Direct branching reactions

Quantum efficiencies for the conversion of BR to M have been reported to be about 0.25 at room temperature [48,278], and not much higher [48] at -40°C. The apparent yield depends, of course, on the photostationary state between BR and K, as well as on any photoconversions involving the other intermediates of the photocycle. Such effects will be minimized with flashes of low intensity and sufficiently short duration, but thermal decay of the intermediates leading directly to BR, and other branching reactions are difficult to assess. 

Reactions (68) and (69) are chain branching and so the system is potentially explosive. However, the pressure dependent reaction, (70), is in direct competition with the H + O2 chain branching reaction leading to a complex pressure and temperature dependence of the explosion limits as shown in Fig. 2.40. 

The excellent diastereoselection of all three of these substrate-directed allylation reactions is consistent with reaction occurring through Felkin transition states analogous to 39 (Fig. 11-6). These examples illustrate the excellent stereochemical control opportunities that exist in ( )-crotylation reactions of a-methyl chiral aldehydes, especially when the -position is branched (as in the (7s)-crotylation of 25 and 32, see above). 

In acid solution, attack at the more highly branched carbon atom tends to occur. Thus when propylene oxide was treated with ethanol, sodium ethoxide directed the reaction almost quantitatively to the formation of the primary ether. Addition of sulfuric acid to the reaction mixture, however, led to the formation of both isomers in approximately equal amounts 20... 

However, the dominant reaction is expected to be the direct displacement reaction (9) with smaller branching ratios for reactions... 

The direct aldol reaction of hydroxyacetone (HA, 39) with an aldehyde leads to either a 1,2-diol (branched) or a 1,4-diol (linear), with the latter being the disfavored... 

Medicine. In humans and animals AdoCbl is the coenzyme of methylmalonyl CoA-mutase that catalyzes the isomerization of methyl-malonyl-CoA into succinyl-CoA (in propionibacteria this reaction runs in the opposite direction). The reaction is linked with the catabolism of amino acids and lipids. If the activity of methylmalonyl CoA-mutase is blocked, the catabolism of some amino acids, fatty acids and thymine is inhibited (Fig. 7.1). Intracellular levels of methylmalonyl-CoA and propionyl-CoA are increased and may affect fatty acids synthesis. In some cases, an increase in the content of C15 and Cn odd-numbered fatty acids and branched-chain fatty acids in glycolipids of the nervous system is observed (Kishimoto et al., 1973). 

The most straigthforward reaction model containing four intermediates is the linear sequential one. In this case each measured time constant gives directly the decay time of one intermediate. Thus only the order of the intermediates in the reaction scheme requires further informations. We have discussed the two possible arrangements of the 0.9ps- and the 3.5ps kinetlc as well as a branched reaction scheme elsewhere /6,7/. Here we present the specific case where the 3.5ps kinetic precedes the 0.9ps-kinetic. The main purpose is to show that the absorption data can really decide between a scheme with four intermediate states of well defined time constants (model I) and one with only three intermediates... 

Direct Aldol Reactions The most frequently applied direct aldol reactions in which unmodified achiral methyl ketones can be directly activated by catalysts were reported by Trost [26] and Shibasaki [27] (Scheme 2.117). Both used bimetalUc catalysts as enzymatic numetics for ketone activation. Shibasaki reported the use of LuLi tris (binaphthoxide) to perform aldol reactions between methyl ketones and branched aldehydes. The underlying notion was that this catalyst mimics enzymatic activity through Lewis acid activity based on the lanthanum ion and Brpnsted basic activity due to the lithium binaphtoxide. The enantiomeric selectivity ranges from modest to excellent ratios. 

In any step of the mechanism, if an active center gives rise to several active centers, each giving rise to a chain, the reaction is said to have direct branching. The branching process may be represented by ... 

Kinetic study of chain reactions with direct branching... 

Figure 12.8. Reaction possibilities of an active center in a chain reaction with direct branching...

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