Functional group activation reactive intermediates

Electrophilic aromatic substitution is a situation in which it is useful to discuss TS structure in terms of a reaction intermediate. The ortho, para, and meta directing effects of aromatic substituents were among the first structure-reactivity relationships to be developed in organic chemistry. Certain functional groups activate aromatic rings toward substitution and direct the entering electrophile to the ortho and para positions, whereas others are deactivating and lead to substitution in the meta position. The bromination of methoxybenzene (anisole), benzene, and nitrobenzene can serve as examples for discussion. 

In recent years, the emphasis of research has been directed more and more toward utilizing nitro compounds as reactive intermediates in organic synthesis. The activating effect of the nitro group is exploited in carrying out many organic reactions, and its facile transformation into various functional groups has broadened the importance of nitro compounds in the synthesis of complex molecules. 

Coenzymes facilitate chemical reactions through a range of different reaction mechanisms, some of which will be discussed in detail in this review. However, in all cases structural features of the coenzyme allow particular reactions to proceed along a mechanistic pathway in which reaction intermediates are more thermodynamically and kinetically accessible. When incorporated into apoen-zyme active sites, the coenzyme reactivity is influenced by a well-defined array of amino acid functional groups. For a given coenzyme, the particular array of amino acids presented by the different apoenzymes can drastically alter the degree of rate acceleration and product turnover and can specify the nature of the reaction catalyzed. 

Late transition-metal hydroamination is the method of choice for the atom economical and functional group-tolerant construction of C—N bonds, and in this context Ir plays a central role (indeed, homogenous transition-metal-catalyzed OHA was discovered with Rh and Ir). However, there is a strong need for the development of better OHA catalyst systems that are applicable to a wider range of substrates and conditions. The characteristics of current Ir based catalyst systems to function via N—H bond activation, though, is a potential handicap to achieve this goal, since it implies highly reactive Ir intermediates that are prone... 

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