Topics in physical organic chemistry

Abstract

Research described in this Thesis (with the main title “Topics in Physical Organic Chemistry”) is, by deliberate decision, of diversified nature, covering three distinct topics in the general field of Physical Organic Chemistry.

I. Correlation of Substituent Effects in 3,4?Diphenylfurazan 2?oxides In earlier work carried out in these laboratories on the correlation of substituent effects in 3,4?diphenylfurazan 2?oxides substituted at the p-positions of either or both phenyl rings (systems 1, 2 and 3), employing the dual substituent parameter treatment, the tendency was to interpret results in terms of possible maintenance of a higher dihedral angle, on the average, by the C?3 phenyl than the C?4 phenyl with respect to the furazan 2?oxide ring. Differences in transmission of substituent effects (polar and resonance), manifested as differences in ^13C shifts of C?1?, C?1?, C?3 and C?4 in systems 1, 2 and 3, were seen as attributable to a difference in transmission of substituent effects from C?4? and from C?4?, the ipso?centers of the differently oriented phenyls. However, a difference in transmission could also have arisen from a difference in the abilities of the C?3–N?2(O)–O?1 and C?4–N?5–O?1 groupings, which constitute the furazan?2?oxide system, to control the transmission of effects. Effects due to such a difference and those due to a difference in the dihedral angles cannot, apparently, be separated. But an analysis of the modes of transmission of substituent effects, as detailed in Chapter X of this Thesis, has led to premises that indicate that certain weightage needs to be accorded to directionality of transmission arising from a difference in transmissivity of the two constituent groups of the furazan?2?oxide ring. The first steps in the critical reappraisal of the earlier interpretations of correlation of substituent effects in systems 1, 2 and 3, which had been prepared according to known procedures, were a careful re?examination of ^13C line assignments and an assessment of the correlative significance of changes in the line positions attendant on change of substituent. Resonance transmission to C?1? in systems 1 and 3 was found to be higher than to C?1? in system 2, and that to C?1? higher than that to C?1? in system 3 by about the same extent that it is higher at C?1 in 4?substituted ??t?butylstyrenes than at C?1 in 4?substituted styrenes. This was in apparent accordance with the C?3 and C?4 phenyls in systems 1, 2 and 3 maintaining degrees of twist with respect to the furazan 2?oxide ring similar to the twist of the phenyls with respect to the vinyl grouping in ??t?butylstyrene and styrene, respectively. But transmission of the polar effect to C?1? in system 2 was found to be higher than to C?1? in system 1, contrary to what is found in 4?substituted styrenes and ??t?butylstyrenes where that to C?1 is lower in the former. An explanation has been suggested on the basis that the “permanent” (mesomeric) delocalization from the N?oxide oxygen towards C?3 in the furazan?2?oxide system makes for a difference in the transmission of polar effects by the two groupings constituting the system. If this explanation is correct, a recently suggested hypothesis—that polar effects are modified by resonance effects—can be said to have received support in a fact of observation.

II. Thermal Regio?isomerization of Hydroborated Oleic Systems The possibility that the ??position of methyl oleate can be activated through a sequence of hydroboration and thermal isomerization of the derived hydroborated product had been examined by Fore and Bickford. These authors reported that, under the circumstance that little or no reduction of the ester function occurs when methyl oleate and diborane are taken in the ratio 3:1, little or no regio?isomerization takes place when the hydroborated product is heat?treated. Following this report, Logan suggested—based on results with hydroboration/regio?isomerization of oleyl alcohol—a role for oxygen atoms in oxygenated materials like alcohols or ketones, present in or added to the reaction mixture, in retarding regio?isomerization through coordination of the boron atom in the formed alkylboranes. It was of interest to examine if, after attempting complete hydroboration of methyl oleate by taking diborane in large excess, thermal regio?isomerization could be engineered in the alkylborane–borinate esters of structural type 4 that could be assumed to have been formed. The formation of such products depends on hydroboration, in independent reactions, of the double bond and the ester function occurring at comparable rates in methyl oleate. More probable would be the formation of products through intra? and intermolecular reactions in which primarily formed RBH? and R?BH species participate. Available information indicated that the carboxylic acid function undergoes hydroboration more rapidly than an ester function, the former at a rate comparable with that of a double bond. It therefore appeared that products of type 4, resulting from “double attack”, would be more likely in the hydroboration of oleic acid (in the presence of excess diborane) than from methyl oleate. After reassessment of previous results, experiments with methyl oleate and oleic acid using excess diborane and varying conditions of thermal isomerization were carried out. With diborane in excess, conversion of both ester and acid functions seemed complete from IR and ^1H NMR evidence. Regio?isomerization appeared more extensive with oleic acid than with methyl oleate and greater with tri?n-butyl borate than with diglyme as medium. Formation of ??activated product (1,18?octadecanedioate) was also observed in several experiments. An overall impression is gained that boron has a poor tendency to regio?isomerize in entities incorporating the structural feature C–B–O–C. Such entities, some possibly macrocyclic, could predominate in the hydroborated mixtures. Two different mechanisms—one involving dehydroboration/rehydroboration and the other involving a lower?energy ??complex transfer—have been suggested for regio?isomerization of boron in alkylboranes. Without deciding the question, results with the oleic systems appear to indicate that the ??complex mechanism may be selectively suppressed when oxygen?containing materials are present in the reaction mixture. Premises, experimental details and discussion of results form the subject matter of Chapter II.

III. Reaction Kinetics (a) E–Z Isomerization of Hydrazono Compounds The hydrazono structure 5 is established for products of condensation of phenyl diazonium salts with active methylene compounds under basic conditions. One geometric isomer (e.g., the E form 5a) can be isolated free from the Z form (5b). Interconversion can be achieved by heating their solutions in a hydrogen?acceptor solvent such as acetone. The predominance of the Z form is attributed to internal N–H···O hydrogen bonding and additional stabilization in a six?membered ring. Earlier studies showed that E–Z isomerization about C=N has a lower activation energy (17–27 kcal/mol) than geometric isomerization about C=C (60–70 kcal/mol), indicating a lateral shift (nitrogen inversion) pathway. Later studies on iminocarbonates claimed rotation as pathway based on lower activation energies (13–17 kcal/mol), but this remains debated. Kinetics of E ? Z isomerization were followed using IR spectroscopy, as carbonyl bands of the two forms were well?separated. The activation barrier was found to be ~21 kcal/mol and the entropy of activation ~–12 eu, consistent with a highly ordered, solvent?associated transition state. (b) Reappraisal of Ferricyanide Oxidation Kinetics Earlier kinetic studies on oxidation of benzildioxime 7a by alkaline ferricyanide yielded several unusual observations—non?first?order kinetics, altered behaviour in presence of ferrocyanide, pH?dependent increases in rate, and order shifts at higher ferricyanide concentration. A reappraisal has been undertaken using additional data from substituted systems 7b and 7c, bearing electron?withdrawing and electron?releasing groups. This work is summarized in Chapter IV.