Selenyl halides

Terminal alkenes react with selenenyl halides with Markovnikov regioselectivity.64 However, the (J-selenyl halide addition products readily rearrange to the isomeric products.65... 

The reaction of primary amines with arenesulfenyl halides leads to the formation of sulfonamides (12). ° These compounds are most stable when the aryl group has electron-withdrawing substituents at Ae 2- and 4-positions. Selenamides are formed in an analogous manner but are somewhat less stable aliphatic amines give isolable compounds, but most anilines react with selenyl halides to give products of ring substitution. An example of an isolable selenamide is compound (13), which was prepared (79%) from the amine and 2-nitrobenzeneselenyl chloride. This compound was used as an intermediate in the preparation of 7a-methoxycephalosporins. 

Phenylsclenenyl triflate shows highly electrophilic character since it reacts even at — 78 °C with unsaturated carboxylic acids to afford the corresponding lactones. In some cases, the reaction of selenyl halides in the presence of the nucleophilie halide anion causes unfavorable side reactions, such as addition of the halogen atom, and decreased stereoselectivity. On the other hand, reactions of Ar-(phenylseleno)imides, which are not effected by the nucleophile, require a strong acid catalyst because of their lower reactivity. 

Only S and Se are known to form well-defined oxo halides. These are of the three main types (a) the thionyl and selenyl halides, SOX2 and SeOX2 (b) the sulfuryl halides, SOzX2, and their one selenium analog, Se02F2 and (c) a number of more complex sulfur oxo chlorides and oxo fluorides. 

The thionyl and selenyl halides are stable in a vacuum at ordinary temperatures and below, but when strongly heated they decompose, usually to a mixture of products including the dioxide, free halogen, and lower halides. The structures are pyramidal (Fig. 15-13), the S and Se atoms using sets of, roughly, sp3 hybrid orbitals, one of which holds the unshared pair. The S—O bonds in the thionyl halides are evidently resonance hybrids of the canonical structures (15-XIIa, b, c). In b and c the multiple bonding results s—6 s=o <— - s=o... 

The thionyl and selenyl halides can function as weak Lewis bases, using lone pairs on oxygen and also, more surprisingly, as weak Lewis acids, using vacant d orbitals. The structure of the compound SeOCl2 2py is shown in Fig. 15-13(b). 

The transformation of saturated ketones into a,/8-unsaturated ketones is often carried out by the introduction of an a-seleno-substituent, followed by oxidation and elimination. An alternative method for selenation of ketones (and esters) has been described wherein the lithium enolate reacts with selenium metal (rather than a selenyl halide) followed by methyl iodide (Scheme 29). Yields are comparable to the classical method. Phenylselenyl chloride reacts with enamines derived from aldehydes to give a-phenylselenoaldehydes, and hence o(,/3-unsaturated aldehydes by oxidative elimination [equation (21)]. ... 

Palladium pincer complex 115-catalyzed selenylation of propargyl, allyl, benzyl, and benzoyl halides could be achieved under mild reaction conditions employing trimethylstannylphenylselenide 116 as selenylating agent <2005JOC9215>. 

If one of the reactions in a radical chain sequence is too slow to compete effectively with radical-radical reactions, the chain will collapse. Slow reactions of simple silanes such as Et3SiH with alkyl radicals precludes their use in the tin hydride method. Although quite reactive with alkyl radicals, thiols and selenols fail in the tin hydride method because the thiyl and selenyl radicals do not react rapidly with organic halide precursors. Nonetheless, it is possible to use thiols and selenols in tin hydride sequences when a Group 14 hydride is used as a sacrificial reducing agent. The thiyl or selenyl radical reacts with the silane or stannane rapidly, and the silicon- or tin-centered radical thus formed reacts rapidly with the organic halide [8], In practice, benzeneselenol in catalytic amounts has been used in radical clock studies where BusSnH served as the sacrificial reductant [9]. 

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