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Formaldehyde, a highly reactive carbonyl species, has been widely used in day-to-day life owing to its numerous applications in essential commodities, etc.; the extrusion of formaldehyde from these sources basically leads to increased formaldehyde levels in the environment. Additionally, formaldehyde is endogenously produced in the human body via several biological processes. Considering the adverse effects of formaldehyde, it is highly important to develop an efficient and reliable method for monitoring formaldehyde in environmental and biological samples. Several chemodosimeters (reaction-based sensing probes) have been designed and synthesized to selectively detect the presence of formaldehyde utilizing the photophysical properties of molecules. In this review, we have comprehensively discussed the recent advances in the design principles and sensing mechanisms of developed probes and their biological/environmental applications in selective formaldehyde detection and imaging endogenous formaldehyde in cells. We have summarized the literature based on three different categories: (i) the Schiff base reaction, (ii) the 2-aza-Cope sigmatropic rearrangement reaction and (iii) miscellaneous approaches. In all cases, reactions are accompanied by changes in color and/or emission that can be detected by the naked eye.

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Distal C-H bond functionalization of heterocycles remained extremely challenging with covalently attached directing groups (DG). Lack of proper site for DG attachment and inherent catalyst poisoning by heterocycles demand alternate routes for site selective functionalization of their distal C-H bonds. Utilizing non-productive coordinating property to hold the heterocycle into the cavity of a template system in a host-guest manner, we report distal C-H alkylation (C-5 of quinoline and thiazole, C-7 of benzothiazole and benzoxazole) of heterocycles. Upon complexation with heterocyclic substrate, nitrile DG in template directs the metal catalyst towards close vicinity of the specific distal C-H bond of the heterocycles. Our hypothesized pathway has been supported by various X-ray crystallographically characterized intermediates.

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α,ß-Alkenyl carboxylic acids undergo CuII -mediated decarboxylative annulation reactions with aliphatic cyclic ketones to provide synthetically valuable di-heterocycles. The annulation process tolerates a variety of aliphatic ketones and heterocyclic alkenyl carboxylic acids, producing substituted fused furan derivatives with complete regioselectivity. The current protocol offers a synthetically applicable pathway to construct a variety of oligo-heterocycles through Cu-mediated single-electron transfer and decarboxylation. Notably, synthesis of relatively inaccessible di-heterocycles has been achieved successfully using this protocol.

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An efficient method has been developed to afford highly C-5 selective olefination of thiazole derivatives utilizing a bifunctional template in an intermolecular fashion. Coordinative interaction between the substrates and the metal chelated template backbone plays a crucial role in high C-5 selectivity. Excellent selectivity for the C-5 position was observed while mono substituted (2- or 4-) or even more challenging unsubstituted thiazoles were employed.

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Palladium(II)-catalyzed meta-selective C-H allylation of arenes has been developed utilizing synthetically inert unactivated acyclic internal olefins as allylic surrogates. The strong σ-donating and π-accepting ability of pyrimidine-based directing group facilitates the olefin insertion by overcoming inertness of the typical unactivated internal olefins. Exclusive allyl over styrenyl product selectivity as well as E stereoselectivity were achieved with broad substrate scope, wide functional-group tolerance, and good to excellent yields. Late-stage functionalisations of pharmaceuticals were demonstrated. Experimental and computational studies shed light on the mechanism and point to key steric control in the palladacycle, thus determining product selectivities.

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Directing group assisted ortho-C-H activation has been known for the last few decades. In contrast, extending the same approach to achieve activation of the distal meta- and para-C-H bonds in aromatic molecules remained elusive for a long time. The main challenge is the conception of a macrocyclic transition state, which is needed to anchor the metal catalyst close to the target bond. Judicious modification of the chain length, the tether linkage, and the nature of the catalyst-coordinating donor atom has led to a number of successful studies in the last few years. This Review compiles the significant achievements made in this field of both meta- and para-selectivity using covalently attached directing groups, which are systematically classified on the basis of their mode of covalent attachment to the substrate as well as their chemical nature. This Review aims to create a more heuristic approach for recognizing the suitability of the directing groups for use in future organic transformations.

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Achieving site-selective C-H functionalization of arene is a fundamental challenge, as it is mainly controlled by the electronic nature of the molecules. A chelation-assisted C-H functionalization strategy overcomes the selectivity issues by utilizing distance and geometry of covalently attached directing groups (DGs). This strategy requires stoichiometric DG installation/removal and a suitable functional group on which to tether the DG. Such strategies are ineffective for small heterocycles unless suitable functional groups are added. Moreover, heterocycles are not the judicious choice as substrates owing to the possibilities of catalyst deactivation. Inspired by recent developments, this work demonstrates the utilization of a chelating template backbone bearing covalently attached directing groups, which enables site-selective remote C-H functionalization of heterocycles. The observed selectivity is the outcome of non-covalent interactions between the heterocycles and bifunctional template backbone.

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With the growing interest in renewable energy and global warming, it is important to minimize the usage of hazardous chemicals in both academic and industrial research, elimination of waste, and possibly recycle them to obtain better results in greener fashion. The studies under the area of mechanochemistry which cover the grinding chemistry to ball milling, sonication, etc. are certainly of interest to the researchers working on the development of green methodologies. In this review, a collection of examples on recent developments in organic bond formation reactions like carbon-carbon (C-C), carbon-nitrogen (C-N), carbon-oxygen (C-O), carbon-halogen (C-X), etc. is documented. Mechanochemical syntheses of heterocyclic rings, multicomponent reactions and organometallic molecules including their catalytic applications are also highlighted.

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The discovery of a direct method for the synthesis of three-ring heterocyclic carbazoles from unactivated arenes and anilides by a metal-free (organic) intermolecular dehydrogenative annulation reaction under ambient laboratory conditions is reported. Iodine(III) was used as the sole reagent either stoichiometrically from inexpensive phenyliodine diacetate or organocatalytically by in situ generation from PhI-mCPBA. In a single step, three C(sp2)-H bonds and one N(sp3)-H bond are functionalized from two different arenes for tandem C-C and C-N bond formation reactions.

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Iodinium cation (I(+) or IOAc) was produced from the combination of phenyliodine diacetate (PIDA) and iodine. I(+) facilitated the direct vicinal difunctionalization of olefins to α-azido, α-trideuteriomethoxy, α-2,2,2-trifluoroethoxy and α-acyloxy alkyl iodides via cation-π interaction at room temperature and under transition-metal free conditions.

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A solvent-free cross-coupling method for oxidative amidation of aldehydes and alcohols via a metal-free radial pathway has been demonstrated. The proposed methodology uses the TBAI-TBHP combination which efficiently induces metal-free C-H activation of aldehydes under neat conditions at 50 °C or ball-milling conditions at room temperature.

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