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Constructing highly proficient C–X (X = O, N, S, etc.) and C–C bonds by leveraging TMs (transition metals) (Fe, Cu, Pd, Rh, Au, etc.) and enzymes to catalyze carbene insertion into X–H/C(sp2)–H is a highly versatile strategy. This is primarily achieved through the in situ generation of metal carbenes from the interaction of TMs with diazo compounds. Over the last few decades, significant advancements have been made, encompassing a wide array of X–H bond insertions using various TMs. These reactions typically favor a stepwise ionic pathway where the nucleophilic attack on the metal carbene leads to the generation of a metal ylide species. This intermediate marks a critical juncture in the reaction cascade, presenting multiple avenues for proton transfer to yield the X–H inserted product. The mechanism of C(sp2)–H insertion reactions closely resembles those of X–H insertion reactions and thus have been included here. A major development in carbene insertion reactions has been the use of engineered enzymes as catalysts. Since the seminal report of a non-natural "carbene transferase" by Arnold in 2013, "P411", several heme-based enzymes have been reported in the literature to catalyze various abiological carbene insertion reactions into C(sp2)–H, N–H and S–H bonds. These enzymes possess an extraordinary ability to regulate the orientation and conformations of reactive intermediates, facilitating stereoselective carbene transfers. However, the absence of a suitable stereochemical model has impeded the development of asymmetric reactions employing a lone chiral catalyst, including enzymes. There is a pressing need to investigate alternative mechanisms and models to enhance our comprehension of stereoselectivity in these processes, which will be crucial for advancing the fields of asymmetric synthesis and biocatalysis. The current review aims to provide details on the mechanistic aspects of the asymmetric X–H and C(sp2)–H insertion reactions catalyzed by Fe, Cu, Pd, Rh, Au, and enzymes, focusing on the detailed mechanism and stereochemical model. The review is divided into sections focusing on a specific X–H/C(sp2)–H bond type catalyzed by different TMs and enzymes. [ABSTRACT FROM AUTHOR]

Fe-based catalysts as well as enzymes typically yield low stereoselectivity for carbene insertion into X–H bonds. Here, we have utilized DFT methods to understand the mechanism and unusually high enantioselectivity in an Fe-spiroBox catalyzed carbene insertion reaction into the O–H bond of aliphatic alcohols. Our transition state model shows a unique binding of the reaction intermediates to the chiral catalyst enabled by weak non covalent interactions that is absent in other X–H insertion reactions. [ABSTRACT FROM AUTHOR]

Bisphenol A (BPA), a common plasticizer, is categorized as a neurotoxic compound. Its impact on individuals exhibits sex-linked variations. Several biological and environmental factors impact the degree of toxicity. Moreover, nutritional factors have profound influence on toxicity outcome. BPA has been demonstrated to be an obesogen. However, research on the potential role of obesity as a confounding factor in BPA toxicity is lacking. We studied the neurodegenerative effects in high-fat diet (HFD)-induced obese female rats after exposure to BPA (10 mg/L via drinking water for 90 days). Four groups were taken in this study – Control, HFD, HFD + BPA and BPA. Cognitive function was evaluated through novel object recognition (NOR) test. Inflammatory changes in brain, and changes in hormonal level, lipid profile, glucose tolerance, oxidative stress, and antioxidants were also determined. HFD + BPA group rats showed a significant decline in memory function in NOR test. The cerebral cortex (CC) of the brain showed increased neurodegenerative changes as measured by microtubule-associated protein-2 (MAP-2) accompanied by histopathological confirmation. The increased level of neuroinflammation was demonstrated by microglial activation (Iba-1) and protein expression of nuclear factor– kappa B (NF-КB) in the brain. Obesity also caused significant (p < 0.05) increase in lipid peroxidation accompanied by reduced activities of antioxidant enzymes (glutathione S-transferase, catalase and glutathione peroxidase) and decrease in reduced-glutathione (p < 0.05) when compared to non-obese rats with BPA treatment. Overall, study revealed that obesity serves as a risk factor in the toxicity of BPA which may exacerbate the progression of neurological diseases. [ABSTRACT FROM AUTHOR]

