Reductive C–O, C–N, and C–S Cleavage by a Zirconium Catalyzed Hydrometalation/β-Elimination Approach

Zr-Catalyzed Assembly of 1,1,1-Triborylalkanes from Alkenes and HBpin

Multiboronated compounds play more and more significant roles in the chemical community, and 1,1,1-triborylalkanes have emerged as versatile building blocks in organic synthesis. However, efficient strategies for the assembly of such compounds are very rare. A general and atom-economical synthesis of 1,1,1-triborylalkanes from various alkenes with pinacolborane (HBpin) is reported for the first time. The success of this transformation relies on the use of the commercially available Schwartz reagent (Cp2ZrHCl) as a catalyst, enabling sequential dehydrogenative borylation and hydroboration. This reaction demonstrates excellent selectivity, high yields, and broad functional group tolerance. Mechanistic investigations reveal that the process begins with two consecutive dehydrogenative borylations catalyzed by zirconium, producing a 1,1-diborylalkene intermediate, which subsequently undergoes hydroboration with HBpin to afford 1,1,1-triborylalkanes. Notably, this approach allows for remote 1,1,1-triboration of internal alkenes.

Methoxide-Enabled Zirconium-Catalyzed Migratory Alkene Hydrosilylation

A zirconocene dichloride-catalyzed alkene hydrosilylation is reported that can be applied to non-activated and conjugated terminal and internal alkenes. It involves a catalytic Zr-walk process and leads to a selective conversion to the linear product. Lithium methoxide serves as mild catalyst activating agent, which significantly increases the applicability and operational simplicity in comparison to earlier zirconium(II)-based protocols. Supported by additional experiments and calculations, a mechanism via zirconium(IV) intermediates is proposed. Due to the benign nature and ready-availability of the zirconium catalyst, the reaction is an attractive alternative to established alkene hydrosilylation methods.

Efficient synthesis of amides from secondary alcohols and CH3CN promoted by Fe(NO3)3·9H2O

The Ritter reaction is the most attractive method for synthesizing amides, and various acids have been used to promote this reaction. Compared to these acids, Fe(NO3)3·9H2O is less toxic and costly, and it shows relatively high Lewis acidity and great catalytic activity. In this study, a simple and efficient protocol involving Fe(NO3)3·9H2O as an additive for the synthesis of amides was developed. Various secondary alcohols could be reacted with CH3CN to obtain their corresponding products, with CH3CN being used as a reactant and solvent. This protocol was found to be applicable to a wide range of alcohols and nitrile substrates. In general, it was found that substrates containing electron-donating-groups offered the corresponding amides in good to excellent yields, while those with electron-withdrawing groups offered low to moderate yields. Meanwhile, this approach was scalable to the gram level, offering an attractive opportunity for further application in organic synthesis.

Catalytic Asymmetric Ring-Opening Reactions of Unstrained Heterocycles Using Cobalt Vinylidenes

Cobalt catalysts promote highly enantioselective ring-opening reactions of 2,5-dihydrofurans using vinylidenes. The products are acyclic organozinc compounds that can be functionalized with an electrophile. The proposed mechanism involves the generation of a cobalt vinylidene species that adds to the alkene by a [2+2]-cycloaddition pathway. Ring-opening then occurs via outer-sphere β-O elimination assisted by coordination of a ZnX2 Lewis acid to the alkoxide leaving group. DFT models reveal that competing inner-sphere syn β-H and β-O elimination pathways are suppressed by the geometric constraints of the metallacycle intermediate. These models rationalize the observed stereochemical outcome of the reaction.

Zirconium-Based Catalysts in Organic Synthesis

Zirconium is a silvery-white malleable and ductile metal at room temperature with a crustal abundance of 162 ppm. Its compounds, showing Lewis acidic behavior and high catalytic performance, have been recognized as a relatively cheap, low-toxicity, stable, green, and efficient catalysts for various important organic transformations. Commercially available inorganic zirconium chloride was widely applied as a catalyst to accelerate amination, Michael addition, and oxidation reactions. Well-designed zirconocene perfluorosulfonates can be applied in allylation, acylation, esterification, etc. N-Chelating oganozirconium complexes accelerate polymerization, hydroaminoalkylation, and CO₂ fixation efficiently. In this review, the applications of both commercially available and synthesized zirconium catalysts in organic reactions in the last 5 years are highlighted. Firstly, the properties and application of zirconium and its compounds are simply introduced. After presenting the superiority of zirconium compounds, their applications as catalysts to accelerate organic transformations are classified and presented in detail. On the basis of different kinds of zirconium catalysts, organic reactions accelerated by inorganic zirconium catalysts, zirconium catalysts bearing Cp, and organozirconium catalysts without Cp are summarized, and the plausible reaction mechanisms are presented if available.

