Reversible Carbon−Carbon Bond Formation between 1,3-Dienes and Aldehyde or Ketone on Nickel(0)

Dimer involvement and origin of crossover in nickel-catalyzed aldehyde-alkyne reductive couplings

The mechanism of nickel(0)-catalyzed reductive coupling of aldehydes and alkynes has been studied. Extensive double-labeling crossover studies have been conducted. While previous studies illustrated that phosphine- and N-heterocyclic carbene-derived catalysts exhibited differing behavior, the origin of these effects has now been evaluated in detail. Many variables, including ligand class, sterics of the ligand and alkyne, temperature, and ring size being formed in intramolecular versions, all influence the extent of crossover observed. A computational evaluation of these effects suggests that dimerization of a key metallacyclic intermediate provides the origin of crossover. Protocols that proceed with crossover are typically less efficient than those without crossover given the thermodynamic stability and low reactivity of the dimeric metallacycles involved in crossover pathways.

Paraformaldehyde and methanol as C1 feedstocks in metal-catalyzed C-C couplings of π-unsaturated reactants: beyond hydroformylation

Ruthenium-catalyzed reductive couplings of paraformaldehyde with dienes, alkynes, and allenes provide access to products of hydrohydroxymethylation that cannot be formed selectively under the conditions of hydroformylation. In certain cases, the regioselectivity of the CC coupling can be inverted by using nickel catalysts. With iridium catalysts, methanol engages in redox-neutral regioselective hydrohydroxymethylations.

Catalytic bismetallative multicomponent coupling reactions: scope, applications, and mechanisms

Catalytic reactions have played an indispensable role in organic chemistry for the last several decades. In particular, catalytic multicomponent reactions have attracted significant attention due to their efficiency and expediency towards complex molecule synthesis. The presence of bismetallic reagents (e.g. B-B, Si-Si, B-Si, Si-Sn, etc.) in this process renders the products enriched with various functional groups and multiple stereocenters. For this reason, catalytic bismetallative coupling is considered an effective method to generate the functional and stereochemical complexity of simple hydrocarbon substrates. This review highlights key developments of transition-metal catalyzed bismetallative reactions involving multiple π components. Specifically, it will highlight the scope, synthetic applications, and proposed mechanistic pathways of this process.

Iridium-catalyzed annulation of salicylimines with 1,3-dienes

Iridium-catalyzed annulation of salicylimines with 1,3-dienes gave high yields of the corresponding 4-aminochromanes with high stereoselectivity. The use of a chiral diene ligand enabled the asymmetric reaction to give 4-aminochromanes with high enantioselectivity.

Reversible carbon-carbon bond formation between carbonyl compounds and a ruthenium pincer complex

This communication describes the reversible reaction of a ruthenium pincer complex with a variety of carbonyl compounds. Both NMR spectroscopic and X-ray crystallographic characterization of isomeric carbonyl adducts are reported, and the equilibrium constants for carbonyl binding have been determined.

Nickelalactones with an allyl subunit – the effect of penta-coordination on structures and stability

Nickelalactone derivatives with an allyl subunit containing different mono- and bidentate ligands were synthesized and characterized. The introduced neutral ligands strongly affect the structures of these versatile molecules and can be used to modulate their stabilities.

Ruthenium catalyzed hydrohydroxyalkylation of isoprene with heteroaromatic secondary alcohols: isolation and reversible formation of the putative metallacycle intermediate

Heteroaromatic secondary alcohols react with isoprene to form products of hydrohydroxyalkylation in the presence of ruthenium(0) catalysts generated from Ru3(CO)12 and tricyclohexylphosphine, enabling direct conversion of secondary to tertiary alcohols in the absence of premetalated reagents or stoichiometric byproducts. The putative oxaruthenacycle intermediate has been isolated and characterized, and reversible metallacycle formation has been demonstrated.

Chiral ditopic cyclophosphazane (CycloP) ligands: synthesis, coordination chemistry, and application in asymmetric catalysis

A series of dichlorocyclophosphazanes [{ClP(μ-NR)}2 ] containing chiral and achiral R groups was obtained from simple commercially available amines and PCl3 . Their condensation reactions with axially chiral biaryl diols yielded ansa-bridged chiral cyclophosphazane (CycloP) ligands. This highly modular methodology allows extensive elaboration of the ligand set, in which the chirality can be introduced at the diol bridge and/or the amido R group. This provides the possibility to observe match and mismatch effects in catalysis. A series of twenty CycloP ligands was synthesized and characterized by multinuclear NMR spectroscopy, HRMS, elemental analysis, and in selected cases, single-crystal X-ray diffraction. These studies show that all of the ditopic CycloP ligands are C2 symmetric, rendering their metal coordination sites symmetry equivalent. Two well-established enantioselective reactions were explored by using late-transition metal CycloP complexes as catalysts; the gold-catalyzed hydroamination of γ-allenyl sulfonamides and the asymmetric nickel-catalyzed three-component coupling of a diene and an aldehyde. The steric demands of the CycloP ligands have a subtle influence on the reactivity and selectivity observed in both reactions. Good enantiomeric ratios (e.r.) as high as 89:11 in the gold-catalyzed reaction and 92:8 in the nickel-catalyzed bis-homoallylation reaction were observed.

