50 - New Catalysts for Efficient Use of Fossil Resources: A New Strategy for the Preparation of Alkyl and Vinyl Arenes
University of Virginia
Alkyl arenes, such as ethylbenzene, are important chemical precursors that are produced by hydroarylation of olefins (that is, the addition of an aromatic C_H bond across an olefin double bond). The starting materials for ethylbenzene synthesis, benzene and ethylene, are inert and require a catalyst to form ethylbenzene with sufficient rates. Current industrial methods to synthesize this alkyl arenes are energy intensive, exhibit poor selectivity and generate chemical waste. An alternative strategy involves the use of molecular transition metal complexes that can be rationally tuned to achieve desired selectivity. Cationic platinum complexes supported by 4,4'-disubstituted-2,2'-bipyridyl (bpy) ligands have been shown to be effective catalysts for olefin hydroarylation. Increased donor ability of 4,4'-disubstituted bpy ligands increases the selectivity for ethylbenzene over the production of styrene as well as catalyst longevity. Thus, we sought to prepare platinum complexes that are supported by N-heterocyclic carbene ligands, which are known to be strongly donating. Complexes such as [(IMes*)2Pt(Me)][BAr'4] (IMes = 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene, Ar' = 3,5-bis(trifluoromethyl)phenyl), [(DtBuC)Pt(Ph)(NCMe)][BAr'4] (DtBuC = 1,1'-methylene-3,3'-di-tert-butylimidazol-2,2'-diylidine), and [(IPr)Pt(I)(€-I)]2 (IPr = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene) were isolated as catalyst precursors. Surprisingly, these complexes have been found to favor styrene production. It is proposed that styrene is produced via a ?-hydride elimination mechanism, which results in a Pt_H complex that leads to catalyst decomposition.