"Dry" carbonyl Diels-Alder and carbonyl-ene reactions with microwaves and reactants supported on graphite were dramatically accelerated with respect to homogeneous conditions both involving classical or microwave heating. Al-though the reactions were carried out in an open reactor, a control of the evaporation of volatile reagents was possible, probably due to their retention or confinement by graphite.Chemistry on solid supports, almost unnatural for organic reactions, has been developed rapidly during the last twenty years [']. The idea of microwave (MW) heating in this reaction type has recently stimulated additional interest[*]. We have recently described "dry" by graphite support MW irradiation and reported a high acceleration of some well-known difficult Diels-Alder reactions. The same process was applied here to two thermal cycloadditions involving carbonyl derivatives as electrophiles, the carbonyl Diel~-Alder[~>~] and the carbonyl-ene r e a~t i o n [~,~] , which are also known to be difficult. Moreover, in this study we tested the possible retentive behavior of graphite towards organic reagentsL7]. To this end, volatile dienes were used in Diels-Alder reactions.At first we treated 2,3-dimethyl-l,3-butadiene (DMB) (1) with ethyl glyoxylate (2) and ethyl mesoxalate (3). With the latter, reactions occurred without a catalyst, giving the expected adduct 5 in 75% yield after only MW irradiation for 20 X 1 min (IP = 30 W; t , , , = 165°C). In the case of conventional heating, it is necessary to heat at 150°C for 4 h in a sealed tube to obtain a satisfactory yield ['].
MeWith ethyl glyoxylate, a weaker dienophile than 3, a catalyst (ZnC12) was necessary, as under conventional heating conditions [lO]. Adduct 6 was isolated in 87% yield after MW irradiation for 10 X 1 rnin (IP = 30 W, t , , , = 165°C). As above, conventional heating conditions are more drastic" 'I. Taking glyoxylic acid (4) as dienophile, we obtained the adduct 7 after MW irradiation for 10 X 1 min (IP = 60 W; t , , , = 249°C) in 54% yield. This reaction has not yet been examined by conventional heating, and 7 was previously prepared from S91.The ene reaction or Alder reaction["] is a cycloaddition which requires more energy than the Diels-Alder reaction. However, it can compete with it, in particular for C-alkylated dienes such as DMB. The ene reaction was not detected in the above examples. In 1992, the investigation of a specific activation by MW led us to study the carbonylene reaction in homogeneous medium ['3]. A kinetic approach showed that an athermal (or no thermal) specific effect agrees more with a myth than with reality, as was also true for the esterification of 2,4,6-trimethylbenzoic acid with 2-p r o p a n~l [ '~~] and the Diels-Alder reaction of anthracene with ethyl maleate [14b].In the present work, we take up again some examples of carbonyl-ene reactions under conditions of coupling MW and graphite. The enophile 3 was allowed to react with (-)-P-pinene (8) and 1-decene (9). In homogeneous medium, 8 and 3 react in 6 h in CC14...
