The thermodynamically leveraged conversion of ethers and alcohols to saturated hydrocarbons is achieved efficiently with low loadings of homogeneous M(OTf)n + heterogeneous Pd tandem catalysts (M = transition metal; OTf = triflate; n = 4). For example, Hf(OTf)4 mediates rapid endothermic ether ⇌ alcohol and alcohol ⇌ alkene equilibria, while Pd/C catalyzes the subsequent, exothermic alkene hydrogenation. The relative C-O cleavage rates scale as 3° > 2° > 1°. The reaction scope extends to efficient conversion of biomass-derived ethers, such as THF derivatives, to the corresponding alkanes.

No abstract

Selective ether/ester C–O bond hydrogenolysis of an acetylated lignin model is achieved using a thermodynamically leveraged tandem catalytic strategy. Acetylation serves to (1) solubilize both lignin and lignin models and to (2) modify the reactivity of pendant hydroxy groups to promote more selective C–O cleavage.

Metal-catalyzed stereoselective reactions are a central theme in organic chemistry research. In these reactions, the stereoselection is achieved predominantly by introducing chiral ligands at the metal catalyst’s center. For decades, researchers have sought better chiral ligands for asymmetric catalysis and have made great progress. Nevertheless, to achieve optimal stereoselectivity and to catalyze new reactions, new chiral ligands are needed. Due to their high metal affinity, hydroxamic acids play major roles across a broad spectrum of fields from biochemistry to metal extraction. Dr. K. Barry Sharpless first revealed their potential as chiral ligands for asymmetric synthesis in 1977: He published the chiral vanadium-hydroxamic-acid-catalyzed, enantioselective epoxidation of allylic alcohols before his discovery of Sharpless Asymmetric Epoxidation, which uses titanium-tartrate complex as the chiral reagent. However, researchers have reported few highly enantioselective reactions using metal-hydroxamic acid as catalysts since then. This Account summarizes our research on metal-catalyzed asymmetric epoxidation using hydroxamic acids as chiral ligands. We designed and synthesized a series of new hydroxamic acids, most notably the C2-symmetric bis-hydroxamic acid (BHA) family. V-BHA-catalyzed epoxidation of allylic and homoallylic alcohols achieved higher activity and stereoselectivity than Sharpless Asymmetric Epoxidation in many cases. Changing the metal species led to a series of unprecedented asymmetric epoxidation reactions, such as (i) single olefins and sulfides with Mo-BHA, (ii) homoallylic and bishomoallylic alcohols with Zr- and Hf-BHA, and (iii) N-alkenyl sulfonamides and N-sulfonyl imines with Hf-BHA. These reactions produce uniquely functionalized chiral epoxides with good yields and enantioselectivities.

To reduce global reliance on fossil fuels, new renewable sources of energy that can be used with the current infrastructure are required. Biomass represents a major source of renewable carbon based fuel; however, the high oxygen content (∼40%) limits its use as a conventional fuel. To utilize biomass as an energy source, not only with current infrastructure, but for maximum energy return, the oxygen content must be reduced. One method to achieve this is to develop selective catalytic methods to cleave C-O bonds commonly found in biomass (aliphatic and aromatic ethers and esters) for the eventual removal of oxygen in the form of volatile H2O or carboxylic acids. Once selective methods of C-O cleavage are understood and perfected, application to processing real biomass feedstocks such as lignin can be undertaken. This Laboratory previously reported that recyclable "green" lanthanide triflates are excellent catalysts for C-O bond-forming hydroalkoxylation reactions. Based on the virtues of microscopic reversibility, the same lanthanide triflate catalyst should catalyze the reverse C-O cleavage process, retrohydroalkoxylation, to yield an alcohol and an alkene. However, ether C-O bond-forming (retrohydroalkoxylation) to form an alcohol and alkene is endothermic. Guided by quantum chemical analysis, our strategy is to couple endothermic, in tandem, ether C-O bond cleavage with exothermic alkene hydrogenation, thereby leveraging the combined catalytic cycles thermodynamically to form an overall energetically favorable C-O cleavage reaction. This Account reviews recent developments on thermodynamically leveraged tandem catalysis for ether and more recently, ester C-O bond cleavage undertaken at Northwestern University. First, the fundamentals of lanthanide-catalyzed hydroelementation are reviewed, with particular focus on ether C-O bond formation (hydroalkoxylation). Next, the reverse C-O cleavage/retrohydroalkoxylation processes enabled by tandem catalysis are discussed for both ether and ester C-O bond cleavage, including mechanistic and computational analysis. This is followed by recent results using this tandem catalytic strategy toward biomass relevant substrates, including work deconstructing acetylated lignin models, and the production of biodiesel from triglycerides, while bypassing the production of undesired glycerol for more valuable C3 products such as diesters (precursors to diols) in up to 47% selectivity. This Account concludes with future prospects for using this tandem catalytic system under real biomass processing conditions.