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

2019-01-05T21:05:02Z (GMT) 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(CH<sub>2</sub>)<sub>3</sub>SO<sub>3</sub>Na, 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 <i>electrophilic</i> CMD (<i>e</i>CMD) is introduced to distinguish this mechanism. More O'Ferrall-Jencks analysis further suggests <i>e</i>CMD should be a general mechanism manifested by many metal complexes. A preliminary classification of complexes into those favoring <i>e</i>CMD or standard CMD is proposed, which should be informative for studies toward tunable catalyst-controlled reactivity.