Mechanistic transformation methods

When this type of transform is applied mechanistically to 85, retron generation is simple, for example by the change 85 => 86, and the sequence 86 => 90 disconnects two rings and provides an interesting synthetic pathway. Radical intermediate 88, which is disconnected at p-CC bond a to produce 89, may alternatively be disconnected at the P-CC bond b which leads to a different, but no less interesting, pathway via 91 to the acyclic precursor 92. The analysis in Chart 11 is intended to illustrate the mechanistic transform method and its utility it is not meant to be exhaustive or complete. 

Yagci Y, Tasdelen MA (2006) Mechanistic transformations involving living and con-trolled/living polymerization methods. Prog Polym Sci 31 1133-1170... 

Yagci, Y. Tasdelen, M.A. Mechanistic transformations involving living and controlled/living polymerization methods. Prog. Polym. Sci. 2006,31 (12), 1133-1170. 

The first and second synthetic methods are performed under the same polymerization mechanism whereas the third and fourth methods can involve mechanistic transformations. 

Figure 8 Main mechanistic transformation reactions in living and/or controlled polymerization methods. ATRP, atom transfer radical polymerization RAFT, reversible addition-fragmentation chain transfer NMRP, nitroxide-mediated free radical polymerization CROP, cationic ring-opening polymerization AROP, anionic ring-opening poiymerization.

As stated previously, the iniferter method is not suitable for aU monomers. Therefore, the application of a single mode polymerization technique for the synthesis of block copolymers is severely limited, and suffers from the requirement for care in the order of monomer addition. In order to extend the range of monomers for the synthesis of block copolymers, a mechanistic transformation approach was proposed, by which the polymerization... 

Another method for conducting cyclizations catalytic in Cp2TiCl is shown in Scheme 14. It relies on the thermodynamically favorable ring closure of THF from 5-titanoxy radicals [81,82]. This step is mechanistically related to the oxygen rebound steps of oxidation reactions. While the scope of this transformation remains to be established, the presence of substituted THF-derivatives in many natural products renders the method potentially attractive. 

The ruthenium-, rhodium-, and palladium-catalyzed C-C bond formations involving C-H activation have been reviewed from the reaction types and mechanistic point of view.135-138 The activation of aromatic carbonyl compounds by transition metal catalyst undergoes ortho-alkylation through the carbometallation of unsaturated partner. This method offers an elegant way to activate C-H bond as a nucleophilic partner. The rhodium catalyst 112 has been used for the alkylation of benzophenone by vinyltrimethylsilane, affording the monoalkylated product 110 in 88% yield (Scheme 34). The formation of the dialkylated product is also observed in some cases. The ruthenium catalyst 113 has shown efficiency for such alkylation reactions, and n-methylacetophenone is transformed to the ortho-disubstituted acetophenone 111 in 97% yield without over-alkylation at the methyl substituent. 

Subsequently, List reported that although the method described above was not applicable to the reduction of a,P-unsaturated ketones, use of a chiral amine in conjunction with a chiral anion provided an efficient and effective procedure for the reduction of these challenging substrates [210]. Transfer hydrogenation of a series of cyclic and acyclic a,P-unsaturated ketones with Hantzsch ester 119 could be achieved in the presence of 5 mol% of valine tert-butyl ester phosphonate salt 155 with outstanding levels of enantiomeric control (Scheme 64). A simple mechanistic model explains the sense of asymmetric induction within these transformations aUowing for reliable prediction of the reaction outcome. It should also be noted that matched chirality in the anion and amine is necessary to achieve high levels of asymmetric induction. 

As an alternative to the Tauber method, 2,2 -diaminobiphenyl (509) was transformed to carbazole (1) using diazotization conditions. This reaction proceeds intra-molecularly by displacement of nitrogen. With this procedure, only moderate yields of carbazoles were obtained. The reaction is mechanistically similar to the Graebe-Ullmann synthesis (494) (Scheme 5.6). 

Three methods for the preparation of nitroalkenes are outlined as shown. Describe in mechanistic terms how each of these transformations might occur. 

The time course of the emission intensity is proportional to the concentration of the HEI and, in the case of isolated HEIs, the rate constant for its decomposition is obtained from the CL emission curves. Isothermal kinetic measurements give rise to the activation parameters for the whole transformation and can be used for mechanistic discussion in the case of isolated HEI ° . Specific CL methods have also been used to determine... 

A number of very important synthetic methods are based on ring interconversions of six-membered heterocyclic systems. As with the transformations of five- into six-membered rings, examples are known of all of the possible atom fragments, but [5 + 1] and [4+2] processes are by far the most important. As described in Chapter 2.02, a number of [6+0] photoisomerizations of diazines have been discovered, but while these are of considerable mechanistic and theoretical importance they are of little preparative significance. 

In an effort to obtain a salvinorin derivative possessing an oe-diol system which can be transformed into the dibenzoate ester required for the exciton chirality CD method, salvinorin A (1) or B (2) was treated with sodium borohydride in various protic solvents. The products having the la,2a-diol group were obtained in high yield. However, this reduction was accompanied by extensive isomerization at C-8. While mechanistic details for this unexpected observation remain to be established at this time, the isomerization at C-8 appears to be the result of the base-promoted clevage of the C-8/9 bond under the reaction conditions followed by the reclosure to provide the 8-epimer prior to the reduction of the 1-ketone. Furthermore, attempts to obtain the 1,2-dibenzoate derivative of the major reduction product 3 under various benzoylating conditions invariably produced only the 2-monobenzoate. 

For a detailed analysis of monomer reactivity and of the sequence-distribution of mers in the copolymer, it is necessary to make some mechanistic assumptions. The usual assumptions are those of binary, copolymerization theory their limitations were discussed in Section III,2. There are a number of mathematical transformations of the equation used to calculate the reactivity ratios and r2 from the experimental results. One of the earliest and most widely used transformations, due to Fineman and Ross,114 converts equation (I) into a linear relationship between rx and r2. Kelen and Tudos115 have since developed a method in which the Fineman-Ross equation is used with redefined variables. By means of this new equation, data from a number of cationic, vinyl polymerizations have been evaluated, and the questionable nature of the data has been demonstrated in a number of them.116 (A critique of the significance of this analysis has appeared.117) Both of these methods depend on the use of the derivative form of,the copolymer-composition equation and are, therefore, appropriate only for low-conversion copolymerizations. The integrated... 

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