Ortho-hydrogen, reactivity

The reactivity of iridium is often associated with that of rhodium however, as shown in Scheme 36, iridium is more favorable toward activating the C-H bond and so this metal has been widely used for this reaction. Reaction of [Ir(COD)Cl]2 with one equivalent of the pyridine-fimctionalizedNHC (232) gave in quantitative yield the complex (233) by cleavage of the metallic precursor dimer and orthometallation of the pyridine moiety (Scheme 36). In contrast, the same reaction carried ont with [Rh(COD)Cl]2 as the metal sonrce afforded (234), for which a strong interaction between the rhodinm center and the ortho hydrogen of the pyridine was observed. Treatment of (234) with a sodinm salt did not lead to a C-H activation, bnt led to the coordination of the pyridine onto the metal center yielding the air stable (235). 

Although these reactions are quite impressive, kinetic measurements have shown that 32 and 42 are less reactive dienophiles than other comparably strained cyclic olefins [35b, 35 c]. It can be concluded that the inherent high reactivity of the strained double bonds in 32 and 42 is partially compensated by the steric hindrance which the ortho-hydrogen atoms on the phane-arene units exert on the approach of the cycloaddend. 

With organometallic compounds as bases in aprotic solvents, the acidity of the ortho hydrogen is the dominant factor and the reactivity order, owing to the bond polarity effect, is F > Cl > Br >... 

One of the main differences between the reactivity of Sm(Cp )2 and Sm(Tp 2 2 with similar substrates is the former tends to form binuclear complexes while the latter forms mononuclear Sm complexes. An interesting example of the different reactivity is in the reaction of Sm(Tp 2)2 with diphenylditelluride (Hillier et al., 1999). Reaction of diphenylditelluride with two equivalents of Sm(Tp 2)2 as a slurry in toluene produced [Sm(Tp 2)(XePh)]. Examination of the product by NMR showed that the ortho hydrogens of the phenyl group were deshielded indicative of a close interaction between the TePh moiety and the Sm(Tp 2)+ cation. Surprisingly, addition of 1.5 equivalents of diphenylditelluride with Sm(Tp 2)2 under identical conditions provided a product consistent with the structure [Sm(Tp 2 2(TePh)3] (eq. (144)). Analysis by NMR showed that the spectrum was similar... 

Hu, S. and Neckers, D.C., Photocycloaddition and ortho-hydrogen abstraction reactions of methyl arylglyoxylates structure dependent reactivities,/. Chem. Soc., Perkin Trans. 2, 1771,1999... 

Unprotected arylamines are so reactive toward halogenation that it is difficult to limit the reaction to monosubstitution Generally halogenation proceeds rapidly to replace all the available hydrogens that are ortho or para to the ammo group... 

At first glance it appears that these systems do conform fully to the discussion above this is an oversimplification, however. The ortho and para hydrogens in phenol are not equal in reactivity, for example. In addition, the technology associated with these polymers involves changing the reaction conditions as the polymerization progresses to shift the proportions of several possible reactions. Accordingly, the product formed depends on the nature of the catalyst used, the proportions of the monomers, and the temperature. Sometimes other additives or fillers are added as well. 

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