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Organometallic Compounds

Organometallic Compounds. Chapter 15. 15.1 Organometallic Compounds. Organometallic compound: A compound that contains a carbon-metal bond. In this chapter, we focus on organometallic compounds of Mg, Li, and Cu.

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Organometallic Compounds

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  1. OrganometallicCompounds Chapter 15

  2. 15.1 Organometallic Compounds • Organometallic compound: A compound that contains a carbon-metal bond. • In this chapter, we focus on organometallic compounds of Mg, Li, and Cu. • 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. • We discuss several more recent reactions of organometallic compounds in Chapter 24.

  3. M g B r B r M g Butylmagnesium bromide (an alkyl Grignard reagent) B r M g M g B r Phenylmagnesium bromide (an aryl Grignard reagent) A. Grignard Reagents • Grignard reagent: An organomagnesium compound is a very versatile reagent: • prepared by addition of an alkyl, aryl, or alkenyl (vinylic) halide to Mg metal in diethyl ether or THF. ether + 1-Bromobutane ether + Bromobenzene

  4. RMgX and RLi • Grignard reagents dissolve as coordination compounds solvated by ether. • Ethylmagnesium bromide, EtMgBr.

  5. RMgX and RLi • Organolithium reagents are: • prepared by reaction of an alkyl, aryl, or alkenyl halide with lithium metal. • Essentially any halide can be used to make RMgX or RLi.

  6. Difference in Percent Ionic C-M Electronegativity character* Bond C - L i C - M g C - A l C - Z n C - S n C - C u C - H g E - E C M E C RMgX and RLi, Table 15.1 • The carbon-metal bonds in RMgX and RLi are polar covalent bonds. 2.5 - 1.0 = 1.5 60 2.5 - 1.2 = 1.3 52 2.5 - 1.5 = 1.0 40 2.5 - 1.6 = 0.9 36 2.5 - 1.8 = 0.7 28 2.5 - 1.9 = 0.6 24 2.5 - 1.9 = 0.6 24 x 100 *Percent ionic character =

  7. RMgX and RLi • RMgX and RLi are valuable in synthesis as good nucleophiles. • The carbon bearing the halogen is transformed from an electrophile to a nucleophile.

  8. RMgX and RLi • The most common use of RMgX or RLi is: • Addition to the electrophilic carbon of C=O groups of aldehydes, ketones, carboxylic esters, and acid chlorides to form a new carbon-carbon bonds. • Reaction with halides cause elimination.

  9. B. RMgX and RLi • RMgX and RLi are also strong bases. • Reaction with proton acids: • RMgX or RLi react readily to remove the proton from any of the compounds below.

  10. C. RMgX and RLi • Reaction of RMgX or Ri with oxiranes (epoxides): • Reaction of RMgX or RLi with an oxirane followed by protonation gives a primary alcohol with a carbon chain two carbons longer than the original chain.

  11. RMgX and RLi • Reaction with oxiranes (epoxides), cont. • The major product corresponds to SN2 attack of RMgX or RLi on less hindered carbon of the epoxide as discussed in the ether chapter.

  12. 15.2 A.Gilman Reagents • Lithium diorganocopper reagents, known more commonly as Gilman reagents are: • prepared by treating an alkyl, aryl, or alkenyl lithium compound with Cu(I) iodide.

  13. B. Gilman Reagents • Coupling with organohalogen compounds: • A new carbon-carbon bond forms by coupling with alkyl and alkenyl chlorides, bromides, and iodides. • Example

  14. Gilman Reagents • Coupling with a vinylic halide is stereospecific; the configuration of the carbon-carbon double bond is retained.

  15. Gilman Reagents • A variation on the preparation of a Gilman reagent is to use a Grignard reagent with a catalytic amount of a copper(I) salt.

  16. C. Gilman Reagents • Reaction with epoxides shows: • regioselective ring opening.

  17. 15.3 A.Carbenes and Carbenoids • Carbene, R2C: A neutral molecule in which a carbon atom is surrounded by only six valence electrons. • Methylene, the simplest carbene, is • prepared by photolysis or thermolysis of diazomethane. • Methylene prepared in this manner is so nonselective that it is of little synthetic use.

  18. B. Carbenes and Carbenoids • Dichlorocarbene is: • prepared by treating chloroform with potassium tert-butoxide.

  19. Carbenes and Carbenoids • Dichlorocarbene: • reacts with the pi bond of an alkene to give a dichlorocyclopropane.

  20. C H I I C H Z n I Z n ( C u ) 2 2 2 Diiodo- Iodomethylzinc iodide Zinc-copper methane (Simmons-Smith reagent) couple C. Carbenes and Carbenoids • Simmons-Smith reaction provides: • a way to add methylene to an alkene to form a cyclopropane. • Generation of the Simmons-Smith reagent : • The Simmons-Smith reagent is a carbenoid (carbene-like) compound. • This organozinc compound reacts with a wide variety of alkenes to give cyclopropanes. + diethyl ether

  21. Carbenes and Carbenoids • Use of the Simmons-Smith reagent:

  22. Carbenes and Carbenoids • Mechanism of the Simmons-Smith reaction: • The organozinc compound reacts with an alkene by a concerted mechanism.

  23. Organometallic Compounds End Chapter 15

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