Regio- and Stereoselective Coupling of Heteroaryl-Substituted Alkynes: New Insights into the Mechanism of Zirconium-Mediated Cyclodimerization of Alkynes and a Facile Route to 3-Methylenecyclobutenes

详细信息    查看全文

• 作者：Zhiquan Song ; Yuxue Li ; Meina Liu ; Liqing Cong ; Yuanhong Liu
• 刊名：Organometallics
• 出版年：2006
• 出版时间：October 9, 2006
• 年：2006
• 卷：25
• 期：21
• 页码：5035 - 5044
• 全文大小：367K
• 年卷期：v.25,no.21(October 9, 2006)
• ISSN：1520-6041

文摘

The cyclodimerization of (2-pyridyl)alkynes tethered with an amino group mediated by zirconiumwas achieved cleanly under controlled reaction conditions. This methodology provided an efficient routefor the synthesis of tetrasubstituted cyclobutenes with high regio- and diastereoselectivity when the reactionwas quenched by water. However, by quenching with saturated NaHCO₃ solution, a 3-methylenecyclobutene derivative was readily constructed. The effect of other quenching reagents on the productdistribution was also investigated. This cyclization/elimination was influenced strongly by the nature ofthe substituent on the amino group. The reaction worked well with aryl/methyl-, aryl/benzyl-, or diaryl/aminomethyl-substituted alkynes, giving generally good yields of the corresponding products. Conversely,the phenyl/hydrogen or dialkyl (except 1k) substitutions on nitrogen gave either lower chemical yieldsor different products. The zirconium intermediate was isolated successfully, which has been fullycharacterized by NMR experiments and mass analysis. Extensive NMR experiments (DEPT, COSY,HMQC, and HMBC) revealed its structure is in agreement with that of zirconacyclopentadiene, and thisstructure was further confirmed by DFT calculations. Subsequent cyclization to yield a cyclobutadieneskeleton was induced by the attack of the coordinative species. This is different with the reactions of latetransition metal mediated cyclodimerization of alkynes (e.g., Co) via direct reductive elimination. Toaccount for the formation of 3-methylenecyclobutenes, an E1cb reaction pathway was suggested.