On the competitive intramolecular H-bonding in alcohols with two accepting sites

Ordered Hydrogen-Bonded Alcohol Networks Confined in Lewis Acid Zeolites Accelerate Transfer Hydrogenation Turnover Rates

The disruption of ordered water molecules confined within hydrophobic reaction pockets alters the energetics of adsorption and catalysis, but a mechanistic understanding of how nonaqueous solvents influence catalysis in microporous voids remains unclear. Here, we use kinetic analyses coupled with IR spectroscopy to study how alkanol hydrogen-bonding networks confined within hydrophobic and hydrophilic zeolite catalysts modify reaction free energy landscapes. Hydrophobic Beta zeolites containing framework Sn atoms catalyze the transfer hydrogenation reaction of cyclohexanone in a 2-butanol solvent 10× faster than their hydrophilic analogues. This rate enhancement stems from the ability of hydrophobic Sn-Beta to inhibit the formation of extended liquid-like 2-butanol oligomers and promote dimeric H-bonded 2-butanol networks. These different intraporous 2-butanol solvent structures manifest as differences in the activation and adsorption enthalpies and entropies that comprise the free energy landscape of transfer hydrogenation catalysis. The ordered H-bonding solvent network present in hydrophobic Sn-Beta stabilizes the transfer hydrogenation transition state to a greater extent than the liquid-like 2-butanol solvent present in hydrophilic Sn-Beta, giving rise to higher turnover rates on hydrophobic Sn-Beta. Additionally, reactant adsorption within hydrophobic Sn-Beta is driven by the breakup of intraporous solvent-solvent interactions, resulting in positive enthalpies of adsorption that are partially compensated by an increase in the solvent reorganization entropy. Collectively, these results emphasize the ability of the zeolite pore to regulate the structure of confined nonaqueous H-bonding solvent networks, which offers an additional dimension to modulate adsorption and reactivity.

PCM Study of the Solvent and Substituent Effects on the Conformers, Intramolecular Hydrogen Bonds and Bond Dissociation Enthalpies of 2-Substituted Phenols

A PCM continuum model, at the DFT/B3LYP level, is used to study the solvent and substituent effects on the conformers, intramolecular hydrogen bond (HB) enthalpies, (Delta H(intra)s), and O-H bond dissociation enthalpies, (BDEs), in 2-substituted phenols, 2-X-ArOH, in the liquid phase. Two electron-donating (edg) and three electron-withdrawing (ewg) substituents are chosen, involved in a variety of biochemical transformations. Seven solvents, differing in their H-bonding ability and polarity, are selected to model different environmental situations. Very good correlations are found between the computed R(O-H) and nu(O-H) values in solution for all non-HB 2-X-ArOH, showing that the former can be used as an universal molecular descriptor for the latter and vice-versa. In all 2-X-ArOH, the HB parent conformer is the most stable in all media, closely matching frequency experimental data in CCl4. However, for all 2-X-ArO*, the most stable conformer either forms a "reverse"-HB or a HB is not formed, due to the long distance or steric effects. Changes in the stability, in solution, are observed for some of the 2-X-ArO* conformers. The intramolecular HB-strength in solution, Delta H(S,intra), varies significantly with the size of the HB ring formed and the nature of the substituents. Reasonable correlations, derived between the two energetic parameters (BDE(aw,sol) and Delta H(S,intra)) and the solvent ( and a), and/or molecular, [R(O-H) and nu(O-H)] ones, allow for an approximate estimation of the two former from the four latter. 2-X(edg) decrease BDEs (hence, increase the antioxidant efficiency of the solute, too) in all media; 2-X(ewg) present an opposite result. Moreover, an isodesmic reactions study affords total stabilization effect (TSE) values (identical to the Delta[BDE(aw)]s), which are mainly governed by the stabilization of the phenolic radical (SPR) than that of the parent molecule (SPP). Quantitative correlations between the two effects in the TSE in both the gas and the liquid phases are also given. Unlike in the protic solvents, the better stabilization of the radical than the parent species, derived for the 2-X(edg)-ArOH in the aprotic, apolar, and/or low polar solvents, could account well for their smaller BDE(sol)s. An effective antioxidant in solution should involve either one of the two edg in any one of the two latter solvents.

The Reactivity of o-Hydroxybenzyl Alcohol and Derivatives in Solution at Elevated Temperatures

The reactivity of o-hydroxybenzyl alcohol (o-HBA, 1), as a model compound for lignin, has been studied in various solvents between 390 and 560 K. Both in polar and apolar solvents the benzylic cation is the reactive intermediate. In alcoholic solvents, the benzylic cation reacts with the solvent to give the corresponding ethers. Relative reaction rates have been determined for different alcohols; a factor of 14 is encountered between the most (methanol) and least (tert-butyl alcohol) reactive ones. The etherification is reversible, in contrast to the electrophilic aromatic substitution with phenol and anisole, for which k(PhOH) = 1 x 10(5) M(-)(1) s(-)(1) and k(anisole) = 1 x 10(4) M(-)(1) s(-)(1), at 424 K. In apolar hydroaromatic solvents, 7H-benz[de]anthracene, 9,10-dihydroanthracene, and 9,10-dihydrophenanthrene, the formation of o-cresol proceeds via hydride transfer from the solvent to the benzylic cation; rate constants at 555 K are 2 x 10(6), 5 x 10(4), and 5 x 10(3) M(-)(1) s(-)(1), respectively.