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range of 45 to 50, all of these organometallic reagents behave as strong bases and re- act rapidly with even fairly weak acids, such as water and alcohols, as illustrated in the following equation. Therefore, the solvents that are used for their preparation must be scrupulously dried, and compounds containing OH or NH groups must be avoided. Even a 1-alkyne is acidic enough to react with a Grignard reagent. This reaction is the most common method of preparing Grignard reagents derived from these alkynes: PROBLEM 18.8 Show the products of these reactions: 18.6 Addition of Organometallic Nucleophiles The reaction of Grignard reagents and organolithium compounds with aldehydes and ketones is perhaps the most useful method for the preparation of alcohols. The reaction is conducted under basic conditions and proceeds according to the following general mechanism: Because these organometallic reagents are powerful nucleophiles, the reaction is irreversible. After the addition is complete, acid is added in the workup step to pro- O . . . . O W W R R'±C±R" R'±C±R" R±MgX . . . . ± ± . . W W R R'±C±R" O±H . . . . H±A (workup) c) CH 3 CH 2 Br 1) Mg, ether 2) CH 3 CH 2 CPCH b) Br 1) Mg, ether 2) D 2 O a) CH 3 CHCH 2 CH 3 W Cl 1) Mg, ether 2) H 2 O H±CPC±MgBr H±CPC±H Ethyne Ethane CH 3 CH 2 MgBr W H CH 3 CH 2 Ethylmagnesium bromide CH 3 CH 2 CH 2 CH 2 MgBr Butylmagnesium bromide Butane CH 3 CH 2 CH 2 CH 2 H±OH W H Mg(OH)Br 18.6 ADDITION OF ORGANOMETALLIC NUCLEOPHILES 753

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Page 1: ADDITION OF ORGANOMETALLIC NUCLEOPHILESaskthenerd.com/NOW/CH18/18_6_7_11.pdf · 18.6 Addition of Organometallic ... The reaction of an organometallic reagent with formaldehyde

range of 45 to 50, all of these organometallic reagents behave as strong bases and re-act rapidly with even fairly weak acids, such as water and alcohols, as illustrated inthe following equation. Therefore, the solvents that are used for their preparationmust be scrupulously dried, and compounds containing OH or NH groups must beavoided.

Even a 1-alkyne is acidic enough to react with a Grignard reagent. This reaction is themost common method of preparing Grignard reagents derived from these alkynes:

PROBLEM 18.8Show the products of these reactions:

18.6 Addition of Organometallic Nucleophiles

The reaction of Grignard reagents and organolithium compounds with aldehydes andketones is perhaps the most useful method for the preparation of alcohols. The reactionis conducted under basic conditions and proceeds according to the following generalmechanism:

Because these organometallic reagents are powerful nucleophiles, the reaction isirreversible. After the addition is complete, acid is added in the workup step to pro-

O..

. .

OW

W

R

R'±C±R"R'±C±R"

R±MgX

..

. .

± ±

..W

W

R

R'±C±R"

O±H..

. .–

H±A

(workup)

c) CH3CH2Br1) Mg, ether

2) CH3CH2CPCH

b)

Br1) Mg, ether

2) D2Oa) CH3CHCH2CH3

WCl

1) Mg, ether

2) H2O

H±CPC±MgBrH±CPC±H

Ethyne Ethane

� CH3CH2MgBrW

H

� CH3CH2

Ethylmagnesium bromide

CH3CH2CH2CH2MgBr

Butylmagnesium bromide Butane

CH3CH2CH2CH2� H±OHW

H

� Mg(OH)Br

18.6 � ADDITION OF ORGANOMETALLIC NUCLEOPHILES 753

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tonate the alkoxide ion and produce the alcohol. Reactions employing formaldehydeas the carbonyl component produce primary alcohols, those using other aldehydesproduce secondary alcohols, and those using ketones produce tertiary alcohols. So-lutions of hydrochloric or sulfuric acid are commonly used in the protonation step.However, to avoid alkene formation, the weaker acid, ammonium chloride (pKa � 9)in aqueous solution, is used when the product is a tertiary or other alcohol that readily undergoes acid-catalyzed E1 elimination (Section 10.13). Of course, if thealkene is the desired product, the elimination can be accomplished during theworkup without isolating the alcohol. A number of examples are provided in the fol-lowing equations.

