Presentation on theme: "15 15-1 Organic Chemistry William H. Brown & Christopher S. Foote."— Presentation transcript:
Organic Chemistry William H. Brown & Christopher S. Foote
Organometallic Compounds Chapter 15
Organometallic Compounds Organometallic compound: Organometallic compound: a compound that contains a carbon-metal bond We focus on organometallic compounds of Mg, Li, Cu, Zn, Pd, and Ru these classes illustrate the usefulness of organometallics in modern synthetic organic chemistry they illustrate how the use of organometallics can bring about transformations that cannot be accomplished in any other way
Organometallic Compounds Oxidative addition: Oxidative addition: a reagent adds to a metal or metal compound, causing its coordination to increase by two Reductive elimination: Reductive elimination: a reagent is eliminated from a metal compound, causing its coordination to decrease by two Ligand: Ligand: a Lewis base bonded to a metal in a coordination compound
Grignard Reagent Grignard reagent: Grignard reagent: an organomagnesium compound prepared by addition of an alkyl, aryl, or alkenyl (vinylic) halide to Mg metal in diethyl ether or THF
RMgX and RLi Grignard reagents dissolve as coordination compounds solvated by ether ethylmagnesium bromide, EtMgBr
RMgX and RLi Organolithium reagents prepared by reaction of an alkyl, aryl, or alkenyl halide with lithium metal
RMgX and RLi The carbon-metal bonds in RMgX and RLi are polar covalent
RMgX and RLi RMgX and RLi are valuable in synthesis as nucleophiles the carbon bearing the halogen is transformed from an electrophile to a nucleophile their most valuable use is addition to the electrophilic carbon of a C=O group to form a new carbon-carbon bond
RMgX and RLi Reaction with protic acids RMgX and RLi are strong bases
RMgX and RLi Reaction with protic acids RMgX and RLi react readily with these proton donors
RMgX and RLi Reaction with oxiranes (epoxides) reaction of RMgX or RLi with an oxirane followed by protonation increases chain length by two carbons
RMgX and RLi Reaction with oxiranes (epoxides) the major product corresponds to S N 2 attack of RMgX or RLi on less hindered carbon of the epoxide
Gilman Reagents Lithium diorganocopper reagents, known more commonly as Gilman reagents prepared by treating an alkyl, aryl, or alkenyl lithium compound with Cu(I) iodide
Gilman Reagents Coupling within organohalogen compounds form new carbon-carbon bonds by coupling with alkyl chlorides, bromides, and iodides
Gilman Reagents coupling with a vinylic halide is stereospecific; the configuration of an alkene is retained
Gilman Reagents A variation on the preparation of a Gilman reagent is to use a Grignard reagent with a catalytic amount of copper(I) salt
Gilman Reagents Reaction with epoxides regioselective ring opening
Heck Reaction A palladium catalyzed reaction in which the carbon group of a haloalkene or haloarene is substituted for a vinylic H of an alkene
Heck Reaction substitution is highly regioselective; at the less substituted carbon substitution is highly stereoselective; where E,Z isomerism is possible in the product, the E configuration is often formed almost exclusively
Heck Reaction reaction is stereospecific with regard to the haloalkene; the configuration of the double bond is retained
Heck Reaction The catalyst most commonly Pd(II) acetate reduced in situ to Pd(0) reaction of Pd(0) with good ligands gives PdL 2 The organic halogen compound aryl, heterocyclic, and vinylic iodides, chlorides, and bromides alkyl halides with an easily eliminated hydrogen are rarely used because they undergo -elimination to give alkenes OH group, C=O groups of aldehydes & ketones, and esters unreactive under Heck conditions
Heck Reaction The alkene the less the crowding on the alkene, the more reactive it is The base triethylamine, sodium and potassium acetate, and sodium hydrogen carbonate are most common The solvent polar aprotic solvents such as DMF, acetonitrile, DMSO aqueous methanol may also be used The ligand triphenylphosphine is one of the most common
Heck Reaction the usual pattern of acyclic compounds is replacement of a hydrogen of the double bond with an R group if the organopalladium group attacks a double bond so that the R group has no syn H for syn elimination, then the double bond may shift
Carbenes and Carbenoids Carbene, R 2 C: Carbene, R 2 C: a neutral molecule in which a carbon atom is surrounded by only six valence electrons Methylene, the simplest carbene prepared by photolysis or thermolysis of diazomethane methylene prepared in this manner is so nonselective that it is of little synthetic use
Carbenes and Carbenoids Dichlorocarbene prepared by treating chloroform with potassium tert- butoxide
Carbenes and Carbenoids Dichlorocarbene reacts with alkenes to give dichlorocyclopropanes
Carbenes and Carbenoids Simmons-Smith reaction a way to add methylene to an alkene to form a cyclopropane generation of the Simmons-Smith reagent this organozinc compound reacts with a wide variety of alkenes to give cyclopropanes
Carbenes and Carbenoids Simmons-Smith reagent
Carbenes and Carbenoids Simmons-Smith reaction the organozinc compound reacts with an alkene by a concerted mechanism
Stable Nucleophilic Carbenes Stable nucleophilic carbenes certain carbenes with strongly electron-donating groups are particularly stable their stability is enhanced by bulky groups that hinder self-reactions; one such group is the 2,4,6- trimethylphenyl group rather than behaving as electron-deficient reagents like most carbenes, these compounds are nucleophiles because of the strong electron donation by the nitrogens
Stable Nucleophilic Carbenes this carbene is stabilized by the electron-donating nitrogens and the bulky 2,4,6-trimethylphenyl groups
Ring-Closing Alkene Metathesis Alkene metathesis reaction: Alkene metathesis reaction: two alkenes interchange carbons on their double bonds if the reaction involves 2,2-disubstituted alkenes, ethylene is lost to give a single alkene product
Ring-Closing Alkene Metathesis a useful variant of this reaction uses a starting material in which both alkenes are in the same molecule, and the product is a cycloalkene
Ring-Closing Alkene Metathesis a particularly useful alkene methathesis catalyst consists of ruthenium, Ru, complexed with a nucleophilic carbene and another carbenoid ligand. In this example, the other carbenoid ligand is a benzylidene group.
Ring-Closing Alkene Metathesis Like the Heck reaction, alkene metathesis involves a catalytic cycle addition of a metalocarbenoid to the alkene gives a four-membered ring elimination of an alkene in the opposite direction gives a new alkene
Prob 15.9 Complete these reactions involving Gilman reagents.
Prob Show reagents to synthesize this target molecule from cyclohexane.
Prob Complete these equations.
Prob Account for the stereospecificity of this reaction.
Prob Account for the stereospecificity of this Heck reaction; that is, that the E alkene is formed exclusively.
Prob Account for the formation of these isomeric alkenes in this Heck reaction.
Prob Complete these Heck reactions.
Prob Account for the formation of 3-phenylcyclohexene and the fact that no 1-phenylcyclohexene is formed.
Prob Account for the formation of this product and the cis stereochemistry of its ring junction.
Prob Account for the formation of the following product, including the cis stereochemistry at the ring junction.
Prob Show how Exaltolide can be synthesized from the given starting material. Give the structure of R.
Prob Propose a synthesis of spiro[2.2]pentane from organic compounds of three carbons or less.
Prob Predict the product of each alkene metathesis reaction.