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Development and Evaluation of a Borohydride- Palladium System for Selective Reduction of the C=C Bond of Α,Β-Unsaturated Carbonyl Compounds Kerstin L

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Development and Evaluation of a Borohydride- Palladium System for Selective Reduction of the C=C Bond of Α,Β-Unsaturated Carbonyl Compounds Kerstin L International Journal of Undergraduate Research and Creative Activities Volume 7 Article 5 4-13-2015 Development and Evaluation of a Borohydride- palladium System for Selective Reduction of the C=C Bond of α,β-unsaturated Carbonyl Compounds Kerstin L. Amezcua Pacific nU iversity, [email protected] Tony J. Mull Pacific nU iversity, [email protected] Amy L. Mayhugh Pacific nU iversity, [email protected] David B. Cordes Pacific nU iversity, [email protected] Recommended Citation Amezcua, Kerstin L.; Mull, Tony J.; Mayhugh, Amy L.; and Cordes, David B. (2015) "Development and Evaluation of a Borohydride- palladium System for Selective Reduction of the C=C Bond of α,β-unsaturated Carbonyl Compounds," International Journal of Undergraduate Research and Creative Activities: Vol. 7, Article 5. DOI: http://dx.doi.org/10.7710/2168-0620.1037 Development and Evaluation of a Borohydride-palladium System for Selective Reduction of the C=C Bond of α,β-unsaturated Carbonyl Compounds Peer Review This work has undergone a double-blind review by a minimum of two faculty members from institutions of higher learning from around the world. The faculty reviewers have expertise in disciplines closely related to those represented by this work. If possible, the work was also reviewed by undergraduates in collaboration with the faculty reviewers. Abstract Selective reduction of the carbon-carbon double bond of α,β-unsaturated carbonyl compounds is most commonly and reliably effected using a palladium metal catalyst together with molecular hydrogen from a pressurized tank. Sodium borohydride, like other hydrides, is ordinarily associated with reduction of the more polarized carbonyl of such compounds. However, we have developed an alternative means of employing sodium borohydride in combination with palladium metal to selectively reduce the carbon-carbon double bonds of these compounds. In this survey study, we introduce sodium borohydride as an alternative hydrogen source for such selective, palladium-catalyzed reductions. We also compare the results of this new, heterogeneous borohydride-palladium method with that of traditional palladium-catalyzed hydrogenation. A third method using only sodium borohydride with no palladium is included for comparison. Keywords chemistry, palladium, borohydride, catalyst, reduction, hydrogenation Acknowledgements The uthora s wish to acknowledge the Murdock Charitable Trust and the Pacific nivU ersity Research Institute for Science and Mathematics (PRISM) for financial support. We also wish to thank the reviewers for their helpful comments and insights. Editor's Note: Dr. David B. Cordes, Associate Professor, Department of Chemistry, Pacific nivU ersity, served as the faculty mentor for this work. Amezcua et al.: Sodium Borohydride: An Alternative to Hydrogen Gas for Selective, Palladium-catalyzed Reductions INTRODUCTION conception of the palladium-hydrogen system, it is understood that several distinct To the alchemists of past centuries, structural Pd-H phases may exist, sometimes the skillful and selective transformation of simultaneously, depending on the hydrogen one substance into another might be but a concentration, pressure, and temperature single step in their efforts to prepare the (Lewis, 1967). Since palladium retains Philosophers’ Stone (Principe, 2013). The much of its form and structural properties Philosophers’ Stone was, in turn, thought to even as it absorbs very large volumes of promote the transformation of metals such hydrogen, these Pd-H phases are sometimes as tin and lead into gold. Today, expanding referred to as ‘solid solutions’ and the range and improving the quality of ‘palladium hydrides’. functional group transformations continues There is general agreement on the to occupy a primary place in the work of basic steps for the palladium-catalyzed organic chemists. Of all the various organic hydrogenation of alkenes and related reactions that have been described, it is fair compounds as described by Horiuti and to say that reduction is one of the most Polyani in 1934 (Mattson et al., 2013; common and fundamentally important. Polyani & Horiuti, 1934). Key steps involve Catalytic reduction of carbon-carbon adsorption of the unsaturated substrate to the double and triple bonds with hydrogen gas metal surface by way of its C=C pi-bond was first demonstrated with a platinum system. With addition of hydrogen gas (H2), metal catalyst in 1874, but was developed as absorption and dissociation of molecular a general method by Sabatier, who won the hydrogen occurs both on the surface and, to 1912 Nobel Prize