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The Activating Effect of Strong Acid for Pd-Catalyzed Directed C-H Activation by Concerted Metalation-Deprotonation Mechanism.

ABSTRACT: A computational study on the origin of the activating effect for Pd-catalyzed directed C-H activation by the concerted metalation-deprotonation (CMD) mechanism is conducted. DFT calculations indicate that strong acids can make Pd catalysts coordinate with directing groups (DGs) of the substrates more strongly and lower the C-H activation energy barrier. For the CMD mechanism, the electrophilicity of the Pd center and the basicity of the corresponding acid ligand for deprotonating the C-H bond are vital to the overall C-H activation energy barrier. Furthermore, this rule might disclose the role of some additives for C-H activation.

SUBMITTER: Jiang H

PROVIDER: S-EPMC8271562 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Oligothiophene Synthesis by a General C-H Activation Mechanism: Electrophilic Concerted Metalation-Deprotonation (eCMD).

Project description: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(CH2)3SO3Na, 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 ?-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.

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C–H bond activation via concerted metalation–deprotonation at a palladium(iii) center† † Electronic supplementary information (ESI) available. CCDC 2129670–2129674. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc00034f

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