Pd-Catalyzed Decarbonylative Cross-Couplings of Aroyl Chlorides

Ligand Relay for Nickel-Catalyzed Decarbonylative Alkylation of Aroyl Chlorides

Transition metal-catalyzed direct decarboxylative transformations of aromatic carboxylic acids usually require high temperatures, which limit the substrate's scope, especially for late-stage applications. The development of the selective decarbonylative of carboxylic acid derivatives, especially the most fundamental aroyl chlorides, with stable and cheap electrophiles under mild conditions is highly desirable and meaningful, but remains challenging. Herein, a strategy of nickel-catalyzed decarbonylative alkylation of aroyl chlorides via phosphine/nitrogen ligand relay is reported. The simple phosphine ligand is found essential for the decarbonylation step, while the nitrogen ligand promotes the cross-electrophile coupling. Such a ligand relay system can effectively and orderly carry out the catalytic process at room temperature, utilizing easily available aroyl chlorides as an aryl electrophile for reductive alkylation. This discovery provides a new strategy for direct decarbonylative coupling, features operationally simple, mild conditions, and excellent functional group tolerance. The mild approach is applied to the late-stage methylation of various pharmaceuticals. Extensive experiments are carried out to provide insights into the reaction pathway and support the ligand relay process.

Palladium-Catalyzed Decarbonylative Coupling of (Hetero)Aryl Boronate Esters with Difluorobenzyl Glutarimides

This report describes the Pd-catalyzed decarbonylative coupling of difluorobenzyl glutarimides with (hetero)aryl boronate esters to yield difluorobenzyl-substituted (hetero)arene products. The use of PAd2Bu as the phosphine ligand in combination with neopentylboronate ester nucleophiles proved critical for the selective formation of the decarbonylative coupling product versus analogous difluorobenzyl ketone. This transformation is effective for electronically diverse (hetero)aryl boronate esters and substituted difluorobenzyl glutarimides.

A machine-learning tool to predict substrate-adaptive conditions for Pd-catalyzed C-N couplings

Machine-learning methods have great potential to accelerate the identification of reaction conditions for chemical transformations. A tool that gives substrate-adaptive conditions for palladium (Pd)-catalyzed carbon-nitrogen (C-N) couplings is presented. The design and construction of this tool required the generation of an experimental dataset that explores a diverse network of reactant pairings across a set of reaction conditions. A large scope of C-N couplings was actively learned by neural network models by using a systematic process to design experiments. The models showed good performance in experimental validation: Ten products were isolated in more than 85% yield from a range of couplings with out-of-sample reactants designed to challenge the models. Importantly, the developed workflow continually improves the prediction capability of the tool as the corpus of data grows.

Mechanism-Driven Development of Group 10 Metal-Catalyzed Decarbonylative Coupling Reactions

