Mild and selective Pd-Ar protonolysis and C-H activation promoted by a ligand aryloxide group

Addition and Oxidation Reactivity of a Pentacoordinate Nickelacyclobutane

Nickelacyclobutanes are reactive intermediates in catalytic cycles including cyclopropanation and insertion reactions. The stoichiometric study of these intermediates has shown that their reactivity is highly influenced by the coordination environment of the nickel center. A pentacoordinated nickelacyclobutane embedded in a diphosphine pincer ligand has been shown to selectively undergo various reactions with exogenous ligands, including [2+2] cycloreversion and carbene transfer to an isocyanide. Herein, we investigate the reactivity of the pentacoordinated nickelacyclobutane towards addition and oxidation reactions. Addition reactions lead to ring-opening to form stable square planar Ni(II) compounds, while metal oxidation enhances [2+2] cycloreversion. DFT calculations are used to shed light on the different mechanisms.

Large Isotope Effects in Organometallic Chemistry

The kinetic isotope effect (KIE) is key to understanding reaction mechanisms in many areas of chemistry and chemical biology, including organometallic chemistry. This ratio of rate constants, k_H /k_D , typically falls between 1-7. However, KIEs up to 105 have been reported, and can even be so large that reactivity with deuterium is unobserved. We collect here examples of large KIEs across organometallic chemistry, in catalytic and stoichiometric reactions, along with their mechanistic interpretations. Large KIEs occur in proton transfer reactions such as protonation of organometallic complexes and clusters, protonolysis of metal-carbon bonds, and dihydrogen reactivity. C-H activation reactions with large KIEs occur with late and early transition metals, photogenerated intermediates, and abstraction by metal-oxo complexes. We categorize the mechanistic interpretations of large KIEs into the following three types: (a) proton tunneling, (b) compound effects from multiple steps, and (c) semi-classical effects on a single step. This comprehensive collection of large KIEs in organometallics provides context for future mechanistic interpretation.

Experimental and Computational Studies on the Ruthenium-Catalyzed Dehydrative C-H Coupling of Phenols with Aldehydes for the Synthesis of 2-Alkylphenol, Benzofuran, and Xanthene Derivatives

The cationic Ru-H complex [(C₆H₆)(PCy₃)(CO)RuH]⁺BF₄^- (1) was found to be an effective catalyst for the dehydrative C-H coupling reaction of phenols and aldehydes to form 2-alkylphenol products. The coupling reaction of phenols with branched aldehydes selectively formed 1,1-disubstituted benzofurans, while the coupling reaction with salicylaldehydes yielded xanthene derivatives. A normal deuterium isotope effect was observed from the coupling reaction of 3-methoxyphenol with benzaldehyde and 2-propanol/2-propanol-d₈ (k_H/k_D = 2.3 ± 0.3). The carbon isotope effect was observed on the benzylic carbon of the alkylation product from the coupling reaction of 3-methoxyphenol with 4-methoxybenzaldehyde (C(3) 1.021(3)) and on both benzylic and ortho-arene carbons from the coupling reaction with 4-trifluorobenzaldehdye (C(2) 1.017(3), C(3) 1.011(2)). The Hammett plot from the coupling reaction of 3-methoxyphenol with para-substituted benzaldehydes p-X-C₆H₄CHO (X = OMe, Me, H, F, Cl, CF₃) displayed a V-shaped linear slope. Catalytically relevant Ru-H complexes were observed by NMR from a stoichiometric reaction mixture of 1, 3-methoxyphenol, benzaldehyde, and 2-propanol in CD₂Cl₂. The DFT calculations provided a detailed catalysis mechanism featuring an electrophilic aromatic substitution of the aldehyde followed by the hydrogenolysis of the hydroxy group. The calculations also revealed a mechanistic rationale for the strong electronic effect of the benzaldehdye substrates p-X-C₆H₄CHO (X = OMe, CF₃) in controlling the turnover-limiting step. The catalytic C-H coupling method provides an efficient synthetic protocol for 2-alkylphenols, 1,1-disubstituted benzofurans, and xanthene derivatives without employing any reactive reagents or forming wasteful byproducts.

Dynamics of the efficient cyclometalation of the undercoordinated organoplatinum complex [Pt(COD)(neoPh)]+ (neoPh = 2-methyl-2-phenylpropyl)

The cyclometalation reaction of [Pt(COD)(κ¹-neoPh)]⁺ (neoPh = 2-methyl-2-phenylpropyl) to [Pt(COD)(κ²-neoPh)] was studied experimentally and mechanistically using DFT and MD simulations.

Selectivity in carbon–carbon coupling reactions at palladium(IV) and platinum(IV)

The isomerization and reductive elimination reactions from octahedral organometallic complexes of palladium(IV) and platinum(IV) usually occur through five-coordinate intermediates that cannot be directly detected. This paper reports a computational study of five-coordinate complexes of formulae [PtMe3(bipy)]+, [PtMe2Ph(bipy)]+, and [PtMe(CH2CMe2C6H4)(bipy)]+ (M = Pd or Pt, bipy = 2,2′-bipyridine), particularly with respect to reactivity and selectivity in reductive elimination. All of the complexes are predicted to have square pyramidal structures with the bipy and two R groups in the equatorial positions and one R group in the axial position, and axial–equatorial exchange occurs by a pairwise mechanism, with the transition state having a pinched trigonal bipyramidal (PTBP) stereochemistry, with one nitrogen and two R groups in the trigonal plane. The activation energy for isomerization is lower than that for reductive elimination in all cases. For the complexes [MMe2Ph(bipy)]+, the activation energies for reductive elimination with Me–Me or Me–Ph coupling are similar. For the complexes [MMe(CH2CMe2C6H4)(bipy)]+, the reductive elimination with Me–C6H4 bond formation from the isomer with the methyl group in the axial position is predicted and is attributed to it having the best conformation of the Me and C6H4 groups for C–C bond formation. In all cases, the selectivity for reductive elimination is similar for M = Pd or Pt, but reactivity is higher for M = Pd. The relevance of this work to selectivity in catalysis is discussed.

A twisted macrocyclic hexanuclear palladium complex with internal bulky coordinating ligands

A macrocyclic hexanuclear palladium complex, which accumulates coordination sites on metals inside the cavity, was found to take a uniquely-twisted conformation when six molecules of a bulky pyridine derivative coordinated to the palladium.