Syntheses and Thermolysis of Arylpalladium Hydroxide Complexes: Implications for C(sp2)–OH Bond-Forming Reductive Elimination To Generate Phenol Derivatives

Combined Experimental and Computational Mechanistic Investigation of the Palladium-Catalyzed Decarboxylative Cross-Coupling of Sodium Benzoates with Chloroarenes

Reported herein is a mechanistic investigation into the palladium-catalyzed decarboxylative cross-coupling of sodium benzoates and chloroarenes. The reaction was found to be first-order in Pd. A minimal substituent effect was observed with respect to chloroarene, and the reaction was zero-order with respect to chloroarene. Palladium-mediated decarboxylation was assigned as the turnover-limiting step based on an Eyring plot and density functional theory computations. Catalyst performance was found to vary based on the electrophile, which is best explained by catalyst decomposition at Pd(0). The 1,5-cyclooctadiene (COD) ligand contained in the precatalyst CODPd(CH₂TMS)₂ (Pd1) was shown to be a beneficial additive. The bench-stable Buchwald complex XPhosPdG2 could be used with exogenous COD and 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl (XPhos) instead of complex Pd1. Adding exogenous XPhos significantly increased the catalyst turnover number and enhanced reproducibility.

Bonding Energetics of Palladium Amido/Aryloxide Complexes in DMSO: Implications for Palladium-Mediated Aniline Activation

Thermodynamic knowledge of the metal-ligand (M-L) σ-bond strength is crucial to understanding metal-mediated transformations. Here, we developed a method for determining the Pd-X (X=OR and NHAr) bond heterolysis energies (ΔG_het (Pd-X)) in DMSO taking [(tmeda)PdArX] (tmeda=N,N,N',N'-tetramethylethylenediamine) as the model complexes. The ΔG_het (Pd-X) scales span a range of 2.6-9.0 kcal mol^-1 for ΔG_het (Pd-O) values and of 14.5-19.5 kcal mol^-1 for ΔG_het (Pd-N) values, respectively, implying a facile heterolytic detachment of the Pd ligands. Structure-reactivity analyses of a modeling Pd-mediated X-H bond activation reveal that the M-X bond metathesis is dominated by differences of the X-H and Pd-X bond strengths, the former being more influential. The ΔG_het (Pd-X) and pK_a (X-H) parameters enable regulation of reaction thermodynamics and chemoselectivity and diagnosing the probability of aniline activation with Pd-X complexes.

Gold Catalyzed Decarboxylative Cross-Coupling of Iodoarenes

This report details a decarboxylative cross-coupling of (hetero)aryl carboxylates with iodoarenes in the presence of a gold catalyst (>25 examples, up to 96% yield). This reaction is site specific, which overcomes prior limitations associated with gold catalyzed oxidative coupling reactions. The reactivity of the (hetero)aryl carboxylate correlates qualitatively to the field effect parameter (F_ortho). Reactions with isolated gold complexes and DFT calculations support a mechanism proceeding through oxidative addition at a gold(I) cation with decarboxylation being viable at either a gold(I) or a silver(I) species.

Development of efficient palladium catalysts for alkoxycarbonylation of alkenes

Herein, we report a general and efficient Pd-catalysed alkoxycarbonylation of sterically hindered and demanding olefins including a variety of tri-, tetra-substituted and 1,1-disubstituted alkenes. In the presence of 1,3-bis(tert-butyl(pyridin-2-yl)phosphanyl)propane L3 or 1,4-bis(tert-butyl(pyridin-2-yl)phosphanyl)butane L4 the desired esters are obtained in good yields and selectivities. Similar transformation is obtained using tertiary ether as showcased in the carbonylation of MTBE to the corresponding linear ester in high yield and selectivity.

Dismantling of Vinyl Ethers by Pentanuclear [(iPr3 P)Ni]5 H6 : Facile Cooperative C-O, C-C and C-H Activation Pathways

The [(iPr₃ P)Ni]₅ H₆ cluster (1) and H₂ C=CHOtBu react at room temperature to form the new pentanuclear NiH carbide [(iPr₃ P)Ni]₅ H₄ (C)(CO) (3), along with an equivalent of isobutylene. This transformation requires the activation of multiple unreactive bonds, including C-H, C-C, and C(sp³ )-O bond cleavage. Analysis of the reaction mixture by ¹ H NMR revealed the production of two additional paramagnetic species, assigned as [(iPr₃ P)Ni]₄ H₄ (C-CH₃ )NiOtBu (4 a) and [(iPr₃ P)Ni]₄ H₄ (C-CH₂ OtBu)NiOtBu (5 a), which arise from C(sp² )-O bond cleavage and CH bond rearrangements. The reaction of 1 with H₂ C=CHOSiMe₂ CH₂ Ph produced the isolable 4 a analogue [(iPr₃ P)₄ Ni₅ ]H₄ (CCH₃ )(OSiMe₂ CH₂ Ph) (4 c). An isolable analogue of 5 a was obtained from the reaction of 1 with H₂ C=CHOAd (Ad=1-admantyl), which produced [(iPr₃ P)₄ Ni₅ ]H₄ (CCH₂ OAd)(OAd) (5 d). The utilization of both cluster faces and vertices for bonding substrate fragments in these transformations demonstrates the remarkable flexibility of the robust Ni₅ H₄ core in the cooperative activation of multiple C-O, C-C and C-H bonds.

