Experimental and DFT Studies Explain Solvent Control of C鈥揌 Activation and Product Selectivity in the Rh(III)-Catalyzed Formation of Neutral and Cationic Heterocycles

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• 作者：David L. Davies ; Charles E. Ellul ; Stuart A. Macgregor ; Claire L. McMullin ; Kuldip Singh
• 刊名：Journal of the American Chemical Society
• 出版年：2015
• 出版时间：August 5, 2015
• 年：2015
• 卷：137
• 期：30
• 页码：9659-9669
• 全文大小：648K
• ISSN：1520-5126

文摘

A range of novel heterocyclic cations have been synthesized by the Rh(III)-catalyzed oxidative C鈥揘 and C鈥揅 coupling of 1-phenylpyrazole, 2-phenylpyridine, and 2-vinylpyridine with alkynes (4-octyne and diphenylacetylene). The reactions proceed via initial C鈥揌 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鈥揌 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鈥揘 coupled products) is therefore favored by more polar solvents. An alternative non-directed C鈥揌 activation is also possible that does not involve acetate displacement and so becomes favored in low polarity solvents, leading to C鈥揅 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鈥揘 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鈥揘⁺ > C鈥揘 > C鈥揅 (i.e., involving the formation of cationic C鈥揘, neutral C鈥揘, and neutral C鈥揅 coupled products, respectively).