Modes of Participation

Owing to the nucleophilicity of amines, the reaction of electrophiles, such as halogens, with olefinic amines may take a different course than that followed by alcohols or ethers, where concerted addition-cyclization normally occurs (see Chapter 4). For example, the bromination of 1-allylpyrazole (74) leads to the dibromide (75), which may be converted to diazapentalene (76) by N-5 cyclization and dehydrobromination. In 

Alternatively, the Bordwell-Shiner-Sneen ion-pair assistance mechanism (see p. 248), i.e., Eq. (47), was put forth. In the latter case, 

To reach a decision on the mechanism for participation by nitrogen in (80a), Grob et al. prepared and solvolyzed the series of tertiary aryl-substituted p-nitrobenzoates (83) and (84) (Table 11). A plot of log k for 

There is need for caution in accepting the above conclusion. The guidelines for the interpretation of data produced from the probes used by Grob et al. in arriving at their conclusion are essentially based on traditional Sj l and S 2 mechanisms and not on the existance of hidden ion-pair return. If hidden ion-pair formation is as common as envisioned by some, the interpretations of m and p values may be based, at least in part, on the reaction of ion pairs and not neutral substrates. The direction of the curvature of the log k versus plot for (83a)-(83d) (if the curvature is real) could be taken as evidence for the ion-pair mechanism. Hence all that can be said until the question of the general involvement of ion pairs is resolved is that nitrogen participation is demonstrated. 

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The differences in the mode of participation of nitrogen-containing, complex neighboring groups under neutral and basic conditions has been reviewed by Winstein and coworkers S a number of examples in the carbohydrate field supplement these observations. 

When describing nucleophilic participation it is frequently convenient to use the symbol G-n, where G is the participating group and n the size of the ring that is formed in the transition state. Equations (3)-(5) therefore depict examples of AcO-5, Ar-3, and N-5 participation, respectively. The mode of participation shown in Eq. (4) may be more precisely symbolized as Ari-3 participation the subscript 1 symbolizes that the nucleophilic attack is carried out by carbon-1 of the phenyl group. An example of Ar2-6 participation leading to a cyclic product is shown in Eq. (8). ... 

As shown from the kinetic data earlier in this chapter, ring closure to four-membered azacarbocyclic rings (N-4 participation) is quite slow relative to ring closure to higher or lower homologs. Nevertheless, R2N-4 participation does occur. In fact, this mode of participation is shown to be more efficient than HO-3 participation in l-alkylamino-3-chloro-2-propanols as l-alkyl-3-azetidinols are the exclusive products [Eq. (26)]. ... 

In addition, because of the mode of participation of the electrodes, modular arrangements can be made to cope with energy- or power-oriented requirements. 

The first classical trajectory study of iinimoleciilar decomposition and intramolecular motion for realistic anhannonic molecular Hamiltonians was perfonned by Bunker [12,13], Both intrinsic RRKM and non-RRKM dynamics was observed in these studies. Since this pioneering work, there have been numerous additional studies [9,k7,30,M,M, ai d from which two distinct types of intramolecular motion, chaotic and quasiperiodic [14], have been identified. Both are depicted in figure A3,12,7. Chaotic vibrational motion is not regular as predicted by tire nonnal-mode model and, instead, there is energy transfer between the modes. If all the modes of the molecule participate in the chaotic motion and energy flow is sufficiently rapid, an initial microcanonical ensemble is maintained as the molecule dissociates and RRKM behaviour is observed [9], For non-random excitation initial apparent non-RRKM behaviour is observed, but at longer times a microcanonical ensemble of states is fonned and the probability of decomposition becomes that of RRKM theory. 

The mode of action of plasticizers can be explained using the Gel theory [35 ]. According to this theory, the deformation resistance of amorphous polymers can be ascribed to the cross-links between active centres which are continuously formed and destroyed. The cross-links are constituted by micro-aggregates or crystallites of small size. When a plasticizer is added, its molecules also participate in the breaking down and re-forming of these cross-links. As a consequence, a proportion of the active centres of the polymer are solvated and do not become available for polymer-to-polymer links, the polymer structure being correspondingly loosened. 

In many of the normal modes of vibration of a molecule the main participants in the vibration will be two atoms held together by a chemical bond. These vibrations have frequencies which depend primarily on the masses of the two vibrating atoms and on the force constant of the bond between them. The frequencies are also slightly affected by other atoms attached to the two atoms concerned. These vibrational modes are characteristic of the groups in the molecule and are useful in the identification of a compound, particularly in establishing the structure of an unknown substance. 

Some metabolites of curcumin (particularly tetrahydrocurcumin) may also participate in producing the observed effects of curcumin in different models because these metabolites display greater stabilities than the parent curcumin molecule at physiological pH. Recent data show similar modes of action of curcumin metabolites regarding antioxidant enzyme induction and inhibition of multidrug-resistant proteins. " Additional data indicate that curcumin may even act against other types of diseases such as atherosclerosis " " and Alzheimer s disease. " - " ... 

There are several criteria based on which chemical reactors are classified (1) mode of operation, (2) number and type of phases participating, (3) thermal conditions, and (4) design features. 

The theory of stereoselectivity found in intramolecular hydrogen migration in polyenes was disclosed by Woodward and Hoffmann 51>. The HO—LU interaction criterion is very conveniently applied to this problem 64>. The LU of the C—H sigma part participates in interaction with the HO of the polyene n part. The mode of explanation is clear-cut... 

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

The small cluster threshold behavior, suggested by the kinetics scheme presented earlier, is apparent in this data set. Assuming (1) the RRK form for k n, (2) the addition reactions are not activated, and (3) the number of participating modes are independent of the element type, the M-CO bond strengths can be grouped by energy. For... 

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