Catalytic Enantioselective 6π Photocyclization of Acrylanilides

Enantioselective Photocatalysis Using a Privileged Al-Salen Complex

ConspectusEnantioselective catalysts that exhibit broad generality are disruptive innovators in contemporary synthesis and are considered to be "privileged" on account of their expansive reactivity/selectivity profiles. Operating in the ground state, these species simultaneously regulate reactivity and orchestrate the translation of chiral information with exquisite efficiency: achieving parity in higher-energy (excited-state) scenarios remains a frontier in contemporary catalysis. Advancing this field will require new structure-activation guidelines to be delineated that reflect the energetic realities of achieving chiral induction in non-ground-state environments, thereby expediting the discovery of privileged photocatalysts. Earth-abundant aluminum-salen (Al-salen) complexes, which have a venerable history in ground-state enantioselective catalysis, show great promise in reconciling this disparity on account of their well-defined photophysical properties. In this Account, the potential of these catalysts in engaging various substrates via discrete activation modes to furnish optically enriched products with high levels of reliability is discussed. The deployment of commercial Al-salen complexes in the single electron transfer (SET)-enabled deracemization of cyclopropyl ketones is an exemplar. Irradiation of a commercial Al-salen complex augments the function of the catalyst to enable efficient deracemization (up to 98:2 e.r.), thereby eliminating the need for directing units. In stark contrast to conventional deracemization approaches that are predicated on C(sp3)-H deprotonation/reprotonation sequences, the transformation is characterized by a key C(sp3)-C(sp3) bond cleavage/cyclization process. Subsequent downstream manipulations of the enantioenriched products demonstrate the synthetic utility of the methodology. To illustrate mechanistic diversity using the same Al-salen complex, an enantioselective photocyclization under the auspices of energy transfer (EnT) catalysis is described. The photocyclization of acrylanilides under operationally simple conditions facilitates access to a diverse group of heterocyclic products (up to quantitative yield and 96:4 e.r.) using an Al-salen as the sole chiral operator. Collectively, these mechanistically distinct scenarios illustrate that light activation is a powerful strategy to augment the reactivity arsenal of a ubiquitous small molecule catalyst that is considered to be privileged in the ground state. The mechanistic foundations of reaction development are surveyed (combined experimental and computational approach), together with a perspective on the impact of this enabling technology in chiral functional molecule discovery. This Account serves to emphasize the synthetic utility of leveraging photochemical activation to mitigate intrinsic constraints of processes that might be considered to be thermochemically challenging.

Enantioselective Excited-State Nazarov Reaction: A Relay Strategy of Electrocyclization and Parallel Kinetic Resolution

We report herein the enantioselective photoinduced Nazarov reaction using a relay strategy of an electrocyclization, followed by parallel kinetic resolution (PKR). No enantioselectivity was observed during electrocyclization due to weak coordination between chiral ligands and the substrate's carbonyl group. However, PKR was successfully achieved in the deprotonation step with a bifunctional chiral thiourea ligand. Regioselective deprotonation, controlled by the chiral N,N-dimethyl amine motif, produced cis-6,5,6-fused tricyclic stereoisomers with high ee and yields.

Dynamic Vertical Triplet Energies: Understanding and Predicting Triplet Energy Transfer

A computational approach for modeling and predicting triplet energy sensitization of organic molecules is described, which involves sampling the instantaneous, vertical energy gaps over molecular vibrational motions. This approach provides new theoretical support for the hot-band mechanism of energy transfer, in which the energy difference between donor and acceptor can be lessened by geometric distortions. We demonstrate excellent predictive performance against experimental triplet energies, with R2 = 0.97 and a mean absolute error (MAE) of 1.7 kcal/mol, for a collection of 24 small organic molecules, whereas a static, adiabatic description performs significantly worse (R2 = 0.51, MAE = 9.5 kcal/mol). Using this approach, it is possible to quantitatively predict the correct E/Z-isomerism of alkenes under energy transfer, for which adiabatic calculations predict the wrong outcome.

