Zum Einfluß sterischer Effekte auf die Selektivität radikalischer CC‐Verknüpfung

Abstract

Sperrige Substituenten R' und R2 am Radikal 5 verringern die Additionsgeschwindigkeit an Fumarsaure-diethylester starker als die Additionsgeschwindigkeit an Acrylsaure-methylester (s. Tab. 1). Der Vergleich mit dem von sterischen Effekten nur wenig beeinfluaten H-Einfang zeigt, dal3 5 e (R2 = t-C4%) mindestens 235mal langsamer mit Fumarsaure-diethylester reagiert als 5a (Rz = CH,) (s. Tab. 2). Deswegen steigt die Stereoselektivitat der cyclischen Radikale 1 (n = 1,2) um den Faktor 5 -7, wenn bei der CC-Verkniipfungsreaktion Fumarsaureester anstelle von Acrylsaureester eingesetzt wird. The Influence of Sterie Effeets on the Selectivity of Radical CC Bond Formation ReactionsBulky substituents R' and R2 at radical 5 decrease the rate of addition to diethyl fumarate to a larger extent than to methyl acrylate ( Table 1). The comparison with H-abstraction, which is only slightly influenced by steric effects, shows that 5 e (R2 = l-C4H,) reacts at least 235 times slower with diethyl fumarate than 5a (R2 = CH,) ( Table 2). Therefore, the stereoselectivity of cyclic radicals 1 (n = 1,Z) increases if the CC bond formation reaction is carried out with diethyl fumarate instead of methyl acrylate.Kurzlich konnten wir zeigenlj, daR die Stereoselektivitat des Alkenangriffs auf cyclische Radikale 1 dem Reaktivitats-Selektivitatsprinzip gehorcht: Mit sinkender Reaktivitat endstandiger Alkene 2 (Y = H) nimrnt der trans-Angriff zu, weil sich in spateren Ubergangszustanden die unterschiedliche Abschirmung des Radikalzentrums starker bemerkbar macht als in friiheren obergangszustanden.Fiihrt man am angegriffenen C-Atom des AIkens 2 einen Substituenten ein (Y # H), dann sollte durch zunehmende Gruppenhaufung im cis-ubergangszustand die trans-Stereoselektivittit der CC-Verkniipfungsreaktion ebenfalls erhoht werden. Um dies zu iiberpriifen, haben wir den Einflul3 sterischer Effekte auf die Selektivitat von Radikalen im Fumarsaure-diethylester/Acrylsauremethylester-Konkurrenzsystem gemessen und mit der Stereoselektivitat cyclischer Radikale verglichen.

Determination of absolute rate constants for elementary reactions in radical polymerization of diethyl fumarate

“…However, DEF as monomer is more reactive toward poly(St) radical than MA, in agreement with the result of addition of small radicals. 27 Recently, Fukuda and co-workers have emphasized the importance of the penultimate effect on the propagation rate constant of copolymerization.32 The steric factor of the DEF unit and the highly alternating tendency might result in the penultimate effect on copolymer composition. According to their study, however, copolymer composition is not sensitive to the penultimate effect, because the numerators and denominators of the rx and r2 expressions involve approximately the same amount of effect from the penultimate unit.33 Careful reexamination of the results of the copolymerization of DEF with St did not show significant improvement with the penultimate treatment, and the contribution of the penultimate unit was not estimated definitely.…”

Free radical reactions of organomercurials

“…This study has also been extended to a-and /3-functionalized radicals; in this case the steric effects play an important role in the reaction rate. 106 Especially interesting is the kinetics in the reaction of radicals generated by "the mercury method" with substituted styrenes:107,108 an important correlation between the reaction rate constant and the corresponding a values is found for the aromatic ring substituents.…”

“…The reaction indicated in Scheme 40 can be carried out without isolating the mercurial 41, in a one-pot process, the yields being lower in the in situ procedure due to the resulting byproducts and the necessary purification of the final products 42.132 The selectivity of a-acetoxyalkyl radicals has been investigated: when the groups R1 and R2 in the mercurial 41 are bulky, the reaction rate is lower with diethyl fumarate than with methyl acrylate. 106 Likewise, the relative nucleophilicity of different radicals of type 41 has been studied by using competition reactions with other primary, secondary, and tertiary radicals: the acetoxyalkyl radical turned out to be less nucleophilic than cyclohexyl radical, a logical consequence of the ability of the acetoxy group for attracting electrons. (i) d-Oxygenated Radicals.…”

Synthetic applications of radical C-C bond forming reactions

“…For example, with cyclopentyl radicals, the selective trans addition of alkenes increases with increasing bulk of the substituents at the radicals and the alkene [2,13]. Six-membered cyclic radicals are predominately attacked from the equatorial side, when the neighboring substituents are equatorial.…”