Intermolecular [2 + 2] Cycloaddition of 1,4-Dihydropyridines with Olefins via Energy Transfer

Photocatalyzed Selective Hydrocarbonation of Alkenes with Hantzsch Esters toward 4-Alkyl-Hantzsch Esters

Here, we describe a mild photoredox-neutral reaction system that enables the selective hydrocarbonation of alkenes with Hantzsch esters, affording structurally diverse 4-alkyl-Hantzsch esters. This straightforward protocol can be performed under an air atmosphere without the need for any transition metals. The synthetic potential of this method is well exemplified by the late-stage structural modification of a series of pharmaceutically relevant complex molecules.

Dearomative Construction of 2D/3D Frameworks from Quinolines via Nucleophilic Addition/Borate-Mediated Photocycloaddition

Dearomative construction of multiply-fused 2D/3D frameworks, composed of aromatic two-dimensional (2D) rings and saturated three-dimensional (3D) rings, from readily available quinolines has greatly contributed to drug discovery. However, dearomative cycloadditions of quinolines in the presence of photocatalysts usually afford 5,6,7,8-tetrahydroquinoline (THQ)-based polycycles, and dearomative access to 1,2,3,4-THQ-based structures remains limited. Herein, we present a chemo-, regio-, diastereo-, and enantioselective dearomative transformation of quinolines into 1,2,3,4-THQ-based 6-6-4-membered rings without any catalyst, through a combination of nucleophilic addition and borate-mediated [2+2] photocycloaddition. Detailed mechanistic studies revealed that the photoexcited borate complex, generated from quinoline, organolithium, and HB(pin), accelerates the cycloaddition and suppresses the rearomatization that usually occurs in conventional photocycloaddition. Based on our mechanistic analysis, we also developed further photoinduced cycloadditions affording other types of 2D/3D frameworks from isoquinoline and phenanthrene.

Synthesis of cyclohepta[b]indoles via gold mediated energy transfer photocatalysis

Photocatalysis involving energy transfer (EnT) has become a valuable technique for building intricate organic frameworks mostly through [2+2]-cycloaddition reactions. Herein, we report a synthetic method leading to functionalized cyclohepta[b]indoles, an important structural motif in natural products and pharmaceuticals, using gold-mediated energy transfer photocatalysis. The scope of this operationally simple and atom-economical strategy is presented. Density functional theory studies were employed in order to gain insights into the mechanism of formation of the cyclohepta[b]indole core.

Visible-Light-Driven [2 + 2] Photocycloaddition for Constructing Dimers of N,N'-Diacyl-1,4-dihydropyrazines: Experimental and Theoretical Investigation

In this study, the visible-light-driven [2 + 2] photocycloaddition of 1,4-dihydropyrazines in solution was reported. The N,N'-diacyl-1,4-dihydropyrazines with different substituents showed completely different reactivity under the irradiation of a 430 nm blue light-emitting diode (LED) lamp. N,N'-Diacetyl-1,4-dihydropyrazine and N,N'-dipropionyl-1,4-dihydropyrazine were the only compounds capable of undergoing a [2 + 2] photocycloaddition reaction, yielding syn-dimers and cage-dimers (known as 3,6,9,12-tetraazatetraasteranes) with overall yields of 76 and 83%, correspondingly. The substituent-reactivity effect on [2 + 2] photocycloaddition of N,N'-diacyl-1,4-dihydropyrazines was investigated by density functional theory calculations. The results show that the substituents have little influence on Gibbs free energy for the [2 + 2] photocycloaddition and mainly affect the excited energy, reaction sites, and the triplet excited-state structures of 1,4-dihydropyrazines, which are closely related to whether the reaction occurs. The results offer insights into the photochemical reactivity of 1,4-dihydropyrazines and an approach for constructing dimers of N,N'-diacyl-1,4-dihydropyrazines through a solution-based visible-light-driven [2 + 2] photocycloaddition, especially for the construction of 3,6,9,12-tetraazatetraasteranes. Compared with the solid-state [2 + 2] photocycloaddition of 1,4-dihydropyrazine, this photocycloaddition will be an efficient and environmentally friendly method for synthesizing tetraazatetraasteranes with the advantages of milder reaction conditions, simple operation, adjustable reaction amounts by omitting the cocrystal growth step, etc.

Construction of 3,12-Diazatetracyclododecane-dienes through Unexpected Visible-Light-Induced Radical Cascade Cyclization

A novel visible-light-induced cascade radical cyclization reaction of 3-cyano-4-aryl-1,4-dihydropyridines for the construction of 3,12-diazatetracyclododecane-diene derivatives is reported for the first time. In the presence of 410 nm blue LED lamp as the light source and ethanol as the solvent, the reactions proceed smoothly to afford photocyclization products in good yields. The process is carried out through the breaking of original C═C double bonds and the formation of three new single bonds in one pot and proved to be able to tolerate different substituents.