Transition-metal-catalyzed cross-coupling reactions are widely used in both academia and industry for the construction of carbon-carbon and carbon-heteroatom bonds. The vast majority of cross-coupling reactions utilize aryl (pseudo)halides as the electrophilic coupling partner. Carboxylic acid derivatives (RC(O)X) represent a complementary class of electrophiles that can engage in decarbonylative couplings to produce analogous products. This decarbonylative approach offers the advantage that RC(O)X are abundant and inexpensive. In addition, decarbonylative coupling enables both intramolecular (between R and X of the carboxylic acid derivative) as well as intermolecular bond-forming reactions (in which an exogeneous nucleophile is coupled with the R group derived from RC(O)X). In these intermolecular reactions, the X-substituent on the carboxylic acid can be tuned to facilitate both oxidative addition and transmetalation, thus eliminating the need for an exogeneous base. This Account details our group's development of a diverse variety of base-free decarbonylative coupling reactions catalyzed by group 10 metals. Furthermore, it highlights how catalyst design can be guided by stoichiometric organometallic studies of these systems.Our early studies focused on intramolecular decarbonylative couplings that transform RC(O)X to the corresponding R-X with extrusion of CO. We first identified Pd and Ni monodentate phosphine catalysts that convert aryl thioesters (ArC(O)SR) to the corresponding thioethers (ArSR). We next expanded this reactivity to fluoroalkyl thioesters, using readily available fluoroalkyl carboxylic acids as the fluoroalkyl (RF) source. A Ni-phosphinoferrocene catalyst proved optimal, and the large bite angle bidentate ligand was necessary to promote the challenging RF-S bond-forming reductive elimination step.We next pursued intramolecular decarbonylative couplings of aroyl halides. Palladium-based catalysts bearing dialkylbiaryl ligands (e.g., BrettPhos) were identified as optimal for converting aroyl chlorides (ArC(O)Cl) to aryl chlorides (ArCl). These ligands were selected based on their ability to facilitate the key C-Cl bond-forming reductive elimination step of the catalytic cycle. In contrast, all attempts to convert aroyl fluorides [ArC(O)F)] to aryl fluorides (ArF) were unsuccessful with either Pd- or Ni-based catalysts. Organometallic studies of the Ni-system show that C(O)-F oxidative addition and CO deinsertion proceed smoothly, but the resulting nickel(II) aryl fluoride intermediate fails to undergo C-F bond-forming reductive elimination.In contrast to its inertness to reductive elimination, this nickel(II) aryl fluoride proved highly reactive toward transmetalation. The fluoride ligand serves as an internal base, such that no additional base is required. We leveraged this "transmetalation active" intermediate to achieve base-free Ni-catalyzed intermolecular decarbonylative coupling reactions between aroyl fluorides and boron reagents to access both biaryl and aryl-boronate ester products. By tuning the electrophile, transmetalating reagent, and catalyst, this same approach also proved applicable to base-free intermolecular decarbonylative fluoroalkylation (between difluoromethylacetyl fluoride and arylboronate esters) and aryl amination (between phenol esters and silyl amines).Moving forward, a key goal is to identify catalyst systems that enable more challenging bond constructions via this manifold. In addition, CO inhibition remains a major issue leading to the requirement for high temperatures and high catalyst loadings. Identifying catalysts that are resistant to CO binding and/or approaches to remove CO under mild conditions will be critical for making these reactions more practical and scalable.

Palladium-Catalyzed Unimolecular Fragment Coupling of N-Allylamides via Elimination of Isocyanate

Transition metal-catalyzed unimolecular fragment coupling (UFC) is defined as processes that forge new chemical bonds through the extrusion of molecules, such as CO and CO₂, and the subsequent recombination of the remaining fragments. Herein, we report on a new UFC reaction that involves the palladium-catalyzed elimination of an isocyanate fragment from an amide, with the formation of carbon-carbon and carbon-heteroatom bonds. An organometallic intermediate that is relevant to the catalytic reaction was characterized by X-ray crystallography. This UFC reaction enables the late-stage transformation of an amide functionality, allowing amides to be used as a convertible directing or protecting group.

Decarbonylative Pd-Catalyzed Suzuki Cross-Coupling for the Synthesis of Structurally Diverse Heterobiaryls

Heteroaromatic biaryls are core scaffolds found in a plethora of pharmaceuticals; however, their direct synthesis by the Suzuki cross-coupling is limited to heteroaromatic halide starting materials. Here, we report a direct synthesis of diverse nitrogen-containing heteroaromatic biaryls by Pd-catalyzed decarbonylative Suzuki cross-coupling of widely available heterocyclic carboxylic acids with arylboronic acids. The practical and modular nature of this cross-coupling enabled the straightforward preparation of >45 heterobiaryl products using pyridines, pyrimidines, pyrazines, and quinolines in excellent yields. We anticipate that the modular nature of this protocol will find broad application in medicinal chemistry and drug discovery research.