Protein aggregation is invariably associated with the inflammation as a factor in Alzheimer's disease (AD). We investigated the interaction between downstream factors of endoplasmic reticulum (ER) stress pathway and inflammation, with implications in cognitive impairment in AD. Amyloid-β (Aβ)(1–42) was administered by bilateral intracerebroventricular (icv) injection in the brain of adult male Wistar rats to experimentally develop AD. The cognitive impairment was assessed by measuring behavioral parameters such as Morris water maze and novel object recognition tests. Levels of pro-inflammatory cytokines such as interleukin (IL)-1β and tumor necrosis factor (TNF)-α and anti-inflammatory cytokines IL-4 and IL-10 were measured by the enzyme-linked immunosorbent assay (ELISA) in different rat brain regions. Inflammatory marker proteins such as cyclo-oxygenase (COX)-2 and phosphorylation of nuclear factor kappa B (NF-КB) (p65) were measured by the western blotting. Gene expression of ER stress downstream factors such as ATF-4, CHOP, and GADD-34 was analyzed by qRT-PCR. Histological studies were performed to check Aβ accumulation and neuronal degeneration. Integrated stress response inhibitor (ISRIB) was used to confirm the specific role of ER stress–mediated inflammation in cognitive impairment. Administration of Aβ(1–42) resulted in alteration in levels of inflammatory cytokines, inflammatory proteins, and mRNA levels of ER stress downstream factors. ISRIB treatment resulted in attenuation of Aβ(1–42)-induced ER stress, inflammation, neurodegeneration, and cognitive impairment in rats. These results indicate that ER stress–mediated inflammation potentiates the cognitive impairment in AD. An understanding of cascade of events, interaction of ER stress which was a hallmark of the present investigation together with inflammation and modulation of downstream signalling factors could serve as potent biomarkers to study AD progression. Schematic representation of interaction between ER stress and inflammation. Administration of Aβ(1–42) resulted in ER stress which caused the activation of factors of PERK pathway, inflammation, neuronal degeneration, and cognitive impairment. ISRIB treatment caused downregulation of ATF-4 and attenuation of inflammation indicating a role of ER stress–mediated inflammation in the cognitive impairment in AD. The site of action of ISRIB is shown in blue color [ABSTRACT FROM AUTHOR]

Multicomponent reactions provide an excellent approach toward quaternary carbon centres utilizing convergent chemical reactions in a highly selective manner under one‐pot conditions. The reactivity of substrates and metal catalysts is carefully controlled, precluding the formation of side‐products. In this context, transition metal catalyzed reactions of onium ylides generated via diazo precursors and nucleophiles with a third component (an electrophile) have garnered significant attention. Using an allylic precursor as the electrophilic partner provides opportunities to construct all‐carbon quaternary centres. Furthermore, the presence of an allyl fragment in the multicomponent product serves as a pivotal handle for carrying out subsequent modifications. Several recent studies have employed Rh, Pd, and dual Rh/Pd catalytic systems in multicomponent reactions involving allylic alkylation that proceed via a synergistic or relay pathway. Although not significantly successful, in a few cases, asymmetric induction is achieved through chiral phosphoric acids or chiral phosphine ligands. Limited substrate and catalyst scope and the underlying mechanistic complexities have posed formidable challenges, slowing the advancement of asymmetric reactions. This review provides details of multicomponent reactions using readily available substrates like diazo compounds, allylic carbonates, and nucleophiles (R‐OH, R‐NH2,,etc.) forming complex organic compounds. Our primary objective is to discuss the mechanistic issues that may facilitate the progress in asymmetric reactions in this field. [ABSTRACT FROM AUTHOR]

The mechanism of Rh2(S‐NTTL)4 catalyzed carbene insertion into C(3)−H of indole is investigated using DFT methods. Since the commonly accepted enol mechanism cannot account for enantioinduction, a concerted oxocarbenium pathway was proposed in an earlier work using a model catalyst. However, after considering the full catalytic system, this study finds that akin to other reactions, here, too, the enol pathway is of lower energy, which now naturally raises a conundrum regarding the mode of chiral induction. Herein, a new water promoted mechanistic pathway involving a metal‐associated enol intermediate hydrogen bonding and stereochemical model are proposed to solve this puzzle. It is shown how the catalyst bowl‐shaped structure along with substrate‐catalyst binding is crucial for achieving high levels of enantioselectivity. A stereodetermining water‐assisted proton transfer is proposed and confirmed through deuterium‐labeling experiments. The water molecules are held together by H‐bonding interactions with the carboxylate ligands that is reminiscent of enzyme catalysis. Although several previous studies have aimed at understanding the mechanism of metal catalyzed carbene insertion reactions, the origin of high stereoinduction especially with chiral metal complexes remains unclear, and till date there is no transition state model that can explain the high enantioselectivity with such chiral Rh complexes. The metal‐associated enol pathway is currently underrepresented in catalytic cycles and may play a crucial role in catalyst design. Since the enol pathway is commonly adopted in other metal‐catalyzed X−H insertion reactions involving a diazoester, the presented results are not specific to the current reaction. Therefore, this study could provide the direction for achieving high levels of enantioselectivity which is otherwise difficult to achieve with a single metal catalyst. [ABSTRACT FROM AUTHOR]

