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

Herein we report the employment of the quintuply bonded dichromium amidinates [Cr{κ(2) -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-η(6) :η(6) -1,3,5-(Me3 Si)3 C6 H3 )[Cr{κ(2) -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.