No abstract

Criteria for the mechanism of 1,3-dipolar cycloadditions which lead to 5-membered rings are provided by the stereoselectivity observed with cis-trans isomeric dipolarophiles, by the efect of solvent and substituents on the rate constants, by the activation parameters, and by orientation phenomena. A concerted addition, which can also be described in terms of molecular orbitals and in which the two new o-bonds are formed simultaneously, although not necessarily at equal rates, ofers the best explanation of the experimental facts,The concept of "cycloaddition" gives a formal description of an overall reaction but not a mechanistic interpretation. Any discussion of the mechanism of the 1,3dipolar addition must be viewed against the background of mechanistic interpretation of other cycloadditions which lead to three-, four-, or six-membered rings. Recent studies have shown that cyclopropanes can be formed from an alkene + carbene via at least two mechanisms. Likewise, two reaction paths exist for the coupling of two alkenes to give cyclobutanes, and different stereoselectivities are observed. A preceding review [I], described various 1,3-cycloadditions which follow the scheme 3 + 2 -+ 5. Our studies of the mechanism of addition have been carried out exclusively with 1,3-dipoles which have internal octet stabilization according to the definition given elsewhere [ 11 ; hence the following discussion is limited to 1,3-dipolar cycloadditions of this type. A. Confrontation of Possible MechanismsThe principal question is whether the two newo-bonds, which lead to the uncharged five-membered ring on fusion of the 1,3-dipole a-b-c with the dipolarophile d-e, are closed simultaneously or one after the other. d Ae [*I Extended version of a lecture given at

rnDie zahlenmarJige Errnittlung der Dipolarophilen-Aktivitaten gestattet es, das AusmarJ der mechanistischen Gerneinsamkeit der Cycloadditionen verschiedener 1.3-Dipole zu beurteilen. Die Aktivitatsreihe gegeniiber Diphenylnitrilimin3), einem Glied der Reihe der Nitriliumbetaiue, sei hier rnit derjenigen des Phenylazids verglichen, das den Diazoniumbetainen4) angehort. Nach AbschluR unserer Versuche (1963) lag noch keine kinetische Studie der Azid-Addition vor. Inzwischen berichteten Scheiner und Mitarbb.5) iiber die Kinetik der Arylazid-Addition an Norbornen. Bailey und Whites) verglichen Pikryl-und Phenylazid i n der Reaktivitat gegenuber cyclischen und offenkettigen Alkenen. A. MethodeDie kinetische Verfolgung der kid-Addition griindet man zweckmaisig auf die sinkende Konzentration des organischen Azids. Die Reduktion des unverbrauchten Azids mit alkalischem Arsenit 7) und volumetrischer Messung des entbundenen Stickstoffs bewahrte sich nicht, da die Addukte a.P-ungesattigter Carbonylverbindungen unter diesen Bedingungen ebenfalls Stickstoff verlieren.

Einer sehr groJen Zahl s p e z i e l l e r Synthesewege in die heterocyclische

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