Title: Multiple Mechanisms Mapped in Aryl Alkyl Ether Cleavage via Aqueous Electrocatalytic Hydrogenation over Skeletal Nickel

Abstract

We present here detailed mechanistic studies of electrocatalytic hydrogenation (ECH) in aqueous solution over skeletal nickel cathodes to probe the various paths of reductive catalytic C–O bond cleavage among functionalized aryl ethers relevant to energy science. Heterogeneous catalytic hydrogenolysis of aryl ethers is important both in hydrodeoxygenation of fossil fuels and in upgrading of lignin from biomass. The presence or absence of simple functionalities such as carbonyl, hydroxyl, methyl, or methoxyl groups is known to cause dramatic shifts in reactivity and cleavage selectivity between sp3 C–O and sp2 C–O bonds. Specifically, reported hydrogenolysis studies with Ni and other catalysts have hinted at different cleavage mechanisms for the C–O ether bonds in α-keto and α-hydroxy β-O-4 type aryl ether linkages of lignin. Our new rate, selectivity, and isotopic labeling results from ECH reactions confirm that these aryl ethers undergo C–O cleavage via distinct paths. For the simple 2-phenoxy-1-phenylethane or its alcohol congener, 2-phenoxy-1-phenylethanol, the benzylic site is activated via Ni C–H insertion, followed by beta elimination of the phenoxide leaving group. But in the case of the ketone, 2-phenoxyacetophenone, the polarized carbonyl π system apparently binds directly with the electron rich Ni cathode surface without breaking the aromaticity of the neighboringmore » phenyl ring, leading to rapid cleavage. Substituent steric and electronic perturbations across a broad range of β-O-4 type ethers create a hierarchy of cleavage rates that supports these mechanistic ideas while offering guidance to allow rational design of the catalytic method. Furthermore, on the basis of the new insights, the usage of cosolvent acetone is shown to enable control of product selectivity.« less

Authors:

Zhou, Yuting  ^[1];

• Search DOE PAGES for author "Zhou, Yuting"
• Search DOE PAGES for ORCID "0000-0002-8751-9686"

Klinger, Grace E. ^[1]; Hegg, Eric L.  ^[1];

• Search DOE PAGES for author "Hegg, Eric L."
• Search DOE PAGES for ORCID "0000-0003-2055-5495"

Saffron, Christopher M. ^[1]; Jackson, James E.  ^[1]

• Search DOE PAGES for author "Jackson, James E."
• Search DOE PAGES for ORCID "0000-0002-4506-7415"

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+ Show Author Affiliations

1. Michigan State Univ., East Lansing, MI (United States)

Publication Date:

Tue Feb 04 00:00:00 EST 2020

Research Org.:

Great Lakes Bioenergy Research Center (GLBRC), Madison, WI (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Biological and Environmental Research (BER); National Science Foundation (NSF)

OSTI Identifier:

1637672

Grant/Contract Number:  

SC0018409; FC02-07ER64494; 1603347

Resource Type:

Accepted Manuscript

Journal Name:

Journal of the American Chemical Society

Additional Journal Information:

Journal Volume: 142; Journal Issue: 8; Journal ID: ISSN 0002-7863

Publisher:

American Chemical Society (ACS)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; bond cleavage; ethers; alcohols; crystal cleavage; ketones