Transition metal-catalyzed arylation of unstrained C-C single bonds

Carbon-carbon bond activation is one of the most challenging and important research areas in organic chemistry. Selective C-C bond activation of unstrained substrates is difficult to achieve owing to its inert nature and competitive side reactions, but the ubiquitous presence of C-C bonds in organic molecules makes this transformation attractive and of vital importance. Moreover, transition metal-catalyzed arylation of unstrained C-C single bonds can realize the cleavage of old C-C bonds and introduce important aryl groups into the carbon chain to construct new C-C bonds at the same time, providing a powerful and straightforward method to reconstruct the skeleton of the molecules. In recent years, considerable progress has been made in the area of direct arylation of C-C bonds, and β-C elimination or oxidative addition strategies play key roles in these transformations. This review summarizes recent achievements of transition metal-catalyzed arylation of unstrained C-C bonds, demonstrated by various kinds of substrates including alcohol, nitrile and carbonyl compounds, and each example is detailed by its corresponding mechanism, catalytic system and scope of the substrate.

Facile Conversion of Molecularly Complex (Hetero)aryl Carboxylic Acids into Alkynes for Accelerated SAR Exploration

1,2,3-Triazoles are well-established bioisosteres for amides, often installed as a result of structure-activity-relationship (SAR) exploration. A straightforward approach to assess the effect of the replacement of an amide by a triazole would start from the carboxylic acid and the amine used for the formation of a given amide and convert them into the corresponding alkyne and azide for cyclization by copper-catalyzed alkyne-azide cycloaddition (CuAAC). Herein, we report a functional-group-tolerant and operationally simple decarbonylative alkynylation that allows the conversion of complex (hetero)aryl carboxylic acids into alkynes. Furthermore, the utility of this method was demonstrated in the preparation of a triazolo analog of the commercial drug moclobemide. Lastly, mechanistic investigations using labeled carboxylic acid derivatives clearly show the decarbonylative nature of this transformation.

Catalytic Carbochlorocarbonylation of Unsaturated Hydrocarbons via C-COCl Bond Cleavage*

Here we report a palladium‐catalysed difunctionalisation of unsaturated C−C bonds with acid chlorides. Formally, the C−COCl bond of an acid chloride is cleaved and added, with complete atom economy, across either strained alkenes or a tethered alkyne to generate new acid chlorides. The transformation does not require exogenous carbon monoxide, operates under mild conditions, shows a good functional group tolerance, and gives the isolated products with excellent stereoselectivity. The intermolecular reaction tolerates both aryl‐ and alkenyl‐substituted acid chlorides and is successful when carboxylic acids are transformed to the acid chloride in situ. The reaction also shows an example of temperature‐dependent stereodivergence which, together with plausible mechanistic pathways, is investigated by DFT calculations. Moreover, we show that benzofurans can be formed in an intramolecular variant of the reaction. Finally, derivatisation of the products from the intermolecular reaction provides a highly stereoselective approach for the synthesis of tetrasubstituted cyclopentanes.

Peri-Selective Direct Acylmethylation and Amidation of Naphthalene Derivatives Using Iridium and Rhodium Catalysts

An iridium-catalyzed acylmethylation and a rhodium-catalyzed amidation of naphthalene derivatives are reported, adopting sulfoxonium ylides and dioxazolones as carbene and nitrene transfer agents, respectively. The use of SMe group as a directing group was key to ensure the peri-selective functionalization, and it can be easily removed or diversely transformed to other synthetically useful functionalities after the catalysis.

Palladium-Catalyzed Decarbonylative Iodination of Aryl Carboxylic Acids Enabled by Ligand-Assisted Halide Exchange

We report an efficient and broadly applicable palladium-catalyzed iodination of inexpensive and abundant aryl and vinyl carboxylic acids via in situ activation to the acid chloride and formation of a phosphonium salt. The use of 1-iodobutane as iodide source in combination with a base and a deoxychlorinating reagent gives access to a wide range of aryl and vinyl iodides under Pd/Xantphos catalysis, including complex drug-like scaffolds. Stoichiometric experiments and kinetic analysis suggest a unique mechanism involving C-P reductive elimination to form the Xantphos phosphonium chloride, which subsequently initiates an unusual halogen exchange by outer sphere nucleophilic substitution.

