Palladium-catalyzed intermolecular carboazidation of allenes with aryl iodides and trimethylsilyl azide

A new method for the synthesis of highly substituted allyl azides by carboazidation of allenes catalyzed by palladium complexes is described. Treatment of 1,1-dimethylallene (1a) with an aromatic iodide ArI 2 (4-CH3COC6H4I, 4-CH3OC6H4I, 3-CH3OC6H4I, 4-CH3C6H4I, 3-CH3C6H4I, 3-C2H5OCOC6H4I, 4-C2H5OCOC6H4I, 4-ClC6H4I, 3-BrC6H4I or 1-iodothiophene), (CH3)3SiN3 and KOAc in the presence of Pd(dba)2 (dba = dibenzylideneacetone) in DMF at 70 C gave two regioisomers (CH3)2CC(Ar)CH2N3 3 and (CH3)2(N3)CC(Ar)CH2 4 in good to excellent yields. The observed regioand stereoisomer ratios of the allyl azides from these reactions are close to the equilibrium ratio of these isomers at ambient temperature due to a rapid 1,3-shift of the azido group. Reduction of a mixture of 3a and 4a (80 :20, Ar = 4-C6H4COMe) by PPh3–H2O afforded only the sterically less hindered (CH3)2C C(Ar)CH2NH2 in 95% yield further supporting a rapid 1,3-azide shift of 3a and 4a. Mono-substituted allenes RCHCCH2 also undergo carboazidation with aryl iodides and (CH3)3SiN3 in good to excellent yields. For n-butyl-, cyclopentyland cyclohexylallene (1b–d), carboazidation gives three isomers 5, 6 and 7 in ca. 1 : 1 :1 ratio. For phenyland phenoxyallene (1e and f), the reaction produces two stereoisomers Z(5) and E-CHR C(Ar)CH2N3 (6), where R = Ph and PhO; the regioisomer RCH(N3)C(Ar) CH2 (7) was not observed. In contrast, carboazidation of tert-butylallene (1g) afforded regioisomer t-BuHC(N3)C(Ar) CH2 as the major product (~90% yield). Based on known palladium-allene and -allyl chemistry, a mechanism is proposed to account for this palladium-catalyzed three-component assembling reaction.