Grotjahn, Douglas B

Gustafson, Jeffery L

Larsen, Casey R

Nair, Reji

Sharma, Abhinandini

eScholarship - University of California

UC San DiegoUC San Diego Previously Published WorksTitleExtensive Isomerization of Alkenes Using a Bifunctional Catalyst:Â  An Alkene ZipperPermalinkhttps://escholarship.org/uc/item/2pj3b28mJournalJournal of the American Chemical Society, 129(31)ISSN0002-7863 1520-5126AuthorsGrotjahn, Douglas BLarsen, Casey RGustafson, Jeffery Let al.Publication Date2007-08-01DOI10.1021/ja073457i Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaExtensive Isomerization of Alkenes Using a Bifunctional Catalyst: An AlkeneZipperDouglas B. Grotjahn,* Casey R. Larsen, Jeffery L. Gustafson, Reji Nair, and Abhinandini SharmaDepartment of Chemistry and Biochemistry, 5500 Campanile DriVe, San Diego State UniVersity,San Diego, California 92182-1030Received May 15, 2007; E-mail: grotjahn@chemistry.sdsu.eduRedox isomerizations are examples of atom-economical pro-cesses1 in which one site in an organic substrate is oxidized withconcomitant reduction of another site. One especially well-studiedexample is the conversion of allylic alcohols to aldehydes orketones,2,3 which involves movement of the alkene double bond4-7over two positions (eq 1,n ) 2). Far fewer catalysts exist for themovement of a more remote double bond (> 2). For example,Kirchner et al. reported8 that, although [CpRu(PR3)(CH3CN)2]+(1a-1c) were improved catalysts for allylic alcohol isomerizationrelative to CpRu(PR3)2Cl, it failed in the case of 3-buten-1-ol (n )3) or alkenes devoid of alcohol functionality. The apparent recordfor alkene isomerization of any kind is over 20 positions on thehydrocarbon CH3(CH2)19CHdCH(CH2)19CH3 by stoichiometricamounts of the reagent Cp2Zr(H)(Cl).9 The apparent record forcatalyzeddouble bond movement is on 9-decene-1-ol (nine posi-tions,n ) 9) using Fe3(CO)12.10,11However, 30 mol % was required,which means that nearly a mole of metal was used per mole ofalkenol.Here we highlight the discovery and development of a novelcatalyst (2) whose notable and attractive features include thefollowing: (1) ability to isomerize a variety of heterofunctionalizedalkene derivatives, (2) formation of (E)-isomer products of highpurity, (3) useful activity in many cases at low loadings (2-5 mol%), and (4) a heterocyclic ligand which appears to act not merelyas a hemilabile group but rather as an internal base. Finally, weshow that bifunctional catalyst2 is truly an alkene zipper12 byreportingthe catalyzed moVement of an alkene double bond oVer30 positionsalong an alkyl chain (eq 1,n ) 30).Initial screening experiments were conducted on the alkenes1-pentene and pent-4-en-1-ol, representing two different classes ofsubstrate, one lacking any polar or protic group and the otherfeaturing the alkene and OH separated by three carbons (eq 1,n )4). Such substrates were inert to1a-1c.13 From these preliminarystudies, which included phosphines with both imidazol-2-yl andpyrid-2-yl substituents on P as well ast-Bu, i-Pr, Ph, and Me groups,the phosphine ligand14,15 in 2 emerged as a superior choice for itsability to promote isomerization of both substrates.To prepare2, [CpRu(CH3CN)3]+PF6- in acetone was allowedto react with the required ligand, forming a mixture of1d andchelate2.16 This mixture could be used but for purposes of catalystcharacterization and further testing as described in this paper wasconverted to>90% pure2 by repeated coevaporation of addedacetone with the acetonitrile liberated. Complex2 showed diagnosticNMR data consistent with a chiral structure and four unique,diastereotopic methyl groups.Because pent-4-en-1-ol (13a) represented a more challengingclass of substrate, its isomerization to pentanal (14a) was optimizedwith respect to solvent, catalyst amount, and reaction temperature.At the 2 mol % level, heating at 70°C was advised, whereas byusing 5 mol %, reactions