The ability of the tungsten dearomatization agent, {TpW(NO)(PMe3)}, to activate a dihapto-coordinated benzene ring towards addition reactions was investigated. In the past, the instability of the η2-benzene complex under acidic conditions has limited the ability to enact organic reactions. In comparison, the dihapto-benzene complexes of osmium and rhenium dearomatization agents undergo 1,4-addition reactions with a carbon electrophile followed by a nucleophile, resulting in cis-3,6-disubstituted 1,4-cyclohexadienes. Recently, it was discovered that protonation of TpW(NO)(PMe3)(η2-benzene) with a mild Brønsted acid at reduced temperatures, followed by nucleophilic addition, results in the selective formation of 1,3-diene complexes, with the metal still in conjugation to the uncoordinated double bond. In the case of benzene, with no substituents on the ring to influence the regioselectivity of addition reactions, the powerful influence of the asymmetric tungsten fragment is demonstrated by the formation of one major allyl complex (&gt;12:1) upon protonation. In situ addition of a nucleophile to the asymmetric benzenium complex leads to a 1,3-diene product (&gt;12:1), with the nucleophile adding selectively anti to the metal. This 1,2-addition reaction has been demonstrated with a range of nucleophiles, including protected enolates, amines, cyanide, and hydrides. Futhermore, the 1,3-diene products are susceptible to further tungsten-promoted addition reactions at the uncoordinated double bond. The range of reagents, as well as, the regioselectivity of the second tandem addition reaction is examined. These reactions represent the first examples of dihapto dearomatization of benzene where a total of four metal controlled addition reactions were realized. Additionally, this dearomatization chemistry provides new synthetic routes to quickly build up structural and functional complexity from a simple benzene ring.

Tungsten-promoted dearomatization of benzene: A regio and stereoselective route to functionalized cyclohexenes

255th National Meeting (2018)

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