catalysts Article Alkanethiolate-Capped Palladium Nanoparticles for Regio- and Stereoselective Hydrogenation of Allenes Ting-An Chen and Young-Seok Shon * Department of Chemistry and Biochemistry, California State University Long Beach, 1250 Bellflower Blvd., Long Beach, CA 90840, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-562-985-4466 Received: 3 September 2018; Accepted: 25 September 2018; Published: 29 September 2018 Abstract: Colloidal Pd nanoparticles capped with octanethiolate ligands have previously shown an excellent selectivity toward the mono-hydrogenation of both isolated and conjugated dienes to internal alkenes. This paper reports an efficient stereoselective mono-hydrogenation of cumulated dienes (allenes) to either Z or E olefinic isomers, depending on the substitution pattern around C=C bonds. Kinetic studies indicate that the reaction progresses through the hydrogenation of less hindered C=C bonds to produce internal Z olefinic isomers. In the cases of di-substitued olefinic products, this initial hydrogenation step is followed by the subsequent isomerization of Z to E isomers. In contrast, the slow isomerization of Z to E isomers for tri-substituted olefinic products results in the preservation of Z stereochemistry. The high selectivity of Pd nanoparticles averting an additional hydrogenation is steered from the controlled electronic and geometric properties of the Pd surface, which are the result of thiolate-induced partial poisoning and surface crowding, respectively. The high activity of colloidal Pd nanoparticle catalysts allows the reactions to be completed at room temperature and atmospheric pressure. Keywords: selective hydrogenation; catalysis; nanoparticle; cumulated diene; allene; ligand-capped; semi-heterogeneous 1. Introduction Selective hydrogenation of small alkynes and dienes is an important chemical process because such compounds are reactive monomers for polymerization that decreases the purity of pyrolysis petroleum products [1–3]. Compared to a variety of catalysts investigated for selective hydrogenation of alkynes and conjugated/isolated dienes, only a limited number of studies has been reported for the selective hydrogenation of allenes that have the unique orthogonal π orbital of cumulene. Besides the complete hydrogenation of two π bonds, the partial hydrogenation of allenes can involve either 1,2- or 2,3-hydrogenation. In addition, the products for selective hydrogenation of allenes are often stereoisomers (cis and trans), further complicating the catalysis results. The isomerization of allene groups to alkynes was also observed for several catalytic systems during the hydrogenation of allenes [4]. Earlier studies reported in the literature indicated that the selectivity of supported metal catalysts among different chemo-, regio-, and stereoselective hydrogenation products is generally low for this challenging allene hydrogenation reaction [5–10]. For example, palladium on alumina catalyst, which was synthesized by the hydrothermal method, converted allenes into alkenes, with a relatively low and time-dependent selectivity [9,10]. It has been shown that the pressure of the hydrogen gas would affect the catalytic selectivity of palladium on alumina, which has been correlated to the hydride concentration on the palladium surface. With the lower pressure of H2 and at the earlier stage of buta-1,2-diene hydrogenation, cis-but-2-ene was the major reaction product. However, Catalysts 2018, 8, 428; doi:10.3390/catal8100428 www.mdpi.com/journal/catalysts Catalysts 2018, 8, 428 2 of 10 Catalysts 2018, 8, x FOR PEER REVIEW 2 of 10 with the increased hydrogen pressure above 100 torr or at the later stage of the catalytic reaction, reaction,the yields thefor yields butane, for a full butane, hydrogenation a full hydrogenation product, would product, greatly increasewould greatly with the increase formation with of some the formationtrans-but-2-ene of some [10]. trans During-but-2-ene the hydrogenation [10]. During reaction the hydrogenat of buta-1,2-diene,ion reaction the isomerizationof buta-1,2-diene, products, the isomerizationbut-1-yne and products, buta-1,3-diene, but-1-yne were and also buta-1,3-diene, observed [9]. were also observed [9]. SelectiveSelective mono-hydrogenationmono-hydrogenation of substitutedof substitu allenested allenes could produce could fourproduce different four stereoisomeric different stereoisomericalkenes depending alkenes on depending the regio- andon the stereoselective regio- and stereoselective addition of hydrogen, addition of as hydrogen, shown in as Scheme shown1. inMost Scheme heterogeneous 1. Most heterogeneouscatalysts, including catalysts, supported including Pd, supported Pt, Rh, and Pd, Ir, Pt, produced Rh, andcis Ir,-1,2-hydrogenation produced cis-1,2- hydrogenationproducts as major products products, as major with products, maximum with yieldsmaximum ranging yields from ranging 70% from to 98%, 70% to that 98%, are that highly are highlydependent dependent on the on reaction the reaction time and time hydrogen and hydrogen pressure pressure [5,6,11 [5,6,11].]