The Multifaceted Palladium Chemistry of 2,2′-Diphosphinotolanes

The 2,2′-bis­(diisopropylphosphino)­tolane 1 was reacted with various palladium precursors to afford a surprisingly diverse array of palladium­(II) and palladium­(I) complexes. The conventional cationic [PCCP]­PdCl+ pincer-type complex 2 was readily obtained by starting from 1 and an in situ generated PdCl+ fragment. Upon chloropalladation of 2 via reaction with a chloride source, a novel vinyl–palladium pincer complex (3) featuring a new C–Cl bond was produced quantitatively. The newly formed C–Cl bond in 3 may be cleaved by addition of AgBF4, which led to the regeneration of 2. Treatment of 1 with (MeCN)2PdCl2 afforded 3 in a single step, while the analogous reaction between 1 and [(η3-allyl)­PdCl]2 did not result in chloride or allyl addition. Instead, the alkyne was attacked by one of the phosphines to afford the cationic CArY-MIC palladium complex 4 (CArY-MIC = cyclic (aryl)­(ylidic) mesoionic carbene). Upon reaction of 1 with dibenzylidene acetone palladium(0) precursors, two alkynes were coupled to afford the novel palladol 5. When Pd­(CNR)2 (R = tBu, Cy) was employed instead of the latter palladium(0) precursors, yet another product was isolated: namely, a dinuclear Pd­(I)–Pd­(I) complex. In this complex, both palladium atoms are located within the pocket of the ligand and stabilized by isonitrile coligands (CNR with R = tBu for 6a and R = Cy for 6b). The latter isonitriles may be exchanged reversibly for carbon monoxide, as shown by in situ NMR spectroscopy. The finding that five significantly different palladium complexes (2–6) have been isolated by starting from a single phosphino alkyne is discussed in the context of phosphino–palladium-catalyzed alkyne transformations.将2,2′-双(二异丙基膦基)二苯乙炔（2,2′-bis(diisopropylphosphino)tolane）1与多种钯前驱体进行反应，得到了结构多样、数目可观的钯(II)与钯(I)配合物。以1和原位生成的PdCl+片段为起始原料，可便捷制备得到常规的阳离子型[PCCP]PdCl+钳型（pincer-type）配合物2。通过向2中加入氯化物源进行氯钯化反应，可定量得到一种带有全新C-Cl键的新型乙烯基-钯钳型配合物3。向3中加入AgBF4可断裂其新形成的C-Cl键，从而实现配合物2的再生。将1与(MeCN)₂PdCl₂反应，可一步合成配合物3；而1与[(η³-烯丙基)PdCl]₂的类似反应则未发生氯化物或烯丙基的加成，取而代之的是其中一个膦基进攻炔烃，得到了阳离子型环式(芳基)(叶立德)介离子卡宾（cyclic(aryl)(ylidic)mesoionic carbene，简称CArY-MIC）钯配合物4。将1与二苄叉丙酮钯(0)前驱体反应时，两分子炔烃发生偶联，得到新型palladol配合物5。若使用Pd(CNR)₂（R为叔丁基tBu、环己基Cy）替代上述钯(0)前驱体，则可分离得到另一产物——双核Pd(I)-Pd(I)配合物。在该配合物中，两个钯原子均处于配体的空腔内，并由异腈共配体稳定：对应配合物6a的CNR中R为tBu，配合物6b的CNR中R为Cy。经原位核磁共振(NMR)光谱表征证实，这类异腈配体可与一氧化碳发生可逆交换。本文围绕膦-钯催化炔烃转化的研究背景，对以下发现展开讨论：从单一膦基炔烃出发，可分离得到五种结构差异显著的钯配合物（2~6）。