Expanding the Role of Dimeric Species: On-Cycle Involvement, Improved Stability, and Control of Stereo-Specificity. A Case Study of Atom-Economic Catalytic Hydrothiolation

Abstract

A common assumption that dimeric metal complexes in many catalytic systems represent a resting state and are not directly involved in catalytic processes was revised in a combined experimental and theoretical study. On-cycle participation of dimeric metal complexes, rather than typically assumed off-cycle involvement, was revealed, and advantageous performance in terms of improved selectivity was observed. The conceptual rationalization for the participation of dimeric species in the catalytic cycle was developed. The Pd-catalyzed hydrothiolation process (where strong Pd−S binding is well established and a persistent opinion for the inactive/poisoning role of dimeric species is presumed) was evaluated as a challenging system to test the concept. Activation of an (NHC)Pd(Cl)(acac) precatalyst (NHC�N-heterocyclic carbene and acac�acetylacetonate) under the reaction conditions produced monomeric (NHC)Pd(SPh) 2 or dimeric (NHC) 2 Pd 2 (SPh) 4 species depending on the steric bulkiness of the NHC ligand. Dimeric complexes possessed higher selectivity and tolerated disulfide impurities in contrast to monomeric complexes. Quantum chemical modeling suggested that dimeric catalysis proceeds through the opening of only one (μ-SPh)−Pd bridging bond with retention of the dimeric structure. The second bridging bond is maintained, which prevents the monomerization of the complex. Catalytically active species were detected in a hydrothiolation reaction by high-resolution mass spectrometry and NMR spectroscopy. Proving the opportunity for productive homogeneous catalysis via strongly coordinated dimeric metal species opens new opportunities for catalyst design in the increased nuclearity dimension.

Photocatalytic iodosulfonylation of internal alkynes under green conditions

“…Methods for creating carbon-heteroatom bonds, including C-S bonds, are in high demand in modern organic chemistry. [1][2][3][4][5][6][7][8][9][10] Sulfur-containing compounds are widely used as catalysts, 11,12 reagents and synthetic intermediates, [13][14][15][16][17][18][19] as well as for the preparation of biologically active compounds 20 and functional materials. 21,22 One important class of organosulfur compounds are sulfones, especially vinyl sulfones (Scheme 1).…”

“…Methods for creating carbon–heteroatom bonds, including C–S bonds, are in high demand in modern organic chemistry. 1–10 Sulfur-containing compounds are widely used as catalysts, 11,12 reagents and synthetic intermediates, 13–19 as well as for the preparation of biologically active compounds 20 and functional materials. 21,22…”

(CAAC)Pd(py) Catalysts Disproportionate to Pd(CAAC)2

“…However, the zero-valent intermediate (NHC)­Pd 0 (py) has not been isolated so far. Based on isotopic labeling, MS spectrometry, nuclear magnetic resonance (NMR), and FE-SEM analysis, Ananikov and colleagues reported the formation of Pd 0 nanoparticles in combination with (NHC) 2 Pd and referred to this mixture as “catalytic cocktail” . They concluded that the presence of strong bases induces the reduction of the PEPPSI precursor complex, with the formation of Pd 0 nanoparticles and O–NHC coupling with urea formation in case of oxygen-derived bases…”

“… 10 Furthermore, Ananikov et al showed that the dissociation of the NHC does not compromise the activity of the catalyst and that the presence of ureas might even boost turnover. 8 , 9 Cyclic (alkyl)(amino) carbenes (CAACs) stabilize low-valent compounds and radicals due to their π-acidity. 11 Although not yet quantified, they are also expected to show a strong trans-effect.…”