Catalytic Asymmetric β-Oxygen Elimination

A catalytic enantioselective β-O-elimination reaction is reported in the form of a zirconium-catalyzed asymmetric opening of meso-ketene acetals. Furthermore, a regiodivergent β-O-elimination is demonstrated. The reaction proceeds under mild conditions, at low catalyst loadings, and produces chiral monoprotected cis-1,2-diols in good yield and enantiomeric excess. The combination with a Mitsunobu reaction or a one-pot hydroboration/Suzuki reaction sequence then gives access to additional diol and aminoalcohol building blocks. A stereochemical analysis supported by DFT calculations reveals that a high selectivity in the hydrozirconation step is also important for achieving high enantioselectivity, although it does not constitute the asymmetric step. This insight is crucial for the future development of related asymmetric β-elimination reactions.

A uniformly anchored zirconocene complex on magnetic reduced graphene oxide (rGO@Fe3O4/ZrCp2Cl x (x = 0, 1, 2)) as a novel and reusable nanocatalyst for synthesis of N-arylacetamides and reductive-acetylation of nitroarenes

In this study, a crafted zirconocene complex on rGO@Fe3O4 as a novel magnetic nanocatalyst was synthesized and then characterized using FT-IR, SEM, EDX, VSM, ICP-OES, TGA, BET and MS analyses. Next, catalytic activity of the prepared nanocomposite rGO@Fe3O4/ZrCp2Cl x (x = 0, 1, 2) towards successful reduction of aromatic nitro compounds to arylamines using N2H4·H2O (80%) was investigated. The examined nanocatalyst also showed perfect catalytic activity for reductive-acetylation of aromatic nitro compounds to the corresponding N-arylacetamides without isolation of the prepared in situ amines using the N2H4·H2O/Ac2O system. Furthermore, acetylation of the commercially available arylamines to the corresponding N-arylacetamides was carried out by acetic anhydride in the presence of the rGO@Fe3O4/ZrCp2Cl x (x = 0, 1, 2) nanocomposite. All reactions were carried out in refluxing EtOH as a green solvent to afford the products in high yields. The obtained results exhibited that the nanocomposite of rGO@Fe3O4/ZrCp2Cl x (x = 0, 1, 2) showed a great catalytic activity in comparison to rGO and rGO@Fe3O4 as the parent constituents. Recovery and reusability of rGO@Fe3O4/ZrCp2Cl x (x = 0, 1, 2) were also examined for 8 consecutive cycles without significant loss of the catalytic activity. This establishes the sustainable anchoring of the zirconocene complex on the surface and mesopores of the rGO@Fe3O4 nanohybrid system.

Expanding Zirconocene Hydride Catalysis: In Situ Generation and Turnover of ZrH Catalysts Enabling Catalytic Carbonyl Reductions

Despite the wide use and popularity of metal hydride catalysis, methods utilizing zirconium hydride catalysts remain underexplored. Here, we report the development of a mild method for the in situ preparation and use of zirconium hydride catalysts. This robust method requires only 2.5-5 mol % of zirconocene dichloride in combination with a hydrosilane as the stoichiometric reductant and does not require careful air- or moisture-free techniques. A key finding of this study concerns an amine-mediated ligand exchange en route to the active zirconocene hydride catalyst. A mechanistic investigation supports the intermediacy of an oxo-bridged dimer precatalyst. The application of this method to the reduction of a wide variety of carbonyl-containing substrates, including ketones, aldehydes, enones, ynones, and lactones, is demonstrated with up to 92% yield and exhibits broad functional group tolerability. These findings open up alternative avenues for the catalytic application of chlorozirconocenes, potentially serving as the foundation for broader applications of zirconium hydride catalysis.

Homogeneous Palladium-Catalyzed Selective Reduction of 2,2′-Biphenols Using HCO2H as Hydrogen Source

An efficient homogeneous palladium-catalyzed selective deoxygenation of 2,2′-biphenols by reduction of aryl triflates with HCO2H as the hydrogen source is reported. This protocol complements the current method based on heterogeneous Pd/C-catalyzed hydrogenation with hydrogen gas. This process provided the reduction products in good to excellent yields, which could be readily converted to various synthetically useful molecules, especially ligands for catalytic synthesis.

