Seleno reactivity

Tellurocarbonyl derivatives R C(=Te)OR and telluroamides, e.g. PhC(= Te)NMe2 (mp 73°) have been prepared and shown to be similar to, though more reactive than, the corresponding seleno derivatives. 

The 5-position was the most reactive in bromination of 1,3-disubstituted l//-thieno- and -seleno-[3,2-c/]pyrazoles (59), whereas the isomeric 2H-[3,2-c] derivatives (60) reacted at the 6-position [73JOU2216 77CS( 12)1 ]. 

Q -Phenylthiomethyl-jS-hydroxy esters (50) can be prepared, predominantly as the iyn-isomer, by a stereoselective one-pot Michael-aldol tandem reaction. The seleno analogue can similarly be prepared (again, mainly syn), using PhSeLi in diethyl ether, but phenoxide ion is not sufficiently reactive for this sequence. 

This unusual reactivity was subsequently extended to a broad variety of allylic and propargylic amines, in combination with other ruthenium(II) metal fragments, allowing the isolation of aminoaUenylidene complexes 33-35 [47 9] (Fig. 6). In addition, the S3mthesis of the thio-allenylidene [44] and seleno-aUenylidene [50] derivatives 36 from the in situ generated butatrienylidene tra s-[RuCl (=C=C=C=CH2)(L2)2] (L2 = dppm, dppe) and the corresponding aUyl sulfides... 

Only a few electrophilic reactions of selenolopyridines have been reported. In deuteriodeprotonation of selenolo[3,2-6]pyridine the 3-position is the preferred site of attack (78JCS(P2)86l>. In selenolo[3,2-6]pyridine and thieno[3,2-6]pyridine the C-2/C-3 reactivity ratio is ca. 10-3 whereas for furo[3,2-6]pyridine a value of ca. 10-5 has been determined. Logarithmic partial rate factors (Figure 13) show that seleno o[3,2-6 ]pyridine is the most reactive compound. As in the case of (9) and (261), the deuteriodeprotonation of (426) takes place on the protonated species. Both se enolo[2,3-6]- and [3,2-6]pyridine (423, 426) on treatment with potassium nitrate and concentrated sulfuric acid give yield the corresponding 3-nitro derivatives in 50% yield (10 °C, 3 h). 

Iron(III) salts, and Grignard reagent reactivity, 9, 49 Iron seleno-terephthalates, for fungus and molds, 12, 458 Iron silicides, formation, 6, 19... 

The hydrolysis of seleno- and telluropyrylium dyes involves nucleophilic addition of water to the substrate. a,f3-LJnsaturated selones and -tellones 60 are intermediates in this reaction (Equation 22) <1997JOC4692, 1998JOC5716>. Diketones are the primary hydrolysis products isolated in high yields from these reactions. The tellurium derivative hydrolyzes most rapidly over the pH range 3-12, with the sulfur analogue least reactive under these conditions. Seleno- and telluropyrylium dyes are important both for their biological activities and potential industrial applications (see Section 7.11.8). 

Me3SiCl, it is possible to record the initial stereospecificity of the Se-Li exchange - at least 96% retention. Proof that this is the result of stereospecificity and not stereoselectivity comes from related reactions which showed that, in diselenoacetals, attack of BuLi at the axial heteroatom is preferred. These experiments also provided evidence that, though a seleno-ate complex is probably an intermediate in the sequence, it is not the final reactive species itself, since the ratios of axial and equatorial products were not dependent on the nature of the starting selenide (SePh or SeMe) - i.e. the selenium is lost before the stereoselective step. 

Alternative reaction pathways exploring different synthetic possibilities have been studied. For instance, electron-rich dihydroazines also react with isocyanides in the presence of an electrophile, generating reactive iminium species that can then be trapped by the isocyanide. In this case, coordination of the electrophile with the isocyanide must be kinetically bypassed or reversible, to enable productive processes. Examples of this chemistry include the hydro-, halo- and seleno-carba-moylation of the DHPs 270, as well as analogous reactions of cyclic enol ethers (Scheme 42a) [223, 224]. p-Toluenesulfonic acid (as proton source), bromine and phenylselenyl chloride have reacted as electrophilic inputs, with DHPs and isocyanides to prepare the corresponding a-carbamoyl-(3-substituted tetrahydro-pyridines 272-274 (Scheme 42b). Wanner has recently, implemented a related and useful process that exploits M-silyl DHPs (275) to promote interesting MCRs. These substrates are reacted with a carboxylic acid and an isocyanide in an Ugi-Reissert-type reaction, that forms the polysubstituted tetrahydropyridines 276 with good diasteroselectivity (Scheme 42c) [225]. The mechanism involves initial protiodesilylation to form the dihydropyridinum salt S, which is then attacked by the isocyanide, en route to the final adducts. 

Addition of phenylethynethiolate or -selenate to the carbene complex 185 (M = Cr or W, R = Me or Et, E = S or Se) affords an anionic adduct (186) which, unexpectedly, contains a reactive thioketene function. Electrophiles can attack either at the sulfur or at the carbon a thereto to produce coordinated carbenes or thio- and seleno aldehydes and esters [H4J45). 

Stick, R V, Tilbrook, D M G, Williams, S J, The selective activation of telluro- over seleno-(i-D-glucopyranosides as glycosyl donors a reactivity scale for various telluro, seleno and thio sugars, Aust. J. Chem., 50, 237-240, 1997. 

Reactivity cf l,l-bis(seleno)-l-alkyl metals and a-selenocctrbonyl compounds with enals and enones... 

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