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Organometallic Compounds

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Organometallic Compounds Organometallic Compounds Dr. Ajay Kumar Das Associate Professor Department of Chemistry MLT College Saharsa [email protected] 9431863881 Organometallic Reagents in Synthesis Organometallic and other C-C bond forming reactions in some representative syntheses: Li = lithium reagent, Mg = Grignard reagent, Cu = organocopper reagent, P = Wittig reagent, Li/P Na/P K/P Horner-Wadsworth-Emmons, Pd/Sn = Stille coupling, Pd/Zn = Negishi coupling, Li/Si = Peterson olefination, Zr/Al = Tebbe reagent, B = organoboron reagent, R = Radical addition/cyclization. Isoamijiol (14-deoxy) Ruguluvasines A and B Majetich, G.; Song, J. S.; Ringold, C.; Nemeth, G. A. Liras, S.; Lynch, C. L.; Fryer, A. M.; Vu, B. T.; Martin, S. F. Tetrahedron Lett. 1990, 31, 2239 J. Am. Chem. Soc 2001, 123, 5918. R OH O H Li Si Pd/Sn Li O NHMe Si Cu R = Radical K HN Shahamin K Pironetin Lebsack, A. D.; Overman, L. E.; Valentkovitch, R. J. Dias, L. C.; Oliveira, L. G.; Sousa, M. A. J. Am. Chem. Soc. 2001, 123, 4851. O Org. Lett. 2003, 5, 265 AcO O LI H Li Li H O Li OMe OH O Cationic cyclization olefin Li Li P/Na OAc B B B Li Cu H Penostatin A (Deoxy) Morphine Snider, B. B.; Liu, T. Taber, D. F.; Neubert, T. B.; Rheingold, A. L. J. Org. Chem. 2000, 65, 8490-8498. J. Am. Chem. Soc. 2002, 124, 12416 P/Li H K O Li LI N H Diels Alder (hetero) O O H P K OH Carbene P/Li Li C7H15 OH Okinellin B Laurenyne Schmitz, W. D.; Messerschmidt, N. B. Overman, L. E.; Thompson, A. S. J. Am. Chem. Soc. J. Org. Chem. 1998, 63, 2058 1988, 110, 2248 Li H Cl O O Li/Si Li Li Cr Pd/Zn K/P O Cationic Zr/Al cyclization olefin O K Si Mg OH Hirsutene Dysidiolide D. P. Curran, D. M. Rakiewicz Madnuson, S. R.; Sepp-Lorenzino, L.; Rosen, N.; J. Am. Chem. Soc., 1985, 107, 1448. Danishefsky, S. J. J. Am. Chem. Soc. 1998, 120, 1615. Cu Li R H Li Pd R R = Radical cyclization • LI Li Li Cu O H H Claisen O OH 53% OH Tedanolide (13-deoxy) Smith, A. B.; Adams, C. M.; Barbosa, S. A. L.; Degnan, A. P. J. Am. Chem. Soc 2003, 125, 350 Li Li P1 Li1 P1 + S S PPh3 S S Br Br 1 + Li2 Li P1 O PPh3 O OTIPS OH P3 B2 B2 H O OH OMe OPMB L 3 P Li2 + O OMe O Li1 Li 1 PPh3 P2 O P S S Li3 B 1 B3 3 CO2Me Li P2 OH O O O O iPr B-Enolate O O B-Enolate CO2 O O N O N B CO2iPr O 2 Ph 3 B B1 Ph B Organometallic Reactions in Partial Synthesis of Spongistatin 1 Smith, A. B. et al Tetrahedron Lett. 1997, 38, 8667, 8761, 8675 CO2iPr O B(Ipc)2 (Mg) CO2iPr B(Ipc) B Major disconnections 2 O OH O O OBn HO Li B HO Li O Li H H H S S Li(Cu) O O H B Li OMe LI Li Li O HO Li O BnO Li OH H S S O Li H O PhSO2 Li O H Li B O OH O H Li Li O TESO Cl B PhSO2 OTES AcO Li Li OAc OTBS OH Spongistatin 1 S S Li t Li Li Li SiMe2 Bu PhSO2 S S S S PhSO2 OTES OTBS OPMB Classes of Nucleophilic Organometallic Reagents Strong Carbanion, M + Weak Lewis Acid High nucleophilicity C M+ R_ Li, R_ Na, R_ K, (R_ MgX) Weak Carbanion, M+ Lewis Acid Stereochemical control C _M _ _ _ _ _ Nucleophilic catalysis Cyclic transition states R B, R Al, R Zn, R Ti, R-SiX3, (R MgX) Weak Carbanion, M+ Non-Lewis Acidic Regiochemical control C _M R_ Si, R_ Sn, R_ Hg, various ate complexes Isomerically stable : Weak Carbanion, M+ Lewis Base Unusual Reactivity patterns C _M High selectivity towards electrophiles R2Cu , Pd° Balancing the Reactivity of Nucleophile and Electrophile N + E+ N_ E O O H + + HX X R R Activate the nucleophile: O O Li + Me2N R R BuLi Br Activate the electrophile: O + O H + R + HCl R AlCl3 O Cl R Assemble on a transition metal (mildly activate both E and N): O O Pd(0) SnMe3 + + Me3SnCl Cl R R Preparation of Organolithium Reagents 1. Reduction of carbon-X bonds with lithium metal R-X + 2Li° R-Li + LiX MeLi PhLi n-Bu-Li t-BuLi s-Bu-Li X = Cl, Br, I, SPh O O 2. Metalation (Li/H exchange) OMe S Ph R-C≡C-Li R-H + R'Li R-Li + R'-H Li Li O Li 3. Lithium-metalloid exchange (Li/M) Bn O O RO Li R-M + R'Li R-Li + R'M BnO Li BnO Li M = Br, I, SnBu 3, HgCl, SePh, TePh H 4. Addition of RLi to C-C multiple bonds. Li Li Li R'Li R R' R R R Ph PhSO2 