The Development of PtII Complexes as Catalyst Precursors for Transition Metal Mediated Olefin Hydroarylation

Olefin hydroarylation is an important C−C bond forming reaction for the synthesis of alkyl arenes. The transformation is traditionally catalyzed using acid-based methodologies, which suffer several deficiencies inherent to the reaction mechanism. Selective transition metal catalysts for olefin hydroarylation are potentially viable alternatives. However, such a process has been difficult to realize as catalysts for olefin hydroarylation via a non-acidic pathway must be able to mediate two fundamentally different reactions in the catalytic cycle, olefin insertion into M−Ar bonds and aromatic C−H activation. While several examples of transition metal mediated olefin hydroarylation have been realized for activated substrates, such as substituted olefins and arenes possessing heteroatomic functionality, the synthesis of alkyl arenes from unactivated substrates (e.g., ethylene and benzene) are more scarce. Current examples based on RuII and IrIII have displayed marginal efficiency for catalytic olefin hydroarylation using unactivated substrates, but relatively rapid decomposition and reduced activity with substituted substrates such as alpha-olefins limit a broader application of this methodology.
Extension of catalytic olefin hydroarylation to PtII catalyst precursors has been investigated. Initial studies focus on formally cationic bipyridyl ligated PtII complexes [(tbpy)Pt(Ph)(L)][BAr'4] (tbpy = 4,4′-tBu2-2,2′-bipyridyl; L = THF, NC5F5 or NCMe; Ar′ = 3,5-(CF3)2-C6H3). These complexes were found to be active for catalytic hydrophenylation of ethylene, and mechanistic evidence presented supports an operative pathway that incorporates ethylene insertion and benzene C−H activation into a single catalytic cycle.
The bipyridyl scaffold presents an excellent opportunity to develop catalyst structure/activity relationships for future catalyst design. Electronic effects on catalyst activity and selectivity have been isolated with negligible influence on the catalyst steric profile by incorporating various functionality in the 4,4′-positions of bipyridine. Moreover, the consequence of steric perturbations have been evaluated through modulating the dipyridyl chelate ring size and incorporating steric hinderance in the 6,6′-positions of the pyridyl rings.
In addition, compatibility with functionalized substrates with bipyridyl based catalyst precursors and ligand sets outside the dipyridyl motif have been investigated for efficacy.