Cascade Syntheses Routes to the Centrocountins

Cascade and domino reactions that proceed through multiple steps in one pot and include multiple bond formations are promising methods for the rapid and efficient generation of complex molecular architectures, including the scaffolds of classes of complex natural product. We describe the development of various one‐pot cascade reaction sequences to yield centrocountins, which are tetracyclic indole derivatives with the basic scaffold of numerous polycyclic alkaloids. The mechanistic investigation of a sequence employing readily available alkynes and 3‐formylchromones as starting materials provided evidence that this one‐pot synthesis proceeds through at least twelve consecutive transformations and includes at least nine different chemical reactions, making it the longest cascade reaction sequence known to date. We describe the scope and limitations of the cascade synthesis approaches and the development of an enantioselectively catalyzed centrocountin synthesis.     

Wenn die Rührgeschwindigkeit die Produktverteilung bestimmt: Selektivität mischungsmaskierter Reaktionen

Many chemical reactions are so fast relative to mixing that significant conversion takes place before homogenization down to the molecular scale is reached. In such cases, the bond‐breaking and bond‐forming events mainly occur in regions where concentrations gradients are still present. Consequently the rate of mixing can have a pronounced effect on the kinetics and the product distribution in competitive chemical reactions. This review gives an account of such mixing‐sensitive single‐phase liquid‐liquid chemical reactions where all reactants and products are soluble, because a better understanding of the coupling between mixing and chemical reaction is needed to optimize the product distribution by choosing the appropriate reactor geometry and mixing conditions. Additionally these reactions have the potential to be employed as an exploratory microprobe to investigate the local flow characteristics or the mixing process at the molecular level and to evaluate the mechanism of chemical reactions.     

Synthesis of Three‐Membered and Four‐Membered Heterocycles with the Assistance of Photochemical Reactions

Some transformations are not possible with ground‐state reactions even in the presence of a catalyst; hence, they are performed under photochemical conditions. Electron transfer occurred even with the photochemical excitement of one molecule where redox reaction is not possible at the ground state. The side products were obtained from ground‐state reactions. For C─C bond formation during photochemical reactions, there was no requirement of any chemical activation of the substrates. Therefore, these reactions are presented here for the synthesis of three‐membered and four‐membered heterocycles in the context of sustainable processes.     

Automated discovery of chemically reasonable elementary reaction steps

Due to the significant human effort and chemical intuition required to locate chemical reaction pathways with quantum chemical modeling, only a small subspace of possible reactions is usually investigated for any given system. Herein, a systematic approach is proposed for locating reaction paths that bypasses the required human effort and expands the reactive search space, all while maintaining low computational cost. To achieve this, a range of intermediates are generated that represent potential single elementary steps away from a starting structure. These structures are then screened to identify those that are thermodynamically accessible, and then feasible reaction paths to the remaining structures are located. This strategy for elementary reaction path finding is independent of atomistic model whenever bond breaking and forming are properly described. The approach is demonstrated to work well for upper main group elements, but this limitation can easily be surpassed. Further extension will allow discovery of multistep reaction mechanisms in a single computation. The method is highly parallel, allowing for effective use of modern large‐scale computational clusters. © 2013 Wiley Periodicals, Inc.     

LED‐Illuminated NMR Studies of Flavin‐Catalyzed Photooxidations Reveal Solvent Control of the Electron‐Transfer Mechanism

Mechanistic insights into chemical photocatalysis are mainly the domain of UV/Vis spectroscopy, because NMR spectroscopy has been limited by the type of illumination so far. An improved LED‐based illumination device can be used to obtain NMR reaction profiles of photocatalytic reactions under synthetic conditions and perform both photo‐CIDNP and intermediate studies. Flavin‐catalyzed photooxidations of alcohols show the potential of this setup. After identical initial photoreaction steps the stabilization of a downstream intermediate is the key to the further reaction mechanism and the reactivity. As a chemical photocatalyst flavin can act either as a one‐ or a two‐electron mediator when the stability of the zwitterionic radical pair is moldulated in different solvents. This demonstrates the importance of downstream intermediates and NMR‐accessible complementary information in photocatalytic reactions and suggests the control of photoorganic reactions by solvent effects.     

A Nickel‐Catalyzed Carbonyl‐Heck Reaction

The use of transition‐metal catalysis to enable the coupling of readily available organic molecules has greatly enhanced the ability of chemists to access complex chemical structures. In this work, an intermolecular coupling reaction that unites organotriflates and aldehydes is presented. A unique catalyst system is identified to enable this reaction, featuring a Ni⁰ precatalyst, a tridentate Triphos ligand, and a bulky amine base. This transformation provides access to a variety of ketone‐containing products without the selectivity‐ and reactivity‐related challenges associated with more traditional Friedel–Crafts reactions. A Heck‐type mechanism is postulated, wherein the π bond of the aldehyde takes the role of the olefin in the insertion/elimination steps.     

