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Research Summary (Adam B. Powell)
My primary research as a postdoctoral fellow has focused on heterogeneous catalyst devel...
Next the substrate scope of this catalyst
was examined (Table 1). A variety of activated
(benzylic/allylic) alcohols were ...
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Postdoctoral Research Summary

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Postdoctoral Research Summary

  1. 1. Research Summary (Adam B. Powell) My primary research as a postdoctoral fellow has focused on heterogeneous catalyst development and methodology. Specifically, I have discovered a bench-friendly oxidative esterification catalyst composed of heterogeneous palladium and main-group element promoters (Scheme 1). Esters are widely utilized as flavoring agents, medicines and renewable fuels. Reported catalysts for this transformation are limited to homogeneous catalysts with high Pd or Ru loadings and activated substrates (i.e. benzylic and allylic alcohols). Therefore, to lower catalyst loading and increase scope we sought to develop a heterogeneous catalyst capable of accessing aliphatic (in addition to benzylic and allylic) methyl esters. Previous work by the Kimura group highlighted that pre-made catalysts composed of Pd and various combinations of main-group additives, including selenium, tellurium, lead, bismuth, and antimony could promote the aerobic oxidation of aliphatic alcohols to carboxylic acids.1 I discovered that the addition of similar additives including tellurium metal (Te) or bismuth nitrate (Bi(NO3)3) to commercially-available Pd catalysts could significantly increase formation of methyl ester product. (Figure 1). Interestingly, combination of Bi(NO3)3 and Te additives with Pd/Charcoal (Char) accessed a highly active oxidative esterification catalyst. Control experiments confirm the necessity of the all catalyst components (including O2). Next we sought to better understand the behavior of these additives by monitoring the progress of the reaction. A time course that tracks the oxidative esterification of 1-octanol with MeOH was acquired in the absence (Figure 2a) and presence of the bismuth and tellurium additives (Figure 2b). Without these additives, product formation reached 20 % after 2 h and slowly increased over a 12 h period to a maximum of 40 %. With these additives, however, methyl octanoate was achieved in 50 % yield after 1 h with 80 % yield obtained after 4 h. A maximum yield of 90 % was obtained after 12 h with excellent mass balance retained with throughout the course of the reaction. The timecourses indicate that the reaction progress ceases around 8-12 h. For 1-octanol, the additives clearly increased both the rate of product formation and overall yield of the transformation. Scheme 1. Aerobic Oxidative Esterification of Primary Alcohols with Methanol Figure 1. Effect of Additives for Pd-Cat Oxidative Esterification Figure 2. Time Courses of Pd/Char-Catalyzed Esterification of 1- Octanol with (a) no additives (b) 5 mol % Bi, 2.5 mol % Te (a) Without Bi(NO3)3/Te (b) With Bi(NO3)3/Te
  2. 2. Next the substrate scope of this catalyst was examined (Table 1). A variety of activated (benzylic/allylic) alcohols were subjected to the optimized catalyst conditions. High yields of methyl esters were obtained for a variety of para-substituted benzyl alcohols as well as and electron-deficient substrates. Nitro and olefin functional groups are known to undergo transfer hydrogenation with Pd0 catalysts, but reasonable yields could be obtained by lowering the reaction temperature to 25 °C. In addition to 1-octanol, other aliphatic alcohols with pyridyl groups, ethers, protected secondary amines or even protected primary amines were oxidatively esterified in high yields. Several of the substrates in Table 1 are inaccessible by reported methods and the scope afforded by this new catalyst vastly expands the utility of this method relative to earlier precedents. Future directions for this project include adapting this catalyst for other oxidative transformations, as well as designing a robust palladium- bismuth-tellurium catalyst that can be implemented in flow-based applications. Other postdoctoral work centered on the synthesis of catalytically relevant ruthenium complexes that can be tethered to conductively-doped diamond surfaces. In solution, these ruthenium complexes have been reported to perform selective epoxidations2 , but high catalyst loadings and low turnover frequencies have restricted their use. Attaching the catalyst to an electrode surface could allow for much lower catalyst loadings, simplify electron-transfer kinetics and facilitate recycling. Published examples of electrode-tethered metal complexes, however, are limited to catalytically-inert architectures such as ferrocene.3 The synthesis of this tetherable ruthenium epoxidation catalyst was accomplished, followed by attachment via click coupling (Scheme 2). The tethered complex was fully characterized and efforts to perform electrocatalysis are currently underway. My primary Ph.D dissertation research focused on the construction of polymeric materials for electrochromic applications. Polymeric frameworks are believed to increase the stability/longevity of electroactive N-heterocyclic carbenes (NHCs) metal complexes, but previous methods utilized the NHC in forming the polymer chain. This prevented transitional metal binding and thus rendered them electrochromically inactive. I designed a polymerizable architecture that positioned the NHCs orthogonal to bis(bithiophene) substituents making them available for metal binding and subsequent polymerization (Scheme 3). In contrast to previous methods a variety of metals including silver, gold and iridium could then be rapidly integrated in high yields into the main-chain scaffold. The identity of the attached metal was found to modulate the electrochromic activity of these materials. References 1) Kimura, H.; Kimura, A.; Kokubo, I.; Wakisaka, T.; Mitsuda, Y. Appl. Catal. A:Gen. 1993, 95, 143-169. 2) Dakkach, M.; Fontrodona, X.; Parella, T.; Atlamsani, A; Romero, I.; Rodriguez, M. Adv. Synth. Catal. 2011, 353, 231-238. 3) Devadoss, A.; Chidsey, C. J. Am. Chem. Soc. 2007, 129, 5370-5371. Table 1. Abbreviated Scope of the Pd/Char, Bi(NO3)3, Te Oxidative Esterification Catalyst Systema a Reaction carried out on 1 mmol scale and 1 M concentration. All yields isolated. b Reaction run at 25 °C. Scheme 3. Electropolymerization of Bis(bithiophene) Metal NHCs Scheme 2. Click Coupling of Ruthenium NHC Complex to Diamond Electrode Surface