chapter 14

636 Chapter 14 Organometallic Compounds

With electrons fl owing from ethylene to zirconium, the ZrOCH 3 bond weakens, the carbons of ethylene become positively polarized, and the methyl group migrates from zirconium to one of the carbons of ethylene. Cleavage of the ZrOCH 3 bond is accom-panied by formation of a � bond between zirconium and one of the carbons of ethylene in step 3. The product of this step is a chain-extended form of the active catalyst, ready to accept another ethylene ligand and repeat the chain-extending steps. Before coordination polymerization was discovered by Ziegler and applied to pro-pene by Natta, there was no polypropylene industry. Now, more than 10 10 pounds of it are prepared each year in the United States. Ziegler and Natta shared the 1963 Nobel Prize in Chemistry: Ziegler for discovering novel catalytic systems for alkene polymerization and Natta for stereoregular polymerization. We’ll see more about Ziegler–Natta polymerization in Chapter 27 when we examine the properties of synthetic polymers in more detail.

14.17 SUMMARY

Section 14.1 Organometallic compounds contain a carbon–metal bond. They are named as alkyl (or aryl) derivatives of metals.

Butyllithium

CH3CH2CH2CH2Li

Phenylmagnesium bromide

C6H5MgBr

Section 14.2 Carbon is more electronegative than metals and carbon–metal bonds are polarized so that carbon bears a partial to complete negative charge and the metal bears a partial to complete positive charge.

HC Na�C�

Sodium acetylide has anionic bond between carbon

and sodium.

��C Li��

H

H H

Methyllithium has a polarcovalent carbon–lithium

bond.

Section 14.3 See Table 14.3


Section 14.5 Organolithium compounds and Grignard reagents are strong bases and react instantly with compounds that have OOH groups.

R HR M �� O R�M�H O R�

These organometallic compounds cannot therefore be formed or used in solvents such as water and ethanol. The most commonly employed solvents are diethyl ether and tetrahydrofuran.

Section 14.6 See Tables 14.2 and 14.4



Section 14.9 When planning the synthesis of a compound using an organometallic reagent, or indeed any synthesis, the best approach is to reason backward from the product. This method is called retrosynthetic analysis. Retrosynthetic analysis of 1- methylcyclohexanol suggests it can be prepared by the reaction of methylmagnesium bromide and cyclohexanone.

CH3MgBr

Methylmagnesiumbromide

�O

Cyclohexanone1-Methylcyclohexanol

CH3

OH

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14.17 Summary 637



Section 14.12 Carbenes are species that contain a divalent carbon; that is, a carbon with only two bonds. One of the characteristic reactions of carbenes is with alkenes to give cyclopropane derivatives.

Certain organometallic compounds resemble carbenes in their reactions and are referred to as carbenoids. Iodomethylzinc iodide (Section 14.11) is an example.


638 Chapter 14 Organometallic Compounds

TABLE 14.4 Carbon–Carbon Bond-Forming Reactions of Organometallic Reagents (Continued)

Reaction (section) and comments General equation and specifi c example

Synthesis of alcohols using organolithium reagents (Section 14.7) Organolithium reagents react with aldehydes and ketones in a manner similar to that of Grignard reagents to produce alcohols.

Aldehydeor ketone

R�CR�

OX

Alkyllithium

RLi

Alcohol

RCOHW

W

R�

R�

�1. diethyl ether

2. H3O�

3,3-Dimethyl-2-butanone

CH3CC(CH3)3

OX

�1. diethyl ether

2. H3O�Li

Cyclopropyllithium

CC(CH3)3W

W

OH

CH3

2-Cyclopropyl-3,3-dimethyl-

2-butanol (71%)

Synthesis of acetylenic alcohols (Section 14.8) Sodium acetylide and acetylenic Grignard reagents react with aldehydes and ketones to give alcohols of the type

CPC±C±OH.W

W Aldehydeor ketone

RCR�

OX

Sodiumacetylide

NaCPCH

Acetylenic alcohol

HCPCCR�W

W

OH

R

�1. NH3, �33°C

2. H3O�

2-Butanone

CH3CCH2CH3

OX

Sodiumacetylide

NaCPCH �1. NH3, �33°C

2. H3O�

3-Methyl-1-pentyn-3-ol(72%)

HCPCCCH2CH3

W

W

OH

CH3

Preparation of alkanes using lithium dialkylcuprates (Section 14.10) Two alkyl groups may be coupled together to form an alkane by the reaction of an alkyl halide with a lithium dialkylcuprate. Both alkyl groups must be primary (or methyl). Aryl and vinyl halides may be used in place of alkyl halides.

