Studies in heterocyclic and organomercury chemistry

Abstract

The subject of the following studies is the synthetic and mechanistic chemistry of certain heterocyclic and organomercury compounds. The thesis entitled “STUDIES IN HETEROCYCLIC AND ORGANOMERCURY CHEMISTRY” is divided into four chapters. Chapter I Chapter I describes a new synthetic route to the 1H?4,1,2?benzothiadiazine ring system. The formation of this ring system is difficult by conventional heterocyclization strategies, and the known routes have several limitations. By a new strategy, readily available 1,2,3?benzothiadiazole has been converted into 3?substituted?1H?4,1,2?benzothiadiazines, via the quaternary salt of benzothiadiazole (Scheme I). The structure of the quaternary salt and the mechanism of the ring expansion have been studied. It was found that the ring expansion most probably takes place via the N??ylid intermediate (shown in Scheme I). It was also observed that the initially formed N??quaternary salt isomerises to the more stable N??quaternary salt.

Chapter II In Chapter II, the first systematic attempts to prepare the 3,1,2?benzoxadiazine ring system are described. This ring system is of interest because, apart from being a novel heterocycle, it is also an internal diazoether. Furthermore, several unsubstantiated synthetic claims regarding this ring system exist in the literature. The ring system was approached by the strategy described in Scheme II. However, when 2?aminobenzyl alcohol was diazotized and the resulting product treated with base, the major products isolated were benzaldehyde and benzyl alcohol (along with traces of benzoic acid). Deuterium?labelling and cross?over studies showed that benzaldehyde and benzyl alcohol were formed predominantly by an intermolecular hydride transfer. The intermediacy of a diazene and the mechanism of the hydride transfer are discussed.

Chapter III Chapter III consists of two sections. Section A Section A describes two unsuccessful approaches toward the synthesis of the 4,1,2?benzothiadiazine ring system. One approach involves, formally, the insertion of a nitrogen atom into a benzothiazoline ring; the other involves the reaction between nitrile oxides and 2?aminothiophenol (both approaches shown in Scheme III). Section B Section B discusses studies on the alkylation of 1H?naphtho[1,8?cd]triazine and 1H?benzotriazole. Alkylation of these heterocycles is known to occur at the N?? and N??positions (Scheme IV). The factors controlling the site of alkylation have been studied and are discussed.

Chapter IV Chapter IV describes a study intended to explore a new strategy for olefin inversion. The strategy involves anti?oxymercuration of olefins, followed by deoxymercuration in a syn?fashion (Scheme V). A general method of deoxymercuration to yield olefins in good yields was developed. When this strategy was applied to cis?2?heptene, cis?2?pentene, and cis? and trans?stilbenes, mixtures of cis? and trans?olefins were recovered. It is proposed that deoxymercuration takes place in both syn? and anti?fashions, the syn?pathway requiring a novel four?membered ring transition state incorporating a mercury atom. Several interesting mechanistic points are discussed.