Fe catalyzed carbene insertion reactions present an efficient route for direct C−H functionalization. The use of Fe(III) in place of the widely used Fe(II) presents several benefits. However, the mechanistic understanding of Fe(III) severely lags behind Fe(II) complexes. One of the major unsolved issues relates to the formation of bridged versus terminal metallocarbenes. Even though the oxidized bridged carbenoid complexes have been isolated and found to be thermodynamically more stable, they are generally considered a dead end for the catalytic cycle. In the current report, the formation and the subsequent reactions of the bridged carbenoid complexes for an Fe(TPP)Cl catalyzed C(sp2)−H insertion are investigated. Using DFT calculations, it is observed that both mono and bis oxidized bridged carbenoid complexes can participate in the catalytic cycle. Importantly, for the first time, a mechanistic pathway showing that these bridged species are not a dead end in Fe catalysis is presented. Their existence in other reactions might be more prevalent than what is currently believed. The current study will have important implications in utilizing Fe(III) complexes for other insertion reactions, especially for heme containing enzymes which necessarily need to be carried out under anaerobic/reducing conditions. [ABSTRACT FROM AUTHOR]

Heterobiaryls serve as relevant structural motifs in many fields of high applicative importance such as drugs, agrochemicals, organic functional materials etc. Cross‐dehydrogenative coupling involving direct oxidation of two C−H bonds to construct a C−C bond is actively being pursued as a more benign and 'greener' alternative for synthesizing heterobiaryls. Herein, we report a Cu(I)‐catalyzed cross‐dehydrogenative coupling of indoles and furans, two of the most important aromatic heterocycles using air as the terminal oxidant. The reaction proceeds with regio‐ and chemoselectivity to give the cross‐coupled products in good to excellent yields generally. A broad substrate scope with respect to both the coupling partners has been demonstrated to prove the generality of this reaction. This represents the hitherto unexplored cross‐dehydrogenative coupling methodology to obtain an indole‐furan biaryl motif. [ABSTRACT FROM AUTHOR]

DFT methods are used to probe the mechanism of a newly developed Ir-quinoid catalyzed C(sp3)–H functionalization of 1,4 dienes. The lowest energy pathway proceeds via an old-school concerted C–H insertion as opposed to a unique hydrogen atom transfer process proposed previously. The concertedness of the reaction shows an intriguing dependence on sterics of the diene leading to either inserted or dehydrogenated products. We use these new insights to tune the axial ligand, and design a more efficient catalyst. [ABSTRACT FROM AUTHOR]

Although the construction of axially chiral C−C bonds leading to the atroposelective synthesis of biaryls and allied compounds are well‐known, the related synthesis of compounds bearing axially chiral C−N bonds are relatively rare. Described herein is the N‐heterocyclic carbene‐catalyzed atroposelective synthesis of N‐aryl succinimides having an axially chiral C−N bond via the desymmetrization of N‐aryl maleimides. The NHC involved intermolecular Stetter‐aldol cascade of dialdehydes with prochiral N‐aryl maleimides followed by oxidation afforded N‐aryl succinimides in good yields and ee values. Preliminary studies on rotation barrier for the C−N bond, the temperature dependence, and detailed DFT studies on mechanism are also provided. [ABSTRACT FROM AUTHOR]

Although the construction of axially chiral C−C bonds leading to the atroposelective synthesis of biaryls and allied compounds are well‐known, the related synthesis of compounds bearing axially chiral C−N bonds are relatively rare. Described herein is the N‐heterocyclic carbene‐catalyzed atroposelective synthesis of N‐aryl succinimides having an axially chiral C−N bond via the desymmetrization of N‐aryl maleimides. The NHC involved intermolecular Stetter‐aldol cascade of dialdehydes with prochiral N‐aryl maleimides followed by oxidation afforded N‐aryl succinimides in good yields and ee values. Preliminary studies on rotation barrier for the C−N bond, the temperature dependence, and detailed DFT studies on mechanism are also provided. [ABSTRACT FROM AUTHOR]

