stereo chemistry i i

8/17/2019 Stereo Chemistry i i

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Stereoselective and stereospecific reactions.

* *CH3CH=CHCH3 + Br 2 CH3CHCHCH3

Br Br 2-butene 2,3-dibromobutane

2 geometric isomers 3 stereoisomers cis- and trans- (S,S )-, ( R,R)-, and ( R,S )-

meso-


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H CH3 CH3 CH3 \ / \ /

C = C C = C

/ \ / \

CH3 H H H

trans-2-butene cis-2-butene

CH3 CH3 CH3

H Br Br H H Br

Br H H Br H Br

CH3 CH3 CH3

(S,S) (R,R) meso


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CH3

H CH3 \ / H Br

C = C + Br 2

/ \ H Br

CH3 H CH3trans-2-butene meso-2,3-dibromobutane

only product

A reaction tat !ie"ds #redominate"! one stereoisomer (or

one #air o$ enantiomers) o$ se%era" diastereomers is ca""ed a

stereose"ecti%e reaction. &n tis case te meso- #roduct is

#roduced and not te oter t'o diastereomers.


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CH3 CH3

H H \ / H Br Br H C = C + Br 2 +

/ \ Br H H Br CH

3

CH3

CH3 CH3cis-2-butene (S,S )- ( R,R)-2,3-dibromobutane

racemic modification only products

A reaction in 'ic stereocemica""! di$$erent mo"ecu"es

react di$$erent"! is ca""ed a stereos#eci$ic reaction. &n tis

case te cis- and trans- stereoisomers gi%e di$$erent

#roducts.


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e $act tat te addition o$ a"ogens to a"enes is bot

stereose"ecti%e and stereos#eci$ic gi%es us additiona"

in$ormation about te stereocemistr! o$ te addition and te

mecanism $or te reaction.


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C C CC anti-addition

C C C C syn-addition


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C C

H

CH3

H

H3C

C C

CH3

H

H

H3C

CH3

H Br

CH3

Br H

CH3

Br H

CH3

H Br

CH3

H Br

CH3

H Br

+

Br 2

Br 2

&s te addition o$ Br 2 syn or anti


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H H

\ / H

C = C H C C CH3 anti-addition o$ 2 / \ CH3 to te cis-isomer

CH3 CH3

ote0 must rotate about C-C to get to te 1iscer #roection

CH3 H H

H C C CH3 C C H H CH3

CH3 CH3 H

CH3


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H CH3

\ / CH3

C = C H C C H anti-addition o$ 2 / \ CH3 to te trans-isomer

CH3 H


CH3

CH3 H H H C C H C C H

CH3

CH3 CH3 H

CH


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CH3

H Br

CH3

Br H

H

H

H3C

H3C

anti-

Br

H3C H

Br

HH3CBr

H3C H

H

H3C Br

Br 2

H

CH3H

H3CBr 2

anti-

Br

H CH3

Br

HH3C

H

H3C Br

H

H3C Br

CH3

H Br

CH3

H Br

rotate about C2-C3 to get toFischer projection!


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&n determining 'eter a stereose"ecti%e addition is syn- or

anti- !ou cannot sim#"! "oo at te 1iscer #roection.

4emember it is o$ten necessar! to rotate about a carbon-

carbon bond to get a mo"ecu"e into te con$ormation tatcorres#onds to te 1iscer #roection 5se !our mode" it to

%eri$!


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What does the stereochemistry tell us about the mechanismof addition of halogens to alenes

"# C C + $ $

%&'

C C

$

+ $

(halonium ion(

C C

$

2# $ + C C

$

$

anti-addition


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H!dro6!"ation o$ a"enes0

* *CH3CH=CHCH3 + 78n9: CH3CH-CHCH3

9H 9H 2-butene 2,3-butanedio"

2 geometric isomers 3 stereoisomers


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cis-2-butene + 78n9: 2,3-butanedio" m# 3:oC

trans-2-butene + 78n9: 2,3-butanedio" m# ;


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cis-2-butene + 78n9: meso-2,3-di!dro6!butane m# 3:o

CH3

H 9H

H 9H

CH3

trans-2-butene + 78n9: (S,S ) ( R,R)-2,3-di!dro6!butane m# ;<o

CH3 CH3

H 9H + H9 H H9 H H 9H

CH3 CH3

stereose"ecti%e and stereos#eci$ic

C C

H

H3

C CH3

H

C C

H

H3C H

CH3


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&s !dro6!"ation 'it 78n9: syn- or anti-