Mechanistic origin of cross-coupling selectivity in Ni-catalysed Tishchenko reactions

Mechanistic studies have been performed for the recently developed, Ni-catalysed selective cross-coupling reaction between aryl and alkyl aldehydes. A mono-carbonyl activation (MCA) mechanism (in which one of the carbonyl groups is activated by oxidative addition) was found to be the most favourable pathway, and the rate-determining step is oxidative addition. Analysing the origin of the observed cross-coupling selectivity, we found the most favourable carbonyl activation step requires both coordination of the aryl aldehyde and oxidative addition of the alkyl aldehyde. Therefore, the stronger π-accepting ability of the aryl aldehyde (relative to alkyl aldehyde) and the ease of oxidative addition of the alkyl aldehyde (relative to aryl aldehyde) are responsible for the cross-coupling selectivity.

Aldehyde binding through reversible C-C coupling with the pincer ligand upon alcohol dehydrogenation by a PNP-ruthenium catalyst

Primary alcohol dehydrogenation by a PNP-Ru(II) catalyst was probed by low-temperature NMR experiments. Facile dehydrogenation occurred at -30 °C, but the resulting aldehydes were not found in solution, as they were trapped by the catalyst through a new mode of metal-ligand cooperation involving Ru-O coordination and an unusual, highly reversible C-C coupling with the PNP pincer ligand.

Nickel-catalyzed dehydrogenative [4 + 2] cycloaddition of 1,3-dienes with nitriles

Pyridines, which comprise one of the most important classes of the six-membered heterocyclic compounds, are widely distributed in nature, and the transition-metal-catalyzed [2 + 2 + 2] cycloaddition reaction of two alkynes and a nitrile is one of the most powerful methods for preparing versatile, highly substituted pyridine derivatives. However, the lack of chemo- and regioselectivity is still a crucial issue associated with fully intermolecular [2 + 2 + 2] cycloaddition. The present study developed the Ni(0)-catalyzed intermolecular dehydrogenative [4 + 2] cycloaddition reaction of 1,3-butadienes with nitriles to give a variety of pyridines regioselectively.

Stereoselective borylative ketone-diene coupling

In the presence of catalytic Ni(cod)(2) and P(t-Bu)(3), ketones, dienes, and B(2)(pin)(2) undergo a stereoselective multicomponent coupling reaction. Upon oxidation, the reaction furnishes 1,3-diols as the major reaction product.

Nickel-catalyzed cycloaddition of anthranilic acid derivatives to alkynes

A nickel-catalyzed cycloaddition has been developed where readily available anthranilic acid derivatives react with alkynes to afford substituted indoles. The reaction involves oxidative addition of Ni(0) to an ester moiety, which allows intermolecular addition to alkynes via decarbonylation and 1,3-acyl migration.

Ni-catalyzed borylative diene-aldehyde coupling: the remarkable effect of P(SiMe3)3

The nickel-catalyzed reaction of carbonyls and dienes was accomplished in a regio- and stereoselective fashion employing a stoichiometric amount of bis(pinacolato)diboron. In the presence of P(SiMe(3))(3), this reductive coupling furnishes allyl boronic esters which are regioisomeric to those obtained with PCy(3) as the ligand. The coupling product may be subject to oxidation, which furnishes the derived 1,3-diol, or allylation with an additional aldehyde, which furnishes the derived 1,6-diol in a stereoselective fashion.

Origins of regioselectivity and alkene-directing effects in nickel-catalyzed reductive couplings of alkynes and aldehydes

The origins of reactivity and regioselectivity in nickel-catalyzed reductive coupling reactions of alkynes and aldehydes were investigated with density functional calculations. The regioselectivities of reactions of simple alkynes are controlled by steric effects, while conjugated enynes and diynes are predicted to have increased reactivity and very high regioselectivities, placing alkenyl or alkynyl groups distal to the forming C-C bond. The reactions of enynes and diynes involve 1,4-attack of the Ni-carbonyl complex on the conjugated enyne or diyne. The consequences of these conclusions on reaction design are discussed.