Kempe et al. and Milstein et al. have recently advanced the dehydrogenative coupling methodology to synthesize pyrroles from secondary alcohols (e.g., 3) and β-amino alcohols (e.g., 4), using PNP-Ir (1) and PNN-Ru (2) pincer complexes, respectively. We herein present a DFT study to characterize the catalytic mechanism of these reactions. After precatalyst activation to give active 1A/2A, the transformation proceeds via four stages: 1A/2A-catalyzed alcohol (3) dehydrogenation to give ketone (11), base-facilitated C-N coupling of 11 and 4 to form an imine-alcohol intermediate (18), base-promoted cyclization of 18, and catalyst regeneration via H2 release from 1R/2R. For alcohol dehydrogenations, the bifunctional double hydrogen-transfer pathway is more favorable than that via β-hydride elimination. Generally, proton-transfer (H-transfer) shuttles facilitate various H-transfer processes in both systems. Notwithstanding, H-transfer shuttles play a much more crucial role in the PNP-Ir system than in the PNN-Ru system. Without H-transfer shuttles, the key barriers up to 45.9 kcal/mol in PNP-Ir system are too high to be accessible, while the corresponding barriers (<32.0 kcal/mol) in PNN-Ru system are not unreachable. Another significant difference between the two systems is that the addition of alcohol to 1A giving an alkoxo complex is endergonic by 8.1 kcal/mol, whereas the addition to 2A is exergonic by 8.9 kcal/mol. The thermodynamic difference could be the main reason for PNP-Ir system requiring lower catalyst loading than the PNN-Ru system. We discuss how the differences are resulted in terms of electronic and geometric structures of the catalysts and how to use the features in catalyst development.

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Density functional theory (DFT) calculations (B3LYP and M06) have been utilized to study a newly reported Heck-type reaction that uses an allylic or alkenyl alcohol as substrate and palladium as catalyst in the form of a chelate with a chiral pyridine oxazoline (PyrOx) ligand. The reaction not only controls the regio- and enantioselectivities of arylation of the C═C bond, but also forms the carbonyl functionality up to four bonds away from the aryl substituent via tandem C═C bond migration and enol-to-keto conversion. Computations performed on representative reaction systems allow us to propose a detailed mechanism with several key steps. Initial oxidation of palladium(0) by aryldiazonium generates active arylpalladium(II) species that bind the C═C bond of an allylic or alkenyl alcohol. The activated C═C bond inserts into the palladium-aryl moiety to attain aryl substitution and a chiral carbon center, and the resulting complex undergoes β-hydride elimination to give a new C═C bond that can repeat the insertion/elimination process to move down the carbon chain to form an enol that tautomerizes to a highly stable carbonyl final product. The calculations reveal that the C═C bond migratory insertion step determines both the regioselectivity and the enantioselectivity of arylation, with the former arising mainly from the electronic effect of the hydroxyl group on the charge distribution over the C═C bond and the latter originating from a combination of steric repulsion, trans influence, and C-H/π dispersion interactions.

Polyols pathway probed: Density functional theory computations reveal that the methyltrioxorhenium-catalyzed deoxydehydration of polyols follows pathway C, which is energetically more favorable than the previously proposed pathways A and B. In addition to serving as solvent/reductant, the alcohol also acts as a shuttle to greatly facilitate various hydrogen-transfer steps.

An olympicenyl radical, a spin 1/2 hydrocarbon radical with C 2v symmetry and uneven spin distribution, remains elusive despite the considerable theoretical research interest. Herein, we report syntheses of two air-stable olympicenyl radical derivatives, OR1 and OR2, with half-life times (τ1/2) in air-saturated solution of 7 days and 34 days. The high stability was ascribed to kinetic blocking of reactive sites with high spin densities. X-ray crystallographic analysis revealed unique 20-center–2-electron head-to-tail π-dimer structures with intermolecular distances shorter than the sum of van der Waals radius of carbon. The ground state of the π-dimers was found to be singlet, with singlet–triplet energy gaps estimated to be −2.34 kcal/mol and −3.28 kcal/mol for OR1 and OR2, respectively, by variable-temperature electron spin resonance (ESR) spectroscopy. The monomeric radical species were in equilibrium with the π-dimer in solution, and the optical and electrochemical properties of the monomers and π-dimers in solution were investigated by UV–vis–NIR spectroscopy and cyclic voltammetry, revealing a concentration-dependent nature. Theoretical calculations illustrated that upon formation of a π-dimer the local aromaticity of each monomer was enhanced, and spatial ring current between the monomers was present, which resulted in an increment of aromaticity of the interior of the π-dimer.