Formation of Pyrazol-1,3,4-Thiadiazoles through 1,3-Dipolar Cycloadditions of 3-Thioxo-[1,2,4]-Triazepin-5-one with Nitrilimines: An Experimental and Computational study

In this work the results of experimental and computational study of the title compounds and some ancillary compounds are reported. Two bicyclic pyrazol-1,3,4-thiadiazole derivatives were synthesized by reaction between 6-dimethylaminomethylene-3-thioxo-[1,2,4]- triazepin-5-one 1 and several nitrilimines 2a-f to give corresponding spirocycloadducts 3a-f, which undergo a rapid rearrangement leading to the new bicyclic compounds, 4a-f and 5a-f. These obtained bicyclic products were characterized by 1H and 13C NMR spectroscopy and finally by X-ray crystallography. Theoretical calculations have been carried out using DFT methods to rationalize the formation of the two new bicyclic compounds. Two reaction types are involved in the formation of the compounds 4a-f and 5a-f. The first one is a 1,3-dipolar cycloaddition reaction between 1 acting as dipolarophile and 2a-f as dipoles. The results indicate that the cycloaddition between 1 and 2g, as model of 2a-c, takes place via a high asynchronous bond-formation process. The regioselectivity obtained from the calculations is in complete agreement with the formation of the unique spirocycloadducts 3a-f. The second reaction leading to the formation of the final products is a domino process that is initiated by the quick and irreversible cleavage in a catalytic acid environment of triazepenic ring

Strain Effects in Protonated Carbonyl Compounds. An Experimental and ab Initio Treatment of Acyclic Carboxamides and Ketones

Strain effects have been quantitatively evaluated for a set of 22 compounds including ketones (R2CO), carboxamides (RCONH2), and N,N-dimethylcarboxamides (RCONMe2), where R = Me, Et, i-Pr, t-Bu, 1-adamantyl (1-Ad), in their neutral and protonated forms. To this end, use was made of the gas-phase proton affinities and standard enthalpies of formation of these compounds in the gas phase, as determined by Fourier transform ion cyclotron resonance mass spectrometry (FT ICR) and thermochemical techniques, respectively. The structures of 1-AdCOMe and (1-Ad)2CO were determined by X-ray crystallography. Quantum-mechanical calculations, at levels ranging from AM1 to MP2/6-311+G(d,p)//6-31G(d), were performed on the various neutral and protonated species. Constrained space orbital variation (CSOV) calculations were carried out on selected protonated species to further assess the contributions of the various stabilizing factors. Taking neutral and protonated methyl ketones as references, we constructed isodesmic reactions that provided, seemingly for the first time, quantitative measures of strain in the protonated species. A combination of these data with the results of theoretical calculations (which also included several “computational experiments”) lead to a unified, conceptually satisfactory, quantitative description of these effects and their physical link to structural properties of the neutral and protonated species.This work was supported by grants PB 93-0289-C02 and PB-93-0142-C03-01 from the Spanish D.G.I.C.Y.T. Work by H.H. was supported by the Moroccan Ministry of Education and C.S.I.C

Effects of Charge and Substituent on the S∙∙∙N Chalcogen Bond

Neutral complexes containing a S···N chalcogen bond are compared with similar systems in which a positive charge has been added to the S-containing electron acceptor, using high-level ab initio calculations. The effects on both XS···N and XS+···N bonds are evaluated for a range of different substituents X = CH3, CF3, NH2, NO2, OH, Cl, and F, using NH3 as the common electron donor. The binding energy of XMeS···NH3 varies between 2.3 and 4.3 kcal/mol, with the strongest interaction occurring for X = F. The binding is strengthened by a factor of 2–10 in charged XH2S+···NH3 complexes, reaching a maximum of 37 kcal/mol for X = F. The binding is weakened to some degree when the H atoms are replaced by methyl groups in XMe2S+···NH3. The source of the interaction in the charged systems, like their neutral counterparts, is derived from a charge transfer from the N lone pair into the σ*(SX) antibonding orbital, supplemented by a strong electrostatic and smaller dispersion component. The binding is also derived from small contributions from a CH···N H-bond involving the methyl groups, which is most notable in the weaker complexes

Structure, Basicity, and Thermodynamic Properties of 3,5-Bis(trifluoromethyl)-1,2,4-triazole with Regard to 1,2,4-Triazole: The Trifluoromethylation Effect

Thermodynamic properties (enthalpies of sublimation, gas-phasebasicity) of 3,5-bis (trifluoromethyl)- 1,2,4-triazole have been measured. These properties are discussed in the larger frame work of two o the rtriazoles, the parent compoundand 3(5)-(trifluoromethyl)-l,2,4-triazole, thanks to abinitio calculations atthe MP2/6-3lG*//6-31G* level of accuracy. The calculations provide(i) an excellent description of the vibrational frequencies of 1H-1,2,4-triazole (ii) age o metry for this compound more consistent with the microwaveresults;(iii)adescription of the trifluoromethyl group as a substituentintheazoleseries;(iv)anexcellentaccord with the experimental protonaffinity, and(v) an understanding of the interesting properties of the title compound.Work by E.B. was supported by agrant from CSIC.This work was partially supported by the DGICYT Project No.PB90-0228-C02. M.E. gratefully acknowledges a post-doctoral fellowship from the Ministerio de Educación y Ciencia of Spain.Peer reviewe

Activation of the disulfide bond and chalcogen-chalcogen interactions: An experimental (FTICR) and computational study

Dimethyldisulfide (I) is the simplest model of the biologically relevant family of disubstituted disulfides. The experimental study of its gas-phase protonation has provided, we believe for the first time, a precise value of its gas-phase basicity. This value agrees within 1 kJ mol−1 with the results of G3 calculations. Also obtained for the first time was the reaction rate constant for the bimolecular reaction between I and its protonated form, IH+, to yield methanethiol and a dimethyldithiosulfonium ion. This constant is of the order of magnitude of the collision limit. A computational mechanistic study based on the energetic profile of the reaction, completed with Fukui's and Bader's treatments of the reactants and transition states fully rationalizes the regioselectivity of the reaction as well as the existence of a shallow, flat Gibbs energy surface for the reaction. The mechanistic relevance of the chalcogen–chalcogen interaction and the CH⋅⋅⋅S bonds has been demonstrated.This work was supported by grants BQU2003-05827 and BQU2003-00894 of the Spanish DGIYT, and by the Project MADRISOLAR. Ref.: S-0505/PPQ/0225 of the Comunidad Autónoma de Madrid