Dr. Wolf's CHM 201 & 202 7-1 7.12 Molecules with Multiple Chirality Centers.

7-1Dr. Wolf's CHM 201 & 202

7.127.12

MoleculesMolecules

withwith

Multiple Chirality CentersMultiple Chirality Centers

Dr. Wolf's CHM 201 & 202 7-2

maximummaximum number of stereoisomers = 2 number of stereoisomers = 2nn

where where nn = number of structural units capable of = number of structural units capable of stereochemical variationstereochemical variation

structural units include chirality centers and cis structural units include chirality centers and cis and/or trans double bondsand/or trans double bonds

number is reduced to less than 2number is reduced to less than 2nn if meso forms if meso forms are possibleare possible

How many stereoisomers?How many stereoisomers?How many stereoisomers?How many stereoisomers?

Dr. Wolf's CHM 201 & 202 7-3

ExampleExampleExampleExample

4 chirality centers4 chirality centers

16 stereoisomers16 stereoisomers

OO

HOCHHOCH22CCH—H—CCH—H—CCH—H—CCHCHHCH

OHOH OHOH OHOH OHOH

7-4Dr. Wolf's CHM 201 & 202

HOHO OHOH

HH

HH

HOHO

HH33CC

HH

HHCHCH22CHCH22COCO22HH

CHCH33

HH

CHCH33

11 chirality centers11 chirality centers

221111 = 2048 stereoisomers = 2048 stereoisomers

one is "natural" cholic acidone is "natural" cholic acid

a second is the enantiomer of a second is the enantiomer of natural cholic acidnatural cholic acid

2046 are diastereomers of cholic 2046 are diastereomers of cholic acidacid

Cholic acid Cholic acid Cholic acid Cholic acid

Dr. Wolf's CHM 201 & 202 7-5

maximummaximum number of stereoisomers = 2 number of stereoisomers = 2nn

where where nn = number of structural units capable of = number of structural units capable of stereochemical variationstereochemical variation

structural units include chirality centers and cis structural units include chirality centers and cis and/or trans double bondsand/or trans double bonds

number is reduced to less than 2number is reduced to less than 2nn if meso forms if meso forms are possibleare possible

How many stereoisomers?How many stereoisomers?How many stereoisomers?How many stereoisomers?

Dr. Wolf's CHM 201 & 202 7-6

3-Penten-2-ol3-Penten-2-ol

HOHO HH

EE RR

HH OHOH

EE SS

HHHOHO

ZZ RR

HH OHOH

SS

How many stereoisomers?How many stereoisomers?How many stereoisomers?How many stereoisomers? ZZ

7-7Dr. Wolf's CHM 201 & 202

7.13 7.13

Chemical Reactions That Chemical Reactions That

Produce Diastereomers Produce Diastereomers

7-8Dr. Wolf's CHM 201 & 202

In order to know understand stereochemistry of In order to know understand stereochemistry of product, you need to know two things:product, you need to know two things:

(1) stereochemistry of alkene (cis or trans; Z or E)(1) stereochemistry of alkene (cis or trans; Z or E) (2) stereochemistry of mechanism (syn or anti)(2) stereochemistry of mechanism (syn or anti)

Stereochemistry of Addition to AlkenesStereochemistry of Addition to AlkenesStereochemistry of Addition to AlkenesStereochemistry of Addition to Alkenes

CC CC ++ E—YE—Y CC CCEE YY

7-9Dr. Wolf's CHM 201 & 202

BrBr22

RR

SS RR

SS

mesomeso

anti addition to anti addition to transtrans-2-butene gives meso -2-butene gives meso diastereomerdiastereomer

Bromine Addition to trans-2-ButeneBromine Addition to trans-2-ButeneBromine Addition to trans-2-ButeneBromine Addition to trans-2-Butene

7-10Dr. Wolf's CHM 201 & 202

BrBr22

RR

RR SS

SS

50%50% 50%50%

Bromine Addition to cis-2-ButeneBromine Addition to cis-2-ButeneBromine Addition to cis-2-ButeneBromine Addition to cis-2-Butene

anti addition to anti addition to ciscis-2-butene gives racemic mixture -2-butene gives racemic mixture of chiral diastereomerof chiral diastereomer

