Although nitrogen-coordinated complexes of pyridine are ubiquitous in organometallic chemistry, other coordination modes are substantially less common. Especially unusual are η2-(C,C)-coordinated complexes, with only a few examples found in the literature. Dihapto-coordination is of considerable interest, as this binding mode promotes addition reactions that yield dearomatized products. Past work has demonstrated extensive organic chemistry with a tungsten η2-N-acetylpyridinium complex. More limited chemistry with η2 -tungsten complexes of pyridines bearing electron-donating substituents has also been reported. This work explores the synthesis and organic reactivity of dihapto-bound pyridine complexes of a second-row metal, molybdenum, with the goal of producing substituted tetrahydropyridines via regio- and stereoselective additions. Given that molybdenum generally forms weaker bonds to arenes compared to analogous third-row dearomatization agents, it was uncertain whether such η2-pyridine complexes could be successfully isolated and made to undergo organic transformations. MoTp(NO)(DMAP)(η2-2-(trifluoromethyl)pyridine) was investigated, as little is known about the effect of an electron-withdrawing substituent on dihapto-coordinate pyridine reactivity. Methylation followed by treatment with nucleophiles, including Grignard reagents, organozinc compounds, hydrides, and cyanide, allows the formation of various substituted dihydropyridine complexes. In the presence of acid, some of these dihydropyridine complexes undergo a ring-opening reaction. Furthermore, the free dihydropyridines can be liberated from the metal by oxidation with ferrocenium salts, allowing access to non-aromatic derivatives difficult to prepare by traditional methods.

Molybdenum-promoted dearomatization of pyridines

2019 Spring National Meeting

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