could be completed at 25°C within 1 d.Acetone emerged as the solvent of choice, although dichlo-romethane could also be used. The effects of a substrate hydroxylgroup were apparent by noting that under the optimized conditions,isomerization of 1-pentene to (E)-2-pentene required only minutesat room temperature using 2 mol % catalyst (Scheme 1).Substrate scope was explored. Table 1 shows results which arein most cases unoptimized for a particular substrate but are designedto give an idea of the capabilities and limitations of the catalyst. Ingeneral, full isomerization of substrates with polar groups (OH,amide) required heating at 70°C, whereas partial isomerization ofthese compounds or full isomerization of nonpolar substrates couldbe done at 25°C. Most dramatically, alkenol isomerization over 9,22, and even 30 positions could be achieved (entries 9-12). Thesilyl ether11 (entry 8) gave enol ether (E)-12 in which only tracesScheme 1. Reactants and ProductsPublished on Web 07/18/20079592 9 J. AM. CHEM. SOC. 2007 , 129, 9592-9593 10.1021/ja073457i CCC: $37.00 © 2007 American Chemical Society(<2%) of (Z)-isomer are detected in 500 MHz1H NMR spectra.Remarkably, diallyl ether (entry 2) gave (E,E)-4 in high yieldwithout Claisen rearrangement of the intermediate allyl propenylether17-19 and without any detectable (Z,E)-isomer, providing rapidentry under neutral conditions to a little-explored class of com-pounds which have typically been made as mixtures using strongbase.20Preliminary experiments are thus far consistent with the mecha-nistic hypothesis advanced in Scheme 2. Rather surprisingly,bubbling ethylene (∼25 equiv) into an acetone solution of2 leadsto formation offree acetonitrileand an ethylene complex similarto 15, rather than a chelate-opened species. Therefore, we proposethat the first step of the catalytic cycle is exchange of acetonitrilefor alkene, forming15. The role of the basic nitrogen of theimidazole could then be deprotonation at an allylic position, forming16with an anionic allyl and a protonated heterocyclic ligand, whichcould then return the proton to either end of the allyl moiety andpromote isomerization. Preliminary experiments using propeneinstead of ethylene show formation of15 but no other detectablecomplexes; thus, the concentration of16 during the catalysis mustbe low.Additional compelling evidence for the role of the heterocyclein the catalyst21 is that complex1e is 330 times slower atisomerizing 1-pentene than is2 and, moreover, is 10000 timesslower at isomerizing pent-4-en-1-ol. Our mechanistic hypothesisfor high (E)-selectivity is that either the transition states (e.g., from15 to allyl complex16) or intermediates (e.g.,16) leading to (Z)-alkene are sufficiently high in energy to leave the system shuttlingbetween terminal and (E)-alkene(s), creating “semi-thermodynamic”mixtures consisting of almost exclusively these latter isomers. Themechanistic hypothesis in Scheme 2 helps rationalize the unusualability of 2 to convert geraniol to (E)-isogeraniol rather than to thealdehyde, because formation of the enol isomer would require thecatalyst to form an allyl intermediate like16-Z. Finally, we notethat, since isomerization of the longer substrates presumably occursby a random walk of the double bond up and down the chain enroute to the most stable isomer, the observed movement of thedouble bond is actually just a lower estimate of the number of timesthe catalyst acts on a substrate.In summary, catalyst2 is unusual in its activity (in many cases2-5 mol % is sufficient), selectivity, and unprecedented ability tomove unhindered double bonds over many positions. Ongoingstudies are designed to elucidate further the mechanism of actionand the capabilities of this bifunctional catalyst design, and thesewill be reported in due course.Acknowledgment. The NSF is thanked for support of this andrelated work, and Dr. LeRoy Lafferty is thanked for assisting withNMR experiments.Supporting Information Available: Details of compound prepara-tion, characterization, and use of the catalysts. This material is availablefree of charge via the Internet at http://pubs.acs.org.References(1) Trost, B. M.Acc. Chem. Res.2002, 35, 695-705.