A Case of Concerted Formation of Five and Six Member Rings; Solvolytic Behavior of 1-Aryl-1-chloro-4,8,13,17,21-pentamethyl-4,8,12,16,20-docosapentaenes

Chlorides 1 (1-aryl-1-chloro-4,8,13,17,21-pentamethyl-4,8,12,16,20-docosapentaenes) with various phenyl substituents were prepared, and solvolysis rates were measured in 80% (v/v) aqueous ethanol, and in 97% (w/w) aqueous 2,2,2-trifluoroethanol. Hammett ρ+ values obtained are -2.18, -2.13, resp., suggesting a concerted bicyclization in the rate determining step

Nucleofugalities of neutral leaving groups in 80% aqueous acetonitrile

Nucleofugalites of tetrahydrothiophene, dimethyl sulfide and differently substituted pyridines in 80 % aqueous acetonitrile have been derived from the SN1 solvolysis rate constants of the corresponding X,Y-substituted benzhydryl derivatives (1–10). In sol-volysis of sulfonium ions in 80 % aqueous acetonitrile, where acetonitrile is a good cation solvator, the solvation of the reactant ground state is an important rate determining variable since the positive charge is almost entirely located on the leaving group. As a consequence, reaction rates of sulfonium ions are more sensitive to the substrate structure in 80 % aqueous acetonitrile than in pure and aqueous alcohols, which are less efficient as cation solvators. In solvolysis of pyridinium ions the solvation of the reactant ground state is less important, since the positive charge is considerably distributed between the carbon at the reaction center and the leaving group. In such cases the important rate determining variable is solvation of the transition state. Slower reactions of pyridinium sub-strates progress over later, carbocation-like transition states in which the solvation is more important, so those substrates solvolyze slightly faster in aqueous acetonitrile than in methanol (k80AN > kM). Faster reactions proceed over earlier TS in which the solvation is diminished, so those substrates solvolyze somewhat faster in methanol than in aqueous acetonitrile (k80AN < kM). This work is licensed under a Creative Commons Attribution 4.0 International License

Mechanism of 1,2-Hydride Shift in Some Carbocations Involved in Steroid Biosynthesis

The mechanism of 1,2-hydride shift in protosteryl C(20) cation (1A) and in dammarenyl C(20) cation (2A) was investigated by the semi-empirical AM1 method and ab initio quantum Chemical calculations (HF/3-21G level). Stationary points 1A/1B and 2A/2B, and the corresponding transition hydrido-bridged structures 1TS and 2TS were located on the energy surface. Process 1A&#8594;1B turned out to be energetically more favorable than process 2A&#8594;2B by ca. 9 kcal mol-1, mostly due to the unfavorable steric repulsive interaction between the methyl group at C(14) and the &beta;-oriented side chain at C(17) in 1A and the lack of CC-hyperconjugative stabilization in 1A. The exothermicity of processes 1A&#8594;1B and 2A&#8594;2B was increased by subsequent introduction of substituents (H, Me, i-Pr, and t-Bu) at C(14). The more pronounced trend in 1A&#8594;1B proves that the origin of the relative stability of 1B comes from the steric interactions in 1A. Introduction of the halogen atom (F, Cl, and Br), due to its -I effect and relatively small size, changed the direction of the equilibrium 1AY/1BY, and 1AY was found to be by ca. 3 kcal mol-1 more stable than 1BY

Reactivity of Some Tertiary Chlorides with Methoxy and Olefinic Neighboring Group

Solvolysis rates of tertiary chlorides 1-6 and their hexadeuterated analogs 1-dg-6-đe were measured in ethanol (q> = 80%) and 2,2,2- trifluoroethanol (w = 97%). The (5-deuterium KIE calculated showed that chloride 1 solvolyzes with n-participation of the methoxy group in the rate determining step, while chloride 2 solvolyzes mainly by way of a kc process. Based on the reduced P-deuterium isotope effect, it was also concluded that in the solvolysis of chloride 5 7t-electrons take part in the rate-determining step

Reactivity of Acetates in Aqueous Alcohols

A series of X,Y-substituted benzhydryl acetates (1-3) were subjected to solvolysis in various binary methanol/water and ethanol/water mixtures at 25 °C. The LFER equation log k = sf(Ef + Nf) was used to derive the nucleofuge-specific parameters (Nf and sf) for an SN1 type reaction in 60 % and 80 % aqueous methanol. It has been established that X,Y-substituted benzhydryl acetates produce almost parallel correlation lines, so the average sf = 0.9 has been taken for calculation of the nucleofugality parameters from a single rate constant in some solvent mixtures. In comparison with other leaving groups for which the nucleofugality has already been determined, acetates are the poorest nucleofuges

The Reactivity of Benzoates in Mixtures of Water and Aprotic Solvents

Nucleofugalities of pentafluorobenzoate (PFB), 2,4,6-trifluorobenzoate (TFB), 2-nitrobenzoate, and 3,5-dinitrobenzoate (DNB) leaving groups have been derived from the solvolysis rate constants of the corresponding X,Y-substituted Z-benzhydryl benzoates in 60 % and 80 % aqueous acetonitrile and 60 % aqueous acetone, by applying the LFER equation: log k = f s (Ef + Nf). The experimental barriers (ΔG‡ exp) for solvolyses of 13 dianisylmethyl Z-substituted benzoates in these solvents correlate very well with the ΔH‡ calc of the model epoxy ring formation (calculated earlier by the B3LYP-PCM quantum-chemical method). Using the ΔG‡ exp vs. ΔH‡ calc correlation, and taking f s based on similarity, the nucleofugality parameters for about 70 benzoates have been determined in 60 % and 80 % aqueous acetonitrile and 60 % aqueous acetone. Z-Substituents on the phenyl ring have larger impact on the solvolytic reactivity in the less polar solvent than in the more polar solvent due to the more carbocation-like transition state. (doi: 10.5562/cca2179

Deuterium and Carbon-13 Scrambling Processes in Isopropyl Cation

The degenerate rearrangement processes in i-propyl cation were studied using the isotopically double-labeled 2-propyl-2-13C1-2-dI cation (A), which was obtained from the corresponding chloride using the molecular beam technique. The first step in the rearrangement is a shift of one of the methyl hydrogens to form n-propyl cation (or a species in its vicinity on the energy surface), followed by rotation of the methylene group. Rotation in one direction (Process I) leads to formation of isopropyl cation with an interchange of the methine proton with a methyl proton while rotation in the other direction (Process II) results in the formation of intermediate protonated cyclopropane. Through corner-to-corner proton shifts, the isotopes are completely scrambled before the reverse process returns the ion to isopropyl. Relative rates of Processes I and II were determined on the basis of the experimentally established composition of the mixture of isotopomers obtained in an early stage of scrambling, using Runge-Kutta integration to simulate the kinetics. It was found that Process I was faster (ki/ki = 3.4), meaning, according to the proposed mechanism, that hydrogen scrambles a little more rapidly than carbon