Correlating Reactivity and Selectivity to Cyclopentadienyl Ligand Properties in Rh(III)-Catalyzed C-H Activation Reactions: An Experimental and Computational Study

Enantiodivergent Pd-catalyzed C-C bond formation enabled through ligand parameterization

Despite the enormous potential for the use of stereospecific cross-coupling reactions to rationally manipulate the three-dimensional structure of organic molecules, the factors that control the transfer of stereochemistry in these reactions remain poorly understood. Here we report a mechanistic and synthetic investigation into the use of enantioenriched alkylboron nucleophiles in stereospecific Pd-catalyzed Suzuki cross-coupling reactions. By developing a suite of molecular descriptors of phosphine ligands, we could apply predictive statistical models to select or design distinct ligands that respectively promoted stereoinvertive and stereoretentive cross-coupling reactions. Stereodefined branched structures were thereby accessed through the predictable manipulation of absolute stereochemistry, and a general model for the mechanism of alkylboron transmetallation was proposed.

Mild complexation protocol for chiral CpxRh and Ir complexes suitable for in situ catalysis

Mild complexations of chiral cyclopentadienes with rhodium(i) and iridium(i) precursors enable user-friendly in situ complex formation for catalytic applications.

A practical complexation method for chiral cyclopentadienyl (Cp^x) iridium and rhodium complexes is described. The procedure uses the free Cp^xH with stable and commercially available rhodium(i) and iridium(i) salts without base or additive. The conditions are mild and do not require the exclusion of air and moisture. A salient feature is the suitability for in situ complexations enhancing the user-friendliness of Cp^x ligands in asymmetric catalysis. DFT-calculations confirm an intramolecular proton abstraction pathway by either the bound acetate or methoxide. Furthermore, the superior facial selectivity of the proton abstraction step enabled the development of TMS-containing trisubstituted Cp^x ligands which display improved enantioselectivities for the benchmarking dihydroisoquinolone synthesis.

Enantiodivergent Desymmetrization in the Rhodium(III)-Catalyzed Annulation of Sulfoximines with Diazo Compounds

Rh^III - and Ir^III -catalyzed asymmetric C-H functionalization reactions of arenes have relied on the employment of chiral Rh^III /Ir^III cyclopentadienyl catalysts, the introduction of chiral carboxylic acids to achiral Cp*RhX₂ catalysts, and the integration of both strategies. Despite considerable progress, each reaction only provided a specific configuration of the enantioenriched product when using a particular chiral catalyst. Reported in this work is the enantiodivergent coupling of sulfoximines with various diazo compounds by Rh^III -catalyzed desymmetrizing annulation. The enantiodivergence was enabled by a judicious choice of achiral carboxylic acids, and the enantioselectivity correlates with the steric bias of the carboxylic acid and the sulfoximine.

Rhodium(III)-Catalyzed Meta-Selective C-H Alkenylation of Phenol Derivatives

Rhodium(III)-catalyzed remote meta-selective-C-H alkenylation of phenol derivatives has been developed using a traceless organosilicon template as the directing group. This transformation proceeds smoothly with good yields and high meta-selectivities toward a series of phenol and alkene substrates. In addition, this protocol provides an effective strategy for late-stage transformations of various meta-alkenylated aromatic compounds.

Enantioselective fluorination of homoallylic alcohols enabled by the tuning of non-covalent interactions

The study of the enantioselective fluorination of homoallylic alcohols via chiral anion phase transfer (CAPT) catalysis using an in situ generated directing group is described. Multivariate correlation analysis, including designer π-interaction derived parameters, revealed key structural features affecting the selectivity at the transition state (TS). Interpretation of the parameters found in the model equation highlights the key differences as well as similarities for the reaction of homoallylic and allylic substrates. A similar T-shaped π-interaction was found to occur between the substrate and the catalyst. The tuning of this crucial interaction by identification of the best combination of phosphoric acid catalyst and boronic acid directing group allowed for the development of a methodology to access γ-fluoroalkenols in typically high enantioselectivities (up to 96% ee).

Stereodivergent Rhodium(III)-Catalyzed cis-Cyclopropanation Enabled by Multivariate Optimization

The design of stereodivergent transformations is of great interest to the synthetic community as it allows funneling of a given reaction pathway toward one stereochemical outcome or another by only minor adjustments of the reaction setup. Herein, we present a physical organic approach to invert the sense of induction in diastereoselective cyclopropanation of alkenes with N-enoxyphthalimides through rhodium(III) catalysis. Careful parametrization of catalyst-substrate molecular determinants allowed us to interrogate linear-free energy relationships and establish an intuitive and robust statistical model that correlates an extensive number of data points in high accuracy. Our multivariate correlations-steered mechanistic investigation culminated with a robust and general diastereodivergent cyclopropanation tool where the switch from trans- to cis-diastereoinduction is attributed to a mechanistic dichotomy. Selectivity might be determined by the flexibility of rhodacyclic intermediates derived from ring-opened versus -unopened phthalimides, induced by both their respective ring size and the Sterimol B1 parameter of the CpX ligand on rhodium.

