Enhanced Catalytic Activity of Nickel Complexes of an Adaptive Diphosphine–Benzophenone Ligand in Alkyne Cyclotrimerization

Abstract

Adaptive ligands, which can adapt their coordination mode to the electronic structure of various catalytic intermediates, offer the potential to develop improved homogeneous catalysts in terms of activity and selectivity. 2,2′-Diphosphinobenzophenones have previously been shown to act as adaptive ligands, the central ketone moiety preferentially coordinating reduced metal centers. Herein, the utility of this scaffold in nickel-catalyzed alkyne cyclotrimerization is investigated. The complex [( p -tol L1 )Ni(BPI)] ( p -tol L1 = 2,2′-bis(di( para -tolyl)phosphino)-benzophenone; BPI = benzophenone imine) is an active catalyst in the [2 + 2 + 2] cyclotrimerization of terminal alkynes, selectively affording 1,2,4-substituted benzenes from terminal alkynes. In particular, this catalyst outperforms closely related bi- and tridentate phosphine-based Ni catalysts. This suggests a reaction pathway involving a hemilabile interaction of the C=O unit with the nickel center. This is further borne out by a comparative study of the observed resting states and DFT calculations.

Nickel-Catalyzed Site- and Stereoselective Reductive Alkylalkynylation of Alkynes

“…and DMA as solvent under ambient conditions (Table 1, entry 1). Switching the reducing agent to Zn or tetrakis(dimethylamino)ethylene (TDAE) led to poor yields of 4a with excessive by-product formation from 1a cyclotrimerization 65 and 2a homocoupling [63][64] (Table 1, In order to enhance the catalytic efficiency and/or suppress the undesired formation of diyne by-products, [63][64] various additives were experimented as detailed in Table 1, entries 710.…”

“…Instead, the alkyne 1a was fully consumed in cyclotrimerization 65 to form arene side products, and homocoupling of bromoalkyne 2a to give diyne was detected (Fig. 4a).…”

“…Our motivation to pursue this approach is twofold: (1) The greater variety of N-(acyloxy)phthalimides accessible from aliphatic carboxylic acids (vs. alkyl halides) means that diverse aliphatic units (tertiary, secondary, primary) can be incorporated; (2) The ability of NHPI esters to promote challenging alkylalkynylations in which alkyl halides fail to deliver, by minimizing rampant undesired pathways arising from homocoupling [63][64] and cross-coupling 14,16 of the alkynyl halide and NHPI ester reactants, as well as alkyne cyclotrimerization 65 (see Fig. 4 for further discussion).…”

De Novo Construction of Substituted Terephthalates via Phosphine Catalyzed Domino Benzannulation Reactions

“…Among of those, transition‐metal catalysts have demonstrated great potentials to carry out the preparation of functionalized benzenes. In recent advances, Moret's group achieved [2+2+2] cyclotrimerization with the use of Ni(0) complexes, selectively affording carboxylate substituted terephthalates from terminal alkynes (Scheme 1a) [9] . Although a small number of such methods have been exploited, multistep strategies and restriction on the substitution pattern still limit the generality.…”

“…In recent advances, Moret's group achieved [2 + 2 + 2] cyclotrimerization with the use of Ni(0) complexes, selectively affording carboxylate substituted terephthalates from terminal alkynes (Scheme 1a). [9] Although a small number of such methods have been exploited, multistep strategies and restriction on the substitution pattern still limit the generality. The pronounced limitation of these strategies, coupled with the increasing emphasis on sustainability, necessitates the development of inclusive green approaches toward the construction of polysubstituted aromatic compounds.…”