Electrophilic behaviour of 3-methyl-2-methylthio-1,3,4-thiadiazolium salts: A multimodal theoretical approach

2005 ARKIVOC  
Dedicated to Professors Pedro Molina and José Elguero on the occasion of their respective 60 th and 70 th birthdays (Abstract The chemical reactivity of 3-methyl-2-methylthio-1,3,4-thiadiazolium salts towards a variety of nucleophiles has been examined from several theoretical points of view including thermochemical analysis of the global reactions, the evaluation of aromaticity-related parameters -ASE (Aromatic Stabilization Energy), NICS (Nucleus Independent Chemical Shift), HOMA (Harmonic
more » ... , HOMA (Harmonic Oscillator Model of Aromaticity) -for the resulting products, the qualitative application of the HSAB-(Hard-and Soft-Acid and Bases) principle, quantitative calculation of DFT-(Density Functional Theory)-derived local and group properties such as softness and philicity, as well as the estimation of interaction energies. Two well-distinguished reaction paths are characterized depending on the nucleophilicity of the reagent. has analogous features) that should account, on the basis of the HSAB-principle, for the more favorable donor-acceptor interaction with hard nucleophiles. Indeed, the HOMO of simple O-or N-nucleophiles such as H 2 O, MeOH, NH 3 or MeNH 2 lies slightly below the energy of the 1h LUMO. On the contrary, both LUMO+1 and LUMO+5 have significant coefficients over the Smethyl C-atom with antibonding character for the S-C(methyl) bond. Thus, these softer acceptor orbitals are expected to be involved in the S-demethylation processes by the action of softer nucleophiles like S-nucleophiles, and carboxylate or halide salts. Similarly, LUMO+11 of 1h would be involved in N-demethylation reactions leading to 3b, which has never been observed experimentally. Figure 3. Relevant acceptor orbitals (0.035 isovalue) of 1h calculated at the B3LYP/6-31+G* level. It is noteworthy that LUMO+4 (-3.727 eV) exhibits high coefficients similarly distributed at the S-and N-methyl C atoms, with rough sp 3 -antibonding symmetry, as well as two highly Issue in Honor of Prof. J. Elguero and P. Molina ARKIVOC 2005 (ix) 415-437
doi:10.3998/ark.5550190.0006.935 fatcat:ngkbofpyhvdmthkmeo3dceop6a