++

7-11Dr. Wolf's CHM 201 & 202

RCORCO33HH

RR

RR SS

SS

syn addition to syn addition to transtrans-2-butene gives racemic -2-butene gives racemic mixture of chiral diastereomermixture of chiral diastereomer

Epoxidation of trans-2-ButeneEpoxidation of trans-2-ButeneEpoxidation of trans-2-ButeneEpoxidation of trans-2-Butene

50%50% 50%50%

++

7-12Dr. Wolf's CHM 201 & 202

RR

SS RR

SS

Epoxidation of cis-2-ButeneEpoxidation of cis-2-ButeneEpoxidation of cis-2-ButeneEpoxidation of cis-2-Butene

syn addition to syn addition to ciscis-2-butene gives meso -2-butene gives meso diastereomerdiastereomer

RCORCO33HH

mesomeso

7-13Dr. Wolf's CHM 201 & 202

Of two stereoisomers of a particular starting Of two stereoisomers of a particular starting material, each one gives differentmaterial, each one gives differentstereoisomeric forms of the productstereoisomeric forms of the product

Related to mechanism: terms such asRelated to mechanism: terms such assyn addition and anti addition refer tosyn addition and anti addition refer tostereospecificitystereospecificity

Stereospecific reactionStereospecific reactionStereospecific reactionStereospecific reaction

7-14Dr. Wolf's CHM 201 & 202

..

transtrans-2-butene-2-butene

ciscis-2-butene-2-butene

transtrans-2-butene-2-butene

ciscis-2-butene-2-butene brominationbromination antianti 22RR,3,3RR + 2 + 2SS,3,3SS

brominationbromination

epoxidationepoxidation

epoxidationepoxidation

antianti

synsyn

synsyn

mesomeso

mesomeso

22RR,3,3RR + 2 + 2SS,3,3SS

Stereospecific reactionsStereospecific reactionsStereospecific reactionsStereospecific reactions

7-15Dr. Wolf's CHM 201 & 202

A single starting material can give two or moreA single starting material can give two or morestereoisomeric products, but gives one of themstereoisomeric products, but gives one of themin greater amounts than any otherin greater amounts than any other

++

CHCH33

HH

CHCH33

HH

68%68% 32%32%

Stereoselective reactionStereoselective reactionStereoselective reactionStereoselective reaction

CHCH33

CHCH22

HH CHCH33

HH

CHCH33

HH

HH22

PtPt

7-16Dr. Wolf's CHM 201 & 202

7.14 7.14

Resolution of EnantiomersResolution of Enantiomers

Separation of a racemic mixture into its two Separation of a racemic mixture into its two enantiomeric formsenantiomeric forms

Dr. Wolf's CHM 201 & 202 7-17

enantiomersenantiomers

C(+)C(+)C(+)C(+) C(-)C(-)C(-)C(-)

StrategyStrategyStrategyStrategy

Dr. Wolf's CHM 201 & 202 7-18


C(+)C(+)C(+)C(+) C(-)C(-)C(-)C(-)

2P(+)2P(+)

C(+)C(+)P(+)C(+)C(+)P(+) C(-)C(-)P(+)C(-)C(-)P(+)

diastereomersdiastereomers


Dr. Wolf's CHM 201 & 202 7-19


C(+)C(+)C(+)C(+) C(-)C(-)C(-)C(-)

2P(+)2P(+)

C(+)C(+)P(+)C(+)C(+)P(+) C(-)C(-)P(+)C(-)C(-)P(+)


C(+)C(+)P(+)C(+)C(+)P(+)

C(-)C(-)P(+)C(-)C(-)P(+)


Dr. Wolf's CHM 201 & 202 7-20


C(+)C(+)C(+)C(+) C(-)C(-)C(-)C(-)

2P(+)2P(+)

C(+)C(+)P(+)C(+)C(+)P(+) C(-)C(-)P(+)C(-)C(-)P(+)


