Competing reaction pathways

Two different alkenes can be brought to reaction to give a [2 -I- 2] cycloaddition product. If one of the reactants is an o, /3-unsaturated ketone 11, this will be easier to bring to an excited state than an ordinary alkene or an enol ether e.g. 12. Consequently the excited carbonyl compound reacts with the ground state enol ether. By a competing reaction pathway, the Patemo-Buchi reaction of the 0, /3-unsaturated ketone may lead to formation of an oxetane, which however shall not be taken into account here ... 

Surprisingly, no marked effect of the substituent R2 on the product ratio was observed whereas in the similar reaction of 2 -hydroxychalcone dibromides a great influence of substituent R2 on the competing reaction pathways was found (ref. 4). Noteworthy, no formation of the azide E (regioisomer of 2) was detected. 

SCHEME 2.4 Competing reaction pathways in the benzylation of cytosine (R = H) and 1-methyl cytosine (R = Me) explored by computational DFT means. 

Scheme 39.3 Product control by affecting the relative rates of competing reaction pathways in scC02. Path A is preferred in scC02, whereas path B prevails in conventional solvents.

The catalytic reaction conditions required some optimization. This was due to competing reaction pathways. The interception of trans-11 results in the formation of the organotitanium intermediate 44, as shown in Scheme 17. Thus, 2 equiv. of Cp2TiCl are consumed and a complete conversion in the presence of 10 mol% Cp2TiCl2 cannot be achieved because catalyst regeneration is prevented. Similar considerations apply for czs-11. 

Butynone 93 and tetrasubstituted allene 94 combine to form acetyl cyclopenta-diene 95 (Eq. 13.30) [32]. The annulation does not proceed in satisfactory yield in the case of unsubstituted trimethylsilyl allene 96 (Eq. 13.31) [32]. This is presumably due to diminished stabilization of the cationic intermediates in the case of 96, which allows competing reaction pathways to erode the yield. 

For example, in one pathway H-atom elimination reactions generate the enol intermediate, which eventually rearranges to 2-butanone. In a second competing reaction pathway, H-atom addition results in direct hydrogenation to the saturated alcohol 2-butanol. 

However, more than one reaction pathway may exist, in which case the rate equation will contain sums of terms representing the competing reaction pathways. For example, one of the oxidation reactions that convert the atmospheric pollutant sulfur dioxide to sulfuric acid (a component of acid rain) in water droplets in clouds involves dissolved ozone, O3 (see Sections 8.3 and 8.5) ... 

The theoretical analysis of chemical activation reactions is similar to the Lindemann theory of unimolecular and association reactions. There are a number of competing reaction pathways. Depending on total pressure, concentrations of the participating species, and temperature, the outcome of the competition can change. 

Carbenes 100, generated by UV irradiation of methyl (alkoxysilyl)diazoacetates 99, furnish l-oxa-2-silacyclopentanes 101 by 1,5-C,H insertion (equation 25)60. In order to suppress competing reaction pathways of the carbene intermediate (e.g. formation of a ketazine with excess diazo compound), the photolysis was carried out at high dilution, but, even then, yields were rather modest. Carbene insertion at CH2 seems to occur much more easily than at CH3 a preference of 3 0.6 1 for methylene insertion can be calculated from the isolated yields after correction for the number of C—H bonds. It should be noted that for both carbenes, 96 and 100, C,H insertion occurs only in the 1,5 mode whereas no 1,3-, 1,4- or 1,6-C,H insertion products could be detected. 

Despite the obvious synthetic potential of this formal cycloaddition or annulation reaction, its application has remained little known because of the competing reaction pathways due to 1,2- versus 1,4-addition as well as C-addition versus O-addition [Scheme 4], Moreno-Manas reported a... 

Although the use of cyclic enals improved the overall product distribution by suppressing the 1,4-addition pathway,33 34 a general solution remained elusive. To solve the competing reaction pathway problem or to improve the pathway that would ultimately lead to the 2H-pyran 33 involved extensive experimental modifications. Our solution eventually involved the utilization of preformed a,P-unsaturated iminium salts instead of generating them in situ.35-38... 

Radical fragmentation of 2-nitrophenyl-azo-trityl resin was studied in the presence of various radical acceptor solvents to elucidate possible radical reaction pathways. When using benzene as solvent, only 2-nitro-bi-phenyl was formed as the product of radical substitution reaction (SNR) in 67% yield. Hydrogen-radical abstraction from the polymer backbone (e.g., from the benzylic units of polystyrene) was completely suppressed. When toluene was used as solvent, a mixture of the following products was obtained nitrobenzene, 4-methyl-2 -nitrobisphenyl, 2-methyl-2 -nitro-bisphenyl, and 3-methyl-2 -nitrobisphenyl (9 9 1 1). In the case of toluene, the nitro-aryl radicals undergo H-abstraction with radical substitution as a competing reaction pathway. These results indicate that H-abstraction... 

Scheme 40 Competing reaction pathways for the reaction of C60 and DEAP.

The driving force for this reaction is the formation of N2, an exceptionally stable molecule. The carbocations generated in this manner react like others we have seen by nucleophilic attack to give substitution, by proton loss to give elimination, and by rearrangement. Because of the many competing reaction pathways, alkanediazonium salts usually decompose to give complex mixtures of products. Therefore, the diazo-tization of primary alkylamines is not widely used for synthesis. 

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