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)], 3^R, react with additional CO to afford cycle-containing products, L(CO)Rh$$ ), 5^R, that ultimately release highly functionalized organic heterocycles of the form $$ =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.