Luciana Luchetti

Cyclization of 2-(3-halopropyloxy)phenoxide ions in functionalized surfactants

Abstract Observed rate constants (kobs) of cyclization of 2-(3-halopropyloxy)phenoxide (halo = bromo, iodo; PhBr7 and Ph17, respectively) in aqueous micelles of N-hexadecyl-N-(2-hydroxyethyl)-N,N-dimethylammonium bromide (1) depend on [1] and [NaOH]. The rate profiles go through a maximum and, at high [1], the catalytic effect is modest. By contrast, kobs in aqueous micelles of N-hexadecyl-N-(2-hydroxyethyl)-N,N-dimethylammonium sulfate (2) increases monotonically with increasing surfactant concentration and becomes constant at high [2]. 1H, 13C, and 14N NMR spectra of 1 and 2 show that micelles of 1 and 2 do not differ in dimension and shape and that the average conformations of 1 and 2 are similar.

Surfactant control of the ortho/para ratio in the bromination of anilines. 2.

Abstract We report the bromination of N,N-dimethylaniline in aqueous suspension of cetylpyridinium tribromide, cetylquinuclidinium tribromide, laurylquinuclidinium tribromide, cetyltributylammonium tribromide, cetyldimethyl-2-hydroxyethylammonium tribromide, 1,2-bis(cetyl-N,N-dimethylammonium)ethane tribromide, 1,3-bis(cetyl-N,N-dimethylammonium)propane tribromide, 1,4-bis(cetyl-N,N-dimethylammonium)butane tribromide, (2S,3S)-2,3-dimethoxy-1,4-bis(N-cetyl-N,N-dimethylammonium)butane tribromide. The nature of the head groups of the surfactant, as well as the length of the hydrophobic chain, influence the regioselectivity of the reaction and permits a hypothesis on the structure of the aggregates.

Single electron transfers in zinc-promoted reactions. The mechanisms of the clemmensen reduction and related reactions

Abstract A parallel study of the Clemmensen reaction of benzophenone, 1 , benzhydryl chloride, 2 , and dichlorodiphenylmethane, 3 , suggests new hypotheses about the mechanism of this reduction of diarylketones. Two main pathways have been recognized, both involving ionic and non-ionic reactions. The devised mechanisms provide a tool for interpreting literature data not understood hitherto, concerning Clemmensen-type as well as other zinc-promoted reactions.

Tunable microwave-assisted aqueous conversion of seaweed-derived agarose for the selective production of 5-hydroxymethyl furfural/levulinic acid

A simple, highly efficient and tunable microwave-assisted conversion of seaweed-derived agarose into 5-hydroxymethyl furfural (5-HMF) or levulinic acid (LA), depending on the reaction conditions, is reported. The proposed system could be fine-tuned for the selective production of 5-HMF (ca. 51% yield) in the absence of any catalyst or alternatively to levulinic acid (64% yield) with the simple addition of 1% v/v sulfuric acid, both under optimized aqueous microwave irradiation conditions (10 min, 180 °C). Additionally, galactose could also be generated as the main product (45–77%) depending on the temperature and the addition of catalytic amounts of sulfuric acid. A plausible reaction mechanism is provided in which the formation of intermediates from the hydrolysis of agarose is critical to obtain 5-HMF and LA as final products.

Surfactant effects upon decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion in dichloromethane. The role of head group size

Abstract Spontaneous decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion in CH2Cl2 is speeded by cationic surfactants (C16H33NR3Br, R = Me, Et, n-Pr, n-Bu; C16H33NMe3X, X = Br, ClO4, C12H25OSO3; CTABr, CTAClO4, CTADS, cetylquinuclidinium bromide, CQBr, and p-octyloxybenzyltrimethylammonium bromide, pOOTABr) and by Bu4NBr. First order rate constants reach limiting values (kmax) which increase with increasing bulk of the head group, and decrease on addition of water. Values of kmax in CH2Cl2 with and without a given amount of added water correlate with conductivities for most of the surfactants. It appears that reaction is fastest in small ionic clusters, and growth of these clusters and especially formation of “water-pool” reverse micelles slows reaction. Variations of NMR 1H chemical shifts on addition of H2O to CD2Cl2 show that CTABr readily generates “water-pool” micelles, but they do not form as the bulk of the head group is increased. There is little effect upon the conformations of the hexadecyl residues, based on 13C chemical shifts, which were separated for most of the methylene groups.

