Exocyclic substituents

Discussion of these compounds is divided into isomers of aromatic compounds, and dihydro and tetrahydro derivatives. The isomers of aromatic azoles are a relatively little-studied class of compounds. Dihydro and tetrahydro derivatives with two heteroatoms are quite well-studied, but such compounds become more obscure and elusive as the number of heteroatoms increases. Thus dihydrotriazoles are rare dihydrotetrazoles and tetrahydro-triazoles and -tetrazoles are unknown unless they contain doubly bonded exocyclic substituents. 

Nitration of a substituted quinoxaline usually occurs in the homocyclic ring and occasionally in an exocyclic substituent such as a phenyl group. The actual position(s) of nitration can seldom be forecast with any confidence analogy with reported nitrations is still the best guide. Accordingly, the following examples are classified, not by nitration sites but by the reagents used. 

Despite the numerous reports concerning NHC-Ru olefin metathesis initiators, a complex incorporating a carbene that has only one exocyclic substituent adjacent to the carbenic centre was not reported until 2008. Studies by Grubbs and co-workers led to the development of ruthenium-based catalysts bearing such carbene ligands, in this case incorporating thiazole-2-ylidenes [63] (Fig. 3.19). 

In addihon to this so-called 1,3-interaction between an exocyclic substituent and the hydrogens in 3-position, steric interactions can cause extra strain also in 1,2-disubstituted rings as the tra s-l,2-dimethyl-cyclohexane in Fig. 7.10(c). The... 

The oxidative addition of one equivalent of X2 (X=C1, F) to S4N4 under mild conditions produces 1,5-S4N4X2.106 The structure of 1,5-S4N4C12 (32) consists of a folded eight-membered ring [d(S- -S) = 2.45 A] with the exocyclic substituents in exo,endo positions. The reaction of 32 with (Me3SiN)2S is the best route to the binary sulfur nitride S5N6 (11).42... 

Aromatic molecules containing freely rotating substituents usually tend to fluoresce less intensely than those without these substituents. This results from the introduction into each electronic state of rotational and vibrational substates by the exocyclic substituents. At very low temperatures, the quantum yields of... 

The p-carbon signals in heterocyclohexanes 52 (143) respond to the electronegativity of endocyclic substituents similarly to those of carbon atoms in p positions with respect to exocyclic substituents. The slopes of chemical shift vs. electronegativity plots for the p-methylene groups in 52 and 53 are negative (Table 10). For the quaternary P-carbon atom in 53, however, a slight positive slope is observed (164). 

Thus, in the endocyclic case, interaction effects can at best be estimated, whereas if exocyclic substituents are involved and the molecular framework is conformationally rigid, a reliable quantitative measure is obtainable. 

This structiire may be compared with those of the recently reported isoelectronic carbon analogues RC(NSH)2CR(13, R=Me2N,Ph). In the carbon compounds the effect of the exocyclic substituent on ring conformation is remarkable. Thus the phenyl derivative is completely planar and has structural parameters consistent with a 10 TT-electron system while the dimethylamino derivative has a folded structiire with a transannular S-S distance of 2.I38 (Figure 3). 

Spherical polar coordinates are used for conformational representation of pyranose rings in the C-P system. Unlike the free pseudorotation of cyclopentane, the stable conformations of cyclohexane conformers are in deeper energy wells. Even simong the (less stable) equatorial (6 = 90 ) forms, pseudorotation is somewhat hindered. Substitutions of heteroatoms in the ring and additions of hydroxylic or other exocyclic substituents further stabilize or destabilize other conformers compared to cyclohexane. A conformational analysis of an iduronate ring has been reported based on variation of < ) and 0 (28), and a study of the glucopyranose ring... 

Isoelectronic carbon analogs of this 8-membered ring RC(NSN)2CR(R = Ph, MCjN) have been reported recently and their structures are remarkably dependent on the nature of the exocyclic substituent Thus the phenyl derivative is completely planar whereas the Me2N compound has a folded structure with a trans-annular S—S distance of 243 pm (see Sects. 3, 3.2, 3.2.2). 

Appreciable ort/io-lithiation was observed when the exocyclic substituent was a phenyl group, and complete dilithiation could be achieved by using three equivalents of base (Scheme 90). 

This is attributed to the unfavourable steric interactions which arise in the transition state that is required for antiperiplanar migration of the exocyclic substituent.143 Some examples of synthesis of alcohols by hydroboration-oxidation are included in Scheme 4.8. More vigorous oxidizing agents such as Cr(VT) reagents effect replacement of boron... 

Ph, R — COPh, R = H) and dimethyl acetylenedicarboxylate. In this case the exocyclic substituent (19, f) is NCOPh and the transformation involves the elimination of iV-benzoyl-iV -phenylcarbodiimide, Ph-CON=C=NPh, from the intermediate cycloadduct. 

Representatives of this novel class of meso-ionic compounds in which the exocyclic substituent f (see Table I) is a stabilized carbanionoid residue [-C(CN)C02Me or -C(CN)2l have been recently synthesized. Base-catalyzed (potassium carbonate or triethylamine) condensation of AT-acylhydrazines (147) with 3,3-dichloroacrylonitriles (165) yield the greenish-yellow meso-ionic l,3,4-oxadiazol-2-enes (164) directly. 

R> = Me, R = R3 = Ph, K = H, R = N=CHAr), which with ammonia in chloroform solution give novel derivatives (224) of meso-ionic l,2,4-triazol-3-imines (216). These compounds are of interest in that they belong to a new type of meso-ionic heterocycle in which the exocyclic substituent f (see Table I) is a stabilized carbanionoid residue,... 

A third feature which stimulates interest in cyclophosphazenes is their behaviour as ligands towards transition metals [7,8]. Several types of coordination pattern involving either the skeletal ring nitrogen atoms and/or suitable exocyclic substituent donor atoms have been documented (Fig. 2). This area of research has been receiving widespread attention in recent years [7],... 

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