Organometallic routes to cross-conjugated hydrocarbons

Abstract

Molecules with a high degree of unsaturation often have associated with them a concomitant level of instability. In fact, their instability is what hinders the preparation and synthetic utility of large swaths of polyunsaturated frameworks. One form of modulating and controlling that reactivity is the tricarbonyliron group; a functionality that coordinates to a portion of an unsaturated skeleton, often allowing the easy isolation and observation of the unstable compound. This thesis explores the chemistry of cross-conjugated polyenes, and especially their relationship to the tricarbonyliron group, in one review, and three experimental chapters. Chapter 1 reviews the known chemistry of highly reactive polyenes, and their stabilisation through coordination to the tricarbonyliron group. Chapter 2 describes the preparation of tricarbonyliron complexes of the dendralenes. Chapter 3 investigates the curious reactivity of cross-conjugated trienes. Chapter 4 describes a new synthetic strategy towards preparing polyenes that are protected as organometallic complexes. While the ability for the tricarbonyliron group to stabilise reactive polyenes has long been known, their applications in this respect have not been rigorously documented by review. In Chapter 1 we comprehensively review the literature on the tricarbonyliron complexes of unstable molecules, and uncover promising areas for future research. In particular, there remain several hydrocarbons of fundamental interest that have never been synthesised, whose preparation could be realised by using tricarbonyliron protection. The dendralenes are a fundamental family of cross-conjugated oligoalkenes that only recently been accessed on a useful scale. The family of molecules has the power to rapidly form compounds with natural-product like complexity through a cascade of bond-forming reactions, but their synthetic utility is hampered by their instability and lack of selectivity. In Chapter 2 we describe the first targeted preparation of the tricarbonyliron complexes of the [3]-[6]dendralenes. We find that tricarbonyliron complexation not only protects the dendralenes from decomposition, but also selectively activates them to a broad range of reactions. In Chapter 3 we report the first general synthesis of [3]dendralene molecules substituted at the l-position. These compounds were prepared via crossmetathesis on the tricarbonyliron complex of [3]dendralene. With the elusive series of substituted dendralenes in hand, we report the surprising observation that the IE-sub-c1ass undergoes Diels-Alder dimerisation up to 200 times faster than the parent [3]dendralene. This stands in stark contrast to the behaviour of the IZ-, 2-, & 3'-substituted [3]dendralenes, which are invariably more stable than the unsubstituted case. We explore the mechanistic rationale for this behaviour. Finally, in Chapter 4 we use the knowledge gained in the synthetic efforts described in Chapters 2 & 3 to rationally develop a new, general method for the synthesis of polyene complexes via cross-coupling reactions. To verify our approach we use 2- & 2,3- substituted halobutadiene complexes to directly prepare the tricarbonyliron complexes of the dendralenes, as well as some new cross-conjugated frameworks. Show more