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Organometallic Compounds - Reagents with Carbon-Metal Bonds 14.1: Organometallic Nomenclature (Please Read)

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Organometallic Compounds - Reagents with Carbon-Metal Bonds 14.1: Organometallic Nomenclature (Please Read) Chapter 14: Organometallic Compounds - Reagents with carbon-metal bonds 14.1: Organometallic Nomenclature (please read) H H C C - + H3CH2CH2CH2C-Li (H3C)2Cu Li H MgBr Butyllithium vinylmagnesium bromide Dimethylcopper lithium 14.2: Carbon-Metal Bonds in Organometallic Compounds C X C MgX - + !+ !- ! ! _ C X C MgX C Alkyl halides: Carbanions: nucleophile electrophiles react with electrophile 302 Alkyl halides will react with some metals (M0) in ether or THF to form organometallic reagents 14.3: Preparation of Organolithium Compounds Organolithium Compounds 2 Li(0) R-X R-Li + LiX diethyl ether - + ! ! _ very strong bases C Li C very strong nucleophiles organolithium reagents are most commonly used as very strong bases and in reactions with carbonyl compounds M(0) H O R-X R-M 2 R-H + M-OH 303 1 14.4: Preparation of Organomagnesium Compounds: Grignard Reagents Mg(0) R-X R-MgX (Grignard reagent) THF R-X can be an alkyl, vinyl, or aryl halide (chloride, bromide, or iodide) Solvent: diethyl ether (Et2O) or tetrahydrofuran (THF) H CH C CH CH 3 2 O 2 3 O diethyl ether (Et2O) tetrahydrofuran (THF) Alcoholic solvents and water are incompatible with Grignard reagents and organolithium reagents. Reactivity of the alkyl halide: -I > -Br > -Cl >> -F alkyl halides > vinyl or aryl halides 304 The solvent or alkyl halides can not contain functional groups that are electrophilic or acidic. These are incompatible with the formation of the organomagnesium or organolithium reagent. Grignard reagents will deprotonate alcohols 0 _ Mg + H3O HO Br HO MgBr O H HO H _ BrMg Other incompatible groups: -CO2H, -OH, -SH, NH2, CONHR (amides) Reactive functional groups: aldehydes, ketones, esters, amides, halides, -NO2, -SO2R, nitriles 305 2 14.5: Organolithium and Organomagnesium Compounds as Brønsted Bases - Grignard reagents (M = MgX) and organolithium reagents (M = Li) are very strong bases. R-M + H2O R-H + M-OH pKa pKa (CH3)3C-H 71 H2N-H 36 H3CH2C-H 62 H C C H 26 H3C-H 60 Water 16 H H C C 45 H H H H H H 43 H H Hydrocarbons are very weak acids; their conjugate bases are very strong bases. 306 Lithium and magnesium acetylides THF + R C C H H3C-H2C-H2C-H2C-Li R C C Li + H3C-H2C-H2C-H2C-H terminal butyllithium pK > 60 acetylene lithium a acetylide (pKa ~ 26) THF + R C C H H3C-H2C-MgBr R C C MgBr + H3C-H2C-H ethylmagnesium magnesium bromide acetylide 14.6: Synthesis of Alcohols Using Grignard Reagents Grignard reagents react with aldehydes, ketones, and esters to afford alcohols MgX MgX ! - + O O ether O H3O OH ! + R: C C C C R R 307 3 Grignard reagents react with . formaldehyde (H2C=O) to give primary alcohols Br 0 1) H2C=O Mg , ether MgBr + 2) H3O OH aldehydes to give secondary alcohols O OH Br Mg0, ether MgBr 1) H + 2) H3O ketones to give tertiary alcohols OH H H 0 H H O Mg , ether 1) C C C C + H Br H MgBr 2) H3O esters to give tertiary alcohols O C OH OCH2CH3 0 2 Mg , ether 1) C CH 2 H C-Br 2 H C-MgBr 3 3 3 CH3 + 2) H3O 308 14.10: Preparation of Tertiary Alcohols From Esters and Grignard Reagents - mechanism: O 1) THF OH + C 2) then H3O + C OCH2CH3 2 H3C-MgBr CH3 CH3 Reaction of Grignard reagents with CO2 (Lab, Chapter 19.11) _ O O O OH Br MgBr + Mg(0) O=C=O H3O ether 309 4 14.7: Synthesis of Alcohols Using Organolithium Reagents Organolithium reagents react with aldehydes, ketones, and esters in the same way that Grignard reagents do. Li + O ether O H3O OH + R-Li C C C R R 14.8: Synthesis of Acetylenic Alcohols + R C C H + NaNH2 R C C Na + NH3 pKa~ 26 pKa~ 36 + R C C H + H3C(H2C)H2C-Li R C C Li + H3C(H2C)CH3 pKa > 60 + R C C H + H3CH2C-MgBr R C C MgBr + H3CCH3 