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{k 2 -HC(N-2,6i 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.D uring the catalysis,t he ultrashort Cr À Cr quintuple bond underwent reversible cleavage/formation, corroborated by the characterization of two inverted arene sandwich dichromium complexes (m-h 6 :h 6 -1,3,5-(Me 3 Si) 3 C 6 H 3 )[Cr{k 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 s donors,such as THF and 2,4,6-Me 3 C 6 H 2 CN,t he 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 pathwayso ft hese highly regioselective [2+ +2+ +2] cylcotrimerization reactions of terminal alkynes. Scheme 2. Reversible cleavage/formation of the CrÀCr quintuple bond in the cyclotrimerization of Me 3 SiCCH. Scheme 3. No cross reactions between 5 and 8 in THF.

DFT Study of C–C and C–N Coupling on a Quintuple-Bonded Cr2 Template: MECP (Minimum Energy Crossing Point) Barriers Control Product Distribution

“…The singlet ground state of 1b is in line with the previous report of a Cr− Cr quintuple bond with amidinate ligands, where a CASPT2/ CASSCF calculation shows that singlet state lies much lower in energy. 42 The electronic structure of the 18-electron octahedral Cr complex 2 is also obvious. 43 Let us analyze the high-spin ground state electronic structure of the products 3 and 4 (septets) and 5 (quintet).…”

“…This particular method was used to study the alkyne cyclotrimerization mediated by the Cr−Cr quintuple bond. 21 The electronic structure of 1b was further checked by a multiconfigurational CASSCF calculation with the def2-tzvp basis set for all elements on the optimized geometry. A 12-electron−11-orbital active space was used for the calculatiosn.…”

“…Free energies (Δ G ), including thermal corrections and zero-point energies, were obtained from a frequency analysis at 1 atm and 298 K. In addition to M06-L, we have also tested the relative stabilities of different spin states using the dispersion-corrected functional BP86-GD3BJ with the def2-tzvp basis set for Cr and the 6-31G­(d) basis set for other atoms for important intermediates. This particular method was used to study the alkyne cyclotrimerization mediated by the Cr–Cr quintuple bond . The electronic structure of 1b was further checked by a multiconfigurational CASSCF calculation with the def2-tzvp basis set for all elements on the optimized geometry.…”

“… Treatment of 4 equiv of CyNC with 1a resulted in 5a (paramagnetic) with a 40% yield, where one of the cyclohexyl substituents shows a C–H activation (Scheme ). This is in contrast with the reactions of small molecules mediated by Cr–Cr quintuple bonds, which take place on the singlet potential energy surface (PES). − We study computationally the electronic structure of the products (Scheme ) with different spin states, mechanistic pathways for their formation, and MECPs, to understand their product distributions. The enormity of the problem becomes obvious from the complex sequence of reactions involving 50 intermediates, 26 transition states, a range of spin multiplicities, and 3 MECPs among them that we unravel.…”

Regioselectively switchable alkyne cyclotrimerization catalyzed by a Ni(ii)/bidentate P-ligand/Zn system with ZnI2 as an additive

“…In contrast, very limited catalytic systems were found to selectively prepare the 1,3,5-substitution pattern of poly-substituted benzenes, which feature geometric symmetry and electronic uniformity and could have intriguing applications in materials science and biochemistry. 10 The synthesis of 1,3,5-substituted benzenes usually involves expensive reagents, elaborately designed ligands or structurally complicated complexes, as shown in the cases of mononuclear titanium, 11 iron 12 and indium 13 complexes, dinuclear chromium 14 and niobium 15 catalysts and polymer-supported cobalt 16 catalysts (Scheme 1A (b)). A very recent publication 17 reported the Ni(II)-B(OH) 2 co-catalyzed cycloaddition reaction of terminal alkynes for the highly regioselective synthesis of 1,3,5-substitution products at a relatively high temperature (Scheme 1A (c)).…”