The reaction of an organometallic reagent with formaldehyde produces a primaryalcohol:

Reactions with other aldehydes produce secondary alcohols. Although the presence ofmost other functional groups (OH, NH, carbonyl groups, and so on) must be avoidedbecause they react with Grignard or organolithium reagents, ether and alkene groupscan be present:

In the next example the bromine, being more reactive than chlorine, selectively reactsto form the Grignard reagent. The product alcohol is especially prone to E1 elimination(the carbocation would be stabilized by resonance), so the weak acid, ammonium chlo-ride, is used in the workup step.

CHCH3

OHW

(88%)1) CH3CH

OX

2) NH4Cl H2O

MgEt2O

Br

Cl

m-Bromochlorobenzene

MgBr

Cl Cl

2) H3O+

1)

(90%)

OX

C±H

OCH3

MgBr

H±C

OCH3

WOH

p-Methoxybenzaldehyde

(69%)1) HCH

O

2) H3O+

Cl MgCl CH2OH

Mg

Et2O

X

754 CHAPTER 18 � ADDITIONS TO THE CARBONYL GROUP

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Reactions with ketones give tertiary alcohols. These are very prone to E1 elimination,so weak acid is used in the workup.

Organolithium reagents work well in any of these reactions:

If the product resulting from elimination of water from the alcohol is desired, then theworkup can be conducted by using a stronger acid so that the reaction proceeds directlyto the alkene without the isolation of the alcohol.

Acetylenic Grignard reagents are commonly prepared by reaction of the appropriate 1-alkyne with an alkyl Grignard reagent, such as ethylmagnesium bromide.

The carbon of carbon dioxide is electrophilic, similar to a carbonyl carbon. Grig-nard reagents react with carbon dioxide to form salts of carboxylic acids:

Carbondioxide

R±MgX R±C±O

OX

C

O

OX

....

..

. .

..

. .

.. ..

. . R±C±O±H

OX

..

. .. .

. .

H±A

±±

H±CPC±H H±CPC±MgBrCH3CH2MgBr�

(69%)

Ethyne Ethylmagnesiumbromide

� CH3CH3

2) NH4Cl H2O

1)H

O

H

OH

(48%)1) PhMgBr

2) H3O+

O

�-Tetralone

Ph

(72%)2) NH4Cl

H2O

1) Li

4-Methyl-2-pentanone

O HO

(75%)

Cyclopentanone

OCH2CH3HO

1) CH3CH2MgBr2) NH4Cl

H2O

18.6 � ADDITION OF ORGANOMETALLIC NUCLEOPHILES 755

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Acidification of the reaction mixture produces a carboxylic acid. Examples are providedin the following equations:

PROBLEM 18.9Show the products of these reactions:

g) CH3CPCH1) CH3CH2MgBr

3) H3O+2) CH3CH

OX

f)1) Mg, ether

3) H3O+2) CO2

Br1) CH3Li2) NH4Cl, H2O

e)

O

1) Mg, ether

3) H2SO4, H2O2) CH3CH2CCH2CH3

OX

d)

Br

c)1)

2) H3O+HMgCl

O

b)1) Mg, ether

3) H3O+2) HCH

OX

Cla)1) CH3CH2MgBr2) NH4Cl, H2O

±

OX

CCH3

(85%)

1) Mg, ether2) CO23) H3O+

(70%)

OX

C±OH

OX

C±OH1) Mg, ether2) CO23) H3O+

Br

1-Bromonaphthalene

Chlorocyclohexane

Cl

756 CHAPTER 18 � ADDITIONS TO THE CARBONYL GROUP

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The Grignard reaction is an important and versatile way to prepare alcohols. When-ever an alcohol is encountered as a synthetic target, this reaction should be consideredbecause it forms a carbon–carbon bond, building the alcohol from smaller compounds,and it often allows more than one route to the desired product. For example, supposethe target is 2-phenyl-2-butanol. This alcohol can be prepared by three different Grig-nard reactions:

Because the product is a tertiary alcohol, each of these reactions must be acidified witha weak acid (NH4Cl/H2O) to avoid elimination. Which of these pathways is the best de-pends on a number of factors, such as the availability of the ketone and the halideneeded to prepare the Grignard reagent and the yield of the reaction.