. Most Most of these of these catalysts catalysts produced produced full fullhydrogenation hydrogenation products products as major as major products products after anafte extendedr an extended reaction reaction time. Hydrogenationtime. Hydrogenation of allenes of alleneswith homogeneous with homogeneous catalysts, catalysts, such assuch chlorotris(triphenylphosphine) as chlorotris(triphenylphosphine) rhodium, rhodium, allylpalladium allylpalladium (II), (II),and palladiumand palladium (bis(arylimino)acenaphthene) (bis(arylimino)acenaphthene) complexes, complexe also broughts, also about brought the cis about-1,2-hydrogenation the cis-1,2- hydrogenationreaction as its primaryreaction pathwayas its primary [12–14 pathway]. With the[12–14]. good With selectivity the good toward selectivity the cis -1,2-hydrogenationtoward the cis-1,2- hydrogenationproduct, the homogeneous product, the palladiumhomogeneous (bis(arylimino)acenaphthene) palladium (bis(arylimino)acenapht complex washene) used complex for the was selective used forhydrogenation the selective ofhydrogenation 1,2-allenyl phosphonates of 1,2-allenyl tophosphonates (Z)-1-alkenyl to phophonates (Z)-1-alkenyl [ 14phophonates]. [14]. R1H2C R2 R1 CH2R2 Mono- R2 R1 Hydrogenation H • H R2 R1 R H C CH R 1 2 2 2 SchemeScheme 1. 1.SelectiveSelective mono-hydrogenation mono-hydrogenation of substituted of substituted allenes produces allenes four produces different fourstereoisomeric different alkenes.stereoisomeric alkenes. WithWith the the emergence emergence of of the the α-amino-amino vinylphosphonate vinylphosphonate as as an an important important building building block block for for the the preparationpreparation of of biologically biologically active active molecules molecules [15], [15], the the research research on on developing developing heterogeneous heterogeneous catalysts catalysts forfor selectiveselective hydrogenationhydrogenation of ofα -aminoα-amino allenephosphonates allenephosphonates to α -aminoto α-amino vinylphosphonates vinylphosphonates has drawn has drawninterest interest [16]. The [16]. most The efficient most efficient heterogeneous heterogene catalystous catalyst for 1,2-hydrogenation for 1,2-hydrogenation was determined was determined to be the toPd/C be the (10 Pd/C wt.%) (10 in wt.%) the presence in the presence of quinoline. of quino Dependingline. Depending on the sizeon the and size functionality and functionality of the aminoof the aminogroup, group, however, however, the stereoselectivity the stereoselectivity between between Z and Z E and isomers E isomers varied varied dramatically, dramatically, ranging ranging from from58:42 58:42 to 96:4, to in96:4, favor in favor of Z isomers.of Z isomers. The yields The yields for 1,2-hydrogenation for 1,2-hydrogenation were aswere high as ashigh 97% as for 97%α-amino for α- aminoallenephosphonates, allenephosphonates, but also but as also low as as low 43% as for 43% bulkier for bulkier substrates. substrates. Recently,Recently, thethe firstfirst successful successful and and highly highly efficient effici 2,3-reductionent 2,3-reduction of substituted of substituted allenes wasallenes reported was reportedby Dong by et Dong al. [17 et]. al. The[17]. Rh-HThe Rh-H based based homogeneous homogeneous catalyst catalyst with with a a chiral chiral ligand ligand waswas able to to affordafford the the chemoselective chemoselective reduction, reduction, producing producing chir chiralal benzylic benzylic motifs motifs with with good good enantioselectivity. enantioselectivity. TheThe mechanistic studies using isotope labeling revealed that that the the hydride hydride is is added added directly directly to to the the branchedbranched position position (3-position) (3-position) of of allenes allenes for for 2,3-reduction. 2,3-reduction. DespiteDespite the the recent recent development, development, a facile a facile method method that produces that produces more th moreermodynamically thermodynamically stable E-alkenesstable E-alkenes directly from directly allenes from in allenesa single instep a is single currently step lacking is currently in the literature. lacking in Alkanethiolate- the literature. cappedAlkanethiolate-capped palladium nanoparticles palladium (PdNP) nanoparticles have shown (PdNP) a havehigh shownselectivity a high for selectivityboth isolated for bothand conjugatedisolated and dienes conjugated to form dienes mono-hyd to