Fe(II)-catalyzed imidation of allyl sulfides and subsequent [2,3]- sigmatropic rearrangement. Preparation of α-branched N-tert-butyloxycarbonyl (Boc)-protected N-allylamines

Article abstract of DOI:10.1021/jo991569p

Allyl aryl sulfides 1 and 5 were shown to undergo an imidation/[2,3]- sigmatropic rearrangement reaction upon treatment with N-tert- butyloxycarbonyl azide (BocN₃) and catalytic amounts of FeCl₂ in CH₂Cl₂. The N-Boc-protected N-allyl sulfenamides 3 and 21 were obtained in yields between 48 and 75% (12 examples). Whereas the reaction is well suited for the transformation of α-unbranched sulfides to α-branched sulfenamides, the enantiomerically pure α-branched sulfides 10 and 13 reacted sluggishly. The corresponding sulfenamides 22 and 23 were obtained in only moderate enantiomeric excess (36-39% ee). A reaction mechanism is proposed that postulates the intermediacy of an N-Boc-substituted Fe(IV)-nitrene complex 14 acting as the imidation reagent in the catalytic cycle. Possible side reactions are discussed. The benzenesulfenamides 3 were further converted into N-Boc-N-allylamines 4 by removal of the phenylsulfanyl group. Bu₃SnH in benzene was found to be the reagent of choice for the deprotection of α- branched amines that bear a secondary allyl substituent (five examples, 71- 93% yield). This method failed for the α-branched amines 3i-k with a tertiary allyl substituent. The phenylsulfanyl group was finally removed with P(OEt)₃/NEt₃ in CH₂Cl₂ (three examples, 43-62% yield).