85846-50-0Relevant 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.
Carbonyl ylides and carbenes from thermolysis of oxadiazolines. Substituent effects on intramolecular and intermolecular reactions of carbonyl ylides
Bekhazi, Michel,Warkentin, John
, p. 619 - 624 (2007/10/02)
Thermolysis of a 2-methoxy-Δ3-1,3,4-oxadiazoline involves loss of N2 with formation of carbonyl ylide.The fate of the carbonyl ylide depends on its enviroment and on the other substituents present.Thus, the ylides from 2-methoxy-2,5,5-trimethyl-Δ3-1,3,4-oxadiazoline (1) and from 2-methoxy-2-(4-methoxyphenyl)-5,5-dimethyl-Δ3-1,3,4-oxadiazoline (2) are trapped very efficiently by methanol.However, the ylide from 1 is trapped much less efficiently than that from 2 by dimethylacetylene dicarboxylate, cis-1,2-dichloroethylene, or norbornadiene.A major competitive process in the case of 1 is fragmentation of the ylide to carbonyl compounds and carbenes, the latter being trapped by alkenes to form cyclopropanes.An intramolecular 1,4-H transfer is also competitive under some conditions.The ylide from 2 does not appear to fragment, nor does it undergo the 1,4-H transfer, but it cyclizes efficiently to the oxirane in the absence of trapping agents.Preliminary estimates of rate constants for cyclization of ylide from 2 to form the oxirane (kcycl31 deg C = 1.3*1E-6 s-1) and for its additions to norbornadiene and to dimethylacetylene dicarboxylate (1*1E-5 M-1s-1 and 1*1E-9 M-1s-1, respectively are reported.If it is assumed that the ylide from 1 would add to dimethylacetylene dicarboxylate with a similar rate constant, then the yield for that process can be used to place a lower limit of 1010s-1 on the rate constant for fragmentation of that ylide at 31 deg C.
