Organic Reactions Volume 4 - Sciencemadness Dot Org

DIENE SYNTHESIS II 87
or even prevent, the primary reaction. This competing reaction has
been noted in the reaction of allyl amine with cyclopentadiene, of 1nitropentene-1
with cyclopentadiene, and of o-methoxycinnamic acid
with butadiene. Some dienophiles (acids 26 and acid chlorides *) have
been found to promote the polymerization of dienes. This troublesome
reaction cannot be eliminated entirely, but its effect upon the yield may
be minimized by the use of somewhat more than an equimolar amount
of the diene (1.5 moles) and by diluting the reaction mixture with an
inert solvent (benzene, toluene, or xylene 13 ).
Polymerization of the dienophile may account for a further decrease
in the yield. The dienophile in the reaction, acrolein + 2-methoxyand
2-ethoxy-butadiene, 8 ' 26 has been stabilized with hydroquinone. 6
A number of secondary reactions of the addition products have been
discussed previously. These changes involve: addition of a second mole
of the diene to the primary adduct (cyclopentadiene + vinyl acetate and
vinyl chloride, furan + acetylenedicarboxylic acid and its methyl ester);
rearrangement and the addition of a second mole of the dienophile to
the adduct; 14 elimination of the bridge system which may or may not
be accompanied by dehydrogenation (ethyl cinnamate + 1,3-diphenylisobenzofuran,
ethyl acetylenedicarboxylate + cyclohexadiene, styrene
+ piperylcyclone, 1,2-dihydronaphthalene + acecyclone); dehydrogenation
by the solvent 66 (cinnamic acid + methyleneanthrone);
and the reverse Diels-Alder reaction (see Chapter 1).
Traces of mineral acid may alter the course of the reaction. It has
been reported 6 - 7 that, after several months at room temperature, such
unifunctional dienophiles as acrolein, crotonaldehyde, methyl vinyl
ketone, and vinyl phenyl ketone failed to react with furan and 2~methylfuran.
Traces of sulfuric acid or sulfur dioxide, sulfonic acids or their
chlorides, but not formic acid or esters of sulfonic acids, promote the
so-called "substitution-addition reaction"—exemplified by the formation
of /3-(2-furyl-)propionaldehyde from furan and acrolein. This
exothermic reaction may proceed one stage further by a similar mechanism.
|l J_H * H.JI ILcH2CH2CO2H *
O T)
CH2=CHCO2H
HO2CCH2CH2-J^ ^LCH2CH2CO2H
66 Bergmann, Haskelberg, and Bergmann, J. Org. Chem., 7, 303 (1942).