The dimerization of 1,3-dienes with the nickel-ligand catalyst: evidence for a multi-step mechanism

Nickel-catalyzed coupling reaction of alkyl halides with aryl Grignard reagents in the presence of 1,3-butadiene: mechanistic studies of four-component coupling and competing cross-coupling reactions

We describe the mechanism, substituent effects, and origins of the selectivity of the nickel-catalyzed four-component coupling reactions of alkyl fluorides, aryl Grignard reagents, and two molecules of 1,3-butadiene that affords a 1,6-octadiene carbon framework bearing alkyl and aryl groups at the 3- and 8-positions, respectively, and the competing cross-coupling reaction. Both the four-component coupling reaction and the cross-coupling reaction are triggered by the formation of anionic nickel complexes, which are generated by the oxidative dimerization of two molecules of 1,3-butadiene on Ni(0) and the subsequent complexation with the aryl Grignard reagents. The C-C bond formation of the alkyl fluorides with the γ-carbon of the anionic nickel complexes leads to the four-component coupling product, whereas the cross-coupling product is yielded via nucleophilic attack of the Ni center toward the alkyl fluorides. These steps are found to be the rate-determining and selectivity-determining steps of the whole catalytic cycle, in which the C-F bond of the alkyl fluorides is activated by the Mg cation rather than a Li or Zn cation. ortho-Substituents of the aryl Grignard reagents suppressed the cross-coupling reaction leading to the selective formation of the four-component products. Such steric effects of the ortho-substituents were clearly demonstrated by crystal structure characterizations of ate complexes and DFT calculations. The electronic effects of the para-substituent of the aryl Grignard reagents on both the selectivity and reaction rates are thoroughly discussed. The present mechanistic study offers new insight into anionic complexes, which are proposed as the key intermediates in catalytic transformations even though detailed mechanisms are not established in many cases, and demonstrates their synthetic utility as promising intermediates for C-C bond forming reactions, providing useful information for developing efficient and straightforward multicomponent reactions.

Multicomponent Coupling Reaction of Perfluoroarenes with 1,3-Butadiene and Aryl Grignard Reagents Promoted by an Anionic Ni(II) Complex

An anionic Ni complex was isolated and its structure determined by X-ray crystallography. With such an anionic complex as a key intermediate, a regio- and stereoselective multicomponent coupling reaction of perfluoroarenes, aryl Grignard reagents, and 1,3-butadiene in a 1:1:2 ratio was achieved, resulting in the formation of 1,6-octadiene derivatives containing two aryl groups, one from the perfluoroarene and the other from the aryl Grignard reagent, at the 3- and 8-positions, respectively.

Mechanism, reactivity, and selectivity of nickel-catalyzed [4 + 4 + 2] cycloadditions of dienes and alkynes

Density functional theory (DFT) calculations with B3LYP and M06 functionals elucidated the reactivities of alkynes and Z/E selectivity of cyclodecatriene products in the Ni-catalyzed [4 + 4 + 2] cycloadditions of dienes and alkynes. The Ni-mediated oxidative cyclization of butadienes determines the Z/E selectivity. Only the oxidative cyclization of one s-cis to one s-trans butadiene is facile and exergonic, leading to the observed 1Z,4Z,8E-cyclodecatriene product. The same step with two s-cis or s-trans butadienes is either kinetically or thermodynamically unfavorable, and the 1Z,4E,8E- and 1Z,4Z,8Z-cyclodecatriene isomers are not observed in experiments. In addition, the competition between the desired cooligomerization and [2 + 2 + 2] cycloadditions of alkynes depends on the coordination of alkynes. With either electron-deficient alkynes or alkynes with free hydroxyl groups, the coordination of alkynes is stronger than that of dienes, and alkyne trimerization prevails. With alkyl-substituted alkynes, the generation of alkyne-coordinated nickel complex is much less favorable, and the [4 + 4 + 2] cycloaddition occurs.

An approach to mimicking the sesquiterpene cyclase phase by nickel-promoted diene/alkyne cooligomerization

Artificially mimicking the cyclase phase of terpene biosynthesis inspires the invention of new methodologies, since working with carbogenic frameworks containing minimal functionality limits the chemist's toolbox of synthetic strategies. For example, the construction of terpene skeletons from five-carbon building blocks would be an exciting pathway to mimic in the laboratory. Nature oligomerizes, cyclizes, and then oxidizes γ,γ-dimethylallyl pyrophosphate (DMAPP) and isopentenyl pyrophosphate (IPP) to all of the known terpenes. Starting from isoprene, the goal of this work was to mimic Nature's approach for rapidly building molecular complexity. In principle, the controlled oligomerization of isoprene would drastically simplify the synthesis of terpenes used in the medicine, perfumery, flavor, and materials industries. This article delineates our extensive efforts to cooligomerize isoprene or butadiene with alkynes in a controlled fashion by zerovalent nickel catalysis building off the classic studies by Wilke and co-workers.