Palladium-catalyzed decarbonylative and decarboxylative cross-coupling of acyl chlorides with potassium perfluorobenzoates affording unsymmetrical biaryls

This paper describes the synthesis of unsymmetrical biaryls by the palladium-catalyzed cross-coupling reaction of acyl chlorides with potassium perfluorobenzoates. This transformation is unique in that it involves simultaneous decarbonylation and decarboxylation under redox-neutral conditions. Compared to conventional cross-coupling protocols for the synthesis of unsymmetrical biaryls, the two reactants in this synthetic strategy can be readily prepared from abundant and inexpensive aromatic carboxylic acids.

Investigation of the Selectivity of the Palladium-Catalyzed Aroylation and Arylation of Stannyl Glycals with Aroyl Chlorides

The selectivity of the palladium-catalyzed aroylation and arylation of 1-tributylstannyl glycals with aroyl chlorides was investigated. The selectivity was controlled by the palladium catalyst, and high selectivity was achieved via ligand modification of the palladium catalyst. The reaction catalyzed by Pd(OAc)2 provided aroyl C-glycals with high selectivity, whereas the reaction catalyzed by Pd(PPh3)4 produced aryl C-glycals with diminished selectivity. The scope and limitation of the selectivity in this reaction are discussed.

Palladium-catalysed carboformylation of alkynes using acid chlorides as a dual carbon monoxide and carbon source

Hydroformylation, a reaction that installs both a C-H bond and an aldehyde group across an unsaturated substrate, is one of the most important catalytic reactions in both industry and academia. Given the synthetic importance of creating new C-C bonds, the development of carboformylation reactions, wherein a new C-C bond is formed instead of a C-H bond, would bear enormous synthetic potential to rapidly increase molecular complexity in the synthesis of valuable aldehydes. However, the demanding complexity inherent in a four-component reaction, utilizing an exogenous CO source, has made the development of a direct carboformylation reaction a formidable challenge. Here, we describe a palladium-catalysed strategy that uses readily available aroyl chlorides as a carbon electrophile and CO source, in tandem with a sterically congested hydrosilane, to perform a stereoselective carboformylation of alkynes. An extension of this protocol to four chemodivergent carbonylations further highlights the creative opportunity offered by this strategy in carbonylation chemistry.

A direct synthesis method towards spirocyclic indazole derivatives via Rh(iii)-catalyzed C–H activation and spiroannulation

A rhodium(iii)-catalyzed [4 + 1] spiroannulation of N-aryl phthalazine-diones (pyridazine-diones) with diazo compounds to construct spirocyclic indazole derivatives with diverse structures is described.

Deoxygenative Tri- and Difluoromethylthiolation of Carboxylic Acids with Benzothiazolium Reagents

Deoxygenative syntheses of fluorinated thioesters directly from carboxylic acids have been developed employing benzothiazolium reagents. The process using BT-SCF₃ represents an attractive approach toward these SCF₃-containing compounds that avoids the use of metal ^-SCF₃ salts or preactivated acyl electrophiles. Moreover, the in situ activation of BT-SCF₂H allows for an unprecedented nucleophilic difluoromethylthiolation reaction. DFT calculations support a mechanistic scenario involving a four-membered transition state where acyl substitution occurs without the formation of an unstable free ^-SCF₂H anion.

Synthesis of indoline-fused eight-membered azaheterocycles through Zn-catalyzed dearomatization of indoles and subsequent base-promoted C-C activation

A cascade reaction involving the Zn-catalyzed dearomatization of indoles, base-promoted ring-expansion and intramolecular S_NAr reaction has been developed. This process realized a novel, atom economical and efficient synthesis of indoline-fused eight-membered azaheterocycles in a one pot manner.

