Solvolytic Behavior of Aryl and Alkyl Carbonates. Impact of the Intrinsic Barrier on Relative Reactivities of Leaving Groups.

The effect of negative hyperconjugation on the solvolytic behavior of carbonate diesters has been investigated kinetically by applying the LFER equation log k = s(E + N). The observation that carbonate diesters solvolyze faster than the corresponding carboxylates and that the enhancement of aromatic carbonates is more pronounced indicates that the negative hyperconjugation and π-resonance within the carboxylate moiety is operative in TS. The plots of ΔG vs approximated ΔG° for solvolysis of benzhydryl aryl/alkyl carbonates and benzhydryl carboxylates reveal that a given carbonate solvolyzes over the higher Marcus intrinsic barrier and over the earlier transition state than carboxylate that produces an anion of similar stability.

A DFT-based model for calculating solvolytic reactivity. The nucleofugality of aliphatic carboxylates in terms of Nf parameters.

The most comprehensive nucleofugality scale, based on the correlation and solvolytic rate constants of benzhydrylium derivatives, has recently been proposed by Mayr and co-workers (Acc. Chem. Res.

Method for estimating S(N)1 rate constants: solvolytic reactivity of benzoates.

Nucleofugalities of pentafluorobenzoate (PFB) and 2,4,6-trifluorobenzoate (TFB) leaving groups have been derived from the solvolysis rate constants of X,Y-substituted benzhydryl PFBs and TFBs measured in a series of aqueous solvents, by applying the LFER equation: log k = s(f)(E(f) + N(f)). The heterolysis rate constants of dianisylmethyl PFB and TFB, and those determined for 10 more dianisylmethyl benzoates in aqueous ethanol, constitute a set of reference benzoates whose experimental ΔG(‡) have been correlated with the ΔH(‡) (calculated by PCM quantum-chemical method) of the model epoxy ring formation. Because of the excellent correlation (r = 0.

Effect of the leaving group and solvent combination on the LFER reaction constants.

Fine effects that influence the variations of the reaction constants s(f) in LFER log k = s(f)(N(f) + E(f)) have been summarized here. Increasing solvent polarity in the series of binary mixtures increases the solvolysis rates for the same factor for all benzhydryl derivatives in which the solvation of the leaving group moiety in the transition state is substantial, i.e.

A practical guide for estimating rates of heterolysis reactions.

Chemists are well trained to recognize what controls relative reactivities within a series of compounds. Thus, it is well-known how the rate of ionization of R-X is affected by the stabilization of the carbocation R(+), the nature of the leaving group X(-), or the solvent ionizing power. On the other hand, when asked to estimate the half-life of the ionization of a certain substrate in a certain solvent, most chemists resign.

Leaving group property of dimethyl sulfide.

The LFER equation log k = s(f)(E(f) + N(f)) was used to derive the nucleofuge specific parameters (N(f) and s(f)) for dimethyl sulfide in the series of aqueous alcohols, using the S(N)1 solvolysis rate constants obtained for X-substituted benzhydryl dimethyl sulfonates 1-5. The slope parameters (s(f)) are practically independent of the solvent used, while N(f) parameters slightly decrease as the polarity of the solvent increases.

Solvolytic reactivity of heptafluorobutyrates and trifluoroacetates.

A series of X,Y-substituted benzhydryl heptafluorobutyrates (1-6-HFB) and trifluoroacetates (1-6-TFA) were subjected to solvolysis in various methanol/water, ethanol/water, and acetone/water mixtures at 25 degrees C. The LFER equation log k = s(f)(E(f) + N(f)) was used to derive the nucleofuge-specific parameters (N(f) and s(f)) for S(N)1-type reaction. In comparison with TFA, the HFB leaving group is a better nucleofuge for less than 0.

Nucleofugality of phenyl and methyl carbonates.

A series of X,Y-substituted benzhydryl phenyl carbonates 1 and X,Y-substituted benzhydryl methyl carbonates 2 were subjected to solvolysis in different methanol/water, ethanol/water, and acetone/water mixtures at 25 degrees C. The LFER equation, log k = sf(Ef + Nf), was used to derive the nucleofuge-specific parameters (Nf and sf) for phenyl carbonate (1LG) and methyl carbonate (2LG) leaving groups in a given solvent in SN1 type reaction. Kinetic measurements showed that phenyl carbonates solvolyze one order of magnitude faster than methyl carbonates.

