Jones M., Fleming S.A. Organic Chemistry

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Additions to Alkenes 2

and Additions to Alkynes

409

10.1 Preview

10.2 Addition of H

and X

Reagents

10.3 Hydration through Mercury

Compounds: Oxymercuration

10.4 Other Addition Reactions

Involving Three-Membered

Rings: Oxiranes and

Cyclopropanes

10.5 Dipolar Addition Reactions:

Ozonolysis and the Synthesis

of Carbonyl ( )

Compounds

10.6 Addition Reactions of Alkynes:

Addition

10.7 Addition of X

Reagents to

Alkynes

10.8 Hydration of Alkynes

10.9 Hydroboration of Alkynes

10.10 Hydrogenation of Alkynes:

Alkene Synthesis through syn

Hydrogenation

10.11 Reduction by Sodium in

Ammonia: Alkene Synthesis

through anti Hydrogenation

10.12 Special Topic: Three-

Membered Rings in

Biochemistry

10.13 Summary

10.14 Additional Problems

NATURAL DYES The red dragonfly may be one of several insects that produce

the red cochineal dye, a tricyclic compound with many multiple bonds.

410 CHAPTER 10 Additions to Alkenes 2 and Additions to Alkynes

Neal Stephenson (b. 1959) is the author of the brilliant Snow Crash, as well as The Diamond Age, Zodiac: The

Eco-Thriller, and, of course, Cryptonomicon.

A red dragonfly hovers above a backwater of the stream, its wings

moving so fast that the eye sees not wings in movement but a probability

distribution of where the wings might be, like electron orbitals: a quantum-

mechanical effect that maybe explains why the insect can apparently

teleport from one place to another, disappearing from one point and

reappearing a couple of meters away, without seeming to pass through the

space in between. There sure is a lot of bright stuff in the jungle. Randy

figures that, in the natural world, anything that is colored so brightly must

be some kind of serious evolutionary badass.

—NEAL STEPHENSON,

CRYPTONOMICON

10.1 Preview

We continue the discussion of additions to π systems in this chapter. Most reactions

we see here follow the pattern seen in Chapter 9, in which a Lewis acid (usually a

cation) is formed, and subsequently captured by a nucleophile,often an anion. Other

reactions follow a different path. We begin with one of these, hydrogenation, the

addition of H

. We then move on to reactions involving X

reagents, which often

proceed through the formation of an intermediate nonisolable, three-membered

ring. Next, we take up additions in which the three-membered ring is stable, and

finish with reactions forming different-sized rings. We will also discuss the related

additions to alkynes. Throughout this chapter, attention is paid to what all these

reactions allow us to do in the way of synthesis, which is rather a lot.

ESSENTIAL SKILLS AND DETAILS

1. Addition reactions have stereochemical consequences. In Chapters 3 and 9, where we first

encountered additions, we focused mainly on regiochemistry, the direction of addition.

Here,when we describe additions of X

and XY reagents, the stereochemistry, syn or anti, of

the reaction can also be followed easily, and it is essential that you keep this factor in mind.

2. Addition reactions can take place through either open or cyclic intermediates, and you

will learn to assess the factors that control which way the reaction goes. If an open

cation is well stabilized, usually by resonance, it is likely to be favored. If not, the cyclic

intermediate generally prevails.

3. Open cations permit addition of a nucleophile to each lobe of the empty orbital. Cyclic

ions require opening in S

2 fashion, leading to overall trans addition.

4. Epoxides open differently in acid and base. In base, the nucleophile adds to the less

hindered position, whereas in acid the nucleophile becomes attached to the more

substituted, more hindered side.

10.2 Addition of H

and X

Reagents

In this section, we see two very different reactions of diatomic reagents, the addition

of H

and of X

. In the first reaction, the concerted addition of hydrogen to the

double bond of an alkene, a catalyst is required to get things going. The second

reaction, halogen addition, fits better into the pattern seen in Chapter 9, in which

a two-step process is followed.