Divergent ring-opening coupling between cyclopropanols and alkynes under cobalt catalysis

Cobalt catalysts promote ring-opening coupling between cyclopropanols and unactivated alkynes to give β-alkenyl ketones or cyclopentenols with exquisite chemoselectivity control.

Cobalt–diphosphine catalysts promote ring-opening coupling reactions between cyclopropanols and unactivated internal alkynes, affording either β-alkenyl ketones or multisubstituted cyclopentenol derivatives in good yields with good to excellent regioselectivities. The chemoselectivity between these β-alkenylation and [3 + 2] annulation reactions, which likely share a cobalt homoenolate as a key catalytic intermediate, is exquisitely controlled by the reaction conditions, with the solvent being a major controlling factor. The reactions are proposed to involve ring opening of cobalt cyclopropoxide into homoenolate, migratory insertion of the alkyne into the Co–C bond, and protodemetalation or intramolecular carbonyl addition of the resulting alkenylcobalt species. The feasibility of these reaction steps was supported by DFT calculations.

Crystal structure of the tetra-meth-yl(pheneth-yl)cyclo-penta-dienylmolybdenumtricarbonyl dimer

The structure of the dimer bis-{tricarbon-yl[η5-tetra-meth-yl(pheneth-yl)cyclo-penta-dien-yl]molybdenum}(Mo-Mo), [Mo2(C17H21)2(CO)6], at 102 K has triclinic (P ) symmetry. The reaction between tetra-meth-yl(pheneth-yl)cyclo-penta-diene and molybdenum hexa-carbonyl in refluxing xylenes for 18 h led to a 56% yield of the dimer as a red solid. The asymmetric unit of the structure is the tetra-meth-yl(pheneth-yl)cyclo-penta-dienylmolybdenumtricarbonyl moiety and the entire dimeric mol-ecule is generated by inversion symmetry. The Mo-Mo bond length is 3.2773 (3) Å, a value slightly above the mean value for all [CpMo(CO)3]2 compounds listed in the CSD and slightly below the mean for [Cp*Mo(CO)3]2 complexes.

Delineating Physical Organic Parameters in Site-Selective C-H Functionalization of Indoles

Site-selective C-H functionalization is a great challenge in homogeneous transition-metal catalysis. Herein, we present a physical organic approach to delineate the origin of regioselective amidation of N-acylindoles through Ir(III) catalysis. Bulkiness of N-directing groups of indole substrates and electronics of carboxylate additives were identified as two major factors in controlling C2 and C7 selectivity, and their microscopic mechanisms were studied with DFT-based transition state analysis. Computational insights led us to interrogate a linear free energy relationship, and parametrization of molecular determinants enabled the establishment of an intuitive yet robust statistical model that correlates an extensive number of validation data points in high accuracy. This mechanistic investigation eventually allowed the development of a new C2 amidation and alkenylation protocol of indoles, which affords the exclusive functionalization at the C2 position with up to >70:1 selectivity.

Ir(III)-Catalyzed Carbocarbation of Alkynes through Undirected Double C-H Bond Activation of Anisoles

A novel, electron-deficient cyclopentadienyl iridium(III) catalyst enables sequential cleavage of arene C(sp²)-H and methoxy C(sp³)-H bonds of anisoles, generating reactive metalacycles that insert difluoroalkynes to afford chromenes under mild reaction conditions. This transformation is an arylalkylation of an alkyne-a carbocarbation-via a nonchelate-assisted cleavage of two C-H bonds.

Predictive and mechanistic multivariate linear regression models for reaction development

Multivariate Linear Regression (MLR) models utilizing computationally-derived and empirically-derived physical organic molecular descriptors are described in this review. Several reports demonstrating the effectiveness of this methodological approach towards reaction optimization and mechanistic interrogation are discussed. A detailed protocol to access quantitative and predictive MLR models is provided as a guide for model development and parameter analysis.

Formation of quaternary centres by copper catalysed asymmetric conjugate addition to β-substituted cyclopentenones with the aid of a quantitative structure-selectivity relationship

A new asymmetric conjugate addition method was developed for β-substituted cyclopentenones to form quaternary centres using alkylzirconocene nucleophiles giving up to 97% yield and 92% ee.

A new asymmetric conjugate addition method was developed for β-substituted cyclopentenones to form quaternary centres using alkylzirconocene nucleophiles giving up to 97% yield and 92% ee. Key to the reaction's success was the design of suitable phosphoramidite ligands which was aided by a linear quantitative structure–selectivity relationship (QSSR). QSSR models were created from the ligand screening data (a total of 36 ligands) which revealed important electronic and steric requirements and led to the synthesis of more enantioselective ligands. DFT calculations of competing transition structures enable the interpretation of the electronic and steric terms present in the QSSR models.

