A theoretical study on the stability and spectra of cycloaddition products: Methylene-β-lactam isomers

Abstract

The theoretical thermodynamic stability was calculated for products of cycloaddition of vinylimine to ketene and isocyanic acid to allene by using the MP2, DFT(B3PW91), and HF methods involving the 6-311++G** basis set. These cycloaddition reactions could yield fourteen different molecules; practically however, free Gibbs energies calculated for the considered molecules suggest that only 4-and 3-methylene-b-lactams can be formed in observable amounts. For these particular isomers the heats of formation, geometry, rotational constants, harmonic vibrational frequencies, infrared intensities, Raman activities, depolarisation ratios, and 1 H, 13 C, 17 O and 15 N NMR spectra were evaluated and discussed. The PED analysis carried out for theoretical IR spectra allowed us to assign vibrations of the two isomers. The IR and NMR assignments form a basis for the elucidation of future experimental data.

An ab Initio Study on the Allene−Isocyanic Acid and Ketene−Vinylimine [2 + 2] Cycloaddition Reaction Paths

“…25 We demonstrated that out of the 11 possible products of such a reaction, 4-methyleneβ-lactam is the one most stable thermodynamically. 25 However, no other theoretical nor experimental paper has dealt with this reaction. On the other hand, we did not consider the mechanism of this reaction, and this seems to be desirable as the experimental findings report 2-iminooxetanes to be the main product of the catalyzed cycloaddition reaction of vinylimine with ketenes or aldehydes.…”

“…Also, the ketene−vinylimine cycloaddition reaction could possibly yield 4-methylene-β-lactams . We demonstrated that out of the 11 possible products of such a reaction, 4-methylene-β-lactam is the one most stable thermodynamically . However, no other theoretical nor experimental paper has dealt with this reaction.…”

“…In both isocyanic acid−allene and ketene−vinylimine reactions, 14 molecules could possibly form, however, out of them, only the 3-methylene-2-azetidinone molecule is known . Quite recently, the chemistry of azetin-3-ones has been reviewed, and some halogen derivatives of 1,2-oxazetidines and iminooxetanes have been discovered and characterized.…”

“…23 However, the MP2/6-31+G* calculations have shown that the inclusion of both electron correlation and diffuse functions led to a nonplanar TS, and therefore, a [ π 2 s + ( π 2 s + π 2 s )] mechanism was proposed instead of the [ π 2 s + π 2 a ]. 24 Reactions of unsubstituted allenes yielded 4-methylene-βlactams, [23][24][25] however, inclusion of substituents in the allene moiety yielded the 3-methylene isomers. The results were concordant with experimental findings.…”

“…On the other hand, we did not consider the mechanism of this reaction, and this seems to be desirable as the experimental findings report 2-iminooxetanes to be the main product of the catalyzed cycloaddition reaction of vinylimine with ketenes or aldehydes. 30 In both isocyanic acid-allene and ketene-vinylimine reactions, 14 molecules could possibly form, 25 however, out of them, only the 3-methylene-2-azetidinone molecule is known. 31 Quite recently, the chemistry of azetin-3-ones has been reviewed, 32 and some halogen derivatives of 1,2-oxazetidines 33 and iminooxetanes 34 have been discovered and characterized.…”