Gabriele Renzi

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.

Anomalous stereochemistry of gas-phase acid-induced ring opening in 1-phenylcyclohexene oxide

Abstract The first experimental evidence for an entropy-driven frontside displacement in a gas-phase cationic nucleophilic substitution has been provided by the results concerning the ring opening of 1-phenylcyclohexene oxide catalized by gaseous acids.

Gas-phase acid-induced ring opening in diastereoisomeric 9,10-oxides derived from trans-1,2,3,4,4a,10a-hexahydrophenanthrene

Abstract The stereochemical outcome of the two diastereoisomeric rigid benzocondensed epoxides 2a and 2b in the acid-induced nucleophilic attack by MeOH in the gas-phase was studied and compared with the corresponding results obtained with the mobile epoxides of type 1. The stereochemistry of the ring opening process in these systems (1, 2a, b) appears to be dependent on the extent of the positive charge developed at the benzylic carbon. A rationalization which implies different benzylic carbocationic species is proposed, while the hypothesis of the intermediacy of fully developed benzylic carbenium ions does not appear to be supported by the present results.

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.

Substituent effects on the regio- and stereoselectivity of gas-phase acid-induced ring opening in 1-arylcyclohexene oxides

Abstract Comparative analysis of the ratio of the isomeric monomethyl ethers of 1-arylcyclohexane-1,2-diols formed in the gas-phase and solvalytic acid-induced methanolysis of several 1-arylcyclohexene oxides indicates the intrinsic electronic factors determining the regio- and stereochemical course of the nucleophilic attack, related to the partial degree of carbocationic character at the reaction centre in the substitution transition state.

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.

Aromatic substituent effect on the stereoselectivity of the gas-phase acid-induced ring opening in 9,10-oxides derived from trans-1,2,3,4,4a,10a-hexahydrophenanthrene

Abstract The effect of the aromatic ring substituents on the product distribution of the gas-phase acid-induced ring opening of benzocondensed epoxides 2a and 2b with MeOH was examined and compared with results from methanolysis. The observed syn/anti diastereoselectivity is strictly dependent on the type of the aromatic ring substituent (6-OCH3 or 7-Br), and a very satisfactory Hammett-type linear correlation was found for both epoxides. A razionalization of the results is given.

Reaction of diphenyl disulphide with 1,8-dehydronaphthalene

Abstract The reaction of diphenyl disulphide with 1,8-dehydronaphthalene gives benzo [kl] thioxantene(5), 1-naphthyl-phenyl-sulphide(6) and naphtho-[1,8-bc] thiet(7). Formation of these compounds is explained via the intermediacy of the radical species (8).

Mercury(II) derivatives of azoles

Abstract The reaction between mercury(II) chloride and azoles(pyrazole, 3,5-dimethylpyrazole, benzotriazole, 1,2,4-triazole) afforded mainly N-chloro mercury derivatives, although other reactions, such as the well known ring mercuration or the formation of a complex, were observed simultaneously. Therefore, no simple generalization explains the observed reactivity of azoles towards mercury(II) chloride, each reaction investigated being a different case.

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.