Electronic and Steric Tuning of a Prototypical Piano Stool Complex: Rh(III) Catalysis for C-H Functionalization

The history of transition metal catalysis is heavily steeped in ligand design, clearly demonstrating the importance of this approach. The intimate relationship between metal and ligand can profoundly affect the outcome of a reaction, often impacting selectivity, physical properties, and the lifetime of a catalyst. Importantly, this metal-ligand relationship can provide near limitless opportunities for reaction discovery. Over the past several years, transition-metal-catalyzed C-H bond functionalization reactions have been established as a critical foundation in organic chemistry that provides new bond forming strategies. Among the d-block elements, palladium is arguably one of the most popular metals to accomplish such transformations. One possible explanation for this achievement could be the broad set of phosphine and amine based ligands available in the chemist's toolbox compatible with palladium. In parallel, other metals have been investigated for C-H bond functionalization. Among them, pentamethylcyclopentadienyl (Cp*) Rh(III) complexes have emerged as a powerful mode of catalysis for such transformations providing a broad spectrum of reactivity. This approach possesses the advantage of often very low catalyst loading, and reactions are typically performed under mild conditions allowing broad functional group tolerance. Cp*Rh(III) is considered as a privileged catalyst and a plethora of reactions involving a C-H bond cleavage event have been developed. The search for alternative cyclopentadienyl based ligands has been eclipsed by the tremendous effort devoted to exploring the considerable scope of reactions catalyzed by Cp*Rh(III) complexes, despite the potential of this strategy for enabling reactivity. Thus, ligand modification efforts in Rh(III) catalysis have been an exception and research directed toward new rhodium catalysts has been sparse. Recently, chiral cyclopentadienyl ligands have appeared allowing enantioselective Rh(III)-catalyzed C-H functionalization reactions to be performed. Alongside chiral ligands, an equally important collection of achiral cyclopentadienyl-derived ligands have also emerged. The design of this new set of ligands for rhodium has already translated to significant success in solving inherent problems of reactivity and selectivity encountered throughout the development of new Rh(III)-catalyzed transformations. This Account describes the evolution of cyclopentadienyl ligand skeletons in Rh(III)-catalysis since the introduction of pentamethylcyclopentadienyl ligands to the present. Specific emphasis is placed on reactivity and synthetic applications achieved with the new ligands with the introduction of achiral mono-, di-, or pentasubstituted cyclopentadienyl ligands exhibiting a stunning effect on reactivity and selectivity. Furthermore, an underlying question when dealing with ligand modification strategies is to explain the reason one ligand outperforms another. Conjecture and speculation abound, but extensive characterization of their steric and electronic properties has been carried out and information about electronic and steric properties of the ligands all contribute to our understanding and give crucial pieces to solve the puzzle.

Regiodivergent Iridium(III)-Catalyzed Diamination of Alkenyl Amides with Secondary Amines: Complementary Access to γ- or δ-Lactams

Alkenyl N-pivaloylhydroxamates undergo an Ir(III)-catalyzed diamination of the alkene with simple exogenous secondary amines under extraordinarily mild reaction conditions. The regioselectivity of the diamination is controlled by the solvent and the electronics of the cyclopentadienyl (Cpx) ligand on Ir. On the basis of a set of mechanistic experiments, we propose that the relative rates of Ir(V)-nitrenoid formation versus attack on the amido-Ir-coordinated alkene by the exogenous amine determine the outcome of the reaction.

Multidimensional Correlations in Asymmetric Catalysis through Parameterization of Uncatalyzed Transition States

The study of the oxidative amination of tetrahydroisoquinolines under chiral-anion phase-transfer (CAPT) catalysis by multidimensional correlation analysis (MCA) is revisited. The parameterization of the transition states (TSs) for the uncatalyzed reaction, the introduction of conformational descriptors, and the use of computed interaction energies and distances as parameters allowed access to a considerably simplified mathematical correlation of substrate and catalyst structure to enantioselectivity. The equation obtained is suggestive of key interactions occurring at the TS. Specifically, the CAPT catalyst is proposed to coordinate the intermediate iminium cation by P=O⋅⋅⋅H-O hydrogen-bonding and N⋅⋅⋅H-C electrostatic interactions. The conformational freedom of the benzyl substituent of the substrate was also found to be important in providing an efficient mode of molecular recognition.

Quantitative Modeling of Bis(pyridine)silver(I) Permanganate Oxidation of Hydantoin Derivatives: Guidelines for Predicting the Site of Oxidation in Complex Substrates

The bis(pyridine)silver(I) permanganate promoted hydroxylation of diketopiperazines has served as a pivotal transformation in the synthesis of complex epipolythiodiketopiperazine alkaloids. This late-stage C-H oxidation chemistry is strategically critical to access N-acyl iminium ion intermediates necessary for nucleophilic thiolation of advanced diketopiperazines en route to potent epipolythiodiketopiperazine anticancer compounds. In this study, we develop an informative mathematical model using hydantoin derivatives as a training set of substrates by relating the relative rates of oxidation to various calculated molecular descriptors. The model prioritizes Hammett values and percent buried volume as key contributing factors in the hydantoin series while correctly predicting the experimentally observed oxidation sites in various complex diketopiperazine case studies. Thus, a method is presented by which to use simplified training molecules and resulting correlations to explain and predict reaction behavior for more complex substrates.