Synthesis and Characterization of Ru(II) Complexes as Potential Catalysts in Olefin Hydroarylation

The production of alkyl arenes from benzene and simple olefins is a continually growing market. However, current acid base technologies (i.e., Friedel-Crafts & Zeolites) have significant drawbacks. For example, Friedel-Crafts alkylation of arenes exhibits extensive polyalkylation, and thus requires an additional high temperature transalkylation process to increase the selectivity for monoalkylated products. Furthermore, Friedel Crafts is selective for the Markovnikov addition products when alpha-olefins are employed. Additionally, the ability to recycle the catalyst is impossible due to degradation during product isolation. Our strategy is to use transition metal catalysts for olefin hydroarylation that proceeds via an alternative mechanism which combines both olefin insertion and C–H activation which could potentially overcome these challenges.
Extension to previous studies conducted by our group on a series of complexes with the motif TpRu(L)Ph(NCMe) [Tp = hydridotris(pyrazolyl)borate, L= neutral two electron donor] were investigated. These studies demonstrated that an electron poor metal center was needed to strike promote olefin insertion over olefin C–H activation. Therefore, the electron donating properties of where explored using cyclic voltammetry of a variety of Ru(II) complexes contain a wide range of phosphites and phosphines. It was determined that the metal center is less electron rich with than L = PMe3, P(OCH2)3CEt, PPh3, and P(OMe)3; however, the metal center is still more electron rich than when L = CO. The Ru(II) complex TpRu[ ](NCMe)Ph has been synthesized and isolated. This complex was shown to be both capable of activating C–H bonds and an active catalyst for ethylene hydrophenylation.
Additionally, rather than varying the electron density of the metal center via the neutral two electron ligand one could alter the electron density by replacing the anionic Tp ligand with neutral analogues, such as tri(pyrazolyl)alkanes. Complexes with tris(pyrazolyl)alkanes were synthesized, characterized and tested for olefin hydrophenylation the results of these experiments will be discussed herein.