10.2 Addition of H

and X

Reagents 411

No reaction

Insoluble—

heterogeneous catalysis

Pd/C

Ph =

RhCl(PPh

)

Wilkinson’s catalyst

Soluable—

homogeneous catalysis

FIGURE 10.1 Hydrogen gas and an

alkene react very quickly in the

presence of a metal catalyst such as

palladium adsorbed on charcoal

(Pd/C).The rhodium-based

Wilkinson’s catalyst is typical of

soluble molecules that also catalyze

hydrogenation.

10.2a Hydrogenation When an alkene is mixed with hydrogen gas, nothing

happens (unless one is careless with oxygen and a match). If the two reagents are

mixed in the presence of any of a large number of metallic catalysts, however, an

efficient addition of H

converts the alkene into an alkane. This reaction is called

hydrogenation, and it can be achieved by either of two catalytic processes. First,

there is heterogeneous catalysis, in which an insoluble catalyst, often palladium

(Pd) on charcoal, is used. Many other insoluble metallic catalysts work as well in

heterogeneous catalysis. A second process is homogeneous catalysis, in which a

soluble catalyst, often the rhodium-based Wilkinson’s catalyst, after Geoffrey

Wilkinson (1921–1996), is employed (Fig. 10.1).

PROBLEM 10.1 Use bond dissociation energies (BDE) (Table 8.2, p. 337) to

calculate the exothermicity or endothermicity of the hydrogenation of ethylene

to ethane.

Nonmetallic reducing agents can be used in alkene hydrogenation as well.

Diimide ( ) is a good example. It is not stable under normal conditions

and is made only as it is needed. Diimide exists in two forms, cis and trans, but only

the less stable cis form is active in hydrogenation (Fig. 10.2).

(95%)

HN NH

WEB 3D

trans Form

cis Form

....

Diimide

A SPECIFIC EXAMPLE

THE GENERAL CASE

H C

FIGURE 10.2 An effective nonmetallic

reducing agent for alkenes is diimide

(HNNH).

412 CHAPTER 10 Additions to Alkenes 2 and Additions to Alkynes

94.5% cis

PtO

WEB 3D

FIGURE 10.3 Metal-catalyzed

addition to a cycloalkene shows that

deuterium is added in syn fashion.

Detached

alkane

Metal surface

Adsorbed

hydrogen

Adsorbed

alkene

Metal surface

FIGURE 10.4 Partial bonding to the

metal surface is followed by hydrogen

transfer to the alkene.

Ru/C

Less substituted (less sterically hindered,

faster to react)

More substituted (more hindered, slower to react)

FIGURE 10.5 It is possible to hydrogenate less-substituted double bonds in the presence of more-hindered,

more-substituted double bonds.The catalyst in this example is a ruthenium–carbon complex.

There are many kinds of selectivity in the hydrogenation reaction. Not all

carbon–carbon double bonds are hydrogenated at the same rate, and the differences

in rate often allow hydrogenation of one double bond in the presence of another.

Generally, the more sterically hindered a double bond, the more slowly it is hydro-

genated. This rate change is probably related to ease of adsorption on the catalyst

surface. Large groups hinder adsorption and therefore slow the rate of hydrogena-

tion. One can often hydrogenate a less substituted double bond in the presence of

more substituted double bonds, as shown in Figure 10.5.

We have to pay attention to the drawing conventions (codes) here. We do not

always draw out all of the carbon and hydrogen atoms—we are beginning to adopt

the convention most used by chemists—the “stick”drawings that show only the car-

bon framework of the molecule. Figure 10.5 shows two levels of code, but we will

often lapse into the most schematic representations of molecules.

If hydrogenation is carried out with a cycloalkene,transfer of hydrogen takes place

in predominately syn fashion. It is a syn addition because both new hydrogens are

delivered to the same side of the alkene. In Figure 10.3, D

is used so we can see

the stereochemical outcome of the reaction.

