Cyclization reactions bimolecular reaction

The attachment of an electron to an organic acceptor generates an umpolung anion radical that undergoes a variety of rapid unimolecular decompositions such as fragmentation, cyclization, rearrangement, etc., as well as bimolecular reactions with acids, electrophiles, electron acceptors, radicals, etc., as demonstrated by the following examples.135"137... 

Homolytic substitution reactions including homolytic allylation, radical [2,3]-migrations and stereochemical reactions been reviewed. The review also highlights the possible applications of homolytic substitution reactions. ni reactions at silicon (by carbon-centred radicals in the a-position of stannylated silyl ethers) are efficient UMCT reactions producing cyclized alkoxysilanes. Bimolecular reactions can also be facilitated in good yield (Schemes 32 and 33). ... 

Redox and homolytic substitution reactions almost never directly form C—C, C—N and C—O bonds. Such bonds are generated in radical addition reactions (Scheme 14). Intermolecular addition reactions are presented in this chapter. Cyclization reactions have important similarities with, and differences from, bimolecular additions, and they are presented in Chapter 4.2 of this volume. Falling under the umbrella of addition reactions are radical eliminations (the reverse of addition) and radical migrations (which are usually, but not always, comprised of an addition and an elimination). 

A bimolecular reaction has been proposed for the reaction with hydroxide ion and imidazole in aqueous acetonitrile and aqueous EtOH solutions of the 1,2,3-benzoxathiazole 2,2-dioxides (323) under various pressures.295 The effect of A -mcthyl substituents in the cyclization of 2-methoxycarbonylphenylsulfamides (324) to give ( H)-2,, 3-bcnzothiadiazin-4(3//)-onc 2,2-dioxides has been examined.296... 

At this time, no absolute rate constants have been determined for a reaction of an aminium cation radical. However, for synthetic utility, one needs to consider the relative rate constants for competing reactions. Competition between two unimolecular reactions depends only upon the relative rate constants for the processes. For competition between a unimolecular and a bimolecular reaction whose rate constants are comparable, product distributions can easily be controlled by the concentration of the second species in the ratio of rate laws. The ratio of reaction products from cyclization (unimolecular) versus hydrogen atom trapping before cyclization (bimolecular) can be expressed by the equation %(42 + 65)/%41 = Ar/(A H[Y - H]) (Scheme 20). Competition between two bimolecular reactions is dependent on the relative rate constants for each process and the effective, or mean, concentration of each reagent. The ratio of the products from H-atom transfer trapping of the cyclized radical versus self-trapping by the PTOC precursor can be expressed by the equation %42/%65 = (kH /kT) ([Y - H]/[PTOC]). 

Both direct and sensitized irradiations of cyclohexene 78Z to give trans-anti-transcis-trans-, and cis-anti-cis- 2 + 2]-cyclodimers 80-82 in different ratios [59]. This photodimerization is believed to proceed through the initial formation of the highly strained ( )-isomer 78E, which is followed by the thermal concerted and/or stepwise cyclodimerization with 78Z, although no direct evidence for the intervention of 78E has been obtained and the cyclization mechanism(s) involved are not very clear. In this photocyclodimerization, the enantiodifferentiation occurs not in the cyclodimerization but in the initial photoisomerization step hence we classify this formally bimolecular reaction as a unimolecular enantiodifferenti-ating photosensitization. 

A simple example of the formation of a six-membered ring by unimolecular cyclization is provided by an alternative synthesis of the chloride of compound (93) from the pyrrolidine (95) (Equation (39)) <44JOC359>, while bimolecular reaction of piperidine (96) gives the diazoniadispiro derivative (4) (Equation (40)) <65JOC82l>. A seven-membered ring can be formed if the stereochemistry of the starting material is suitable, as in compound (97) (Equation (41)) <63JOC2843> there is no obvious reason for the difference in yield between pyrrolidine and piperidine derivatives. 

There are various possibilities that aU arise from the presence of a carbonyl group and a Grignard in the same molecule. These two would react together. They might cyclize to form a four-membered ring or a bimolecular reaction might lead to a dimer and perhaps polymerization. 

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