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H 9 9 CH3 9H 9H

\ / C = C H C C CH3 syn-o6idation o$

/ \ CH3 H te trans-isomer

CH3 H


9H 9H CH3

H 9H H C C CH3 H9 C C H H9 H

CH3 H

CH3 CH3 H 9H

CH3


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H 9 9 H 9H 9H

\ /

C = C H C C H syn-o6idation o$

/ \ CH3 CH3 te cis-isomer

CH3 CH3

ote0 no rotation necessar! to get to 1iscer #roection

9H 9H CH3 H H

H C C H H9 C C 9H H 9H CH3 CH3

CH3 CH3 H 9H

CH3


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cis-2-butene + HC93H 2,3-butanedio" m# ;<oC

trans-2-butene + HC93H 2,3-butanedio" m# 3:oC

2,3-butanedio" m# ;


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96idation 'it 78n9: syn-o6idation

cis-2-butene meso-2,3-di!dro6!butane

trans-butene (S,S )- ( R,R)-2,3-di!dro6!butane

96idation 'it HC929H gi%es te o##osite

cis-2-butene (S,S )- ( R,R)-2,3-di!dro6!butane

trans-2-butene meso-2,3-di!dro6!butane

96idation 'it HC929H is anti-o6idation.


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C C !dro6!"ation 'it 78n9: is syn- because o$ an intermediate

9 9 #ermanganate addition #roduct.

8n

9 9

C C !dro6!"ation 'it HC929H 9 is anti- because o$ an intermediate

e#o6ide.


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* *CH2-CH-CH-CH=9

9H 9H 9H

1our carbon sugar, an a"dotetrose.

'o cira" centers, $our stereoisomers


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CH9 CH9

H 9H H9 H

H 9H H9 H

CH29H CH29H

D-erythrose L-erythrose

CH9 CH9

H9 H H 9H

H 9H H9 H

CH29H CH29H

D-threose L-threose


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erythro-”

threo-”


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* *C>H?CHCHC>H? + 79H(a"c) C>H?CH=CC>H? Br CH3 CH3

;-bromo-;,2-di#en!"#ro#ane ;,2-di#en!"#ro#ene

: stereoisomers 2 stereoisomers

( E )- ( Z )-

dehydrohalogenation of an alkyl halide ia @2 mechanism


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C>H? C>H? C>H? C>H?

CH3

H H CH3

CH3

H H CH3

Br H H Br H Br Br H

C>H? C>H? C>H? C>H?

erythro- threo-

C>H? CH3 C>H? C>H? \ / \ /

C = C C = C

/ \ / \ H C>H? H CH3

( E )- ( Z )-


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C>H? C>H?

CH3 H H CH3 79H(a"c)

Br H H Br

C>H? C>H?

erythro- C>H? C>H? \ / C = C / \ H CH3

( Z )-


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C>H? C>H?

CH3 H H CH3 79H(a"c)

H Br Br H

C>H? C>H?

threo- C>H? CH3 \ / C = C / \ H C>H?

( E )-


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@2 is bot stereose"ecti%e and stereos#eci$ic.

; anti-e"imination o$ te H Br 0

C>H?

Br

CH3 H CH3 CH3 H C>H? C C H \ /

Br H C>H? C = C

H / \

C>H? C>H? C>H? H9-

erythro- ( Z )-


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C>H? Br

CH3 H CH3 CH3 C>H?

C>H? C C C>H? \ /

H Br H C = C H / \

C>H? C>H? CH3 H9-

threo- ( E )-

9nce again, !ou must rotate about te CC bond in te

1iscer #roection to get te H Br anti to one anoter.


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@2 is an anti-e"imination. e !drogen and te a"ogen must

be on o##osite sides o$ te mo"ecu"e be$ore te @2 e"imination

can tae #"ace. is maes sense as bot te base and te

"ea%ing grou# are negati%e"! carged. ere$ore te! 'ou"dtr! to be as $ar a#art as #ossib"e. &n addition, te "ea%ing

grou# is "arge and tere is more room $or te remo%a" o$ te

adacent #roton i$ it is on te o##osite side $rom te "ea%ing

grou#.


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8ecanism = e"imination, bimo"ecu"ar E2

; anti-e"imination

base)

C

$

C

H

C C + H)base + )$

%&'


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CH3

H &

CH3

H Br

CH3

& H

CH3

Br H

base C

C

C

CCH3H

H3

C H & CH3

H CH3

CH3

& H

CH3

H Br

CH3

H &

CH3

Br H

baseC

C

C

CCH

3H

H3C & H CH3

H CH3

+

+


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Addition o$ a"ogensto a"enes

anti-addition

H!do6!"ation 'it78n9

:

syn-o6idation

H!dro6!"ation 'itHC929H

anti-o6idation

e!droa"ogenationo$ a"!" a"ides @2 anti-e"imination


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stereos#eci$ic and stereose"ecti%e #rob"ems

tt#0//cemistr!2.csud.edu/organic/s!nanti/starts!nanti.tm"

tt#0//cemistr!2.csud.edu/organic/s!nanti/starts!nanti.tm"
http://chemistry2.csudh.edu/organic/synanti/startsynanti.htmlhttp://chemistry2.csudh.edu/organic/synanti/startsynanti.html

stereo chemistry i i

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