Carbon-centered radicals acetates

Hydroxy alkyl) mercury(II) acetates and NaBH4 react to form carbon-centered radicals through the reaction steps shown in Figure 1.14. When methyl acrylate is present in the reaction mixture, these radicals can add to the C=C double bond of the ester (Figure 1.15). The addition takes place via a reaction chain, which comprises three propagation steps. 

Free radical cycUzations. Cobaloxime in combination with a sacrificial Zn foil anode has been used to effect the reductive generation of carbon-centered radicals from bromoacetals in electrochemical proces.ses. When an unsaturated side chain is present in such acetals, cyclization may occur. 

The reaction of carboxylic acids with the PhI(OAc)2-iodine system may result in a decarboxylation ieading to the intermediate formation of a carbon-centered radical, which can be further oxidized to a carbocation and trapped by a nucleophile. This process has been utilized in several syntheses [97, 615,616, 617]. In a typical example, the oxidative decarboxylation of uronic acid derivatives 568 in acetonitrile under mild conditions affords acetates 569 in good yields (Scheme 3.225) [615]. A similar oxidative decarboxylation has been be used for the synthesis of 2-substituted pyrrolidines 571 from the cyclic amino acid derivatives 570 [616,617]. 

A different form of retardation occurs when a radical species formed from transfer (S in Scheme 4.3) reinitiates at a slow rate. In addition to the slower reaction rate with monomer to form a polymer radical, the termination of S with other radicals in the system may also need to be considered (Scheme 4.8). Explicit balances must be written for S, and the extra mechanisms must be included when deriving expressions for [Ptot], Rpoi, and DP . As solvent/transfer agent is generally not completely consumed, the retardation effect will last the duration of the polymerization (curve b in Figure 4.2). The degree of retardation depends on the value of which can vary with monomer type many carbon-centered radicals show much lower reactivity toward vinyl esters (for example, vinyl acetate) than (meth)acrylates [3]. 

The use of silanethiols as polarity-reversal catalysts is highly recommended in the addition of nucleophilic carbon-centered radicals to double bonds. This is especially so for the radical-chain addition of aldehydes to alkenes. In this particular reaction, tri-organosilanethiols are very efficient catalysts for the addition of butanal to both electron-rich isopropenyl acetate, or to electron-deficient ethyl crotonate. The results obtained with HSTIPS are given in eqs 23 and 24. ... 

First, one had to check that the mechanism of action was correct. The product of co-ozonlysis of O-methyl-2-adamantanone oxime with 1,4-cyclohexanedione afforded on treatment with ferrous acetate a secondary carbon-centered free radical that was trapped with the usual spin trap, 2,2,6,6-tetramethylpiperidine-Ar-oxide (TEMPO), and involved a /3-scission of the adamantane fragment, thus proving that the attack of the Fe(ll) species occurred on the less-hindered peroxide bond oxygen atom (Scheme 85) <2004NAT900, 2005JOC513>. 

In contrast to most other oxygen-centered radicaLs e.g. benzoyloxy (3.4.2.2.1), hydroxy (3.4.2.3)1, t-butoxy radicals and other r-alkoxy radicals (3.4.2.1.2) show relatively high regiospecificity in reactions with carbon-carbon double bonds (Table 3.8). Nonetheless, significant amounts of head addition are observed with the halo-olcfins, simple alkcncs, vinyl acetate and methyl acrylate. " Head addition is generally not observed with 1,1-disubstituted monomers. The exception is vinylidene niioride" where head addition predominates (Section... 

According to the classical definition. Barton esters are mixed anhydrides of carboxylic acids with thio-hydroxamic acid such as I (Scheme Ij. This class of compound was originally developed to allow the transformation of carboxylic acids to a convenient source of radicals for synthetic application. Even now, they are one of the most important entries to C-radicals. Over time, the scope of the reaction was broadened, allowing the generation of heteroatom-centered radicals, particularly oxyl-, aminyl-, and iminyl radicals of synthetic interest. For these transformations, carbonates and carbamates (II), acetates (IV), and ethers (V) were developed (Scheme 1). Finally, oxalates (III) were used for deoxygenation of secondary and tertiary alcohols. The radical fragmentation reaction of these compounds can be carried out either by irradiation or by thermal activation. Both methods are discussed here briefly. 

For efficient cationic polymerization of vinyl monomers, it is necessary that the carbon-carbon double bond be the strongest nucleophile in the molecule. Thus vinyl acetate would be classed as an electron-donor-type monomer (Section 7.10.2) but it cannot be polymerized cationically because the carbonyl group complexes the active center. (It is polymerized only by free radicals anionic initiators attack the ester linkage.)... 

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