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 -(PPr2)-Pr-i 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)((SnBu3)-Bu-n)(H) and 1(iPr)(SnPh3)(H) are obtained from the room temperature reactions with (Bu3SnH)-Bu-n and Ph3SnH, respectively.