Reversible Cleavage/Formation of the Chromium-Chromium Quintuple Bond in the Highly Regioselective Alkyne Cyclotrimerization

Abstract

Herein we report the employment of the quintuply bonded dichromium amidinates [Cr{(2)-HC(N-2,6-(Pr2C6H3)-Pr-i)(N-2,6-R2C6H3)}](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 (-(6):(6)-1,3,5-(Me3Si)(3)C6H3)[Cr{(2)-HC(N-2,6-(Pr2C6H3)-Pr-i)(N-2,6-R2C6H3)}](2) (R=Pr-i (5), Me (8)). In the presence of sigma donors, such as THF and 2,4,6-Me3C6H2CN, the bridging arene 1,3,5-(Me3Si)(3)C6H3 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.