LED Light Sources in Organic Synthesis: An Entry to a Novel Approach

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In recent years, photocatalytic technology has shown great potential as a low-cost, environmentally friendly, and sustainable technology. Compared to other light sources in photochemical reaction, LEDs have advantages in terms of efficiency, power, compatibility, and environmentally friendly nature. This review highlights the most recent advances in LED-induced photochemical reactions. The effect of white and blue LEDs in reactions such as oxidation, reduction, cycloaddition, isomerization, and sensitization is discussed in detail. No other reviews have been published on the importance of white and blue LED sources in the photocatalysis of organic compounds. Considering all the facts, this review is highly significant and timely.

Visible light-assisted Ni-/Ir-catalysed atom-economic synthesis of spiro[furan-3,1'-indene] derivatives

An atom-economic, efficient, and highly convenient construction of spiro[furan-3,1'-indene] skeletons from isocyanides and 1,5-enynes by synergistic nickel- and iridium-photocatalysis is reported. Spirocyclization was developed under practical and mild conditions, which features excellent functional group tolerance, gram-scale synthesis and representative synthetic transformations for the obtained products and broad substrate scope. Primary mechanistic studies demonstrated that the reaction proceeds through energy-transfer-mediated excitation of intermediate catalytic species.

Catalytic Asymmetric [3 + 2] Annulation of Hantzsch Esters with Racemic N-Sulfonylaziridines

Hantzsch esters (HEs) served as two-carbon partners in a copper(I)-catalyzed enantioselective [3 + 2] annulation with racemic 2-(hetero)aryl-N-sulfonyl aziridines via kinetic resolution to provide pyrrolo[2,3-b]tetrahydropyridines containing multiple contiguous stereogenic centers including all-carbon quaternary centers in excellent yields and enantiopurities and moderate-to-excellent diastereoselectivities. Mainly dependent upon the structures of the aziridines, a competitive hydrogenolysis process with HEs as the hydrogen source was also observed in some cases.

Regio- and diastereoselective dearomatizations of N-alkyl activated azaarenes: the maximization of the reactive sites

An unprecedented base-promoted multi-component one-pot dearomatization of N-alkyl activated azaarenes was developed, which enabled the synthesis of complex and diverse bridged cyclic polycycles with multiple stereocenters in a highly regio- and diastereoselective manner. Besides, we realized the step-controlled dearomative bi- and trifunctionalization of quinolinium salts. These transformations not only achieved the maximization of the reaction sites of pyridinium, quinolinium and isoquinolinium salts to enhance structural complexity and diversity, but also opened up a new reaction mode of these N-activated azaarenes. A unique feature of this strategy is the use of easily accessible and bench-stable N-alkyl activated azaarenes to provide maximum reactive sites for dearomative cascade cyclizations. In addition, the salient characteristics including high synthetic efficiency, short reaction time, mild conditions and simple operation made this strategy particularly attractive.

An unprecedented base-promoted multi-component one-pot dearomatization of N-alkyl activated azaarenes was developed to construct complex and diverse bridged cyclic polycycles with multiple stereocenters in a highly regio- and diastereoselective manner.

Aldolase-Catalyzed Asymmetric Synthesis of N-Heterocycles by Addition of Simple Aliphatic Nucleophiles to Aminoaldehydes

Nitrogen heterocycles are structural motifs found in many bioactive natural products and of utmost importance in pharmaceutical drug development. In this work, a stereoselective synthesis of functionalized N-heterocycles was accomplished in two steps, comprising the biocatalytic aldol addition of ethanal and simple aliphatic ketones such as propanone, butanone, 3-pentanone, cyclobutanone, and cyclopentanone to N-Cbz-protected aminoaldehydes using engineered variants of d-fructose-6-phosphate aldolase from Escherichia coli (FSA) or 2-deoxy-d-ribose-5-phosphate aldolase from Thermotoga maritima (DERA Tma ) as catalysts. FSA catalyzed most of the additions of ketones while DERA Tma was restricted to ethanal and propanone. Subsequent treatment with hydrogen in the presence of palladium over charcoal, yielded low-level oxygenated N-heterocyclic derivatives of piperidine, pyrrolidine and N-bicyclic structures bearing fused cyclobutane and cyclopentane rings, with stereoselectivities of 96-98 ee and 97:3 dr in isolated yields ranging from 35 to 79%.

