Activate neighboring functional groups

However, the most relevant synthetic trait of this molecule is its capacity to activate neighboring functional groups embedded in its structural framework (Schemes 5.4-5.6,5.10-5.13, and 5.15). In one of the earliest examples devised to this end, cleavage of the PMB ethers 2 and 15 with 2,3-dihydro-5,6-dicyano-l,4-benzoquinone (DDQ) in dichloromethane/H20 emulsions selectively afforded the expected primary alcohol 16 [25,26] or a formyl function 17 [23,37] (Scheme 5.4). The latter transformation is not typically observed starting from the corresponding sulfur-free allyl PMB ethers [38]. 

Selective addition of alkenes and alkynes to aromatic compounds has also been performed by ruthenium-catalyzed aromatic C-H bond activation. Carbon-carbon bond formation occurs at the ortho positions of aromatic compounds, assisted by the neighboring functional group chelation. The reaction, catalyzed by RuH2(CO)(PPh3)3, was efficient with aromatic and heteroaromatic compounds, with various functional groups, and a variety of alkenes and alkynes [ 121 ] (Eq. 90). Activation of vinylic C-H bonds can occur in a similar manner. 

A ruthenium complex such as Ru3(CO)12 can activate the C-H bond of sp carbons on the condition that a neighboring functional group can coordinate to the metal to favor intramolecular C-H bond activation [123] (Eq. 92). 

While catalytic tritiodehalogenation of aromatic halides plays a dominant role in the preparation of substrates of high specific activity, aliphatic halides are more often treated with strong tritide reagents (see Section 4.3). However, sp halides activated by a neighboring functional group such as aryl-, heteroaryl, or carbonyl are often viable substrates for catalytic... 

In principle, the reactivity of a functional group should not be altered when it is attached to a polymer ( 1). However, special effects may be encountered when a reagent is attracted to a polymer or repelled from it, when the polymer-bound reactive group is activated or inhibited by a neighboring group or when the local polarity of the polymer domain differs from that of the bulk solvent. A review of studies of such effects... 

Generally these globular dendritic architectures offer several advantages over other kinds of organic polymers, such as the full exposure of the catalytic centers to the environment. In contrast to linear or cross-Hnked polymeric supports, which can partially hide catalytic centers, the functional groups are located on the surface of the dendritic nanoparticle and diffusional Hmitations are less relevant Furthermore the close proximity of the catalytic centers on the surface of the dendritic polymer can enhance the catalytic activity by multiple complexation or even cooperativity. This behavior is described as positive dendritic effect. However, in some cases a negative dendritic effect was observed, which is caused by an undesired interaction or electron transfer between the neighboring catalytic centers on the surface of the dendrimer [70]. 

As a neighboring group the oxadiazole ring does not appreciably modify the chemistry of many functional groups, but there is frequently a difference in activity of the same group at C-3 and C-5 (Section V, F). The transformations shown in Eq. (53), for example,23 are not different... 

Dihydrouracil, an isomer of glycine anhydride, has two kinds of carbon-bound hydrogen atoms. Those activated by the neighboring NH-group react much more readily (90%) than those next to the carbonyl function (ca. 5% Schuch-mann et al. 1984 for details, see Chap. 10). Thus, a high regioselectivity is again observed. 

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