Visible Light-Induced 6π-Heterocyclization/Dehydroaromatization for Synthesis of Indoloquinolinone Skeletons

In this work, we report a protocol for the synthesis of an indoloquinolinone skeleton using visible light-induced energy transfer. This method avoids the premodification of substrates and exhibits high yields. For gram-scale reactions, only 0.01 mol % (100 ppm) of photosensitizer is required for rapid conversion. Mechanistic studies revealed that this reaction differs from conventional 6π photocyclization reactions; undergoing a process involving 6π cyclization due to energy transfer and dehydrogenation due to product self-catalysis has been experienced.

Photoreactions of Semi-Stiff-Diarylethenes Based on the Cyclohexenol Motif

Nonsymmetric diarylethenes with an additional "stiff" cyclohexenol ring undergo various types of tandem transformations launched by light-induced 6π-photocyclization. Among these, there are two novel reactions (formal [1,3]-H migration and complete aromatization to an anthracene derivative) as well as photorearrangement and formal methane elimination. This diverse reactivity demonstrates the great potential of semi-stiff-diarylethenes in synthetic photochemistry.

Visible light-induced Mallory reaction of tertiary benzanilides via iminium intermediates

The Mallory reaction, which involves the photocyclization of stilbenes/diarylethenes and their analogues into polycyclic aromatics, is of significant synthetic importance. However, its application to tertiary benzanilides has not been explored to date. Besides, most of the reported Mallory reactions require ultraviolet irradiation. In this study, we show the first Mallory reaction of tertiary benzanilides promoted by visible light via iminium intermediates formed in situ from tertiary benzanilide, Tf2O (triflic anhydride) and pyridine. UV/vis absorption spectroscopy combined with density functional theory (DFT) calculations revealed that the formation of the iminium intermediate decreased the HOMO-LUMO energy gap, thereby enhancing visible light absorption. This study provides a rapid and practical approach for the preparation of the phenanthridinone skeleton and provides a new idea for the design of new visible light photoswitches.

Catalytic enantioselective synthesis of 2-pyrazolines via one-pot condensation/6π-electrocyclization: 3,5-bis(pentafluorosulfanyl)-phenylthioureas as powerful hydrogen bond donors

A new conjugate-base-stabilized carboxylic acid (CBSCA) containing a 3,5-bis(pentafluorosulfanyl)phenylthiourea functionality catalyses challenging one-pot condensations/6π-electrocyclizations of hydrazines and α,β-unsaturated ketones under mild conditions. Structurally diverse N-aryl 2-pyrazolines are obtained in good yields and enantioselectivities. The superior performance of 3,5-bis(SF5)phenylthioureas over the widely used 3,5-bis(CF3)phenylthioureas is further demonstrated in the Michael addition of dimethyl malonate to nitrostyrene, using a new Takemoto-type catalyst.

Accessing 3,4-Dihydroquinolinone from N-Arylacrylamides via Tetralone-Mediated Energy Transfer and 1,3-Hydrogen Shift

Herein, we introduce a tetralone-mediated photocyclization method of N-arylacrylamides. The protocol proceeds smoothly to deliver a diverse set of 3,4-dihydroquinolinones in moderate to good yields with excellent functional group compatibility and readily allows for late-stage modifications of a number of complex drug molecules. Mechanistic studies reveal that the present systems with ultraviolet light irradiation enable the cyclization via energy transfer and exclusive 1,3-hydrogen shift.

Regiodivergent Ring-Expansion of Oxindoles to Quinolinones

The development of divergent methods to expedite structure-activity relationship studies is crucial to streamline discovery processes. We developed a rare example of regiodivergent ring expansion to access two regioisomers from a common starting material. To enable this regiodivergence, we identified two distinct reaction conditions for transforming oxindoles into quinolinone isomers. The presented methods proved to be compatible with a variety of functional groups, which enabled the late-stage diversification of bioactive oxindoles as well as facilitated the synthesis of quinolinone drugs and their derivatives.

