π-Allylnickel Intermediates in Organic Synthesis

Enantioselective Iron-Catalyzed Cross-[4+4]-Cycloaddition of 1,3-Dienes Provides Chiral Cyclooctadienes

Chiral cyclooctadienes are a frequently occurring scaffold in natural products and specialty chemicals, and are used as ligands in asymmetric catalysis. Accessing substituted cyclooctadienes in an efficient asymmetric fashion has been notoriously challenging. We report an iron-catalyzed enantioselective cross-[4+4]-cycloaddition of 1,3-dienes to form substituted cyclooctadienes under very mild conditions. A highly tailored chiral α-diimine iron complex is key for the success of the transformation providing a balanced performance between reactivity, excellent cross-selectivity and very high enantioselectivity. Steric maps of the complexes help accounting for the observed selectivity. The developed method allows rapid and atom-economic access to novel differently functionalized cyclooctadienes in very high yields and enantioselectivities.

Synthesis, Characterisation and Reactions of Truly Cationic NiI -Phosphine Complexes

The recently published purely metallo-organic Ni^I salt [Ni(cod)₂ ][Al(OR^F )₄ ] (1, cod=1,5-cyclooctadiene, R^F =C(CF₃ )₃ ) provides a starting point for a new synthesis strategy leading to Ni^I phosphine complexes, replacing cod ligands by phosphines. Clearly visible colour changes indicate reactions within minutes, while quantum chemical calculations (PBE0-D3(BJ)/def2-TZVPP) approve exergonic reaction enthalpies in all performed ligand exchange reactions. Hence, [Ni(dppp)₂ ][Al(OR^F )₄ ] (2, dppp=1,3-bis(diphenylphosphino)propane), [Ni(dppe)₂ ][Al(OR^F )₄ ] (3, dppe=1,3-bis(diphenyl-phosphino)ethane), three-coordinate [Ni(PPh₃ )₃ ][Al(OR^F )₄ ] (4) and a remarkable two-coordinate Ni^I phosphine complex [Ni(PtBu₃ )₂ ][Al(OR^F )₄ ] (5) were characterised by single crystal X-ray structure analysis. EPR studies were performed, confirming a nickel d⁹ -configuration in complexes 2, 4 and 5. This result is supported by additional magnetization measurements of 4 and 5. Further investigations by cyclic voltammetry indicate relatively high oxidation potentials for these Ni^I compounds between 0.7 and 1.7 V versus Fc/Fc⁺ . Screening reactions with O₂ and CO gave first insights on the reaction behaviour of the Ni^I phosphine complexes towards small molecules with formation of mixed phosphine-CO-Ni^I complexes and oxidation processes yielding new Ni^I and/or Ni^II derivatives. Moreover, 4 reacted with CH₂ Cl₂ at RT to give a dimeric Ni^II ylide complex (4 c). As CH₂ Cl₂ is a rather stable alkyl halide with relatively high C-Cl bond energies, 4 appears to be a suitable reagent for more general C-Cl bond activation reactions.

[Ni(cod)2][Al(OR(F))4], a Source for Naked Nickel(I) Chemistry

The straightforward synthesis of the cationic, purely organometallic Ni(I) salt [Ni(cod)2](+)[Al(OR(F))4](-) was realized through a reaction between [Ni(cod)2] and Ag[Al(OR(F))4] (cod = 1,5-cyclooctadiene). Crystal-structure analysis and EPR, XANES, and cyclic voltammetry studies confirmed the presence of a homoleptic Ni(I) olefin complex. Weak interactions between the metal center, the ligands, and the anion provide a good starting material for further cationic Ni(I) complexes.

Ni(0)-Promoted Hydroxycarboxylation of 1,2-Dienes by Reaction with CO2 and O2

[structure: see text]. A novel method for the preparation of hydroxy carboxylic acid derivatives has been developed by O2-oxidation of pi-allylnickel intermediates generated by Ni(0)-mediated coupling of 1,2-dienes with CO2.

[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.