Improving the Industrial Feasibility of Metal-Free Hydrogenation Catalysts Using Chemical Scavengers

B(C6F5)3-Catalyzed Dehydrogenation of Pyrrolidines to Form Pyrroles

Pyrroles are important N-heterocycles found in medicines and materials. The formation of pyrroles from widely accessible pyrrolidines is a potentially attractive strategy but is an underdeveloped approach due to the sensitivity of pyrroles to the oxidative conditions required to achieve such a transformation. Herein, we report a catalytic approach that employs commercially available B(C6F5)3 in an operationally simple procedure that allows pyrrolidines to serve as direct synthons for pyrroles. Mechanistic studies have revealed insights into borane-catalyzed dehydrogenative processes.

Green Chemistry in the Synthesis of Pharmaceuticals

The principles of green chemistry (GC) can be comprehensively implemented in green synthesis of pharmaceuticals by choosing no solvents or green solvents (preferably water), alternative reaction media, and consideration of one-pot synthesis, multicomponent reactions (MCRs), continuous processing, and process intensification approaches for atom economy and final waste reduction. The GC's execution in green synthesis can be performed using a holistic design of the active pharmaceutical ingredient's (API) life cycle, minimizing hazards and pollution, and capitalizing the resource efficiency in the synthesis technique. Thus, the presented review accounts for the comprehensive exploration of GC's principles and metrics, an appropriate implication of those ideas in each step of the reaction schemes, from raw material to an intermediate to the final product's synthesis, and the final execution of the synthesis into scalable industry-based production. For real-life examples, we have discussed the synthesis of a series of established generic pharmaceuticals, starting with the raw materials, and the intermediates of the corresponding pharmaceuticals. Researchers and industries have thoughtfully instigated a green synthesis process to control the atom economy and waste reduction to protect the environment. We have extensively discussed significant reactions relevant for green synthesis, one-pot cascade synthesis, MCRs, continuous processing, and process intensification, which may contribute to the future of green and sustainable synthesis of APIs.

Amide Iridium Complexes As Catalysts for Transfer Hydrogenation Reduction of N-sulfonylimine

Sulfonamide moieties widely exist in natural products, biologically active substance, and pharmaceuticals. Here, an efficient water-soluble amide iridium complexes-catalyzed transfer hydrogenation reduction of N-sulfonylimine is developed, which can be carried out under environmentally friendly conditions, affording a series of sulfonamide compounds in excellent yields (96-98%). In comparison with organic solvents, water is shown to be critical for a high catalytic transfer hydrogenation reduction in which the catalyst loading can be as low as 0.001 mol %. These amide iridium complexes are easy to synthesize, one structure of which was determined by single-crystal X-ray diffraction. This protocol gives an operationally simple, practical, and environmentally friendly strategy for synthesis of sulfonamide compounds.

Transition-Metal-Free Site-Selective γ-C(sp2)-H Monoiodination of Arenes Directed by an Aliphatic Keto Group

A general γ-C(sp²)-H iodination method directed by an aliphatic keto group has been developed under transition-metal-free conditions for the first time, generating iodoarenes in good to excellent yields with excellent site selectivity. This protocol features a wide range of aryl-substituted ketones, short reaction times, mild reaction conditions, and scalable synthetic procedures. A possible reaction mechanism was also proposed based on several control experiments.

B(C6F5)3-Catalyzed Direct C3 Alkylation of Indoles and Oxindoles

The direct C3 alkylation of indoles and oxindoles is a challenging transformation, and only a few direct methods exist. Utilizing the underexplored ability of triaryl boranes to mediate the heterolytic cleavage of α-nitrogen C–H bonds in amines, we have developed a catalytic approach for the direct C3 alkylation of a wide range of indoles and oxindoles using amine-based alkylating agents. We also employed this borane-catalyzed strategy in an alkylation-ring opening cascade.

Reversible Activation of Water by an Air- and Moisture-Stable Frustrated Rhodium Nitrogen Lewis Pair

[Cp*Rh(κ³ N,N',P-L)][SbF₆ ] (Cp*=C₅ Me₅ ), bearing a guanidine-derived phosphano ligand L, behaves as a "dormant" frustrated Lewis pair and activates H₂ and H₂ O in a reversible manner. When D₂ O is employed, a facile H/D exchange at the Cp* ring takes place through sequential C(sp³ )-H bond activation.

