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

preprint
revised on 05.01.2019, 17:38 and posted on 05.01.2019, 21:05 by Long Wang, Brad Carrow
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 classic concerted metalation-deprotonation (CMD) model. Enhanced positive charge build-up on the substrate results from this perturbation, which rationalizes experimental trends strongly favoring π-basic sites. The term electrophilic CMD (eCMD) is introduced to distinguish this mechanism. 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.

Funding

Aliphatic Effects in Transition Metal Catalysis

National Institute of General Medical Sciences

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History

Email Address of Submitting Author

bcarrow@princeton.edu

Institution

Princeton University

Country

USA

ORCID For Submitting Author

0000-0003-4929-8074

Declaration of Conflict of Interest

We declare no competing financial interests.

Version Notes

eCMD_Final_v2 : Corrected a file corruption that caused deletion of pages 9–11 during conversion from Microsoft Word to pdf file format.

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