C-substituent

A rule of thumb is that a C=C substituent stabilizes a carbocation about as well as two methyl groups Al though allyl cation (H2C=CHCH2 ) is a primary carbocation it is about as stable as a typical secondary carbocation such as isopropyl cation (CH3)2CH-"... 

Substituent R Approximate time for complete dimerization at 18 C Substituent R Approximate time for complete dimerization at 18 °C... 

A choice can be made between these two with the help of published C substituent effects for the substituents (-NH2, -NH3, -COOR see Section 2.5.4) on the benzene ring in A and B ... 

Fig. 12.4. PMO representation ofp-orbital interactions with C=C (a), C=0 (b), and OR (c) substituents. Form of SOMO (singly occupied molecular orbital) is ...

Little is known about the influence of C-substituents on the position of the annular tautomeric equihbrium 24a 24b 24c. According to MNDO... 

Prinzbach and Limbach have studied the valence isomerism between N-substituted azepines 14b and benzeneimines 14c (76CB3505) although 14b is much more stable (actually it is the only form detected by NMR), the compound could react, depending on R, as 14c with diazomethane. Later, Prinzbach et al. reported the study of the equilibrium 14b (90% )/14c (10%) in the case of R = p-tosyl [the compound has the following C-substituents 3,6-dichloro-4,5-di(methoxycarbonyl)] in the solid state (X-ray) only 14b is present [86CB616],... 

Substitution of aldehydic H by a ring or ring system is indicated simply with a C-substituent prefix. 

Some ylides 92 among them C-silylated ones have been synthesized in order to compare the stabilization influence on the carbanionic center of various C-substituents (I, SiMej, Ph3P ) [ 113]. It appears from this study that the stabilization due to the electron-withdrawing C-substituents (R or R ) is not so negligible by comparison with the hyperconjugative stabilization between the ylidic carbon and the phosphonium group [114]. This is particularly true for the iodine substituent. 

Important literature is available for this type of ylides which are usually thermally stable in the case of phosphorus, arsenic or stilbene C-substituents. This is different for ylides C-substituted by nitrogen atoms which have a destabilizing effect. 

Primary 1,2-diamines can be obtained by hydrogenolysis of the benzylic N-auxiliary bond over Pd/C, e.g., the preparation of 249d. The cleavage is not selective in the presence of other benzylic bonds in the molecule, e.g., aryl substituents at Cl - C2. However, in the presence of methoxyphenyl or dimethoxyphenyl C substituents, the N substituents can be selectively removed, as exemplified by the preparation of the primary 1,2-diamine... 

More significant is to change the nature of the C and C substituents. Remaining in the class of [IrCl(CO)2(NHC)] complexes, the replacement of the H atoms on the C and C atoms of the IPr complex by Cl atoms resulted in the rather high shift of the average CO stretching frequency from 2023.9 to 2028.3 cm", see Fig. 1.12a [30], Similar effects have been found in the closely related [RhCl(CO)3(NHC)]... 

Direct C-Harylation of purines in position 8 by diverse aryl iodides has been achieved with Pd catalysis in the presence of Cul and CS2CO3. The methodology is general and efficient and was applied in the consecutive regioselective synthesis of 2,6,8-trisubstituted purines bearing three different C-substituents in combination with two cross-coupling reactions... 

X-Ray crystallography has been commonly used to confirm structures of new 1,2,5-oxadiazoles. X-ray data show that the heterocyclic ring is planar with C2v symmetry. The delocalization of 7i-bonds depends on the C-substituents. 

As we have already mentioned, nucleophilic displacements of ring C-substituents in 1,3,4-oxadiazoles are seldom reported. The reactions occur only for compounds containing very good leaving groups, as shown in Scheme 12 <2001JA6179, 2004MI1343>. 

The stereoselectivity of these cycloaddition reactions is influenced by the steric hindrance of both, the IV- and C -substituent of the nitrone, that is, the selectivity increases as the nitrogen substituent of the nitrone becomes bulkier. As shown in Table 2.17, the highest diastereoselectivity in these reactions was observed with A-benzyl nitrones (479b) and (480). 

Introduction of the azido group into the azine heterocycle can also be accomplished starting with azine derivatives bearing a five-membered heterocycle as a leaving group in this position. Table 8 shows three such conversions where the leaving groups are isoxazolone (entry a), imidazole (entry b), and triazole (entry c) substituents. 

Herndon, W. C. Substituent Effects in Photochemical Cycloaddition Reactions. 46, 141-179... 

There is a continual tendency for the values of cr/ and aR (and other substituent chemical shifts for a series of mono-substituted benzenes in very dilute solution in cyclohexane, carbon tetrachloride or deuteriochloroform were the basis for a redefinition of the aR scale and some amendment of a R values. However, the value for the nitro group was confirmed as 0.15. 

Figure 1.5. Newman projections about the C —bond, showing the possible positions of the phenol group (R) in relation to the C substituents. The rotamer assignments I, II, and III and the corresponding values of the torsion angle Xi are indicated for tyrosine (and its derivatives) of the l configuration.

Iwasa, J., Fujita, T., and Hansch, C. Substituent constants for aliphatic functions obtained from partition coefficients. J. Med. C/tem., 8(2) 150-153, 1965. 

Generally, C-substituents on thianthrenes behave as normal aryl sulfide substituents, and few abnormalities have been noted (see later). The reaction of thianthrene sulfoxides and sulfones, too, are representative of those classes of functional groups. 

Some carbon-13 chemical shifts of benzotriazoles polysubstituted at both N- and C-atoms of the benzotriazole ring are listed in Table 8. They are generally assigned by comparison to literature values and by careful selective decoupling experiments. The contribution of C-substituents to the chemical shifts of benzotriazoles is much large than that of V-substituents. 

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