Selenium—nitrogen bonds reactions with

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>. 

In this chapter, we will review the use of ylides as enantioselective organocata-lysts. Three main types of asymmetric reaction have been achieved using ylides as catalysts, namely epoxidation, aziridination, and cyclopropanation. Each of these will be dealt with in turn. The use of an ylide to achieve these transformations involves the construction of a C-C bond, a three-membered ring, and two new adjacent stereocenters with control of absolute and relative stereochemistry in one step. These are potentially very efficient transformations in the synthetic chemist s arsenal, but they are also challenging ones to control, as we shall see. Sulfur ylides dominate in these types of transformations because they show the best combination of ylide stability [1] with leaving group ability [2] of the onium ion in the intermediate betaine. In addition, the use of nitrogen, selenium and tellurium ylides as catalysts will also be described. 

Addition Reactions with Formation of Carbon-Nitrogen Bonds Addition Reactions with Formation of Carbon-Sulfiir or Carbon-Selenium Bonds Addition Reactions with Formation of Carbon-Halogen Bonds Cleavage Reactions... 

Reaction with selenium at 190-200 °C is not much used now because some double-bond migration also occurs. More often sodium nitrite-nitric acid (a source of nitrogen dioxide) or a sulphur-containing compound (3-mercaptopropionic acid, 2-mercapto-ethanoic acid, 2-mercaptoethylamine, thiophenol, or an arylsulphonic acid) is used. 

In some reactions intramolecular chalcogen nitrogen interactions may lead to stereochemical control. For example, selenenyl bromides react with C=C double bonds, providing a convenient method of introducing various functional groups. The reaction proceeds readily, but affords a racemic mixture. The modified reagent 15.22 contains a chiral amine in close interaction with the selenium atom. It reacts with olefins affording up to 97% ee of isomer A (Scheme 15.2). ... 

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... 

The 4,6-diamino-1,3,5-triaza-2-phosphapentalenes are deep red crystalline solids. For reactions, the 4,6-bis(diethylamino) derivative (99) was used (Scheme 34). The compound is not oxidized by air and does not react with sulfur or selenium. Alkylation takes place at the nitrogen atom adjacent to phosphorus. With diethylamine and alcohols no reaction is observed <86CB3213,87PS(30)780>. On simultaneous oxidation by sulfur or selenium, however, alcohols add to the P=N bond yielding... 

The use of hypervalent iodine reagents for heteroatom-heteroatom bond forming reactions is well established in the context of classical oxidation chemistry [1-11]. For example, oxidations of anilines to azobenzenes, thiols to disulfides, and sulfides to sulfoxides with aryl-A3-iodanes were documented decades ago [1-5]. During the last ten years, particular attention has also been given to oxidative transformations of compounds derived from heavier elements, including the interception of reaction intermediates or initially formed products with external nucleophiles. A second important development is the utilization of sulfonyliminoiodanes, ArI = NS02R, for heteroatom-nitrogen bond formation, especially for imidations of sulfur, selenium, phosphorus and arsenic com-... 

The reaction of cycloalkeno-l,2,3-selenadiazole 183 with a mixture of [Pd2(dba)3] and trialkylphosphine in toluene under reflux for 1 h gave the novel complexes 51 in 36-55% yields (dba = dibenz[ ] anthracene) <2004POL2967, 1998CC1305>. The molecular structure of complex 51 (n = 1, R = Bu) was determined by X-ray crystallography. The proposed mechanism is shown in Scheme 18. Insertion of palladium(O) into the selenium-nitrogen bond of 1,2,3-selenadiazole occurs, followed by 1,3-dipolar addition of a selenaketocarbene 185 formed in situ by thermal elimination of dinitrogen with the elimination of a trialkylphosphine. 

The SRN1 process has proven to be a versatile mechanism for replacing a suitable leaving group by a nucleophile at the ipso position. This reaction affords substitution in nonactivated aromatic (ArX) compounds, with an extensive variety of nucleophiles ( u ) derived from carbon, nitrogen, and oxygen to form new C—C bonds, and from tin, phosphorus, arsenic, antimony, sulfur, selenium, and tellurium to afford new C-heteroatom bonds. 

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