The stereospecific 1,2‐migration of boronate complexes is one of the most representative reactions in boron chemistry. This process has been used extensively to develop powerful methods for ...asymmetric synthesis, with applications spanning from pharmaceuticals to natural products. Typically, 1,2‐migration of boronate complexes is driven by displacement of an α‐leaving group, oxidation of an α‐boryl radical, or electrophilic activation of an alkenyl boronate complex. The aim of this article is to summarize the recent advances in the rapidly expanding field of electrophile‐induced stereospecific 1,2‐migration of groups from boron to sp2 and sp3 carbon centers. It will be shown that three different conceptual approaches can be utilized to enable the 1,2‐migration of boronate complexes: stereospecific Zweifel‐type reactions, catalytic conjunctive coupling reactions, and transition metal‐free sp2–sp3 couplings. A discussion of the reaction scope, mechanistic insights, and synthetic applications of the work described is also presented.
Electrophile‐induced stereospecific 1,2‐migration of boronate complexes has been successfully applied in many asymmetric reactions. Typically, the electrophilic species provides the activation required for the boronate complex to undergo the desired migration of a group from boron to the adjacent sp2‐ or sp3‐hybridized carbon. Such activating species range from stoichiometric classical electrophiles to transition metal complexes present in catalytic quantities.
Photoredox‐catalyzed methylcyclobutanations of alkylboronic esters are described. The reactions proceed through single‐electron transfer induced deboronative radical addition to an electron‐deficient ...alkene followed by single‐electron reduction and polar 4‐exo‐tet cyclization with a pendant alkyl halide. Key to the success of the methodology was the use of easily oxidizable arylboronate complexes. Structurally diverse cyclobutanes are shown to be conveniently prepared from readily available alkylboronic esters and a range of haloalkyl alkenes. The mild reactions display excellent functional group tolerance, and the radical addition‐polar cyclization cascade also enables the synthesis of 3‐, 5‐, 6‐, and 7‐membered rings.
Arylboronate complexes formed from alkylboronic esters and phenyllithium were found to undergo facile single‐electron oxidation to form alkyl radicals. The novel use of these complexes as radical precursors enabled the development of a photoredox‐catalyzed cyclobutane synthesis proceeding through a radical‐polar crossover mechanism.
The use of pyridinium‐activated primary amines as photoactive functional groups for deaminative generation of alkyl radicals under catalyst‐free conditions is described. By taking advantage of the ...visible light absorptivity of electron donor–acceptor complexes between Katritzky pyridinium salts and either Hantzsch ester or Et3N, photoinduced single‐electron transfer could be initiated in the absence of a photocatalyst. This general reactivity platform has been applied to deaminative alkylation (Giese), allylation, vinylation, alkynylation, thioetherification, and hydrodeamination reactions. The mild conditions are amenable to a diverse range of primary and secondary alkyl pyridiniums and demonstrate broad functional group tolerance.
Electron donor–acceptor complexes between pyridinium‐activated primary amines and Hantzsch ester or triethylamine undergo catalyst‐free photoinduced single‐electron transfer with visible light. Fragmentation leads to alkyl radicals that could be intercepted with a variety of acceptors. This deaminative radical generation was applied to catalyst‐free Giese, allylation, vinylation, alkynylation, thioetherification, and hydrodeamination reactions.
1,3‐Disubstituted bicyclo1.1.1pentanes (BCPs) are valuable bioisosteres of para‐substituted aromatic rings. The most direct route to these structures is via multicomponent ring‐opening reactions of ...1.1.1propellane. However, challenges associated with these transformations mean that difunctionalized BCPs are more commonly prepared by multistep reaction sequences with BCP‐halide intermediates. Herein, we report three‐ and four‐component 1,3‐difunctionalizations of 1.1.1propellane with organometallic reagents, organoboronic esters, and a variety of electrophiles. This process is achieved by trapping intermediate BCP‐metal species with boronic esters to form boronate complexes, which are versatile intermediates whose electrophile‐induced 1,2‐metallate rearrangement chemistry enables a broad range of C−C bond‐forming reactions.