The mechanistic details of hydrogenation remain vague. Presumably, the metal

surface binds both hydrogen and alkene, weakening the π bond of the alkene and

the σ bond of hydrogen. The alkane is formed by addition of a weakened hydrogen

molecule to the coordinated alkene. As the transfer is completed, the product

alkane detaches from the metal surface (Fig. 10.4).

CONVENTION ALERT

10.2 Addition of H

and X

Reagents 413

(Not observed)

catalyst

Now let’s consider the energetics of hydrogenation. Is it likely to be exothermic

or endothermic? As shown in Problem 10.1, a simple and common technique for

making such estimates uses bond energies.In the hydrogenation reaction,two bonds

are breaking: the hydrogen–hydrogen σ bond and the π bond of the alkene. Two

bonds are also being made in this reaction: the two new σ bonds in the alkane prod-

uct.If we know bond energies (Table 8.2,p.337), we can estimate ΔH ° for this reac-

tion.We made this estimate in Problem 10.1, and Figure 10.6 repeats the calculation.

The process is exothermic by about 30 kcal/mol.

Two C

H σ bonds =

~200 kcal/mol

π bond = 66 kcal/mol

σ bond = 104 kcal/mol

Bonds broken Bonds made

200 – 170 = 30 kcal/mol exothermic

H = –30 kcal/mol

FIGURE 10.6 In a typical

hydrogenation, the π bond of the

alkene and the σ bond of hydrogen

are broken.Two new carbon–

hydrogen bonds are made.The

hydrogenation of an alkene is

exothermic by approximately

30 kcal/mol.

Table 10.1 collects some measured heats of hydrogenation. We can see that the

crude bond energy calculation of Figure 10.6 has actually done quite well; the heats

of hydrogenation of simple alkenes are quite close to our calculation of 30 kcal/mol.

PROBLEM 10.2 Hydrogenation of β-pinene gives only one of the possible

diastereomeric products, as shown below. Explain why.

TABLE 10.1 Some Heats of Hydrogenation

Alkene (kcal/mol)

32.7

30.0

30.3

28.6

27.6

28.4

26.9

29.6

27.3

26.4

28.0

26.6(CH

)

C(CH

)

trans CH

cis CH

(CH

)

CHCH

(CH

)

trans CH

cis CH

≤H

414 CHAPTER 10 Additions to Alkenes 2 and Additions to Alkynes

10.2b Additions of Halogens Alkene halogenation is the reaction between

and an alkene and it results in addition of a halogen atom to each of the carbons

of the double bond. Molecular bromine and chlorine (Br

and Cl

) do not need a

catalyst to add to alkenes to give these vicinal dihalides (Fig. 10.7). Groups are

described as vicinal when they are on adjacent atoms.

PROBLEM 10.3 Use the values in Table 10.1 to estimate the relative energies of the

hexenes. Compare your estimates with those generated from heats of formation

in Chapter 3 (p. 115).

PROBLEM 10.4 A mixture of an alkene and hydrogen gas has an energy that is

some 30 kcal/mol above the alkane product.Why doesn’t the reaction occur spon-

taneously? Use an Energy versus Reaction progress diagram to help explain in

general the role of the metal catalyst (Hint: See p. 381).

C C

Vicinal (1,2-)

dihalides

Vicinal (1,2-)

dihalides

Br Br

Cl Cl

FIGURE 10.7 Addition of Br

and Cl

to yield vicinal dihalides.

trans -1,2-Dibromo-

cyclopentane

cis -1,2-Dibromo-

cyclopentane

(

Observed

)(

Not observed

)

Br Br Br

WEB 3D

FIGURE 10.8 Anti addition of Br

to cyclopentene to give the trans

product.

Halogen addition is stereospecifically anti; the groups add to opposite sides of

the alkene. For example, addition to cyclopentene gives only trans-1,2-dibromo-

cyclopentane (Fig. 10.8).

