Electrophilic Metal Complexes in Bond Activations and Catalysis

Studies of electrophilic of late metal (e.g., platinum) complexes focus on C-H and N-H bond activations, particularly in the context of catalytic hydroarylation and hydroamination. For example, platinum complexes containing anionic analogs of bipyridine (e.g., 3,5-diphenyl-2-(2-pyridyl)pyrrolide, PyPyr) were used to explore C-N bond formation via couplings of amido and olefinic groups at platinum. Observed reactions proceed via external nucleophilic attack of amides onto a coordinated ethylene.1 In a collaboration with Professor Alex Bell in Chemical Engineering, mechanistic studies on hydroaminations of olefins with sulfonamides targeted a series of platinum bis(triflate) catalysts of the type L2Pt(OTf)2. Kinetic and mechanistic experiments suggest that catalytic hydroamination occurs by formation of a highly acidic species of the type [L2Pt(NH2R)(OTf)]+OTf+, which then undergoes proton transfer to a coordinated olefin. This work has broad implications for interpretation of results from catalytic reactions involving bond activations mediated by electrophilic, late metal complexes.2

Recent Projects

A Bergman-Tilley collaboration demonstrated that similar Pt(II) hydroarylation catalysts operate via a proton-transfer mechanism.3 However, as discussed in our recent review,4 hydroarylation catalyzed by d8 metal complexes of this type engage in a variety of mechanistic pathways. For example, platinum complexes (PyInd)PtPh(SR2) (PyInd = 2,2′-pyridyl-indolate) mediate hydroarylation via olefin insertion into a Pt–C bond; a mechansitic investigation from this laboratory revealed key details on this process and competing (off-cycle) reaction steps.5 

In one aspect of this research, the highly strained spiro-hydrocarbon substrate 1 (below), initially designed as a mechanistic probe for hydroarylation, was observed to undergo catalytic transformation mediated by (Me2bpy)PtPh(NTf2), to generate a pentahydroindene. This unusual catalytic mechanism involves an initial, masked C–H bond activation which initiates a series of subsequent C–C bond cleavage events.6 

References

1. "Nucleophilic Attack of Amides onto Coordinated Ethylene in Platinum Complexes Supported by a Chelating Pyridyl-Pyrrolide Ligand: Azaplatinacyclobutane and Vinylamine Complexes." J. L. McBee and T. D. Tilley. Organometallics 2010, 29, 184-192. DOI: 10.1021/om900569b

2. "Mechanistic Studies of Hydroamination of Norbornene with Electrophilic Platinum Complexes: The Role of Proton Transfer." J. L. McBee, A. T. Bell and T. D. Tilley. J. Am. Chem. Soc. 2008, 130, 16562-16571. DOI: 10.1021/ja8030104

3. "Disambiguation of Metal and Bronsted Acid Catalyzed Pathways for Hydroarylation with a Platinum(II) Catalyst." M. Bowring, R. G. Bergman and T. D. Tilley, Organometallics 2011, 30, 1295-1298. DOI: 10.1021/om2000458

4. "Hydroarylation of Olefins with Complexes Bearing d8 Metal Centers." B. A. Suslick and T. D. Tilley, Chapter 4 in Catalytic Hydroarylation of Carbon-Carbon Multiple Bonds, T. B. Gunnoe, L. Ackerman and L. G. Habgood, Eds., Wiley-VCH, 2017. DOI: 10.1002/9783527697649.ch4 ISBN: 978-3-527-34013-2. http://www.wiley.com/WileyCDA/WileyTitle/productCd-3527340130.html

5. "Olefin Hydroarylation Catalyzed by (Pyridyl-Indolate)Pt(II) Complexes: Catalytic Efficiencies and Mechanistic Aspects." B. A. Suslick, A. L. Liberman-Martin, T. C. Wambach and T. D. Tilley, ACS Catal. 2017, 7, 4313-4322. DOI: 10.1021/acscatal.7b01560

6. "Platinum-Catalyzed C-C Activation Induced by C-H Activation." M. Bowring, R. G. Bergman and T. D. Tilley, J. Am. Chem. Soc. 2013, 135, 13121-13128. DOI: 10.1021/ja406260j