om060505j_si_001.cif (19.28 kB)
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
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posted on 2006-10-09, 00:00 authored by Zhiquan Song, Yuxue Li, Meina Liu, Liqing Cong, Yuanhong LiuThe cyclodimerization of (2-pyridyl)alkynes tethered with an amino group mediated by zirconium
was achieved cleanly under controlled reaction conditions. This methodology provided an efficient route
for the synthesis of tetrasubstituted cyclobutenes with high regio- and diastereoselectivity when the reaction
was 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 product
distribution was also investigated. This cyclization/elimination was influenced strongly by the nature of
the 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 yields
or different products. The zirconium intermediate was isolated successfully, which has been fully
characterized 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 this
structure was further confirmed by DFT calculations. Subsequent cyclization to yield a cyclobutadiene
skeleton was induced by the attack of the coordinative species. This is different with the reactions of late
transition metal mediated cyclodimerization of alkynes (e.g., Co) via direct reductive elimination. To
account for the formation of 3-methylenecyclobutenes, an E1cb reaction pathway was suggested.
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DEPTmass analysisAlkyneNMR experimentscyclodimerizationFacile RouteExtensive NMR experimentsmethylenecyclobutenechemical yieldsquenching reagentsarylHMQCproduct distributionalkynecyclobutadiene skeletonCOSYHMBCcoordinative speciestransition metalE 1cb reaction pathwayzirconium1 kreductive eliminationreaction conditionsSubsequent cyclizationNaHCO 3 solutionDFT calculationstetrasubstituted cyclobutenes
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