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Organometalic Compounds Electrophilic Carbon Organometalic Compounds Electrophilic Carbon Carbon is an electrophile when it is attached to an electron-withdrawing group. Nucleophilic Carbon Carbon is a nucleophile when it is attached to a metal. Many Metals Can Be Used to Make Organometallic Compounds An organometallic compound contains a carbon–metal bond. Organomagnesium and organolithium compounds are two of the most common organometallic compounds. Nomenclature Name the alkyl group, then the metal. Organolithium Compounds Reactions of Alkyl Halides: Grignard Reagents • Reaction of RX with Mg in ether or THF • Product is RMgX – an organometallic compound (alkyl-metal bond) – R is alkyl 1°, 2°, 3°, aryl, alkenyl – X = Cl, Br, I Organomagnesium Compounds Organomagnesium compounds are also called Grignard reagents. The Synthesis of a Grignard Reagent is Carried Out in an Ether Solvent The solvent provides electrons so magnesium can complete its octet. Organomagnesium and Organolithium Compounds React as if They Were Carbanions Organomagnesium and Organolithium Compounds Will React with a Proton of an Acidic Group How to Prepare a Deuterated Hydrocarbon Transmetallation An organometallic compound will undergo transmetallation (metal exchange) if a less polar carbon–metal bond will be formed. The more polar the carbon–metal bond, the more reactive the organometallic compound. Organometallic Coupling Reactions • Alkyllithium (RLi) forms from RBr and Li metal • RLi reacts with copper iodide to give lithium dialkylcopper (Gilman reagents) • Lithium dialkylcopper reagents react with alkyl halides to give alkanes Utility of Organometallic Coupling in Synthesis • Coupling of two organometallic molecules produces larger molecules of defined structure • Aryl and vinyl organometallics also effective • Coupling of lithium dialkylcopper molecules proceeds through trialkylcopper intermediate The Alkyl Group of an Organocuprate Replaces a Halogen A coupling reaction joins two CH-containing groups. The Configuration of the Double Bond is Retained Organocuprates can be used to prepare compounds that cannot be prepared by using nucleophilic substitution reactions. Other Functional Groups Can Be Present The product has two more carbons than the alkyl group of the organocuprate. The Suzuki and Heck Reactions (palladium-catalyzed coupling reactions) Both reactions replace the halogen of a vinylic halide or an aryl halide with a carbon-containing group. Suzuki and Heck Reactions both reactions start by inserting a palladium between the alkyl group and the halogen The Suzuki Reaction Couples a Vinylic or Aryl Halide with an Organoboron Compound the organoboron compound can be an alkyl, alkenyl,or aryl group The Heck Reaction Couples a Vinylic or Aryl Halide with an Alkene The R group of the halide replaces a vinylic hydrogen. The Mechanism The first step is oxidative addition. The fourth step is transmetallation. The last (fifth) step is a reductive elimination. Preparing Alkyl- and Alkenyl-Boron Compounds (hydroboration of a terminal alkene or alkyne) Preparing an Aryl-Organoboron Compound The Mechanism High yields are obtained only if Z is an electron withdrawing group. Why High Yields Are Obtained if Z is an Electron Withdrawing Group the electron withdrawing group makes the carbon more electrophilic Alkene Metathesis The double bond breaks and the fragments are rejoined. Each new double bond is formed between two sp2 carbons that were not previously joined. attaching sp2 carbons to form new alkenes Alkene Metathesis attaching sp2 carbons to re-form the reactant alkene Alkene Metathesis Requires a Grubbs Catalyst Alkene Metathesis Forms E and Z Isomers Two Different Alkenes Can Be Used as Starting Materials in Alkene Metathesis Ring-Closing Metathesis Mechanism for Alkene Metathesis Phase 1 The first phase creates two metal-containing intermediates. In each intermediate, the metal is bonded to an sp2 carbon. Mechanism for Alkene Metathesis Phase 2 In the second phase, each intermediate reacts with the starting material to form a new alkene and the other metal-containing intermediate. Alkyne Metathesis The preferred catalyst for alkyne metathesis is a Schrock catalyst (a catalyst that contains molybdenum or tungsten). Oxidation and Reduction in Organic Chemistry • In organic chemistry, we say that oxidation occurs when a carbon or hydrogen that is connected to a carbon atom in a structure is replaced by oxygen, nitrogen, or halogen – Not defined as loss of electrons by an atom as in inorganic chemistry • Oxidation is a reaction that results in loss of electron density at carbon (as more electronegative atoms replace hydrogen or carbon) • Oxidation: break C–H (or (C–C) and form C–O, C–N, C–X Reduction Reactions • Organic reduction is the opposite of oxidation – Results in gain of electron density at carbon (replacement of electronegative atoms by hydrogen or carbon) – Reduction: form C–H (or C–C) and break C–O, C–N, C–X Oxidation Levels • Functional groups are associated with specific levels Let’s Work a Problem How would you carry out the following syntheses? .
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