Reversible O–H bond activation by an intramolecular frustrated Lewis pair

The interactions of the O–H bonds in alcohols, water and phenol with dimethylxanthene-derived frustrated Lewis pairs (FLPs) have been probed.

Water and a Borohydride/Hydronium Intermediate in the Borane-Catalyzed Hydrogenation of Carbonyl Compounds with H2 in Wet Ether: A Computational Study

We have computationally evaluated water as an active Lewis base (LB) and introduced the borohydride/hydronium intermediate in the mechanism of B(C₆F₅)₃-catalyzed hydrogenation of carbonyl compounds with H₂ in wet/moist ether. Our calculations extend the known frustrated Lewis pair mechanism of this reaction toward the inclusion of water as the active participant in all steps. Although the definition of the zero-energy point interweaves in comparison of the scenarios with and without water, we will be able to show that (i) water (hydrogen bonded to its molecular environment) can, in principle, act as a reasonably viable LB in cooperation with the borane Lewis acid such as B(C₆F₅)₃ but relatively a strong borane-water complexation can be the hindering factor; (ii) the herein-proposed borohydride/hydronium intermediates with the hydronium cation having three OH···ether hydrogen bonds or a combination of the OH···ether/OH···ketone hydrogen bonds appear to be as valid as the previously considered borohydride/oxonium or borohydride/oxocarbenium intermediates; (iii) the proton-coupled hydride transfer from the borohydride/hydronium to a ketone (acetone) has a reasonably low barrier. Our findings could be useful for better mechanistic understanding and further development of the aforementioned reaction.

(Dicyclohexyl(2-(dimesitylboryl)phenyl)phosphine: en route to stable frustrated Lewis pairs-hydrogen adducts in water

The new ansa-phosphinoborane (dicyclohexyl(2-(dimesitylboryl)phenyl)phosphine was synthesized via an one-pot protocol in 67% yield. The compound has been characterized by 1H, 13C, 11B and 31P NMR, and its solid-state structure determined by a single crystal X-ray diffraction analysis. The ansa-phosphinoborane does not react with molecular hydrogen or water at room or elevated temperature. According to performed DFT studies, heterolytic splitting of water or hydrogen by the phosphinoborane are both endergonic but close in thermodynamics. In polar solvents, such as in methanol or acetonitrile, addition of hydrogen is energetically more favorable than of water.

N-Methyl-Benzothiazolium Salts as Carbon Lewis Acids for Si-H σ-Bond Activation and Catalytic (De)hydrosilylation

N-Me-Benzothiazolium salts are introduced as a new family of Lewis acids able to activate Si-H σ bonds. These carbon-centred Lewis acids were demonstrated to have comparable Lewis acidity towards hydride as found for the triarylboranes widely used in Si-H σ-bond activation. However, they display low Lewis acidity towards hard Lewis bases such as Et3 PO and H2 O in contrast to triarylboranes. The N-Me-benzothiazolium salts are effective catalysts for a range of hydrosilylation and dehydrosilylation reactions. Judicious selection of the C2 aryl substituent in these cations enables tuning of the steric and electronic environment around the electrophilic centre to generate more active catalysts. Finally, related benzoxazolium and benzimidazolium salts were found also to be active for Si-H bond activation and as catalysts for the hydrosilylation of imines.

An innovation for development of Erlenmeyer–Plöchl reaction and synthesis of AT-130 analogous: a new application of continuous-flow method

The first micro-flow Erlenmeyer–Plöchl azlactone reaction and synthesis of N-benzoylglycine carbamide were established.

Facile Protocol for Water-Tolerant "Frustrated Lewis Pair"-Catalyzed Hydrogenation

Despite rapid advances in the field of metal-free, “frustrated Lewis pair” (FLP)-catalyzed hydrogenation, the need for strictly anhydrous reaction conditions has hampered wide-scale uptake of this methodology. Herein, we report that, despite the generally perceived moisture sensitivity of FLPs, 1,4-dioxane solutions of B(C₆F₅)₃ actually show appreciable moisture tolerance and can catalyze hydrogenation of a range of weakly basic substrates without the need for rigorously inert conditions. In particular, reactions can be performed directly in commercially available nonanhydrous solvents without subsequent drying or use of internal desiccants.

