Graziella Roselli

Regiochemical Control of the Ring Opening of 1,2-Epoxides by Means of Chelating Processes. Part 14: Regioselectivity of the Opening Reactions with MeOH of Remote O-Substituted 1,2-Epoxycyclohexanes under Gas-Phase Operating Conditions.

Abstract The regiochemical behavior of cyclohexene oxides bearing a remote O -functionality was determined in the gas-phase in opening reactions with MeOH, using a gaseous acid (D 3 + , C n H 5 + , Me 2 F + ) as the promoting agent. The results obtained indicate the incursion in the opening process in the gas phase of H + (or D + )-mediated chelated bidentate species completely absent in the corresponding reactions (methanolysis) carried out in the condensed phase.

Aromatic substituent effect on the stereoselectivity of the condensed- and gas-phase acid-induced methanolysis in 2-aryloxiranes derived from 3,4-dihydronaphthalene and trans-1,2,3,4,4a,10a-hexahydrophenanthrene bearing a tertiary benzylic oxirane nucleophilic centre

The ring-opening reactions with MeOH of the title benzocondensed 2-aryl oxiranes 6 and 7a,b both in the condensed (methanolysis) and in the gas phase were examined, obtaining in all cases a good Hammett-type linear correlation. Results indicate that the secondary or tertiary nature of the benzylic oxirane carbon is not responsible for the different stereochemical behavior so far encountered in different 2-aryl oxirane systems under the same operating conditions.

Regiochemical control of the ring opening of aziridines by means of chelating processes. Part 3: regioselectivity of the opening reactions with methanol of remote O-substituted regio- and diastereoisomeric activated aziridines under condensed- and gas-phase operating conditions

Abstract The regiochemical behavior of the pairs of diastereoisomeric activated aziridines 1 – 8 deriving from the cyclohexane system, bearing a remote O -functionality, was determined in the acid methanolysis in the condensed phase (cd-phase) and in the reaction with MeOH in the gas-phase using a gaseous acid (D 3 + ) as the promoting agent. The results obtained in the opening process of the cis diastereoisomers indicate the constant incursion in the gas phase of D + (corresponding to H + )-mediated chelated bidentate species able to modify the regiochemical result found in the methanolysis in the cd-phase.

Gas-phase facial diastereoselectivity of equatorial and axial 4-chloro-adamant-2-yl cations

The acid-catalyzed addition of CH3(18)OH to 2-methylene-adamantanes bearing a chlorine atom in the 4-equatorial (1e) or 4-axial (1a) position has been investigated in the gas phase, at 760 Torr, in the 40-120 degrees C temperature range. Two different experimental approaches were employed: (1) by adding neutral CH3(18)OH to the 2-methyl-4-Cl-adamant-2-yl cation, generated by protonation of the corresponding 2-methylene-4-Cl-adamantane (the extracomplex reaction) and (2) by reaction of 2-methylene-4-Cl-adamantane with CH3(18)OH2+, generated by methylation of H2(18)O (the intracomplex reaction). The crucial role of the nature of the noncovalent intermediates involved along the reaction coordinates emerges from the difference between the results obtained in the extracomplex and intracomplex reactions for both substrates investigated. The kinetic and stereochemical results indicate that the 4-Cl substituent plays a different role depending on its equatorial or axial orientation. Examination of the experimental results in the light of MP2/6-31G* theoretical calculations provides important information about the intrinsic factors governing the facial diastereoselectivity of trigonal carbocations. The effects due to differential face solvation phenomena emerge from the comparison of the present gas-phase results with those obtained from strictly related studies in solution.

Regiochemical control of the ring opening of 1,2-epoxides by means of chelating processes. Part 15: Regioselectivity of the opening reactions with MeOH of remote O-substituted regio- and diastereoisomeric pyranosidic epoxides under condensed- and gas-phase operating conditions

Abstract The regiochemical behavior of pairs of regio- and diastereoisomeric epoxides derived from the 3,4,5,6-tetrahydro-2 H -pyrane system, bearing an acetal group as the remote functionality, was determined in the acid methanolysis in the condensed phase (cd-phase) and in the reaction with MeOH in the gas-phase using a gaseous acid (D 3 + ), as the promoting agent. With only one exception, the results obtained in the opening process of these epoxides indicate the incursion in the gas-phase of D + -mediated chelated bidentate species able to modify the regiochemical result found in the methanolysis in the cd-phase.