Chiral Recognition and Dynamic Thermodynamic Resolution of Sulfoxides by Chiral Iridium(III) Complexes

The
optically active Ir­(III) complex Λ-[Ir­(ppy)2(MeCN)2]­(PF6) (ppy is 2-phenylpyridine) with a chiral-at-metal
was first demonstrated to preferentially react with (R)-configuration sulfoxides 2-(alkylsulfinyl)­phenol (HLO-R, R = Me,
Et, iPr, and Bn) rather than (S)-configuration sulfoxides under thermodynamic equilibrium
due to the hydrogen-bonding interaction and the differences in the
steric interference, and thus act as a highly efficient enantioreceptor
for resolution of sulfoxide enatiomers. Treatment of Λ-[Ir­(ppy)2(MeCN)2]­(PF6) with 2 equiv of rac-HLO-R offered (S)-HLO-R in yields of
46–47% with 97–99% enantiomeric excess (ee) values and
Λ-[Ir­(ppy)2{(S)-LO-R}] complex in
yields of 89–93% with 98% diastereomeric excess (de). The (R)-HLO-R chiral sulfoxides were obtained by the acidolysis
of Λ-[Ir­(ppy)2{(S)-LO-R}] complexes
with trifluoroacetic acid (TFA) in the presence of coordinated solvent
MeCN in yields of 45–47% with 98–99% ee values. Moreover,
the enantioreceptor Λ-[Ir­(ppy)2(MeCN)2]­(PF6) can be recycled in a yield of 86–91% with
complete retention of the configuration at metal center and can be
reused in a next reaction cycle without loss of reaction activity
and enantioselectivity. The absolute configurations at the metal centers
and sulfur atoms were determined by X-ray crystallography.

具有金属中心手性（chiral-at-metal）的光学活性Λ构型三(2-苯基吡啶)二(乙腈)合铱(III)六氟磷酸盐（Λ-[Ir(ppy)₂(MeCN)₂](PF₆)，其中ppy为2-苯基吡啶（2-phenylpyridine)），首次被证实可在热力学平衡条件下，凭借氢键相互作用与空间位阻差异，优先与(R)构型亚砜2-(烷基亚磺酰基)苯酚（HLO-R，R=甲基、乙基、异丙基、苄基）发生反应，而非(S)构型亚砜，因此可作为高效的对映选择性受体用于亚砜对映体的拆分。将Λ-[Ir(ppy)₂(MeCN)₂](PF₆)与2当量的外消旋HLO-R进行反应，可得到收率46%~47%、对映体过量值（enantiomeric excess，ee）97%~99%的(S)-HLO-R，同时生成收率89%~93%、非对映体过量值（diastereomeric excess，de）98%的Λ-[Ir(ppy)₂{(S)-LO-R}]配合物。通过三氟乙酸（TFA）在配位溶剂乙腈（MeCN）存在下对Λ-[Ir(ppy)₂{(S)-LO-R}]配合物进行酸解，可得到收率45%~47%、对映体过量值98%~99%的(R)-HLO-R手性亚砜。此外，该对映选择性受体Λ-[Ir(ppy)₂(MeCN)₂](PF₆)可被回收，回收收率达86%~91%，其金属中心的构型完全保持，且可用于后续反应循环，不会损失反应活性与对映选择性。金属中心与硫原子的绝对构型通过X射线晶体学得以确定。