–

CHH

Br Addition

Possibly analogous

addition (?)

C C

–

FIGURE 10.9 A first-guess

mechanism for the reaction of an

alkene and bromine. For analogy, it

draws on the mechanism of hydrogen

bromide addition to an alkene.

If we take the addition of hydrogen bromide to an alkene as a model for the

alkene halogenation reaction, we can build a mechanism quite quickly. Just as the

alkene reacts with hydrogen bromide to produce a carbocation and bromide (Br



so reaction with bromine–bromine might give a brominated cation and bromide

(Fig.10.9).The π bond of the alkene acts as a nucleophile,displacing Br



from bromine.

Addition of bromide to the carbocation would give the dibromide product.

Alkene halogenation

10.2 Addition of H

and X

Reagents 415

There are cases of bromination in which the mechanism shown in Figure 10.9 is

exactly correct, but for most alkenes it is not quite right. First, carbocation rearrange-

ments (p. 386) are not observed in most bromination and chlorination reactions. For

example, addition of bromine to 3,3-dimethyl-1-butene gives only 1,2-dibromo-3,3-

dimethylbutane, not 1,3-dibromo-2,3-dimethylbutane, as would be expected if an

open cation were involved (Fig.10.10).We already know that if a cycloalkene is used,

the product is exclusively the trans-1,2-dibromide. The cis compound cannot be

detected (Fig. 10.8).

Tertiary

carbocation

Secondary

carbocation

–

migration

This product is the only

observed dibromide

Not observed

–

CCH

Br Br

FIGURE 10.10 Rearrangements of

carbocations are not observed in

most halogenation reactions.

If the addition involved an open cation, we would expect the bromide ion to add

from both sides to produce a mixture of products. The yields of cis and trans com-

pounds would not be equal, but both compounds should be formed (Fig. 10.11).

Because only anti addition is observed, our initial proposed mechanism cannot be

completely correct. So our mechanism needs modification. How do we explain the

simultaneous preference for anti addition and absence of rearrangement? It seems

that an open carbocation, which would lead to cationic rearrangements and permit

both syn and anti addition, must be avoided in some way.

–

Path a

(a)

Path b

(b)

Br Br

FIGURE 10.11 An open carbocation

must lead to both syn and anti addition.

What if the initial displacement of Br



from Br

were to take place in a

symmetrical fashion (Fig. 10.12)? Once again, the filled π orbital of the alkene

acts as nucleophile in an S

2 reaction in which Br



is displaced from Br

As the figure shows,a bromine-containing three-membered ring, a bromonium ion

(brom bromine; onium positive), is formed. An open carbocation is avoided==

Symmetrical displacement A bromonium ion

–

FIGURE 10.12 A symmetrical attack

of the alkene on bromine yields a

cyclic bromonium ion along with

bromide, Br



.The π bond is the

nucleophile in this S

2 reaction.

416 CHAPTER 10 Additions to Alkenes 2 and Additions to Alkynes

and rearrangements cannot occur. But what of the preference for anti addition? Look

closely at the second step of the reaction, the opening of the bromonium ion by the

nucleophile Br



(Fig. 10.13). This reaction is an S

2 displacement, and therefore

must take place with inversion, through addition from the rear as we saw so often

in Chapter 7. Overall anti addition of the two bromine atoms is assured.

WEB 3DWEB 3D

trans -1,2-Dibromo-

clo

entane

–

FIGURE 10.13 This anti addition

of the two bromine atoms requires

the formation of trans-

1,2-dibromocyclopentane.

Mirror

Enantiomers!

(a)

(b)

(a)

(b)

–

WORKED PROBLEM 10.5 In Figure 10.13, the reaction starts with achiral molecules.

Yet the product, as drawn in the figure, is a single enantiomer. Clearly something is

awry. What is it? Hint: Convention Alert! (p. 157).