Decarbonylative Suzuki-Miyaura Cross-Coupling of Aroyl Chlorides

Herein, we report a catalyst system for Pd-catalyzed decarbonylative Suzuki-Miyaura cross-coupling of aroyl chlorides with boronic acids to furnish biaryls. This strategy is suitable for a broad range of common aroyl chlorides and boronic acids. The synthetic utility is highlighted in the direct late-stage functionalization of pharmaceuticals and natural products capitalizing on the presence of carboxylic acid moiety. Extensive mechanistic and DFT studies provide key insight into the reaction mechanism and high decarbonylative cross-coupling selectivity.

Nickel-Catalyzed Decarbonylative Synthesis of Fluoroalkyl Thioethers

This report describes the development of a nickel-catalyzed decarbonylative reaction for the synthesis of fluoroalkyl thioethers (R_FSR) from the corresponding thioesters. Readily available, inexpensive, and stable fluoroalkyl carboxylic acids (R_FCO₂H) serve as the fluoroalkyl (R_F) source in this transformation. Stoichiometric organometallic studies reveal that R_F-S bond-forming reductive elimination is a challenging step in the catalytic cycle. This led to the identification of diphenylphosphinoferrocene as the optimal ligand for this transformation. Ultimately, this method was applied to the construction of diverse fluoroalkyl thioethers (R_FSR), with R = both aryl and alkyl.

Synthesis of N-methylated amines from acyl azides using methanol

The transformation of acyl azide derivatives into N-methylamines was developed using methanol as the C1 source via the one-pot Curtius rearrangement and borrowing hydrogen methodology. Following this protocol, various functionalised N-methylated amines were synthesized using the (NNN)Ru(ii) complex from carboxylic acids via an acyl azide intermediate. Several kinetic studies and DFT calculations were carried out to support the mechanism and also to determine the role of the Ru(ii) complex and base in this transformation.

Nickel-Catalyzed Decarbonylation of Acylsilanes

Nickel-catalyzed decarbonylation of acylsilanes is developed. In sharp contrast to cross-coupling reactions of acylsilanes, in which the silyl group serves as a leaving group, the silyl group is retained in the product in this decarbonylation reaction. Although the strong binding of the dissociated CO to the nickel center frequently hinders catalyst turnover in nickel-mediated decarbonylative reactions, this reaction can be catalyzed by nickel complexes bearing a CO ligand.

Synthesis of Indolizine Derivatives Triggered by the Oxidative Addition of Aroyl Chloride to Pd(0) Complex

An efficient synthesis of indolizine derivatives from propargylic pyridines and aroyl chlorides was developed. The 5-endo-dig cyclization was initiated by the in situ formed acylpalladium species from the facile oxidative addition of aroyl chloride to Pd(0) complex. This transformation successfully occurred in the presence of an N-nucleophilic moiety and acid chlorides, a good electrophilic partner, affording highly functionalized indolizines in good-to-excellent yields.

Nickel-Catalyzed Intramolecular Desulfitative C-N Coupling: A Synthesis of Aromatic Amines

A nickel-catalyzed intramolecular C-N coupling reaction via SO₂ extrusion is presented. The use of a catalytic amount of BPh₃ allows the transformation to take place under much milder conditions (60 °C) than previously reported C-N coupling reactions by CO or CO₂ extrusion (160-180 °C). In addition, this method displays good functional group tolerance and versatility, as it can be applied to the synthesis of dialkyl aryl amines, alkyl diaryl amines, and triaryl amines. The robustness of the desulfitative C-N coupling is demonstrated by three high-yielding gram-scale reactions.