Cycloaddition of P−C Single Bonds: Stereoselective Formation of Benzo‐1,3,6,2‐trioxaphosphepine Complexes via a Ditopic van der Waals Complex

While phosphaalkenes and phosphanes are known to participate in [4+n] cycloaddition reactions, P?C single bonds are inert in this respect. Herein, reactions of oxaphosphirane complexes with tetrachloro‐ortho‐benzoquinone are presented that reveal a stereoselective reaction of the endocyclic P?C bond to afford benzo‐1,3,6,2‐trioxaphosphepine complexes. High‐level DFT calculations provide evidence that the final product is derived from a sequence of three consecutive steps involving a ditopic van der Waals complex.     

The Broad Aryl Acid Specificity of the Amide Bond Synthetase McbA Suggests Potential for the Biocatalytic Synthesis of Amides

Amide bond formation is one of the most important reactions in pharmaceutical synthetic chemistry. The development of sustainable methods for amide bond formation, including those that are catalyzed by enzymes, is therefore of significant interest. The ATP‐dependent amide bond synthetase (ABS) enzyme McbA, from Marinactinospora thermotolerans, catalyzes the formation of amides as part of the biosynthetic pathway towards the marinacarboline secondary metabolites. The reaction proceeds via an adenylate intermediate, with both adenylation and amidation steps catalyzed within one active site. In this study, McbA was applied to the synthesis of pharmaceutical‐type amides from a range of aryl carboxylic acids with partner amines provided at 1–5 molar equivalents. The structure of McbA revealed the structural determinants of aryl acid substrate tolerance and differences in conformation associated with the two half reactions catalyzed. The catalytic performance of McbA, coupled with the structure, suggest that this and other ABS enzymes may be engineered for applications in the sustainable synthesis of pharmaceutically relevant (chiral) amides.     

Palladium‐Mediated Direct Disulfide Bond Formation in Proteins Containing S‐Acetamidomethyl‐cysteine under Aqueous Conditions

One of the applied synthetic strategies for correct disulfide bond formation relies on the use of orthogonal Cys protecting groups. This approach requires purification before and after the deprotection steps, which prolongs the entire synthetic process and lowers the yield of the reaction. A major challenge in using this approach is to be able to apply one‐pot synthesis under mild conditions and aqueous media. In this study, we report the development of an approach for rapid disulfide bond formation by employing palladium chemistry and S‐acetamidomethyl‐cysteine [Cys(Acm)]. Oxidation of Cys(Acm) to the corresponding disulfide bond is achieved within minutes in a one‐pot operation by applying palladium and diethyldithiocarbamate. The utility of this reaction was demonstrated by the synthesis of the peptide oxytocin and the first total chemical synthesis of the protein thioredoxin‐1. Our investigation revealed a critical role of the Acm protecting group in the disulfide bond formation, apparently due to the generation of a disulfiram in the reaction pathway, which significantly assists the oxidation step.     

β‐Selective C‐Glycosylation and its Application in the Synthesis of Scleropentaside A

C‐Glycosides are carbohydrates that bear a C?C bond to an aglycon at the anomeric center. Due to their high stability towards chemical and enzymatic hydrolysis, these compounds are widely used as carbohydrate mimics in drug development. Herein, we report a general and exclusively β‐selective method for the synthesis of a naturally abundant acyl‐C‐glycosidic structural motif first found in the scleropentaside natural product family. A Corey–Seebach umpolung reaction as the key step in the synthesis of scleropentaside A and analogues enables the β‐selective construction of the anomeric C?C bond starting from unprotected carbohydrates in only four steps. The one‐pot approach is highly atom‐efficient and avoids the use of toxic heavy metals.     

Oxidative Interception of the Hydroamination Pathway: A Gold‐Catalyzed Diamination of Alkenes

A complimentary diamination of alkenes by using homogeneous gold catalysts is described. The reaction is one of very few examples of homogeneous gold oxidation catalysis and proceeds with high selectivity under mild conditions. Individual steps of the suggested catalytic cycle were investigated on isolated model gold complexes, and new pathways for gold‐catalyzed amination reactions were established. The key step is an intramolecular alkyl–nitrogen bond formation from a gold(III) intermediate. This process validates the concept of reductive elimination from high oxidation catalyst states for this type of C? N bond forming reactions.     