PRACTICE PROBLEM 18.2

Show two ways to prepare 2-butanol using Grignard reagents.

Strategy

When you encounter an alcohol as a synthetic target, consider using a Grignard reac-tion to synthesize it. The carbon bonded to the hydroxy group was the electrophilic car-bon of the carbonyl group of the reactant. One of the alkyl groups bonded to this carbonwas the nucleophilic carbon of the Grignard reagent or alkyllithium reagent. There areoften several ways to accomplish the synthesis, depending on which alkyl group isadded as the nucleophile. Remember to work up the reaction with the weak acid NH4Clif the product alcohol is prone to E1 elimination.

Solution

The carbon (blue) bonded to the hydroxy group in the alcohol comes from the carbonylcarbon electrophile of the starting material. This carbon is also bonded to a methylgroup and an ethyl group. We can add the ethyl group, using the reaction of ethylmag-

CH3CH2CHCH3

2-Butanol

OHW

H3C±C±CH2CH3

OHW

W

Ph

CH3CCH2CH3

� PhMgBr

OX

PhCCH2CH3

� CH3MgI

OX

PhCCH3

� CH3CH2MgBr

OX

2-Phenyl-2-butanol

18.6 � ADDITION OF ORGANOMETALLIC NUCLEOPHILES 757

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nesium bromide and ethanal, or we can add the methyl group, using methylmagnesiumiodide and propanal.

PROBLEM 18.10Suggest syntheses of these compounds using Grignard reagents:

PROBLEM 18.11When the Grignard reaction shown in this equation is attempted, the products are ben-zene and the starting hydroxyaldehyde. Explain this result.

18.7 Addition of Phosphorus Ylides; The Wittig Reaction

In Chapters 9 and 10 the use of elimination reactions to prepare alkenes was described.The major problem with that method is that a mixture of alkenes is often produced, re-sulting in lower yields and separation problems. The Wittig reaction provides an alter-

� CH3CHCH2CH

OHW

OX

Br

OHW

OX

1) Mg, ether

3) H3O+2) CH3CHCH2CH

g) (3 ways)

OH

f)

CPCHHO

e) (2 ways)

OH

c)

OHW

CH2CH2

b)

d) Ph3COH

CO2H

(2 ways)a)

OH

OX

HCCH3

OX

CH3CH2CH1) CH3CH2MgBr

2) H3O+1) CH3MgI

2) H3O+CH3CH2CHCH3

OHW

758 CHAPTER 18 � ADDITIONS TO THE CARBONYL GROUP

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native method for the synthesis of alkenes. It is especially useful because it results incarbon–carbon bond formation and the position of the double bond is completely con-trolled. Georg Wittig shared the 1979 Nobel Prize in chemistry for developing this re-action. (He shared the award with H. C. Brown, who developed the hydroborationreaction; see Section 11.7.)

The nucleophile used in this reaction is called an ylide. It is a carbanion that isbonded to a positive phosphorus group that helps to stabilize it:

The ylide is prepared by deprotonating a triphenylalkylphosphonium salt with a strongbase, commonly an organometallic base such as butyllithium or phenyllithium. The hy-drogens on the carbon that is bonded to the phosphorus of the salt are somewhat acidicbecause the carbanion of the conjugate base (the ylide) is stabilized by the inductive ef-fect of the positive phosphorus atom. In addition, a resonance structure with five bondsto phosphorus makes a minor contribution to the structure and provides some additionalstabilization. The triphenylalkylphosphonium salt can be prepared by an SN2 reactionof triphenylphosphine with the appropriate alkyl halide (see Section 10.9).

PROBLEM 18.12Show a preparation of this phosphonium salt from an alkyl halide:

The reaction of the ylide with an aldehyde or ketone results in the formation of analkene with the double bond connecting the carbonyl carbon of the reactant to the an-ionic carbon of the ylide, as shown in the following example:

The by-product is triphenylphosphine oxide. The ylide is a strong nucleophile, so theequilibrium greatly favors the products and the reaction is irreversible.

