The Catalytic Magic of the 3Rs: Ruthenium, Rhenium, and Rhodium
Although there has been a lot of interest in organic catalysts this century, the traditional type of metal-based catalyst is making a comeback, but in a different guise. Now the rarest of metals are finding a role, such as rhenium, ruthenium, and rhodium in groups 7, 8 and 9 of the periodic table, respectively. These metals are rare. Their concentrations in the Earth’s crust are: ruthenium, 1.0 part per billion (ppb); rhenium, 0.4 ppb; and rhodium, 0.2 ppb. (Gold is 1.1 ppb.)
A leader in the field is Michael Krische of the Center for Green Chemistry and Catalysis, University of Texas at Austin. Back in 1998, Krische wrote a seminal paper on the subject of metal catalysts (see B.M. Trost, M.J. Krische, Synlett, 1: 1-16, 1998). That paper has since accumulated 350 citations. Krische tells ScienceWatchthat a re-awakening of interest in these metals is not surprising, because they have much to offer.
“They display incredible functional group compatibility and selectivity, and can perform at exceptionally low catalyst loadings,” says Krische. “They are eminently amenable to process optimization and some have been used on a large scale. For example, rhodium is used in hydroformylation and the Monsanto process of methanol carbonylation. There are also practical reasons for choosing these metals in that they are easy to handle outside a glove-box, being relatively stable to air and moisture.”
The main reason is their unique contribution to the synthesis of complex molecules. This is particularly important for alcohols, which are key intermediaries in many chemical reactions. Alcohols are not compatible with main-group metals or the lighter transition metals, but they can be activated successfully by the heavier ones. The result can be the formation of new carbon-to-carbon links exactly where they are required whilst leaving the alcohol group itself intact. (See recent work by Krische: A.R. Dechert-Schmitt, et al., Angew. Chem. Int. Ed., 52 , 3195-8, 2013.)
So which chemists are researching the 3Rs? We turn to Thomson Reuters Web of Science to find the most-cited papers of the past three years. The table lists the top four papers that have gathered the most citations for each of these metals.
Rare-Metal Catalysts Most-cited papers, 2011-2014
|1||J.W Tucker, et al., “Shining light on photoredox catalysis: theory and synthetic applications”, J. Org. Chem., 77 (4), 1617-1622, 2012. [Boston University, Massachusetts]||139|
|2||T. Ueyama, et al., “Ruthenium-catalyzed oxidative vinylation of heteroarene carboxylic acid with alkenes via regioselective C-H bond cleavage”, Organic Lett., 13 (4), 706-708, 2011. [Osaka University., Japan]||107|
|3||A. Prades, et al., “Oxidation and oxidative coupling catalyzed by triazolylidene ruthenium complexes”, Organometallics, 30 (5), 1162-1167, 2011. [University College Dublin, Ireland]||92|
|4||L. Ackermann, A.V. Lygin, “Cationic ruthenium(II) catalysts for oxidative C-H/N-H bond functionalizations of aniline with removable directing groups: systhesis of indoles in water, Organic Lett., 14 (3), 764-767, 2012. [University of Gottingen, Germany]||81|
|Source: Thomson Reuters Web of Science|
Heading the ruthenium list is a paper from the research group of Corey Stephenson of Boston University, Massachusetts. His choice of catalyst is tris(2,2’-bipyridyl)ruthenium chloride, aka Ru(bipy)32+, in which there are three bipyridyl ligands surrounding a ruthenium(II) ion. It is a photoredox catalyst which means it can promote some remarkable reactions with the help of light of wavelength 452 nm (blue). This provides the energy to move an electron from the metal to the outer ligands and thereby make it available to stimulate carbon-hydrogen (C-H) bonds so they will undergo reaction.
Activating C-H by a ruthenium catalyst is also reported in paper #2, which comes from Tetsuya Satoh at Osaka University and shows how molecules in which there are carboxylic acid groups can be made to react with alkenes and attach these at specific locations without involving the acid component.
