Reactions of a Dimethylxanthene-Derived Frustrated Lewis Pair with Silanes and Stannanes

The reactivity towards Si−H and Sn−H bonds of a dimethylxanthene-based frustrated Lewis pair featuring −PⁱPr₂ and −B(C₆F₅)₂ functions in the 4-, and 5-positions is reported. In the case of silanes, greater steric bulk appears to impede reactivity, with only PhSiH₃ and SiH₄ showing unambiguous evidence for Si−H bond activation. In the case of PhSiH₃, the silylphosphonium borohydride 1^iPr(SiH₂Ph)(H) is formed by Si−H bond activation across the P/B manifold. 1^iPr(SiH₂Ph)(H) loses PhSiH₃ in hydrocarbon solution and therefore can only be crystallized from the neat silane. On heating, it undergoes a metathesis process leading to installation of the −SiH₂Ph function at the 5-position of the xanthene scaffold, with accompanying migration of the boryl group (as Piers’ borane, HB(C₆F₅)₂) to the phosphine. Similar chemistry is observed with SiH₄ (although occurring much more readily), and such processes are thought to be mechanistically similar to related rearrangements occurring with the boranes HBpin and HBcat. By contrast, the activation of the weaker Sn−H bonds found in stannanes occurs more readily, even for bulkier substrates, and the stannyl-phosphonium borohydrides 1^iPr(SnⁿBu₃)(H) and 1^iPr(SnPh₃)(H) are obtained from the room temperature reactions with ⁿBu₃SnH and Ph₃SnH, respectively.