Engineered heme enzymes such as myoglobin and cytochrome P450s metalloproteins are gaining widespread importance due to their efficiency in catalyzing non-natural reactions. In a recent strategy, the naturally occurring Fe metal in the heme unit was replaced with non-native metals such as Ir, Rh, Co, Cu, etc., and axial ligands to generate artificial metalloenzymes. Determining the best metal–ligand for a chemical transformation is not a trivial task. Here we demonstrate how computational approaches can be used in deciding the best metal–ligand combination which would be highly beneficial in designing new enzymes as well as small molecule catalysts. We have used Density Functional Theory (DFT) to shed light on the enhanced reactivity of an Ir system with varying axial ligands. We look at the insertion of a carbene group generated from diazo precursors via N2 extrusion into a C–H bond. For both Ir(Me) and Fe systems, the first step, i.e., N2 extrusion is the rate determining step. Strikingly, neither the better ligand overlap with 5d orbitals on Ir nor the electrophilicity on the carbene centre play a significant role. A comparison of Fe and Ir systems reveals that a lower distortion in the Ir(Me)-porphyrin on moving from the reactant to the transition state renders it catalytically more active. We notice that for both metal porphyrins, the free energy barriers are affected by axial ligand substitution. Further, for Fe porphyrin, the axial ligand also changes the preferred spin state. We show that for the carbene insertion into the C–H bond, Fe porphyrin systems undergo a stepwise HAT (hydrogen atom transfer) instead of a concerted hydride transfer process. Importantly, we find that the substitution of the axial Me ligand on Ir to imidazole or chloride, or without an axial substitution changes the rate determining step of the reaction. Therefore, an optimum ligand that can balance the barriers for both steps of the catalytic cycle is essential. We subsequently used the QM cluster approach to delineate the protein environment's role and mutations in improving the catalytic activity of the Ir(Me) system. [ABSTRACT FROM AUTHOR]

The first catalytic enantioselective Fischer indolization of prochiral diketones containing enantiotopic carbonyl groups is developed and shown to proceed through dynamic kinetic resolution (DKR). Catalyzed by the combination of a spirocyclic chiral phosphoric acid and ZnCl2 (Lewis acid assisted Brønsted acid), this direct approach combines 2,2‐disubstituted cyclopentane‐1,3‐diones with N‐protected phenylhydrazines to furnish cyclopenta[b]indole derivatives containing an all‐carbon quaternary stereocenter with good to excellent enantioselectivities. [ABSTRACT FROM AUTHOR]

The first catalytic enantioselective Fischer indolization of prochiral diketones containing enantiotopic carbonyl groups is developed and shown to proceed through dynamic kinetic resolution (DKR). Catalyzed by the combination of a spirocyclic chiral phosphoric acid and ZnCl2 (Lewis acid assisted Brønsted acid), this direct approach combines 2,2‐disubstituted cyclopentane‐1,3‐diones with N‐protected phenylhydrazines to furnish cyclopenta[b]indole derivatives containing an all‐carbon quaternary stereocenter with good to excellent enantioselectivities. [ABSTRACT FROM AUTHOR]

The diverse reactivity of N‐heterocyclic carbenes (NHCs) in organocatalysis is due to the possibility of different modes of action. Although NHC‐bound enolates and dienolates are known, the related NHC‐bound cross‐conjugated aza‐trienolates remain elusive. Herein, we demonstrate the NHC‐catalyzed formal [6+2] annulation of nitrogen‐containing heterocyclic aldehydes with α,α,α‐trifluoroacetophenones leading to the formation of versatile pyrrolooxazolones (29 examples). The catalytically generated cross‐conjugated aza‐trienolates (aza‐fulvene type) underwent smooth [6+2] annulation with electrophilic ketones to afford the product in moderate to good yields under mild conditions. Preliminary DFT studies on the mechanism are also provided. [ABSTRACT FROM AUTHOR]

Rational enzyme design presents a major challenge that has not been overcome by computational approaches. One of the key challenges is the difficulty in assessing the magnitude of the maximum possible catalytic activity. In an attempt to overcome this challenge, we introduce a strategy that takes an active enzyme (assuming that its activity is close to the maximum possible activity), design mutations that reduce the catalytic activity, and then try to restore that catalysis by mutating other residues. Here we take as a test case the enzyme haloalkane dehalogenase (DhlA), with a 1,2-dichloroethane substrate. We start by demonstrating our ability to reproduce the results of single mutations. Next, we design mutations that reduce the enzyme activity and finally design double mutations that are aimed at restoring the activity. Using the computational predictions as a guide, we conduct an experimental study that confirms our prediction in one case and leads to inconclusive results in another case with 1,2-dichloroethane as substrate. Interestingly, one of our predicted double mutants catalyzes dehalogenation of 1,2-dibromoethane more efficiently than the wild-type enzyme. [ABSTRACT FROM AUTHOR]

