[Ni⁰L]-Catalyzed Cyclodimerization of 1,3-Butadiene:  A Density Functional Investigation of the Influence of Electronic and Steric Factors on the Regulation of the Selectivity

Abstract

We present a comprehensive theoretical investigation of the influence of the ligand L on the regulation of the product selectivity for the [Ni(0)L]-catalyzed cyclodimerization of 1,3-butadiene. The investigation was based on density functional theory (DFT) and a combined DFT and molecular mechanics (QM/MM) approach for the real [bis(butadiene)Ni(0)L] catalysts with L = PMe(3), I; PPh(3), II; P((i)Pr)(3), III; and P(OPh)(3), IV. The role of electronic and steric effects has been elucidated for all crucial elementary steps of the entire catalytic cycle. Allylic isomerization, allylic enantioface conversion, as well as oxidative coupling are shown to be influenced to a minor extent by electronic and steric effects. In contrast, the ligand's properties have a distinct influence on the preestablished equilibrium between the eta(3),eta(1)(C(1)) and bis-eta(3) forms 2 and 4, respectively, of the [(octadienediyl)Ni(II)L] complex and on the rate-determining reductive elimination following competing routes for generation of either VCH, cis-1,2-DVCB, or cis,cis-COD. Electronic factors are shown to predominantly determine the position of the kinetically mobile 2 right harpoon over left harpoon 4 equilibrium. 4 is the prevailing species for ligands L that are pi-acceptors (L = P(OPh)(3)) or weak sigma-donors (L = PPh(3)), while stronger sigma-donors (L = PMe(3), P((i)Pr)(3)) displace the equilibrium to the left. Steric bulk on the ligand as well as its pi-acceptor ability act to facilitate the reductive elimination, while sigma-donor abilities serve to retard this process. Electronic and steric factors are found to not influence uniformly the reductive elimination routes with either 2 or 4 involved. The regulation of the product selectivity is elucidated on the basis of both thermodynamic and kinetic considerations.

Density Functional Study on the Mechanism of the Vanadium-Catalyzed Oxidation of Sulfides by Hydrogen Peroxide

“…The region absent in our model is likely to have mainly a steric role, and it will be possible to introduce it through QM/MM calculations . These calculations have shown the ability to handle enantioselectivity and regioselectivity in a number of cases, − but the they require a previous detailed knowledge of the electronic characteristics of the system.…”

The influence of N-substitution on the reductive elimination behaviour of hydrocarbyl–palladium–carbene complexes—a DFT study

“…The influence that steric and electronic properties of spectator ligands in general have on the rate of reductive elimination reactions is well known. [24][25][26][27][28][29][30][31] As a spectator, NHC ligands generate an electron-rich metal centre that enhances oxidative addition of substrates, while the steric bulk incorporated on nitrogen aids to promote the converse reductive elimination. 23,32 For example, Suzuki catalysis is promoted by steric bulk on the carbene, which aids the reductive elimination step in the catalytic cycle.…”

Computational Rationalization of the Dependence of the Enantioselectivity on the Nature of the Catalyst in the Vanadium-Catalyzed Oxidation of Sulfides by Hydrogen Peroxide

“…In this work, we present a computational study with the IMOMM method on the origin of enantioselectivity in the process depicted in Figure . Computational chemistry has been shown to be a good complement to experiment for the elucidation of reaction mechanisms in homogeneous catalysis. − In particular, quantum mechanics/molecular mechanics (QM/MM) schemes such as IMOMM 42 and ONIOM have been shown to be very efficient in the introduction of the steric effects associated to bulky ligands usually present in enantioselective catalysis. − …”