Chemoselectivity sensitizers effect

In addition to the steric issues in chemoselectivity, which are intrinsic to the reaction of a given set of substrates, solvent and sensitizer effects are of importance for a direct control on selectivity because they can be controlled. The reaction of 8a and 13c catalyzed by sensitizer 6 shows an overall increase in cross-product formation with increasing solvent polarity. Entries 4 and 5 in Table 4.1 demonstrate that use of 3a reduces the formation of cross-products in polar solvents. This difference can be rationalized by the fact that after ET by 6, a strongly complexed ion pair is formed, whereas in the case of 3a, the sensitizer is neutral and can dissociate more easily from the radical cation formed. 

More recent work has focused on understanding the mechanism or mechanisms of selectivity. Some of these studies have been performed on well-characterized catalysts about which particle size information is available. Still others have been performed on single crystals. So conclusions may be reached about the effects on chemoselectivity of planes, edges, and corners that are related to particle size (structure sensitivity). A number of these studies, mostly on Pt, are summarized in Table 2.6. Since these studies have usually been performed in the vapor phase, information about solvent effects and their possible influence on chemoselectivity is unavailable. 

Amidine derivatives are effective dehalogenation inhibitors for the chemoselective hydrogenation of aromatic halonitro compounds with Raney nickel catalysts. The best modifiers are unsubstituted or N-alkyl substituted formamidine acetates and dicyandiamide which are able to prevent dehalogenation even of very sensitive substrates. Our results indicate that the dehalogenation occurs after the nitro group has been completely reduced i.e. as a consecutive reaction from the halogenated aniline. A possible explanation for these observations is the competitive adsorption between haloaniline, nitro compound, reaction intermediates and/or modifier. The measurement of the catalyst potential can be used to determine the endpoint of the desired nitro reduction very accurately. 

A surprising steric sensitivity is frequently observed for these radical cation reactions. As was shown previously during the discussion of chemoselectivity, the variable positioning of bulky substituents has effects on periselectivity as well. DFT calculations on the influence of diene substitutions for the neutral reaction have demonstrated a behavior similar to that of the radical cation reaction. Although... 

In an application of (Z)-selective alkene formation to enolizable aldehydes, it was noted that the combination of LiCl and DBU was effective for deprotonation by lithium complexation of the Still phosphonate. In this example, the cyclopropyl aldehyde (176) reacted chemoselectively in the presence of the ketone (equation 43). In addition, the ( )-alkene could be synthesized by lithium coordination with a standard HWE methyl phosphonate. As this example illustrates, the trifluoroethyl phosphonate can fill an important void by providing trisubstituted alkenes with sensitive substrates in go< selectivity. From the examples of Marshall and Oppolzer it appears that the application of the reaction to higher order trisubstituted alkenes is selective for the (Z)-isomer. The magnitude of the selectivity is substrate specific and dependent on the rapid rate of eo firo-a-oxyphosphonate decomposition. 

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