Introducing an α-Keto Ester Functional Group through Pt-Catalyzed Direct C-H Acylation with Ethyl Chlorooxoacetate

Platinum-catalyzed selective C-H acylation of 2-aryloxypyridines with ethyl chlorooxoacetate provides an efficient way of introducing an α-keto ester functional group. The reaction is oxidant-free, additive-free, and, more significantly, free of any decarbonylative side reactions. The reaction tolerates a variety of substituents from strongly electron-donating to strongly electron-withdrawing groups. Double acylation is feasible for 2-phenoxypyridine and its derivatives with only one substituent at the para position. Although the reaction of 2-(2-methylphenoxy)pyridine with ethyl malonyl chloride did not produce the desired β-keto ester, the reaction with ethyl succinyl chloride proceeded smoothly to give the γ-keto ester. Ethyl chlorooxoacetate is much more reactive than ethyl succinyl chloride in this Pt-catalyzed C-H acylation reaction.

Nickel or Palladium-Catalyzed Decarbonylative Transformations of Carboxylic Acid Derivatives

Carboxylic acid derivatives containing acyl halides, anhydrides, esters, amides and acyl nitriles are highly appealing electrophiles in transition-metal-catalyzed carbon-carbon bond-forming reactions due to their ready availability and low cost, which can provide divergent transformations of carboxylic acids into other value-added products. In this Minireview, we focus on the recent advances of decarbonylative transformations of carboxylic acid derivatives in carbon-carbon bond formations using Ni or Pd catalysts. A series of reaction types, product classifications and reaction pathways are presented herein, which show the advantageous features of carboxylic acid derivatives as alternative to aryl or alkyl halides in terms of reactivity and compatibility. The well-accepted mechanism of nickel- or palladium-catalyzed decarbonylative transformations involves initial oxidative addition of carboxylic acid derivatives, followed by decarbonylation or transmetalation (or insertion), and reductive elimination to generate the products, thereby regenerating the catalysts.

Deoxofluorination of (Hetero)aromatic Acids

Diverse trifluoromethyl-substituted compounds were synthesized by deoxofluorination of cinnamic and (hetero)aromatic carboxylic acids with sulfur tetrafluoride. The obtained products were used as starting materials in the preparation of novel fluorinated amino acids, anilines, and aliphatic amines - valuable building blocks for medicinal chemistry and agrochemistry.

A practical chlorination of tert-butyl esters with PCl3 generating acid chlorides

For the first time, using PCl3, a range of tert-butyl esters is chlorinated successfully, allowing access of both aromatic acid chlorides and aliphatic acid chlorides in good yields. The method features simple reaction conditions and wide substrate scope. Various tert-butyl esters including aryl esters, alkenyl esters, and alkyl esters were tolerated well in the reaction. A plausible mechanism is proposed.

Deoxygenative trifluoromethylthiolation of carboxylic acids

A deoxygenative trifluoromethylthiolation method produces trifluoromethyl thioesters from readily available carboxylic acids.

Here we describe a deoxygenative trifluoromethylthiolation method that yields trifluoromethyl thioesters from readily available carboxylic acids. The method is built upon an “umpolung” strategy where triphenylphosphine is used to first activate an electrophilic trifluoromethylthiolating reagent and then serves as an oxygen acceptor for the deoxygenation. The method is mild, efficient, broad-scope, and tolerant. It can be applied for the late-stage functionalization of numerous natural products and drug molecules containing a carboxylic acid group. The trifluoromethyl thioesters can be converted into trifluoromethyl thioethers by Pd-catalyzed decarbonylation.

N-Heterocyclic Carbene Ligand-Controlled Chemodivergent Suzuki-Miyaura Cross Coupling

Two N-heterocyclic carbene ligands provide orthogonal chemoselectivity during the Pd-catalyzed Suzuki-Miyaura (SM) cross-coupling of chloroaryl triflates. The use of SIPr [SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene] leads to selective cross-coupling at chloride, while the use of SIMes [SIMes = 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene] provides selective coupling at triflate. With most chloroaryl triflates and arylboronic acids, ligand-controlled selectivity is high (≥10:1). The scope of this methodology is significantly more general than previously reported methods for selective SM coupling of chloroaryl triflates using phosphine ligands. Density functional theory studies suggest that palladium's ligation state during oxidative addition is different with SIMes compared to SIPr.

