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This review analyzes recent mechanistic studies that have provided new insights into how the structure of a metal complex influences the rate and selectivity of base-assisted C-H cleavage. Partitioning a broader mechanistic continuum into classes delimited by the polarization between catalyst and substrate during C-H cleavage is postulated as a method to identify catalysts favoring electrophilic or nucleophilic reactivity patterns, which may be predictive based on structural features of the metal complex (i.e., oxidation state, d-electron count, charge). Multi-metallic cooperativity and polynuclear speciation also provide new avenues to affect energy barriers for C-H cleavage and site selectivity beyond the limitations of single metal catalysts. An improved understanding of mechanistic nuances and structure-activity relationships on this important bond activation step carries important implications for efficiency and controllable site selectivity in non-directed C-H functionalization.

Zerovalent palladium complexes are ubiquitous active species in modern cross-coupling reactions that comprise many premier methods for the construction of C–C and C–heteroatom bonds in organic synthesis. While palladium(0) complexes stabilized by two or more dative ligands are widely known, the most active form of Pd(0) coordinated by a single ancillary ligand (“monoligated Pd(0)”) has long eluded direct characterization. We report the synthesis and unambiguous solution- and solid-state characterization of functionally 12-electron Pd(0) complexes coordinated by a single tri(1-adamantyl)phosphine (PAd3) ligand. Access to these fleeting intermediates was achieved by enabling B-to-Pd transmetalation reactions that occur at cryogenic temperature. This work opens new avenues to experimentally interrogate highly reactive on-cycle Pd(0) catalysts and their structure-dependent reactivity and speciation, which should be broadly informative in continuing studies of catalytic processes featuring the prevalent Pd(0)/Pd(II) redox couple.

The origin of switchable site selectivity during Pd-catalysed C–H alkenylation of heteroarenes has been examined through More O’Ferrall–Jencks, isotope effect, and DFT computational analyses, which indicate substitution of ionic thioether for pyridine dative ligands induces a change from selectivity-determining C–H cleavage to C–C bond formation, respectively.

Oxidative C–H/C–H coupling is a promising synthetic route for the streamlined construction of conjugated organic materials for optoelectronic applications. Broader adoption of these methods is neve...

Oxidative C–H/C–H coupling is a promising synthetic route for the streamlined construction of conjugated organic materials for optoelectronic applications. Broader adoption of these methods is nevertheless hindered by the need for catalysts that excel in forging core semiconductor motifs, such as ubiquitous oligothiophenes, with high efficiency in the absence of metal reagents. We report a (thioether)Pd-catalyzed oxidative coupling method for the rapid assembly of both privileged oligothiophenes and challenging hindered cases, even at low catalyst loading under Ag- and Cu-free conditions. A combined experimental and computational mechanistic study was undertaken to understand how a simple thioether ligand, MeS(CH(2))(3)SO(3)Na, leads to such potent reactivity toward electron-rich substrates. The consensus from these data is that a concerted, base-assisted C–H cleavage transition state is operative, but thioether coordination to Pd is associated with decreased synchronicity (bond formation exceeding bond breaking) versus the “standard” concerted metalation-deprotonation (CMD) model that was formalized by Fagnou in direct arylation reaction. Enhanced positive charge build-up on the substrate results from this perturbation, which rationalizes experimental trends strongly favoring π-basic sites. The term electrophilic CMD (eCMD) is introduced to distinguish this mechanism from the standard model, even though both mechanisms locate in a broad concerted continuum. More O’Ferrall-Jencks analysis further suggests eCMD should be a general mechanism manifested by many metal complexes. A preliminary classification of complexes into those favoring eCMD or standard CMD is proposed, which should be informative for studies toward tunable catalyst-controlled reactivity.

The reactions of 2-methyltetrahydropyran (2-MTHP, C6H12O) on Ni2P/SiO2 provide insights on the interactions between a cyclic ether, an abundant component of biomass feedstock, with a transition-metal phosphide, an effective hydrotreating catalyst. At atmospheric pressure and a low contact time, conditions similar to those of a fast pyrolysis process, 70% of products formed from the reaction of 2-MTHP on Ni2P/SiO2 were deoxygenated products, 2-hexene and 2-pentenes, indicating a good oxygen removal capacity. Deprotonation, hydrogenolysis, dehydration, and decarbonylation were the main reaction routes. The reaction sequence started with the adsorption of 2-MTHP, followed by ring-opening steps on either the methyl substituted side (Path I) or the unsubstituted side (Path II) to produce adsorbed alkoxide species. In Path I, a primary alkoxide was oxidized at the α-carbon to produce an aldehyde, which subsequently underwent decarbonylation to 2-pentenes. The primary alkoxide could also be protonated to give a ...

