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Mechanisms of the reaction of 1,2,4-triazolinediones (4-RTAD, R = phenyl and methyl) with some olefins have been examined. The principal products are those of an ene reaction (eq 1) and diazetidine formation (overall, (2 + 2) addition, eq 2 and 3). The diazetidine from adamantylideneadamantane and MeTAD shows (X-ray analysis) a central carbon-carbon bond length of 1.635 Á. Rates of reaction increase with increasing alkylation (range: k(tetramethylethylene)/k(1-hexene) = 30000). The intramolecular isotope effects kK/kD for reaction of PhTAD with cis-, trans-, and gem-tetramethylethylene-d6 (Stephenson isotope effect test) are 1.1, 3.7, and 5.6, ascribed to rate-determining conversion of olefin and RTAD to an intermediate (considered to be an aziridinium imide, 2). Solvent effects are small and similar for the ene reaction, diazetidine formation (2 + 2), and a Diels-Alder reaction. The unexpectedly slow rate of reaction of norbornene, the moderate rate of reaction of adamantylideneadamantane, and the relative rates of various cis-trans olefin pairs are consistent with a "perpendicular planes" approach of RTAD and olefin (Figure 4a,c). Some comparisons are made between RTAD and other electrophiles. Product select!vities in the ene reactions of PhTAD and singlet oxygen differ markedly (see Schemes I and III). Some striking similarities are noted in the sensitivity of the rate to substituent for the doubly bonded electrophiles RTAD and singlet oxygen as well as for the singly bonded electrophile Br-Br. Features in common with these electrophiles are low lying LUMO's and relatively small gaps between the HOMO and LUMO levels of each of these electrophiles.Triazolinedione, RTAD, is a molecule of special interest and versatility. It reacts rapidly with conjugated dienes in a Diels-Alder fashion,2® with various olefins in an ene fashion to afford JV-allylurazoles (eq l),2b,c,d and with some olefins in what is overall a (2 + 2) reaction to afford 1,2-diazetidines2® (e.g., eq 2)-a set of three reaction types also shown by singlet oxygen.2* RTAD also reacts with certain strained single bonds.2® Another notable feature is the reaction of RTAD with some olefins to afford products of ionic rearrangement.2h_k In this paper we describe some aspects of products and rates in RTAD-olefin reactions.RTAD-Olefin Reactions: Evidence for Intermediates. Trapping experiments (external traps2e,3®,b and internal traps3®) have pointed to the presence of an intermediate, usually formulated as a 1,4-dipole (1). We 1 (1) (a) This work has been supported by the National Science Foundation. (b) Presented in part at the symposium "Stereochemistry and Reactivity in ir Systems" at Texas Christian University, May, 1982. Acknowledgement is made to Verlag Chemie International, Inc. covering reproduction of some of the material from the chapter by F. D. Greene in the symposium volume: "Stereochemistry and Reactivity of Systems

An X-ray structure determination shows that trimesitylborane assumes a propeller conformation in the crystal lattice. The low-temperature -nmr spectra of diverse triarylboranes which have substituents other than hydrogen in all six ortho positions are consistent with this geometry in solution. At higher temperatures, the spectra reflect rapid stereoisomerizations, which are interpreted in terms of two-ring flip mechanisms. These processes have activation energies of ca. [14][15][16] kcal/mol, and separation of stereoisomers at ambient temperatures is therefore precluded. he conformations and conformational isomeriza-

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