Reversibility effects on the stereoselectivity of Pt(II)-Mediated cascade poly-ene cyclizations
Reversibility effects on the stereoselectivity of Pt(II)-Mediated cascade poly-ene cyclizations
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DOI：
10.1021/ja075518i
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发表时间：
2008-01-16
影响因子：
15
通讯作者：
Gagne, Michel R.
中科院分区：
文献类型：
--
作者：
Feducia, Jeremiah A.;Gagne, Michel R.
含亲核陷阱的1，5-二烯与亲电三膦钳状连接的铂(II)配合物通过铂(n(2)-烯烃)中间体环合生成多环铂-烷基化合物。对于富电子的三膦配体，观察到铂(n(2)-烯烃)和铂-烷基之间的平衡。平衡的位置对配体的碱性、共轭酸强度、溶剂的极性和环的大小很敏感。在配体缺乏电子且不促进逆环化的情况下，观察到遵循Stork-Eschenmoser假设的动力学产物(E-烯烃进行反环连接)。当逆环化很快时，观察到多步重排产生的替代热力学产物(顺式-[6，5]-自行车)。在动力学和热力学条件下，遥远的甲基取代基导致了高度的非对映选择性反应。在三烯醇底物的情况下，来自C环取代基的长程不对称诱导被大大减弱，并且仅观察到适度的非对映选择性(类似于2：1)。数据表明，对于三环化，远程立体控制是在新生环的组织过程中发展起来的非对映选择相互作用的结果。相反，自行车的非对映选择性是可逆级联环化的结果。
Cyclization of 1,5-dienes bearing nucleophilic traps with electrophilic trisphosphine pincer ligated Pt(II) complexes results in the formation of a polycyclic Pt-alkyl via a pt(n(2)-alkene) intermediate. With electron-rich triphosphine ligands, an equilibrium between the pt(n(2)-alkene) and Pt-alkyl was observed. The position of the equilibrium was sensitive to ligand basicity, conjugate acid strength, solvent polarity, and ring size. In cases where the ligand was electron poor and did not promote retrocyclization, the kinetic products adhering to the Stork-Eschenmoser postulate were observed (E-alkenes give trans-ring junctions). When retrocyclization was rapid, alternative thermodynamic products resulting from multistep rearrangements were observed (cis-[6,5]-bicycles). Under both kinetic and thermodynamic conditions, remote methyl substituents led to highly diastereoselective reactions. In the case of trienol substrates, long-range asymmetric induction from a C-ring substituent was considerably attenuated and only modest diastereoselectivity was observed (similar to 2:1). The data suggest that for a tricyclization, the long-range stereocontrol results from diastereo-selecting interactions that develop during the organization of the nascent rings. In contrast, the bicyclization diastereoselectivities result from reversible cascade cyclization.