CHAPTER 16 CONJUGATE ADDITION

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1 HAPTER 16 NJUGATE ADDITIN

2 onjugated Pi Bonds onjugated double bonds are separated by only one single bond. Isolated double bonds are separated by two or more single bonds.

3 arbonyl ompounds with onjugated Pi Bonds A carbon-carbon double bond can also be conjugated pi bond of a carbonyl group in an aldehyde, ketone, carboxylic acid and its derivatives. H 2 H H 3 H H H H 3 All are onjugated Pi Bonds

4 H Examples onjugated onjugated onjugated Isolated

5 1,2-Addition H 2 H H H 2 Addition occurs here examples or here H 3 H H H HH 3 l 2 H 3 H l H H HH 3 l

6 1,4-Addition = onjugate addition H 2 H H H 2 H 2 H H Addition occurs here example

7 1,2 and 1,4- Addition to onjugated Mechanism Dienes

8 ther electrophilic reagents add to conjugated dienes in similar fashion

9 Kinetic ontrol versus Thermodynamic ontrol of a hemical Reaction In addition of HBr to 1,3-butadiene the temperature of reaction greatly affects the distribution of 1,2 and 1,4 products.

10 Heating the 1,2-addition product leads to an equilibrium which favors the 1,4-addition product Because equilibrium conditions favor the 1,4-addition product it must be the most stable

11 1,2-addition product is formed faster and is the major product at low temperatures due to DG of 1,2-addition product is lower than for 1,4-addition product The reaction is said to be under kinetic control At higher temperatures when an equilibrium is established, the most stable product predominates 1,4-addition product is more stable and is the major product at high temperatures The reaction is said to be under thermodynamic control The 1,4 product is most stable because it leads to a disubstituted double bond 1,2-addition product has a less stable monosubstituted double bond

12 1,4-Addition Polymers X H 3 H 2 H H 2 isoprene catalyst 1,4-addition H 3 H 3 H 2 HH 2 H 2 HH 2 polyisoprene X Natural rubber = polyisoprene with all cis double bonds Hard polymer (gutta-percha) = trans polyisoprene

13 DIELS- ALDER REATIN

14 The Diels-Alder Reaction concerted three pairs of electrons move at one time + heat ring-forming reaction diene + dienophile cyclohexene NTE: = gain of bond order = loss of bond order

15 EXAMPLES F DIELS-ALDER REATIN H 3 H 3 H 3 H 3 H 3 H 3 Diene Dienophile A yclohexene

16 THE DIENE MUST BE ABLE T ADPT THE S-IS NFRMATIN rotate + reacts s-trans conformation s-cis conformation does not react reacts normally +

17 Some s-cis Dienes can be used in Diels Alder Reaction All are s-cis

18 MRE DIELS-ALDER REATINS

19

20 cis cis

21 If the dienophile is an alkyne a cyclohexadiene is formed. H 3 H 3 H 3

22 1- Electrophilic 1,4-Addition to α,β- Unsaturated arbonyl ompounds Weak Nu: catalyzed by H + Mechanism H 2 H R H H 2 H R + + H + H H 2 H R H H 2 H R + H 2 H 2 R u product Nu Nu reactant H H H 2 H R H 2 H R Nu Nu H 2 H R

23 Under acid condition Nu: attacks the β- carbon Examples H 2 HH H 3 H HH 3 Hl H 2 l H + H 2 H H 2 H 3 HH 2 H 3 H H + H 3 H H 3

24 2- Nucleophilic Addition to α,β unsaturated carbonyl compounds H 2 H R - H 2 H R + arbonyl is + Nu: attacks here β carbon is + ; Nu: - attacks here + H 2 H R -

25 Example H H 2 H H N HN H 2 HH 3 N Attack of N - on the carbonyl carbon Mechanism H 2 H H N H 2 HH 3 N or H 2 H 2 H 3 N - HN H H 2 HH 3 N

26 Attack of N - on the β carbon - Mechanism H 2 H H 3 - N H H 2 H H 3 enol N H 2 H 2 H 3 N H + H 2 H H 3 N - H 2 H H 3 N -

27 Which of the two addition reactions occurs? A mixture of products results but the major product depends on the nature of nucleophiles 1. Stronger bases attack 1,2 (at =). eg. LiAlH 4, NaBH 4, RMgX 2. Weaker bases attack 1,4 ( at =) e.g. N -, NH 3, RNH 2 3. uprates (R 2 uli) attack 1,4 (at =).

28 Stronger bases attack 1,2 (at =) H H 2 H 3 1) H 3 H 2 MgBr 2) H +, H 2 H 3 H HH 3 H 2 H 3 H 3 1) LiAlH 4 2) H +, H 2 1) NaBH 4 2) H +, H 2 H H 3 H HH 3 H H H 2 HH 3 H 3

29 Weaker bases attack 1,4 ( at =) H 3 H HH 3 H 2 N - H 3 HN NH 3 H 3 H H 2 H 3 NH 2 H 3 N H 3 H 2 HH 3 (H 3 ) 2 HH 3 H 3 NH 2 (H 3 ) 2 H 2 H 3 NHH 3

30 Lithium dialkylcuprates attack 1,4 at = H 3 1) (H 3 ) 2 uli 2) H +, H 2 H 3 H 3 H 3 H HH 3 1) (H 3 ) 2 uli 2) H +, H 2 H 3 H H 2 H 3 H 3

31 α,β- Unsaturated carboxylic acid A conjugated carboxylic acid behaves like conjugated aldehydes and ketones H 2 H H N - H 2 H 2 H N 2 H H 2 H NH 3 NH 4 l H 2 H NH 2 H 2 2 H

32 Examples H H +, H 2 H H H 2 H 3 NH 2 - NHH 3