A simple approach for the catalytic conversion of primary alcohols into their corresponding esters and amides, with evolution of H2 gas using in situ formed ruthenium PNP- and PNN-pincer catalysts, is presented. The evaluation showed conversions for the esterification with turnover numbers as high as 4300, and 4400 for the amidation

Ben-David, Y.

Leitner, W.

Milstein, D.

Prechtl, M. H. G.

Theyssen, N.

Wobser, K.

English

Kölner UniversitätsPublikationsServer

Direct coupling of alcohols to form esters and amides with evolution of H-2 using in situ formed ruthenium catalysts

Martin H. G. Prechtl

Kathrin Wobser

Nils Theyssen

Yehoshoa Ben-David

David Milstein

Walter Leitner

Crossref

Catalysis Science & Technology

Direct coupling of alcohols to form esters and amides with evolution of H2 using in situ formed ruthenium catalysts

Extract: Do you think that social marketing is more ethical than commercial marketing? If you do, you are in good company, as straw polls we have taken at conferences show that virtually everyone (95% or more) thinks so. But what justifies this belief? Social marketers may be well meaning,and perhaps more so than commercial marketers, but is there anything about social marketing that makes it inherently ethical?We argue that social marketing is not inherently ethical and that there are unique ethical concerns that confront, or perhaps ought to confront,social marketers. As commercial and social marketers use the same marketing tools, social marketers cannot claim to be inherently more ethical than commercial marketers in terms of the 'means'. We may well hope that social marketers subscribe to a high standard of ethics in their use of marketing tools, as has been urged by some (Kirby &amp; Andreasen,2001; Kotler &amp; Andreasen, 2007; Murphy &amp; Bloom, 1992), but there is certainly no assurance that they do so simply because they are social marketers

Holden, Stephen

Cox, Damian

Bond University Research Portal

Social marketing: Immunizing against unethical practice

Prechtl, Martin H. G.

Wobser, Kathrin

Theyssen, Nils

Ben-David, Yehoshoa

Milstein, David

Leitner, Walter

Publikationsserver der RWTH Aachen University

Prechtl, M.

MPG.PuRe

Direct coupling of alcohols to form esters and amides with evolution of H<sub>2</sub> using in situ formed ruthenium catalysts