Recent advances in well-defined, late transition metal complexes that make and/or break C-N, C-O and C-S bonds

Chemical transformations that result in either the formation or cleavage of carbon-heteroatom bonds are among the most important processes in the chemical sciences. Herein, we present a review on the reactivity of well-defined, late-transition metal complexes that result in the making and breaking of C-N, C-O and C-S bonds via fundamental organometallic reactions, i.e. oxidative addition, reductive elimination, insertion and elimination reactions. When appropriate, emphasis is placed on structural and spectroscopic characterization techniques, as well as mechanistic data.

Computational study of C(sp3)-O bond formation at a PdIV centre

This report describes a computational study of C(sp³)-OR bond formation from Pd^IV complexes of general structure Pd^IV(CH₂CMe₂-o-C₆H₄-C,C')(F)(OR)(bpy-N,N') (bpy = 2,2'-bipyridine). Dissociation of ^-OR from the different octahedral Pd^IV starting materials results in a common square-pyramidal Pd^IV cation. An S_N2-type attack by ^-OR (^-OR = phenoxide, acetate, difluoroacetate, and nitrate) then leads to C(sp³)-OR bond formation. In contrast, when ^-OR = triflate, concerted C(sp³)-C(sp²) bond-forming reductive elimination takes place, and the calculations indicate this outcome is the result of thermodynamic rather than kinetic control. The energy requirements for the dissociation and S_N2 steps with different ^-OR follow opposing trends. The S_N2 transition states exhibit "PdCO" angles in a tight range of 151.5 to 153.0°, resulting from steric interactions between the oxygen atom and the gem-dimethyl group of the ligand. Conformational effects for various OR ligands and isomerisation of the complexes were also examined as components of the solution dynamics in these systems. In all cases, the trends observed computationally agree with those observed experimentally.

Structural, Kinetic, and Computational Characterization of the Elusive Arylpalladium(II)boronate Complexes in the Suzuki-Miyaura Reaction

The existence of the oft-invoked intermediates containing the crucial Pd-O-B subunit, the "missing link", has been established in the Suzuki-Miyaura cross-coupling reaction. The use of low-temperature, rapid injection NMR spectroscopy (RI-NMR), kinetic studies, and computational analysis has enabled the generation, observation, and characterization of these highly elusive species. The ability to confirm the intermediacy of Pd-O-B-containing species provided the opportunity to clarify mechanistic aspects of the transfer of the organic moiety from boron to palladium in the key transmetalation step. Specifically, these studies establish the identity of two different intermediates containing Pd-O-B linkages, a tri-coordinate (6-B-3) boronic acid complex and a tetra-coordinate (8-B-4) boronate complex, both of which undergo transmetalation leading to the cross-coupling product. Two distinct mechanistic pathways have been elucidated for stoichiometric reactions of these complexes: (1) transmetalation via an unactivated 6-B-3 intermediate that dominates in the presence of an excess of ligand, and (2) transmetalation via an activated 8-B-4 intermediate that takes place with a deficiency of ligand.

New Approach to the Polymerization of Disubstituted Acetylenes by Bulky Monophosphine-Ligated Palladium Catalysts

Bulky monophosphine-ligated Pd complexes served as unprecedented admirable catalysts for the polymerization of a disubstituted acetylene. The moderately high polymer yields and cis content of the formed polyacetylene contrasted with those observed for traditional Mo catalyst-based polymer. These Pd catalysts are strong tools to promote the understanding of the structure-property relationships of disubstituted acetylenes.

Suzuki-Miyaura cross-coupling of unprotected, nitrogen-rich heterocycles: substrate scope and mechanistic investigation

The Suzuki-Miyaura cross-coupling of unprotected, nitrogen-rich heterocycles using precatalysts P1 or P2 is reported. The procedure allows for the reaction of variously substituted indazole, benzimidazole, pyrazole, indole, oxindole, and azaindole halides under mild conditions in good to excellent yields. Additionally, the mechanism behind the inhibitory effect of unprotected azoles on Pd-catalyzed cross-coupling reactions is described based on evidence gained through experimental, crystallographic, and theoretical investigations.