Tetravinylallene and other pi-bond rich hydrocarbons

Abstract

This thesis explores the synthesis and chemistry of novel pi-bond rich molecules, in particular the multivinylallenes and the [13]annulene carbocation in one review and four experimental chapters. Chapter 1 describes the impetus for exploring the structural space of conjugated hydrocarbons, Chapter 2 provides an overview of the chemistry of vinylallenes, a class of conjugated allenes. Chapter 3 describes the first preparation of the parent tetravinylallene, as well as the first examples of substituted tetravinylallenes, in publication format. Chapter 4 investigates the first synthesis of the parent trivinylallene and includes a study of the first substituted trivinylallenes. Chapter 5 details an improved synthesis of the divinylallenes and a study of the multivinylallene family of molecules. Chapter 6 describes syntheses towards the [13]annulene carbocation, an annulene proposed to have Mobius topology.

While the synthesis and application of vinylallenes have long been known, they have only been reviewed as examples within the larger allene literature. In Chapter 2 an overview of the literature on vinylallenes is presented. In particular this examines their synthesis, reactivity and synthetic applications.

Tetravinylalllene is the highest member of the multivinylallene family that has not previously been reported. In Chapter 3 we describe the first synthesis of the parent tetravinylallene molecule using a cross-coupling reaction to incorporate the final vinyl functionality. The approach was extended to the synthesis of the first substituted tetravinylallenes. The half-lives of these molecules were compared to other known pi-bond rich hydrocarbons. The reactivity of tetravinylallenes in pericyclic cascades was also explored, which points to the potential synthetic application of these molecules.

Trivinylallene is another member of the multivinylallene family that has also not been reported. Chapter 4 describes the first synthesis of the parent trivinylallene molecule using a cross-coupling approach similar to that used in the synthesis of tetravinylallene. This method was then applied to the synthesis of the first substituted trivinylallenes, investigating the effects of different substitution patterns as well as stereo-electronic effects. The reactivity of trivinylallene in pericyclic reactions is described, as well as comparison with select substituted trivinylallene analogs. Computational analysis of these molecules was performed to rationalise the observed reactivity.

With access to the higher members of the multivinylallene family for the first time, a study of this family of molecules was sought. Chapter 5 initially describes an improved synthesis of the previously reported divinylallenes. This, in conjunction with data from Chapters 3 and 4, enabled a comparison of the properties of the multivinylallenes using NMR and UV-Visible spectroscopy to identify trends within this family of molecules.

Aromaticity is an important concept in organic chemistry, with an evolving definition. While the idea of Mobius aromaticity has often been theorised, the unambiguous synthesis of a Mobius annulene remains unresolved. Finally, Chapter 6 describes syntheses towards the [13]annulene carbocation, a molecule proposed to have Mobius topology. The synthesis towards this molecule focuses on the use of the irontricarbonyl group protecting group strategy in order to target [13]annulenone, another molecule that is yet to be synthesised, as an advanced intermediate. Two different approaches to this molecule were investigated. The first focused on a symmetric disconnection, while the second approach utilised a non-symmetric disconnection to target the 13-membered ring.