Synthesis and Kinetic Stabilization of [n]Cumulenes
Creators: Bozheng, Sun
Publisher: name: University of Alberta Library
Year: 2022

<p>The studies of [n]cumulenes have marked the 100th anniversary of Brand’s report of tetraphenylbutatriene in 1921. During the past century, hundreds of different [n]cumulenes have been studied, and [n]cumulenes are depicted as good candidates for the development of novel materials. [n]Cumulenes also serve as model compounds to study the 1-dimensional carbon allotrope, commonly referred to as carbyne. The synthesis and application of [n]cumulenes, however, have been hindered often by poor kinetic stability, especially for extended derivatives with n ≥ 7, which is challenging to be isolated.<br>
    This thesis focuses on the design, synthesis, characterization, and comparison of [n]cumulenes. Chapter 1 gives a brief introduction to [n]cumulenes, such as the definitions used in the discussions and examples of the synthesis of known derivatives. More importantly, the persistence and reactivity of [n]cumulenes are discussed to explain why kinetic stabilization is needed. The orbital structure and the spectroscopic determination of the HOMO-LUMO gaps of [n]cumulenes also are discussed to account for such reactivity.<br>
    Chapter 2 includes the synthesis of a series of cyclic tetraaryl[5]cumulenes, which have been incorporated into different ring sizes. These compounds provide a platform to study how the ring strain affects a cumulenic framework. The reactivity and the spectroscopic characteristics of the cyclic derivatives are examined as a function of ring strain. Additionally, the frontier molecular orbitals are investigated on the basis of DFT computations, and the results are used to compare with and to supplement the experimental observations.<br>
    Chapter 3 deals with the syntheses of a [3]- and a [5]cumulene using an alkyl substituted aryl endgroup, which has been designed to afford [n]cumulenes that favor the formation of uniform crystalline films. The intended products have been synthesized successfully, and the effects of the alkyl substituent on the spectroscopic characteristics of [n]cumulenes are discussed. The formation of uniform crystalline films using the [3]cumulene synthesized in this chapter, however, has not been successful to date.<br>
    Chapter 4 introduces the attempts to synthesize tetraphenyl[4]cumulene ([4]Ph) through reductive elimination reactions. These reactions, if successful, are expected to afford fewer side reactions as well as higher yields of [4]cumulenes than the methods used previously. While the intermediates we designed are synthesized successfully, the desired product ([4]Ph) has not been isolated from the subsequent reductive elimination reactions.<br>
    Chapter 5 describes the rationale for the design of five new endgroups to afford kinetically stabilized [n]cumulenes. The corresponding [n]cumulenes for three endgroups (3,5-di-tert-buty-2-methylphenyl, ortho-isopropylphenyl, and ortho-tert-butylphenyl) have been synthesized successfully with n = 3, 5, 7, 9, 11, and 13. The attempts to synthesize derivatives of the other two endgroups (4,6-di-tert-buty-2-methylphenyl, 2,4,6-tri-tert-butylphenyl) have not been successful. A comparison of the persistence of [9]cumulenes, functionalized with different endgroups, reveals a trend of kinetic stability as a function of the ortho substituents of the aryl endgroups. Namely, aryl endgroups with more bulky substituents at the ortho positions usually afford more stable [n]cumulenes.<br>
Chapter 6 discusses the characterization of [n]cumulenes synthesized in Chapter 5. Thanks to the improved kinetic stability, many analyses of the [9]-, [11]-, and [13]cumulenes are possible for the first time. UV-vis spectroscopy has been used to study the HOMO-LUMO gaps of [n]cumulenes with n = 3, 5, 7, 9, 11, and 13. The trend of absorptions, as a function of n, is examined and compared to the trends provided from a series of [n]cumulenes limited to shorter derivatives. The convergence of [n]cumulenes to cumulenic carbyne, however, is not observed from the results of UV-vis or NMR spectroscopic analyses.<br>
    General methods, experimental procedures, characterization of the new compounds, and the spectra appended can be found in Chapter 7.</p>