Progress in Electrochemically Empowered C-O Bond Formation: Unveiling the Pathway of Efficient Green Synthesis.

(C-X) bonds (X=C, N, O) are the main backbone for making different skeleton in the organic synthetic transformations. Among all the sustainable techniques, electro-organic synthesis for C-X bond formation is the advanced tool as it offers a greener and more cost-effective approach to chemical reactions by utilizing electrons as reagents. In this review, we want to explore the recent advancements in electrochemical C-O bond formation. The electrochemically driven C-O bond formation represents an emerging and exciting area of research. In this context, electrochemical techniques offers numerous advantages, including higher yields, cost-efficient production, and simplified work-up procedures. This method enables the continuous and consistent formation of C-O bonds in molecules, significantly enhancing overall reaction yields. Furthermore, both intramolecular and intermolecular C-O bond forming reaction provided valuable products of O-containing acyclic/cyclic analogue. Hence, carbonyl (C=O), ether -O-), and ester (-COOR) functionalization in both cyclic/acyclic analogues have been prepared continuously via this innovative pathway. In this context, we want to discuss one-decade electrochemical synthetic pathways of various C-O bond contains functional group in chronological manner. This review focused on all the synthetic aspects including mechanistic path and has also mentioned overall critical finding regarding the C-O bond formation via electrochemical pathways.

Benzo[a]acridinylmethyl esters as pH sensitive fluorescent photoactive precursors: synthesis, photophysical, photochemical and biological applications.

A newsworthy class of carboxylate esters based on the (benzo[a]acridin-12-yl)methyl (BAM) chromophore has been shown to perform dual functions as a "pH sensitive fluorescent probe" and a "phototrigger" for acids. The photophysical properties of all the BAM ester conjugates were investigated and found to be highly sensitive to solvent polarity, H-bonding capability and pH of the environment. On irradiation using UV light (≥410 nm), BAM ester conjugates underwent heterolytic cleavage of C-O bonds resulting in efficient release of carboxylic and amino acids. Interestingly, the newly synthesized BAM chromophore was also explored for the construction of a drug delivery system (DDS). In the current DDS, the BAM chromophore plays two important roles: (i) a "fluorophore" for cell imaging and (ii) a "phototrigger" for the drug release. In vitro biological studies revealed that the newly developed BAM based DDS has a good biocompatibility, cellular uptake properties and efficient photoregulated anticancer drug release ability.

Fluorescent photoremovable precursor (acridin-9-ylmethyl)ester: synthesis, photophysical, photochemical and biological applications.

A series of carboxylic acids including amino acids were protected as their corresponding fluorescent ester conjugates by coupling with an environment sensitive fluorophore 9-methylacridine. Photophysical properties of all the ester conjugates along with the protecting group have been investigated. Interestingly, the emission properties of the ester conjugates and 9-methylacridine were found to be highly sensitive to polarity, H-bonding and pH of the environment. Photolysis of all the ester conjugates was carried out using UV light above 360 nm and it was found that in every case the corresponding carboxylic acids were released in high chemical yield. Further, intercalation and the preferred binding mode of acridine-9-methanol and its ester conjugates with DNA were studied. In vitro biological studies revealed that acridine-9-methanol has good biocompatibility, cellular uptake property and cell imaging ability.

Photoremovable protecting groups as controlled-release device for sex pheromone.

Photoremovable protecting groups (PRPGs) were demonstrated as a delivery device for controlled release of pheromone under both UV light (≥350 nm) and direct sunlight irradiation. In the present work, (Z)-11-hexadecen-1-ol (sex pheromone of Chilo infuscatellus Snellen) was chemically caged by four different photoremovable protecting groups (7-hydroxy-4-hydroxymethylcoumarin, 1-pyrenemethanol, 9-anthracenemethanol and 2-(hydroxymethyl)anthraquinone) individually. Photophysical studies showed that the caged pheromone with coumarin, pyrene and anthracene derivatives exhibited strong fluorescence. Controlled release of (Z)-11-hexadecen-1-ol was achieved by irradiating the caged compounds in aqueous ethanol both under UV and sunlight. Further, to mimic the environmental conditions, controlled release of (Z)-11-hexadecen-1-ol was also studied in soil medium under direct sunlight. Thermogravimetric analysis showed that caging of (Z)-11-hexadecen-1-ol by PRPGs significantly reduced its volatility. Bioassay experiments indicated that PRPGs are harmless to soil bacteria (Azotobacter sp. and Pseudomonas aeruginosa) and in vitro cytotoxicity studies on eukaryotic L929 cells showed that PRPGs are also non-toxic. Field bioassays were performed using caged pheromone against maize stalk borer (Chilo partellus) and the results showed that the caged alcohol is effective in a number of moths catches instead of free alcohol in a blend for a longer period of time. Our studies indicated that use of PRPGs as delivery device for controlled release of pheromone by sunlight holds great interest for field applications.

