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

Abstract

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)CH₂N₃3 and (CH₃)₂(N₃)CC(Ar)CH₂4 in good to excellent yields. The observed regio- and 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)₂CC(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-, cyclopentyl- and cyclohexylallene (1b–d), carboazidation gives three isomers 5, 6 and 7 in ca. 1∶1∶1 ratio. For phenyl- and phenoxyallene (1e and f), the reaction produces two stereoisomers Z- (5) and E-CHRC(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.