Theresa Sperger

Computation and Experiment: A Powerful Combination to Understand and Predict Reactivities

Computational chemistry has become an established tool for the study of the origins of chemical phenomena and examination of molecular properties. Because of major advances in theory, hardware and software, calculations of molecular processes can nowadays be done with reasonable accuracy on a time-scale that is competitive or even faster than experiments. This overview will highlight broad applications of computational chemistry in the study of organic and organometallic reactivities, including catalytic (NHC-, Cu-, Pd-, Ni-catalyzed) and noncatalytic examples of relevance to organic synthesis. The selected examples showcase the ability of computational chemistry to rationalize and also predict reactivities of broad significance. A particular emphasis is placed on the synergistic interplay of computations and experiments. It is discussed how this approach allows one to (i) gain greater insight than the isolated techniques, (ii) inspire novel chemistry avenues, and (iii) assist in reaction development. Examples of successful rationalizations of reactivities are discussed, including the elucidation of mechanistic features (radical versus polar) and origins of stereoselectivity in NHC-catalyzed reactions as well as the rationalization of ligand effects on ligation states and selectivity in Pd- and Ni-catalyzed transformations. Beyond explaining, the synergistic interplay of computation and experiments is then discussed, showcasing the identification of the likely catalytically active species as a function of ligand, additive, and solvent in Pd-catalyzed cross-coupling reactions. These may vary between mono- or bisphosphine-bound or even anionic Pd complexes in polar media in the presence of coordinating additives. These fundamental studies also inspired avenues in catalysis via dinuclear Pd(I) cycles. Detailed mechanistic studies supporting the direct reactivity of Pd(I)-Pd(I) with aryl halides as well as applications of air-stable dinuclear Pd(I) catalysts are discussed. Additional combined experimental and computational studies are described for alternative metals, these include the discussion of the factors that control C-H versus C-C activation in the aerobic Cu-catalyzed oxidation of ketones, and ligand and additive effects on the nature and favored oxidation state of the active catalyst in Ni-catalyzed trifluoromethylthiolations of aryl chlorides. Examples of successful computational reactivity predictions along with experimental verifications are then presented. This includes the design of a fluorinated ligand [(CF3)2P(CH2)2P(CF3)2] for the challenging reductive elimination of ArCF3 from Pd(II) as well as the guidance of substrate scope (functional group tolerance and suitable leaving group) in the Ni-catalyzed trifluoromethylthiolation of C(sp(2))-O bonds. In summary, this account aims to convey the benefits of integrating computational studies in experimental research to increase understanding of observed phenomena and guide future experiments.

Highly Efficient CSeCF3 Coupling of Aryl Iodides Enabled by an Air‐Stable Dinuclear PdI Catalyst

Building on our recent disclosure of catalysis at dinuclear Pd(I) sites, we herein report the application of this concept to the realization of the first catalytic method to convert aryl iodides into the corresponding ArSeCF3 compounds. Highly efficient C-SeCF3 coupling of a range of aryl iodides was achieved, enabled by an air-, moisture-, and thermally stable dinuclear Pd(I) catalyst. The novel SeCF3 -bridged dinuclear Pd(I) complex 3 was isolated, studied for its catalytic competence and shown to be recoverable. Experimental and computational data are presented in support of dinuclear Pd(I) catalysis.

Remote C−H alkylation and C−C bond cleavage enabled by an in situ generated palladacycle

The direct and selective functionalization of C−H bonds of arenes is one of the most challenging yet valuable aims in organic synthesis. Despite notable recent achievements, a pre-installed directing group proved to be essential in most of the methodologies reported so far. In this context, the use of a transient directing group that can be generated in situ has attracted attention and demonstrated the great potential of this strategy. Here we report the use of an in situ generated palladacycle to accomplish remote-selective C−H alkylation reactions of arenes. Following the C−H functionalization event, the alkylated aryl ring undergoes a formal migration to provide diversely substituted benzofuran and indole scaffolds. Computational studies revealed that a palladium(IV) intermediate is not involved in the alkylation step. The aryl migration was found to proceed through a sequential C−C bond cleavage, insertion and β-hydride-elimination process. The increasing steric bulk that builds up during the C−H functionalization step drives the unusual C−C bond cleavage in a non-strained system. Existing methods for C–H activation depend on pre-installed directing groups, the removal of which poses a practical limitation on the use of these reactions in synthesis. Now, a remote-selective C−H alkylation reaction of arenes using an in situ generated spiropalladacycle has been shown to furnish benzofurans and indoles without the need for a directing group.

