Pincer complexes, Non-innocent ligands and Cooperative catalysis

Metal–ligand–Lewis acid multi-cooperative catalysis: a step forward in the Conia-ene reaction
A. Clerc, E. Marelli, N. Adet, J. Monot, B. Martín-Vaca, D. Bourissou
Chem. Sci., 2021,12, 435-441

Palladium pincer complexes featuring an unsymmetrical SCN indene-based ligand with a hemilabile pyridine sidearm
P. Brunel, C. Lhardy, S. Mallet-Ladeira, J. Monot, B. Martin-Vaca, D. Bourissou
Dalton Trans., 2019, 48, 9801-9806.

A new unsymmetrical indene-based pro-ligand featuring thiophosphinoyle and methylpyridine sidearms 2 was prepared. Coordination and cyclometalation in the presence of [PdCl₂(PhCN)₂] and PS-DIEA afforded three well-defined 2-indenyl SCN pincer complexes 3a–c. The lability of the pyridine moiety has been evidenced upon treatment with triphenylphosphine and 2,6-dimethylphenylisocyanide. In addition, reversible C–Pd bond cleavage has been demonstrated under Brønsted acid/base conditions. The indenediide SCN pincer complex 4 was prepared by deprotonation of 3a in the presence of triphenylphosphine. Preliminary catalytic tests on the cycloisomerization of 4-pentynoic acid have underlined the impact of the pyridine sidearm on the catalytic activity.

Valorization of CO₂: Preparation of 2-​Oxazolidinones by Metal-​Ligand Cooperative Catalysis with SCS Indenediide Pd Complexes
P. Brunel, J. Monot, C. E. Kefalidis, L. Maron, B. Martin-Vaca,* D. Bourissou*
ACS Catal. 2017, 7, 2652-2660.

The capture and utilization of CO₂ to prepare high-value compounds is very attractive chemically and highly desirable socially. Indenediide-based Pd SCS pincer complexes are shown here to promote the carboxylative cyclization of propargylamines leading to 2-oxazolidinones under mild conditions (0.5–1 bar of CO2, DMSO, 40–80 °C, 1–5 mol % Pd loading). The indenediide Pd complex is competitive with known catalysts. It proved successful for a wide range of propargylamines, including hitherto challenging substrates such as secondary propargylamines bearing tertiary alkyl groups at nitrogen, primary propargylamines, and propargylanilines. Thorough experimental (NMR) and computational (DFT) investigations were undertaken to gain mechanistic insights. Accordingly, (i) the resting state of the catalytic cycle is a Pd DMSO complex; (ii) the indenediide backbone and Pd center act in concert to activate the carbamic acid intermediate and promote its cyclization; (iii) proton shuttling is essential to lower the activation barriers of the initial amine carboxylation as well as of the proton transfers between the ligand backbone and the organic fragments at Pd.

Platinum pincer complexes featuring an SCS indenediide backbone have been prepared and evaluated in the catalytic cycloisomerization of alkynoic acids and N-tosyl alkynylamides. One of the platinum complexes significantly outperforms its palladium analogue for the formation of 6- and 7-membered rings. The catalytic system takes advantage of the alkynophilicity of Pt and of the non-innocent character of the indenediide framework. Like for Pd, the catalytic performance is significantly improved by using an H-bond donor additive such as pyrogallol. For the first time, a large variety of ω-unsaturated δ- and ε-lactones/lactams could be prepared with high selectivities and in very good yields.

The mechanism of alkynoic acid cycloisomerization with SCS indenediide Pd pincer complexes has been investigated experimentally and computationally. These studies confirmed the cooperation between the Pd center and the backbone of the pincer ligand, and revealed the involvement of a second molecule of substrate. It acts as a proton shuttle in the activation of the acid, it directs the nucleophilic attack of the carboxylic acid on the π-coordinated alkyne and it relays the protonolysis of the resulting vinyl Pd species. A variety of H-bond donors have been evaluated as external additives, and polyols featuring proximal hydroxyl groups, in particular catechol derivatives, led to significant catalytic enhancement. The impact of 4-nitrocatechol and 1,2,3-benzenetriol is particularly striking on challenging substrates such as internal 4- and 5-alkynoic acids. Endo/exo selectivities up to 7.3/1 and 60-fold increase in reactivity were achieved.

New SCS Pd pincer complexes featuring a non-innocent indenediide backbone show high catalytic activity in cycloisomerization. A variety of alkylidene lactams (5 to 7 membered rings) have been prepared efficiently from N-tosyl alkynylamides and good results have also been obtained with challenging alkynoic acids.

Indenediide Pd complexes 1a-c are shown to very efficiently catalyze the cycloisomerization of alkynoic acids into alkylidene lactones via metal-ligand cooperation (TON up to 2000). 1a-c are competent towards a broad range of alkynoic acids, including functionalized and internal ones, and give access to 5- as well as 6- and 7-membered lactones in excellent yields and with very high selectivities.