N-phenylmaleic imide

The partially hydrogenated phenanthrene derivative 18 (entry 4) is a very moderate diene due to the steric crowding caused by the substituents and the anulated rings, and it reacts even with highly reactive dienophiles such as maleic anhydride (MA) or N-phenylmaleic imide only at high pressure. The minor product 20 in the reaction with MA obviously stems from diene 21. This can be explained by a double-bond isomerization 18 - 21 prior to the cycloaddition, certainly catalyzed by traces of acid present in the MA. In the absence of acid only the Diels-Alder adduct 22 derived from diene 18 was observed. In the reaction of diene 23 with MA (entry 5) a similar sequence of steps was observed. A [1,5] shift of the C—O bond in 23, again certainly acid-catalyzed, produces the diene 26 followed by the Diels-Alder reaction with MA to give 24 and 25. 

Elemental and NMR analyses indicated that the homopolymers contained equimolar amounts of cyclopentadiene and N-phenylmale-imide, irrespective of polymerization temperature within the 120-260 C range. The homopolymers from the exo and endo adducts had softening points of 240 and 265-280 C, respectively. The polymer from the polymerization of the endo adduct at 120 C had a molecular weight of 1645 (vapor pressure osmometry). 

Grieger and Eckert [21, 49] considered two explanations of the ratio O > 1 in the Diels-Alder reaction of isoprene with maleic anhydride a larger dipole moment of the transition state or secondary orbital interactions which can only occur in endo Diels-Alder reactions. The findings that the difference between the activation volumes of many endo and exo Diels-Alder reactions is small (AAV < 1-2 cm mol ) and that the activation volume of retro Diels-Alder reaction of the endo cycloadduct between dimethylfulvene and N-phenylmaleic imide is positive and that of the retro Diels-Alder reaction of the corresponding exo cycloadduct is negative (Scheme 2.6), rule out that secondary orbital interactions are important and induce a larger contraction of the volume of the endo transition state. 

Scheme 10.1. Diels-Alder reaction between isoprene and N-phenylmaleic imide.

Fulvenallenes 4 react with dienophiles, such as tetrafluoroethylene or N-N-phenylmaleic acid imide, across their cyclopentadiene double bonds to give [4+2] cycloadducts 5. 

---