Selenium nitrogen—sulfur bonds

Furthermore, the strongly metallic character of selenium weakens the C-Se bond and thus favors reactions involving opening of the ring. The basicity of the three heterocycles is approximately in the same order, the nitrogen atom of selenazole and thiazole possessing much the same properties as the heteroatom of pyridine. Of the two carbon atoms ortho to nitrogen, that is, the 2-carbon and the 4-carbon, only the one in the 2-position is fairly active as a result of its interaction with selenium or sulfur. The 4- and 5-positions of thiazole and selenazole are more susceptible to electrophilic substitution than the 3- and 5-positions of pyridine. This is particularly true of the 5-position of selenazole. Thus it can be said that the 2- and 5-positions of the selenazoles and thiazoles... 

This approach is used exclusively for the synthesis of the isomeric annelated isothiazoles and isoselenazoles to form the nitrogen-sulfur(selenium) bond and has been discussed in CHEC-II(1996) <1996CHEC-II(7)49>. A wide variety of oxidative cyclizations have been reported, some of which incorpotate an amination. Eor example, reaction of 47 with chloramine gives the corresponding isothiazolo[5,4-7]thiophene 40 (Equation 9) <2000P65>. 

Complexes Containing Multiple Bonds to Sulfur, Selenium, Nitrogen and Carbon Atoms 979... 

Cleavage of a carbon to sulfur bond by a lithium reagent is a well known entry to organolithium reagents. This process has been used to synthesize a-nitrogen carbanions, and hence SMA derivatives.67 165 This study has been extended to tellurium and selenium starting materials as well.166... 

Metallaheterocycles with the metal atom capable of supporting a carbon-metal double bond, and with oxygen, nitrogen, sulfur, or selenium as the second heteroatom are the only structures capable of full conjugation. These compounds show furan-like aromaticity with the heteroatom /i-electrons participating. No examples of this type of ring system were uncovered, undoubtedly due to the stability of the metalla-carbon double bonds required for their formation. 

Identify the most common number of covalent bonds and lone pairs for the atoms of each of the following elements hydrogen, the halogens (group 17), oxygen, sulfur, selenium, nitrogen, phosphorus, and carbon. 

Sulfur-nitrogen, sulfur-phosphorus, and sulfur-selenium 2three-electron bonded systems are further examples in this series of heteroatom-centered radical ions. In principle, the same trends and considerations apply as have been outlined for all the S S and S X species mentioned so far. Therefore, only a few additional remarks will be made here. 

The accessibility of the +4 and +6 oxidation states for sulfur and, to a lesser extent, selenium gives rise to both acyclic and cyclic molecules that have no parallels in N-O chemistry. Thus there is an extensive chemistry of chalcogen diimides RN=E=NR (E = S, Se, Te) (Section 10.4). In the case of Te these unsaturated molecules form dimeric structures reflecting the increasing reluctance for the heavier chalcogens to form multiple bonds to nitrogen. The acyclic molecule N=Sp3,... 

---