(2) Uma, R.; Crevisy, C.; Gree, R.Chem. ReV. 2003, 103, 27-51.(3) Kuznik, N.; Krompiec, S.Coord. Chem. ReV. 2007, 251, 222-233.(4) Herrmann, W. A.; Prinz, M. InApplied Homogeneous Catalysis withOrganometallic Compounds, 2nd ed.; Cornils, B., Herrmann, W. A., Eds.;Wiley-VCH: Weinheim, Germany, 2002; Vol. 3, pp 1119-1130.(5) Negishi, E.-I. InHandbook of Organopalladium Chemistry for OrganicSynthesis; Negishi, E.-i., Ed.; John Wiley & Sons: Hoboken, NJ, 2002;Vol. 2, pp 2783-2788.(6) Suzuki, H.; Takao, T. InRuthenium in Organic Synthesis; Murahashi,S.-I., Ed.; Wiley-VCH: Weinheim, Germany, 2004; pp 309-331.(7) Otsuka, S.; Tani, K. InTransition Metals for Organic Synthesis, 2nd ed.;Beller, M., Bolm, C., Eds.; Wiley-VCH: Weinheim, Germany, 2004; Vol.1, pp 199-209.(8) Rüba, E.; Simanko, W.; Mauthner, K.; Soldouzi, K. M.; Slugovc, C.;Mereiter, K.; Schmid, R.; Kirchner, K.Organometallics1999, 18, 3843-3850.(9) Gibson, T.; Tulich, L.J. Org. Chem.1981, 46, 1821-1823.(10) Iranpoor, N.; Mottaghinejad, E.J. Organomet. Chem.1992, 423, 399-404.(11) In eq 1, isomerization to the enol requires a movement overn-1 positions,all of which can be reasonably attributed to catalysis by metal. The finalenol-keto tautomerization may or may not involve metal catalyst but forsimplicity is included in the total count ofn.(12) The term “zipper” has also been applied to alkyne isomerization bysuperstoichiometric amounts of strong bases: Brown, C. A.; Yamashita,A. J. Am. Chem. Soc.1975, 97, 891-892. Bifunctional reactivity may beinvolved, but salts of simple amines also work: Abrams, S. R.; Nucciarone,D. D.; Steck, W. F.Can. J. Chem.1983, 61, 1073-1076.(13) Slugovc, C.; Rueba, E.; Schmid, R.; Kirchner, K.Organometallics1999,18, 4230-4233.(14) Grotjahn, D. B.; Gong, Y.; Zakharov, L. N.; Golen, J. A.; Rheingold, A.L. J. Am. Chem. Soc.2006, 128, 438-453.(15) For examples of ligands we have made and tested here, see: Grotjahn,D. B.; Incarvito, C. D.; Rheingold, A. L.Angew. Chem., Int. Ed.2001,40, 3884-3887. Grotjahn, D. B.; Lev, D. A.J. Am. Chem. Soc.2004,126, 12232-12233. Grotjahn, D. B.; Gong, Y.; DiPasquale, A. G.;Zakharov, L. N.; Rheingold, A. L.Organometallics2006, 25, 5693-5695.(16) See Supporting Information for details.(17) Trost, B. M.; Zhang, T.Org. Lett.2006, 8, 6007-6010.(18) Schmidt, B.J. Mol. Catal. A: Chem.2006, 254, 53-57.(19) Nelson, S. G.; Bungard, C. J.; Wang, K.J. Am. Chem. Soc.2003, 125,13000-13001.(20) Taskinen, E.; Virtanen, R.J. Org. Chem.1977, 42, 1443-1449.(21) Placement of the base in the catalyst is crucial: adding 1-methyl-4-tert-butylimidazole to the 1-pentene isomerization by1e made the reactioneven slower.JA073457ITable 1. Scope and Limitations Using 2aentry reactant product2(%)temp(°C) timeyield(%)1 1-pentene (E)-2-pentene 2 25 15 min 952 3 (E, E)-4 2 25 40 min 963 5 (E)-6 2 25 4 h 75b4 7 8 2 25 26 h 70:30c5 9 10 2 25 4 d 61d6 13a 14a 2 70 1 h 95e7 13b 14b 2 70 1 h 978 11 (E)-12 5 70 4 h 909 13c 14c 5 70 4 h 8410 13d 14d 5 70 4 h 9711 13e 14e 20 70 3.6 d 91b12 13f 14f 30 70 3 d 81ba Acetone-d6 solvent, yields determined by1H NMR and internal standardunless otherwise indicated.b Isolated.c Equilibrium ratio of7 and8, alsoreached from either side within 2-4 h using 2 mol % of2 at 70 °C.d Unreacted9 (29%) and an unidentified isomer (10%) also present.e Isomerization could also be completed using 5 mol % of2 for 1 d at25 °C.Scheme 2. Mechanistic HypothesisC O M M U N I C A T I O N SJ. AM. CHEM. SOC. 9 VOL. 129, NO. 31, 2007 9593

Extensive Isomerization of Alkenes Using a Bifunctional Catalyst:Â An Alkene Zipper

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