Substrate reactivity trends

Substrate reactivity trends - The faster sulfido transfer to the metal in comparison to the oxo-transfer is obvious and appears to be general. Within the oxygen atom transfer series a substrate trend from MegNO to PH3PO can be derived in aeeordance with work by the Holm Especially the difference between M-oxides (fast) and 5-oxides (slow) is clearly evident. Holm and eoworkers established the following reactivity trend ... 

Absolute rate data for Friedel-Crafts reactions are difficult to obtain. The reaction is complicated by sensitivity to moisture and heterogeneity. For this reason, most of the structure-reactivity trends have been developed using competitive methods, rather than by direct measurements. Relative rates are established by allowing the electrophile to compete for an excess of the two reagents. The product ratio establishes the relative reactivity. These studies reveal low substrate and position selectivity. 

The catalyzed hydrogenation of an aldehyde- vs. a ketone-carbonyl is invariably faster because of steric effects (23), and the data for 6 vs. 10 are in line with this (eqs. 4 and 5). Thus, conversions of 6a-c after 0.5 h at standard conditions are 86, 47, and 97%, respectively, while corresponding values for lOa-c after 4 h are 78, 36 and 49%, respectively. Indeed, the aldehydes can be reduced at 25 °C under otherwise identical conditions (6b gives 38% conversion after 4 h, and 6c gives 99% after 15 h). The above reactivity trend for the ketones lOa-c shows that the hydrogenation rates depend on the substituent para to the carbonyl functionality and increase in the order H > OMe > OH. For the aldehyde susbtrates, the more limited data (substrate 6 with R = H and R = OMe was not available) suggest a similar para-substitucnt effect (at least OMe > OH). Note that this is the reverse trend to that observed for reduction of the activated C=C systems described above. 

Evaluation of the only appropriate Fukui function is required for investigating an intramolecular reaction, as local softness is merely scaling of Fukui function (as shown in Equation 12.7), and does not alter the intramolecular reactivity trend. For this type, one needs to evaluate the proper Fukui functions (/+ or / ) for the different potential sites of the substrate. For example, the Fukui function values for the C and O atoms of H2CO, shown above, predicts that O atom should be the preferred site for an electrophilic attack, whereas C atom will be open to a nucleophilic attack. Atomic Fukui function for electrophilic attack (fc ) for the ring carbon atoms has been used to study the directing ability of substituents in electrophilic substitution reaction of monosubstituted benzene [23]. In some cases, it was shown that relative electrophilicity (f+/f ) or nucleophilicity (/ /f+) indices provide better intramolecular reactivity trend [23]. For example, basicity of substituted anilines could be explained successfully using relative nucleophilicity index ( / /f 1) [23]. Note however that these parameters are not able to differentiate the preferred site of protonation in benzene derivatives, determined from the absolute proton affinities [24],... 

The indole oxidation has been shown to proceed via the hydroperoxide intermediate 9 (126), but whether this is formed via coordination catalysis, for example, as suggested in Reaction 41 for a phenol substrate (10— 12,13,14) (124), or via Haber-Weiss initiation, poses the same problem encountered in the organometallic type systems. A reactivity trend observed for Reaction 40 using tetraphenyl-porphyrin complexes (Co(II) Cu(II) Ni(II)) is reasonable in that the Co(II) system is known to give 1 1 02-adducts (at least, at low temperatures) but the reactivity trend also was observed for the catalyzed decomposition rate of 9. It is interesting to note that in Reac-... 

A membrane-induced structure-reactivity trend that may be exploited to achieve selective processes has been recently observed in polymeric catalytic membranes prepared embedding polyoxotungstates, W(VI)-oxygen anionic clusters having interesting properties as photocatalysts, in polymeric membranes [17]. These catalytic membranes have been successfully apphed in the photooxidation of organic substrates in water providing stable and recyclable photocatalytic systems. 

The stability constants decrease in the order Ki>K2>K3 and K >K4>Ks for oxidation of saturated [5] and unsaturated [5,28] hydrocarbons respectively and their thermodynamic parameters [5,28] are found to be nicely in line with their stability. The order of the reactivity [5,26,28] of the substrates was found to be cis-cyclooctene > cyclohexene > styrene > trans-stilbene > ldamantane > cyclohexane. The reactivity trend was supported by activation parameters which were higher for saturated hydrocarbons [5] than that of unsaturated hydrocarbons [5,28]. 

The efficiency of secondary amine catalysts is often eroded when moving from aldehydes to ketones as the donor carbonyl substrates, a trend that can be explained in terms of either the greater difficulty in the generation of the intermediate enamine species or their attenuated reactivity. To alleviate this situation, primary amines have emerged as a complementary family of amine catalysts. For instance, proline and related chiral secondary amines are not useful catalysts for the a-amination of aromatic enolizable ketones. As in other similar situations involving ketones as substrates, primary amines proved to be superior catalysts, although in these cases the presence of an acid co-catalyst seems to be crucial for reactivity. For instance (Scheme 11.4), primary amines derived from cinchona alkaloids can efficiently... 

Studies of oxidation reactions of differing substrate types have been described. The nature of oxidant species in HCIO4 and H2SO4 media has been examined by investigation of catalysis of the Ce -Hg reaction. It is suggested that at 2.OM-HCIO4 the iridium(iv) is hydrolysed with protonation constant of 0.4 for [Ir(H20)s0H] +. Most kinetic studies, however, refer to reaction of the hexachloro and hexabromo ions. The oxidation of thiourea (tu), iV,iV -dimethyl-thiourea (dmtu), and 2-imidazolidinethione (it) follows a rate law second order in [substrate] and first in [Ir "]. The rate of oxidation follows the reactivity trend established previously for aquo-metal ions. The mechanism proposed involves rapid pre-equilibria followed by disulphide radical formation,... 

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