Rhodium-mediated Assembly of New Heterocycles: From Borylenes to Oxaboroles

Base-stabilized rhodium borylene complex κ2-L(CO)Rh(BMes), 2; κ2-L = κ2-NN’-Rh,κ1-N-B-(2,5-[iPr2P=N(4-iPrC6H4)]2-N’(C4H2)-); Mes = mesityl, reacts with a series of alkynes (PhC≡C—R; R = Ph, Me, CO2Et, H) to yield unique structures whereby the alkyne has regioselectively added across boron and the carbon atom of a CO ligand. The resulting complexes, LRh[C(O)C(Ph)C(R)B(Mes)], 3R, react with additional CO to afford cycle-containing products, L(CO)Rh([[EQUATION]]), 5R, that ultimately release highly functionalized organic heterocycles of the form [[EQUATION]]=NPipp (Pipp = 4-iPrC6H4), 6. These oxaboroles, which were assembled from a primary hydroborane, CO, an alkyne, and an azide-generated NPipp, are structurally analogous to two of the five boron-containing therapeutics approved by the FDA.