Chalcogen protonation

There are several systematic nuclear magnetic resonance studies of the interaction between the substituents and the protons and ring atoms of five-membered heterocycles. In some 2-substituted furans, thiophenes, selenophenes, and tellurophenes there is a linear correlation between the electronegativity of the chalcogen and several of the NMR parameters.28 As there also is a good correlation between the shifts of the corresponding protons and carbons in the four heterocycles, the shifts of unknown selenophene and tellurophene derivatives can be predicted when those of thiophene are known. This is of special interest for the tellurophene derivatives, since they are difficult to synthesize. In the selenophene series, where a representative set of substituents can be introduced in the 2- as well as in the 3-position, the correlation between the H and 13C shifts and the reactivity parameters according to Swain and Lupton s two-parameter equation... 

Hence, chalcogenoamidophosphinic acids of general formula R2P(E)NHR (27) can be considered as dichalcogenophosphinic acids with one of the chalcogen atoms replaced by an amido group. These amido species are still normally referred to as acids due to their acidic amide protons. They are accessible from reaction of thiophosphinic chlorides and primary amines (Equation 43) or from reaction of aminophosphines with elemental chalcogen... 

Chalcogenic acids, R E(0)0H, are also tricoordinate and considered to have pyramidal structures. However, no studies on their optical activity have been reported since facile racemization of chalcogenic acids may occur via achiral chalcogenate anions, which are formed by the extrusion of a proton and/or by an intra- or intermolecular proton transfer reaction. 

The NMR spectra of the selenolopyridines (423-426) (Table 13) reveal effects which have already been observed for selenophene (B-73MI31704). Compared with furopyridines (Table 4) and thienopyridines (Table 11) the chemical shifts (8) of the protons in the a-position to the chalcogen atom are found in the order S < O < Se whereas for the /3-protons the order O < S < Se is observed. The protons H-7 in the fused [3,2-6]pyridines (9,261,426)... 

When coordinated to the metal in the rf form heteroaidehyde ligands react readily with electrophiles. As expected, carbon electrophiles add to the chalcogen atom. Very likely, the site of the initial attack of protons is also the chalcogen atom. However, rapid succeeding rearrangement and addition of an anion usually lead to chalcogenolato complexes. 

Enium Ions of Other Group 16 Elements. Furukawaetal.316,320 have obtained enium ions of the heavier chalcogen elements stabilized by intramolecular complexation with dimethylamino groups (322). Resonances of the benzylic and methyl protons in the 1H NMR spectrum of cation 322a are shifted downfield... 

A theoretical study of the acidity (proton loss) of the porphyrin 266 and some related systems was carried out by means of the B3LYP/6-31+G(d,p) computational level. These include, on one hand, porphyrin fragments 267 and pyrrole 11 (X = H) and the monoanion and dianion of 266 and, on the other hand, chalcogen-related porphyrins 268,269, and 270 and their anions, as well as cyclic nitrogen derivatives 271, 272, 273, and 274, simplified models of porphyrin, dibenzo[14]annulene, phthalocyanine, and porphycene. 

The H NMR spectra of various 1,3-dichalcogenolylium ions of structure (38) are compiled in Table 10. The chemical shifts of all ring protons are quite sensitive to the adjoining chalcogen atoms. The chemical shift of the proton attached to C-2 is 511.43 in 1,3-dithiolylium fluoroborate (75TL1259) and is 515.00 in 1,3-ditellurolylium fluoroborate (82TL1531), while the protons attached to C-4 and C-5 resonate at 59.45 in 1,3-dithiolylium fluoroborate and at 511.81 in 1,3-ditellurolylium fluoroborate. 

If four molar equivalents of LDA are used in the metallation [48], all four protons of TTF can be replaced (19). Numerous products can be prepared from this intermediate [48—52] (Scheme 6). This is the preferred route for preparing tetrahalotetrathiafulvalenes pathways involving electrophilic aromatic substitution are not practical in general. Chalcogen insertion (Se, Te) offers a good pathway to mixed chalcogen donors. For... 

Reactions of Ru3(CO)i2 in alkaline solution with chalcogen compounds led to H2Ru3(CO)9X (X = S, Se, Te) derivatives " the synthesis of H2Ru3(CO)9S was also achieved by protonation of FlRu3(CO)ioSEt with concentrated FI2SO4 cluster cations are formed first, then dilution with water leads to the neutral complex. " ... 

Variable temperature NMR studies have been extensively used to examine conformational changes to the rings, and are discussed in Section 9.33.4. Since nearly all papers use proton and carbon spectra to characterize the compounds discussed above, relatively little data will be given here, only that which is influenced by the chalcogen or is unusual. 

Reductive Metalation. The powerful reductive nature of this reagent makes it an important tool for lithium-heteroatom exchange reactions. Thus, it was established early on that (phenylthio)alkanes can be converted into their requisite alkyl-lithium species. This has become the method of choice over generation by lithium metal alone. The resultant alkyllithium species can either be quenched with a proton source (eq 1), or intercepted with an electrophile. This has subsequently evolved into a powerful technique, since the reaction is general for all chalcogens (eq 2f and halides (eq 3). ... 

---