Hypervalent azido- and cyanosilicate derivatives, prepared in situ by the reaction of trimethylsilyl azide or trimethylsilyl cyanide, respectively, with tetrabutylammonium fluoride, are effective sources of nucleophilic azide or cyanide. Primary and secondary alkyl halides and sulfonates undergo rapid and efficient azide or cyanide displacement in the absence of phase transfer catalysts with the silicate derivatives. Application of these reagents to the stereoselective synthesis of glycosyl azide derivatives is reported.

General reaction conditions for the synthesis of aryl(trialkoxy)silanes from aryl Grignard and lithium reagents and tetraalkyl orthosilicates (Si(OR)(4)) have been developed. Ortho-, meta-, and para-substituted bromoarenes underwent efficient metalation and silylation at low temperature to provide aryl siloxanes. Mixed results were obtained with heteroaromatic substrates: 3-bromothiophene, 3-bromo-4-methoxypyridine, 5-bromoindole, and N-methyl-5-bromoindole underwent silylation in good yield, whereas a low yield of siloxane was obtained from 2-bromofuran, and 2-bromopyridine failed to give silylated product. The synthesis of siloxanes via organolithium and magnesium reagents was limited by the formation of di- and triarylated silanes (Ar(2)Si(OR)(2) and Ar(3)SiOR, respectively) and dehalogenated (Ar-H) byproducts. Silylation at low temperature gave predominantly monoaryl siloxanes, without requiring a large excess of the electrophile. Optimal reaction conditions for the synthesis of siloxanes from aryl Grignard reagents entailed addition of arylmagnesium reagents to 3 equiv of tetraethyl- or tetramethyl orthosilicate at -30 degrees C in THF. Aryllithium species were silylated using 1.5 equiv of tetraethyl- or tetramethyl orthosilicate at -78 degrees C in ether.

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Cohalogenation of alkenes constitutes one of the most important classes of reaction used to form a carbon heteroatom bond in a regio-, chemo-, and stereoselective manner.1 Since the pioneering work of Hassner 2 considerable attention has been given to the haloazidation of the alkenic double bond by using bromine azide 4a or iodine azide 4b as active reagent.3 This method constitutes a very useful procedure for introducing a nitrogen functionality into a carbon skeleton, leading to vinyl azides, 4 amines, 5 and heterocycles, 6 particularly aziridines. 7In continuation of studies devoted to ligand transfer reactions from iodine(III) onto halide ions, 8 we investigated the use of the azide group as a mobile ligand. This method would create haloazide-like species under much milder conditions, namely, in an organic solvent, than commonly applied. A two-phase system by the interaction of Br 2 or NBS with NaN 3 in the presence of acid 7,9 is often required for the preparation of bromine azide. Alternatively, the reagent system NBS/TMSN 3 in DME/H 2 O has been developed. 10 Iodine azide has been generated from sodium azide and iodine chloride in polar solvents. 11However, as a result of its explosive character, its use has often been hampered.Thus, (diacetoxyiodo)benzene (1) was reacted with tetraethylammonium bromide (2a) in dichloromethane at room temperature and presumably gave tetraethylammonium [di(acyloxy)bromate (I)] (3a) (Scheme 1). 12Treatment of this solution with TMSN 3 followed by addition of alkenes 7-11 led to the corresponding bromoazidation products 12, 14, 16, 18, and 19 (Table 1). From these observations it is reasonable to assume that either tetraethylammonium [bis(azido)bromate (I)] (6a) or bromine azide 4a is formed under these conditions (Table 1). When tetraethylammonium iodide (2b) was employed, the corresponding 1,2-iodo azides 13, 15, and 17 were generated instead, again presumably via the iodate(I) † Present address: Department of Chemistry, Jahangivnagar University, Savar, Dhakar 1342, Bangladesh.(1) Reviews: (a) Block, E.; Schwan, A. L. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Semmelhack, M. F., Eds.; Pergamon Press: Oxford, 1991; Vol. 4, 329-362. (b) (6) For the construction of tetrazoles via the "Hassner-Ritter" reaction see: (a) Ranganathan, S.; Ranganathan, D.; Mehrotra, A. K. Tetrahedron Lett. 1973, 14, 2265-2268 Devaprabhakara, D. Tetrahedron Lett. 1975, 16, 257-260. (7) (a) Van Ende, D.; Krief, A. Angew. Chem. 1974, 86, 311-312; Angew. Chem., Int. Ed. Engl. 1974, 13, 279-280. (b) (12) Szá ntay, C.; Blaskó, G.; Bá rczai-Beke, M.; Péchy, P.; Dörnei, G. Tetrahedron Lett. 1980, 21, 3509-3512. (13) For recent studies on dialkyl and diphenyl halogen-ate complexes refer to: (a) Schulze, V.; Brönstrup, M.; Böhm, V. P. W.; Schwerdtfeger, P.; Schimeczek, M.; Hoffmann, R. W.

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