Reaction of electrophiles with unsaturated systems: triazolinedione-olefin reactions

Abstract

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

Asymmetric polymerization via cycloaddition reactions

“…[32][33][34][35] In order to explain the above result, we propose the following mechanism for the Diels-Alder reaction of chiral monomer 3 with PhTD.…”

Solvent‐Dependent Changes in the Triazolinedione–Alkene Ene Reaction Mechanism

“…In transition state TS 3 , hyperconjugative effects involving the six hydrogen atoms of the two methyl groups, as opposed to the six deuterium atoms in TS 4 , are expected to give a normal and large b-secondary KIE (k H /k D % 1.05-1.10 per deuterium), as is found experimentally. Similar isotope effects have been reported in the dipolar cycloaddition of tetracyanoethylene to 2,4-hexadiene.…”

“…Unlike the results in non-protic solvents, the large, non-stereochemically dependent kinetic isotope effects measured in the RTAD-alkene ene reactions of TMEs and [ 4 Si by using the residual solvent peak as an internal standard. Isomeric purities were determined by 1 H NMR and by GC on a 50 m HP-5 capillary column connected to a 5971 A MS detector.…”

“…(1)] apart from being synthetically useful, [2] has attracted considerable mechanistic [3][4][5][6][7][8][9] and theoretical attention [10] for many years. A number of mechanisms and key intermediates ( Figure 1) have been proposed for this reaction.…”

“…Among these, the formation of a closed, three-membered aziridinium imide (AI) intermediate ( Figure 1) was the most popular, and initially found support in the results of trapping experiments. [11,12] Subsequently, isotope effect measurements on deuterium-labeled tetramethylethylenes (TMEs) [3,4] and 2-butenes [5,6] also suggested the formation of an AI intermediate in the rate-determining step of the reaction. Thus, large intramolecular kinetic isotope effects (KIEs) were found in the ene reactions of RTADs with the cis-related methyl and deuteriomethyl groups in substrates 1 and 2 (H/D isotopic competition; Scheme 1), whereas only a small isotope effect was observed with the trans-related groups in compound 3 (no isotopic competition).…”