15-2 15.1 Organometallic Compounds Organometallic compound: 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.
15-3 A. Grignard Reagents Grignard reagent: 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. Mg + 1-Bromobutane Butylmagnesium bromide (an alkyl Grignard reagent) ether Br MgBr + MgBrMgBr BromobenzenePhenylmagnesium bromide (an aryl Grignard reagent)
15-4 RMgX and RLi Grignard reagents dissolve as coordination compounds solvated by ether. Ethylmagnesium bromide, EtMgBr.
15-5 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.
15-6 RMgX and RLi, Table 15.1 The carbon-metal bonds in RMgX and RLi are polar covalent bonds. 2.5 - 1.2 = 1.3 2.5 - 1.0 = 1.5 2.5 - 1.5 = 1.0 2.5 - 1.6 = 0.9 2.5 - 1.8 = 0.7 2.5 - 1.9 = 0.6 Difference in Electronegativity C-M Bond Percent Ionic character* 60 52 40 36 28 24 C-Li C-Mg C-Al C-Zn C-Sn C-Hg *Percent ionic character = E C - E M E C x 100 2.5 - 1.9 = 0.624 C-Cu
15-7 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.
15-8 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.
15-9 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.
15-10 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.
15-11 RMgX and RLi Reaction with oxiranes (epoxides), cont. The major product corresponds to S N 2 attack of RMgX or RLi on less hindered carbon of the epoxide as discussed in the ether chapter.
15-12 15.2 A. Gilman Reagents 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.
15-13 B. Gilman Reagents Coupling with organohalogen compounds: A new carbon-carbon bond forms by coupling with alkyl and alkenyl chlorides, bromides, and iodides. ExampleExample
15-14 Gilman Reagents Coupling with a vinylic halide is stereospecific; the configuration of the carbon-carbon double bond is retained.
15-15 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.
15-16 C. Gilman Reagents Reaction with epoxides shows: regioselective ring opening.
15-17 15.3 A. 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, is prepared by photolysis or thermolysis of diazomethane. Methylene prepared in this manner is so nonselective that it is of little synthetic use.
15-18 B. Carbenes and Carbenoids Dichlorocarbene is: prepared by treating chloroform with potassium tert-butoxide.
15-19 Carbenes and Carbenoids Dichlorocarbene: reacts with the pi bond of an alkene to give a dichlorocyclopropane.
15-20 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. CH 2 I 2 Zn(Cu) ICH 2 ZnI + diethyl ether Diiodo- methane Iodomethylzinc iodide (Simmons-Smith reagent) Zinc-copper couple
15-21 Carbenes and Carbenoids Use of the Simmons-Smith reagent:
15-22 Carbenes and Carbenoids Mechanism of the Simmons-Smith reaction: The organozinc compound reacts with an alkene by a concerted mechanism.