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 Pr 2 C 6 H 3 )(N‐2,6‐R 2 C 6 H 3 )}] 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‐(Me 3 Si) 3 C 6 H 3 )[Cr{κ 2 ‐HC( N ‐2,6‐ i Pr 2 C 6 H 3 )( N ‐2,6‐R 2 C 6 H 3 )}] 2 (R= i Pr ( 5 ), Me ( 8 )). In the presence of σ donors, such as THF and 2,4,6‐Me 3 C 6 H 2 CN, the bridging arene 1,3,5‐(Me 3 Si) 3 C 6 H 3 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.