A Titanium-Catalyzed Reductive α-Desulfonylation

A titanium(III)-catalyzed desulfonylation gives access to functionalized alkyl nitrile building blocks from α-sulfonyl nitriles, circumventing traditional base-mediated α-alkylation conditions and strong single electron donors. The reaction tolerates numerous functional groups including free alcohols, esters, amides, and it can be applied also to the α-desulfonylation of ketones. In addition, a one-pot desulfonylative alkylation is demonstrated. Preliminary mechanistic studies indicate a catalyst-dependent mechanism involving a homolytic C-S cleavage.

Cobalt-catalyzed deoxygenative triborylation of allylic ethers to access 1,1,3-triborylalkanes

Polyborylated organic compounds have been emerging as versatile building blocks in chemical synthesis. Here we report a selective cobalt-catalyzed deoxygenative 1,1,3-triborylation reaction of allylic ethers with pinacolborane to prepare 1,1,3-triborylalkane compounds. With naturally abundant and/or synthetic cinnamic methyl ethers as starting materials, we have achieved the synthesis of a variety of 1,1,3-triborylalkanes (25 examples). The synthetic utility of these 1,1,3-triborylalkanes is demonstrated through site-selective allylation, protodeborylation, and consecutive carbon-carbon bond-forming reactions. Mechanistic studies including deuterium-labeling and control experiments suggest that this 1,1,3-triborylation reaction proceeds through initial cobalt-catalyzed deoxygenative borylation of allylic ethers to form allylic boronates followed by cobalt-catalyzed 1,1-diborylation of the resulting allylic boronates.

Zirconium-Catalyzed Atom-Economical Synthesis of 1,1-Diborylalkanes from Terminal and Internal Alkenes

A general and atom‐economical synthesis of 1,1‐diborylalkanes from alkenes and a borane without the need for an additional H₂ acceptor is reported for the first time. The key to our success is the use of an earth‐abundant zirconium‐based catalyst, which allows a balance of self‐contradictory reactivities (dehydrogenative boration and hydroboration) to be achieved. Our method avoids using an excess amount of another alkene as an H₂ acceptor, which was required in other reported systems. Furthermore, substrates such as simple long‐chain aliphatic alkenes that did not react before also underwent 1,1‐diboration in our system. Significantly, the unprecedented 1,1‐diboration of internal alkenes enabled the preparation of 1,1‐diborylalkanes.

Transition-metal mediated carbon-sulfur bond activation and transformations: an update

Carbon-sulfur bond cross-coupling has become more and more attractive as an alternative protocol to establish carbon-carbon and carbon-heteroatom bonds. Diverse transformations through transition-metal-catalyzed C-S bond activation and cleavage have recently been developed. This review summarizes the advances in transition-metal-catalyzed cross-coupling via carbon-sulfur bond activation and cleavage since late 2012 as an update of the critical review on the same topic published in early 2013 (Chem. Soc. Rev., 2013, 42, 599-621), which is presented by the categories of organosulfur compounds, that is, thioesters, thioethers including heteroaryl, aryl, vinyl, alkyl, and alkynyl sulfides, ketene dithioacetals, sulfoxides including DMSO, sulfones, sulfonyl chlorides, sulfinates, thiocyanates, sulfonium salts, sulfonyl hydrazides, sulfonates, thiophene-based compounds, and C[double bond, length as m-dash]S functionality-bearing compounds such as thioureas, thioamides, and carbon disulfide, as well as the mechanistic insights. An overview of C-S bond cleavage reactions with stoichiometric transition-metal reagents is briefly given. Theoretical studies on the reactivity of carbon-sulfur bonds by DFT calculations are also discussed.

Nickel-Catalyzed Transformation of Alkene-Tethered Oxime Ethers to Nitriles by a Traceless Directing Group Strategy

Nickel-catalyzed transformation of alkene-tethered oxime ethers to nitriles using a traceless directing group strategy has been developed. A series of alkene-tethered oxime ethers derived from benzaldehyde and cinnamyl aldehyde derivatives were converted into the corresponding benzonitriles and cinnamonitriles in 46-98% yields using the nickel catalyst system. Control experiments showed that the alkene group tethered to an oxygen atom on the oximes via one methylene unit plays a key role as a traceless directing group during the catalysis.

Nickel-Catalyzed Removal of Alkene Protecting Group of Phenols, Alcohols via Chain Walking Process

An efficient nickel-catalyzed removal of alkene protection group under mild condition with high functional group tolerance through chain walking process has been established. Not only phenolic ethers, but also alcoholic ethers can be tolerated with the retention of stereocenter adjacent to hydroxyl group. The new reaction brings the homoallyl group into a start of new type of protecting group.