C(+)C(+)P(+)C(+)C(+)P(+)

C(-)C(-)P(+)C(-)C(-)P(+)

P(+)P(+)

P(+)P(+)

C(+)C(+)C(+)C(+)

C(-)C(-)C(-)C(-)


7-21Dr. Wolf's CHM 201 & 202

7.157.15Stereoregular PolymersStereoregular Polymers

atacticatactic

isotacticisotactic

syndiotacticsyndiotactic

Dr. Wolf's CHM 201 & 202 7-22

Atactic PolypropyleneAtactic PolypropyleneAtactic PolypropyleneAtactic Polypropylene

random stereochemistry of methyl groups random stereochemistry of methyl groups attached to main chain (stereorandom)attached to main chain (stereorandom)

properties not very useful for fibers etc.properties not very useful for fibers etc.

formed by free-radical polymerizationformed by free-radical polymerization

Dr. Wolf's CHM 201 & 202 7-23

Isotactic PolypropyleneIsotactic PolypropyleneIsotactic PolypropyleneIsotactic Polypropylene

stereoregular polymer; all methyl groups onstereoregular polymer; all methyl groups onsame side of main chainsame side of main chainuseful propertiesuseful propertiesprepared by coordination polymerization prepared by coordination polymerization under Ziegler-Natta conditionsunder Ziegler-Natta conditions

Dr. Wolf's CHM 201 & 202 7-24

Syndiotactic PolypropyleneSyndiotactic PolypropyleneSyndiotactic PolypropyleneSyndiotactic Polypropylene

stereoregular polymer; methyl groups alternate stereoregular polymer; methyl groups alternate side-to-side on main chainside-to-side on main chain

useful propertiesuseful properties

prepared by coordination polymerization under prepared by coordination polymerization under Ziegler-Natta conditionsZiegler-Natta conditions

7-25Dr. Wolf's CHM 201 & 202

7.167.16Chirality CentersChirality Centers

Other Than CarbonOther Than Carbon

7-26Dr. Wolf's CHM 201 & 202

SiliconSiliconSiliconSilicon

Silicon, like carbon, forms four bonds in its stable Silicon, like carbon, forms four bonds in its stable compounds and many chiral silicon compounds compounds and many chiral silicon compounds have been resolvedhave been resolved

SiSi SiSidd dd

aabb

cc

aabb

cc

7-27Dr. Wolf's CHM 201 & 202

Nitrogen in aminesNitrogen in aminesNitrogen in aminesNitrogen in amines

Pyramidal geometry at nitrogen can Pyramidal geometry at nitrogen can produce a chiral structure, but enantiomers produce a chiral structure, but enantiomers equilibrate too rapidly to be resolvedequilibrate too rapidly to be resolved

NN NN:: ::

aabb

cc

aabb

cc

very fastvery fast

7-28Dr. Wolf's CHM 201 & 202

Phosphorus in phosphinesPhosphorus in phosphinesPhosphorus in phosphinesPhosphorus in phosphines

Pyramidal geometry at phosphorus can produce a Pyramidal geometry at phosphorus can produce a chiral structure; pyramidal inversion slower than chiral structure; pyramidal inversion slower than for amines and compounds of the type shown have for amines and compounds of the type shown have been resolvedbeen resolved

PP PP:: ::

aabb

cc

aabb

cc

slowslow

7-29Dr. Wolf's CHM 201 & 202

Sulfur in sulfoxidesSulfur in sulfoxidesSulfur in sulfoxidesSulfur in sulfoxides

Pyramidal geometry at sulfur can produce a chiral Pyramidal geometry at sulfur can produce a chiral structure; pyramidal inversion is slow and structure; pyramidal inversion is slow and compounds of the type shown have been resolvedcompounds of the type shown have been resolved

SS SS:: ::

aabb

O_O_

aabb

O_O_

slowslow

++ ++

Dr. Wolf's CHM 201 & 202 7-30

End of Chapter 7

Dr. Wolf's CHM 201 & 202 7-1 7.12 Molecules with Multiple Chirality Centers.

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