Abstract
The intermediacy of short-lived isoindenes, generated in the course of metallotropic or silatropic shifts over the indene skeleton, can be shown by Diels-Alder trapping with tetracyanoethylene, leading to the complete elucidation of the dynamic behaviour of a series of polyindenylsilanes. Cyclopentadienones, bearing ferrocenyl and multiple phenyl or naphthyl substituents undergo [4 + 2] cycloadditions with diaryl acetylenes or triphenylcyclopropene to form the corresponding polyarylbenzenes or cycloheptatrienes. The heptaphenyltropylium cation, [C7Ph7+], was shown to adopt a nonplanar shallow boat conformation. In contrast, the attempted Diels-Alder reaction of tetracyclone and phenethynylfluorene yielded electroluminescent tetracenes. Finally, benzyne addition to 9-(2-indenyl)anthracene, and subsequent incorporation of a range of organometallic fragments, led to development of an organometallic molecular brake.
Highlights
While Diels-Alder cycloadditions have been the cornerstone of much elegant natural product chemistry, as in the first total synthesis of a steroid or in the preparation of molecules having unprecedented symmetry such as cubane by Eaton [2], triquinacene by Paquette [3], and cubic graphite by Pascal [4], or exhibiting novel materials properties as in Mullen’s superacenes [5], they can play a role in the elucidation of reaction mechanisms
A combination of 1 H-1 H COSY, 1 H-13 C and 1 H-29 Si shift-correlated NMR data revealed the unequivocal assignment of the proton and carbon-13 nuclei in all the different indenyl ring environments, and 1D-selective inversion and 2D-EXSY spectra allowed the elucidation of their molecular dynamics [17]
In ferrocenyl-penta(β-naphthyl)benzene, 30, (Figure 14), the orientations of the peripheral β-naphthyl rings paralleled the behaviour of the phenyls in C6 Ph5 Fc whereby each ring was rotated more than its preceding neighbour
Summary
While Diels-Alder cycloadditions have been the cornerstone of much elegant natural product chemistry, as in the first total synthesis of a steroid (cortisone by Woodward [1]) or in the preparation of molecules having unprecedented symmetry such as cubane by Eaton [2], triquinacene by Paquette [3], and cubic graphite by Pascal [4], or exhibiting novel materials properties as in Mullen’s superacenes [5], they can play a role in the elucidation of reaction mechanisms. Diels-Alder reactions to trap proposed short-lived intermediates, to provide convenient high yield routes to sterically crowded organic and organometallic molecules and, even when they do not proceed as anticipated, to lead to unexpected novel products
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