In paper #3 the focus is on the ruthenium catalyst itself, in this case Ru(II) with 1,2,3-triazolylidene ligands attached. The new catalysts were fully characterized, including X-ray structure determinations, and then used very effectively in a variety of reactions such as converting amines (NH2) to imines (=NH) and (COH) alcohols to aldehydes (CHO). This paper is by Eduardo Peris at University College Dublin, Ireland.
Paper #4 is from Lutz Ackermann and Alexander Lygin of Gottingen University, Germany, and reveals reactions involving Ru(II) catalysts which can be performed in water. Indeed, this is the best solvent in which to convert anilines, in which the NH2 is attached to a benzene ring, to indoles, in which the nitrogen atom becomes incorporated in a second ring system.
|5||F.W. Patureau, et al., “Rhodium-catalyzed oxidative olefination of C-H bonds in acetophenones and benzamides, Angew Chemie Int. Edn., 50 (5), 1064-1067, 2011. [University of Münster, Germany]||159|
|6||S.H. Park, et al., “Rhodium-catalyzed selective olefination or arene esters via C-H bond activation”, Organic Lett., 13 (9), 2372-2375, 2011. [Korean Advanced Institute of Science & Technology, Taejon, South Korea]||110|
|7||S. Mochida, et al., “Rhodium-catalyzed regioselective olefination directed by a carboxylic group”, J. Org. Chem., 76 (9), 3024-3033, [Osaka University, Osaka, Japan]||72|
|8||K. Muralirajan, et al, “Regioselective synthesis of indenols by rhodium-catalyzed C-H activation and carbocyclization of rayl ketones and alkynes”, Angew. Chem. Int. Edn., 50 (18), 4169-4172, 2011. [National Tsing Hua University, Hsinchu, Taiwan]||72|
|Source: Thomson Reuters Web of Science|
Rhodium can activate C-H bonds and cause them to react. Leading the rhodium citations is paper #5 from a group at the University of Münster, Germany, headed by Frank Glorius. This work was done in collaboration with Frederic Patureau of the Technical University of Kaiserlautern, Germany. More than 35 such reactions are described in which C-H bonds are induced to react with olefins. Their mechanism is as yet unclear but appears to involve rhodium(V) at some stage. The exchange of H and D between reaction partners also reveals some interesting aspects of catalytic behavior.
The Glorius/Patureau collaboration also features in a paper on forming aryl-aryl bonds by linking benzene rings by means of the catalyst Rh(III)Cp (see Angew Chemie Int. Edn., 51 (9), 2247-2251, 2012), which is about forming aryl-to-aryl links and which has already attracted 59 citations. (A useful summary of rhodium-catalyzed C-H reactions is given by Patureau and colleagues in the magazine Aldrichimica Acta [(45 (2), 31-41, 2012], already cited more than 100 times.)
Paper #6 is the work of Sukbok Chan, based at the Korean Advanced Institute of Science & Technology, Taejon. It too focuses on the rhodium activation of C-H bonds that are part of a benzene ring and adjacent to groups that would otherwise dominate the molecule’s chemical reactivity, in this case its reactions with alkenes. Paper #7 is another from Tetsuya Satoh and also examines reacting alkenes with benzoic acids, and in one-pot reactions.
Paper #8 from the group led by Chien-Hong Chen of the National Tsing Hua University at Hsinchu, Taiwan, concerns the joining together of aromatic ketones and alkynes by the activation of ortho C-H bonds leading to ring formation. The reaction, at 120oC, takes only one hour to go to completion.
