Kinetic study of base-promoted elimination reactions of some 1,1,1-trihalo-2,2-bis(dimethoxyphenyl)ethanes in alcoholic solutions

Abstract

The base-promoted elimination reactions of 1,1,1-trichloro-2,2-bis(dimethoxyphenyl)ethanes were investigated. The bis(3,4-dimethoxyphenyl)ethane was found to be more reactive than the bis(2,5-dimethoxyphenyl)ethane and the latter more reactive than the bis(2,4-dimethoxyphenyl)ethane. Kinetic data relative to 1,1,1-trihalo(chloro or bromo)-2,2-bis(3,4-dimethoxyphenyl)ethanes show that the tribromo reacts faster than trichloro derivative and that the reactions are general-base promoted with Brønsted b values of about 0.6. A kinetic isotope effect, with k H /k D ratio ranging from 3.5 to 5.7, for the base-promoted elimination reaction of 1,1,1-trichloro-2,2-bis(3,4-dimethoxyphenyl)ethane was found. Tunneling occurs for methoxide and ethoxide ion-promoted eliminations. Activation parameters for alkoxy-promoted elimination show a similar trend for chloride and bromide derivatives. The data collected seem to confirm that there is contiguity between E1cB irr and E2 mechanisms.

An Ionic Liquid Dependent Mechanism for Base Catalyzed β-Elimination Reactions from QM/MM Simulations

“…E1cb versus E2 Mechanism. A representative free energy map from the QM/MM/MC calculations in methanol (Figure 1) predicts a concerted E2 transition structure with a large amount of proton transfer characteristic of an E1cb-like mechanism, but not the irreversible stepwise E1cb route proposed in recent publications 12,13,16 as no carbanion intermediate was located. The differences in the results may be rationalized through the disparities in base strengths used, where the base reported MeO − is an extremely strong base relative to the cyclic amines and could potentially accelerate proton removal but not affect the carbanion decomposition.…”

“…However, pyrrolidine was reported to induce elimination of the current tribromo-ethane reactant faster than a methoxide/methanol reaction despite the difference in basicity . Experimental studies have proposed that the transition structure should be strikingly similar for E1cb and E2 mechanisms. , For example, Gandler and Jencks speculated in 1982 about a theoretical potential energy surface for the elimination reactions of (2-arylethyl)quinulidinium ions where the transition state for the E1cb mechanism converts into an E2 mechanism as a substituent change to a β-phenyl group caused the carbanion to become less stable and cease to exist (Figure ) . In addition to the β-phenyl groups present in the current reaction, Br as a leaving group generally tends to favor the E2 mechanism, as does the use of a moderate-strength base, such as the cyclic amines piperidine and pyrrolidine, in polar solvents .…”

“…The E2 mechanism is a one-stage process in which the base attacks the β-hydrogen and abstracts it with a concomitant cleavage of the α-C–Br bond; in the E1cb process, the removal of the β-hydrogen is rate limiting and generally reversible, and can be detected by isotopic exchange studies (Scheme ) . Despite the obvious differences in the mechanisms, experimentally distinguishing between the irreversible E1cb and E2 mechanisms for dehydrohalogenation reactions can be notoriously difficult. ,− The present work applied mixed quantum and molecular mechanical (QM/MM) calculations utilizing Monte Carlo sampling and free-energy perturbation theory (MC/FEP) to the β-eliminations in methanol, [BMIM][BF 4 ], and [BMIM][PF 6 ] to investigate the proposed mechanism change and its origins. Two secondary cyclic amine bases, pyrrolidine and piperidine, were studied as intriguingly no primary or tertiary amines were found to experimentally induce eliminations.…”

Study of Aromatic Nucleophilic Substitution with Amines on Nitrothiophenes in Room-Temperature Ionic Liquids: Are the Different Effects on the Behavior of para-Like and ortho-Like Isomers on Going from Conventional Solvents to Room-Temperature Ionic Liquids Related to Solvation Effects?

“…The structure of the cation of a RTIL plays in turn an important role, as also the hydrogen-bond donor ability of the solvent appears to be very important. In fact, a change in the cation component of an ionic liquid, going from bmim to bm 2 im, can cause a different reactivity and mechanism. 7d, Herein, for the pyrrolidino debromination of 1a in [bm 2 im][BF 4 ], a different dependence of k obs on amine concentration was observed ( k obs = ( k 1 k 3 / k - 1 )[AmH] 2 = k III [AmH] 2 ). This could be explained by the fact that bm 2 im, a cation less able to form a hydrogen bond, needs a second amine molecule to assist the leaving group departure.…”

“…Recent studies have demonstrated that changes in the nature of the cation and/or of the counterion bring about notable variations in the properties (e.g., electrophilicity or nucleophilicity) of the reacting molecules as well as in the mechanism of reaction . Accordingly, we recently proposed a shift from the E1 cB (in MeOH)10a to the E2 mechanism (in RTILs) 10b for the amine-promoted dehydrobromination of the 1,1,1-tribromo-2,2-bis(phenyl-substituted)ethanes in ionic liquids.…”