Sterically Hindered Free Radicals, 21. 1,2- and 1,4-Additions of Diphenylmethyl Radicals to Substituted Acrylonitriles

✍ Scribed by Zarkadis, Antonios K. ;Neumann, Wilhelm P. ;Dünnebacke, Dieter ;Peñeñory, Alicia ;Stapel, Ralf ;Stewen, Ulrich

• Publisher: Wiley (John Wiley & Sons)
• Year: 1993
• Tongue: English
• Weight: 830 KB
• Volume: 126
• Category: Article
• ISSN: 0009-2940
• DOI: 10.1002/cber.19931260518

✦ Synopsis

Radical additions 1 Acrylonitriles 1 Diphenylmethyl radicals / Steric effects Additions of the substituted diphenylmethyl radicals Arl-Ar'CR 2 (R = CMe3, SiMe3, GeMe3, SnMe3, OSiMe3, CF3, CO'Me, CN) to various acrylonitriles CHZ=C(X)CN 3 (X = SMe, SiPr, StBu, OAc, OSiMe,, OSiEt3, OMe, OEt) lead to 1,2-5 or 1,4-adducts 6 (ketenimines), depending mainly on the steric hindrance by the substituents R and X. Bulky substituents like tBu in 2 and tBuS in the acrylonitrile favour the formation of the extended and nearly strainless ketenimine system 6 (1,Cadduct); smaller substituents like OSiMe, (radical 2) and SMe, OAc, OSiMe3, OSiEt3, OMe, OEt (acrylonitrile) allow isolation of the sterically crowded 1,2-adducts. Substituents of intermediate bulkiness like CF3 (radical 2) and SiPr (acrylonitrile) give a mixture of 1,2-adducts (6cb, hb) and di-mers ? of the adduct radicals 4 (?cb, hb). The voluminous tBu group directly bound to the olefin (3j, k) prevents addition.

The latter is generally reversible, and the various adducts 5, 6, or ? dissociate to the adduct radicals 4 and/or fragment to the initial radicals 2 at temperatures which reflect the steric strains of the corresponding substituents R, X. The complete inertness, even toward the electron-rich olefin 3i, of the electrophilic a-carbonyl-substituted radicals 2qs (R = CHO, COMe, COPh) in the above additions is discussed. Additions of the radicals 2a -c, f, i, n, o to the conjugated olefin 3n are described and are in accordance with the conclusion that steric effects predominate in adduct formation, whereas electronic effects are of distinct but minor importance.