for this work; see Figure 1 some degree, in the subsurface volume of (Sabatier & Senderens, 1897; Wilde, 1874). the metal to form so-called palladium- Palladium, however, soon became the hydrides (Pd-H). These palladium hydrides preferred metal catalyst. In fact, the unique are understood to be the active species in properties of palladium combined with palladium-catalyzed hydrogenations. hydrogen so impressed early researchers that Reagents such as sodium they referred to the combination as a unique borohydride and lithium aluminum hydride metallic compound or alloy and called it represent an entirely different approach to “hydrogenium”. chemical reduction as they harness the nucleophilicity of hydride ions (Nystrom & Brown, 1947; Schlesinger et al., 1953; Seyden-Penne, 1997). Hydride nucleophiles (:H-) attack electrophiles and positively polarized moieties such as the carbon atom of a carbonyl or imine. The mechanisms for both hydride reduction and palladium- catalyzed hydrogenation are depicted in Figure 1. Palladium pioneers: Thomas Graham, Paul Scheme 1. Sabatier, and Herbert C. Brown In this study, we examined the chemoselectivity of several methods for the Thomas Graham hypothesized that reduction of α,β-unsaturated carbonyl structural changes were occurring with new compounds. Their properties and reactivity Pd-H bonds formed as the hydrogen was vary dramatically depending on the presence added and broken as the hydrogen departed of structural and electronic features such as (Graham, 1868, 1869). In the modern 1 DOI: http://dx.doi.org/10.7710/2168-0620.1037 International Journal of Undergraduate Research and Creative Activities, Vol. 7 [2015], Art. 5 Scheme 1. Representative mechanisms for reduction/hydrogenation: a) hydride reduction of a ketone to tetrahedral intermediate; b) Horiuti-Polyani mechanism for alkene reduction to an alkane with a traditional Pd- H2 system. aromatic rings and donating or withdrawing While reduction of the C=C bond is groups conjugated with or in proximity to thermodynamically and kinetically favorable the α,β−unsaturated carbonyl system (Al- (Bond, 1962), non-selective reduction and Rifai, Rucker, & Amslinger, 2013; the formation of unwanted side-products Amslinger, 2010). Selective reduction of will often be a problem. Selectivity for these compounds is required to gain access reduction of a bifunctional molecule largely to the great variety of their saturated and depends on the rate constants for the partially saturated analogs. A notable competing reactions in question, but it can example is the NAPDH-dependent also depend on competition between biochemical reduction of testosterone to the functional groups for adsorption onto active potent analogue dihydrotestosterone by the catalytic sites (Rylander, 1994). Possible 5α-reductase enzyme, as shown in Scheme 2 products for the reduction of α,β-unsaturated (Bull et al., 1996). carbonyl compounds, illustrated for chalcone, are shown in Scheme 3. While hydrogen gas in combination with a Pd catalyst reliably reduces the C=C bond of many α,β-unsaturated compounds, hydride reagents tend to show selectivity for the reduction of the more polar carbonyl function (Meyer, 1981). High selectivity for Scheme 2. Selective reduction of testosterone to the reduction of the C=O bond of α,β- dihydrotestosterone by the NAPDH-dependent 5α- reductase enzyme. 2 DOI: http://dx.doi.org/10.7710/2168-0620.1037 Amezcua et al.: Sodium Borohydride: An Alternative to Hydrogen Gas for Selective, Palladium-catalyzed Reductions Scheme 4. Palladium-catalyzed reduction of vinyl cyclohexane to ethyl cyclohexane using sodium borohydride. variety of alkenes and alkynes. A typical reaction is shown in Scheme 4. Literature reports and our Scheme 3. Some possible fates for reduction of observations suggest that this reaction takes α,β−unsaturated compounds as illustrated for the place by way of catalytic hydrogenation on reduction of chalcone. the metal surface using hydrogen that has been produced in situ from the unsaturated ketones and aldehydes is most decomposition of borohydride (Osby, notably achieved by Luche reduction in 3+ Heinzman, & Ganem, 1986; Satyanarayana which the carbonyl is activated by a Ce & Periasamy, 1984). It is known that catalyst (Gemal & Luche, 1981). Many palladium metal catalyzes the hydrolysis of other strategies for the selective reduction of sodium borohydride at the metal surface to the C=O bond have been described, but only liberate hydrogen which may then reside a handful of hydride-based methods for within or on the surface of the metal catalyst selective C=C bond reduction have been in the form of palladium-hydrides (Pd-H) introduced (Aramini, Brinchi, Germani, & (Brown & Brown, 1962b; Demirci & Garin, Savelli, 2000; Hutchins, Rotstein, Natale, 2008; Guella, Zanchetta, Patton, & Miotello, Fanelli, & Dimmel, 1976; Keinan & 2006; Kaufman & Sen, 1985; Titova, Greenspoon, 1986; Ranu & Samanta,
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