The mechanism for this reaction is shown in Figure 18.2. The first part of the mecha-nism is similar to the others that have been presented so far: the nucleophile (the negativecarbon of the ylide) attacks the carbonyl carbon. However, unlike the previous cases, this

(86%)� Ph3P±CH2

+ –. .

� Ph3PœO

O CH2

DMSO

Ph3P±CH2PhCl

–+

+H3C±I

I–

Ph3P±CH3 (99%)Ph3P .. �SN2

Phosphoniumsalt

Minorcontributor

Butyllithium Ylide

Ph±P±CH3+ + –

I–

PhW

W

Ph

BuLi� Ph3P±CH2

. .

Ph3P CH2±±

Electrostatic potential

map of Ph3P–CH2

–+

18.7 � ADDITION OF PHOSPHORUS YLIDES; THE WITTIG REACTION 759

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reaction proceeds beyond this step. The phosphorus, acting as a Lewis acid, bonds to thebasic oxygen and ultimately leads to its removal from the product. The overall result is theformation of a double bond between the carbonyl carbon and the nucleophilic carbon ofthe ylide, while both of the carbon–oxygen bonds of the carbonyl group are broken.

Previous reactions in this chapter have involved only addition of the nucleophile anda hydrogen to the carbonyl group. In this reaction, addition is followed by eliminationof the oxygen to form a double bond between the carbonyl carbon and the nucleophile.Such an addition––elimination reaction occurs when the nucleophile has or can gener-ate (by the loss of a proton or a phosphorus group) a second pair of electrons that canbe used to form a second bond to the electrophilic carbon. In the case of the Wittig re-action, the phosphorus and the oxygen are eliminated to form the alkene. The forma-

760 CHAPTER 18 � ADDITIONS TO THE CARBONYL GROUP

CH2±PPh3

+–

. .CH2

Ph3PœO

A betaineCyclohexanone

An ylide

An oxaphosphetaneMethylenecyclohexane

WW

WO±PPh3

±±CH2

OWW ±

PPh3

±±CH2

....

. .

O ..

. .

+

This reaction begins in the same manner as other reactions that proceed by the mechanism for basic conditions. The nucleophile, the negative carbon of the ylide, attacks the carbonyl carbon.

However, the reaction does not stop at this intermediate, known as a betaine.

Rather, the oxygen forms a bond to the phos-phorus in a Lewis acid–base reaction, forming a four-membered ring. (The mechanism for this reaction is still being investigated, and it may be that these first two steps occur in a concerted manner.)

The resulting four-membered ring, containinging an oxygen and a phosphorus, is called anoxaphosphetane.

Under the reaction conditions it spontaneously fragments to give the product alkene and tri-phenylphosphine oxide.

The reaction proceeds directly to thealkene. The intermediate betaine and oxaphosphetane are not isolated.

1

1

2

2

3

3

Active Figure 18.2

MECHANISM OF THE WITTIG REACTION. Test yourself on the concepts in this figure atOrganicChemistryNow.

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tion of the strong phosphorus–oxygen bond helps make this step of the reaction favor-able. (Note that the previous reactions in this chapter, employing hydride, cyanide, andorganometallic nucleophiles, did not have any way to form a second bond between thenucleophile and the electrophilic carbon, so only addition occurred.)

Because the location of the double bond in the product is well defined, the Wittig re-action provides probably the most useful general method for the preparation of alkenes.Some examples are provided in the following equations:

Because the electron pair of the carbanion of the ylide in the following example isstabilized by resonance delocalization with the carbonyl group, it is a weaker nu-cleophile. Such ylides react readily with aldehydes but do not react well with ketones.

