Triplet Energy Transfer from Ruthenium Complexes to Chiral Eniminium Ions: Enantioselective Synthesis of Cyclobutanecarbaldehydes by [2+2] Photocycloaddition

Abstract

Chiral eniminium salts, prepared from a,b-unsaturated aldehydes and a chiral proline derived secondary amine, underwent, upon irradiation with visible light, a rutheniumcatalyzed (2.5 mol %) intermolecular [2+2] photocycloaddition to olefins, which after hydrolysis led to chiral cyclobutanecarbaldehydes (17 examples, 49-74 % yield), with high diastereo-and enantioselectivities. Ru(bpz) 3 (PF 6 ) 2 was utilized as the ruthenium catalyst and laser flash photolysis studies show that the catalyst operates exclusively by triplet-energy transfer (sensitization). A catalytic system was devised with a chiral secondary amine co-catalyst. In the catalytic reactions, Ru(bpy) 3 (PF 6 ) 2 was employed, and laser flash photolysis experiments suggest it undergoes both electron and energy transfer. However, experimental evidence supports the hypothesis that energy transfer is the only productive quenching mechanism. Control experiments using Ir(ppy) 3 showed no catalysis for the intermolecular [2+2] photocycloaddition of an eniminium ion.

Triplet Energy Transfer Photocatalysis: Unlocking the Next Level

“…As demonstrated by Yoon and co-workers, Lewis acids can significantly decrease excited-state energies, 39 which is likely expandable to further classes of substrates or activators, as already demonstrated by LUMO lowering of enones through eniminium ion formation. 40 This merger of the unique reactivity modes of photo-and Lewis acid catalysis or amine organocatalysis underlines the value of dual catalytic approaches (Figure 7). Within the recent years, dual-EnT catalysis has emerged not only to activate otherwise unreactive substrate classes but also to enable novel synthetic (dis)connections.…”

“…In this case, the Lewis acid not only induces chirality but also lowers the olefin triplet energy through bidentate coordination, thereby ''switching on'' the initial EnT event (Figure 3B). 39 Similarly, Bach and co-workers 40 demonstrated how covalent pre-activation (through iminium formation) allows for activating acyclic enones for sensitized (enantioselective) cycloadditions.…”

Chiral Brønsted acid-controlled intermolecular asymmetric [2 + 2] photocycloadditions

“…Notably, a p-alkoxy-substituted styrene (20) was tolerated under these conditions, whereas the same alkene undergoes rapid polymerization using our previous Lewis acid-catalyzed methods 11 . Various functional groups including protected alcohols (22)(23), a photolabile azide (24), and a protected amine (25) were all tolerated. Heterocycles were also readily transformed including benzofuran-and indole-substituted alkenes (28)(29).…”

“…Pioneering studies by Melchiorre and colleagues 19,20 have demonstrated that secondary amine organocatalysts can activate prochiral enals by chromophore activation: the iminium intermediates absorb at longer wavelengths than the free enal substrates and the excited states have been utilized in a variety of useful organic transformations. Very recently, Bach and colleagues 21,22 demonstrated the first example of triplet activation by secondary amine organocatalysis, demonstrating that the vinyl iminium intermediate undergoes energy transfer faster than the free parent enal and can be the key intermediate in an enantioselective [2 + 2] photocycloaddition. The ability of Brønsted acid catalysts to engage in chromophore and triplet activation mechanisms, however, has been underexplored.…”

A Designed Photoenzyme Promotes Enantioselective [2+2]-Cycloadditions via Triplet Energy Transfer

“…The next step involves energy transfer from the triplet photosensitiser to the substrate in an overall spin-allowed process that returns the photosensitiser to the S0 level and simultaneously promotes the substrate from S0 to the reactive T1 state, which can then undergo a variety of intra-and intermolecular processes. 2,3 Enantioselective versions of photochemical reactions have been developed through dual catalytic strategies that combine an achiral photosensitiser with a photochemically inactive chiral catalyst (Figure 1C), [12][13][14] or by employing chiral photosensitisers (Figure 1D). 10,15 Although powerful, these approaches do not offer a general solution, and many desirable photochemical processes are not amenable to enantioselective catalysis.…”