� R�CH2X RCH2R�

Alkane

R2CuLi

Lithiumdialkylcuprate

Primaryalkyl halide

(CH3)2CuLi

Lithiumdimethylcuprate

C6H5CH2Cl

Benzylchloride

� C6H5CH2CH3

Ethylbenzene (80%)

diethyl ether

The Simmons–Smith reaction (Section 14.11) Methylene transfer from iodomethylzinc iodide converts alkenes to cyclopropanes. The reaction is a stereospecifi c syn addition of a CH 2 group to the double bond.

Iodomethylzinciodide

ICH2ZnI

Alkene

R2CœCR2 �

Zinciodide

ZnI2�diethyl ether

R

RR

R

Cyclopropanederivative

CH2I2, Zn(Cu)

diethyl ether

Bicyclo[3.1.0]hexane(53%)

Cyclopentene

Section 14.13 Transition-metal complexes that contain one or more organic ligands offer a rich variety of structural types and reactivity. Organic ligands can be bonded to a metal by a � bond or through its � system. Metallocenes are transition-metal complexes in which one or more of the ligands is a cyclopentadienyl ring. Ferrocene was the fi rst metallocene synthesized; its electrostatic potential map opens this chapter.

Section 14.14 Organometallic compounds based on transition metals, especially rhodium and ruthenium, can catalyze the hydrogenation of alkenes under homogeneous conditions.


Problems 639

Cinnamic acid 3-Phenylpropanoic acid (90%)

CH CHCOH

O

CH2CH2COH

O

H2

[(C6H5)3P]3RhCl

When a single enantiomer of a chiral catalyst is used, hydrogenations can be carried out with high enantioselectivity.

Section 14.15 The doubly bonded carbons of two alkenes exchange partners on treatment with transition-metal carbene complexes, especially those derived from ruthenium and tungsten.

2R2C CR�2 R�2C CR�2R2C CR2

Metallocarbenecatalyst

�

Among other applications olefi n metathesis is useful in the synthesis of cyclic alkenes, the industrial preparation of propene, and in polymerization.

Section 14.16 Coordination polymerization of ethylene and propene has the biggest economic impact of any organic chemical process. Ziegler–Natta polymerization is carried out using catalysts derived from transition metals such as titanium and zirconium. �-Bonded and �-bonded organometallic compounds are intermediates in coordination polymerization.

PROBLEMS

14.15 Write structural formulas for each of the following compounds. Specify which compounds qualify as organometallic compounds. (a) Cyclopentyllithium (e) Sodium carbonate (b) Ethoxymagnesium chloride (f) Benzylpotassium (c) 2-Phenylethylmagnesium iodide (g) Lithium diisopropylamide (d) Lithium divinylcuprate

14.16 Suggest appropriate methods for preparing each of the following compounds from the starting material of your choice. (a) CH 3 CH 2 CH 2 CH 2 CH 2 MgI (b) CH 3 CH 2 C CMgI (c) CH 3 CH 2 CH 2 CH 2 CH 2 Li (d) (CH 3 CH 2 CH 2 CH 2 CH 2 ) 2 CuLi

14.17 Which compound in each of the following pairs would you expect to have the more polar carbon–metal bond? (a) CH 3 CH 2 Li or (CH 3 CH 2 ) 3 Al (b) (CH 3 ) 2 Zn or (CH 3 ) 2 Mg (c) CH 3 CH 2 MgBr or HC CMgBr

14.18 Write the structure of the principal organic product of each of the following reactions: (a) 1-Bromopropane with lithium in diethyl ether (b) 1-Bromopropane with magnesium in diethyl ether (c) 2-Iodopropane with lithium in diethyl ether (d) 2-Iodopropane with magnesium in diethyl ether (e) Product of part (a) with copper(I) iodide (f) Product of part (e) with 1-bromobutane (g) Product of part (e) with iodobenzene (h) Product of part (b) with D 2 O and DCl (i) Product of part (c) with D 2 O and DCl ( j) Product of part (a) with formaldehyde in diethyl ether, followed by dilute acid (k) Product of part (b) with benzaldehyde in diethyl ether, followed by dilute acid


chapter 14

Documents

iodomethylzinc iodide

zroch3 bond

carbons of ethylene

h o r r h

organolithium compounds

ionic bond

organometallic reagent

divalent carbon