Understanding the Unusual Reduction Mechanism of Pd(II) to Pd(I): Uncovering Hidden Species and Implications in Catalytic Cross-Coupling Reactions

The reduction of Pd(II) intermediates to Pd(0) is a key elementary step in a vast number of Pd-catalyzed processes, ranging from cross-coupling, C-H activation, to Wacker chemistry. For one of the most powerful new generation phosphine ligands, PtBu3, oxidation state Pd(I), and not Pd(0), is generated upon reduction from Pd(II). The mechanism of the reduction of Pd(II) to Pd(I) has been investigated by means of experimental and computational studies for the formation of the highly active precatalyst {Pd(μ-Br)(PtBu3)}2. The formation of dinuclear Pd(I), as opposed to the Pd(0) complex, (tBu3P)2Pd was shown to depend on the stoichiometry of Pd to phosphine ligand, the order of addition of the reagents, and, most importantly, the nature of the palladium precursor and the choice of the phosphine ligand utilized. In addition, through experiments on gram scale in palladium, mechanistically important additional Pd- and phosphine-containing species were detected. An ionic Pd(II)Br3 dimer side product was isolated, characterized, and identified as the crucial driving force in the mechanism of formation of the Pd(I) bromide dimer. The potential impact of the presence of these side species for in situ formed Pd complexes in catalysis was investigated in Buchwald-Hartwig, α-arylation, and Suzuki-Miyaura reactions. The use of preformed and isolated Pd(I) bromide dimer as a precatalyst provided superior results, in terms of catalytic activity, in comparison to catalysts generated in situ.

An Exclusively trans-Selective Chlorocarbamoylation of Alkynes Enabled by a Palladium/Phosphaadamantane Catalyst

Pharmaceutically relevant methylene oxindoles are synthesized by a palladium(0)-catalyzed intramolecular chlorocarbamoylation reaction of alkynes. A relatively underexplored class of caged phosphine ligands is uniquely suited for this transformation, enabling high levels of reactivity and exquisite trans selectivity. This report entails the first transition-metal-catalyzed atom-economic addition of a carbamoyl chloride across an alkyne.

Computationally deciphering palladium-catalyzed reaction mechanisms

Computational studies receive increased attention in the mechanistic exploration of transition metal catalyzed reactions. Especially in Pd catalysis, numerous mechanistic insights could be gained by the use of computational tools to complement experimental studies in order to provide a more detailed mechanistic picture. This includes not only the exploration of novel mechanistic scenarios, but also the comparison of different plausible reaction pathways. The current intense use of calculations in mechanistic studies of Pd‐catalyzed reactions has encouraged constant advancements in the field. However, a number of challenges will be faced in the computational treatment of Pd reactivities, including tackling conformational space, charged molecules, varying ligation states and the description of complexes bearing multiple Pd centers. Their careful consideration may enrich the mechanistic picture of numerous Pd‐catalyzed reactions, but may also encourage further development of computational methodology. WIREs Comput Mol Sci 2016, 6:226–242. doi: 10.1002/wcms.1244

Palladium(I) Dimer Enabled Extremely Rapid and Chemoselective Alkylation of Aryl Bromides over Triflates and Chlorides in Air

Abstract Disclosed herein is the first general chemo‐ and site‐selective alkylation of C−Br bonds in the presence of COTf, C−Cl and other potentially reactive functional groups, using the air‐, moisture‐, and thermally stable dinuclear PdI catalyst, [Pd(μ‐I)PtBu3]2. The bromo‐selectivity is independent of the substrate and the relative positioning of the competing reaction sites, and as such fully predictable. Primary and secondary alkyl chains were introduced with extremely high speed (<5 min reaction time) at room temperature and under open‐flask reaction conditions.

Chemoselective Pd‐Catalyzed C‐TeCF3 Coupling of Aryl Iodides

While the TeCF3 moiety features promising properties and potential in a range of applications, no direct synthetic method exists for its incorporation into aromatic scaffolds. This report features the first direct catalytic method for the formation of C(sp2 )-TeCF3 bonds. The method relies on a Pd/Xantphos catalytic system and allows for the trifluoromethyltellurolation of aryl iodides. Our computational and experimental mechanistic analyses shed light on the privileged activity of Xantphos in this transformation.