Sulfur-aromatic interactions

Fig. 15. A pair of sulfur-aromatic interactions in y-crystallin. The Sy atom is within van der Waals contact of the ring hydrogen atom of Crj2 of tryptophan-68 and the ring hydrogen atom of C81 of phenylalanine-5.

Evidence for a Strong Sulfur-Aromatic Interaction Derived from Crystallographic Data. 

A different approach consists in changing the sulfur-driving interaction (the principle of steric hindrance), to an aromatic-driver interaction such as that present in the nitrogen containing compounds. However in this case, for this concept to work, the hydrogenation-dependence has to be avoided, and hydrogenolysis surface reactivity has to be enhanced. 

Recently Bogolytsin and co-authors (62) found that the associations of sulfur dioxide and other inorganic sulfur derivatives interact with the aromatic nuclei of lignins, and strongly influence their reaction and redox behavior. This association effect also forms a selective pre-association state in the sulfonation reaction in wood pulping. It is evident that a wide range of similar association effects may be present, but they remain to be detected and studied. 

Another series of examples of the efficient combination of several structural effects is demonstrated by Yamashita and coworkers [797, 799-801]. Figure 126 shows different monomers used to prepare the corresponding polymers. In this case, planarization, sulfur-nitrogen interactions, and a combination of different aromatic ring systems is used to obtain polymeric materials with bandgaps down to 0.5 eV, the lowest values reported so far. 

The vertical ionization potentials from the photoelectron spectra of some thiirane and thiirene derivatives are given in Table 3. A Walsh localized scheme of bonding is generally preferred. There is a strong hyperconjugative interaction in thiirene 1,1-dioxides between the occupied C=C tt-MO and the occupied SO2 pure sulfur d-AO. Thiirene oxides are suggested to be less aromatic than cyclopropenones and tropone. 

The nonpolar amino acids (Figure 4.3a) include all those with alkyl chain R groups (alanine, valine, leucine, and isoleucine), as well as proline (with its unusual cyclic structure), methionine (one of the two sulfur-containing amino acids), and two aromatic amino acids, phenylalanine and tryptophan. Tryptophan is sometimes considered a borderline member of this group because it can interact favorably with water via the N-H moiety of the indole ring. Proline, strictly speaking, is not an amino acid but rather an a-imino acid. 

Type G syntheses are typified by the 1,3-dipolar cycloaddition reactions of nitrile sulfides with nitriles. Nitrile sulfides are reactive 1,3-dipoles and they are prepared as intermediates by the thermolysis of 5-substituted-l,3,4-oxathiazol-2-ones 102. The use of nitriles as dipolarophiles has resulted in a general method for the synthesis of 3,5-disubstituted-l,2,4-thiadiazoles 103 (Scheme 11). The thermolysis is performed at 190°C with an excess of the nitrile. The yields are moderate, but are satisfactory when aromatic nitrile sulfides interact with electrophilic nitriles. A common side reaction results from the decomposition of the nitrile sulfide to give a nitrile and sulfur. This nitrile then reacts with the nitrile sulfide to yield symmetrical 1,2,4-thiadiazoles <2004HOU277>. Excellent yields have been obtained when tosyl cyanide has been used as the acceptor molecule <1993JHC357>. 

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