Organometallic Compounds

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
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
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 CO
  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. 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