Short Synthesis of Chiral 4-Substituted (S)-Imidazolinium Salts Bearing Sulfonates and Their Use in γ-Selective Reactions of Allylic Halides with Grignard Reagents
A one-pot reaction of Boc-protected amino alcohols and 2-sulfobenzoic anhydride followed by the addition of a wide variety of primary amines has allowed rapid access to diverse libraries of amidosulfonates 1,2-C6H4(SO3–)(COCHR1CH2NH2+R2) (R1 = Bn, iPr, Ph; R2 = aryl, CHMePh, 1-adamantyl), of which two examples have been characterised by X-ray crystallography. These reactions proceed by SN2 opening of unisolated oxazoline intermediates. The amidosulfonates have been converted to imidazolinium sulfonate zwitterions (N-heterocyclic carbene precursors) in a two-step process, which involved reduction with BH3·SMe2 and cyclisation with HC(OEt)3. Two examples (R1 = Bn, R2 = 2,6-iPrC6H3 and R1 = iPr, R2 = 3,5-Me2C6H3) have been crystallographically characterised, the latter as a PF6 salt. The imidazolinium sulfonate zwitterions (1 mol-%) catalysed additions of RMgBr (R = alkyl) to (E)-ArC(R)=CHCH2Br (R = H, Me). Additions to cinnamyl bromides showed high γ (SN2′) selectivity (up to 18.4:1) and provided significantly enantiomerically enriched products (up to 82 % ee).