Diverse chemoselectivity

Diverse chemoselectivity refers to the selective trapping of several different minor non observable tautomers in chemical reactions [48-50]. 

Thus, similarly to its substituted homolog 23, borabullvalene 54e exhibits diverse chemoselectivities in its reactions with nucleophilic reagents. Moreover, the main interconverting tautomers (23,52,53 and 56,54e, 59) have essentially the same carbon skeletons, but different relative stabilities. Thus, in the borabullvalene series, the most stable tautomer is 23. This is stipulated by the possibility for the 2p-AO in 23 to conjugate with the ir-system of the double bonds. In the corresponding tautomer (56) of the more strained borabarbaralane series such conjugation is impossible, and borabullvalene 54e is the only observable compound. 

Heropoulos et al. (2007) reported bromination reaction (ultrasound and microwave-assisted) of various alkylaryls with NBS (N-bromosuccinimide), either neat or in water with diverse chemoselectivity. Thus, only ring substitution occurs in water with ultrasound, whereas with microwaves both side-chain a-bromination as well as ring substitution occurs. In presence of water, microwaves promote side-chain a-bromination and ring substitution, both. With neat reactant, side-chain a-brominated form was obtained as the major product. 

Tandem azidination- and hydroazidination-Hiiisgen [3 +2] cycloadditions of ynamides are regioselective and chemoselective, leading to the synthesis of chiral amide-substituted 1,2,3-triazoles <06OBC2679>. A series of diversely l-substituted-4-amino-l,2,3-triazoles 132 were synthesized by the copper-catalyzed [3+2] cycloaddition between azides 130 and ynamides 131 <06T3837>. 

The perfluoroacetamide catalysts, rhodium(II) trifluoroacetamidate [Rh2(tfm)4] and rhodium(II) perfluorobutyramidate [Rh2(pfbm)4], are interesting hybrid molecules that combine the features of the amidate and perfluorinated ligands. In early studies, these catalysts were shown to prefer insertion over cycloaddition [30]. They also demonstrated a preference for oxindole formation via aromatic C-H insertion [31], even over other potential reactions [86]. In still another example, rhodium(II) perfluorobutyramidate showed a preference for aromatic C-H insertion over pyridinium ylide formation, in the synthesis of an indole nucleus [32]. Despite this demonstrated propensity for aromatic insertion, the perfluorobutyramidate was shown to be an efficient catalyst for the generation of isomtinchnones [33]. The chemoselectivity of this catalyst was further demonstrated in the cycloaddition with ethyl vinyl ethers [87] and its application to diversity-oriented synthesis [88]. However, it was demonstrated that while diazo imides do form isomtinchnones under these conditions, the selectivity was completely reversed from that observed with rhodium(II) acetate [89, 90]. 

Diverse activation methods of bis(iodozincio)methane (3) for the methylation reactions lead to chemoselective transformations. For instance, the a-keto group is methylated selectively by 3 in the presence of a simple ketone (equation 22)32. The opposite selectivity is observed in the reaction using 3 and /3-TiCl3 (equation 23)32. Aldehyde is methyle-nated selectively by 3 in the presence of simple ketone (equation 24)9. Both aldehyde... 

The utility of reductive amination with NaBHsCN in synthesis is contained in reviews and successful applications have been compiled through 1978. Table 7 provides a variety of examples taken from more recent accounts and chosen to illustrate the versatility and compatibility of the process with diverse structural types and chemoselectivity demands. Thus, esters (entries 2-4, 8-12), amides (entries 3, 6-9, 12), nitro groups (entry 13), alkenes (entry 2), cyclopropyl groups (entry 2), organometallics (entry 5), amine oxides (entry 14) and various heterocyclic rings (entries 1, 3, 5-10) all survive intact. Entry 6 illustrates that deuterium can be conveniently inserted via the readily available NaBDjCN, and entry 15 demonstrates that double reductive amination with diones can be utilized to afford cyclic amines. 

It is well established that sulfur and oxygen anions behave in an opposite way with electrophilic centres during nucleophilic substitutions. The comparison of the reactivity of 1-octanthiol and 1-octanol with DMC (entries 3 and 4, Table 9) outlines the difference in chemoselectivity of RS and RO anions, due to their diverse hardness. Under all of the investigated conditions, alkoxides show a Bac2 reaction mechanism, differently from thiolates, which react via a B,2 type. 

The most common anchors for traceless linkage of arenes are based on silyl linkers due to the well-known protodesilylation of the Si-aryl bond. The first traceless silyl linkers were described independently by Ellman et al. and Veber et al. in 1995 [104, 106], Starting from resin-bound arylstannane 113, Ellman has shown the advantages of this type of detachment in the synthesis of a diverse benzodiazepine library (Scheme 16.27). Improvements in the chemoselectivity of the cleav-... 

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