Contractive annulation — a strategy for the synthesis of small, strained cyclophanes

Biswas, Sourav (2022) Contractive annulation — a strategy for the synthesis of small, strained cyclophanes. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Since the inception of cyclophane chemistry, chemists have been fascinated by the pursuit of ever-smaller and more strained cyclophanes owing to the unusual chemical and physical properties that arise in such systems. The strained nature of these systems poses synthetic challenges and the level of challenge escalates with the level of strain in the cyclophanes. As the degree of strain increases, so does the synthetic challenge and a variety of inventive synthetic approaches to small, strained cyclophanes have been developed. Chapter 1 introduces some important underlying concepts in the area of cyclophane chemistry. A brief summary of different general strategies for the synthesis of cyclophanes is presented. Overall, Chapter 1 serves as a prelude to the subsequent Chapters. Chapter 2 focuses on the development of a new three-stage strategy (contractive annulation) for the synthesis of highly strained cyclophanes that would be very difficult or impossible to access using existing synthetic approaches. The viability of the contractive annulation strategy has been demonstrated by a nine-step synthesis of a strained [2.1]cyclophane from commercially available [2.2]paracyclophane. X-ray crystallographic analysis of the [2.1]cyclophane pointed toward a strained cyclophane framework. The strained nature of the cyclophane was further corroborated by theoretical calculations. Chapter 3 describes the results of the two-directional application of the contractive annulation strategy on [2.2]paracyclophane. As revealed by X-ray crystallographic analysis, the product of the two-directional contractive annulation, a [1.1]cyclophane, was found to have a short interplanar distance. The [1.1]cyclophane is the hitherto smallest member of the [m.n]naphthalenophane family. As expected, DFT calculations indicated that the [1.1]cyclophane is more strained than the [2.1]cyclophane reported in Chapter 2. The emission behavior of the [1.1]cyclophane was tentatively ascribed to an intramolecular excimer formation mechanism. Chapter 4 describes studies on the synthesis and physical properties of [2.2]paracyclophane/9-alkylfluorene hybrids, wherein a solvent-assisted rearrangement reaction of a [2.2]paracyclophane moiety bearing cyclopentadienone was discovered. The rearrangement could be avoided by generating the cyclopentadienone under mild conditions and using benzynes as dienophiles to afford the targeted cyclophanes. One of the [2.2]paracyclophane/9-alkylfluorene hybrids (containing a triphenylene moiety) was found to exhibit dual fluorescence emission. Chapter 5 highlights the synthetic utility of two of the intermediates featured in the synthesis of the [2.1]cyclophane in Chapter 2. One of the intermediates was transformed into [2.2]paracyclophane/quinoxaline hybrids over two steps (oxidation/condensation reactions). An attempt to use the other intermediate for a multistep synthesis of a [2.1]cyclophane bearing a perylene moiety failed. However, the last step of the synthesis met with a serendipitous rearrangement reaction under Scholl reaction conditions to afford an unusual [2.1]cyclophane featuring a 1,1′-binaphthalene moiety. Two plausible mechanistic pathways that have resemblance to those of the Scholl reaction have been delineated to account for the formation of the unexpected [2.1]cyclophane.

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