Facile reductive elimination of ethane from strained dimethylpalladium(II) complexes

Changes in ligand coordination mode induce bimetallic C-C coupling pathways

Carbon-carbon coupling is one of the most powerful tools in the organic synthesis arsenal. Known methodologies primarily exploit monometallic Pd⁰/Pd^II catalytic mechanisms to give new C-C bonds. Bimetallic C-C coupling mechanisms that involve a Pd^I/Pd^II redox cycle, remain underexplored. Thus, a detailed mechnaistic understanding is imperative for the development of new bimetallic catalysts. Previously, a Pd^II-Me dimer (1) supported by L1, which has phosphine and 1-azaallyl donor groups, underwent reductive elimination to give ethane, a Pd^I dimer, a Pd^II monometallic complex, and Pd black. Herein, a comprehensive experimental and computational study of the reactivity of 1 is presented, which reveals that the versatile coordination chemistry of L1 promotes bimetallic C-C bond formation. The phosphine 1-azaallyl ligand adopts various bridging modes to maintain the bimetallic structure throughout the C-C bond forming mechanism, which involves intramolecular methyl transfer and 1,1-reductive elimination from one of the palladium atoms. The minor byproduct, methane, likely forms through a monometallic intermediate that is sensitive to solvent C-H activation. Overall, the capacity of L1 to adopt different coordination modes promotes the bimetallic C-C coupling channel through pathways that are unattainable with statically-coordinated ligands.

The anticancer activity of an air-stable Pd(I)-NHC (NHC = N-heterocyclic carbene) dimer

A new dinuclear Pd(i) complex coordinating two bis(NHC) ligands revealed an unsuspected stability despite the unsaturation of the two metal centres. Even more surprisingly, the compound showed high and selective antiproliferative activity against different cancer cell lines and ovarian cancer tumoroids, and the mechanism of action was different from that of cisplatin.

Rh(DPEPhos)-Catalyzed Alkyne Hydroacylation Using β-Carbonyl-Substituted Aldehydes: Mechanistic Insight Leads to Low Catalyst Loadings that Enables Selective Catalysis on Gram-Scale

The detailed mechanism of the hydroacylation of β-amido-aldehyde, 2,2-dimethyl-3-morpholino-3-oxopropanal, with 1-octyne using [Rh( cis-κ²-_P,P-DPEPhos)(acetone)₂][BAr^F₄]-based catalysts, is described [Ar^F = (CF₃)₂C₆H₃]. A rich mechanistic landscape of competing and interconnected hydroacylation and cyclotrimerization processes is revealed. An acyl-hydride complex, arising from oxidative addition of aldehyde, is the persistent resting state during hydroacylation, and quaternary substitution at the β-amido-aldehyde strongly disfavors decarbonylation. Initial rate, KIE, and labeling studies suggest that the migratory insertion is turnover-limiting as well as selectivity determining for linear/branched products. When the concentration of free aldehyde approaches zero at the later stages of catalysis alkyne cyclotrimerization becomes competitive, to form trisubstituted hexylarenes. At this point, the remaining acyl-hydride turns over in hydroacylation and the free alkyne is now effectively in excess, and the resting state moves to a metallacyclopentadiene and eventually to a dormant α-pyran-bound catalyst complex. Cyclotrimerization thus only becomes competitive when there is no aldehyde present in solution, and as aldehyde binds so strongly to form acyl-hydride when this happens will directly correlate to catalyst loading: with low loadings allowing for free aldehyde to be present for longer, and thus higher selectivites to be obtained. Reducing the catalyst loading from 20 mol % to 0.5 mol % thus leads to a selectivity increase from 96% to ∼100%. An optimized hydroacylation reaction is described that delivers gram scale of product, at essentially quantitative levels, using no excess of either reagent, at very low catalyst loadings, using minimal solvent, with virtually no workup.

Catalytic applications of small bite-angle diphosphorus ligands with single-atom linkers

Diphosphorus ligands connected by a single atom (R₂PEPR₂; E = CR₂, CCR₂and NR) give chelating ligands with very small bite-angles as well as enable access to other properties such as bridging modes and hemilability. ThisPerspectivereviews the properties of diphosphorus ligands featuring a single-atom linker and their applications in catalysis, including transformations of alkenes and transfer hydrogenation and hydrogen-borrowing reactions.