Synthesis, photophysical and photochemical properties of photoacid generators based on N-hydroxyanthracene-1,9-dicarboxyimide and their application toward modification of silicon surfaces.

We have introduced a series of nonionic photoacid generators (PAGs) for carboxylic and sulfonic acids based on N-hydroxyanthracene-1,9-dicarboxyimide (HADI). The newly synthesized PAGs exhibited positive solvachromatic emission (λ(max)(hexane) 461 nm, λ(max)(ethanol) 505 nm) as a function of solvent polarity. Irradiation of PAGs in acetonitrile (ACN) using UV light above 410 nm resulted in the cleavage of weak N-O bonds, leading to the generation of carboxylic and sulfonic acids in good quantum and chemical yields. Mechanism for the homolytic N-O bond cleavage for acid generation was supported by time-dependent density functional theory (TD-DFT) calculations. More importantly, using the PAG monomer N-(p-vinylbenzenesulfonyloxy)anthracene-1,9-dicarboxyimide (VBSADI), we have synthesized N-(p-vinylbenzenesulfonyloxy)anthracene-1,9-dicarboxyimide-methyl methacrylate (VBSADI-MMA) and N-(p-vinylbenzenesulfonyloxy)anthracene-1,9-dicarboxyimide-ethyl acrylate (VBSADI-EA) copolymer through atom transfer radical polymerization (ATRP). Finally, we have also developed photoresponsive organosilicon surfaces using the aforementioned polymers.

Development of 1-hydroxy-2(1H)-quinolone-based photoacid generators and photoresponsive polymer surfaces.

A new class of carboxylate and sulfonate esters of 1-hydroxy-2(1H)-quinolone has been demonstrated as nonionic photoacid generators (PAGs). Irradiation of carboxylates and sulfonates of 1-hydroxy-2(1H)-quinolone by UV light (λ≥310 nm) resulted in homolysis of weak N-O bond leading to efficient generation of carboxylic and sulfonic acids, respectively. The mechanism for the homolytic N-O bond cleavage was supported by time-dependent DFT calculations. Photoresponsive 1-(p-styrenesulfonyloxy)-2-quinolone-methyl methacrylate (SSQL-MMA) and 1-(p-styrenesulfonyloxy)-2-quinolone-lauryl acrylate (SSQL-LA) copolymers were synthesized from PAG monomer 1-(p-styrenesulfonyloxy)-2-quinolone, and subsequently controlled surface wettability was demonstrated for the above-mentioned photoresponsive polymers.

1-(Hydroxyacetyl)pyrene a new fluorescent phototrigger for cell imaging and caging of alcohols, phenol and adenosine.

1-(Hydroxyacetyl)pyrene has been introduced as a new fluorescent phototrigger for alcohols and phenols. Alcohols and phenols were protected as their corresponding carbonate esters by coupling with fluorescent phototrigger, 1-(hydroxyacetyl)pyrene. Photophysical studies of caged carbonates showed that they all exhibited strong fluorescence properties. Irradiation of the caged carbonates by visible light (≥410 nm) in aqueous acetonitrile released the corresponding alcohols or phenols in high chemical (95-97%) and quantum (0.17-0.21) yields. The mechanism for the photorelease was proposed based on Stern-Volmer quenching experiments and solvent effect studies. Importantly, 1-(hydroxyacetyl)pyrene showed as a phototrigger for rapid photorelease of the biologically active molecule adenosine. In vitro biological studies revealed that 1-(hydroxyacetyl)pyrene has good biocompatibility, cellular uptake property and cell imaging ability.

Application of photoremovable protecting group for controlled release of plant growth regulators by sunlight.

We report a novel technique for controlled release of plant growth regulators (PGRs) by sunlight using photoremovable protecting group (PRPG) as a delivery device. In the present work, carboxyl-containing PGRs of the auxin group [indoleacetic acid (IAA) and naphthoxyacetic acid (NOAA)] were chemically caged using PRPGs of coumarin derivatives. Photophysical studies showed that caged PGRs exhibited good fluorescence properties. Irradiation of caged PGRs by sunlight in both aqueous ethanol and soil media resulted in controlled release of PGRs. The results of the bioactivity experiments indicated that caged PGRs showed better enhancement in the root and shoot length growth of Cicer arietinum compared to PGRs after 10days of sunlight exposure. Our results indicated that use of PRPG as a delivery device for controlled release of PGRs by sunlight in soil holds great interest for field application since it can overcome the rapid loss of PGRs in environmental conditions.