An elusive thermal [2 + 2] cycloaddition driven by visible light photocatalysis: tapping into strain to access C2-symmetric tricyclic rings

A mild and operationally simple methodology is reported for the synthesis of cyclobutane rings imbedded within a C2-symmetric tricyclic framework. The method uses visible light and an iridium-based photocatalyst to drive the oft-stated "forbidden" thermal [2 + 2] cycloaddition of cycloheptenes and analogs. Importantly, it generates cyclobutane with four new stereocenters with excellent stereoselectivity, and perfect regioselectivity. The reaction is propelled forward when the photocatalyst absorbs a visible light photon, which transfers this energy to the cycloheptene. Key to success is, upon excitation to the triplet via sensitization from the photocatalyst, the double bond isomerizes to give the transient, highly strained, trans-cycloheptene. The trans-cycloheptene undergoes a strain relieving thermal, intermolecular [π2s + π2a] cycloaddition with another cis-cycloheptene. X-ray analysis reveals that the major product is the head-to-head, C2-symmetric all trans-cyclobutane. Additionally, a dramatic display structural complexity enhancement is observed with the use of chiral cycloheptenols possessing one stereocenter, which results in the formation of cyclobutanes with six contiguous stereocenters with good to excellent diastereocontrol, and can be used to isolate single stereoisomers of stereochemically complex cyclobutanes in good yield.

Synthesis of Cyclobutane-Fused Angular Tetracyclic Spiroindolines via Visible-Light-Promoted Intramolecular Dearomatization of Indole Derivatives

An intramolecular dearomatization of indole derivatives based on visible-light-promoted [2+2] cycloaddition was achieved via energy transfer mechanism. The highly strained cyclobutane-fused angular tetracyclic spiroindolines, which were typically unattainable under thermal conditions, could be directly accessed in high yields (up to 99%) with excellent diastereoselectivity (>20:1 dr) under mild conditions. The method was also compatible with diverse functional groups and amenable to flexible transformations. In addition, DFT calculations provided guidance on the rational design of substrates and deep understanding of the reaction pathways. This process constituted a rare example of indole functionalization by exploiting visible-light-induced reactivity at the excited states.

Hantzsch esters: an emerging versatile class of reagents in photoredox catalyzed organic synthesis

This minireview highlights the recent advances in the chemistry of Hantzsch esters in photoredox catalyzed organic synthesis, with particular emphasis placed on reaction mechanisms.

Construction of Cyclobutanes by Multicomponent Cascade Reactions in Homogeneous Solution through Visible-Light Catalysis

[2+2] Photocycloaddition of two olefins is a general method to assemble the core scaffold, cyclobutane, found in numerous bioactive molecules. A new approach to synthesize cyclobutanes through multicomponent cascade reactions by merging aldol reaction and Witting reaction with visible-light-induced [2+2] cycloaddition is reported. An array of cyclobutanes with high selectivity has been achieved from commercially available aldehydes, ketones (or phosphorus ylide), and olefins with visible-light irradiation of a catalytic amount of (fac-tris(2-phenylpyridinato-C₂ ,N)iridium) ([Ir(ppy)₃ ]) at room temperature. Control experiments and spectroscopic studies revealed that the triplet-triplet energy transfer from the excited [Ir(ppy)₃ ]* to enones, generated in situ from aldehyde and ketone or aldehyde and phosphorus ylide, is responsible for these simple and efficient muticomponent transformations.

Visible-Light-Induced Organic Photochemical Reactions through Energy-Transfer Pathways

Visible-light photocatalysis is a rapidly developing and powerful strategy to initiate organic transformations, as it closely adheres to the tenants of green and sustainable chemistry. Generally, most visible-light-induced photochemical reactions occur through single-electron transfer (SET) pathways. Recently, visible-light-induced energy-transfer (EnT) reactions have received considerable attentions from the synthetic community as this strategy provides a distinct reaction pathway, and remarkable achievements have been made in this field. In this Review, we highlight the most recent advances in visible-light-induced EnT reactions.

Asymmetric synthesis of polysubstituted methylenecyclobutanes via catalytic [2+2] cycloaddition reactions of N-allenamides

A highly enantioselective [2+2] cycloaddition reaction of alkylidene malonates with the internal C[double bond, length as m-dash]C bond of N-allenamides was developed with a MgII/N,N'-dioxide complex as a catalyst. Various polysubstituted methylenecyclobutanes were afforded in good yields (up to 99%) and excellent enantioselectivities (up to 96% ee) under mild conditions. The utility of the donor-acceptor cyclobutane product was demonstrated as a masked 1,4-dipole in the formal [4+2] annulation reaction with a silyl enol ether.

Catalytic Double [2 + 2] Cycloaddition Relay Enabled C-C Triple Bond Cleavage of Yne-Allenones

An unusual catalytic double [2 + 2] cycloaddition relay reaction of yne-allenones with unactivated alkenes and alkynes has been achieved, which enabled C-C triple-bond cleavage to access more than 60 examples of functionalized phenanthren-9-ols with generally good yields. This reaction provides a regioselective and practical method for the construction of carbocyclic ring systems with a high degree of functional group compatibility. Aside from surveying the scope of this transformation, mechanistic details of this process are provided by conducting systematic theoretical calculations.

Hetero-intermolecular [2+2] photocycloaddition of 1,4-dihydropyridines: a combined experimental and DFT study

A detailed DFT mechanistic study of hetero-intermolecular [2+2] photocycloaddition of 1,4-DHPs was carried out for investigating the regioselectivity of such transformation.