We report here the remarkable properties of PAd3, a crystalline air-stable solid accessible through a scalable SN1 reaction. Spectroscopic data reveal that PAd3, benefiting from the polarizability inherent to large hydrocarbyl groups, exhibits unexpected electron releasing character that exceeds other alkylphosphines and falls within a range dominated by N-heterocyclic carbenes. Dramatic effects in catalysis are also enabled by PAd3 during Suzuki-Miyaura cross-coupling of chloro(hetero)arenes (40 examples) at low Pd loading, including the late-stage functionalization of commercial drugs. Exceptional space-time yields are demonstrated for the syntheses of industrial precursors to valsartan and boscalid from chloroarenes with ∼2 × 10(4) turnovers in 10 min.

A new series of palladium catalysts ligated by a chelating bisphosphine monoxide bearing diarylphosphino groups (aryl-BPMO) exhibits markedly higher reactivity for ethylene/methyl acrylate copolymerisation., A new series of palladium catalysts ligated by a chelating bisphosphine monoxide bearing diarylphosphino groups (aryl-BPMO) exhibits markedly higher reactivity for ethylene/methyl acrylate copolymerisation when compared to the first generation of alkyl-BPMO–palladium catalysts that contain a dialkylphosphino moiety. Mechanistic studies suggest that the origin of this disparate catalyst behavior is a change in regioselectivity of migratory insertion of the acrylate comonomer as a function of the phosphine substituents. The best aryl-BPMO–palladium catalysts for these copolymerisations were shown to undergo exclusively 2,1-insertion, and this high regioselectivity avoids formation of a poorly reactive palladacycle intermediate. Furthermore, the aryl-BPMO–palladium catalysts can copolymerise ethylene with other industrially important polar monomers.

Thioether ancillary ligands have been identified that can greatly accelerate the C–H alkenylation of O-, S-, and N-heteroarenes. Kinetic data suggest thioether–Pd-catalyzed reactions can be as much as 800× faster than classic ligandless systems. Furthermore, mechanistic studies revealed C–H bond cleavage as the turnover-limiting step, and that rate acceleration upon thioether coordination is correlated to a change from a neutral to a cationic pathway for this key step. The formation of a cationic, low-coordinate catalytic intermediate in these reactions may also account for unusual catalyst-controlled site selectivity wherein C–H alkenylation of five-atom heteroarenes can occur under electronic control with thioether ligands even when this necessarily involves reaction at a more hindered C–H bond. The thioether effect also enables short reaction times under mild conditions for many O-, S-, and N-heteroarenes (55 examples), including examples of late-stage drug derivatization.

The incorporation of polar functional groups into polyolefins can significantly alter the adhesion, barrier and surface properties, dyeability, printability, and compatibility of the resulting “functional polyolefin”. Thus, the development of methods for the controlled synthesis of functional polyolefins from industrially relevant monomers holds the potential to expand the range of applications available to this already ubiquitous class of materials. In this Perspective, recent advances in transition-metal-catalyzed functional polyolefin synthesis will be reviewed. A common thread among the innovations discussed here is the perturbation of catalyst function by tailored design of the chelating ancillary ligand, aided in many cases by improved mechanistic understanding. Specific topics discussed here include rare examples of catalyst control over the regio- and stereochemistry of polar monomer insertion by phosphine–sulfonato palladium complexes (Drent-type), rate acceleration of insertion polymerization by...

Utilization of palladium catalysts bearing a P-chiral phosphine-sulfonate ligand enabled asymmetric copolymerization of vinyl acetate with carbon monoxide. The obtained γ-polyketones have head-to-tail and isotactic polymer structures. The origin of the regio- and stereoregularities was elucidated by stoichiometric reactions of acylpalladium complexes with vinyl acetate. The present report for the first time demonstrates successful asymmetric coordination-insertion (co)polymerization of vinyl acetate.

