative stepwise mechanisms, in which C2-N3 bond scission of 4 and C5-N4 bond scission of 5 are the rate-determining steps, leading to 1,5-diradical intermediates, can not be excluded on the basis of the evidence.