The most recent paper on rhodium catalysis concerns the addition of silicon to an aromatic ring. Work by John F. Hartwig and Chen Cheng at the University of California, Berkeley (Science, 343(6173): 853-7, 2014) shows that the silicon attaches itself specifically to one of two possible sites on o-cymene and in 82% yield.
|9||J. Qin, et al., “Syntheses, characterization, and properties of rhenium(I) tricarbonyl complexes with tetrathiafulvalene-fused phenanthroline ligands,” Organometallics, 30 (8), 2173-2179, 2011. [Nanjing University, China]||19|
|10||D. Xia, et al., “Rhenium-catalyzed regiodivergent addition of indoles to terminal alkynes”, Organic Lett., 14 (2), 588-591, 2012. [Northwest and Agriculture and Forestry University, Beijing, China]||17|
|11||N.M. West, et al., “Heterobimetallic complexes of rhenium and zinc: potential catalysts for homogeneous syngas conversion”, Organometallics, 30 (10), 26909-2700,. 2011. [Caltech, Pasadena]||15|
|12||L.D. Movsisyan, et al., “Synthesis of polyyned rotaxanes”, Organic Lett. , 14 (13), 3424-3426, 2012. [University of Oxford, Oxford, UK]||13|
|Source: Thomson Reuters Web of Science|
Rhenium is the Cinderella metal as far as catalysis is concerned, as evinced by the relatively few papers in this field and their low citation rate. However, there are signs that things might be changing. Paper #9 from Jin-Ling Zuo of Nanjing University, China, recounts the formation of complexes which were shown by X-ray crystallographic analysis to have the kind of planar molecular structure that has the potential for catalytic behavior.
Paper #10 is from Congyang Wan of the Northwest and Agriculture and Forestry University of Beijing, China. It concerns the reaction of indoles and alkynes, which are simple molecules with a triple bond RC = CH. The indoles have a ring containing a nitrogen atom, and two of these rings then attach themselves to CH end of alkyne, forming what are called bis(indolyl)aklanes. Yields were excellent in the 20 or so different indoles that were investigated.
Paper #11 uses a rhenium complex with diphenylphosphino-bipryidine ligands to convert syngas (hydrogen + carbon monoxide) to produce formaldehyde (H2CO), although whether this is a practical proposition is not investigated. and coordinated to zinc alkyls and halides. It can be used to convert syngas (hydrogen + carbon monoxide) to produce formaldehyde (H2CO) although whether this is a practical proposition is not investigated.
Finally there is paper #12 from Harry Anderson of Oxford University, UK, in which rhenium is really acting as a catalyst, not for synthesis, but for the determination of the crystal structure of rotaxanes. In these a phenanthroline macrocycle acts as the eye of the needle through which are threaded hydrocarbon chains that are held rigid by having alternate triple bonds along their chains.
This rather neatly brings us back to the hidden potential of these rare metals, and to the work of Krische. He has also researched the catalyst abilities of iridium (group 9) which is the rarest non-radioactive metal of them all and found only at an incredible 0.003 parts per trillion in the Earth’s crust.
One of Krische’s most-cited papers involves it (see J. Amer. Chem. Soc., 130 (44), 14891-14899, 2009) and has gathered 87 citations in five years. The iridium catalyst is generated in situ and used to promote so-called carbonyl allylation. In this there is formation of a C-C bond linking an alcohol with a molecule that contains both a vinyl (C=C) group and an ester group. The reaction leaves both the OH group of the alcohol and the C=C bond intact and give 97% selective yield of one stereoisomer.
So where is all this leading? We began this article by quoting Krische, so perhaps he is the best person to have the last word on the subject.
“Because organic molecules are composed of carbon and hydrogen, the development of site-selective C-C bond formations, accompanied by the addition, removal or redistribution of hydrogen, represents an inevitable end-point in the evolution of synthetic methods. I believe that these rarer metals will find a permanent place in the chemist’s arsenal.”
Dr. John Emsley is based at the Department of Chemistry, Cambridge University, UK
The data and citation records included in this report are from Thomson Reuters Web of ScienceTM. Web of ScienceTM is a registered trademark of Thomson Reuters. All rights reserved.