Thermal 1,4-additions of free germylenes Me2Ge to a number of 1,3-dienes proceed concertedly via a [2 + 41 cheletropic mechanism, as reported recently.'a.b For a better understanding of these new pericyclic reactions the question has been raisedla whether in the transition state the HOMOdiene interacts with the LUMO,,,m,Iene ("normal" reaction) or the LUMOdiene with the HOMOgermylene ("inverse" reaction). This question is of general importance, since at present no information is available on the behavior of heavy carbene analogues-silylenes, germylenes, stannylenes-altogether.We wish to report here that free Me2Ge behaves as a donor toward 1,3-dienes, reacting faster with more electron-deficient ones thus exhibiting the "inverse" type of a cheletropic [2 + 41 addition, and not the "normal" one.
HOMO LUMO LUMO HOMO
Normol
Inverse Cheletropic ReactionFollowing the thermal generation, k , , of germylenes MezGe in solution from l,la the fast polymerization, k2, of the latter is competing with the desired disrotatory ring closure with a diene, M : y e J . Am. Chem. SOC. 1984, 106, 3862-3863 Scheme I This sequence is backed up by competition experiments: With a 1 : 1 mixture of the 1,4-diphenylbutadiene and the muconic acid ester at 70 OC as starting materials, exclusively 3b could be detected, and no 3a, whereas this ester and the dinitrile in a similar mixture showed comparable reactivities, k3.
Acknowledgment.We are grateful to the Fonds der Chemischen Industrie for valuable substances and to the Minister fur Wissenschaft und Forschung des Lander Nordnheim Westfalen, Diisseldorf, for support.
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