Isoxazoles theoretical studies

Compared with other potentially aromatic five-membered heterocycles, few theoretical studies on isoxazoles have been reported.262 Del Re, using... 

Theoretical studies on isoxazoles focus on electron density, reactivity, aromaticity, molecular geometry, basicity, and dipole moments. 

Theoretical studies are also done to interpret the synthesis reactions and mechanism of reactions. The regioselectivity of 1,3-dipolar cycloaddition reaction between substituted trimethylstannyl-ethynes and nitrile oxides yielding isoxazoles, was interpreted by the application of frontier electron theory <93CPB478>. By the combination of experimental and molecular orbital (ab initio) studies, a multistep mechanism is proposed for unimolecular radical chemistry of isoxazoles in the gas phase <920MS(27)317>. 

A discussion of conformational studies on few isoxazoles by the use of theoretical methods (PCILO, CNDO/2, Dewar a-n, and EHT), revealed ring and side chain rotation at C-5 position <91HC(49)l>. Theoretical studies on a series of isoxazolyl dihydropyridines (R = Me, Et, iso-Pr, n-C(5)H(11) R = Ph, Me R = H, D R = Me or Et) enabled the estimation of the relative energy and barrier to rotation, for different conformers as a function of both the ring juncture between the... 

A theoretical study using the density functional theory (DFT) method has been performed to rationalize the mechanism of the reactions between 1,3-dialkynes and hydroxylamine or hydrazine for the formation of 3,5-disubstituted isoxazoles or pyrazoles, respectively The computational results support a bimolecular proton transfer as the rate-determining step providing valuable clues for the use of Bronsted acid/base catalysts to promote the cyclization reaction (14OBC7503). 

Theoretical and structural studies have been briefly reviewed as late as 1979 (79AHC(25)147) (discussed were the aromaticity, basicity, thermodynamic properties, molecular dimensions and tautomeric properties ) and also in the early 1960s (63ahC(2)365, 62hC(17)1, p. 117). Significant new data have not been added but refinements in the data have been recorded. Tables on electron density, density, refractive indexes, molar refractivity, surface data and dissociation constants of isoxazole and its derivatives have been compiled (62HC(17)l,p. 177). Short reviews on all aspects of the physical properties as applied to isoxazoles have appeared in the series Physical Methods in Heterocyclic Chemistry (1963-1976, vols. 1-6). 

The aromaticity of isoxazole has been reviewed in terms of theoretical and structural studies,138 139 and the conclusion is that it is slightly less aromatic than oxazole and furan. 

A detailed quantum mechanical study of the mechanism of thermal decomposition of isoxazole has been conducted since previous theoretical predictions appeared to be inconsistent with the experimental results.33 It has been concluded that the mam unimolecular decomposition is through the sequence isoxazole —> NCCH2CHO —> CH3CN + CO and that the minor products, HCN and H2CCO, probably arise via a cyclic carbene as proposed in the experimental study. 

---