Poly(p-xylylene) or parylene has been around for more than a half a century. It is typically deposited by the Gorham method from [2.2] paracyclophane. However, another method is via alpha-derivatives of p-xylene. Both poly(a-bromo-p-xylylene) and poly(a-chloro-p-xylylene) have been successfully deposited by this route through predominately HBr or HCl dehydrohalogenation gas-phase reactions. In the study reported here we synthesize α,α,α-tribromo-p-xylene and a-bromo-α',α'-dichloro-p-xylene as CVD precursors to yield a,a- and a,a'-dihalogenated poly(p-xylylene)s to try to convert them, via post-deposition annealing, to poly(phenylene ethynylene) (PPE). PPE and its intermediate poly(phenylene vinylene) (PPV), have potentially better thermal and oxidative properties, and in addition they are both photoluminescent. The as-deposited and annealed thin films have been characterized by Rutherford backscattering spectrometry (RBS), infrared spectroscopy, and UV-vis spectrophotometry, and electrically tested by dielectric constant, bias-temperature stress (BTS), and leakage current. It is found that the thin films crystallizes concurrent with the formation of PPV and possibly converts to PPE but has much residual chlorine and bromine. However, PPV exhibits excellent stability in contact with copper and is more thermally stable than poly(p-xylyl-ene).

The copolymerization of ethylene with polar vinyl monomers, such as vinyl acetate, acrylonitrile, vinyl ethers, and allyl monomers, was accomplished using cationic palladium complexes ligated by a bisphosphine monoxide (BPMO). The copolymers formed by these catalysts have highly linear microstructures and a random distribution of polar functional groups throughout the polymer chain. Our data demonstrate that cationic palladium complexes can exhibit good activity for polymerizations of polar monomers, in contrast to cationic α-diimine palladium complexes (Brookhart-type) that are not applicable to industrially relevant polar monomers beyond acrylates. Additionally, the studies reported here point out that phosphine-sulfonate ligated palladium complexes are no longer the singular family of catalysts that can promote the reaction of ethylene with many polar vinyl monomers to form linear functional polyolefins.

We report the isolation and reactivity of a series of "ligandless," anionic arylpalladium complexes of the general structure [Pd(Ar)Br(2)](2)(2-) by the reaction of ((t)Bu(3)P)Pd(Ar)(Br) and bromide. These anionic complexes insert olefins at room temperature, and these fast insertions indicate that the anionic complexes are kinetically competent to be intermediates in Heck-Mizoroki reactions conducted under "ligandless" conditions (lacking added dative ligand). Kinetic studies showed that the anionic complexes insert olefins much faster than the corresponding neutral, P(t-Bu)(3)-ligated complexes. Addition of halide to the reaction of the neutral complex ((t)Bu(3)P)Pd(Ar)(Br) and styrene led to a significant rate acceleration, suggesting that the anionic complex forms rapidly and reversibly in situ from the neutral species prior to migratory insertion. These data, along with studies on the regioselectivity for reaction of aryl halides with butyl vinyl ether in the presence of the different starting catalysts, are consistent with the intermediacy of the same anionic, arylpalladium intermediates in Heck reactions catalyzed by palladium complexes containing bulky trialkylphosphine ligands as in reactions conducted under ligandless conditions.

The oxidative addition of PhX (X = I, Br, Cl) to the complexes Pd(P(t)Bu(3))(2) (1), Pd(1-AdP(t)Bu(2))(2) (2), Pd(CyP(t)Bu(2))(2) (3), and Pd(PCy(3))(2) (4) (1-Ad = 1-adamantyl, Cy = cyclohexyl) was studied to determine the effect of steric properties on the coordination number of the species that undergoes oxidative addition and to determine whether the type of halide affects the identity of this species. The kinetic data imply that the number of phosphines coordinated to the complex that reacts in the irreversible step of the oxidative addition process for complexes 1-4 depends more on the halide than on the steric properties of the ligands. The rate-limiting step of the oxidative addition of PhI occurred with L(2)Pd(0) in all cases, as determined by the lack of dependence of k(obs) on [P(t)Bu(3)], [1-AdP(t)Bu(2)], or [CyP(t)Bu(2)] and the inverse dependence of the rate constant on [PCy(3)] when the reaction was initiated with Pd(PCy(3))(3). The irreversible step of the oxidative addition of PhCl occurred with a monophosphine species in each case, as signaled by an inverse dependence of the rate constant on the concentration of ligand. The irreversible step of the oxidative addition of PhBr occurred with a bisphosphine species, as signaled by the zeroth-order or small dependence of the rate constant on the concentration of phosphine. Thus, the additions of the less reactive chloroarenes occur through lower-coordinate intermediates than additions of the more reactive haloarenes.

