Cobalt-mediated two-carbon ring expansion of five-membered rings. Electrophilic carbon-carbon bond activation in the synthesis of seven-membered rings
Cobalt-mediated two-carbon ring expansion of five-membered rings. Electrophilic carbon-carbon bond activation in the synthesis of seven-membered rings
复制标题
DOI：
10.1021/ja047852
复制
发表时间：
2004-08-04
影响因子：
15
通讯作者：
Stryker, JM
中科院分区：
文献类型：
--
作者：
Dzwiniel, TL;Stryker, JM
亲电性钴（III）介导的协调五元环的前所未有的两个碳环的扩张，导致一个显着的通用的新策略，从现成的五元环底物的七元碳环的合成。该反应是金属介导的[5 + 2]环戊烯基/炔环加成反应，通过钴（I）环戊二烯络合物的初始质子化进行，然后重排成Agostic η3-环戊烯基中间体。环状η3-烯丙基残基随后与炔发生迁移偶联，随后无张力的五元环的碳-碳键活化，并发生环化反应生成扩环产物，尽管事件的顺序和密切的机理尚未最终确定。该反应是高度选择性的五元环配体进行活化，大概是一个后果的快速钴介导的环间氢化物转移和动力学偏好的炔插入到较少取代的环戊烯基环。炔插入本身是高度区域选择性的，通过迁移到炔的空间较小端进行。该反应对钴配合物和辅助η5-π取代基都很敏感，但对相当大范围的烷基、芳基和三烷基甲硅烷基取代的末端和内部炔都很敏感。
Electrophilic cobalt(III) mediates an unprecedented two-carbon ring expansion of coordinated five-membered rings, leading to a remarkably general new strategy for the synthesis of seven-membered carbocycles from readily available five-membered ring substrates. The reaction, a metal-mediated [5 + 2] cyclopentenyl/alkyne cycloaddition, proceeds via initial protonation of a cobalt(I) cyclopentadiene complex, followed by rearrangement to an agostic η3-cyclopentenyl intermediate. The cyclic η3-allyl residue then undergoes migratory coupling with alkyne followed by carbon−carbon bond activation of the unstrained five-membered ring and recyclization to the ring expanded product, although the order of events and intimate mechanism has not been conclusively established. The reaction is highly selective with respect to which five-membered ring ligand undergoes activation, presumably a consequence of rapid cobalt-mediated interannular hydride transfer and kinetic preference for alkyne insertion into the less substituted cyclopentenyl ring. The alkyne insertion is itself highly regioselective, proceeding via migration to the sterically smaller end of the alkyne. The reaction is sensitive to both the cobalt counterion and the ancillary η5-cyclopentadienyl substituent but proceeds for a considerable range of alkyl-, aryl-, and trialkylsilyl-substituted terminal and internal alkynes.