A modular synthesis of 1,3‐difunctionalized bicyclo1.1.1pentanes (BCPs) has been achieved through multicomponent couplings between organometallic reagents, 1.1.1propellane, and organoboronic esters. By utilizing the versatile 1,2‐metallate rearrangement chemistry of boronate complexes generated in situ, a range of C−C bond‐forming reactions could be performed without the need for transition‐metal catalysis.
A photochemical method for converting aliphatic alcohols into boronic esters is described. Preactivation of the alcohol as a 2‐iodophenyl‐thionocarbonate enables a novel Barton–McCombie‐type radical ...deoxygenation that proceeds efficiently with visible light irradiation and without the requirement for a photocatalyst, a radical initiator, or tin or silicon hydrides. The resultant alkyl radical is intercepted by bis(catecholato)diboron, furnishing boronic esters from a diverse range of structurally complex alcohols.
Visible light irradiation of 2‐iodophenyl‐thionocarbonates enabled Barton–McCombie‐type radical deoxygenations to proceed in the absence of a photocatalyst, a radical initiator, or tin or silicon hydrides. Applying this strategy to deoxygenative borylation reactions allowed a wide range of structurally diverse aliphatic alcohols to be transformed into synthetically valuable boronic esters.
Herein, we describe the development of a photoredox‐catalyzed decarboxylative radical addition–polar cyclization cascade approach to functionalized cyclopropanes. Reductive termination of ...radical–polar crossover reactions between aliphatic carboxylic acids and electron‐deficient alkenes yielded carbanion intermediates that were intercepted in intramolecular alkylations with alkyl chlorides appended to the alkene substrate. The mild conditions, which make use of a readily available organic photocatalyst and visible light, were demonstrated to be amenable to a broad range of structurally complex carboxylic acids and a wide variety of chloroalkyl alkenes, demonstrating exquisite functional group tolerance.
The development of a fragment coupling‐based cyclopropanation between carboxylic acids and chloroalkyl alkenes is described. The reaction involves a photoredox‐catalyzed decarboxylative radical addition–polar cyclization cascade and uses a readily available organic photocatalyst. The mild conditions provide access to diversely functionalized cyclopropanes from a broad range of structurally complex carboxylic acid and alkene substrates.
Triclosan is an antimicrobial chemical present in consumer products that is frequently detected in aquatic environments. In this research, we investigated the role of a common freshwater microalgae ...species, Euglena gracilis for triclosan uptake and transformation in open–water treatment wetlands. Lab–scale wetland bioreactors were created under various conditions of light (i.e., continuous (white) light, red light, and in the dark), media (i.e., wetland, autoclaved wetland, Milli–Q, and growth media water), and presence or absence of algae. Triclosan and its potential transformation products were identified in the water and algae phases. Triclosan transformation occurred most rapidly with reactors that received continuous (white) light, with pseudo first–order rate constants, k, ranging from 0.035 to 0.292 day−1. This indicates that phototransformation played a major role in triclosan transformation during the day, despite light screening by algae. Algae contributed to the uptake and transformation of triclosan in all reactors, and algae and bacteria both contributed to triclosan biotransformation under dark conditions, representative of nighttime conditions. Some transformation products were formed and further transformed, e.g., triclosan–O–sulfate, methoxy and diglucosyl conjugate of hydroxylated triclosan, and dimethoxy and glucosyl conjugate of 2,4–dichlorophenol, suggesting their minimal accumulation over the 25 days of the experiments. This study shows that the combined action of light, microbes, and algae allows the safe transfer and transformation of triclosan in open–water treatment wetlands.
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•E. gracilis can uptake and transform triclosan under dark conditions.•E. gracilis hindered the overall phototransformation of triclosan.•Role of microbes for nighttime conditions could be significant.•Phototransformation with white light was the dominant removal pathway.•E. gracilis and microbes can biotransform triclosan to triclosan–O–sulfate.
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