PCP"-pincer-ligated iridium complexes have been found to be highly effective catalysts for the dehydrogenation of alkanes. We report a computational and experimental study of the effect on catalytic activity resulting from systematically varying steric crowding by the substitution of methyl groups for the phosphino tert-butyl groups of ( R4 PCP)Ir ( R4 PCP = κ 3 -C 6 H 3 -2,6-(CH 2 PR 2 ) 2 ; R = t Bu or Me). DFT calculations for ( R4 PCP)Ir species (R 4 = t Bu 4 or t Bu 3 Me) indicate that the ratedetermining step in the n-alkane/1-alkene transfer dehydrogenation cycle is β-H elimination by ( R4 PCP)Ir(n-alkyl)(H). It is calculated that the transition state for this step is ca. 10 kcal/mol lower for ( tBu3Me PCP)Ir than for ( tBu4 PCP)Ir (relative to the corresponding free ( R4 PCP)Ir). However, this catalytically favorable effect is calculated to be partially offset by the strong binding of 1-alkene to ( tBu3Me PCP)Ir in the resting state, so the overall barrier is thus lower by only ca. 4 kcal/mol. Further Me-for-t Bu substitutions have a smaller effect on the transition states, and the calculated energy of the olefin-bound resting states is lowered by comparable amounts; therefore these additional substitutions are predicted to have little overall favorable effect on catalytic rates. ( tBu3Me PCP)IrH 4 has been synthesized and isolated, and its catalytic activity has been investigated. It is indeed found to be a more active catalyst precursor than ( tBu4 PCP)IrH 4 for alkane transfer dehydrogenation. ( tBu2Me2 PCP)IrH 4 was also synthesized and as a catalyst precursor is found to afford somewhat lower activity than ( tBu3Me PCP)IrH 4 . However, synthetic precursors of ( tBu2Me2 PCP)IrH 4 tended to yield dinuclear clusters, while complex mixtures were observed during catalysis that were not amenable to characterization. It is therefore not clear if the lesser catalytic activity of ( tBu2Me2 PCP)Ir vs ( tBu3Me PCP)Ir derivatives is due to the energetics of the actual catalytic cycle or due to deactivation of this catalyst via the facile formation of clusters.
A novel method for preparing aromatic compounds containing cyclopropoxy via nucleophilic aromatic substitution reaction (SNAr) of fluoroaromatic compounds with cyclopropanol under relatively mild conditions is presented. As compared to the approaches reported previously for preparing 1-(cyclopropyloxy)-2-nitrobenzene, the one proposed in this work is simplified without sacrificing the yields: When the reaction was performed at 75 °C with Cs2CO3 as the base and DMF as solvent, after 6 h the yield was up to 90%. Finally, various fluoroaromatic compounds were employed as substrates for a test that proves a wide application scope of the method.
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