Mechanism and Site Selectivity in Visible-Light Photocatalyzed C–H Functionalization: Insights from DFT Calculations

Visible-light photocatalyzed (VLPC) late-stage C–H functionalization is a powerful addition to the chemical synthesis toolkit. VLPC has a demonstrated potential for discovery of elusive and valuable transformations, particularly in functionalization of bioactive heterocycles. In order to fully harvest the potential of VLPC in the context of complex molecule synthesis, a thorough understanding of the elementary processes involved is crucial. This would enable more rational design of suitable reagents and catalysts, as well as prediction of activated C–H sites for functionalization. Such knowledge is essential when VLPC is to be employed in retrosynthetic analysis of complex molecules. Herein, we present a density functional theory (DFT) study of mechanistic details in the C–H functionalization of bioactive heterocycles exemplified by the methylation of the antifungal agent voriconazole. Moreover, we show that readily computed atomic charges can predict major site-selectivity in good agreement with experimental studies and thus be informative tools for the identification of active C–H functionalization sites in synthetic planning

Cryptophanes for Methane and Xenon Encapsulation: A Comparative Density Functional Theory Study of Binding Properties and NMR Chemical Shifts

The host–guest chemistry of cryptophanes is an active research area because of its applications in sensor design, targeting small molecules and atoms in environmental and medical sciences. As such, the computational prediction of binding energies and nuclear magnetic resonance (NMR) properties of different cryptophane complexes are of interest to both theoreticians and experimentalists working in host–guest based sensor development. Herein we present a study of 10 known and some newly proposed cryptophanes using density functional theory (DFT) calculations. We benchmark the description of nonbonding interactions by different DFT functionals against spin-component-scaled, second-order Møller–Plesset theory (SCS-MP2) and predict novel host molecules with enhanced affinity toward methane and Xenon, two representative systems of high interest. We demonstrate the power and limitations of the different computational methods in describing the binding and NMR properties of these established and novel host systems. The results show the importance of including dispersion corrections in the DFT functionals. The overall analysis of the dispersion corrections indicated that results obtained from pure DFT functionals should be used cautiously when conclusions are drawn for molecular systems with considerable weak interactions. Proposed analogues of cryptophane-A, where the alkoxy bridges are replaced by alkyl chains, are predicted to display enhanced affinity toward both methane and Xenon

DFT as a Powerful Predictive Tool in Photoredox Catalysis: Redox Potentials and Mechanistic Analysis

Visible-light photoredox catalysis has come forth as a powerful activation mode in chemical synthesis, affording the development of a multitude of new strategies for molecular construction. However, detailed mechanistic knowledge of the various subprocesses involved is lacking, and new tools for addressing this are needed to drive innovation forward in the area. Herein, we describe predictions of ground- and excited-state redox potentials of ruthenium and iridium photocatalysts using nonrelativistic and scalar relativistic zero-order regular approximation density functional theory (DFT) methods. The computed redox potentials were correlated with experimental values and found to reproduce them well. Relativistic corrections were found to be important to reproduce experimental data. Moreover, the computational protocol allows us to estimate redox potentials that are not currently available in the literature or are difficult to determine experimentally. The mechanistic details of the photocatalyzed C–H functionalization of 1-methylindole with diethyl bromomalonate were also studied using the validated DFT method. We demonstrate how DFT can predict the experimentally observed redox behavior of common photocatalysts and mechanistic details of the C–H functionalization process. This work demonstrates that DFT can be a powerful tool for innovation and design in the field of visible-light photoredox catalysis by predicting redox properties and mechanistic behavior

Metal-Free N–H Insertions of Donor/Acceptor Carbenes

Synthetically useful transformations arise from the thermal decomposition of aryldiazoacetates in the presence of primary and secondary amines without the use of a metal catalyst. Thermally generated, free donor/acceptor carbenes directly undergo N–H insertion with amines through selective aza-ylide formation to afford a variety of α-amino esters in 53–96% yields

Scope and Mechanistic Analysis of the Enantioselective Synthesis of Allenes by Rhodium-Catalyzed Tandem Ylide Formation/[2,3]-Sigmatropic Rearrangement between Donor/Acceptor Carbenoids and Propargylic Alcohols

