R-group Decomposition

The concept of activity cliffs and the landscape paradigm have also been applied to R-groups, where an R-cliff occurs when a pair of compounds differs in a single R-group. This is clearly a specialization of the activity cliff concept, placing this type of analysis in the context of analogue series derived via R-group decompositions (43, 44). 

The bulk of Mobius use involves the building and execution of queries and the analysis of the retrieved results for decision support, hi cases where further analysis of data if called for, Mobius can export retrieved data to several standard formats and applications (e.g., Excel, Spotfire). Mobius itself includes a menu of specialized analysis tools (e.g., R-Group decomposition, SAR landscape analysis [10], and an API for defining additional tools). 

Acid-catalyzed, ionic decompositions have been reported for peroxyesters, RC(0)—OOR, in which the R group can form a particularly stable carbonium ion, eg, tropybum ion (213). 

Since the quantum chemical calculations used to parameterize equations 6 and 7 are relatively crude semiempirical methods, these equations should not be used to prove or disprove differences in mechanisms of decomposition within a family of initiators. The assumption made in the present study has been that the mechanism of decomposition of initiators does not change within a particular family of initiators (reactions 1-4). It is generally accepted that trow5-symmetric bisalkyl diazenes (1) decompose entirely by a concerted, synchronous mechanism and that trans-phenyl, alkyl diazenes (2) decompose by a stepwise mechanism, with an intermediate phenyldiazenyl radical (37). For R groups with equal or larger pi-... 

Electron spin resonance (ESR) spectroscopy can be advantageously used to measure the radical concentrations of the nitroxide radicals (XV and XVI) produced, since these are much more stable then the R- radicals. Of greater importance, ESR can be used to determine the structure of R% since the ESR of the nitroxide radical is quite sensitive to the structure of R. (For this purpose, nitroso spin traps are more useful, since the R group in the nitroxide radical is nearer to the lone electron.) This can allow a determination of the structures of radicals first formed in initiator decomposition, the radicals that actually initiate polymerization (if they are not identical with the former) as well as the propagating radicals [Rizzardo and Solomon, 1979 Sato et al 1975],... 

Furthermore, the heterolysis reaction is catalyzed by the addition of anions (organic and inorganic oxy anions, e.g., acetate) (44,107, 110,111). Comparing the acetate effect in the presence of different chelating ligands ([15]aneN4 and nta) to those of the aquated system led to the conclusion that the oxy anions have to occupy the trans position to the R group in order to labilize the M-R bond (trans labilization effect) and thus catalyze the heterolytic decomposition (44,107,110,111). 

The physical properties of sulfonic acids vary greatly depending on the nature of the R-group. Sulfonic acids can be described as having similar acidity characteristics to sulfuric acid. Sulfonic acids are prone to thermal decomposition, i.e., desulfbnation, at elevated temperatures. However, several of the alkane-derived sulfonic acids show excellent thermal stability, as shown in Table 1. Arene-based sulfonic acids are thermally unstable. 

Equation 9.29) and later the second (Equation 9.30). One test is to measure the effect on decomposition rate of changing one of the R groups. If R—N=N—R and R—N=N—R, where R - is a more highly stabilized radical than R, are compared, a lower rate should be found for the unsymmetrical compound in a concerted decomposition in a stepwise one the better radical R- should come off first in either compound, and the rate should be unaffected. The results support the concerted process (Equation 9.28) for the symmetrical azo compounds with R- a well-stabilized radical such as < 2CCN, 2CH, and so forth, but leave... 

Crawford and Tagaki have examined gas-phase decompositions of several azo compounds with R = CH3, /-butyl, or allyl they found that most of these compounds, both symmetric and unsymmetric, decompose by the noncon-certed path (Equation 9.29).87 Seltzer found by studying secondary deuterium isotope effects that in unsymmetrical azo compounds in which one R group is a much better radical than the other, the bond breaking is stepwise.68 In a compound such as 20, substitution of the hydrogen on the a-phenylethyl side gives... 

The alkoxy radicals formed in pathway Eq. 26a have very interesting atmospheric chemistry. The atmospheric fate of alkoxy radicals differs with the nature of the R group. Some alkoxy radicals (e.g., CH3O) are lost solely via reaction with 02, others undergo rapid decomposition via C-C bond scission. Long chain alkoxy radicals can undergo isomerization via intramolecular H-atom abstraction ... 

Radnia and McKennis (53a) present evidence that the correct structures for the complexes claimed to be 22 are actually the disilanylvinyl tetracarbonyl adducts, (R3SiSiRiCH=CH2)Fe(CO)4, bonded in the normal rf-vinyl manner. The NMR nonequivalence of the R groups arises from their diastereotopic nature. Complex 22 may still be an intermediate in the decomposition reaction (84). 

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