Well-Defined and Robust Rhodium Catalysts for the Hydroacylation of Terminal and Internal Alkenes

A Rh-catalyst system based on the asymmetric ligand (t)Bu2PCH2P(o-C6H4OMe)2 is reported that allows for the hydroacylation of challenging internal alkenes with β-substituted aldehydes. Mechanistic studies point to the stabilizing role of both excess alkene and the OMe-group.

Acrylate formation from CO2 and ethylene: catalysis with palladium and mechanistic insight

The first palladium catalyst for the reaction of CO₂, alkenes and a base to acrylates is reported (TON up to 50).

Cyclodextrin and phosphorus(iii): a versatile combination for coordination chemistry and catalysis

The relevance of cyclodextrins equipped with P(iii) atoms in coordination chemistry and homogeneous catalysis is discussed.

TRANSDIP: a trans-chelating ligand tailor-made for probing unusual Pd0 and PdII intermediates

Forty years after Venanzi's seminal studies on trans-spanning diphosphines, a cavity-shaped trans chelator (TRANSDIP) has been used to monitor the stepwise formation of a carbon-carbon bond on a palladium centre. Furthermore, the ligand, which incorporates a cyclodextrin subunit, enabled the synthesis of the first η(2)-dioxygen complex in which the M-O(2) unit is entrapped within a cavity.

Intermolecular hydroacylation: high activity rhodium catalysts containing small-bite-angle diphosphine ligands

Readily prepared and bench-stable rhodium complexes containing methylene bridged diphosphine ligands, viz. [Rh(C(6)H(5)F)(R(2)PCH(2)PR'(2))][BAr(F)(4)] (R, R' = (t)Bu or Cy; Ar(F) = C(6)H(3)-3,5-(CF(3))(2)), are shown to be practical and very efficient precatalysts for the intermolecular hydroacylation of a wide variety of unactivated alkenes and alkynes with β-S-substituted aldehydes. Intermediate acyl hydride complexes [Rh((t)Bu(2)PCH(2)P(t)Bu(2))H{κ(2)(S,C)-SMe(C(6)H(4)CO)}(L)](+) (L = acetone, MeCN, [NCCH(2)BF(3)](-)) and the decarbonylation product [Rh((t)Bu(2)PCH(2)P(t)Bu(2))(CO)(SMePh)](+) have been characterized in solution and by X-ray crystallography from stoichiometric reactions employing 2-(methylthio)benzaldehdye. Analogous complexes with the phosphine 2-(diphenylphosphino)benzaldehyde are also reported. Studies indicate that through judicious choice of solvent and catalyst/substrate concentration, both decarbonylation and productive hydroacylation can be tuned to such an extent that very low catalyst loadings (0.1 mol %) and turnover frequencies of greater than 300 h(-1) can be achieved. The mechanism of catalysis has been further probed by KIE and deuterium labeling experiments. Combined with the stoichiometric studies, a mechanism is proposed in which both oxidative addition of the aldehyde to give an acyl hydride and insertion of the hydride into the alkene are reversible, with the latter occurring to give both linear and branched alkyl intermediates, although reductive elimination for the linear isomer is suggested to have a considerably lower barrier.

Bis(diphenylphosphino)acetonitrile: Synthesis, ligand properties and application in catalytic carbon–carbon coupling

Treatment of acetonitrile (1 equiv.) with n-butyllithium (0.95 equiv.) followed by chlorodiphenylphosphine (0.95 equiv.) under optimised reaction conditions gave a ca. 60% yield of bis(diphenylphosphino)acetonitrile (dppmCN, 1). The bidentate ligand was employed in the synthesis of the four-membered chelate metal complexes (dppmCN)MCl(2) [M = Pd (6a), Pt (6b)] and (dppmCN)RuCp*(Cl) (7). A very active catalyst for bromobenzene/phenylboronic acid Suzuki-Miyaura coupling was in situ generated by treatment of Pd(OAc)(2) with bis(diphenylphosphino)acetonitrile [TOF (1 h) >600000].

