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Selective Transfer Semihydrogenation of Alkynes with Nanoporous Gold Catalysts Yogesh S. Wagh, and Naoki Asao J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 18 Dec 2014 Downloaded from http://pubs.acs.org on December 19, 2014
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The Journal of Organic Chemistry
Selective Transfer Semihydrogenation of Alkynes with Nanoporous Gold Catalysts
Yogesh S. Wagh and Naoki Asao*
WPI-Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Fax: (+)81-22-217-6165 E-mail:
[email protected] Table of Contents Graphic
R
1
R
2
AuNPore cat
H
H
HCO2H, amine cat
R1
R2
20 examples 90-99% yield excellent chemoand stereoselectivity
ABSTRACT A facile, highly chemo- and stereoselective transfer semi-hydrogenation of alkynes to Zolefins has been achieved by use of unsupported nanoporous gold (AuNPore) as a heterogeneous catalyst together with formic acid as a hydrogen donor. A variety of terminal/internal and aromatic/aliphatic alkynes were reduced to the corresponding alkenes in high chemical yields with good functional-group tolerance. The catalyst is robust enough to be reused without leaching. 1
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INTRODUCTION Semihydrogenation of alkynes is one of the most simple and straightforward synthetic methods of alkenes in laboratory as well as in industry.1 This transformation has been established well using Lindlar catalyst with molecular hydrogen, and Z-olefins are produced seletively.2 However, it often suffers from many drawbacks. E/Z isomerization occurs to a considerable extent. The over-reduction of the produced alkenes to alkanes is a serious problem due to the difficulty of the control of a stoichiometric amount of H2. Requirement of toxic lead for deactivation of Pd is obviously undesirable. Furthermore, H2 is highly flammable and readily forms explosive mixtures with air. On the other hand, the catalytic transfer hydrogenation using organic hydrogen donors is attractive because of the operational simplicity and safety.3 Several protocols with homogeneous catalysts have been reported so far,4 but heterogeneous catalytic systems are surprisingly quite limited.5 The first example was reported by Heck in 1978 by use of Pd/C with trialkylammonium formate as a hydrogen source.5a,b Johnstone used a combination of Pd-poisoned catalysts with Hg or Pb and sodium phosphinate.5c In both cases, however, overreduction or E/Z isomerization occurred significantly. Alonso and Yus reported that the nickel nanoparticles were effective for this reaction although substoichiometric amounts of NiCl2 and large excess of Li metal were required.5d,e Cuerva, Cadenas, and Oltra developed a new semihydrogenation system with water as a hydrogen source although excess amounts of Cp2TiCl was needed.5f Liu reported the semihydrogenation with costly Hantzsch ester 1,4dihydropyridine (HEH) as the hydrogen source.5g Furthermore, the recyclability of all these heterogeneous catalysts have not been investigated well. To overcome these limitations, 2
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The Journal of Organic Chemistry
development of selective and cost-effective heterogeneous catalytic system is highly desirable. Recently, unsupported nanoporous gold (AuNPore) has emerged as a potentially green and sustainable catalyst in the chemical process, which can be easily obtained from Au alloys by selective leaching of less noble metals, such as Ag, Cu, and Al.6,7 It has randomly oriented ligaments with a hyperboloid-like shape and nanopore channels, possessing a high density of surface steps and kinks that are active for chemical reactions.8 Since AuNPore has high surface area, high stability for versatile chemicals, high reusability, and no toxic nature, it has been successfully applied to the various types of chemical processes in the gas and liquid phases.9,10 In this context, we recently reported that this material was effective for the transfer semihydrogenation of alkynes.10h Unfortunately, however, this method required excess amounts of costly organosilanes as a hydrogen source. Therefore, the formation of stoichiometric amounts of siloxane was inevitable as a by-product. Also it fails to achieve higher chemical yield of desired alkenes in some cases. Particularly, the reaction with 1,2-dialkyl-substituted internal alkynes does not go well. In continuation with our interest on this material, we herein report a AuNPore-catalyzed transfer semihydrogenation of alkynes by use of HCO2H as a hydrogen donor (Scheme 1). With this method, various terminal as well as internal aromatic/aliphatic alkynes were reduced to the corresponding cis-alkenes with remarkable stereo- and chemoselectivity in high yields. Scheme 1: AuNPore-catalyzed transfer semihydrogenation of alkynes with HCO2H.
RESULT AND DISCUSSION 3
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The transfer semihydrogenation of 4-ethynyl-1,1'-biphenyl (1a) was performed under several conditions and the results are summarized in Table 1. When 1a was treated with HCO2H (1 eq) and Et3N (1 eq) in the presence of AuNPore catalyst (5 mol%) in DMF at 70 °C for 6h, the reaction proceeded smoothly and the corresponding olefin product (2a) was obtained in 99% Table 1. Catalytic transfer semihydrogenation of alkyne 1aa
selectivity entry
solvent
H source
additive
b
yield (%)
(2a/3a) 1
DMF
HCO2H
Et3N (1 eq)
99
99/1
