A Guidebook to Mechanism in Organic Chemistry

Diels-Alder Reaction

CHa=CH—CH=CHa • CHa—CH—CH=CH, (XXXIII)

CHa—CH-^CH=^CHa

H

X

CH,—CH=CH—CHa

H

(XXXIV)

H

Br

I

CHa—CH—CH=CH 2 (XXXVa)

I

H

Br° \:2-addition

*•

Br

I

CH,—CH=CH—CH 8 (XXXV6)

I

H

l:4-addition

to occur preferentially at lower temperatures in non-polar solvents

and 1:4-addition at higher temperatures in polar solvents; the temper­

ature effect is due to the fact that the activation energy for 1:4-addi­

tion is usually higher than that for 1:2-addition (cf. p. 203).

For addition to an unsymmetrical diene the same considerations

apply as in the case of mono-alkenes, thus:

Me-CH=CH-CH=CHa Me— CH==CH*-CH-CH, -* products

H

Me Me

[, -+ CH,

H

=CH, -»• products

(ii) Diels-Alder reaction

The classic example is with butadiene and maleic anhydride

'^6

o

o

A

O