Addition to Carbon-Carbon Double Bondsi.e. l:4-addition, proceeding cis, via a cyclic transition state (cf. the
pyrolysis of esters, p. 208), to yield a cyclic product. It has been used
as a diagnostic test for determining whether the double bonds in a
diene are conjugated or not (though this is normally more readily
determined spectroscopically) and also has considerable synthetic
importance. The reaction is promoted by the presence of electron-
donating substituents in the diene and of electron-withdrawing sub
stituents in the, so-called, dienophile; their presence in the latter is,
indeed, all but imperative for the reaction proceeds very poorly if at all
with a simple double-bonded compound. Other commojdienophiles
are p-benzoquinone, CH 2 =CHCHO and Et0 2 CG=CC0 2 Et.
The reaction is also sensitive to steric effects; thus of the three
1,4-diphenylbutadienes only the trans/trans form undergoes reaction
with maleic anhydride:
SirSTrSBy the reactivity of the diene is promoted when the double
bonds are locked in a cis conformation with respect to each other as in
cyclopentadiene.
Where there is the possibility of more than one product, depending
on which way round the addition takes place, e.g. with maleic anhy
dride and cyclopentadiene, that product is formed in which there is
the maximum concentration of double bonds in the transition state
when the centres that are to react are placed over each other; inter
action between the n electron systems of the two reactants will then be
at a maximum. Thus (XXXVI), the more stable endo structure, is
obtained rather than the exo (XXXVII):Ph(XXXVI) (XXXVII)