Solvent‐Promoted Regio‐ and Stereoselectivity in Ru‐Catalyzed Hydrocarboxylation of Terminal Alkynes: A DFT Study

Abstract

DFT calculations at the M06L‐D3(SMD)/LANL2TZ(f)/6‐311+G(d,p)//M06L/LANL2DZ/6‐31G(d)* level were performed to explore the mechanism, solvent effect, and origin of selectivity in the hydrocarboxylation of terminal alkynes catalyzed by (PCy3)2(CO)RuHCl. This catalytic system offers a unique example in which selectivity in enol‐ester synthesis is controlled in presence of different solvent media under mild reaction conditions. In CH2Cl2 solvent the regioselective gem‐enol‐ester (Pg) was preferred, whereas stereoselective Z‐enol‐ester (PZ) was observed exclusively in THF medium. We have unraveled the complete catalytic mechanism involving the following steps: a) alkyne coordination with subsequent nucleophilic attack and, finally, b) product elimination leading to the Markovnikov product Pg in both solvent media. Herein, the rate‐limiting nucleophilic attack step requires an activation barrier of 20.5 (21.0) kcal mol−1, which is 5.7 (5.6) kcal mol−1 lower than the crucial vinylidene formation step encountered for anti‐Markovnikov addition. Interestingly, a new reaction avenue unfolds under explicit THF medium, in which the inherent coordinating nature of the solvent becomes effective in altering the product regioselectivity. Under such an event, a highly facile ligandassisted proton shuttle (LAPS) mechanism is propagated, leading to vinylidene complex 11A. This intriguing mechanistic scenario facilitates favorably furnishing the PZ product rather than the Pg isomer by 4.3 kcal mol−1.

Mechanistic Insight into the Photoredox-Nickel-HAT Triple Catalyzed Arylation and Alkylation of α-Amino Csp3–H Bonds

“…Since the thermal correction to the Gibbs free energy of each component depends on its concentration in solution, one can incorporate the concentration terms into calculations (details in the SI). − Activation barriers of single electron transfer (SET) steps involve in this work have been calculated using Marcus–Hush theory, and are denoted as Δ G ‡ MH (Table S3 in the SI) . Natural bond orbital (NBO) analysis were performed using the NBO 6 t.0 package as implemented in Gaussian 09…”

Regioselective Hydroalkylation and Arylalkylation of Alkynes by Photoredox/Nickel Dual Catalysis: Application and Mechanism

“…[11,16] Forthe hydroalkylation process,protonation of 11 via intermediate 12 generates the E-isomer 13 (concerted protonation demetallation, CPD) along with the formation of Ni I complex 8. [17] Next, the Z-isomer 15 is obtained via intermediate 14 from the E-isomer 13 through an energy-transfer pathway (Scheme 3a). [3c] Likewise,f or arylalkylation, the generated Ni I intermediate 19 undergoes the oxidative addition of an aryl halide to give intermediate 20,f ollowed by reductive elimination to deliver the antiaddition three-component coupling product 21.A tl ast, the syn-addition three-component coupling product 22 is obtained via the energy-transfer pathway (Scheme 3b).…”

“…Reduction of 9 by the Ir II reductant gives Ni I intermediate 10 , which undergoes 1,2‐migratory alkyne insertion to form Ni I intermediate 11 . For the hydroalkylation process, protonation of 11 via intermediate 12 generates the E ‐isomer 13 (concerted protonation demetallation, CPD) along with the formation of Ni I complex 8 . Next, the Z ‐isomer 15 is obtained via intermediate 14 from the E ‐isomer 13 through an energy‐transfer pathway (Scheme a) .…”

Re(I)-Catalyzed Hydropropargylation of Silyl Enol Ethers Utilizing Dynamic Interconversion of Vinylidene–Alkenylmetal Intermediates via 1,5-Hydride Transfer

“…In most of these reactions, addition of nucleophilic reagents to the central vinylidene carbon generates alkenylmetallic species, which further undergo transformations such as protonation with regeneration of the catalytic species. On the contrary, its reverse reaction, that is, generation of vinylidene complexes from alkenylmetallic species is rather limited even in the stoichiometric reactions, − and elimination of α-hydrogen of alkenylmetallic species as hydride for the generation of vinylidene intermediates has not been utilized for catalytic reactions, except for the α-hydrogen elimination proposed for generation of vinylidene complexes from terminal alkynes . In this paper, we report Re­(I)-catalyzed stereoselective hydropropargylation of silyl enol ethers by utilizing dynamic interconversion of vinylidene–alkenylmetal intermediates through reversible 1,5-hydride transfer of α-hydrogen of alkenylrheniums.…”