Diphenylselenium dibromide

Both diorganotellurium(IV) and diorganoselenium(IV) dibromides are known, stable compounds, which permits a direct comparison of the selenium(IV) and tellurium(IV) compounds. Di-4-chlorophenyltellurium(IV) dibromide (36) and one equivalent of pyridine were essentially unreactive with respect to bromination of either 4-pentenoic acid or 4-pentenol. With either substrate, 36 gives only 2-3% conversion to brominated products after several days of reaction (Fig. 17). In contrast, diphenylselenium(IV) dibromide (1, Fig. 1) and 2-(dimethylaminomethyl)-phenyl phenyl bromoselenonium bromide (37) gave essentially complete bromination of either 4-pentenoic acid or 4-pentenol in 1 h in the presence of one equivalent of pyridine as shown in Fig. 17. 

Fig. 17 Bromination of 4-pentenoic acid and 4-penten-l-ol with bromine/pyridine and with di-4-chlorophenyltellurium(IV) dibromide (36), diphenylselenium(IV) dibromide (1), and A, A -dimethyl-2-(aminomethyl)phenyl phenyl bromoselenonium bromide (37) and one equivalent of pyridine.

The lO-E-4 chalcogen(IV) species diphenylselenium(IV) dibromide (1, Fig. 1) and diphenyltellurium(IV) dibromide (2, Fig. 1) oxidize thiophenol to diphenyl disulfide in nearly quantitative yield as shown in equations (13) and (14). Tellurium(IV) dihalides 6-11 also oxidize thiophenol to diphenyl disulfide and benzene selenol to diphenyl diselenide. Similarly, the 12-Te-5 molecule dioxatellurapentalene 45 (Fig. 19) is a mild oxidant for ethylmercaptan, thiophenol, and benzene selenol giving diethyl disulfide, diphenyl disulfide, and diphenyl diselenide in essentially quantitative yield. As shown in equation (15), 1,1,5,5,9,9-hexachloro-1,5,9-tritelluracyclododecane oxidizes six molecules of thiophenol to diphenyl disulfide and 1,5,9-tritelluracyclododecane in 90% yield. In contrast, 12-Te-5 pertellurane 44 and 12-Se-5 perselenane 46 do not oxidize thiophenol to diphenyl disulfide. Instead, these molecules undergo a nucleophilic addition of thiophenol followed by cleavage of the tellurium-carbon or selenium-carbon bond. ... 

The corresponding selenium(IV) derivatives are less stabilized by the three-center, four-electron bonds in the chalcogen(rV) complex than the tellurium(lV) derivatives. As a consequence, the selenium(lV) derivative is more easily reduced than the tellurium(lV) derivative. Diphenylselenium(lV) dibromide (1) undergoes a two-electron reduction with Ep at - - 0.40 V (vs. SCE) while diphenyltellurium(lV) dibromide (2) undergoes a two-electron reduction with at-1-0.05 V (vs. SCE). ... 

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