Practical and theoretical aspects of Wacker oxidation of tolanophanes: Synthesis and characterization of novel diketonic cyclophanes

Abstract

Novel cyclophanes having 1,2-diketones 2a-c were synthesized by Wacker oxidation of the corresponding tolanophanes 1a-c having various alkyl chain lengths (n = 2-4). Among them, tolanophane 1a with the shortest alkyl chain length (n = 2) having more strained alkyne shows the lowest product selectivity even lower than that of acyclic analogues at various reaction temperatures. In contrast, the oxidation of tolanophane 1b (n = 3) is clean with the highest activity and selectivity of 2b. Theoretical calculations confirm the experimental data. Based on ab initio calculations, the critical step of the reaction pathway is the interaction of oxygen atoms of 1 (in the absence of H 2 O) with Pd ion of intermediate [PdCl 3 (1)] − to give [PdCl 2 (1)] which keeps Pd ion close to the alkyne bond. This step is not observed computationally for 1a because the positions of the oxygen atoms are outside of its central part. This is in good agreement with almost no activity of 1a at room temperature to prove its rigid structure preventing the O…Pd interaction. Nevertheless, the alkyne…Pd interaction of 1b is not detected by NMR measurements which may be due to its too slow interaction at room temperature (35% after 24 h); however, this confirms ab initio calculation data that the preferred coordinating site is oxygen in the reaction pathway. In contrast, cyclic voltammetry measurements distinguish a different behaviour for the palladium complexation of 1. In general, it is found that the stereochemistry of tolanophanes rather than distortion of alkyne bond plays the critical role in the oxidation. The products are new and their structures were characterized.

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