RWTH Publications

This journal is c The Royal Society of Chemistry 2012 Catal. Sci. Technol., 2012, 2, 2039–2042 2039Cite this: Catal. Sci. Technol., 2012, 2, 2039–2042Direct coupling of alcohols to form esters and amides with evolutionof H2 using in situ formed ruthenium catalystsMartin H. G. Prechtl,w*ab Kathrin Wobser,b Nils Theyssen,b Yehoshoa Ben-David,cDavid Milsteinc and Walter Leitner*abReceived 22nd June 2012, Accepted 8th July 2012DOI: 10.1039/c2cy20429kA simple approach for the catalytic conversion of primaryalcohols into their corresponding esters and amides, with evolutionof H2 gas using in situ formed ruthenium PNP- and PNN-pincercatalysts, is presented. The evaluation showed conversions for theesteriﬁcation with turnover numbers as high as 4300, and 4400for the amidation.Catalytic hydrogenation and dehydrogenation reactions play amajor role in industrial processes as well as in academicresearch. In recent years especially progress in the catalyticacceptorless dehydrogenation of primary alcohols is remarkable. Inmost cases ruthenium hydride or iridium complexes were used forthe transformation of primary alcohols into aldehydes.1 Dependingon the reaction conditions, tailor-made catalysts are also applicablefor various tandem- or domino-reactions2 revealing high selectivity.In this respect the formation of esters,3 (aza)Wittig-,4 Aldol-,4 andKnoevenagel-products is known.4 Most remarkably, in thepresence of amines, amides5 are easily accessible. For thispurpose today’s most active catalyst is a ruthenium pincercomplex [Ru(PNN)H(CO)] (1, PNN = {6-[(di-tert-butylphos-phino)methyl]pyridin-2-ylmethyl}-diethylamine; Fig. 1) with acooperative dearomatised pyridine pincer-ligand backbone. It isactive for the direct-synthesis of esters starting from primaryalcohols as well as amides if amines are present as well.5In other attempts it has been shown that certain rutheniumpincer-complexes are suitable to convert alcohols and aminesinto the corresponding coupled imines.6 Moreover, it has beenshown that certain ruthenium and iridium pincer complexesare highly active for the production of hydrogen gas fromisopropanol, for the direct conversion of ethanol into ethylacetate and for the hydrogenation of (chiral) esters rutheniumand osmium pincer complexes were successfully applied.7 Inthe case of the osmium complexes, also selective hydrogenationof unsaturated fatty acid esters to the corresponding unsaturatedalcohols was demonstrated, as well as the dehydrogenativecoupling of aliphatic alcohols to form their esters. In general,ruthenium hydride complexes are often used for hydrogenationof a variety of compounds including ketones, aldehydes,alkynes and alkenes under hydrogen gas or under transferhydrogenation conditions.1 And today it is widely accepted thatin most of these transformations metal dihydrogen complexesare key intermediates.8Based on previous results using the [Ru(PNN)H(CO)]catalyst 1 with a hemi-labile and cooperative pincer-backbone,3now the simple in situ formation of ruthenium dehydrogenationcatalysts (Scheme 1) for the transformation of primary alcoholsinto esters with evolution of two equivalents of dihydrogen gas ispresented (Table 1). As depicted in Scheme 1, the treatment of thereadily available ruthenium precursor [Ru(COD)(2-methylallyl)2](2, COD=1,5-cyclooctadiene) with either the hemi-labile PNN (3)Fig. 1 Complex 1 for the catalytic dehydrogenative coupling ofprimary alcohols to homoesters and amides.Scheme 1 Protocol for the in situ formed precatalysts 5 and 6.a Institut fu¨r Technische und Makromolekulare Chemie,Rheinisch-Westfa¨lische Technische Hochschule Aachen,Worringer Weg 1, 52074 Aachen, Germany.E-mail: leitner@itmc.rwth-aachen.de; Web: www.itmc.rwth-aachen.de;Fax: +49 241 80 22177; Tel: +49 241 80 26480bMax-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1,45470 Mu¨lheim/Ruhr, GermanycDepartment of Organic Chemistry, The Weizmann Institute ofScience, Rehovot, 76100, Israelw Present address: Institut fu¨r Anorganische Chemie, Universita¨t zu Ko¨ln,Greinstr. 