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

Abstract

Additions of the substituted diphenylmethyl radicals ArlAr'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 dimers ? 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 2q -s (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.Over the past decade, free-radical carbon-carbon couplings have become a powerful method for impressive chemo-, regio-, and stereoselectiveIn this context the additions of carbon-centered radicals to unsaturated systems represent one of the most useful applications of freeradical chemistry and are of great importance for polymer synthesis.Experimental resultsp-"] and theoretical calculations['2131 indicate that the addition of small alkyl radicals to alkenes is usually exothermic and irreveresible, with early, unsymmetrical, reactant-like transition states, dominated by SOMO-LUMO (nucleophilic radicals) or SOMO-HOMO (electrophilic radicals) interactions ["]. However, "the complex interplay of polar, steric and bond-strengh terms"['41 seems to govern product f~rmation[~-~'~.Additions of bulky, resonance-stabilized radicals to alkenes are rare in the literature. The trityl radical appears to add only to conjugated systems, like isoprene, whereas it remains inert toward isolated double bond^"^,'^]. We have recently reported our first results on the addition of sterically hindered diphenylmethyl radicals 2a -e to the captodative (c,d)-substituted olefin 3c. A novel, in the light of the c,dconcept an unexpected['71 1P-addition takes place providing the ketenimine adduct 6a['*] (Scheme 1). These radicals retain their nucleophilic character as may be judged from their SOMO energy (IP for Me': 9.84 eV ['9a1, PhCH2: 7.20['9b1, nh2CH: 6.80[19c1, and Ph3C': 7.26['9d1), and the bonds formed when ...

Aromatic Allylation via Diazotization: Metal-Free C−C Bond Formation

“…Similar to the behavior of other radical addition reactions, steric hindrance has an adverse effect on the outcome of the allylation reaction. [33][34][35] When 1o was subjected to the standard conditions no evolution of nitrogen gas could be detected. Even when the temperature was increased to >60 °C, the corresponding allylated product could not be isolated.…”

Thermolysis of Acylazo O-Trimethylsilyl Cyanohydrins: Azoalkanes Yielding Captodatively Substituted Radicals

“…Since the 2-cyanopropyl radicals from AIBN and other cyano substituted radicals − produced a sizable amount of ketenimine, a sample of 3 was thermolyzed in toluene, the solvent was evaporated, and the residue was dissolved in CCl 4 . The IR spectrum of this solution showed only a small band at 2011 cm -1 , ,, indicating either that the ketenimine hardly formed or that it decomposed.…”