title: Experimental and DFT Studies Explain Solvent Control of C-H Activation and Product Selectivity in the Rh(III)-Catalyzed Formation of Neutral and Cationic Heterocycles.
abstract: A range of novel heterocyclic cations have been synthesized by the Rh(III)-catalyzed oxidative C-N and C-C coupling of 1-phenylpyrazole, 2-phenylpyridine, and 2-vinylpyridine with alkynes (4-octyne and diphenylacetylene). The reactions proceed via initial C-H activation, alkyne insertion, and reductive coupling, and all three of these steps are sensitive to the substrates involved and the reaction conditions. Density functional theory (DFT) calculations show that C-H activation can proceed via a heteroatom-directed process that involves displacement of acetate by the neutral substrate to form charged intermediates. This step (which leads to cationic C-N coupled products) is therefore favored by more polar solvents. An alternative non-directed C-H activation is also possible that does not involve acetate displacement and so becomes favored in low polarity solvents, leading to C-C coupled products. Alkyne insertion is generally more favorable for diphenylacetylene over 4-octyne, but the reverse is true of the reductive coupling step. The diphenylacetylene moiety can also stabilize unsaturated seven-membered rhodacycle intermediates through extra interaction with one of the Ph substituents. With 1-phenylpyrazole this effect is sufficient to suppress the final C-N reductive coupling. A comparison of a series of seven-membered rhodacycles indicates the barrier to coupling is highly sensitive to the two groups involved and follows the trend C-N(+) > C-N > C-C (i.e., involving the formation of cationic C-N, neutral C-N, and neutral C-C coupled products, respectively).
authors.affiliation: †Department of Chemistry, University of Leicester, Leicester, LE1 7RH, United Kingdom.
authors.forename: David L
authors.initials: DL
authors.lastname: Davies
authors.affiliation: †Department of Chemistry, University of Leicester, Leicester, LE1 7RH, United Kingdom.
authors.forename: Charles E
authors.initials: CE
authors.lastname: Ellul
authors.affiliation: ‡Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, United Kingdom.
authors.forename: Stuart A
authors.initials: SA
authors.lastname: Macgregor
authors.affiliation: ‡Institute of Chemical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, United Kingdom.
authors.forename: Claire L
authors.initials: CL
authors.lastname: McMullin
authors.affiliation: †Department of Chemistry, University of Leicester, Leicester, LE1 7RH, United Kingdom.
authors.forename: Kuldip
authors.initials: K
authors.lastname: Singh
mesh_terms: D002384:Catalysis; D002412:Cations / Q000138:chemical synthesis / Q000737:chemistry; D006571:Heterocyclic Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D011789:Quantum Theory; D012238:Rhodium / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry
chemical_list: D002412:Cations; D006571:Heterocyclic Compounds; D009942:Organometallic Compounds; D012997:Solvents; D012238:Rhodium