PROBLEM 18.13Show the products of these reactions:

a) PhCH CH3CH2CH±PPh3�

OX

c) HCPCCH Ph3P±CHCH2CH2CH3�

OX

b) CH3CH2PPh3

Br–

1) BuLi2)

d)

OX

X

O

e) PhCH �

OX

OX

2�

. .–

. .–

+

+

Ph3P±CH2

. .–+

+

O

Ph3P±CHCOEt

. .–+CH

CH

OX

CH

OX

� (92%)

OX

Ph3P±CHCCH3

. .–+ benzene

NO2

CHœCHCCH3

NO2

(70%)

Br–

1) BuLi, Et2O

2) PhCH

OX

PhCHœCHCH

OX

PhCHœCHCHœCHPh (84%)� Ph3P±CHPh

. .

+PPh3

–+

CHPh

EtOH

18.7 � ADDITION OF PHOSPHORUS YLIDES; THE WITTIG REACTION 761

Click Coached Tutorial Problemsto practice GrignardReactions and WittigReactions.

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PROBLEM 18.14Explain why the phosphonium salt shown in the following equation can be deproto-nated by using sodium ethoxide, a much weaker base than the butyllithium that is usu-ally needed to deprotonate other phosphonium salts.

PRACTICE PROBLEM 18.3

Show two ways to prepare this compound using a Wittig reaction.

Strategy

When the synthetic target is an alkene, consider using a Wittig reaction, because the lo-cation of the double bond is completely controlled (unlike an E2 elimination, in whicha mixture of products is often formed). One carbon of the alkene was the electrophiliccarbonyl carbon of an aldehyde or ketone and the other carbon of the alkene was the nu-cleophilic carbon of the ylide. Because either carbon of the alkene could come from thenucleophile and the other from the electrophile, there are often two ways to accomplishthe synthesis.

Solution

Break the double bond, making one carbon the nucleophile of an ylide and the othercarbon part of a carbonyl group. The two possible ways to do this in this case are asfollows:

CHPhO

or

� PhCH±PPh3

. .– +

CHPh

. .–

+PPh3

PhCH

OX

CHPh

Ph3P±CH2Ph NaOEt HOEt NaCl� � �

Cl–

Ph3P±CHPh

. .–++

762 CHAPTER 18 � ADDITIONS TO THE CARBONYL GROUP

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PROBLEM 18.15Suggest syntheses of these compounds using the Wittig reaction:

c)

H3C

CHœCHCOCH3

OX

b) CH3CH2CHœCHCCH2CH3

OX

a) PhCH2CHœCHCH2CH3 (2 ways)

18.7 � ADDITION OF PHOSPHORUS YLIDES; THE WITTIG REACTION 763

Focus On

Synthesis of Vitamin AVitamin A, also known as retinol, is essential for vision in mammals (see the Focus Onbox on page 773) and is involved in a number of other important biological functions,such as bone growth and embryonic development. A deficiency in vitamin A leads tonight blindness, in which the eye cannot see in dim light. Our bodies are capable ofconverting compounds such as �-carotene, an orange pigment that is present in manyvegetables, to vitamin A. As its structure suggests, vitamin A is relatively nonpolar andtherefore is not very soluble in water. It accumulates in fat deposits and is not readilyexcreted. For this reason, too much vitamin A is toxic.

The Wittig reaction has proved to be especially useful in the synthesis of naturalproducts, such as vitamin A, which contain a number of carbon–carbon double bonds.An industrial synthesis of vitamin A is outlined in the following equations:

�-Carotene

Oxidative cleavage

Vitamin A (retinol)

UUU

OH2

Continued

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764 CHAPTER 18 � ADDITIONS TO THE CARBONYL GROUP

�-Carotene is also prepared industrially by a Wittig reaction. A dialdehyde is re-acted with two equivalents of the same ylide used in the vitamin A synthesis, as shownin the following equation:

�-Carotene

2PPh3

. .–+

O

H

O

H

Ph3PHCl

OH

OH OH1) HCPCMgBr2) NH4Cl, H2O

H2

Lindlarcatalyst

Vitamin A acetate Phosphonium salt

PPh3

+Cl

�-Ionone

1) CH3O–

2)O

H

OCCH3

OX

OCCH3

OX

�-Ionone can be isolated from natural sources or synthesized in the laboratory. It is reacted with ethynyl magnesium bromide, or some other source of the anion derived from ethyne, to produce an alcohol (see Section 18.6).

