Activation of Benzyl Aryl Carbonates: The Role of Cation−π Interactions

Abstract

Benzyl aryl carbonates can react with a nucleophile to yield an activated electrophile and an aryloxide anion. Previously, we had utilized this in the synthesis of α-nitro esters from nitroalkanes. To further understand the process of activation of these carbonates by nucleophiles, we have performed kinetic studies on the hydrolysis of carbonates using nucleophiles. Rate constants for the hydrolysis were obtained under pseudo-first-order conditions with DABCO as the nucleophile. A comparison of rate constant for hydrolysis of isobutyl phenyl carbonate with benzyl phenyl carbonate shows that the presence of benzyl group results in a 16-fold acceleration of hydrolysis rate. This indicates that the transition state for activation of carbonate is stabilized by cation-π interactions. A comparison of the rate constant for various aromatic rings indicates that electron-donating substituents on the benzyl groups accelerate the rate of hydrolysis. Studies were also carried out with DMAP as nucleophile and the results are presented. Our studies show that stable carbonates can be activated using nucleophiles. Activated acyl groups generated from acid anhydrides have been used in several enantioselective reactions. Our studies show that carbonates can be stable alternatives to acid anhydrides.

Cation−π Interactions in Organic Synthesis

“…This indicates that the transition state TS-329 is stabilized by a cation−π interaction between the ammonium and the phenyl moiety as shown in Scheme 81. 246…”

“…Rajaram et al found that the rate of hydrolysis for p- methoxybenzyl phenyl carbonate 328 with DABCO is 16-fold faster than that for isobutyl phenyl carbonate 327 (Scheme ). This indicates that the transition state TS- 329 is stabilized by a cation−π interaction between the ammonium and the phenyl moiety as shown in Scheme …”

Tandem Reaction of Phenyl α‐Cyano‐α‐arylacetates with Quinone Monoimines

“…As can be seen in Scheme 2, first, deprotonation of phenyl α ‐cyano arylacetate 1 with DABCO generated an enolate and attack of the enolate to quinone monoimine 2 a generated adduct A which underwent aromatization and protonization to give the adduct B . Following nucleophilic addition of DABCO to the carbonyl group [7] afforded zwitterion C . Then C−C cleavage of C produced carbanion D and amonium cation E which was hydrolyzed to give protonized BABCO, phenol and carbon dioxide.…”

Base-Mediated Deuteration of Organic Molecules: A Mechanistic Insight

“…Deuterium-labeled molecules are significant tools for the study of metabolic pathways and reaction mechanisms as interior standards in analytical protocols . Deuterium is generally used in isotope labeling and as the primary source for determining the kinetic isotope effects in mechanistic studies .…”