6, 50939 Ko¨ln, Germany. E-mail: martin.prechtl@uni-koeln.de;http://catalysis.uni-koeln.de; Fax:+49 221 470 1788; Tel:+49 221 470 1981CatalysisScience & TechnologyDynamic Article Linkswww.rsc.org/catalysis COMMUNICATIONDownloaded on 11 December 2012Published on 10 July 2012 on http://pubs.rsc.org | doi:10.1039/C2CY20429KView Article Online / Journal Homepage / Table of Contents for this issue2040 Catal. Sci. Technol., 2012, 2, 2039–2042 This journal is c The Royal Society of Chemistry 2012or the stronger PNP (4) pincer ligand leads presumably to thein situ formation of the precatalysts [Ru(PNN)(2-methylallyl)2] 5and [Ru(PNP)(2-methylallyl)2] 6. This stays in agreement withprevious ﬁndings, where the exchange of COD in [Ru(COD)-(2-methylallyl)2] 2 with chelating phosphine ligands has beendescribed by diﬀerent research groups.9 Thus, for the initialcatalytic experiments we focussed ﬁrst on the stronger coordinatingPNP-ligand 4.Using a precatalyst loading of 1.0 mol%, a variety of aliphaticalcohols (7 and 9–13) show moderate to high conversions into thecorresponding esters. For example, the treatment of 1-hexanol 7with a mixture of 2 and PNP 4 (1.3 eq.), dissolved in toluene, gave499% conversion into the corresponding homoester 8 within15 h under reﬂux (entry 1). Using this protocol with other primaryalcohols, moderate to good conversions and selectivities wereobtained (Table 1, entries 3–6). Consistently, a lowering of thecatalyst loading showed also decreasing conversions, i.e. 33%ester was formed for the treatment of 1-hexanol with the0.1 mol% catalyst in 20 h (entry 2).Encouraged by these results, consequently the PNN ligand 3was tested, since its corresponding ruthenium complex 1 isknown to show a superior activity for this kind of dehydro-genative couplings of primary alcohols.3 And indeed, thetreatment of 7 with 2 and 3 (1.0 and 1.3 mol% respectively)gave 97% yield after 1 h using again toluene as a solvent(Table 1, entry 7). Lowering the catalyst loadings showed stillvery high conversion (98% after 20 h, entry 8) with just0.05 mol% catalyst and still remarkable high conversions(86% after 20 h, entry 9) were obtained with 0.02 mol%catalyst. The conversion of 86% with 0.02 mol% of catalyst 5 isrelated to a turnover number (TON) of 4300 after 20 hours.Complex 1 gave otherwise a TON ofB1000 after six hours undersimilar conditions, and presumably with a higher substrate loadinga similar high TON.3 However, the activity of the in situ formedspecies based on precatalyst 5 is remarkable in comparisonto the one of complex 1. Other primary alcohols showedalso moderate (68%) to very high conversions (99%) withcatalyst loadings as low as 1.0 mol% to 0.05 mol% (Table 1;entries 10–14).To further evaluate the potential of the protocol for thein situ formed catalytic systems, both systems were tested forthe challenging dehydrogenative coupling of primary alcoholswith primary amines (Table 2). This pioneering reactionhas been published in 2007 using catalyst 1.5 And, indeed,treating a toluene solution with our in situ systems (2/3 or 2/4)results in the preferred formation of amides in the latter case(Table 2).The addition of 100 eq. of 1-hexanol 7 and 100 eq. of1-hexylamine 14 to a solution of Ru-2/PNP-4 in tolueneresulted in a high substrate conversion, but relatively poorselectivity for the amide (amide : ester = 44 : 56; Table 2:entry 1). Using the system Ru-2/PNN-3 for these substratesinstead, hexanoic acid hexylamide 15 is formed with highconversions (98%) and much better selectivity (88%, entry 2).A lowering of the catalyst concentration to a value of0.2 mol% still leads to the amide (82%; TON = 410)with almost unchanged selectivity (83%, entry 3). Similarresults were obtained with 1-hexanol 7 and benzylamine 16as substrates (conversion: 91%, 86% amide; entry 4). Incomparison to previously reported in situ catalysts for thedirect amidation of alcohols, we found here a quite activesystem which uses comparably low catalyst loadings withoutthe need for the addition of base. Other direct amidationmethods use higher metal precursor (2–10 mol%) and ligandloadings (2–10mol%) and catalytic active species are only obtainedin the presence of base (8–30 mol%).5b,cFurther experiments with 1-hexylamine 14 as the solesubstrate support the previously proposed mechanism for thistype of dehydrogenative coupling.3,5 Heating the amine intoluene in the presence of Ru-2/PNN-3 gave no products,neither the simple 1-hexylimine, nor one of the possiblecoupling products N-hexyl-hexanamidine or dihexyl amine(coupling under ammonia loss). This result indicates that theTable 1 