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Organometallic Compounds. Organometallic Compounds. An organic compound containing a carbonmetal bond Organolithium compounds and Organomagnesium are two of the most common organometallic compounds. A. Organometallic Mechanisms. - PowerPoint PPT Presentation

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  • OrganometallicCompounds Dr Seemal jelani*

    Dr Seemal jelani

  • Dr Seemal jelani*

    Dr Seemal jelani

  • Organometallic Compounds

    An organic compound containing a carbonmetal bondOrganolithium compounds and Organomagnesium are two of the most common organometallic compoundsDr Seemal jelani*

    Dr Seemal jelani

  • A. Organometallic MechanismsOxidation State: The oxidation state of a metal is defined as the charge left on the metal after all ligands have been removed in their natural, closed-shell configuration This is a formalism and not a physical property!Formalism is the study by analyzing and comparing form and stylethe way objects are made and their purely visual aspects

    d-Electron Configuration: position in the periodic table minus oxidation state.

    Dr Seemal jelani*

    Dr Seemal jelani

  • The d electron count is a Chemistry formalism used to describe the electron configuration of the valence electrons of a transitions metal center in a Coordination complex The d electron count is an effective way to understand the geometry and reactivity of transition metal complexesThe formalism has been incorporated into the two major models used to describe coordination complexes; Crystal field theory and ligand field theoryDr Seemal jelani*

    Dr Seemal jelani

  • 18-Electron RuleA stable complex (with electron configuration of the next highest noble gas) is obtained when the sum of the metal d-electrons, electron donated from the ligand, and the overall charge of the complex equals 18In mononuclear, diamagnetic complexes, the total number of electrons never exceeds 18 (noble gas configuration) The total number of electrons is equal to the sum of d-electrons plus those contributed by the ligands. 18 electrons = coordinatively saturated< 18 electrons = coordinatively unsaturated.

    Dr Seemal jelani*

    Dr Seemal jelani

  • Diamagnetic A diamagnetic compound has all electron spins paired, giving a net spin of zero. They are also weakly repelled by a magnet.

    Descriptive term which indicates that a substance contains no unpaired Electrons and thus is not attracted to a magnetic field

    NH3 is diamagnetic because all of the electrons in NH3 are paired.Dr Seemal jelani*

    Dr Seemal jelani

  • 18-Electron ruleThis was first proposed by N.V. Sigwick who extended the octet theory of G.N. Lewis and using it to explain coordination compoundsLigands were considered Lewis bases and the metal ion in turn acted as a Lewis acidThe sum of all the electrons would assume a noble gas configuration of Kr, Xe or Rn.As with most rules of the thumb, the 18 electron rule is not always obeyed18 electrons are required to fill 5 d-orbitals, 1 s-orbital and 3 p-orbitals of a transition metal and hence reach the configuration of a noble gas.Dr Seemal jelani*

    Dr Seemal jelani

  • Organometallic ReagentsThe Key Concepts:Make a carbon negatively charged/polarlized so it is nucleophilic. Reaction with electrophilic carbons can make carbon-carbon bonds.Dr Seemal jelani*

    Dr Seemal jelani

  • Victor GrignardGrignard ReagentsDiscovered by Victor Grignard in 1900Key factors are ethereal solvent and water-free conditionsAwarded Nobel Prize in 1912Grignard, Victor , 18711935, French chemist. He shared the 1912 Nobel Prize in Chemistry for his work in organic synthesis based on his discovery (1900) of the Grignard Reagent. He taught at the Univ. of Nancy (190919) and at the Univ. of Lyons (from 1919 until the end of his career). The First Organometallic ReagentsDr Seemal jelani*

    Dr Seemal jelani

  • Grignard Reagents An Organomagnesium compound prepared by addition of an alkyl, aryl, or alkenyl (vinylic) halide to Mg metal in diethyl ether or THF+1-Bromobutaneether+etherBromobenzeneDr Seemal jelani*

    Dr Seemal jelani

  • An Alternative to Grignard Reagents are Alkyl LithiumsBoth are prepared from alkyl, vinyl, and aryl halides under anhydrous conditionsDr Seemal jelani*

    Dr Seemal jelani

  • Grignard and Organolithium ReagentsGiven the difference in electronegativity between carbon and magnesium (lithium), the C-Mg (C-Li) bond is polar covalent, with C- and Mg+ ( Li+ ) Grignard and Organolithium reagents behave like carbanions

    Carbanions: an anion in which carbon has an unshared pair of electrons and bears a negative charge

    Dr Seemal jelani*

    Dr Seemal jelani

  • Grignard and Organolithium ReagentsCarbanion: an anion in which carbon has an unshared pair of electrons and bears a negative charge

    Carbanions are strong bases--they are easily quenched by even very weak acids (water, alcohols, amines, amides, carboxylic acids, even terminal alkynes). A limitation to utility!

