Syntheses and structures of chiral rhenium-containing phosphonium salts and phosphines with ReCH2Ar2 or ReCH(R)Ar2 linkages; enantioselective catalysis of intramolecular Morita–Baylis–Hillman and Rauhut–Currier reactions

Abstract

Easily accessed (η⁵-C₅H₅)Re(NO)(PPh₃)(CH₃) (2) is treated with Ph₃C⁺ PF₆⁻ (−78 °C) to generate the methylidene cation [(η⁵-C₅H₅)Re(NO)(PPh₃)(CH₂)]⁺ and then secondary phosphines PAr₂H (Ar = a, Ph; b, p-tol; c, p-C₆H₄OCH₃; d, p-C₆H₄N(CH₃)₂; e, 2-biphen; f, α-naph) to give the phosphonium salts [(η⁵-C₅H₅)Re(NO)(PPh₃)(CH₂PAr₂H)]⁺ PF₆⁻ ([1a–f-H]⁺ PF₆⁻, 87–94%). Additions of t-BuOK yield the phosphines 1a–f (64–91%). Analogous procedures starting with (S)-2 give enantiopure (S)-1a–d in comparable yields. The ethyl complex (S)-(η⁵-C₅H₅)Re(NO)(PPh₃)(CH₂CH₃) is similarly treated with Ph₃C⁺ PF₆⁻ to generate the ethylidene cation (S)-(sc)-[(η⁵-C₅H₅)Re(NO)(PPh₃)(CHCH₃)]⁺ and then PPh₂H to give enantiopure and diastereopure (S_ReS_C)-[(η⁵-C₅H₅)Re(NO)(PPh₃)(CH(CH₃)PPh₂H)]⁺ PF₆⁻ (51%). Addition of t-BuOK yields the corresponding ReCH(CH₃)PPh₂ adduct. The crystal structures of [1c,d-1 PF₆⁻] are determined and analyzed. Most of the ReCH₂PAr₂ species catalyze intramolecular Morita–Baylis–Hillman reactions of R(CO)CHCH(CH₂)_nCH₂CHO (n/R = 1/Ph, 1/S-i-Pr, 2/p-tol 2/Me) in C₆H₆ or C₆H₅Cl (20 °C) to give R(CO)CHCH(CH₂)_nCH₂CHOH (99–86%, 74–38% ee with (S)-1a) or Rauhut–Currier reactions of R(CO)CHCHCH₂CH₂CHCH(CO)R (R = Ph, S-i-Pr) to give R(CO)CCHCH₂CH₂CHCH₂(CO)R (87–82%, 56–42% ee with (S)-1a).