Kinetics of the gas‐phase elimination of isopropyl α‐halo acetates and the taft correlation

Abstract

Thi. gas-phase pyrolysis of three secondary acetates in the temperature range of 290-350°C and the pressure range of 40-236 mm Hg reveals a homogeneous first-order law and unimolecular elimination. The rate constants are given by the following Arrhenius equations: isopropyl fluoroacetate: log kl (sec-l) = (12.83 =t 0.53) -(43,700 f 1400) cal/mo1/2.303RT isopropyl chloroacetate: log k , (sec-I) = (12.63 f 0.33) -(42,800 f 900) cal/mo1/2.303RT isopropyl iodoacetate: log k , (sec-l) = (13.09 f 0.23) -(43,300 j = 600) cal/mo1/2.303RTThe differences in rates I > Br > C1 > F apparently question previous assumptions where the pyi-olyses of esters follow the same sequence as acid strength. The experimental data give a good correlation with u* values, where p* = -1.85 a t 330°C. Some kind of discrete bond polarization a t the C , 4 bond of the acetate in the cyclic transition state seems to be involved in this type of reactions.

Computational studies on thermo-kinetics aspects of pyrolysis of isopropyl acetate and its methyl, bromide and hydroxyl derivatives

“…As a result, the partially negatively charged oxygen atom of acetate (O 3 ) would face greater difficulty in stabilizing its p electrons toward the C=O group's adjacent more π-bonded carbon [ 5 ]. Thus, the more electronegative the substituent, the less stable the oxygen p electrons of the C α −O bond toward the carbonyl carbon and the slower the rate constant [ 88 ].…”

Total Synthesis of (±)-Roserine

“…A single attempt to conduct the imine formation using 0.29 M Ti(OEt) 4 in abs EtOH, followed by reduction with ethanolic NaBH 4 , also gave 12a and 13a in poor yield. Finally, 2,2-dimethoxycyclohexanone was alkylated with isopropyl iodoacetate to give 1,2-cyclohexanedione monoketal 11 in 68% yield. Treatment of 13 with 3,4,5-trimethoxybenzylamine in neat Ti(O i Pr) 4 followed by addition of ethanolic NaBH 4 afforded the cis and trans diastereomers 14a and 15a in a 4:1 ratio.…”