[RhIL]-catalyzed cyclotetramerization of 1,3-butadiene: a theoretical investigation of alternative mechanistic paths for the generation of the [RhIII(octadienediyl)(PR3)]+ complex

A detailed theoretical investigation of alternative mechanistic paths for the formation of the [Rh(III)(octadienediyl)(PiPr3)]+ complex is presented, employing a gradient-corrected density functional theory (DFT) method (BP86). This process represents most likely the first step in the recently reported [Rh(I)L]-catalyzed cyclotetramerization of butadiene (M. Bosch, M. S. Brookhart, K. Ilg and H. Werner, Angew. Chem., Int. Ed., 2000, 39, 2304). The favorable route for oxidative addition under C-C-bond formation starts from the prevalent [Rh(I)(butadiene)2(PiPr3)]+ form of the active catalyst through oxidative coupling between two cis-eta4-butadienes. This affords the [Rh(III)(bis-eta3-anti-octadienediyl)(PiPr3)]+ compound as the kinetic coupling product that consecutively undergoes transformation into the thermodynamically favorable bis-eta3-syn-octadienediyl-Rh(III) isomer via facile allylic conversions occurring in the octadienediyl framework. The computationally predicted energy profile is almost in quantitative agreement with the experimentally determined kinetics and allows a consistent rationalization of the experimental observations.

[Ni0]-Catalyzed Co-oligomerization of 1,3-Butadiene and Ethylene: A Theoretical Mechanistic Investigation of Competing Routes for Generation of Linear and Cyclic C10-Olefins

A detailed theoretical investigation of the mechanism for the [Ni(0)]-catalyzed co-oligomerization of 1,3-butadiene and ethylene to afford linear and cyclic C(10)-olefins is presented. Crucial elementary processes have been carefully explored for a tentative catalytic cycle, employing a gradient-corrected density functional theory (DFT) method. The favorable route for oxidative coupling starts from the prevalent [Ni(0)(eta(2)-butadiene)(2)(ethylene)] form of the active catalyst through oxidative coupling between the two eta(2)-butadienes. The initial eta(3),eta(1)(C(1))-octadienediyl-Ni(II) product is the active precursor for ethylene insertion, which preferably takes place into the syn-eta(3)-allyl-Ni(II) bond of the prevalent eta(3)-syn,eta(1)(C(1)),Delta-cis isomer. The insertion is driven by a strong thermodynamic force, giving rise entirely to eta(3),eta(1),Delta-trans-decatrienyl-Ni(II) forms, with the eta(3)-anti,eta(1),Delta-trans isomer almost exclusively generated. Occurrence of allyl,eta(1),Delta-cis isomers, however, is precluded on both kinetic and thermodynamic grounds, thereby rationalizing the observation that cis-DT and cis,cis-CDD are never formed. Linear and cyclic C(10)-olefins are generated in a highly stereoselective fashion, with trans-DT and cis,trans-CDD as the only isomers, along competing routes of stepwise transition-metal-assisted H-transfer (DT) and reductive CC elimination under ring closure (CDD), respectively, that start from the prevalent eta(3)-anti,eta(1),Delta-trans-decatrienyl-Ni(II) species. The role of allylic conversion in the octadienediyl-Ni(II) and decatrienyl-Ni(II) complexes has been analyzed. As a result of the detailed exploration of all important elementary steps, a theoretically verified, refined catalytic cycle is proposed and the regulation of the selectivity for formation of linear and cyclic C(10)-olefins is elucidated.

[Ni(0)L]-catalyzed cyclodimerization of 1,3-butadiene: a density functional investigation of the influence of electronic and steric factors on the regulation of the selectivity

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.

[Ni(0)L]-catalyzed cyclodimerization of 1,3-butadiene: a comprehensive density functional investigation based on the generic [(C(4)H(6))(2)Ni(0)PH(3)] catalyst

We present a comprehensive theoretical investigation of the mechanism for cyclodimerization of butadiene by the generic [bis(butadiene)Ni(0)PH(3)] catalyst employing a gradient-corrected DFT method. We have explored all critical elementary steps of the whole catalytic cycle, namely, oxidative coupling of two butadienes, reductive elimination under ring closure, and allylic isomerization. Oxidative coupling of two butadienes in the [bis(butadiene)Ni(0)L] complex and reductive elimination in the [(bis(eta(3))-octadienediyl)Ni(II)L] species take place under different stereocontrol, which makes isomerization indispensable. Commencing from a preestablished equilibrium between several configurations of the [(octadienediyl)Ni(II)L] complex, the major cyclodimer products, namely, VCH, cis-1,2-DVCB, and cis,cis-COD, are formed along competing reaction paths via reductive elimination, which is found to be the overall rate-determining step. Careful exploration of different possible conceivable routes revealed that bis(eta(1)) species are not involved as critical intermediates either in reductive elimination or in isomerization along the most feasible pathway. The regulation of the selectivity of the cyclodimer formation based on both thermodynamic and kinetic considerations is outlined.