Intermediates compare

In the case of carbanion and radical intermediates the solvent is less important but the products are partially determined by the resistance of the medium to proton or hydrogen atom abstraction respectively. The increased stability of these intermediates compared with carbonium ions allows the reaction mechanism to be more readily modified by the addition of trapping agents. For example, carbanions are trapped in high yields by the presence of carbon dioxide in the electrolysis medium (Wawzonek and Wearring, 1959 Wawzonek et al., 1955). 

The activation of DMSO toward the addition step can be accomplished by other electrophiles. All of these reagents are believed to form a sulfoxonium species by electrophilic attack at the sulfoxide oxygen. The addition of the alcohol and the departure of the sulfoxide oxygen as part of a leaving group generates an intermediate comparable to C in the carbodiimide mechanism. 

The calculation results listed in Table 1.1 are also able to account for the influence of the bridge on the isospecificity. In fact, in agreement with the experimental results,58 the calculated stereoselectivities for a given jt-ligand are generally smaller for the case of the -C(CH3)2- (2-6) than for the -Si(CH3)2- bridge (10-14), while for the case of the -CH2-CH2- bridge the stereoselectivities are often intermediates (compare, e.g., 5, 13, and 23). 

The selectivity of radical bromination reactions depends, in part, on the increased stability of secondary or tertiary radical intermediates compared with primary radicals. In Section 9.2 we noted that allyl and benzyl radicals were especially... 

For reactions in series the BFB always gives a lower yield of intermediate compared to a fixed bed. 

In alternant hydrocarbons (p. 50) the reactivity at a given position is similar for electrophilic, nucleophilic, and free-radical substitution, because the same kind of resonance can be shown in all three types of intermediate (compare 20,22, and 23). Attack at the position... 

Resorcinol is water-soluble and readily conjugated and eliminated. The chemical has no known potential for formation of electrophilic reactive intermediates comparable to those derived from the other dihydroxybenzenes. Resorcinol was tested in various genetic toxicology assays, including in-vitro bacterial and mammalian assays and in-vivo mammalian assays. It gave negative results in all studies, with the exception of a positive response in the two in-vitro studies that assessed chromosomal aberrations in human lymphocytes from whole blood cultures however, resorcinol did not induce chromosomal aberrations in human fibroblasts. 

The reductive cleavage of the C—O bond in 2- and 4-substituted hydroxy-methylpyridines was mentioned in Part I. The optically active l-(4-pyridyl)-alkanols are reduced in good yield to the optically inactive alkylpyridine, whereas the 2-substituted derivatives were reduced in lower yield to the optically active alkylpyridine (and some tetrahydropyridine derivatives).384,385 The difference in behavior was explained by orientation of the adsorbed pro-tonated pyridine, allowing contact between the electrode and the 2-substituent, but not the 4-substituent. This model was used in later work however, not excluded was the possibility that the higher stability of the p-quinonoid dihydropyridine intermediate, compared to the o-quinonoid dihydropyridine, played a role in the relative ease of reduction of the 4- and 2-substi-tuted pyridines. 

The observed 5-exo-trig cyclization in the above reaction is attributed to the formation of more stabilized malononitrile radical intermediate compared to the trialkyl-substituted radical. The stereochemical outcome of these transformations with regards to the malonodinitrile group is consistently P-sclcctivc this suggests least steric interactions between these substituents as compared to the alternative methyl-methyl interaction in the transition state. 

Propose mechanisms and draw reaction-energy diagrams for the following reactions. Pay particular attention to the structures of any transition states and intermediates. Compare the reaction-energy diagrams for the two reactions, and explain the differences. 

If 6 is reacted with chlorodimethoxyborane instead of dimethoxyborane, the primary product is 8d (established by comparison of its, 3C chemical shifts to those of 8a-c). In CHC13, 8d transforms spontaneously to 9a, probably via 5d as intermediate (compare 8a - 5b). The boron chemical shifts of 9a are close to those calculated for 9auat GIAO- SCF/6-31G //MP2/6-31G. 7... 

An energy diagram is a schematic representation of the energy changes that take place as reactants are converted to products. An energy diagram indicates how readily a reaction proceeds, how many steps are involved, and how the energies of the reactants, products, and intermediates compare. 

An interesting exception to the general rule that phosphonium ylides are more stable than arsonium ylides is provided by PhjP=CHN02 which decomjwses spontaneously at room temperature, whereas the arsenic analogue is stable. This attributed to the greater strength of the P—O bond in the reaction intermediate compared to the As—O bond . ... 

The protonation of an Ni-coordinated cysteine after heterolytic cleavage has also been proposed.85 From the resulting intermediate, the reduction mechanisms evolve through a structurally different Ni-C intermediate (compare Figures 7.7 and 7.8), but similarly regenerates the Ni-SI intermediate. No corresponding low-energy pathway has been found for the initial coordination of H2 to the Ni center. 

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