Tuning the Selectivity of Palladium Catalysts for Hydroformylation and Semihydrogenation of Alkynes: Experimental and Mechanistic Studies

Abstract

Here, we describe a selective palladium catalyst system for chemodivergent functionalization of alkynes with syngas. In the presence of an advanced ligand L2 bearing 2-pyridyl substituent as a built-in base, either hydroformylation or semihydrogenation of diverse alkynes occurs with high chemo- and stereoselectivity under comparable conditions. Mechanistic studies, including density functional theory (DFT) calculations, kinetic analysis, and control experiments, revealed that the strength and concentration of acidic cocatalysts play a decisive role in controlling the chemoselectivity. DFT studies disclosed that ligand L2 not only promotes heterolytic activation of hydrogen similar to frustrated Lewis pair (FLP) systems in the hydrogenolysis step for hydroformylation but also suppresses CO coordination to promote semihydrogenation under strong acid conditions. This switchable selectivity provides a strategy to design new catalysts for desired products.

Palladium‐Catalyzed Hydroformylation of Alkenes and Alkynes

“…In contrast to the transition-metal-catalyzed hydroformylation of olefins, which is among the most extensively studied catalytic reactions, hydroformylation of alkynes gained rather limited attention over the years. [60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78] This disparity might be surprising, considering that the products, the α,β-unsaturated aldehydes are important intermediates in organic synthesis, particularly for the synthesis of bioactive compounds, flavors, and fragrances. One challenge that hinders the development of efficient hydroformylation protocols is the high reactivity of starting materials and products.…”

“…During the past two decades, several methods of rhodium-catalyzed hydroformylation of alkynes were reported, demonstrating that the selective production of α,β-unsaturated aldehydes from alkynes is feasible. [62][63][64][65][66]68,69,71,72,[75][76][77]79] Hereafter, only the palladium-catalyzed hydroformylation of alkynes will be discussed.…”

“…In 2020, Beller and coworkers reported acid-controlled chemoselective transformations of alkynes under syngas to produce selectively either α,β-unsaturated aldehydes or (E)alkenes in the presence of the same palladium-based catalysts. [77] The authors devised a palladium catalyst bearing special ligand L15 equipped with a 2-pyridyl substituent acting as a built-in base. Remarkably, using PTSA as the co-catalyst, the reactions of internal alkynes occurred to form α,β-unsaturated aldehyde products with high regio-and stereoselectivities (chemoselective hydroformylation; Scheme 11, blue path), while the same reaction but in the presence of CF 3 SO 3 H in place of PTSA furnished (E)-alkene products predominantly (chemoselective hydrogenation; Scheme 11, red path).…”

“…Besides the above-mentioned Pd-based protocols, methods involving other catalysts, including Co- [60] and Rh-based complexes [61][62][63][64][65][66]68,69,71,72,[75][76][77] , were reported to undergo hydroformylation of internal alkynes. However, the counterpart methods for terminal alkynes with adequate activity and chemoselectivity remain rather rare.…”

“…In contrast to the transition‐metal‐catalyzed hydroformylation of olefins, which is among the most extensively studied catalytic reactions, hydroformylation of alkynes gained rather limited attention over the years [60–78] . This disparity might be surprising, considering that the products, the α,β‐unsaturated aldehydes are important intermediates in organic synthesis, particularly for the synthesis of bioactive compounds, flavors, and fragrances.…”