    Dr Seemal jelani*

    Dr Seemal jelani

  • Carbanion: an anion in which carbon has an unshared pair of electrons and bears a negative chargeCarbanions are strong bases--they are easily quenched by even very weak acids (water, alcohols, amines, carboxylic acids, amides, even terminal alkynes). A limitation to utility!

    Dr Seemal jelani*

    Dr Seemal jelani

  • LimitationsCant make Grignards with acidic or electrophilic functional groups present in the molecule: R2NH pKa 38-40Terminal AlkynespKa 25ROHpKa 16-18Carbonyls& NitrospKa 11-27

    Dr Seemal jelani*

    Dr Seemal jelani

  • Carbanion: an anion in which carbon has an unshared pair of electrons and bears a negative chargeCarbanions are also great nucleophiles This is the reason for their great utility!

    Dr Seemal jelani*

    Dr Seemal jelani

  • Key Point:

    Great nucleophiles that add efficiently to electrophilic carbons, such as epoxides and carbonyl group of aldehydes, ketones and esters However, their basicity can be a limitation!

    Epoxides illustrate how many common organic functional groups contain electrophilic carbonsDr Seemal jelani*

    Dr Seemal jelani

  • Carbanions (nucleophiles) can react with electrophilic carbon centers in favorable casesThe net result is a carbon-carbon Grignards and Organolithium reagents react with many oxygen-containing electrophiles, but not with alkyl halides.

    Dr Seemal jelani*

    Dr Seemal jelani

  • formaldehyde to give primary alcohols aldehydes to give secondary alcoholsketones to give tertiary alcoholsesters to give tertiary alcoholsCO2 to give acids epoxides to give primary alcoholsGrignard reagents react productively with:The one we are choosing for the sake of initial illustrationDr Seemal jelani*

    Dr Seemal jelani

  • Epoxides: The Example We Want to StressThese is an extremely valuable reaction ConsideredRetrosyn-thetically:d-d+New C-C bondDr Seemal jelani*

    Dr Seemal jelani

  • Back to Work: A Related ExampleDetailed Mechanism Highlighting Retention of StereochemistryKey Points to Note:Attack at least hindered carbonMechanism is SN2-like in initial stepSingle enantiomeric productChiral center not affected by reactionRelief of ring strain helps drive reactionH3O+ breaks up initial saltNew C-C bondDr Seemal jelani*

    Dr Seemal jelani

  • Two More Examples of Additions to EpoxidesNew C-C bondNew C-C bondNote: Stereochemistry at epoxide retained in productDr Seemal jelani*

    Dr Seemal jelani

  • Gilman ReagentsLithium Diorganocopper reagents, known more commonly as Gilman reagentsprepared by treating an alkyl, aryl, or alkenyl lithium compound with Cu(I) iodideDr Seemal jelani*

    Dr Seemal jelani

  • Gilman ReagentsCoupling with Organohalogen compoundsform new carbon-carbon bonds by coupling with alkyl and alkenyl chlorides, bromides, and iodides. (Note that this doesnt work with Grignard or Organolithium reagents They are too basic and do E2 eliminations.)

    ExampleNew C-C bondDr Seemal jelani*

    Dr Seemal jelani

  • Gilman ReagentsCoupling with a vinylic halide is stereospecific; the configuration of the carbon-carbon double bond is retainedNew C-C bondDr Seemal jelani*

    Dr Seemal jelani

  • Gilman ReagentsA variation on the preparation of a Gilman reagent is to use a Grignard reagent with a catalytic amount of a copper(I) saltDr Seemal jelani*

    Dr Seemal jelani

  • Gilman ReagentsReaction with epoxidesregioselective ring opening (attack at least hindered carbon)New C-C bondDr Seemal jelani*

    Dr Seemal jelani

  • Interim Summary of Introduction to Organometallic ReagentsOrganolithium reagents and Grignard reagents are very basic but also great nucleophilesThey react with epoxides at the less hindered site to give a two-carbon chain extended alcoholThey do not couple with alkyl-, aryl-, or vinyl halides.

    Gilman reagents react with epoxides as do Organolithium reagents and Grignard reagents.However, they also add to alkyl-, aryl-, and vinyl halides to make new C-C bonds.Dr Seemal jelani*

    Dr Seemal jelani

  • Back to Grignard ReagentsAddition of a Grignard reagent to formaldehyde followed by H3O+ gives a 1 alcohol

    This sequence (mechanism) is general and important!

    Meanwhile.Dr Seemal jelani*

    Dr Seemal jelani

  • Grignard ReactionsThese are valuable and important reactions Dr Seemal jelani*

    Dr Seemal jelani

  • Grignard reagents react with estersbut species formed is unstable and dissociates under the reaction conditions to form a ketoneCO MgX+dd+R'R'Dr Seemal jelani*

    Dr Seemal jelani

  • Grignard reagents react with estersthis ketone then goes on to react with a second mole of the Grignard reagent to give a tertiary alcoholCO MgX+dd+R'R'CH3OMgX Dr Seemal jelani*

    Dr Seemal jelani

  • ExampleTwo of the groups attached to the tertiary carbon com

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