Pierluigi Caramella

Cycloadditions of nitrile oxides to amidoximes. A general synthesis of 3,5-disubstituted 1,2,4-oxadiazole-4-oxides.

Abstract The cycloaddition of nitrile oxides to amidoximes is a general method for the sinthesis of 3,5-disubstituted 1,2,4-oxadiazole-4-oxides with the same or different substituents. The yields are only moderate since an equivalent amount of the nitrile oxide is consumed by reaction with the amine released in the fragmentation of the primary cycloadducts and reforms the amidoxime. With excess nitrile oxides the 1,2,4-oxadiazole-4-oxides undergo a disproportionation reaction to yield nitroso carbonyl intermediates and 1,2,4-oxadiazoles.

The Remarkable Cis Effect in the Ene Reactions of Nitrosocarbonyl Intermediates

Nitrosocarbonyls are fleeting and highly reactive intermediates that undergo ene reactions in a two-step fashion. The addition steps are rate and product determining and lead to polarized diradicals that readily enter the H-abstraction step yielding the ene products. The addition TSs are reached early, and the stabilizing CH...O contacts drive the reactions to the cis adducts. B3LYP calculations alone do not describe the correct ordering of addition TSs in the ene reaction with trimethylethylene and (E)- and (Z)-3-methyl-2-pentenes. Only at the MPWB1K level of treatment, medium-range noncovalent interactions are successfully recovered, accounting satisfactorily for the experimental selectivities. The more stable and isolable ArNOs exhibit late addition TSs, and distortion energies become dominant driving the reaction exclusively to the Markovnikov adducts.

The influence of electron-withdrawing substituents on the geometries and barriers to inversion of vinyl anions

Abstract Whereas ab initio calculations predict that the vinyl anion is bent and has a high barrier to inversion, substitution of cyano, methoxycarbonyl, and formyl substituents at C-1 lower this barrier. Nevertheless, the first two substituents produce significantly bent anions, while the bent and linear 1-formylvinyl anions are essentially isoenergetic.

Computational Mechanistic Study of Thionation of Carbonyl Compounds with Lawesson’s Reagent

The thionation reaction of carbonyl compounds with Lawessons reagent (LR) has been studied using density functional theory methods and topological analyses. After dissociation of LR, the reaction takes place through a two-step mechanism involving (i) a concerted cycloaddition between one monomer and the carbonyl compound to form a four-membered intermediate and (ii) a cycloreversion leading to the thiocarbonyl derivative and phenyl(thioxo)phosphine oxide. Topological analyses confirmed the concertedness and asynchronicity of the process. The second step is the rate-limiting one, and the whole process resembles the currently accepted mechanism for the lithium salt-free Wittig reaction. No zwitterionic intermediates are formed during the reaction, although stabilizing electrostatic interactions are present in initial stages. Phenyl(thioxo)phosphine oxide formed in the thionation reaction is capable of performing a second thionation, although with energy barriers higher than the first one. The driving force of the thionation reactions is the formation of trimers from the resulting monomers. In agreement with experimental observations, the amides are the most reactive when compared with esters, aldehydes, and ketones and the reaction is slightly influenced by the polarity of the solvent. Whereas for amides and esters substituents have little effect, aldehydes and ketones are influenced by both steric and electronic effects.

Photoelectron spectra of nitrones and nitrile oxides

Abstract The photoelectron spectra of five nitrones—N- t -butyl-, C-phenyl-N-methyl-, C-phenyl-N- t -butyl-, C-mesityl-N-methyl-nitrone and dihydroisoquin

Computational Mechanistic Study of the Julia–Kocieński Reaction

This paper describes the first detailed computational mechanistic study of the Julia-Kocieński olefination between acetaldehyde (1) and ethyl 1-phenyl-1H-tetrazol-5-yl sulfone (2), considered a paradigmatic example of the reaction between unsubstituted alkyl PT sulfones and linear aliphatic aldehydes. The theoretical study was performed within the density functional approach through calculations at the B3LYP/6-311+G(d,p) level for all atoms except sulfur for which the 6-311+G(2df,p) basis set was used. All the different intermediates and transition states encountered along the reaction pathways leading to final E and Z olefins have been located and the relative energies calculated, both for the reactions with potassium- and lithium-metalated sulfones, in THF and toluene, respectively. We have essentially confirmed the complex multistep mechanistic manifold proposed by others; however, the formation of a spirocyclic intermediate in the Smiles rearrangement was excluded. Instead, we found that this step involves a concerted, though asynchronous, mechanism. Moreover, our calculations nicely fit with the diastereoselectivities observed experimentally for potassium- and lithium-metalated sulfones, in THF and toluene, respectively.