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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
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
Scheme 1. Aerobic Oxidative Esterification of Primary Alcohols
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
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
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
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
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.
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
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