Studies in fulvene Chemistry

Abstract

The thesis addresses various facets of fulvene chemistry in four chapters. Chapter 1 presents a brief review of fulvenes as theoretically interesting molecules. Chapter 2 describes detailed studies on the cycloaddition of various fulvenes with different nitrones. Chapter 3 discusses the preparation of a novel class of fulvenes bearing both electron withdrawing and electron donating groups at C (“capto dative” fulvenes). Chapter 4 describes miscellaneous studies largely directed toward exploiting the synthetic potential of fulvenes. ________________________________________ Chapter 2: Fulvene-Nitrone Cycloadditions This study was motivated by the possibility of discovering examples of the rare (6 + 4) cycloaddition category. Most of the few known examples of this type involve fulvenes, with diazomethane and nitrile oxides acting as the 4 components. It was of interest to determine whether nitrones would behave similarly. However, in all the cases studied, only a (2 + 4) mode was observed (1), involving one of the endocyclic double bonds of the fulvene. Fair yields and high regio and stereoselectivities characterised the cycloadditions. Structural assignments were based primarily on NMR (NOESY) and previous X ray crystallographic data. The failure to observe the (6 + 4) mode (2) indicates that the cycloadditions are HOMO(fulvene)-LUMO(nitrone) controlled, since the orbital coefficient at C in fulvenes is greatest in the LUMO. AM1 calculations support this rationale, although other semi empirical calculations indicate a preference for the (6 + 4) mode at least for the parent systems. ________________________________________ Chapter 3: “Capto Dative” Fulvenes Many stable fulvenes bear electron donating groups at C, which are believed to impart aromatic character to the fulvene core. More recently, a few “anti aromatic” fulvenes bearing electron withdrawing groups have been reported. It was therefore of interest to examine whether simultaneous electron donation and withdrawal at C - producing a capto dative effect (3) - might stabilise a diradical form (4). A considerable number of such capto dative fulvenes were synthesised and were found to be relatively stable, in contrast to the general instability of anti aromatic fulvenes. Solvatochromism (UV-visible) and variable temperature NMR studies did not reveal unusual behaviour. However, one member (3, X = OMe, Y = NO) was found to be substantially more reactive than the corresponding monosubstituted analogues (X = H, Y = OMe or NO), forming a dimer-supporting the possibility of a capto dative effect. ________________________________________ Chapter 4: Miscellaneous Studies This chapter describes attempts to exploit the excellent cycloadditive properties of fulvenes for synthesising useful cyclopentanoid synthons. In particular, a synthesis of 3 ethoxycarbonylcyclopentanone (6) was developed via oxidative cleavage of the double bonds in the cycloadduct (5) (from 6,6 dimethylfulvene and ethyl acrylate). In situ decarboxylation of the intermediate keto dicarboxylic acid furnished 6 in 20% overall yield. This route is less expensive than most alternatives (e.g., conjugate addition to cyclopent 2 enone). In another study, a novel methylenation reagent was sought for use in the synthesis of the parent fulvene (via methylenation of cyclopentadiene). The reagent, acetoxymethyl methanesulphonate (AcO-CH-OSOMe), was generated by reacting acetyl methanesulphonate (“Mazur reagent”) with paraformaldehyde. NMR indicated that some (AcO)CH was also formed. Acetoxymethyl methanesulphonate was generally unreactive except toward toluene, which gave an oligomeric product identified as (Ar-CH-Ar)CH, where Ar = p-tolyl and Ar = 2 methyl 5 (p-tolylmethyl)phenyl.