Adriano Pizzabiocca

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.

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.

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).

The thermolysis of N-phenyl-N′-sulphinylhydrazine : an interesting source of phenyl radicals

The thermal decomposition of N-phenyl-N′-sulphinylhydrazine (1) in a number of solvents has been investigated. The isomeric composition of the biphenylic products indicates the intervention of phenyl radicals, which come from the homolysis of (1).

Stereochemlcal effects in the gas-phase pinacol rearrangement of cis-and trans-1-methylcyclohexane-1,2-diol

Abstract The gas-phase pinacol rearrangement of cis and trans -1 -methyl-1,2-cyclohexanediols. promoted by D 3 + and C n H 5 + (n = 1,2), was studied by the radiolytic method in the pressure range 100–760 Torr. Under all conditions, 2-methyl-cyclohexanone is the predominant product, arising from both substrates via different pinacol rearrangements and successive fast isomerization of the corresponding primary intermediates, e.g. 0-protonated 1-methyl-1-cyclopentanecarboxaldehyde. This conclusion is based from kinetic analysis of competition experiments with pinacol as reference substrate, carried out at high pressure (760 Torr) with or without added base (NMe 3 , 3 Torr). showing that the pinacol rearrangement rates are markedly dependent on the stereochemical features of the diol. Accordingly the trans diol rearranges more rapidly than the cis isomer, which in turn isomerizes faster than pinacol, indicating that anti-periplanar CH 2 migration to the vicinal tertiary C-OH 2 + center in trans (k2) is over five times faster than H migration in cis (k3). Analysis Of the relative migrating ability of the different CH 2 moieties In trans (k2 > k1) allowed exclusion of appreciable anchimeric assistance in these gas-phase pinacol rearrangements. The results are compared with relevant gas-phase data and with those concerning the same substrates in acidic solution.

Acid-catalyzed aldehyde-ketone rearrangements in the gas phase. Cyclopentane- and cyclohexanecarboxaldehyde.

Abstract Protonation by gaseous H + 3 , C n H + 5 (n=1,2), s-C 3 H + 7 and t-C 4 H + 9 cations promotes rearrangement of cyclopentane- and cyclohexanecarboxaldehyde to cyclohexanone and cyclopentylmethyl ketone, respectively. The reaction was investigated by radiolytic and mass spectrometric methods.

Evidence for a concerted SN2′ mechanism in the gas-phase acid-induced nucleophilic substitutions on allylic substrates

Gas phase nucleophilic substitution on oxygen-protonated but-1-en-3-ol and trans-but-2-en-1-ol by methanol proceeds via the concerted SN2′ mechanism in competition with the classical SN2 mechanism.

Gas phase acid-induced ring-opening of an α,β-epoxysilane. The first case of complete regio- and stereo-selectivity in an ion–molecule reaction in the gas phase

Complete regio- and stereo-selectivity has been observed in the gas phase acid-induced ring-opening of 1,2-epoxy-1-trimethylsilycyclohexane using two different nucleophiles (i.e. H2O and MeOH), which compares well with that of the same reaction carried out in solution.