ANSWER You were warned about this! Of course there can be no optical activity

produced from a reaction of two achiral reagents. In this reaction, the bromonium

ion must open in two equivalent ways to produce a racemic pair of enantiomers. In

situations such as this one, it is the custom not to draw both enantiomers.

Adamantylideneadamantane

A stable bromonium ion

–

PROBLEM 10.6 One example in which the bromonium ion is stable to further

addition of nucleophiles is the reaction of adamantylideneadamantane with

bromine (see below). Explain why this cyclic ion might be stable.

So, a modification of our earlier addition mechanism suffices to explain the reac-

tion.In contrast to the addition to alkenes we saw in Chapter 9 ( , carbocations,and

boranes),in alkene halogenation (bromination and chlorination) a three-membered ring

is produced as an intermediate.In some, very specialized cases the bromonium ion can

even be isolated, and it seems its intermediacy in most brominations is assured.

CONVENTION ALERT

10.2 Addition of H

and X

Reagents 417

CCl

trans DibromideAcenaphthylene cis Dibromide

PROBLEM 10.7 Addition of Br

to acenaphthylene gives large amounts of the

product of cis addition along with the usual trans dibromide. Give a mechanism

for this bromination and, more important, explain why the mechanism of the

reaction shown below is different from the typical selectivity for only trans addi-

tion. Why is cis product formed in this case?

Stabilized alkene halogenation

In the bromonium ion,there is the full complement of six electrons for the three

σ bonds binding the two carbons and the bromine. The bromonium ion is a nor-

mal compound even though it may look odd at first.To see this clearly, imagine form-

ing the bromonium ion from an open cation.

Bromonium ion

Normally, the nonnucleophilic solvent carbon tetrachloride (CCl

) is used for

the bromination or chlorination of alkenes. But the reaction will also work in pro-

tic solvents such as water and simple alcohols. In these reactions, new products

appear that incorporate molecules of the solvent (Fig. 10.14). How do the OH

or OR groups get into the product molecule? To see the answer, write out the

mechanism and look for an opportunity to make the new products.The first step

Br Br

Br Br Br OH

Cl Cl Cl OCH

CCl

FIGURE 10.14 In the presence of

nucleophilic solvents, addition

reactions of bromine and chlorine

give products incorporating solvent

molecules as well as dihalides.

is the same as in Figure 10.12, in this case formation of the cyclic chloronium ion

(Fig. 10.15).Normally, the chloronium ion is opened in S

2 fashion by the nucleophilic



to give the dichloride.This opening must be the path for dichloride formation in

these examples. But in these reactions there are other nucleophiles present: molecules

of water or alcohol.Of course,they are weaker nucleophiles than the negatively charged



, but they are present as solvent and have an enormous numerical advantage over

the more powerfully nucleophilic chloride, which is present only in relatively low con-

centration (Fig. 10.15).

418 CHAPTER 10 Additions to Alkenes 2 and Additions to Alkynes

–

(91%)

(8%)

/CH

OCH

deprotonation

HOCH

(a)

path a

(b)

path b

A SPECIFIC EXAMPLE

CC CC

–

A chloronium ion

–

THE GENERAL CASE

FIGURE 10.15 Once the chloronium ion is formed, it is vulnerable to attack by any

nucleophile present. When a protic nucleophilic solvent is used in the reaction, it can

attack the cyclic ion to give the solvent-containing product. In this case, addition

of chloride accounts for 8% of the product, whereas the solvent methyl alcohol

produces 91%. Notice that methyl alcohol can produce two products through path a

and path b.

If this mechanism is correct, we can predict that the required inversion in the

opening of the cyclic ion must produce a trans stereochemistry in the products.

So, we have the opportunity to test the mechanism, and it turns out that addi-

tion of Br

(or Cl

) to cyclohexene in the solvent water exclusively produces the

compound resulting from anti addition in which the bromine (or chlorine) and

Halohydrin formation