The triple bond is reduced to a double bond using hydrogen and the Lindlar catalyst (see Section 11.12).

Then, reaction of the alcohol with triphenylphosphine under acidic conditions produces the phosphonium salt by an SN1 reaction. A resonance-stabilized carbocation is formed and reacts at the terminal position of the chain.

The phosphonium salt is more acidic than usual because its conjugate base, the ylide, is stabilizedby resonance involving the double bonds. Therefore, methoxide ion, a weaker base than usual, canbe used to form the ylide. Reaction of the ylide with the aldehyde that has its hydroxy group protectedas an ester produces vitamin A acetate. The acetate group can readily be removed to complete thesynthesis of vitamin A (see Section 10.2).

1

1

2

2

3

3

4

4

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18.8 Addition of Nitrogen Nucleophiles

Amines add to the carbonyl groups of aldehydes or ketones to produce compounds con-taining CN double bonds and water. These nitrogen analogs of aldehydes and ketonesare called imines.

±

OX

CH

(87%)H2O� �

An imineAniline

H2N±

NX

CH

Benzaldehyde

no solvent

18.8 � ADDITION OF NITROGEN NUCLEOPHILES 765

It is interesting to note that an alkene can be prepared by two different Wittig path-ways, depending on which of the doubly bonded carbons was originally the carbon ofthe carbonyl group and which was the carbon of the ylide. Thus, the synthesis of �-carotene has also been accomplished by using the reaction of a diylide with two equiv-alents of an aldehyde, as illustrated in the following equations:

BrBr 2 Ph3P

(90%)

2 PhLi benzene

H2 �

refluxEt2O

Br–PPh3

+

Br–

Ph3P+

Ph3P+

O

51%�-Carotene

PPh3

. .–

. .–+

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18.11 Synthesis

By now, you should be fairly comfortable using retrosynthetic analysis to design thesynthesis of a target molecule. Remember that carbon–carbon bond forming reac-tions are especially important in synthesis. Several extremely useful reactions thatresult in the formation of carbon–carbon bonds have been introduced in this chapter.These are the Grignard reaction to prepare alcohols, the Wittig reaction to preparealkenes, and the conjugate addition of organocuprate reagents to �,�-unsaturatedcarbonyl compounds.

When you are designing a synthesis, the presence of certain structural featuresin the target suggests that the use of certain reactions be considered. Whenever analcohol is the synthetic target, you should consider using a Grignard reaction tomake it. The presence of an alkene suggests the use of the Wittig reaction, and thepresence of a carbonyl compound with a substituent on the �-carbon suggests theuse of a conjugate addition reaction. You do not have to use these reactions, but youshould consider using them. Using retrosynthetic notation, these possibilities aresummarized as follows:

Let’s try some syntheses. Suppose we need to prepare 2-phenyl-l-pentene, startingfrom benzaldehyde:

from

±

X

C

2-Phenyl-1-pentene Benzaldehyde

CH2

CH2CH2CH3

±

X

C

O

H

GrignardreactionW

W

OH

R''

R'''R''

R'

R'

R

R±C±R'X

O

R±C±R'

Wittigreaction

X

C

C

±±

±± R'R

X

O

C±±

Organocupratereagent

CH2

CH2

R

X

O

C±±

W

W

CH

CH2

R

X

O

C±±

X

784 CHAPTER 18 � ADDITIONS TO THE CARBONYL GROUP

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Applying retrosynthetic analysis, the presence of the alkene group in the target suggestsusing a Wittig reaction in its preparation.

The new target is a ketone. We need to somehow add a propyl group to the carbonyl car-bon of benzaldehyde to make this ketone. At this point we do not know of a reactionthat will accomplish this transformation directly, but we recognize that a ketone can beprepared by oxidation of an alcohol. We can prepare the alcohol using the Grignard re-action. Our retrosynthetic analysis is as follows:

We are now ready to write the synthesis in the forward direction:

Let’s try another example. This time our task is to prepare 2-methyl-1-phenylhept-6-en-2-ol from but-3-en-2-one.

