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N-Heterocyclic Carbenes (Nhcs)

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N-Heterocyclic Carbenes (Nhcs) Baran Lab N - H e t e r o c y c l i c C a r b e n e s ( N H C s ) K. J. Eastman An Introduction to N-heterocyclic Carbenes: How viable is resonance contributer B? Prior to 1960, a school of thought that carbenes were too reactive to be smaller isolated thwarted widespread efforts to investigate carbene chemistry. base ! N N N N R R R R Perhaps true for the majority of carbenes, this proved to be an inaccurate X- assessment of the N-heterocyclic carbenes. H longer Kirmse, W. Angerw. Chem. Int. Ed. 2004, 43, 1767-1769 In the early 1960's Wanzlick (Angew. Chem. Int. Ed. 1962, 1, 75-80) first investigated the reactivity and stability of N-heterocyclic carbenes. Attractive Features of NHCs as Ligands for transition metal catalysts: Shortly thereafter, Wanzlick (Angew. Chem. Int. Ed. 1968, 7, 141-142) NHCs are electron-rich, neutral "#donor ligands (evidenced by IR frequency of reported the first application of NHCs as ligands for metal complexes. CO/metal/NHC complexes). Surprisingly, the field of of NHCs as ligands in transition metal chemistry Electron donating ability of NHCs span a very narrow range when compared to remained dormant for 23 years. phosphine ligands In 1991, a report by Arduengo and co-workers (J. Am. Chem. Soc. 1991, Electronics can be altered by changing the nature of the azole ring: 113, 361-363) on the extraodinary stability, isolation and storablility of benzimidazole<imidazole<imidazoline (order of electron donating power). crystalline NHC IAd. NHC-metal complex stability: NaH, DMSO, MeOH N N N N + H2 + NaCl NHCs form very strong bonds with the majority of metals (stronger than Cl- phosphines!) H IAd N-heterocyclic carbenes are electronically (orbital overlap) and sterically (Me vs. Ad) stablilized. N N N N + MLn R R R R Different from traditional carbenes, NHCs are electron rich: (resonance) MLn (robust) (sensitive) R R R N N N PR3 PR3 + MLn N N N MLn R R R A B C 1 Baran Lab N - H e t e r o c y c l i c C a r b e n e s ( N H C s ) K. J. Eastman NHCs are most frequently prepared via deprotonation of the corresponding c) unsymmetrical synthesis of imidazolidinium salts: 1 2 azolium salts (imidazolium, triazolium, tetrazolium, pyrazolium, benz- Ar NH2, Et3N, Ar NH2, O O THF, 0 oC O O toluene, 120 oC O O imidazolium, oxazolium, thiazolium salts - pKa - 21-24). Cl OEt Ar1HN OEt Ar1HN NHAr2 Imidazolium syhthesis: 1) BH -THF, reflux 3 HC(OEt) 1) Existing imidazoles can be alkylated with appropriate electrophiles. 2) conc. HCl 3 - 1 2 X Ar H2N NH2Ar N N 120 oC Ar1 Ar2 2 Cl- 2) The imidazolium ring can be built: d) unsymmetrical synthesis of imidazolium salts: R4 O OEt a) symmetric synthesis: O O O HX, -H O 5 2 Cl- HO R Br A OEt R NH2 N N R R 1 H H R NH2, H H R1NH , cat. HCl, 2 nBuLi, toluene, then A reflux R N N R Cl OEt R4 O OEt H H O 1 5 1 , (AgOTf) R NH R R NH OEt Cl O tBu 1) HCOOH, Ac O MeC(O)OC(O)H 2 2) HCOOH b) unsymmetrical synthesis: 4 5 R O R OAc O O O O 2 - 4 HX, -H2O R X X Ac O, HX R Ac O, HX 1 N N 2 2 R NH2 + NH3 1 N N R1 R2 R R1 N R5 X- O R1 N H H H H H N O O H 2 H base R NH2 N N R1 X H 1 R = alkyl 5 R5 R OH 4 R4 R HX - 2 Furstner et al. Chem. X- N R2 X N R Commun. 2006, 2176-2178 N N 1 R1 R 2 Baran Lab N - H e t e r o c y c l i c C a r b e n e s ( N H C s ) K. J. Eastman Different Monodentate NHC Ligand Classes: 4-membered NHC - stable benzannulated imidazolin-2-ylidene N N 2 R - R1 steric influence is in very close proximity to metal iPr NiPr iPr R1 P - Grubbs N N - bulk required for stability - less reactive/! donating H base iPr iPr N N 2 N N 2 5-membered NHC R R (R1)H X- (R1)H X- R(H) R(H) N N N N R1, R2 R1 R2 R1 R2 - most abuntantly blocked or substituted "abnormal carbene" used as ligands 2,4,6-(Me)3C6H2 IMes SIMes 2,6-(iPr)2C6H3 IPr SIPr 6 and 7-membered NHC cyclohexyl ICy tert-butyl ItBu 2 2 1-adamantyl IAd R R N N N N N N O O - electron richness comparable to tBuP R1 R1 iPr iPr Ad-2 2-Ad - flexibility in steric bulk on rigid tricyclic