Rhodium-catalyzed reactions of tertiary propargylic alcohols with methyl aryl- and styryldiazoacetates result in tandem reactions, consisting of oxonium ylide formation followed by [2,3]-sigmatropic rearrangement. This process competes favorably with the standard O–H insertion reaction of carbenoids. The resulting allenes are produced with high enantioselectivity (88–98% ee) when the reaction is catalyzed by the dirhodium tetraprolinate complex, Rh₂(<i>S</i>-DOSP)₄. Kinetic resolution is possible when racemic tertiary propargylic alcohols are used as substrates. Under the kinetic resolution conditions, the allenes are formed with good diastereoselectivity and enantioselectivity (up to 6.1:1 dr, 88–93% ee), and the unreacted alcohols are enantioenriched to 65–95% ee. Computational studies reveal that the high asymmetric induction is obtained via an organized transition state involving a two-point attachment: ylide formation between the alcohol oxygen and the carbenoid and hydrogen bonding of the alcohol to a carboxylate ligand. The 2,3-sigmatropic rearrangement proceeds through initial cleavage of the O–H bond to generate an intermediate with close-lying open-shell singlet, triplet, and closed-shell singlet electronic states. This intermediate would have significant diradical character, which is consistent with the observation that the 2,3-sigmatropic rearrangement is favored with donor/acceptor carbenoids and more highly functionalized propargylic alcohols

Scope and Mechanistic Analysis of the Enantioselective Synthesis of Allenes by Rhodium-Catalyzed Tandem Ylide Formation/[2,3]-Sigmatropic Rearrangement between Donor/Acceptor Carbenoids and Propargylic Alcohols

Rhodium-catalyzed reactions of tertiary propargylic alcohols with methyl aryl- and styryldiazoacetates result in tandem reactions, consisting of oxonium ylide formation followed by [2,3]-sigmatropic rearrangement. This process competes favorably with the standard O–H insertion reaction of carbenoids. The resulting allenes are produced with high enantioselectivity (88–98% ee) when the reaction is catalyzed by the dirhodium tetraprolinate complex, Rh₂(<i>S</i>-DOSP)₄. Kinetic resolution is possible when racemic tertiary propargylic alcohols are used as substrates. Under the kinetic resolution conditions, the allenes are formed with good diastereoselectivity and enantioselectivity (up to 6.1:1 dr, 88–93% ee), and the unreacted alcohols are enantioenriched to 65–95% ee. Computational studies reveal that the high asymmetric induction is obtained via an organized transition state involving a two-point attachment: ylide formation between the alcohol oxygen and the carbenoid and hydrogen bonding of the alcohol to a carboxylate ligand. The 2,3-sigmatropic rearrangement proceeds through initial cleavage of the O–H bond to generate an intermediate with close-lying open-shell singlet, triplet, and closed-shell singlet electronic states. This intermediate would have significant diradical character, which is consistent with the observation that the 2,3-sigmatropic rearrangement is favored with donor/acceptor carbenoids and more highly functionalized propargylic alcohols

<i>D</i>₂-Symmetric Dirhodium Catalyst Derived from a 1,2,2-Triarylcyclopropanecarboxylate Ligand: Design, Synthesis and Application

Dirhodium tetrakis-(<i>R</i>)-(1-(4-bromophenyl)-2,2-diphenylcyclopropanecarboxylate) (Rh₂(<i>R</i>-BTPCP)₄) was found to be an effective chiral catalyst for enantioselective reactions of aryl- and styryldiazoacetates. Highly enantioselective cyclopropanations, tandem cyclopropanation/Cope rearrangements and a combined C–H functionalization/Cope rearrangement were achieved using Rh₂(<i>R</i>-BTPCP)₄ as catalyst. The advantages of Rh₂(<i>R</i>-BTPCP)₄ include its ease of synthesis, its tolerance to the size of the ester group in the styryldiazoacetates, and its compatibility with dichloromethane as solvent. Computational studies suggest that the catalyst adopts a <i>D</i>₂-symmetric arrangement, but when the carbenoid binds to the catalyst, two of the <i>p</i>-bromophenyl groups on the ligands rotate outward to make room for the carbenoid and the approach of the substrate to the carbenoid

<i>D</i>₂-Symmetric Dirhodium Catalyst Derived from a 1,2,2-Triarylcyclopropanecarboxylate Ligand: Design, Synthesis and Application

Dirhodium tetrakis-(<i>R</i>)-(1-(4-bromophenyl)-2,2-diphenylcyclopropanecarboxylate) (Rh₂(<i>R</i>-BTPCP)₄) was found to be an effective chiral catalyst for enantioselective reactions of aryl- and styryldiazoacetates. Highly enantioselective cyclopropanations, tandem cyclopropanation/Cope rearrangements and a combined C–H functionalization/Cope rearrangement were achieved using Rh₂(<i>R</i>-BTPCP)₄ as catalyst. The advantages of Rh₂(<i>R</i>-BTPCP)₄ include its ease of synthesis, its tolerance to the size of the ester group in the styryldiazoacetates, and its compatibility with dichloromethane as solvent. Computational studies suggest that the catalyst adopts a <i>D</i>₂-symmetric arrangement, but when the carbenoid binds to the catalyst, two of the <i>p</i>-bromophenyl groups on the ligands rotate outward to make room for the carbenoid and the approach of the substrate to the carbenoid