Routes to unique palladium A-frame complexes with a bridging fluoro-ligand

Treatment of a toluene solution of [PdMe(2)(Cy(2)PCH(2)PCy(2))](1) with pentafluoropyridine in the presence of traces of water affords the generation of the A-frame complexes [(PdMe)(2){mu-kappa(2)(P,P)Cy(2)PCH(2)PCy(2)}(2)(mu-F)][SiMeF(4)]() and [(PdMe)(2){mu-kappa(2)(P,P)Cy(2)PCH(2)PCy(2)}(2)(mu-F)][OC(5)NF(4)](2b). If the reaction is performed in an NMR tube equipped with a PFA inliner, complex 2b is produced, only. Treatment of 1 with pentafluoropyridine in the presence of an excess water yields the pyridyloxy complex [PdMe(OC(5)NF(4))(Cy(2)PCH(2)PCy(2))](3). Compound [(PdMe)(2){mu-kappa(2)(P,P)Cy(2)PCH(2)PCy(2)}(2)(mu-F)][FHF](2c) bearing a bifluoride anion instead of SiMeF(4)(-) or OC(5)NF(4)(-) can be generated by reaction of 1 with substoichiometric amounts of Et(3)N.3HF. The analogous complex [(PdMe)(2){mu-kappa(2)(P,P)Ph(2)PCH(2)PPh(2)}(2)(mu-F)][FHF] (5c) has been synthesized by addition of Ph(2)PCH(2)PPh(2) to a solution of [PdMe(2)(Me(2)NCH(2)CH(2)NMe(2))](4) in THF and subsequent treatment of the reaction mixture with Et(3)N.3HF. The structure of the A-frame complex 5c has been determined by X-ray crystallography.

Alcoholysis of acylpalladium(II) complexes relevant to the alternating copolymerization of ethene and carbon monoxide and the alkoxycarbonylation of alkenes: the importance of Cis-coordinating phosphines

The mechanism and kinetics of the solvolysis of complexes of the type [(L-L)Pd(C(O)CH(3))(S)](+)[CF(3)SO(3)](-) (L-L = diphosphine ligand, S = solvent, CO, or donor atom in the ligand backbone) was studied by NMR and UV-vis spectroscopy with the use of the ligands a-j: SPANphos (a), dtbpf (b), Xantphos (c), dippf (d), DPEphos (e), dtbpx (f), dppf (g), dppp (h), calix-6-diphosphite (j). Acetyl palladium complexes containing trans-coordinating ligands that resist cis coordination (SPANphos, dtbpf) showed no methanolysis. Trans complexes that can undergo isomerization to the cis analogue (Xantphos, dippf, DPEphos) showed methanolyis of the acyl group at a moderate rate. The reaction of [trans-(DPEphos)Pd(C(O)CH(3))](+)[CF(3)SO(3)](-) (2e) with methanol shows a large negative entropy of activation. Cis complexes underwent competing decarbonylation and methanolysis with the exception of 2j, [cis-(calix-diphosphite)Pd(C(O)CH(3))(CD(3)OD)](+)[CF(3)SO(3)](-). The calix-6-diphosphite complex showed a large positive entropy of activation. It is concluded that ester elimination from acylpalladium complexes with alcohols requires cis geometry of the acyl group and coordinating alcohol. The reductive elimination of methyl acetate is described as a migratory elimination or a 1,2-shift of the alkoxy group from palladium to the acyl carbon atom. Cis complexes with bulky ligands such as dtbpx undergo an extremely fast methanolysis. An increasing steric bulk of the ligand favors the formation of methyl propanoate relative to the insertion of ethene leading to formation of oligomers or polymers in the catalytic reaction of ethene, carbon monoxide, and methanol.

Vinyl-vinyl coupling on late transition metals through C-C reductive elimination mechanism. A computational study

A detailed density functional study was performed for the vinyl-vinyl reductive elimination reaction from bis-sigma-vinyl complexes [M(CH=CH(2))(2)X(n)]. It was shown that the activity of these complexes decreases in the following order: Pd(IV), Pd(II) > Pt(IV), Pt(II), Rh(III) > Ir(III), Ru(II), Os(II). The effects of different ligands X were studied for both platinum and palladium complexes, which showed that activation barriers for C-C bond formation reaction decrease in the following order: X = Cl > Br, NH(3) > I > PH(3). Steric effects induced either by the ligands X or by substituents on the vinyl group were also examined. In addition, the major factors responsible for stereoselectivity control on the final product formation stage and possible involvement of asymmetric coupling pathways are reported. In all cases DeltaE, DeltaH, DeltaG, and DeltaG(aq) energy surfaces were calculated and analyzed. The solvent effect calculation shows that in a polar medium halogen complexes may undergo a reductive elimination reaction almost as easily as compounds with phosphine ligands.