Redox-Neutral Decarbonylative Cross-Couplings Coming of Age

Major progress has recently been made in the challenging redox-neutral decarbonylative cross-coupling of carboxylic acids. For example, the use of acid fluorides as effective cross-coupling partners has been found to enable control of the decarbonylation selectivity and facilitates challenging Pd⁰ -catalyzed nucleophilic trifluoromethylation and exogenous base-free Suzuki cross-coupling reactions. In another recent advance, the use of acid chlorides in room temperature difluoromethylation and direct decarbonylative cross-coupling of carboxylic acids allows these classical substrates to be used as aryl electrophiles in cross-coupling reactions. Further challenges that are yet to be addressed in redox-neutral decarbonylative cross-couplings are also briefly summarized.

Atom-efficient chlorination of benzoic acids with PCl3 generating acyl chlorides

By using PCl3 as the chlorinating reagent, various carboxylic acids are converted into the corresponding acid chlorides in good yields. This method features high atom efficiency in which nearly all three chlorine atoms of PCl3 are used in the chlorination. Moreover, the work-up procedure is simple since the major side product is phosphonic acid [HP(O)(OH)2] which is non-toxic and easily removed by filtration. A possible mechanism is proposed in this paper.

Nickel-Catalyzed Decarboxylative Alkenylation of Anhydrides with Vinyl Triflates or Halides

Decarboxylative cross-coupling of aliphatic acid anhydrides with vinyl triflates or halides was accomplished via nickel catalysis. This methodology works well with a broad array of substrates and features abundant functional group tolerance. Notably, our approach addresses the issue of safe and environmental installation of methyl or ethyl group into molecular scaffolds. The method possesses high chemoselectivity toward alkyl groups when aliphatic/aromatic mixed anhydrides are involved. Furthermore, diverse ketones could be modified with our strategy.

CO Displacement in an Oxidative Addition of Primary Silanes to Rhodium(I)

The rhodium dicarbonyl {PhB(Ox^Me₂)₂Im^Mes}Rh(CO)₂ (1) and primary silanes react by oxidative addition of a nonpolar Si-H bond and, uniquely, a thermal dissociation of CO. These reactions are reversible, and kinetic measurements model the approach to equilibrium. Thus, 1 and RSiH₃ react by oxidative addition at room temperature in the dark, even in CO-saturated solutions. The oxidative addition reaction is first-order in both 1 and RSiH₃, with rate constants for oxidative addition of PhSiH₃ and PhSiD₃ revealing k_H/ k_D ∼ 1. The reverse reaction, reductive elimination of Si-H from {PhB(Ox^Me₂)₂Im^Mes}RhH(SiH₂R)CO (2), is also first-order in [2] and depends on [CO]. The equilibrium concentrations, determined over a 30 °C temperature range, provide Δ H ° = -5.5 ± 0.2 kcal/mol and Δ S ° = -16 ± 1 cal·mol^-1K^-1 (for 1 ⇄ 2). The rate laws and activation parameters for oxidative addition (Δ H^⧧ = 11 ± 1 kcal·mol^-1 and Δ S^⧧ = -26 ± 3 cal·mol^-1·K^-1) and reductive elimination (Δ H^⧧ = 17 ± 1 kcal·mol^-1 and Δ S^⧧ = -10 ± 3 cal·mol^-1K^-1), particularly the negative activation entropy for both forward and reverse reactions, suggest the transition state of the rate-determining step contains {PhB(Ox^Me₂)₂Im^Mes}Rh(CO)₂ and RSiH₃. Comparison of a series of primary silanes reveals that oxidative addition of arylsilanes is ca. 5× faster than alkylsilanes, whereas reductive elimination of Rh-Si/Rh-H from alkylsilyl and arylsilyl rhodium(III) occurs with similar rate constants. Thus, the equilibrium constant K_e for oxidative addition of arylsilanes is >1, whereas reductive elimination is favored for alkylsilanes.