Theoretical Study of the Mechanism of the Addition of Diazomethane to Ethylene and Formaldehyde. Comparison of Conventional ab Initio and Density Functional Methods

Abstract

The 1,3-dipolar cycloaddition reactions of diazomethane with ethylene and formaldehyde as well as the nitrogen elimination reactions from the cycloadducts have been studied using density functional and conventional ab initio methods. The exothermicity of the reactions is underestimated by DFT methods with respect to CCSD(T) due to an overestimation of the C−N dissociation energy of diazomethane. For the cycloaddition reactions all methods lead to similar transition state geometries, and the potential energy barriers computed using DFT methods are similar to the CCSD(T) ones. On the other hand, for the nitrogen elimination reactions transition state geometries and energy barriers are more dependent on the level of calculation. The results obtained show that for the reaction between diazomethane and ethylene the pyrazoline intermediate is more stable than the reactants and that the Gibbs energy barrier for nitrogen elimination is larger than the barrier corresponding to its formation. On the contrary, for formaldehyde the kinetically most favorable cycloadduct, 1,2,3-oxadiazoline, is less stable than the reactants and has a lower barrier for nitrogen elimination.

Oxadiazoles

“…Ab initio and DFTcalculations indicate that, in the reaction of diazomethane with formaldehyde, the kinetically most favorable cycloadduct is less stable than the reactants and has a lower barrier for nitrogen elimination [17]. Derivative 22 has been postulated as intermediate in the metabolism of some (2-hydroxyethyl)-or (2-haloethyl)nitrosoureas, a class of highly active antitumor agents (Scheme 13.3) [18].…”

“…For 3-methylsydnone, the 14 N and 17 O spectra have also been determined and a complete set of chemical shift values for all the atoms have been reported [45]. According to a synthetic approach that allowed for the independent labeling of the nitrogen atoms (Section 13.2.5.1), the NMR chemical shift for each 15 N has been determined unambiguously.…”

“…The 15 N and 17 O data of different 1,3,4-oxadiazoles have also been used to elucidate their structures. In particular, the 17 O resonances registered for the 1,3,4-oxadiazolium-2-olate have demonstrated that the value centered at 181 ppm, relative to exocyclic oxygen, is that expected for an enolate form instead of that for a carbonylic function [496].…”

Conceptual, Qualitative, and Quantitative Theories of 1,3‐Dipolar and Diels–Alder Cycloadditions Used in Synthesis

“…Blavins et al computed a DH°( 0K) value of 16.4 kcal mol À1 with the CCSD(T)/6-311 + + GA C H T U N G T R E N N U N G (2d,2p) method, [103] while Branchadell found the barrier to be 13.0 kcal mol À1 using the CCSD(T)/ 6-311G** method. [104] The barrier for fulminic acid with acetylene was also investigated with CCSD(T) and G2 variants. All of the high accuracy methods give essentially the same value as the CBS-QB3 value within 0.5 kcal mol…”

Azodicarboxamides vs. Azodicarboxylates in Reactions against Thioisomünchnones: 1,3‐Dipolar Cycloaddition or Nucleophilic Addition?

“…Only in the last decade have several sound computational studies been reported to help understanding the way this reaction occurs. [4][5][6][7][8][9][10][11] Thioisomünchnones (1,3-thiazolium-4-olates or anhydro-4-hydroxy-1,3-thiazolium hydroxides) constitute a group of mesoionic heterocycles that behave as masked dipoles in 1,3-dipolar cycloadditions due to their thiocarbonyl ylide Scheme 1.…”