Radicals continued rearrangements

When dicobalt octacarbonyl, [Co(CO)4]2, is the catalyst, the species that actually adds to the double bond is tricarbonylhydrocobalt, HCo(CO)3. Carbonylation, RCo(CO)3- -CO—>RCo(CO)4, takes place, followed by a rearrangement and a reduction of the C—Co bond, similar to steps 4 and 5 of the nickel carbonyl mechanism shown in 15-30. The reducing agent in the reduction step is tetra-carbonylhydrocobalt HCo(CO)4, ° or, under some conditions, H2. When HCo(CO)4 was the agent used to hydroformylate styrene, the observation of CIDNP indicated that the mechanism is different, and involves free radicals. Alcohols can be obtained by allowing the reduction to continue after all the carbon monoxide is... 

Continuing his studies on the metallation of tetrahydro-2-benzazepine formamidines, Meyers has now shown that the previously unsuccessful deprotonation of 1-alkyl derivatives can be achieved with sec-butyllithium at -40 °C <96H(42)475>. In this way 1,1-dialkylated derivatives are now accessible. The preparation of 3//-benzazepines by chemical oxidation of 2,5- and 2,3-dihydro-l/f-l-benzazepines has been reported <96T4423>. 3Af-Diazepines are also formed by rearrangement of the 5//-tautomers which had been previously reported to be the products of electrochemical oxidation of 2,5-dihydro-lAf-l-benzazepine <95T9611>. The synthesis and radical trapping activities of a number of benzazepine derived nitrones have been reported <96T6519, 96JBC3097>. 

Both intermolecular and intramolecular additions of carbon radicals to alkenes and alkynes continue to be a widely investigated method for carbon-carbon bond formation and has been the subject of a number of review articles. In particular, the inter- and intra-molecular additions of vinyl, heteroatomic and metal-centred radicals to alkynes have been reported and also the factors which influence the addition reactions of carbon radicals to unsaturated carbon-carbon bonds. The stereochemical outcome of such additions continues to attract interest. The generation and use of alkoxy radicals in both asymmetric cyclizations and skeletal rearrangements has been reviewed and the use of fi ee radical reactions in the stereoselective synthesis of a-amino acid derivatives has appeared in two reports." The stereochemical features and synthetic potential of the [1,2]-Wittig rearrangement has also been reviewed. In addition, a review of some recent applications of free radical chain reactions in organic and polymer synthesis has appeared. The effect of solvent upon the reactions of neutral fi ee radicals has also recently been reviewed. ... 

In the context of the potential Cope rearrangement of hexa-1,5-diene radical cations (Section 2.4.1), we mentioned the triplet recombination of radical ion pairs generating a biradical [202, 203]. Because of continuing interest in this type of reaction we briefly mention two additional examples involving radical cationic systems discussed in this review, viz., the isomeric 1,2-diphenylcyclopropane radical cations, cis- and trans- 3 , and norbornadiene radical cation, 91 +. 

The housanes 80a,b are also oxidized by PET, but are more reluctant to rearrange, results illustrating the intriguing chemical fate of radical cations derived from dicyclopentadienes and their isomers. In continuation of their investigation, Adam et al. [239] studied oxidation of 2,3-diazabicyclo[2.2.1]hept-2-enes, 82a-d,... 

The rearranged radical abstracts a hydrogen from aldehyde to continue the chain. 

Many reactions involve a cyclic transition state. Of these, some involve radical or ionic intermediates and proceed by stepwise mechanisms. Pericyclic reactions are concerted, and in the transition state the redistribution of electrons occurs in a single continuous process. In this chapter, we will consider several different types of pericyclic reactions, including electrocyclic transformations, cycloadditions, sigmatropic rearrangements, and the ene reaction. 

Termination is one of those nebulous handwaving terms used to imply that a process is coming to a close. In hpid oxidation, termination is an even fuzzier concept in that, from a practical standpoint, the lipid oxidation chains probably never fully stop. In addition, a specific radical may be terminated and form some product, but if this occurs by H abstraction or rearrangement, another radical is left behind so the chain reaction continues. Net oxidation slows down when H abstractions or other radical quenching processes exceed the rate of new chain production, but it would be difficult indeed to totally stop the entire radical chain reaction. Thus, in the discussion below, termination refers to an individual radical, not the overall reaction. 

As with the 2-methyleneglutarate mutase system, the detailed computational investigation of the methylmalonyl-CoA mutase system is somewhat complex. We therefore continue to use the model system approach, and replace the SCoA and carboxylate groups by hydrogen atoms. This simplification results in the degenerate rearrangement of the 3-propanal radical (8) [69, 76] ... 

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