Chapter 1f - Four-membered rings, containing a single heteroatom other than O, S, Se, Te, or N

Abstract

Publisher Summary Ring systems, both saturated and unsaturated, are now known which involve boron, phosphorus, arsenic, silicon, germanium, tin, and various transition metals, e.g., titanium and zirconimn, as the single heteroatom. Nevertheless, the bulk of the literature is dominated by those systems containing silicon and phosphorus, respectively. A general feature of the chemistry of such four-membered heterocyclic rings is their propensity for ring-opening or ring-expansion reactions, reflecting the considerable inherent bond angle strain. There are several four-membered rings which are becoming increasingly important as synthetic intermediates in the preparation of other four-membered ring heterocyclic systems. This includes titanacyclobutanes and related zirconacyclobutenes. A general route to titanacyclobutanes is provided by the reaction of Tebbe's reagent, a well-known metal-alkylidene alkene metathesis catalyst, with alkenes, in the presence of 4-dimethylaminopyridine. The titanacyclobutene system has been obtained by the reaction of the saturated system with 2-methyl-l-hexen-3-yne in the presence of a base, which results in the formation of three inseparable isomeric systems. The instantaneous isomerism observed is consistent with a rapid and reversible ring-opening process. Titanaeyclobutenes have also been shown to undergo ring-expansion reactions on treatment with alkenes and also with both carbonyl- and nitroso-compounds.