Die Addition von Benzylradikalen an Alkene: Zur Rolle von Radikaldeformationen im Übergangszustand

Abstract

Nichtplanare Radikalzentren im Übergangszustand sind die Ursache dafür, daß Benzyl und para‐substituierte Benzylradikale, wie ESR‐spektroskopisch gezeigt wurde, nur sehr langsam an mono‐ und 1,l‐disubstituierte Alkene addieren; dies folgt unter anderem daraus, daß die para‐Substituenten keinen signifikanten Einfluß auf die Geschwindigkeit der Addition haben. Diese Befunde sind in Einklang mit für die Addition einfacher Alkylradikale an Alkene berechneten Strukturen.

Factors Controlling the Addition of Carbon‐Centered Radicals to Alkenes—An Experimental and Theoretical Perspective

“…Intuitively,15, 20 this seems easier to reach when it is already present in the radical, and therefore pyramidal radicals such as trifluoromethyl should have an advantage. On the other hand, radicals with extended delocalization such as the benzyl radical and cyano‐ or carboxy‐substituted methyl radicals are expected to resist pyramidalization, which by this argument may lead to higher activation energies 21…”

“…Therefore, we first discuss experimental data for this simplest situation and select radicals (Figure 4) which allow a case study for the action of enthalpy and polar effects alone. Besides the methyl radical,58 we include the resonance‐stabilized benzyl radical,21, 59 and three other primary radicals, two with an electron‐acceptor substituent (cyanomethyl60 and tert ‐butoxycarbonylmethyl60) and one with an electron‐donor substituent (hydroxymethyl61)), four tertiary species, two with only donor substituents ( tert ‐butyl35, 36a, 62 and 2 ‐ hydroxy‐2‐propyl36b, 63) and two with two donor and one acceptor substituent (2‐ tert ‐butoxycarbonyl‐2‐propyl64 and 2‐cyano‐2‐propyl65). There are also two radicals with stronger electron‐acceptor substitution, namely the cyclic malonyl radical, 2,2 ‐ dimethyl‐4,6‐dioxo‐1,3‐dioxan‐5‐yl, derived from Meldrum's acid,66 and the trifluoroacetonyl radical 64…”

“…Moreover, the common regression of benzyl and methyl means that the lower reactivity of benzyl is because of the lower exothermicity which is caused by resonance stabilization. Hence, a previous explanation of the low benzyl reactivity21 as being the result of a particularly large resistance to pyramidalization is now abandoned.…”

“…These are also suggested by the extremely facile addition to the very electron‐deficient superalkene C 60 ( E ea =2.65 eV, k 298 =1.4×10 7 M −1 s −1 , E a =13.5 kJ mol −1 ) 97. Overall, however, the nucleophilicity of benzyl is also rather modest, and this explains why para substitution of the benzyl group by electron‐donor or ‐acceptor groups does not change the addition behavior of benzyl markedly, even though these substitutions strongly affect the E i of the radical 21. Hence, one would conclude from Figure 6 that for the additions of methyl and benzyl radicals the reaction enthalpy is the dominating factor, and the slope of the linear regression is also close to the generally accepted α ≈0.25.…”

“…Unfortunately, there are not many absolute rate constants against which such predictions can be checked. However, amongst some other scattered single values, the absolute rate constants are available for the addition of trifluoromethyl and perfluoro‐ n ‐alkyl radicals,20, 67 the n ‐hexenyl and n ‐heptyl radicals,101 the cumyl radical,59 and various p ‐substituted benzyl radicals21 to several alkenes at room temperature. These serve as test cases here.…”