backbone R1 N N R2 - sterics can be modified w/out affecting electronics - rarely seen outside of initial preparation and testing R2 R1 (rare for monodentate ligands) - only complexes of 7-member ring carbene isolable 1 2 R , R = (CH2)4 : IBiox 5 1 2 R , R = (CH2)5 : IBiox 6 NHC as ligands for high-oxidation-state metal complexes and oxidation 1 2 R , R = (CH2)6 : IBiox 7 catalysts: 1 2 R , R = (CH2)7 : IBiox 8 1 2 R , R = (CH2)11 : IBiox 12 Points to consider: R R 1) Phosphines are not used as ancillary ligands in oxidation chemistry. Why? - R = H - very electron rich, but also unstable - R = tBu - very electron rich and stable 2) Are free NHC subject to the same fate as free phosphines? N N 3) Promise held by NHC for oxidation chemistry. R = H R = tBu 3 Baran Lab N - H e t e r o c y c l i c C a r b e n e s ( N H C s ) K. J. Eastman NHC as a ligand for the stabilization of high-oxidation-state metals: Recent independent studies by Stahl and Kawashima show NHCs playing a role as stabilizing ligands in palladium oxidation reactions (J. Am. Chem. Soc. 2001, 123, 7188-7189, J. Am. Chem. Soc. 2004, - stable complex below -20 oC CH3 N N 126, 10212-10213 and J. Am. Chem. Soc. 2005, 127, 7294-7295.) - NHC NOT oxidized at -60 oC Re N X O (NHC)2Re(CH3)O3 II O THF, -60 oC - phosphines are rapidly HOOH Pd SH O N X 2 oxidized with MTO MTO Herrmann et al. J. Organomet. Chem. 1994, 480:C7 2HX Sox + 2 HX N O N A number of additional high-valent metal complexes have been PdII Pd0 stabilized with NHCs: N O N Cl- N N MesN NMes N N N N O2 O Cl Cl O O Cl O Cl W N U Mo N N Mo N Ar O Cl N N N Cl Cl Ar Ar O O O N MesN NMes N N N ArN + O O Pd 2 Pd ArN O Ar Ar N N - first cationic Mo(VI) N complex Ar - stability of these metal-complexed NHCs relative to phosphines will permit further analysis of these fundmental reactions Ph N Cl PMe Ph Mes Cl 2 N Ph N N Re Cl NHCs have been found to be useful with cobalt and nickel as stabilizing Cl PMe Ph V U O 2 N Re N N N Cl ligands for oxidation (allylic oxidations). N O N N N Mes Cl N Ph NPh N However, sterically hindered bis-carbene complexes of Cu(I) undergo Ph Ph Ph Ph rapid oxidation: - first examples of NHC-coordinated metal-oxo complexes characterized R R R by X-ray crystallography N N N O2 Cu1 2 O N N CH2Cl2 N R R R R = ethyl, allyl, benzyl NHCs are not yet universally applicable to metal-mediated oxidation 4 Baran Lab N - H e t e r o c y c l i c C a r b e n e s ( N H C s ) K. J. Eastman Oxidation Reactions Catalyzed by NHC-Coordinated Metal Complexes Oxidative Cleavage of Alkenes: (Crabtree and PersisOrganometallics 2003, Opppenauer-Type Alcohol Oxidation:(Yamaguchi et al. Organometallics 22, 1110-1114) 2004, 23, 1490-1492) N N N R Br N Ir OTf N N Ru OTf nBu Br nBu O N CO OH O R O OH NaIO4 Ph Ph O O - smaller R-substituents promoted catalytic activity " - R = Me - cat. loading = 0.025 mol % w/ 3200 turnovers - Analogs with PPh3 or PnBu3 ligands were not active " Palladium-Catalyzed Aerobic Alcohol Oxidation: (Sigman et al. J. Org. Chem. O 2005, 70, 3343-3352) N N O Ar Ar " O Pd O R R O O OH O " O H H O O /air Ph 2 Ph O - A single NHC ligand can withstand the aerobic oxidation conditions O - small reaction scope - electron-deficient alkenes appear - Suggesting, despite numerous cycling between PdII and Pd0, the to react slower than electron-rich alkenes ligand does not dissociate from the palladium center - evidence supports the NHC-Ru complex remaining in tact Wacker-Type Oxidation: (Muniz, K. Adv. Synth. Catal. 2004, 346, 1425 –1428) during the reaction Oxidation of Methane: (Herrmann and Strassner Angew. Chem. Int. Ed. 2002, 41, 1745-1747) N N IMesPd(TFA)2 toluene, O , 80 oC N Pd N OH 2 O X X - Nitrogen-containing heterocycles have been prepared similarly CH4 + CF3COOH CF3COOCH3 - seven membered NHC ligands have been successfully employed K2S2O8 2 KHSO4 80 oC - Chelating nitrogen ligands did not work under comparable conditions - Yield is only 2-3 times higher than with Pd(OAc)2 without ligands 5 Baran Lab N - H e t e r o c y c l i c C a r b e n e s ( N H C s ) K.
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