Integrating pore sealing and barrier dielectric materials into back-end-of-the-line (BEOL) structures is challenging due to the requirements of high thermal stability, electrical stability (high breakdown, low leakage, and low dielectric constant), and barrier-like properties of the films. With Al etch-back metallization the dielectric needed to be stable to 450 °C due to Al reflow but now the thermal budget is shrinking with Cu metallization. Now the limiting factors are solder and ultra-low k (ULK) dielectric cure temperatures. These temperatures may still require thermal anneals exceeding 400 °C, which is very demanding on organic-based systems. Parylene X, a pore sealing chemical vapor depositable polymer, is deposited a room temperature via a precursor that allows the polymer to be self-initiated and without any by-products. The deposition platform allows controlled penetration of the organic polymer that allows significant fracture toughness improvement for the ULK dielectric. The polymer starts to cross-link at 175 °C with full conversion by 380 °C as measured by FT-IR spectroscopy. The polymer is shown to be stable to 420 °C with a dielectric constant of 2.7 and a leakage current of 0.5 × 10-9 A/cm2 at 0.67 MV/cm using MIMCAP structures. Parylene × was shown to have barrier-like properties in contact with both Ta and Cu metallization using bias-temperature stress (BTS) measurements at 0.5 MV/cm and 150 °C. In a Cu/Ta/parylene X/ULK dielectric stack, Ta a carbide former, can be used as an adhesive layer linking Cu to parylene × or the organic-based surface since parylene × has barrier-like properties.

Poly(p-xylylene) (also known as parylene N) has previously been used to pore seal ultralow k (≤ 2.2) (ULK) dielectrics. The parylene polymers may facilitate the integration of ULK dielectrics by: substantially improving their fracture toughness, hermetically sealing the pores, being able to use standard wet chemical cleans, and minimally impacting the observed dielectric constant, while minimally disrupting current process flow integrations. This paper introduces a new cross-linkable polymer that is deposited using thermal chemical vapor deposition (CVD) on the same tool that is used for parylene N deposition. The polymer, poly(ethynyl-p-xylylene) (parylene X), was deposited at room temperature. A series of 30 min post-deposition anneals in helium shows that the deposited material cross-linked between 200°C and 300°C with full conversion at 380°C for a ~300 A film. After the low molecular weight species out-gassed during anneals at 200°C, there was less than a percent weight loss to 450°C with no change in the optical constants and no optical loss. Previous work with poly(ethyl-p-xylylene) suggests that the dielectric constant of parylene X will be significantly lower than parylene N.

We report that oxidative addition of bromobenzene to Pd(PtBu3)2 occurs by an unusual autocatalytic mechanism. Studies on the effect of various additives showed that the degree of rate acceleration followed the trend: (PtBu3)Pd(Ph)(Br) approximately (HPtBu3)Br < [(PtBu3)Pd(mu-Br)]2 < (PtBu3)2Pd(H)(Br). Studies on the reactions of Pd(PtBu3)2 in the presence of (PtBu3)2Pd(H)(Br) showed that the concentration of (PtBu3)2Pd(H)(Br) decreased only after the Pd(0) complex had been consumed. These data indicated that the catalyst in this process is (PtBu3)2Pd(H)(Br). Thermal decomposition of the three-coordinate oxidative addition product (PtBu3)Pd(Ar)(Br) during the reaction of Pd(PtBu3)2 and bromoarenes ultimately leads to formation of (PtBu3)2Pd(H)(Br). Parallel reactions of bromobenzene with (PtBu3)2Pd(H)(Br) and Pd(PtBu3)2 showed that the bromoarenes reacted considerably faster with the Pd(II) species than with the Pd(0) species. We therefore propose a catalytic cycle for oxidative addition in which PBut3.HBr reacts with the Pd(0) species to form (PtBu3)2Pd(H)(Br), and (PtBu3)2Pd(H)(Br) reacts with the bromoarene, possibly though the anionic species [HPtBu3+][(PtBu3)Pd(Br)-], to form [Pd(PtBu3)(Ar)(Br)].

D Evaluation of a Novel Cu(I) Precursor for Chemical Vapor Deposition D.-X. Ye, B. Carrow, S. Pimanpang, H. Bakhru, G. A. Ten Eyck, G.-C. Wang, and T.-M. Lu Center for Integrated Electronics and Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180-3590, USA Department of Physics, State Univesity of New York-Albany, Albany, New York 12222, USA Department of Electrical, Computer, and Systems Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180-3590, USA