Chapter 15 - An Introduction to Organometallic Compounds

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• ^^Gilman reagents are particularly useful for forming new carboncarbon bonds via a coupling reaction with an alkyl chloride, bromide, or iodide (alkyl fluorides are inactive under these circumstances), as demonstrated by the following production of 2-methyl-1-dodecene.^^

  • It's worth noting that the reaction only transfers one of the Gilman-reagent alkyl groups.
  • Because Gilman reagents are eventually synthesized from halides, efficient coupling of two halides occurs.
  • Gilman reagents are made up of two organic groups linked together by a copper(I) ion, resulting in a negatively charged species that serves as the source of the carbon nucleophile.
  • As the counterion, lithium ion is connected with this negatively charged species.
  • This example shows the reaction of a nucleophile, a Gilman reagent, with an electrophile, a vinylic halide.
  • Vinylic halides are typically nonreactive to nucleophilic displacement.
  • As a result, the lithium diorganocopper reaction demonstrated here is one-of-a-kind.

• Gilman reagents with the highest coupling product yields are those made from methyl, primary alkyl, allylic, vinylic, and aryl halides through the respective organolithium compounds.

  • ^^Secondary and tertiary haloalkanes have lesser yields.^^

• The orientation of the carboncarbon double bond is retained after coupling with a vinylic halide, as demonstrated by the synthesis of trans-5-tridecene.

  

• A metal atom or ion bonds to a surrounding array of molecules known as ligands in inorganic coordination complexes.

  • %%When a ligand directly bonds to a metal, the complex is referred to as an organometallic compound.%%
  • %%The study of chemical compounds having carbon-metal linkages is known as organometallic chemistry.%%

• The carbon-metal bond (C-M) has properties ranging from extremely ionic to covalent. Ionic bonds are more commonly observed with electropositive metals, such as Group 1 or 2 metals.

  • ^^When looking at the periodic table, the C-M bonds usually grow more covalent as you move from left to right in a row.^^
  • Other parameters, like as charge stability, can also impact the degree of ionic or covalent bonding.

• ^^Carbon's partial negative charge makes it basic and nucleophilic; the latter trait may be used in organic synthesis to form carbon-carbon bonds.^^

  • Grignard reagents are named after their discoverer, Victor Grignard, who discovered organomagnesium compounds.
  • Grignard reagents are made by reacting alkyl, aryl, or alkenyl halides (chlorides, bromides, and iodides, not fluorides) in an ether solvent with a little excess of magnesium metal.
  • Grignard reagents' carbon-magnesium bond is polar covalent, with a partial negative charge on carbon, making it nucleophilic and basic.

• An alkyl, aryl, or alkenyl halide is reacted with two equivalents of lithium metal to produce organolithium reagents.

  • In organolithium compounds, the carbon-lithium bond is polar covalent, with a partial negative charge on carbon, making it nucleophilic.
  • Grignard reagents and organolithium compounds react as carbon nucleophiles with a broad variety of electrophilic functional groups, including epoxides (as well as numerous carbonyl-containing species addressed later in the book).

• The image attached below shows a dihalocarbene that is generated by treatment of CHCl3 or CHBr3 with a strong base such as potassium tert-butoxide.

  • Addition of the dihalocarbene to an alkene shows syn stereospecificity

  

• The reaction happens at the less hindered carbon in unsymmetrical epoxides.

  • %%Because a new carbon-carbon bond is generated, these reactions are extremely valuable for synthesis.%%
  • Following an acidic aqueous workup, the initial product generated is an alkoxide salt, which is transformed to an alcohol product.

• The image attached below shows a Treatment of CH2I2 with a zinc-copper couple generates an organozinc compound, known as the Simmons-Smith reagent, which reacts with alkenes to give cyclopropanes.

• %%Because Grignard and organolithium reagents are extremely basic, they will deprotonate functional groups such amines, terminal alkynes, alcohols, thiols, and carboxylic acids.%%

  • Carbenes: refers to neutral compounds that contain a carbon with just six valence electrons; carbenoids are their organometallic-complexed analogues.

• Carbenes are sp2 hybridized, with one sp2 hybrid orbital containing an empty 2p orbital and a lone pair.

  • Carbenes are synthesized by photolysis or thermolysis of diazo compounds such as diazomethane.
  • Dichlorocarbene is made by reacting chloroform with a strong base.

• The Simmons-Smith reagent, which is made from diiodomethane and Zn, is a valuable carbenoid (Cu).

  • Carbenes like methylene are too reactive to be employed in synthesis.
  • ^^Cyclopropanes are formed when dichlorocarbene and the Simmons-Smith reagent react stereospecifically with alkenes.^^

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