Development and Understanding of Transition-Metal Mediated Conversions of Hydrocarbons to Higher Value Chemicals

The development of energy-efficient catalytic processes for the selective conversion of hydrocarbons to higher-value chemicals has been a topic of broad interest in both academia and industry. Achieving such a goal requires fundamental advancements in understanding reaction mechanisms, the effects of reaction conditions, structure-activity correlation based on catalytic identity, as well as probing new catalyst designs.
Hydrogen–deuterium exchange (H/D exchange) is a commonly used method for studying catalytic activation of C–H(D) bonds by transition metal complexes. A series of Lewis acid additives were studied for H/D exchange of toluene-d8 with weak acetic acid (HOAc) using (RPNP)Rh(X) complexes (R = phosphine substituents including cyclohexyl, isopropyl and tert-butyl; X = trifluoroacetate or acetate) as the pre-catalysts. Cu(II) and Ag(I) salts were found to benefit Rh-mediated C–H(D) activation of toluene with meta-para selectivity by facilitating the conversion to active (RPNP)Rh species and stabilizing the Rh catalysts from decomposition to inactive Rh(s). In contrast, non-oxidizing Lewis acid additives, such as B(OMe)3 or NaOAc, were not effective at facilitating Rh-catalyzed toluene C–H activation.
C–H acetoxylation was studied as potential route for conversion of benzene to phenol (after hydrolysis). Using an air-recyclable Cu(II) salt for benzene C–H acetoxylation could potentially lead to a coproduct-free synthetic route for phenol production. Since all of the coproducts are used for recycling of the Cu(II) salt, the overall reaction is the conversion of benzene and dioxygen to phenol. Simple Cu(II) carboxylate salts (e.g., Cu(OAc)2, Cu(OPiv)2, and Cu(OHex)2) were found to be active for C–H acetoxylation of non-activated arenes (i.e., benzene and toluene), as well as related functionalization with OPiv and OHex ester groups (OPiv = pivalate; OHex = 2-ethylhexanoate), with over 80% yield (based on Cu). Combined experimental and computational studies indicate that the arene C–H functionalization likely occurs by a non-radical Cu(II)-mediated organometallic pathway. The presence of water and/or dioxygen during the reaction was found to switch the reaction mechanism to radical pathways. The Cu(II) salts used in the reaction can be isolated, recycled, and reused with little change in reactivity. Also, the Cu(II) salts can be regenerated in situ using O2 and, after the removal of the generated water, the arene C–H acetoxylation and related esterification reactions can be continued.
Catalytic oligomerization for upgrading ethylene to higher olefins is an important commercial method for production of linear α-olefins. By using alkylaluminum(III) compounds or other Lewis acid additives, Ni(II) complexes of the type (RPBP)NiBr (R = tBu or Ph) show activity to produce butenes and higher olefins. (PhPBP)NiBr achieved an optimized turnover frequency (TOF) of 640 mol(ethylene)/{mol(Ni)·s} for the formation of butenes with 41(1)% selectivity for 1-butene. The complex (tBuPBP)NiBr provided a TOF of 68 mol(ethylene)/{mol(Ni)·s} for butenes production with 87.2(3)% selectivity for 1-butene. Using methylaluminoxane as co-catalyst and (tBuPBP)NiBr as the precatalyst, ethylene oligomerization to form C4 through C20 products was achieved while the use of (PhPBP)NiBr as the precatalyst retained selectivity for C4 products. Combined experimental and computational studies point to a mechanism that involves a cooperative B/Ni activation of ethylene to form a key 6-membered borametallacycle intermediate. Thus, a cooperative activation of ethylene by the Ni–B unit of the (RPBP)Ni catalysts is proposed as a key element of the Ni catalysis. In addition, under some conditions, the (RPBP)Ni catalysts can operate without solvent to upgrade ethylene in solid-gas phase reactions.
Previously, we have demonstrated the use of "capping arene" ligands to modulate stoichiometric and catalytic reactions, in which the reactivity can be tuned by adjusting the distance between metal center and the capping arene moiety. In recent efforts, as an extension of the “capping arene” ligands, we are seeking to use a Sb-moiety that can modulate electron density at the transition metal center as a function of the Sb oxidation state. Thus, the possible transformation of Sb–M bonds between L-, X- and Z-type bonding modes offers potential redox flexibility at the transition metal center that could influence the energetics of reactions that involve redox state changes at the transition metal. A series of Sb(III)- and Sb(V)-based preligands with L-type 8-quinolinyl donors have been synthesized and used for coordinate to late transition-metals such as Rh, Pt, Pd, Ir, Ru, Ag, and Cu. The solid-state structure of some complexes was confirmed via X-ray crystallography, which included an example of Z-type complex with a Pt→Sb interaction. One example of the Rh–Sb complexes was found to be active for H/D exchange of benzene-d6 with HOAc, and slow aerobic benzene alkenylation with propylene was observed.