CH3CCHœCH2PhCH2C±CH2CH2CH2CHœCH2 from

But-3-en-2-one2-Methyl-1-phenylhept-6-en-2-ol

W

W

OH

CH3

XO

±CH

OH

CH2CH2CH3

±

X

C

O

H

±

X

C

CH2

CH2CH2CH3

±

X

C

O

CH2CH2CH3

1) CH3CH2CH2MgBr

Na2Cr2O7H2SO4H2O

2) H3O+

Ph3P±CH2

. .–+

W

±CHCH2CH2CH3

±

X

C

O

CH2CH2CH3

±

X

C

O

H

W

OH

±

X

C

CH2

CH2CH2CH3

±

X

C

O

CH2CH2CH3

18.11 � SYNTHESIS 785

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The target is an alcohol, so we should consider using a Grignard reaction. Any of thethree groups attached to the carbon bonded to the hydroxy group could potentially beattached. Comparison of the target to but-3-en-2-one suggests that the benzyl group beadded in the Grignard step. So the first step in our retrosynthetic analysis is as follows:

Our new target is a ketone that is related to but-3-en-2-one by the presence of an allyl group on the �-carbon. This suggests the use of a conjugate addition reaction:

Written in the forward direction, the synthesis is as follows:

Remember that a synthesis can often be accomplished in more than one way. If yoursynthesis is not the same as the one shown in the answer, check to see that your stepsare all correct. If all of the steps in your sequence appear reasonable, then your synthe-sis may be correct—and could even be better than the one in the answer.

PROBLEM 18.26Show syntheses of these compounds from the indicated starting materials:

b)

CHCH3 O

a) CH3CH2CH2CCH2CH3 from

from

CH3CH2CH2CH

CH2CH3

OXW

W

OH

CH3

X

CH3CCHœCH2

OX

CH3CCH2CH2±CH2CHœCH2

O1) (CH2œCHCH2)2CuLi

2) H3O+

1) PhCH2MgCl2) NH4Cl, H2O

PhCH2CCH2CH2CH2CHœCH2

W

W

OH

CH3

X

CH3CCHœCH2

OX

CH3CCH2CH2±CH2CHœCH2

O

X

CH3CCH2CH2CH2CHœCH2

O

PhCH2C±CH2CH2CH2CHœCH2

W

W

OH

CH3

786 CHAPTER 18 � ADDITIONS TO THE CARBONYL GROUP

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Review of Mastery GoalsAfter completing this chapter, you should be able to:

Show the products resulting from the addition to aldehydes and ketones of all of thereagents discussed in this chapter. (Problems 18.27, 18.28, 18.32, 18.33, and 18.36)

Show the products resulting from the addition of certain of these reagents to �,�-unsaturated compounds, noting whether 1,2- or 1,4-addition predominates. (Prob-lems 18.29, 18.32, and 18.36)

Show the mechanisms for any of these additions. (Problems 18.39, 18.41, 18.45,18.47, 18.48, and 18.52)

Predict the effect of the structure of the aldehyde or ketone on the position of theequilibrium for these reactions. (Problems 18.30, 18.31, 18.34, 18.35, 18.58, and18.59)

Use these reactions, in combination with the reactions from previous chapters, tosynthesize compounds. (Problems 18.38, 18.42, and 18.43)

Use acetals as protecting groups in syntheses. (Problem 18.43)

Visual Summary of Key Reactions

The reactions in this chapter begin with the addition of a nucleophile to the carbon of acarbonyl group and an electrophile, usually a proton, to the oxygen. Under basic con-ditions the nucleophile adds first, whereas the proton adds first under acidic conditions.Depending on the nature of the nucleophile, the reaction may stop at this stage or pro-ceed further. Figure 18.7 summarizes the mechanisms followed by the various nucleo-philes. Table 18.2 lists the nucleophiles and the products that result from their reactionswith aldehydes and ketones.

O

c)

d) from

CH3CCH2CH2CH2CH3 from HCCHœCH2

OX

OH

XCH2

O

OH

O

e) from Br

O

VISUAL SUMMARY OF KEY REACTIONS 787

Click Mastery Goal Quiz to testhow well you have met thesegoals.

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