Ruthenium catalysed dehydrogenative coupling of primaryalcohols into esteraNo L Cata [mol%] ROH t [h] Conv. [%] E :Ab1 4 1.0 7 15 499 4982 4 0.1 7 20 33 4983 4 1.0 9 20 90 844 4 1.0 10 20 68 825 4 1.0 11 20 85 866 4 0.2 12 20 62 977 3 1.0 7 1 97 988 3 0.05 7 20 98 999 3 0.02 7 20 86 9910 3 0.1 9 20 99 9911 3 0.05 9 20 83 9912 3 0.1 10 20 68 8213 3 1.0 13 20 80 9614 3 0.05 11 20 97 99a Reaction conditions: Ru (3) : L (3 or 4) = 1 : 1.3, ROH: 1-hexanol 7,1-butanol 9, 1-adamantylmethanol 10, 1-octanol 11, 1-heptanol 12,benzylalcohol 13, reﬂux in toluene under argon ﬂow. b Ratio: E :A =ester : aldehyde.Table 2 Ruthenium catalysed dehydrogenative coupling of primaryalcohols with amines into amidesaEntry L Cat [mol%] ROH RNH2 Conv. [%] A :Eb1 4 1.0 7 14 91 442 3 1.0 7 14 98 883 3 0.2 7 14 82 844 3 1.0 7 16 91 86a Reaction conditions: Ru (2) : L (3 or 4) = 1 : 1.3, ROH: 1-hexanol 7,1-hexylamine 14, benzylamine 16, reﬂux in toluene under argon ﬂowfor 20 hours. b Ratio: A : E = amide : ester.Downloaded on 11 December 2012Published on 10 July 2012 on http://pubs.rsc.org | doi:10.1039/C2CY20429KView Article OnlineThis journal is c The Royal Society of Chemistry 2012 Catal. Sci. Technol., 2012, 2, 2039–2042 2041crucial reaction step is the formation of the aldehyde as thereactive intermediate,3,5 which then reacts with a primaryamine (or primary alcohol) to give the hemi-amidale (orhemi-acetale) which is then dehydrogenated to the corres-ponding amide (or ester). The aldehyde intermediate was alsoidentiﬁed by IR-techniques and trapped in an indirect Wittig-reaction in similar reactions.10 Moreover, it is known thatruthenium hydrides are capable of decarbonylating primaryalcohols under mild conditions (room temperature),11a or atelevated temperature.11b This decarbonylation results in theformation of a ruthenium pincer complex carrying CO as aligand. Such a complex might exhibit a similar reactivityto complex 1 for both dearomatisation/aromatisation ofthe cooperative pyridine backbone under dehydrogenation/hydrogenation conditions with formation of ruthenium hydrideand trans-dihydride complexes.3,5,12 As previously discussedhere, the catalytic activity of catalyst 5 is comparable withcomplex 1. And, this is probably related to a similar structureformed in situ under the applied conditions. However, thereal structures of the in situ formed ruthenium catalysts remainunclear.ConclusionsIn conclusion, the easy applicable conversion of primaryalcohols into their corresponding esters using in situformed catalyst systems was demonstrated. Likewise, primaryalcohols can be coupled with primary amines resulting in amideformation with high selectivity. This protocol allows use of avariety of substrates carrying OH– andNH2– functionalities as thepresented catalysts are sensitive to these functional groups inhydrocarbons.Experimental sectionGeneral informationAll reactions were carried out in ﬂame-dried glassware underargon. All alcohols and amines were obtained from SigmaAldrich or Acros and dried or deoxygenated prior to use.[Ru(COD)(2-methylallyl)2] 3 was used as received from Acros.Toluene was dried over magnesium anthracene. GC analyseswere performed on a HP GC-MS/GC-EI SSQ7000 or on a HP6890 GC System/HP Mass Selective Detector 5973.General procedure for the direct esteriﬁcationExperimental protocol for entry 1 (Table 1: esters): the reactionswere performed under argon-ﬂow in a dried 20 mL three-neckedround bottom ﬂask, equipped with a reﬂux condenser with anargon inlet/outlet, a second argon valve and a screw-cappedadapter with a Teﬂon-coated septum. 10 mg (31.3 mmol) of[Ru(COD)(2-methylallyl)2] 2 were introduced followed bythe addition of 1.3 equivalents of the PNP ligand 4 (16 mg,40.7 mmol). Then, 5 mL of toluene were added via a syringeand the mixture was stirred at 60 1C for 5 hours followed byaddition of 100 equivalents of 1-hexanol 7 (319 mg, 3.1 mmol)via a syringe through the Teﬂon-coated septum. Afterwardsthe reaction mixture was heated to 120 1C for 15 h. The samplewas analysed by GC and GC-MS.General procedure for the direct amidationExperimental protocol for entry 2 (Table 2: amides): thereactions were performed under argon-ﬂow in a dried 20 mLthree-necked round bottom ﬂask, equipped with a reﬂuxcondenser with an argon inlet/outlet, a second argon valveand a screw-capped adapter