A steric deuterium isotope effect in 1,1,3,3-tetramethylcyclohexane.

The equilibrium isotope effect (EIE) for the interconversion of the two chair isotopomers of 1-trideutero-1,3,3-trimethylcyclohexane was predicted using geometry and vibrational force constants derived from electronic structure theory at HF, B3LYP, and MP2 levels as input for the program THERMISTP. Agreement between theory and previously reported NMR results is very good (experimental K(eq) = 1.042 +/- 0.

Stochastic Search for Isomers of the sec-Butyl Cation.

A stochastic search procedure for locating energy minimum structures was applied to the sec-butyl cation. A previously unreported structure 3' with strong H-hyperconjugative stabilization of the carbocation center was found at several levels of theory (HF, B3LYP, and MP2). The theoretical equilibrium isotope effect (EIE) for the monodeutero isotopomer of 3' (Keq = 1.

How fast do R-X bonds ionize? A semiquantitative approach.

The correlation equation log k(25 degrees C) = sf(Nf + Ef), where sf and Nf are nucleofuge-specific parameters referring to leaving group/solvent combinations and Ef are electrofuge-specific parameters referring to the incipient carbocation R+, are used to predict ionization rate constants of alkyl derivatives R--X. We show how to employ the Ef parameters of reference electrofuges and the sf and Nf parameters of reference nucleofuges reported in the preceding article for determining further sf, Nf, and Ef parameters. Since sf is usually close to 1.

Kinetics of the solvolyses of benzhydryl derivatives: basis for the construction of a comprehensive nucleofugality scale.

A series of 21 benzhydrylium ions (diarylmethylium ions) are proposed as reference electrofuges for the development of a general nucleofugality scale, where nucleofugality refers to a combination of leaving group and solvent. A total of 167 solvolysis rate constants of benzhydrylium tosylates, bromides, chlorides, trifluoroacetates, 3,5-dinitrobenzoates, and 4-nitrobenzoates, two-thirds of which have been determined during this work, were subjected to a least-squares fit according to the correlation equation log k(25 degrees C) = sf(Nf + Ef), where sf and Nf are nucleofuge-specific parameters and Ef is an electrofuge-specific parameter. Although nucleofuges and electrofuges characterized in this way cover more than 12 orders of magnitude, a single set of the parameters, namely sf, Nf, and Ef, is sufficient to calculate the solvolysis rate constants at 25 degrees C with an accuracy of +/-16 %.

A new rearrangement process in tert-amyl cation.

13C NMR spectroscopy of the 2-methyl-2-butyl-1-13C cation (13C-labeled tert-amyl cation) indicates that interchange of the inside and outside carbons occurs via a barrier of 19.5 +/- 2.0 kcal/mol.

Isotopic scrambling in Di-13C-labeled 2-butyl cation: evidence for a protonated cyclopropane intermediate.

The (13)C NMR spectrum of 2-butyl-1,2-(13)C(2) cation (1) is unchanged on heating the sample to -78 degrees C, indicating no isomerization to another isotopomer. In contrast, the spectrum of 2-butyl-2,3-(13)C(2) cation (2) shows rapid formation of all of the other isotopomers except 1. These results are consistent with a protonated cyclopropane intermediate in the rearrangement process.

Rearrangement pathways of five-membered ring enlargement in carbocations: quantum chemical calculations and deuterium kinetic isotope effects.

Three plausible routes for the five-membered ring expansion in the equilibrating 2-cyclopentyl-2-propyl and 1-(2-propyl)cyclopentyl cations 1A/1B were located on the PES, all calculated at the MP4/6-31G(d)//MP2/6-31G(d) level of theory. In pathway I, the six-membered transition structure (TS-I) connects the less stable cyclopentyl cation 1A and the 1,2-dimethylcyclohexyl carbocation (2) via a barrier of 16.4 kcal/mol.

Solvolysis of 1,1-dimethyl-4-alkenyl chlorides: evidence for pi-participation.

Tertiary 1,1-dimethyl-4-alkenyl chloride (1) solvolyzes with significantly reduced secondary beta-deuterium kinetic isotope effect (substrate with two trideuteromethyl groups) and has a lower entropy and enthalpy of activation than the referent saturated analogue 4 (k(H)/k(D) = 1.30 +/- 0.03 vs k(H)/k(D) = 1.