Palladium catalysed alkyne hydrogenation and oligomerisation: a parahydrogen based NMR investigation
作者:Joaquín López-Serrano、Simon B. Duckett、John P. Dunne、Cyril Godard、Adrian C. Whitwood
DOI:10.1039/b804162h
日期:——
The role phosphine ligands play in the palladium(II)-bis-phosphine-hydride cation catalysed hydrogenation of diphenylacetylene is explored through a PHIP (parahydrogen induced polarization) NMR study. The precursors Pd(LL′)(OTf)2 (1a–e) [LL′ = dcpe (PCy2CH2CH2PCy2), dppe, dppm, dppp, cppe (PCy2CH2CH2PPh2)] are used. Alkyl palladium intermediates of the type [Pd(LL′)(CHPhCH2Ph)](OTf) (2 and 3) are detected and demonstrated to play an active role in hydrogenation catalysis. Magnetization transfer experiments reveal chemical exchange from the α-H of the alkyl ligand of 2a (LL′ = dcpe) and linkage isomer 2e′ (LL′ = cppe) into trans-stilbene on the NMR timescale. Activation parameters (ΔH≠ and ΔS≠) for this transformation have been determined. These experiments, coupled with GC/MS data, indicate that the catalytic activity is greater in methanol, where it follows the order: dcpe > cppe > dppp > dppe > dppm, than in CD2Cl2. All five of the phosphine systems described are less active than those based on bcope [where bcope is (C8H14)PCH2–CH2P(C8H14)] and tbucope [where tbucope is (C8H14)PC6H4CH2P(tBu)2]. cis, cis-1,2,3,4-Tetraphenyl-buta-1,3-diene is detected as a minor reaction product with Pd(LL′)(PhCH–CHPh–CPhCHPh)+ (4) also being shown to play a role in the alkyne dimerisation step.
通过PHIP(对氢诱导极化)NMR研究探讨了膦配体在钯(II)-双膦-氢阳离子催化苯基乙炔氢化反应中的作用。使用的前体为Pd(LL′)(OTf)₂(1a-e)[LL′ = dcpe(PCy₂CH₂CH₂PCy₂),dppe,dppm,dppp,cppe(PCy₂CH₂CH₂PPh₂)]。检测到类型为[Pd(LL′)(CHPhCH2Ph)](OTf)(2和3)的烷基钯中间体,并证明其在氢化催化中发挥了积极作用。磁化转移实验揭示了来自2a(LL′ = dcpe)和连接异构体2e′(LL′ = cppe)烷基配体的α-H在NMR时间尺度上与反式stilbene之间的化学交换。已确定该转化的活化参数(ΔH≠和ΔS≠)。这些实验结合GC/MS数据表明,在甲醇中催化活性更强,顺序为:dcpe > cppe > dppp > dppe > dppm,相比之下在CD₂Cl₂中活性较低。所描述的五种膦配体系统的活性均低于基于bcope(其中bcope为(C₈H₁₄)PCH₂–CH₂P(C₈H₁₄))和tbucope(其中tbucope为(C₈H₁₄)PC₆H₄CH₂P(tBu)₂)的系统。检测到顺-顺-1,2,3,4-四苯基-丁-1,3-二烯作为次要反应产物,同时Pd(LL′)(PhCH–CHPh–CPhCHPh)⁺(4)在炔烃二聚化步骤中也显示出作用。