Asymmetric Photochemical [2 + 2]-Cycloaddition of Acyclic Vinylpyridines through Ternary Complex Formation and an Uncontrolled Sensitization Mechanism

Abstract

Stereochemical control of photochemical reactions that occur via triplet energy transfer remains a challenge. Suppressing off-catalyst stereorandom reactivity is difficult for highly reactive open-shell intermediates. Strategies for suppressing racemate-producing, off-catalyst pathways have long focused on formation of ground state, substrate-catalyst chiral complexes that are primed for triplet energy transfer via a photocatalyst in contrast to their off-catalyst counterparts. Herein, we describe a strategy where both a chiral catalyst-associated vinylpyridine and a nonassociated, free vinylpyridine substrate can be sensitized by an Ir(III) photocatalyst, yet high levels of diastereo- and enantioselectivity in a [2 + 2] photocycloaddition are achieved through a preferred, highly organized transition state. This mechanistic paradigm is distinct from, yet complementary to current approaches for achieving high levels of stereocontrol in photochemical transformations.

Cp*Rh(III)-Catalyzed Enantioselective C(sp3)–H Amidation of Azine-Linked Cyclobutanes

“…Given the exceptionally high performance of (R)-L27, the effects of other variables, including the metal catalyst, stoichiometry, and additives were also investigated (Table 1, entries [10][11][12][13][14]. No reaction was detected without a metal catalyst (Table 1, entry 10) or with other trivalent group 9 metal catalysts (Table 1, entry 11).…”

“…the formation of only disubstituted cyclobutanes is rarely reported. 14,15 Additionally, due to the scarcity and stability of some olefin precursors, it is challenging to directly obtain cyclobutane products specifically disubstituted at the C1/C2 position through cyclization reactions. Another alternative pathway employs cyclobutene or cyclobutanone or bicyclo[1.1.0]butane 8,16 building blocks to undergo addition or reduction reactions to obtain the targeted cyclobutanes, albeit with increased step count.…”

Construction of gel networksvia[2+2] photocycloaddition

“…With the development of photochemistry, [2+2] cycloaddition upon visible light catalysis [46][47][48][49][50][51] of two olefins can occur mildly and smoothly via single-electron transfer and energy transfer. [52][53][54][55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70][71] Compared with the traditional [2+2] photocycloaddition, visible light-induced photocatalytic reactions occur under extremely mild conditions, with most reactions proceeding at room temperature and highly reactive radical initiators were hardly used. The light source is typically a commercial household light bulb rather than the specialized equipment required for processes employing high-energy UV light.…”

Counterion Effects in [Ru(bpy)3](X)2-Photocatalyzed Energy Transfer Reactions

“…The common method to manipulate excited-state properties of such photocatalysts relies on modifying the ligand scaffold around the metal center: a modus operandi that has provided access to a plethora of photocatalysts covering a broad range of redox potentials and triplet excited state energies. , Importantly, a large majority of these Ir(III) and Ru(II) complexes, but also of emerging earth-abundant transition metal-based luminescent complexes, − are cationic species associated with counterion(s) to ensure charge balance of the molecular photocatalysts. Nevertheless, while photocatalytic reaction cycles are usually modeled considering the properties of the sole organometallic cation, the true nature of the photocatalyst is that of a salt in which the anions could play a role beyond the one of mere spectators. − In this respect, recent reports have brought to light the influence of counterion identity on the efficiency of single electron transfer (SET) processes operated by cationic iridium and ruthenium photoredox catalysts (Figure A). − However, despite the fact that both SET and energy transfer (EnT) photocatalyses rely on the triplet metal-to-ligand charge-transfer ( 3 MLCT) excited state of the photocatalyst, counterion effects on EnT processes have not been demonstrated yet. In triplet–triplet energy transfer (TTEnT) processes, a burgeoning field of research with a wide range of synthetic applications, − the photocatalyst is excited by the direct absorption of visible light and subsequently transfers its triplet excited state energy to a desired substrate, opening otherwise elusive relaxation channels and photoreactive routes, leading to original chemical transformations .…”