Selenonium ions

Trialkyl cations of oxygen, sulfur, and selenenium (and less commonly tellurium) are onium ions by virtue of their having one extra valence, although these three-coordinate ions invite comparison with carbenium ions. These ions (e.g., 28, 29, and 30) are not planar (as opposed to the analogous carbenium ions), but rather are pyramidal. These species have a well-characterized solution acid chemistry concentrated sulfuric acid is required for observation of the trimethyloxonium cation, but the sulfonium and selenonium ions require less strongly acidic solutions. 

The parent hydrotelluronium ion H3Te+ could not be observed in superacid solution of hydrogen telluride, under conditions where the selenonium ion is observed. 

Various organoselenium compounds are able to add in stereospecific anti fashion to alkenes282 [Eq. (4.85)]283 or alkynes282 [Eq. (4.86)] to form stable cyclic selenonium ions. 

Tanzer, D. and K.G. Heumann. 1990. GC determination of dimethyl selenide and trimethyl selenonium ions in aquatic systems using element specific detection. Atmos. Environ. 24 3099-3102. 

Larsen, E.H., Hansen, M., Fan, T., and Vahl, M. 2001. Speciation of selenoamino acids, selenonium ions and inorganic selenium ion by exchange HPLC with mass spectrometric detection and its application to yeast and algae. J. Anal. At. Spectrom. 16(12), 1403-1408. 

Selenonium ions, selenides, selenuranes, and other charged species As expected, alkylation, alkenylation, and arylation of selenides to selenonium salts leads to selenium deshieldings, compare for example (CH3)3Se I , S = 258 relative to S = 0 for (CI 13)280. The magnitude of the deshielding can reach upto ca. 180 250 ppm and depends on the substituent introduced as well as on those already present. However, it seems to be smaller for... 

Due to a strong preference for exo attack, indirect methods are required to install endo substituents. For example, addition reactions which proceed through bridged cationic intermediates cause other nucleophilic species to add from the endo face. Hence, endo halide substitution follows bromonium and selenonium ion formation. A striking example of this phenomenon is the inter-... 

Yet, as the following analysis shows, there was reason to believe that the process might occur in the desired manner. The first point to consider is the n facial preference for selenonium ion formation. In this case, addition to 95 should occur syn to the ring junction hydrogen in order to assure formation of the significantly more stable cis-fused [3.3.0] subunit 112 [14] addition to the opposite face would afford the trans-fused counterpart 113. 

There are very few examples in the literature of fully saturated bicyclic selenonium ions with selenium as the bridgehead atom. The selenonium salts (33) and (34) are used as precursors for the syntheses of medium sized heterocycles containing a selenium atom. 

The synthetic strategy used for bicyclic sulfonium ions shown earlier in the chapter in Scheme 2 is also utilized for bicyclic selenonium ions. As shown in Scheme 10, both 6-6 and 6-5 bicyclic systems can be constructed in good yields <90TL3027>. 

Selenium, bis(diethyldithiocarbamato)-stereochemistry, 60 Selenium, bis(dithiofurancarbamato)-stereochemistry, 60 Selenium(II) complexes bis(dithiochelate), 60 Selenocyanate complexes linkage isomers, 186 Selenonium ions, trifluoro-stereochemistry, 37 Self-exchange reactions, 333 electron transfer rate constants, 347-353, 366 rate constants calculation, 348 rate constants, 362 Semicarbazide, 1,4-diphenylthio-ruthenium determination, 546 Semi-glycinecresol red metallochromic indicator, 557 Semi-xylenol orange metallochromic indicator, 557 Seven-coordinate compounds stereochemistry, 69-83 Sidgwick, Nevil Vincent, 16 Silicon, tris(acetylacetone)-configuration, 195... 

These observations appear to hold for all known alkylaluminum-based liquid clathrates. For example, a selenonium ion-based liquid clathrate having the inclusion formula [(CH3)3Se] [C1A1(CH3)2(C1)A1(CH3)3] (aromatic solvent), can accommodate 8.5 benzene molecules or 8.3 toluene molecules per parent compound. 

Assume that the following reaction proceeds through a bridged selenonium ion. 

What Lewis structure best represents the bridged selenonium ion ... 

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