The mechanism of thermal eliminations. Part 21. Rate data for pyrolysis of 2-ethoxyquinoline, 1- and 3-ethoxyisoquinoline, and 1-ethoxythiazole: correlation of reactivities with π-bond order of the CN bond

Abstract

We have measured the rates of thermal elimination of ethylene from the title compounds between 587.3 and 722.9 K. The reactivities relative to 2-ethoxypyridine at 650 K are: 3-ethoxyisoquinoline (0.21 ), 2ethoxyquinoline (3.1 3), 1 -ethoxyisoquinoline (6.47), 2-ethoxythiazole (63.1 ). These reactivities parallel the Tc-bond order of the C=N bond, though the exceptional reactivity of 2-ethoxythiazole is attributed to additional acceleration through +M electron release from sulphur to nitrogen. This emphasizes the greater relative importance of nucleophilic attack by the nitrogen upon the phydrogen atom as compared with the analogous mechanism for pyrolysis of esters. Because of the semi-concerted nature of the reaction, interruption of aromaticity is much less significant than in, for example, electrophilic aromatic substitution. Thus retention of the benzenoid character of the ring not involved in the elimination is not an important rate-determining feature, as shown by the lower reactivity of 3ethoxyisoquinoline relative to 2-ethoxypyridine. The unimportance of the interruption of aromaticity of the benzenoid ring means that conjugative effects are better relayed to nitrogen in the P-naphthalenelike position (isoquinoline) than in the a-naphthalene-like position (quinoline). This is the reverse of the familiar pattern for reactions of naphthalene-like systems where full charges are involved, and may be an additional factor contributing to the higher reactivity of 1 -ethoxyisoquinoline than of 2-ethoxyquinoline, as may also be the -l effect of the benzo substituent. The conclusions are used to predict elimination rates for alkoxyheterocycles not yet studied.

Mechanism of thermal eliminations. Part 30. Pyrolysis of 2-trimethylsilylethanol and 1-aryl derivatives

“…Evidently the product (of general formula 1) is formed by loss of Me,SiOH from two molecules of the alcohol followed by reaction of Me,SiOH with the initial byproduct, eqn. (7). Consistent with a mechanism involving loss of OH from the x side chain position to give an incipent carbocation is the fact that no by-product at all was formed during preparstion of the 4-trifluoromet hyl-substi tu ted alcohol.…”

Kinetic Study of the Phenolysis of O-Methyl and O-Phenyl O-2,4-Dinitrophenyl Thiocarbonates and O-Ethyl 2,4-Dinitrophenyl Dithiocarbonate

“…O -Bis(phenyl) thiocarbonate 8 (one of the products in the reaction of phenol with PDNPTOC), O -methyl O -phenyl thiocarbonate 7 (one of the products in the reaction of phenol with MDNPTOC), and O -ethyl O -phenyl thiocarbonate (one of the products in the reaction of phenol with EDNPDTC) were prepared as reported.…”

Structure−Reactivity Correlations in the Aminolysis of Ethyl S-Aryl Thiolcarbonates

“…Materials. The ethyl S -aryl thiolcarbonates were synthesized as described and identified by NMR and IR analyses. The amines were purified as reported …”