pKa > 60 310 Recall from Chapter 9.6 _ THF Na + + R C C CH R + NaBr R1 C C R2-H2C-Br 1 2 2 SN2 acetylide anion 1° alkyl halide new C-C bond formed Acetylide anions react with ketones and aldehydes to form a C-C bond; the product is an acetylenic (propargyl) alcohols OH O THF _ + + R1 C C C R1 C C MgBr C + R3 R2 R3 then H3O R2 acetylenic alcohol 311 5 14.9: Retrosynthetic Analysis - the process of planning a synthesis by reasoning backward from the the target molecule to a starting compound using known and reliable reactions. “it is a problem solving technique for transforming the structure of a synthetic target molecule (TM) to a sequence of progressively simpler structures along the pathway which ultimately leads to simple or commercially available starting materials for a chemical synthesis.” The transformation of a molecule to a synthetic precursor is accomplished by: Disconnection: the reverse operation to a synthetic reaction; the hypothetical cleavage of a bond back to precursors of the target molecule. Functional Group Interconversion (FGI): the process of converting one functional group into another by substitution, addition, elimination, reduction, or oxidation 312 Each precursor is then the target molecule for further retrosynthetic analysis. The process is repeated until suitable starting materials are derived. Target Precursors Precursors Starting molecule 1 2 materials Prepare (Z)-2-hexene from acetylene Z-2-hexene 2-Phenyl-2-propanol OH CH3 CH3 313 6 14.11: Alkane Synthesis Using Organocopper Reagents ether H3C _ + 2 CH3Li + CuI Cu Li + LiI H3C Gilman's reagent (dimethylcuprate, dimethylcopper lithium) - R2CuLi = R strong nucleophiles Nucleophilic substitution reactions with alkyl halides and sulfonates (alkylation) H C(H C) H C-I + (H C) CuLi H C(H C) H C-CH 3 2 8 2 3 2 ether 3 2 8 2 3 + CH3Cu + LiI SN2 reaction of cuprates is best with primary and secondary alkyl halides; tertiary alkyl halides undergo E2 elimination. 314 Vinyl and aryl (but not acetylenic) cuprates (0) Br 4 Li , ether Li CuI 2 2 CuLi 2 Br Li CuLi 4 Li(0), ether CuI 2 2 THF CuLi + OTs 2 CuLi THF + I 2 315 7 Reaction of cuprates with aryl and vinyl halides H H (H3C)2CuLi double bond geometry I CH3 is preserved H H Br CH2CH2CH3 (H3CH2CH2C)2CuLi 14.13: Carbenes and Carbenoids Carbene: highly reactive intermediate, 6-electron species. The carbon is sp2 hybridized; it possesses a vacant hybridized p-orbital and an sp2 orbital with a non-bonding pair of electrons 316 Generation and Reaction of Dihalocarbenes: CHCl3 + KOH Cl2C: + H2O + KCl dichlorocarbene Carbenes react with alkenes to give cyclopropanes. Cl Cl H H CHCl3, KOH H H R R R R cis-alkene cis-cyclopropane Br Br H R CHBr3, KOH H R R H R H trans-alkene trans-cyclopropane The cyclopropanation reaction takes place in a single step. There is NO intermediate. As such, the geometry of the alkene is preserved in the product. Groups that are trans on the alkene will end up trans on the cyclopropane product. Groups that are cis on the alkene will end up cis on the cyclopropane product. 317 8 14.12: An Organozinc Reagent for Cyclopropane Synthesis Simmons-Smith Reaction ether CH2I2 + Zn(Cu) I-CH2-Zn-I = H2C: carbene H CH2I2, Zn(Cu) ether H The geometry of the alkene is preserved in the cyclopropanation reaction. CH I , Zn(Cu) H H 2 2 H H R R ether R R cis-alkene cis-cyclopropane CH I , Zn(Cu) H R 2 2 H R R H ether R H trans-alkene trans-cyclopropane 318 14.14: Transition-Metal Organometallic Compounds (please read) 14.15: Homogeneous Catalytic Hydrogenation (please read) H2, Pd/C - The catalyst is insoluble in the reaction media: heterogeneous catalysis, interfacial reaction H2, (Ph3P)3RhCl - The catalyst is soluble in the reaction media: homogeneous catalysis. 14.16: Olefin Metathesis (please read) 14.17: Ziegler-Natta Catalysis of Alkene Polymerization (please read) 319 9.
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