N electron-rich

Ohta s group thoroughly studied the heteroaryl Heck reactions of chloropyrazines and n electron-rich heteroaryls [42-A4]. The substitution occurred at the electron-rich C(5) position of the imidazole ring for the heteroaryl Heck reaction of 2-chloro-3,6-dimethylpyrazine and N-methylimidazole. 

Catenaries and Rotaxanes Containing n-Electron-deficient and n-Electron-rich I 227... 

Catenaries and Rotaxanes Containing. 1-1 leclron-dejiue.nl and n-Electron-rich 231... 

Small shift values for CH or CH2 protons may indicate cyclopropane units. Proton shifts distinguish between alkyne CH (generally SH= 2.5 - 3.2), alkene CH (generally SH = 4.5 - 6) and aro-matic/heteroaromatic CH (SH = 6 - 9.5), and also between n-electron-rich (pyrrole, furan, thiophene, SH = 6 - 7) and rc-electron-deficient heteroaromatic compounds (pyridine, 5H-7.5 - 9.5). 

In contrast to H shifts, l3C shifts cannot in general be used to distinguish between aromatic and heteroaromatic compounds on the one hand and alkenes on the other (Table 2.2). Cyclopropane carbon atoms stand out, however, by showing particularly small shifts in both the 13C and the ]H NMR spectra. By analogy with their proton resonances, the 13C chemical shifts of n electron-deficient heteroaromatics (pyridine type) are larger than those of n electron-rich heteroaromatic rings (pyrrole type). 

Benzothiophene, like thiophene, is a n-electron rich heterocycle. Unlike thiophene, electrophilic attack occurs selectively at the P-position (or 3-position) [10a, Ila] however, substitution at the a-center (2-position) can also be achieved [11b]. The order of positional reactivity, based on nitration, is 3 > 2 > 6 > 5 > 4 > 7 [1 la, 12a]. Also, halogenation under acidic conditions affords the 3-halobenzothiophene [11c, 13a]. In terms of metalation chemistiy, deprotonation occurs preferentially at the 2-position (a) [lid, 13b] metal-halogen exchange is also favored at the a-center over the P-center [12b]. 

Intramolecular addition of the amide group to the triple bond in pyrazoles is more difficult, and results in closure of the 5-lactam rather than the )/-lactam ting. The reaction time of the 4-phenylethynylpyrazole-3-carboxylic acid amide undw the same conditions is extended to 42 h (Scheme 129) (Table XXVII). The cyclization of l-methyM-phenylethynyl-lH-pyrazole-3-carboxylic acid amide, in which the acetylene substituent is located in the n"-electron-rich position of the heterocycle, is the only one complete after 107 h (Scheme 130) (90IZV2089). 

C is a basal plane site characterized by the presence of delocalized n electrons, which thus acts as a Lewis base center. Such sites are probably located in n-electron-rich regions within the basal planes of carbon crystallites (graphenes), away from the edges [63]. Evidence favoring their electron donor-acceptor (EDA) interactions, such as Reaction (3.1), was presented [63] by investigating a polymer-derived microporous carbon and a furnace carbon black, using HCl adsorption isotherms,... 

A few take-home messages can be extracted from the previous pages. The first is that most of the electroactive moieties used in the selected examples are n electron rich and, unless coordination bonds are part of the organizing interactions, these electroactive subunits can be mutually exchanged from one approach to another. In this context, modulation of the way in which r-densities interact is the master key that opens the door to organic materials, and more specifically to molecular wire structures. 

A simple method for o-azaxylylene synthesis by base-induced elimination of hydrogen chloride from carbamate derivatives (for example, 548), prepared with phosgene, has been reported. These intermediates are readily trapped by n-electron-rich alkenes, for example 549, to form hydroquinoline derivatives, 550 [391]. 

In retrospect, donor-acceptor interactions between the n-electron rich aromatic rings and the r-electron deficient domains of Ceo, together with van der Waals forces, provide the major stabilization for the solution complexes. While the complementary of the curvature of the interacting species maximizes the number of intermolecular contact, formation of these concave - convex composites are entropically disfavored due to a more ordered state. The above outlined examples manifest... 

The formation of pseudopolyrotaxanes incorporating cyclobis(paraquat-p-phenylene) (75) and n-electron rich polyethers has been achieved [163-165] through a reliance on a combination of [C—H O] hydrogen bonds, [Ji ... 

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