Light-Activated Olefin Metathesis: Catalyst Development, Synthesis, and Applications.

Or Eivgi,Ravindra S Phatake,Noy B Nechmad,N Gabriel Lemcoff

doi:10.1021/acs.accounts.0c00495

Or Eivgi, Ravindra S Phatake + Show 2 more

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https://doi.org/10.1021/acs.accounts.0c00495

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Abstract

ConspectusThe most important means for tuning and improving a catalyst’s properties is the delicate exchange of the ligand shell around the central metal atom. Perhaps for no other organometallic-catalyzed reaction is this statement more valid than for ruthenium-based olefin metathesis. Indeed, even the simple exchange of an oxygen atom for a sulfur atom in a chelated ruthenium benzylidene about a decade ago resulted in the development of extremely stable, photoactive catalysts. This Account presents our perspective on the development of dormant olefin metathesis catalysts that can be activated by external stimuli and, more specifically, the use of light as an attractive inducing agent.The insight gained from a deeper understanding of the properties of cis-dichlororuthenium benzylidenes opened the doorway for the systematic development of new and efficient light-activated olefin metathesis catalysts and catalytic chromatic-orthogonal synthetic schemes. Following this, ways to disrupt the ligand-to-metal bond to accelerate the isomerization process that produced the active precatalyst were actively pursued. Thus, we summarize herein the original thermal activation experiments and how they brought about the discoveries of photoactivation in the sulfur-chelated benzylidene family of catalysts. The specific wavelengths of light that were used to dissociate the sulfur–ruthenium bond allowed us to develop noncommutative catalytic chromatic-orthogonal processes and to combine other photochemical reactions with photoinduced olefin metathesis, including using external light-absorbing molecules as “sunscreens” to achieve novel selectivities. Alteration of the ligand sphere, including modifications of the N-heterocyclic carbene (NHC) ligand and the introduction of cyclic alkyl amino carbene (CAAC) ligands, produced more efficient light-induced activity and special chemical selectivity. The use of electron-rich sulfoxides and, more prominently, phosphites as the agents that induce latency widened the spectrum of light-induced olefin metathesis reactions even further by expanding the colors of light that may now be used to activate the catalysts, which can be used in applications such as stereolithography and 3D printing of tough metathesis-derived polymers.

Highlights

Since the emergence of well-defined transition metal catalysts, olefin metathesis has evolved to become a handy paintbrush for molecular artists in industry and academia.[5]
Evolution of Light-Induced Olefin Metathesis Catalysts and Applications by the Lemcoff Research Group (Year of Publication in Parentheses) options to form and reshape carbon−carbon double bonds, olefin metathesis is the method of choice for the synthesis of macrocycles[6] and heavy-duty polymeric materials[7] and even for the preparation of simple commodity chemicals from oil waste products and biomass.[8]
The flexible framework of ruthenium-based complexes has opened up many research avenues for the development of task-specific catalysts,[9] such as catalysts embedded with asymmetric ligands for enantioselective olefin metathesis reactions,[10] catalysts that specialize in ring-closing metathesis (RCM) of macrocyclic structures,[11] catalysts for Z-selective[12] and stereoretentive[13] olefin metathesis reactions, and fastinitiating catalysts for ring-opening metathesis polymerization (ROMP).[14]

Summary

■ INTRODUCTION

Since the emergence of well-defined transition metal catalysts, olefin metathesis has evolved to become a handy paintbrush for molecular artists in industry and academia.[5]. The photoinduced metathesis activity of the most efficient complexes was further evaluated with several RCM and ROMP reactions (Figure 7 and Table 1) They showed improved performances compared with 9-cis, this new generation of thermally and photochemically switchable sulfur-chelated catalysts still suffered from some drawbacks. The cis-dichloro configuration of 43 was verified by NMR spectroscopy and single-crystal X-ray diffraction This catalyst could be activated thermally by heating to 80 °C in toluene or photochemically using a 405 nm light-emitting diode (LED) in toluene to complete several RCM and ROMP reactions; the strong chelation impeded a high catalytic activity, and metathesis reactions using this catalyst required long irradiation times and high catalyst loadings (Figure 16). While the experiment that was exposed to visible light yielded a functional and elastic polymer film, the experiment that was exposed first to UV-C yielded a sticky amorphous gel

■ OUTLOOK AND CONCLUSIONS

■ REFERENCES