with a Teﬂon-coated septum.20 mg of [Ru(COD)(2-methylallyl)2] 2 (62.6 mmol) wereintroduced followed by the addition of 1.3 equivalents of thePNN 3 (26 mg, 81.4 mmol). Then, 5 mL of toluene were addedvia a syringe and the mixture was stirred at 60 1C for 5 hoursfollowed by addition of 100 equivalents of 1-hexanol 7(638 mg, 6.2 mmol) and 100 equivalents of 1-hexylamine 14(626 mg, 6.2 mmol) via a syringe through the Teﬂon-coatedseptum. Afterwards the reaction mixture was heated to 120 1Cfor 20 h. The sample was analysed by GC and GC-MS.AcknowledgementsThe authors would like to thank the Max-Planck-Society,the RWTH Aachen, the Fonds der Chemischen Industrieand the German-Israeli-Project Cooperation (DIP-G 7.1) forfunding.Notes and references1 (a) P. J. Jessop and R. H. Morris, Coord. Chem. Rev., 1992,121, 155; (b) C. W. Jung and P. E. Garrou, Organometallics,1982, 1, 658; (c) D. Morton, D. J. Cole-Hamilton, I. D. Utuk,M. Paneque-Sosa and M. Lopez-Poveda, J. Chem. Soc., DaltonTrans., 1989, 489; (d) D. Morton and D. J. Cole-Hamilton,J. Chem. Soc., Chem. Commun., 1988, 1154; (e) L. A. Oro andM. A. Esteruelas, Chem. Rev., 1998, 98, 577; (f) Y. Lin andY. Zhou, J. Organomet. Chem., 1990, 381, 135; (g) S. Burling,M. K. Whittlesey and J. M. J. Williams, Adv. Synth. Catal., 2005,347, 591.2 (a) D. E. Fogg and E. N. dos Santos, Coord. Chem. Rev., 2004,248, 2365; (b) J.-C. Wasilke, S. J. Obrey, R. T. Baker andG. C. Bazan, Chem. Rev., 2005, 105, 1001.3 J. Zhang, G. Leitus, Y. Ben-David and D. Milstein, J. Am. Chem.Soc., 2005, 127, 10840.4 (a) M. G. Edwards and J. M. J. Williams, Angew. Chem., 2002,114, 4934 (Angew. Chem., Int. Ed., 2002, 41, 4740);(b) M. G. Edwards, R. F. R. Jazzar, B. M. Paine, D. J. Shermer,M. K. Whittlesey, J. M. J. Williams and D. D. Edney, Chem.Commun., 2004, 90; (c) P. J. Black, M. G. Edwards and J. M. J.Williams, Eur. J. Org. Chem., 2006, 4367; (d) G. Cami-Kobeci andJ. M. J. Williams, Chem. Commun., 2004, 1072; (e) P. J. Black,G. Cami-Kobeci, M. G. Edwards, P. A. Slatford, M. K. Whittleseyand J. M. J. Williams, Org. Biomol. Chem., 2006, 4, 116.5 (a) C. Gunanathan, Y. Ben-David and D. Milstein, Science, 2007,317, 790; (b) L. U. Nordstrom, H. Vogt and R. Madsen, J. Am.Chem. Soc., 2008, 130, 17672; (c) S. C. Ghosh, S. Muthaiah,Y. Zhang, X. Xu and S. H. Hong, Adv. Synth. Catal., 2009,351, 2643.6 B. Gnanaprakasam, J. Zhang and D. Milstein, Angew. Chem.,2010, 122, 1510 (Angew. Chem., Int. Ed., 2010, 49, 1468).7 (a) M. Nielsen, A. Kammer, D. Cozzula, H. Junge, S. Gladiali andM. Beller, Angew. Chem., 2011, 123, 9767 (Angew. Chem., Int. Ed.,2011, 50, 9593); (b) M. Nielsen, H. Junge, A. Kammer andM. Beller, Angew. Chem., 2012, 124, 5809 (Angew. Chem., Int.Ed., 2012, 51, 5711); (c) D. Spasyuk, S. Smith and D. Gusev,Angew. Chem., 2012, 124, 2826 (Angew. Chem., Int. Ed., 2012,51, 2772).8 (a) M. H. G. Prechtl, Y. Ben-David, D. Giunta, S. Busch,Y. Taniguchi, W. Wisniewski, H. Go¨rls, R. J. Mynott,N. Theyssen, D. Milstein and W. Leitner, Chem.–Eur. J., 2007,13, 1539; (b) M. H. G. Prechtl, M. Holscher, Y. Ben-David,N. Theyssen, R. Loschen, D. Milstein and W. Leitner, Angew.Chem., 2007, 119, 2319 (Angew. Chem., Int. Ed., 2007, 46, 2269);Downloaded on 11 December 2012Published on 10 July 2012 on http://pubs.rsc.org | doi:10.1039/C2CY20429KView Article Online2042 Catal. Sci. Technol., 2012, 2, 2039–2042 This journal is c The Royal Society of Chemistry 2012(c) M. H. G. Prechtl, M. Holscher, Y. Ben-David, N. Theyssen,D. Milstein and W. Leitner, Eur. J. Inorg. Chem., 2008, 22, 3493.9 (a) J. P. Genet, S. Mallart, C. Pinel, S. Juge and J. A. Laﬃtte,Tetrahedron: Asymmetry, 1991, 2, 43; (b) C. Six, K. Beck, A.Wegnerand W. Leitner, Organometallics, 2000, 19, 4639; (c) S. Busch andW. Leitner, Chem. Commun., 1999, 2305; (d) Y. Sun, H.-S. Chan,P. H. Dixneuf and Z. Xie, Organometallics, 2004, 23, 5864.10 M. H. G. Prechtl, Dissertation, RWTH Aachen, 2007.11 (a) L. S. Van Der Sluys, G. J. Kubas and K. G. Caulton, Organo-metallics, 1991, 10, 1033; (b) F. M. A. Geilen, B. Engendahl,M. Hoelscher, J. Klankermayer and W. Leitner, J. Am. Chem.Soc., 2011, 133, 14349.12 C. Gunanathan and D. Milstein, Acc. Chem. Res., 2011, 44,588.Downloaded on 11 December 2012Published on 10 July 2012 on http://pubs.rsc.org | doi:10.1039/C2CY20429KView Article Online

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