Ring-opening reactions. The reactivity of pyrrolidinium and piperidinium ions in solution

Abstract In this paper we report data on the reactivity of 1-phenyl-pyrrolidinium and -piperidinium iodides, and indolinium and tetrahydroquinolinium iodides, with MeO− in methanol and compare them with the corresponding 1,1-dimethylazoniacycloalkane iodides.

Solvent and micellar effects upon the cyclisation of o-3-halopropyloxyphenoxide ions

Rates of intramolecular cyclisations of o-(3-halopropyloxy)phenoxide ion [(1a, b), halogen = Br or I, respectively] increase with decreasing solvent polarity in the order H2O < MeOH < EtOH < PriOH < MeCN, except that reaction of (1a) is faster in H2O than in MeOH, and in H2O (1a) is more reactive than (1b). These effects depend upon the solvation of the oxide ion and the leaving halide ion in the transition state, and relative reactivities kI/kBr indicate the relative importance of these interactions. Micelles of CTAX [C16H33NMe3X, X = Br, Cl, NO3, OH, OTs, or (SO4)0.5] increase the rates by factors of ca. 2 for (1a) and by 4 for (1b), with little effect by the counter-ion. Rate enhancements and kI/kBr increase sharply in the sequence of surfactant head groups N+Me3 < N+ Et3 < N+Pr3 < N+Bu3. Increasing the size of the n-alkyl group apparently decreases hydration of the aryloxide without destabilising the transition state. These micellar medium effects can be related to medium effects in intermolecular reactions in micelles.

Kinetic and spectroscopic investigations of aqueous micelles of cationic surfactants

The aqueous micellar solutions of monocationic surfactants N-hexadecyl-N,N,N-trimethylammonium bromide (CTABr), N-hexadecyl-N,N,N-trimethylammonium nitrate (CTANO3), N,N,N-tributyl-N-hexadecylammonium bromide (CTBABr) and gemini surfactants 1,4-bis(N-hexadecyl-N,N-dimethylammonium)ethane dibromide (C-E-C2Br), 1,4-bis(N-hexadecyl-N,N-dimethylammonium)propane dibromide (C-P-C2Br), and 1,4-bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (C-B-C2Br) were studied with a solvatochromic probe, 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate, better known as Reichard’s ET-30 dye. The local polarity at the probe site (ET) was calculated from the wavelength maximum of the lowest-energy intramolecular charge-transfer ϖ-ϖ* absorption band of ET-30. The results were compared with a kinetic investigation of the cyclization of 2-(3-bromopropyloxy)phenoxide (PhBr7) in micelles; this reaction is a model for SN2 reactions and it depends on medium polarity.

Zinc-promoted reactions. 8. The effect of ring strain in the reduction of 1,2-dibenzoylcycloalkanes

Abstract Ring cleavage was the main route in the Zn reduction of 1 in neat AcOH, while selective carbonyl reduction predominated in the presence of LiCl. The less strained 2 underwent only carbonyl reduction with Zn/AcOH. The Clemmensen reduction of both 1 and 2 resulted mainly in acyclic products. The unstrained 3 was fairly resistant towards reduction, and did not undergo ring cleavage.

Zinc-promoted reactions. Part 5. The behaviour of alkyl substituted 1,3-diketones

Abstract The zinc-promoted reaction of 2,4-pentanedione and related β-dicarbonyl substrates have been investigated under a variety of conditions. The results were explained according to a general mechanism, involving ionic and nonionic pathways.