N-aryl groups

Several types of nitrogen substituents occur in known dye stmetures. The most useful are the acid-substituted alkyl N-substituents such as sulfopropyl, which provide desirable solubiUty and adsorption characteristics for practical cyanine and merocyanine sensitizers. Patents in this area are numerous. Other types of substituents include N-aryl groups, heterocycHc substituents, and complexes of dye bases with metal ions (iridium, platinum, zinc, copper, nickel). Heteroatom substituents directly bonded to nitrogen (N—O, N—NR2, N—OR) provide photochemically reactive dyes. 

Generally, N -acylsulfonamides are less effective than those having a single N -aryl group. One such acyl analogue, sulfabenzamide (130) is prepared simply from sulfanilamide (128) by bisbenzamide formation (to 129) using benzoyl chloride and pyridine,... 

N-Aryl group aids substrate binding orientation r... 

For the formation of the 4-ethynylquinoline complexes a mechanism was proposed involving nucleophilic attack of the terminal carbon of the butatrienylidene ligand at the imine carbon, followed by C—C bond formation between the ortho carbon of the N-aryl group and C3 of the butatrienylidene ligand. Deprotonation finally affords 4-ethynylquinoline complexes (Scheme 3.27). Some preference was observed for quinoline formation with the more electron-rich metal centers, whereas... 

In a preparatively useful process diarylamines can be cyclized photolyti-cally. Substrates having alkyl groups,fluorine, methoxyl, ethylamino, and N-alkyF - " and N-aryl groups have been used. Not all triarylamines could be cyclized, especially... 

In a later study of these reactions, differently substituted thioureas were compared in relation to their effectiveness in producing four-membered rings. Compounds 177 and 178 produced four-membered intermediates that collapsed to the more stable chlorformamidines. This phenomenon is most likely the result of the greater basicity of the N-alkyl than that of the N-aryl group. 

The cycloaddition of diazomethane to SchifT bases from heterocyclic aldehydes and anilines provides a useful route to heterocyclic substituted triazolines. Unlike olefins bearing heterocyclic substituents, the heterocyclic imines can be obtained readily by reaction of the appropriate aldehyde and amine thus the diazomethane-imine addition has greater scope than the olefin-azide reaction. NMR spectroscopic studies of the orientation of addition are in accord with previously reported mechanistic considerations (see, e.g., Scheme 93).329 In addition to the influence of the N-aryl group, the electron-withdrawing power of the heterocyclic substituent on the Schiff-base carbon also has a substantial effect on imine reactivity, in the order 2-quinolyl 2-, 3-, or 4-pyridyl > phenyl > 2-thienyl as 2-furyl.329... 

For the isomerization, strongly basic amines are required as substrates. Allyl-amines bearing N-aryl groups retard the reaction. Allylamides are slow-reacting substrates and require a higher reaction temperature. Thus, N-acetyl-4-methyl-tet-rahydropyridine (16) can only be subjected to isomerization at 150 °C. After a reaction time of 15 h the optically active enamide was obtained in 98 % ee. 

An N-alkyl protecting group is crucial to the regioselectivity of this reaction with N-aryl groups products 333 are obtained, arising from 6-endo cyclisation followed by elimination. 

Recently, the arylation of several specific primary amines have been studied because of the potential biological relevance of the products or products further downstream in a synthetic sequence. For example, cyclopropylamine was shown to be a viable substrate for the coupling under standard conditions [203]. Reactions of 7-azabicyclo[2.2.1]heptane have also been conducted [204] under relatively standard conditions, but with bis(imidazol-2-ylidene) as ligand. Complexes of this ligand and DPPF showed similar catalytic activities, which proved to be superior to those of most bis(phosphine)s. ortfio-Halo anilines were also studied, in this case to provide access to carbolines after use of the halogen as a means of effecting cycliza-tions by an electrophilic or reductive C-C bond formation with the other N-aryl group [205]. 

With electron-donor substituents on the aryl group, the stability of N-arylpentazoles increases (except for orffzo-substituents). With increasing Hammett a values for the para-substituent in the N-aryl group, crystalline Af-arylpentazoles decompose at increasing temperatures between —10 °C for phenyl and 4-55 °C for dimethylamino (64AHC373, 85AGE513). The latter pentazole could even be studied by X-ray diffraction (83CC910). 

Y butenolides 22 were obtained in good yields and as essentially single stereoisomers (Table 5.2). In these studies, the tat leucine derived SchifTbase ligand 18a gave rise to the best diastereo and enantioselectivities, and even with the less expensive valine derived Schiffbase ligand, the products were formed with 97% ee. Removal of the N aryl group was readily accomplished with cerium ammonium nitrate followed by acidic hydrolysis of the intermediate aza quinone. 

Limitations Most of the catalysts currently require imines derived from aromatic amines. N aryl amines are thus easily made (as in the synthesis of SCH48461) but if primary amines are the target, the N aryl group has to be removed (as in the synthesis of colchinol). 

Figure 22 Variation of the N-aryl group in adamantyl-activated catalysts...

Subsequent studies have expanded the scope of this catalyst to include male-imides 5 [49]. In order to obtain enantiomerically enriched cycloadducts with this symmetrical dienophile an unsymmetrical diene was used (Scheme 5) this constitutes the first example of such a process. Ortho substitution on the N-aryl group was found to be crucial to the reaUzation of high enantioselectivity, perhaps to discourage catalyst binding the carbonyl lone pair syn to the M-aryl moiety a transition state analogous to that depicted for the imides (Fig. 15) was invoked. The fact that the dienophile is locked in the s-trans conformer could lend... 

Kamishawa et al. studied the migration of a tricarbonyl chromium from a N-aryl group to a carbazole. The migration is controlled by the dioxolane group. 

N-(2,6-Disubstituted)phenyl-triazolones constitute a very interesting class of atropisomeric compounds (04T4361). They are inhibitors of mitochondrial respiration and it was shown that the biological activity was related to the presence of a substituent in the blocking position of the N-aryl group. 

The presence of cycloalkyl substituents in the 2,6-positions of the N-aryl groups (m) has been found to increase the temperature stability of the iron catalysts over their alkyl analogues (e.g., lb or Ic) [29]. On the basis of a quantum mechanical... 

Figure 5.10 Oligomerization precatalysts 8 and 9 with one or no N-aryl groups.

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