Background: Nalbuphine when used as adjuvant to hyperbaric bupivacaine has improved the quality of perioperative analgesia with fewer side effects. Fentanyl is a lipophilic opioid with a rapid onset following intrathecal injection. It does not cause respiratory depression and improves duration of sensory anesthesia without producing significant side effects. Aim: This study aims to compare the postoperative analgesia of intrathecal nalbuphine and fentanyl as adjuvants to bupivacaine in cesarean section. Methodology: A prospective, randomized, double-blind, and comparative study was conducted on 150 parturients of American Society of Anesthesiologists (ASA) physical status I and II of age group 20–45 years with normal coagulation profile undergoing cesarean section under spinal anesthesia. These patients were randomized into three groups with fifty patients in each group. Group I received 2 ml of 0.5% hyperbaric bupivacaine (10 mg) plus 0.4 ml nalbuphine (0.8 mg), Group II received 2 ml of 0.5% hyperbaric bupivacaine (10 mg) plus 0.4 ml fentanyl (20 μg), and Group III received 2 ml of 0.5% hyperbaric bupivacaine (10 mg) plus 0.4 ml of normal saline. Results: The mean duration of effective analgesia was 259.20 ± 23.23 min in Group I, 232.70 ± 13.15 min in Group II, and 168.28 ± 7.55 min in Group III. The mean number of rescue analgesics required was significantly lower (P < 0.001) in Group I as compared to Group II and III. Conclusion: Both intrathecal nalbuphine 0.8 mg and fentanyl 20 μg are effective adjuvants to 0.5% hyperbaric bupivacaine in subarachnoid block. However, intrathecal nalbuphine prolongs postoperative analgesia maximally and may be used as an alternative to intrathecal fentanyl in cesarean section. [ABSTRACT FROM AUTHOR]

Understanding the origin of the catalytic power of enzymes has both conceptual and practical importance. One of the most important finding from computational studies of enzyme catalysis is that a major part of the catalytic power is due to the preorganization of the enzyme active site. Unfortunately, misunderstanding of the nontrivial preorganization idea lead some to assume that it does not consider the effect of the protein residues. This major confusion reflects a misunderstanding of the statement that the interaction energy of the enzyme group and the transition state (TS) is similar to the corresponding interaction between the water molecules (in the reference system) and the TS, and that the catalysis is due to the reorganization free energy of the water molecules. Obviously, this finding does not mean that we do not consider the enzyme groups. Another problem is the idea that catalysis is due to substrate preorganization. This more traditional idea is based in some cases on inconsistent interpretation of the action of model compounds, which unfortunately, do not reflect the actual situation in the enzyme active site. The present article addresses the above problems, clarifying first the enzyme polar preorganization idea and the current misunderstandings. Next we take a specific model compound that was used to promote the substrate preorganization proposal and establish its irrelevance to enzyme catalysis. Overall, we show that the origin of the catalytic power of enzymes cannot be assessed uniquely without computer simulations, since at present this is the only way of relating structure and energetics. [ABSTRACT FROM AUTHOR]

Insights into chiral induction for an asymmetric sulfoxidation reaction involving a single oxygen atom transfer are gained through analyzing the stereocontrolling transition states. The fitting of the substrate into the chiral cavity of a new class of imidodiphosphoric Brønsted acids, as well as weak CH⋅⋅⋅π and CH⋅⋅⋅O noncovalent interactions, are identified as responsible for the observed chiral induction. [ABSTRACT FROM AUTHOR]

Insights into chiral induction for an asymmetric sulfoxidation reaction involving a single oxygen atom transfer are gained through analyzing the stereocontrolling transition states. The fitting of the substrate into the chiral cavity of a new class of imidodiphosphoric Brønsted acids, as well as weak CH⋅⋅⋅π and CH⋅⋅⋅O noncovalent interactions, are identified as responsible for the observed chiral induction. [ABSTRACT FROM AUTHOR]

The term 'double tooth' is used to describe two developmental dental anomalies of shape: gemination and fusion. Traumatic episodes involving a fused or geminated primary tooth are rare. This report describes an unusual case of crown root fracture in a primary double tooth. [ABSTRACT FROM AUTHOR]