Neighbor group assistance

Benzofuroxan 79 can be generated from 2-nitrophenyl azide 80 (Scheme 49). Neighboring-group assistance within the pyrolysis leads to a one-step mechanism with an activation barrier of 24.6 kcal/mol at the CCSD(T)/6-31 lG(2d,p) level [99JPC(A)9086]. This value closely resembles the experimental one of 25.5 kcal/mol. Based on the ab initio results for this reaction, rate constants were computed using variational transition state theory. 

In some molecules, the loss of halogen is unexpectedly facile and may occur extensively even over platinum, as illustrated by reduction of the dihydro-2-benzazepine 4 to 5. The authors (5i) raised the possibility that such facile loss of halogen may involve neighboring-group assistance from the amide moiety. 

Neighboring-Group Assistance in Free-Radical Reactions... 

This strategy appears to be very attractive because of the possibility of completely solubilizing the support in most of the common solvents. From a chemical perspective, that property allows one to benefit from all the solvent conditions used in classical solution chemistry. This could prove to be very advantageous, especially to obtain stereoselective glycosylation without neighboring-group assistance. Moreover, isolation and purification of the polymer is easily achieved by precipitation usually in diethyl ether or methyl-tert-butyl ether (MTBE) and recrystallisation from ethanol. One major drawback of this type of support is its tendency to solidify at low temperature, thus limiting the variety of temperature conditions. 

For the products formed utilizing the interfacial technique in particular, the fact that high inclusion of the monomeric portion within polymer chains even for "low overall yield" systems may be due to the polymer chains being drawn, or remaining within the critical reaction zone until modification is essentially complete rather than to some neighboring group assistance - their position rather than increased chemical reactivity may be the essential aspect. The relative amount of modification of polymer chains which are in the critical reaction zone may be enhanced... 

For example, glycosylation using 2,3,4,6-tetra-O-acetylglucopyranosyl bromide provided the (3-anomer as the major product due to the neighboring group assistance (Scheme 4.10). However, the preferred a-anomer can be separated from the less active (3-anomer, and assayed individually (Table 4.3). 

Standard organolithium reagents such as butyllithium, ec-butyllithium or tert-butyllithium deprotonate rapidly, if not instantaneously, the relatively acidic hydrocarbons of the 1,4-diene, diaryhnethane, triarylmethane, fluorene, indene and cyclopentadiene families and all terminal acetylenes (1-alkynes) as well. Butyllithium alone is ineffective toward toluene but its coordination complex with A/ ,A/ ,iV, iV-tetramethylethylenediamine does produce benzyllithium in high yield when heated to 80 To introduce metal into less reactive hydrocarbons one has either to rely on neighboring group-assistance or to employ so-called superbases. 

The neighboring group-assisted metalation of arenes and heterocycles is an area too vast to be included into this Chapter. Major aspects have been summarized in Chapter 10 of this book and a comprehensive, though still not exhaustive treatment of this theme can be found in a recent monograph. ... 

NADPH/ NADP+ ratio 776 Naphthalene, resonance energy 299 Nathans, Daniel 84 Natural killer cells 185 Near attack conformation 485 Near-field scanning microscope 130 Neighboring group assistance in displacement reactions 601, 602 Nekoflavin 783 Nematocyst 427 Nematodes 24, 25 Neocarzinostatin 218,224s Neopterin 803... 

Unlike other neighboring group assisted allylborations which require a base such as Et3N,6-7 the allylboration of (3-ketoacids can be carried out in the absence of added base. In fact, when the reaction is run in the presence of one equivalent of Et3N, decarboxylation occurs. A solvent study revealed that decarboxylation is also minimized when the reactions are carried out in ethereal solvents as opposed to less polar solvents such as dichloromethane. 

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