Synthesis and the Thermal and Catalytic Dehydrogenation Reactions of Amine-Thioboranes

A series of trimethylamine-thioborane adducts, Me3N·BH2SR (R = tBu [2a], nBu [2b], iPr [2c], Ph [2d], C6F5 [2e]) have been prepared and characterized. Attempts to access secondary and primary amine adducts of thioboranes via amine-exchange reactions involving these species proved unsuccessful, with the thiolate moiety shown to be vulnerable to displacement by free amine. However, treatment of the arylthioboranes, [BH2–SPh]3 (9) and C6F5SBH2·SMe2 (10) with Me2NH and iPr2NH successfully yielded the adducts Me2NH·BH2SR (R = Ph [11a], C6F5 [12a]) and iPr2NH·BH2SR (R = Ph [11b], C6F5 [12b]) in high yield. These adducts were also shown to be accessible via thermally induced hydrothiolation of the aminoboranes Me2NBH2, derived from the cyclic dimer [Me2N-BH2]2 (13), and iPr2NBH2 (14), respectively. Attempts to prepare the aliphatic thiolate substituted adducts R2NH·BH2SR′ (R = Me, iPr; R′ = tBu, nBu, iPr) via this method, however, proved unsuccessful, with the temperatures required to facilitate hydrothiolation also inducing thermal dehydrogenation of the amine-thioborane products to form aminothioboranes, R2NBH­(SR′). Thermal and catalytic dehydrogenation of the targeted amine-thioboranes, 11a/11b and 12a/12b were also investigated. Adducts 11b and 12b were cleanly dehydrogenated to yield iPr2NBH­(SPh) (22) and iPr2NBH­(SC6F5) (23), respectively, at 100 °C (18 h, toluene), with dehydrogenation also possible at 20 °C (42 h, toluene) with a 2 mol % loading of [Rh­(μ-Cl)­cod]2 in the case of the former species. Similar studies with adduct 11a evidenced a competitive elimination of H2 and HSPh upon thermolysis, and other complex reactivity under catalytic conditions, whereas the fluorinated analogue 12a was found to be resistant to dehydrogenation.