5. Metalation of N-sulfonylhydrazones (Shapiro) Li N NHSO2Ar 2 n-BuLi Effect of Substituents on Carbanion Stability Gas Phase Acidity (kcal/mol) - Type: CH2 -X pKa of H-CH2-X Typical Metalating Agents (CH3)2CH: 10 CH3CH2: Destabilizing (compared to H)* >60 None available CH3: 0 416.6 -CH3 Very Weak** 50-60 sec-BuLi, n-BuLi/TMEDA : n-BuLi/ tBuOK H C=CH: -10 -OR -NR2 -SiR3 H2 Ph: H: Weak*** 40-50 n-BuLi, sec-BuLi, LiTMP NH2: ClCH2: -20 SR PR2 SeR BR2 MeSCH2: Me3SiCH2: HO: CH=CH2 -C≡C-R -Halogen -Ph H2C-CH-CH2: -30 Intermediate 30-40 LDA, n-BuLi, KH Me2PCH2: Me3Si: O O O PhCH2: -40 CN CH3O: - - HC≡C: O N-R NR2 Cl2CH: F: O O O O O (Me2P)2CH: -50 S S Se P R R R R R (Ph)3C: CH3S: Strong 20-30 KO-t-Bu, NaH, LDA -60 O O KH, LiN(TMS) HS: + R + 3 S PR3 R OR R' - H -70 Me3Sn: SO2CF3 Very Strong 10-20 NaOH, KO-t-Bu, DBU -80 + -NO2 -N ≡N Brauman J. Am. Chem. Soc. 1995, 117, 4908. *Alkyl groups are invariably kinetically deactivating. ** These types are not usually prepared by metalation, but by other techniques (Li/Sn, Li/Halg exchange, reduction of halogen or SR). ***Need two of these (X-CH2-X') for easy metalation with LDA. Effect of Substituents on Carbanion Stability K 1. Hybridization In almost all areas of organometallic chemistry the primary subdivision of reactivity types is by the hybridization of the C-M carbon atom (methyl/alkyl, vinyl/aryl, alkynyl). A key second subdivision is the presence of conjugating substituents (allyl/allenyl/propargyl/benzyl). The fractional s-character of the C-H bonds has a major effect on the kinetic and thermodynamic acidity of the carbon acid. Only s-orbitals have electron density at the nucleus, and a lone pair with high fractional s character has its electron density closer to the nucleus, and is hence stabilized. This can be easily seen in the gas-phase acidity of the prototypical C-H types, ethane, ethylene and acetylene, as well as for cyclopropane, where the hybridization of the C-H bond is similar to that in ethylene. CH3-CH3 CH2=CH2 HC≡CH ΔH°acid (kcal/mol) 420 411 406 375 These effects are also clearly evident in solution, with terminal acetylenes and highly strained hydrocarbons easily metalated by strong bases. Li n-BuLi JACS-72-7735 2. Inductive Effects Electron-withdrawing substituents will inductively stabilize negative charge on nearby carbons. These effects are complex, since electronegative substituents interact with carbanions in other ways as well (e.g. O and F substituents have lone pairs, which tend to destabilize adjacent carbanion centers). O O O O O O O O O O + S H S CH3 S OMe S F S NMe3 Ph Ph Ph Ph Ph H H H H H pKa (DMSO) 29.0 31.0 30.7 28.5 19.4 3. Conjugation - π Delocalization Delocalization of negative charge, especially onto electronegative atoms, provides potent stabilizations of carbanionic centers. Since almost all conjugating substituents are also more electronegative than H or CH 3, there is usually a significant inductive contribution to the stabilization. O O N CH4 CH3 H H C H t-BuO pKa (DMSO) ~55 43 26.5 30.3 31.3 A special case is the aromatic stabilization of cyclopentadienide and related indenide and fluorenide anions (Huckel 4n + 2 π electron rule) . pKa (DMSO) 18.0 20.1 22.6 30.1 The aromatic anions (6e π system) show a level of stabilization far above that of normal conjugated systems ΔH°acid (kcal/mol) 356.1 373.9 H K 4. Second and Third Row Element Effects ("d-orbital" effects) All measures of acidity show that there is an unusual level of carbanion stabilization for all second row elements (Cl, S, P, Si, as well as higher elements) when these are bonded to a carbanion center. 6 3 10 0.25 300 CH CH3 CH3 CH3 0.013 CH3 S O N Kinetic acidity 0.25 330 500 14 Isotopic exchange 0.45 24 1 0.2 KNH2/NH3 0.41 6 0.25 0.07 O X Me OMe OPh SPh SePh Bordwell J. Org. Chem. X 1976, 41, 1885 Ph pKa (DMSO) 24.4 22.9 21.1 17.1 18.6 Gas phase acidity FCH3 MeOCH3 Me-CH3 ΔH°acid (kcal/mol) 409 407 420.1 ClCH3 MeSCH3 Me3SiCH3 ΔH°acid (kcal/mol) 395.6 393.2 390.9 ΔΔH°acid 13.4 13.8 19.2 The origin of this stabilization has several components.
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