Radicals carbonyl-substituted

A number of correlations of ionization potentials for substituted benzenes (40-42), benzyl (43), phenoxy (44), and alkyl (45) radicals and substituted pyridines (46) with the simple Hammett equation have been reported. Charton (47) has studied the application of the extended Hammett equation to substituted ethylenes and carbonyl compounds. The sets studied here are reported in Table II (sets 2-10 and 2-11). Results of the correlations are set forth in Table 111. The results obtained are much improved by the exclusion of the values for X = C2 H3, Ac, F, H and OAc from set 2-10 (set 2-lOA) and the value for X = H from set 2-11 (set 2-11 A). The composition of the electrical effect corresponds to that found for the Op constants as is shown by the pR values reported in Table IV. 

The combined influences of polar and steric effects and of the strength of the newly formed bond93 was also recognized in the reaction of a,0-unsaturated carbonyl compounds and similar electron deficient alkenes95 with organomercurials and NaBH4. For the addition of alkyl radicals to substituted styrenes, p assumed a... 

Because the addition steps are generally fast and consequently exothermic chain steps, their transition states should occur early on the reaction coordinate and therefore resemble the starting alkene. This was recently confirmed by ab initio calculations for the attack at ethylene by methyl radicals and fluorene atoms. The relative stability of the adduct radicals therefore should have little influence on reacti-vity 2 ). The analysis of reactivity and regioselectivity for radical addition reactions, however, is even more complex, because polar effects seem to have an important influence. It has been known for some time that electronegative radicals X-prefer to react with ordinary alkenes while nucleophilic alkyl or acyl radicals rather attack electron deficient olefins e.g., cyano or carbonyl substituted olefins The best known example for this behavior is copolymerization This view was supported by different MO-calculation procedures and in particular by the successful FMO-treatment of the regioselectivity and relative reactivity of additions of radicals to a series of alkenes An excellent review of most of the more recent experimental data and their interpretation was published recently by Tedder and... 

Radical carbonylation reaction serves as a powerful tool for the synthesis of a range of carbonyl compounds. Radical carbonylation has been successfully applied to the synthesis of functionalized ketones from alkyl, aryl, and alkenyl halides.The radical aminocarbonylation reaction of alkynes and azaenynes provided efficient routes to 2-substituted acrylamides, lactams, and pyrrolidinones. For example, the aminocarbonylation of 4-pentyn-l-yl acetate 318 initiated by tributyltin hydride (Bu"3SnH) (30mol%) with AIBN (20mol%) gave acrylamide 325 in 92% yield (Scheme 43).A proposed mechanism starts from the addition of tributyltin radical 319 to alkyne... 

Because of the centrality of the carbonyl group in synthesis, carbonyl-substituted radicals are especially useful. The above results indicate that, if planned addition or cyclization reaction of a carbonyl-substituted radical fails due to lack of reactivity of the acceptor, one should consider activation of the alkene not only with electron donors but also with electron acceptors. 

It is well known that kH is similar for all alkyl-substituted radicals but rate constants for reaction of tin hydride with carbonyl-substituted radicals are not known. Substituents can effect the rate constant for hydrogen transfer. For example, the benzyl radical is about 50 times less reactive than a primary alkyl radical. 

Because of the centrality of the carbonyl group in synthesis, the cyclizations of carbonyl-substituted radicals are of special importance. At present, there are no accurately known rate constants for the cyclizations of any carbonyl-substituted radicals, and quantitative statements are risky. Because carbonyl-substituted radicals add more rapidly to simple alkenes than alkyl radicals add, it might be suspected that they cyclize more rapidly as well. However, qualitative evidence does not support this analogy. 

Cyclization of aryl radicals on to either 2- or 3-carbonyl substituted pyrroles occurs preferentially at the 2-position. In the case of the former cyclizations, the yields are poor but this could represent a rapid approach to a range of indole alkaloids. Thus, cyclization of 2-substituted pyrroles 929 by treatment with Bu3SnH (0.02 M) and AIBN (catalytic amounts) as initiator in refluxing toluene for 12 h gave the -exo cyclization products 931 and 932 as the major products judging by H NMR of the crude product (Scheme 181) <1995TL6743>. 

Generation of Carbonyl-Substituted and Related Alkyl Radicals. . 89 Generation of Alkoxy-and Amino-Substituted Alkyl Radicals. .. 90 Acyl and Related Radicals. 92... 

Generation of Carbonyl-Substituted and Related Alkyl Radicals... 

One of the conclusions of this study is that the first ligand substitution proceeds with a much higher coulombic efficiency than the second one, simply because of the increasing bulk at the metal center [317], This trend was known from previous studies of carbonyl substitution by phosphanes in transition-metal carbonyl clusters catalyzed by benzophenone radical anion [318, 319]. In such a case, selective ETC catalysts may be designed as a function of the required exergonicity of the desired initiation step [317]. 

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