Boron azides in Staudinger oxidations and cycloadditions

Staudinger reactions of Cy2BN3 with tri-substituted phosphines (R3P) yielded the boron-nitrogen-phosphorus linked systems Cy2BN=PR3 (R = Et, (t)Bu, Cy, Ph) (1a-1d respectively). Similarly, reaction of (C6F5)2BN3 with the phosphines P(t)Bu3, PPh3, Ph2PC≡CPh and Ph2PC≡CPPh2 yielded (C6F5)2BN=PR3 (2a-d respectively). In contrast, the reaction of (C6F5)2BN3 with Ph2P-C≡Cp-tol in the presence of excess Me3SiN3 afforded the bicyclic product 3 [1-(C6F5)2B-4-(p-tol)-1H-1,2,3-triazole-5-P(NH)Ph2] in which both a Staudinger and a cycloaddition reaction has taken place. However, if the Staudinger reaction is prohibited via phosphine oxidation as in the case of (EtO)2P(O)C≡CP(O)(OEt)2 then the unusual dimeric product 4 [2-(C6F5)2B-4-(P(O)OEt2)-2H-1,2,3-triazole-5-P(O)(OEt)(OB(C6F5)2)] is generated. The structures of 1b-d, 2b-d, 3 and 4 have been determined by X-ray diffraction.