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DIENES

Dienes are with 2 double bonds. IUPAC: Same as , but change -ene to -adiene and use two numbers to locate the two double bonds (number from the end of the chain which makes the smaller of these numbers smaller).

Double bonds separated by more than one are isolated. Compounds with isolated double bonds have the same chemical properties as alkenes.

Double bonds that alternate with single bonds, eg C=C-C=C, are conjugated.

Double bond arrangements like C=C=C are cumulated. Compounds containing two -carbon cumulated double bonds are called .

Conjugated dienes differ from simple alkenes in that they

 are more stable,  undergo 1,4-addition, and  are more reactive. Stability

Alkene Type ∆ Hhydrogenation

RCH=CH2 30 kcal/mole,exotherm.

R2C=CH2, RCH=CHR 28

R2C=CHR 27

∆ For dienes with isolated double bonds, the Hhydrogenation calculated by adding the appropriate values from the table above together is very close to the experimental value.

∆ For conjugated dienes, the experimental Hhydrogenation is smaller than the calculated one, which is based on isolated double bonds.

Conjugated Calculated Experimental ∆ ∆ Hh Hh

CH2=CH-CH=CH2 60 kcal/mole 57

CH2=C(CH3)-C(CH3)=CH2 56 54 Therefore, conjugated dienes are more stable than isolated dienes.

This stability is reflected in the fact that, where possible, conjugated dienes are the preferred diene products of elimination reactions ---

H H C C C C H H H C C H C C KOH, alcohol major product - conjugated C C heat Cl H H H C + C C + H O + KCl Formation of the conjugated C C 2 diene takes precedence over H the Saytzeff rule. minor product - isolated (Saytzeff product)

This stability can be explained by electron delocalization in the conjugated diene

 using the model ---

 or the model --- Energy (Huckel) of Two Isolated vs Conjugated Double Bonds

π4 1.68_

1.0_ π∗ π3 0.68_ Energy 0 π2 -0.68_

-1.0_ π

π1 -1.68_

Two isolated π-bonds. Conjugated π-system. = −4.0 Eπ Eπ = −4.72 Electrophilic Addition to Conjugated Dienes: 1,2- vs. 1,4- Addition ---

1 1 2 4 YZ 2 4 2 1 3 3 3 4 Y YZ Z 1,2-addition 1,4-addition Note the "shift" of the π-bond.

 eg CH3CH=CH-CH=CHCH3 + HCl >

CH3-CH2-CHCl-CH=CHCH3 via 1,2-addition +

CH3-CH2CH=CH-CHCl-CH3 via 1,4-addition Mechanism ---

H H H H

2 CH 4 CH3 HCl 4 3 1 3 CH 1 3 CH3 2 3 H H H H H ordinary 2o carbocation - VERY LITTLE FORMED + H H H H 1 1 2 4 CH3 2 CH 3 3 4 3 CH3 CH3 H H H H H H 2o allylic type of carbocation; ~ stability of 3o carbocation.

Since the positive charge is shared between the labeled 2 & 4 Cl- can attack either. 1,2- vs. 1,4-Addition: Rate vs. Equilibrium Control

2 4 1 3 HBr

Low temp (-80o C) High temp (40o C) 1 1 2 4 2 3 3 4 H 80% H 80% Br Br 40oC 1 1 4 2 2 3 3 4 H 20% H 20% Br Br

At -80o the 1,2- At 40o the 1,2- and 1,4- converts to 1,4-isomer interconvert, giving very slowly. Since 1,2- an equilibrium mixture. predominates here, The 1,4-isomer is more it is formed faster. stable and predominates.

Reaction rates determine Equilibrium constant product here: determines product here: Kinetic Control Thermodynamic Control

In this case, the less stable product is formed faster. How does this happen? Diels-Alder Reaction

X X Y Y

diene dieneophile a substituted (X, Y: typically one or both are electron withdrawing)

There are other variations on this theme, eg---

X X

Y Y Stereochemistry ---

1) With respect to the dieneophile, the addition is stereospecifically syn, eg

H H H H Cl Cl Cl Cl

H Cl + Cl H H Cl Cl H Cl H H Cl

2) With respect to the diene, the addition is stereospecific and syn, eg

C6H5 C6H5 Cl Cl Cl Cl C6H5 C6H5 Cl Cl Cl Cl 3) The diene must be in the s-cis conformation for reaction to occur. [The mechanism is concerted --- no intermediates.]

s-cis s-trans too far close close enough enough too far

Dienes unable to attain the s-cis conformation do not react, eg 4) If the diene is cyclic and the dieneophile has an unsaturated , the endo product is usually favored over the exo product [the reaction is stereoselective; one diasteromer (endo) is favored over another (exo)]. 1 2 3 2 b c 1 3 5 4 b a 5 d c H 4 a e d e H exo- 1 2 3 2 H 1 3 c 5 4 H b b 5 c 4 a d a d e e cyclopentadiene endo-dicyclopentadiene