Selectivity in Nickel-Catalyzed Rearrangements of Cyclopropylen-ynes

Abstract

The Ni/NHC-catalyzed rearrangement of cyclopropylen-ynes is described. Two different heterocycles, cyclopentane- or cycloheptene-based, were obtained. However, by employing ItBu as the NHC ligand, cyclopentane products were obtained selectively under mild conditions.

Iridium Catalyzed Carbocyclizations: Efficient (5+2) Cycloadditions of Vinylcyclopropanes and Alkynes

“…Formation of seven-membered carbocycles from vinylcyclopropanes (VCPs) and alkynes catalyzed by rhodium serves as an illustration. First reported by Wender in 1995, [1] this process has enabled streamlining complex molecule syntheses [2] and provided a mechanistic basis for developing new reactions including higher order cycloadditions [3] and (5 + 2) reactions with other 2p components.[4] Intramolecular versions catalyzed by ruthenium, [5] nickel, [6] and iron [7] have been reported, but the intermolecular reaction that benefits from simple and often commercially available substrates has remained exclusive to rhodium catalysis.[8] A remaining challenge towards improving the efficiency and cost, as well as providing new mechanistic opportunities for interception of intermediates, is the introduction of third-row transition metals as catalysts in this context. [9] To this end, we envisioned that comparatively inexpensive iridium would be a suitable candidate.…”

“…[11b,c] Herein, we report that cationic iridium complexes with cyclooctadiene (cod) [12a,b] or dibenzo [a,e] 6 for the reaction between 4-octyne [13] and commercially available VCP 1 a (Table 1). [14] In a mixture of 1,2-dichloroethane (DCE)/2,2,2-trifluoroethanol (TFE) [15] as solvent (9:1 v/v), the reaction worked remarkably well to give cycloadduct 3 a in an essentially quantitative yield in under two minutes at ambient temperature ( [16] Exchange of the cod ligand in precatalyst 2 a for a more tightly coordinated dbcot ligand [17] in 2 b enabled a preserved turnover rate using only 0.7 mol % of precatalyst (Table 1, entries 6 and 7).…”

Transition Metal-Catalyzed Reactions Using N-Heterocyclic Carbene Ligands (Besides Pd- and Ru-Catalyzed Reactions)

“…By substituting SIPr for an N-alkyl-substituted NHC ligand (ItBu), the skipped-triene product (2) could be prepared selectively from cyclopropylenyne substrates, regardless of substituent size (e.g., R) (Eq. 3) [11]. Thus, skipped-triene products were formed exclusively under mild conditions (room temperature, 2 h).…”

Recent Developments in the [5+2] Cycloaddition

“…[39] In this study, the [5 + 2] cycloadducts were formed as minor products, (e.g., 63c and 64c) along with the corresponding rearrangement cyclopentane products (e.g., 62c) obtained as major products when the size of the substituent on the alkyne (R) was small (e.g., R = Me), while when R was large [e.g., R = t-Bu (61a) or TMS (61b)], the corresponding isomerized seven-membered rings (63a and 64a, and 63b and 64b, respectively) were the sole products isolated in good yields, as shown in Scheme 28. These results were explained by the fact that the selective formation of the [5 + 2] cycloadducts required a bulky alkyne substituent such as t-Bu and trimethylsilyl moieties.…”