Highly Enantioselective 6π Photoelectrocyclizations Engineered by Hydrogen Bonding

Photochemical electrocyclization reactions are valued for both their ability to produce structurally complex molecules and their central role in elucidating fundamental mechanistic principles of photochemistry. We present herein a highly enantioselective 6π photoelectrocyclization catalyzed by a chiral Ir(III) photosensitizer. This transformation was successfully realized by engineering a strong hydrogen-bonding interaction between a pyrazole moiety on the catalyst and a basic imidazolyl ketone on the substrate. To shed light on the origin of stereoinduction, we conducted a comprehensive investigation combining experimental and computational mechanistic studies. Results from density functional theory calculations underscore the crucial role played by the prochirality and the torquoselectivity in the electrocyclization process as well as the steric demand in the subsequent [1,4]-H shift step. Our findings not only offer valuable guidance for developing chiral photocatalysts but also serve as a significant reference for achieving high levels of enantioselectivity in the 6π photoelectrocyclization reaction.

A Lewis Acid-Controlled Enantiodivergent Epoxidation of Aldehydes

Two epoxidation catalysts, one of which consists of two VANOL ligands and an aluminum and the other that consists of two VANOL ligands and a boron, were compared. Both catalysts are highly effective in the catalytic asymmetric epoxidation of a variety of aromatic and aliphatic aldehydes with diazoacetamides, giving high yields and excellent asymmetric inductions. The aluminum catalyst is effective at 0 °C and the boron catalyst at -40 °C. Although both the aluminum and boron catalysts of (R)-VANOL give very high asymmetric inductions (up to 99% ee), they give opposite enantiomers of the epoxide. The mechanism, rate- and enantioselectivity-determining step, and origin of enantiodivergence are evaluated using density functional theory calculations.

Enantioconvergent 6π Electrocyclization Enabled by Photoredox Racemization

This study presents a novel photoredox-enabled enantioconvergent catalytic strategy used to construct chiral 2H-1,3-benzoxazines via an unprecedented oxa-6π electrocyclization utilizing racemic α-substituted glycinates as substrates. The approach leverages a cobalt-based chiral Lewis acid catalyst, which promotes the transformation under thermal or photoredox conditions. While the thermal reaction selectively converts only the (S)-configured glycinates into enantioenriched 2H-1,3-benzoxazines (up to 96:4 e.r.), the addition of 0.5 mol % of a commercially available iridium photocatalyst under visible light irradiation transforms the reaction into an enantioconvergent process. Detailed mechanistic and time course studies of optically pure α-deuterated substrates revealed the presence of an enantiospecific kinetic isotope effect, which helped to clarify the role of both the photo- and chiral Lewis acid catalyst in the reaction sequence. In this dual catalytic system, the photocatalyst promotes a dynamic interconversion between the substrate enantiomers─a process not accessible via ground-state chemistry─while the chiral Lewis acid selectively transforms only the (S)-configured substrates. Further mechanistic evidence for the proposed mechanism is provided by linear free energy relationship analysis, which suggests that the stereodetermining step involves a 6π electrocyclization under both thermal and photoredox conditions.

Asymmetric Synthesis of α-Methylene-γ-Butyrolactones via Tandem Allylboration/Lactonization: a Kinetic Resolution Process

The α-methylene-γ-butyrolactone motif is a widely encountered unit in many natural products and pharmaceutical compounds. Herein, a practical and efficient synthesis of α-methylene-γ-butyrolactones from readily available allylic boronates and benzaldehyde derivatives was developed with chiral N,N'-dioxide/AlIII complex as the catalyst. The key success of this transformation was the kinetic resolution of allylboration intermediate via asymmetric lactonization. This protocol enabled to assemble all of four stereoisomers from the same set of starting materials upon variable lactonization. Taking advantage of the current method as the key step, catalytic asymmetric total synthesis of eupomatilones 2, 5, and 6 was accomplished. Control experiments were carried out to probe into the tandem reaction as well as the origin of stereoselectivities.

Investigating the Effect of Lewis Acid Co-catalysts on Photosensitized Visible-Light De Mayo Reactions

Herein, we describe studies showing that Lewis acid co-catalysts can significantly broaden the scope of alkenes that can be incorporated into the photosensitized visible-light De Mayo reaction. Mechanistic studies suggest that the primary benefit of the Lewis acid is not on substrate sensitization but rather on bond-forming steps downstream of energy transfer, highlighting the diverse effects that Lewis acids can have on sensitized photoreactions.