Stereoselective [2 + 3] cycloaddition of nitrones to platinum-bound organonitriles. First enantioselective synthesis of Δ4-1,2,4-oxadiazolines†
作者:Gabriele Wagner、Matti Haukka
DOI:10.1039/b102291c
日期:——
Coordinated benzonitrile in cis-[PtCl2(R3MeSO)(PhCN)] (R3Â =Â Me, Ph) is sufficiently activated to undergo [2Â +Â 3] cycloaddition with nitrones âO+N(R2)CH(R1) (R1Â =Â Ph, p-C6H4Me, p-C6H4OMe, R2Â =Â Me; R1Â =Â Ph, R2Â =Â CH2Ph) under mild conditions to give the corresponding Î4-1,2,4-oxadiazoline complexes cis-[PtCl2(R3MeSO)NC(Ph)âON(R2)âCH(R1)}] in 68â83% yield. In this reaction, the chiral sulfoxide in cis-[PtCl2(PhMeSO)(PhCN)] induces stereoselective formation of the coordinated Î4-1,2,4-oxadiazoline
leading to mixtures of diastereomeric platinum complexes with a d.e. of 30â60%, which can be enhanced to >90% by fractional crystallization. The major diastereoisomer of [PtCl2(PhMeSO)NC(Ph)âON(Me)âC(H)Ph}] thus obtained was analyzed by X-ray diffraction and shown to have cis-(R,S) configuration.Reaction of cis-[PtCl2(R3MeSO)NC(Ph)âON(R2)âCH(R1)}] with ethane-1,2-diamine results in displacement of the Î4-1,2,4-oxadiazolines from the metal and concomitant formation of water-soluble [Pt(en)2]Cl2 and sulfoxide, which can both be removed by aqueous extraction, and this allows for the first time the isolation of Î4-1,2,4-oxadiazolines in enantiomerically enriched form. Applying this technique to cis-(RS,SC)-[PtCl2(PhMeSO)NC(Ph)âON(Me)âC(H)Ph}]
of 90% d.e. and 79% e.e. in sulfoxide, the corresponding (R)-2-methyl-3,5-diphenyl-Î4-1,2,4-oxadiazoline was obtained with 70% e.e.
顺式-[PtCl2(R3MeSO)(PhCN)](R3Â =Â Me,Ph)中的配位苯腈被充分活化,与硝基化合物âO+N(R2)CH(R1)(R1Â =Â Ph,p-C6H4Me,p-C6H4OMe,R2Â =Â Me)发生[2Â +Â 3]环加成反应;R1Â=ÂPh,R2Â=ÂCH2Ph),得到相应的δ4-1,2,4-噁二唑啉配合物顺式-[PtCl2(R3MeSO)NC(Ph)âON(R2)âCH(R1)}],收率为 68â83% 。在该反应中,顺式-[PtCl2(PhMeSO)(PhCN)]中的手性亚砜诱导了配位δ4-1,2,4-恶二唑啉的立体选择性形成,导致非对映异构体铂络合物混合物的d.e.为30â60%,通过分馏结晶可提高到大于90%。通过 X 射线衍射分析表明,由此获得的[PtCl2(PhMeSO)NC(Ph)âON(Me)âC(H)Ph}]的主要非对映异构体具有顺式(R,S)构型。顺式[PtCl2(R3MeSO)NC(Ph)âON(R2)âCH(R1)}]与乙烷-1,2-二胺反应后,δ4-1,2,4-噁二唑啉从金属中移出,同时形成水溶性[Pt(en)2]Cl2和亚砜、这样就首次分离出了对映体富集形式的δ4-1,2,4-恶二唑啉。将这种技术应用于在亚砜中对映体含量分别为 90% 和 79% 的顺式-(RS,SC)-[PtCl2(PhMeSO)NC(Ph)âON(Me)âC(H)Ph}],得到了相应的(R)-2-甲基-3,5-二苯基δ4-1,2,4-恶二唑啉,对映体含量为 70%。