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Selective Dehydrogenative Coupling of Ethylene to Butadiene via an Iridacyclopentane Complex Yang Gao, Thomas J Emge, Karsten Krogh-Jespersen, and Alan S. Goldman J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11689 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018
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Journal of the American Chemical Society
Selective Dehydrogenative Coupling of Ethylene to Butadiene via an Iridacyclopentane Complex Yang Gao, Thomas J. Emge, Karsten Krogh-Jespersen* and Alan S. Goldman* Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903, United States Supporting Information Placeholder ABSTRACT: An iridium complex is found to catalyze the selective dehydrogenative coupling of ethylene to 1,3-butadiene. The key intermediate, and a major resting state, is an iridacyclopentane that undergoes a surprisingly facile β-H elimination, enabled 3 2 by a partial dechelation (κ -κ ) of the supporting Phebox ligand.
■ INTRODUCTION 1,3-Butadiene is a "platform chemical", with ca. 10 M tons per annum produced for the manufacture of rubbers, polymers 1 and chemicals. Until recently demand for butadiene was largely met through its production as a side-product from the cracking of naphtha, driven primarily by demand for 2 ethylene. The recent abundance of ethane-rich shale gas, however, has shifted the production of ethylene toward the cracking of ethane; this has led to tightening supplies of butadiene while demand continues to increase with growth of the global economy. As a result there is renewed interest in the development of methods for the on-purpose production 3 of butadiene from inexpensive feedstock. On the supply side of this calculus, the same abundance of ethane that has led to decreased butadiene production from naphtha makes ethylene an attractive potential feedstock for butadiene.
of C4 products (entry 1, Table 1) as determined by gas chromatography. (Phebox)Ir(C2H4)2
3
(1)
When an identical solution was heated for a longer time (16 h) the yield of butadiene was higher but the amount of butene as a percentage of C4 products was significantly greater at 21% (entry 2, Table 1). The latter observation indicates that the butadiene observed at the shorter reaction time was not a secondary product formed by the dehydrogenation of butene; it is instead consistent with the converse possibility that the butenes observed are formed as secondary products from butadiene hydrogenation. Table 1. Dehydrogenative Ethylene Coupling Catalyzed a by 1
Herein we report the discovery of a catalyst for the selective formation of butadiene from ethylene, a reaction with little if any precedent. We show that this dehydrogenative coupling proceeds via an iridacyclopentane intermediate that undergoes β-H elimination, a reaction step which also has limited precedent. ■ RESULTS AND DISCUSSION As part of our studies of pincer-ligated iridium catalysts for the dehydrogenation of alkanes we recently reported the synthesis of a new Phebox complex (Phebox = 3,54 dimethylphenyl-2,6-bis-(oxazolinyl)) (Phebox)Ir(C2H4)2 (1). In the course of testing this complex for activity for catalytic alkane dehydrogenation (which was not detected), using ethylene as a potential hydrogen acceptor, the formation of butadiene was observed. For example, upon heating a toluene-d8 solution of 1 (5.0 mM) under ethylene (2 atm) at 100 °C for 4 h, 1,3-butadiene (8.0 mM) and ethane (3.0 mM) were 1 observed in solution by H NMR spectroscopy (eq 1). Surprisingly, only a minimal concentration of butenes (