Functionalization of Intramolecular Frustrated Lewis Pairs by 1,1-Carboboration with Conjugated Enynes

The vicinal P/B frustrated Lewis pair (FLP) Mes2PCH2CH2B(C6F5)2 undergoes 1,1-carboboration reactions with the Me3Si-substituted enynes to give ring-enlarged functionalized C3-bridged P/B FLPs. These serve as active FLPs in the activation of dihydrogen to give the respective zwitterionic [P]H(+)/[B]H(-) products. One such product shows activity as a metal-free catalyst for the hydrogenation of enamines or a bulky imine. The ring-enlarged FLPs contain dienylborane functionalities that undergo "bora-Nazarov"-type ring-closing rearrangements upon photolysis. A DFT study had shown that the dienylborane cyclization of such systems itself is endothermic, but a subsequent C6F5 migration is very favorable. Furthermore, substituted 2,5-dihydroborole products are derived from cyclization and C6F5 migration from the photolysis reaction. In the case of the six-membered annulation product, a subsequent stereoisomerization reaction takes place and the resultant compound undergoes a P/B FLP 1,2-addition reaction with a terminal alkyne with rearrangement.

A unified survey of Si-H and H-H bond activation catalysed by electron-deficient boranes

The bond activation chemistry of B(C6F5)3 and related electron-deficient boranes is currently experiencing a renaissance due to the fascinating development of frustrated Lewis pairs (FLPs). B(C6F5)3's ability to catalytically activate Si-H bonds through η(1) coordination opened the door to several unique reduction processes. The ground-breaking finding that the same family of fully or partially fluorinated boron Lewis acids allows for the related H-H bond activation, either alone or as a component of an FLP, brought considerable momentum into the area of transition-metal-free hydrogenation and, likewise, hydrosilylation. This review comprehensively summarises synthetic methods involving borane-catalysed Si-H and H-H bond activation. Systems corresponding to an FLP-type situation are not covered. Aside from the broad manifold of C=X bond reductions and C=X/C-X defunctionalisations, dehydrogenative (oxidative) Si-H couplings are also included.

Frustrated Lewis pairs: from concept to catalysis

CONSPECTUS: Frustrated Lewis pair (FLP) chemistry has emerged in the past decade as a strategy that enables main-group compounds to activate small molecules. This concept is based on the notion that combinations of Lewis acids and bases that are sterically prevented from forming classical Lewis acid-base adducts have Lewis acidity and basicity available for interaction with a third molecule. This concept has been applied to stoichiometric reactivity and then extended to catalysis. This Account describes three examples of such developments: hydrogenation, hydroamination, and CO2 reduction. The most dramatic finding from FLP chemistry was the discovery that FLPs can activate H2, thus countering the long-existing dogma that metals are required for such activation. This finding of stoichiometric reactivity was subsequently evolved to employ simple main-group species as catalysts in hydrogenations. While the initial studies focused on imines, subsequent studies uncovered FLP catalysts for a variety of organic substrates, including enamines, silyl enol ethers, olefins, and alkynes. Moreover, FLP reductions of aromatic anilines and N-heterocycles have been developed, while very recent extensions have uncovered the utility of FLP catalysts for ketone reductions. FLPs have also been shown to undergo stoichiometric reactivity with terminal alkynes. Typically, either deprotonation or FLP addition reaction products are observed, depending largely on the basicity of the Lewis base. While a variety of acid/base combinations have been exploited to afford a variety of zwitterionic products, this reactivity can also be extended to catalysis. When secondary aryl amines are employed, hydroamination of alkynes can be performed catalytically, providing a facile, metal-free route to enamines. In a similar fashion, initial studies of FLPs with CO2 demonstrated their ability to capture this greenhouse gas. Again, modification of the constituents of the FLP led to the discovery of reaction systems that demonstrated stoichiometric reduction of CO2 to either methanol or CO. Further modification led to the development of catalytic systems for the reduction of CO2 by hydrosilylation and hydroboration or deoxygenation. As each of these areas of FLP chemistry has advanced from the observation of unusual stoichiometric reactions to catalytic processes, it is clear that the concept of FLPs provides a new strategy for the design and application of main-group chemistry and the development of new metal-free catalytic processes.