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Reversible Cleavage/Formation of the Chromium-Chromium Quintuple Bond in the Highly Regioselective Alkyne
Yu-Siang Huang1, Gou-Tao Huang2, Yao-Lun Liu1
1Department of Chemistry and Frontier Research, Center on Fundamental and Applied Sciences and Matters, National Tsing Hua University, 101, Sec. 2, Guang-Fu Road, Hsinchu, 300, Taiwan.
Abstract:
Herein we report the employment of the quintuply bonded dichromium amidinates [Cr{κ2 -HC(N-2,6-i Pr2 C6 H3 )(N-2,6-R2 C6 H3 )}]2 (R=iPr (1), Me (7)) as catalysts to mediate the [2+2+2] cyclotrimerization of terminal alkynes giving 1,3,5-trisubstituted benzenes. During the catalysis, the ultrashort Cr-Cr quintuple bond underwent reversible cleavage/formation, corroborated by the characterization of two inverted arene sandwich dichromium complexes (μ-η6 :η6 -1,3,5-(Me3 Si)3 C6 H3 )[Cr{κ2 -HC(N-2,6-i Pr2 C6 H3 )(N-2,6-R2 C6 H3 )}]2 (R=i Pr (5), Me (8)). In the presence of σ donors, such as THF and 2,4,6-Me3 C6 H2 CN, the bridging arene 1,3,5-(Me3 Si)3 C6 H3 in 5 and 8 was extruded and 1 and 7 were regenerated. Theoretical calculations were employed to disclose the reaction pathways of these highly regioselective [2+2+2] cylcotrimerization reactions of terminal alkynes.
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