Manganese‐Catalyzed Multicomponent Synthesis of Pyrimidines from Alcohols and Amidines

The development of catalytic reactions for synthesizing different compounds from alcohols to save fossil carbon feedstock and reduce CO₂ emissions is of high importance. Replacing rare noble metals with abundantly available 3d metals is equally important. We report a manganese‐complex‐catalyzed multicomponent synthesis of pyrimidines from amidines and up to three alcohols. Our reaction proceeds through condensation and dehydrogenation steps, permitting selective C−C and C−N bond formations. β‐Alkylation reactions are used to multiply alkylate secondary alcohols with two different primary alcohols to synthesize fully substituted pyrimidines in a one‐pot process. Our PN₅P‐Mn‐pincer complexes efficiently catalyze this multicomponent process. A comparison of our manganese catalysts with related cobalt catalysts indicates that manganese shows a reactivity similar to that of iridium but not cobalt. This analogy could be used to develop further (de)hydrogenation reactions with manganese complexes.     

Catalytic Mechanism of the Arylsulfatase Promiscuous Enzyme from Pseudomonas Aeruginosa

To elucidate the working mechanism of the “broad substrate specificity” by the Pseudomonas aeruginosa aryl sulfatase (PAS) enzyme, we present here a full quantum chemical study performed at the density functional level. This enzyme is able to catalyze the hydrolysis of the original p‐nitrophenyl‐sulfate (PNPS) substrate and the promiscuous p‐nitrophenyl‐phosphate (PNPP) one with comparable reaction kinetics. Based on the obtained results, a multistep mechanism including activation of the nucleophile, the nucleophilic attack, and the cleavage of the S? O (P? O) bond is proposed. Regarding the phosphate monoester, the results indicate that only some steps of the promiscuous reaction are identical to those in the native process. Differences concern mainly the last step in which the His115 residue acts as a general base to accept the proton by the O atom of the FGly51 in the PNPS, whereas in PNPP, the Asp317 protonated residue works as a general acid to deliver a proton by a water molecule to the oxygen atom of the C? O bond. The shapes of the relative potential‐ energy surface (PES) are similar in the two examined cases but the rate‐determining step is different (nucleophile attack vs. nucleophile activation). The influence of the dispersion contributions on the investigated reactions was also taken into account.     

Palladium‐Catalyzed Oxidative Carbocyclizations

Palladium‐catalyzed oxidative carbon–carbon bond‐forming annulations, that is, carbocyclization reactions, have recently emerged as efficient and atom‐economical routes to carbo‐ and heterocycles, whereby less functionalized substrates and fewer synthetic steps are needed to obtain a target molecule compared with traditional non‐oxidative carbon–carbon bond‐forming reactions. In this review, the synthetic efforts in palladium‐catalyzed oxidative carbocyclization reactions are summarized.     

Thermodynamics of chemical reactions with COSMO‐RS: The extreme case of charge separation or recombination

Many technically relevant chemical processes in the condensed phase involve as elementary reactive steps the formation of ions from neutral species or, as the opposite, recombination of ions. Such reactions that generate or annihilate charge defy the standard gas phase quantum chemical treatment, and also continuum solvation models are only partially able to account for the right amount of stabilization in solution. In this work, for such types of reaction, a solvation treatment involving the COSMO‐RS method is assessed, which leads to improved results, i.e., errors of only around 10 kJ/mol for both protic and aprotic solvents. The examples discussed here comprise protolysis reactions and organo halide heterolysis, for both of which a comparison with reliable experimental data is possible. It is observed that for protolysis, the quality of results does not strongly depend on the quantum chemical method used for energy calculation. In contrast, in the case of heterolytic carbon‐chlorine bond cleavage, clearly better results are obtained for higher correlated (coupled cluster) methods or the density functional M06‐2X, which is well known for its accuracy if applied to organic chemistry. This hints at least that the right answer is obtained for the right reason and not due to a compensation of errors from gas phase thermodynamics with those from the solvation treatment. Problems encountered with certain critical solvents or upon decomposing Gibbs free energies into heats or entropies of reaction are found to relate mostly to the parameterization of the H‐bonding term within COSMO‐RS. © 2012 Wiley Periodicals, Inc.     

Fluorinated Vinylsilanes from the Copper‐Catalyzed Defluorosilylation of Fluoroalkene Feedstocks

Herein, a copper‐catalyzed C?F bond defluorosilylation reaction of tetrafluoroethylene and other polyfluoroalkenes is described. Mechanistic studies, based on a series of stoichiometric reactions with copper complexes, revealed that the key steps of this defluorosilylation reaction are 1) the 1,2‐addition of a silylcopper intermediate to the polyfluoroalkene and 2) a subsequent selective β‐fluorine elimination, which generates a Cu?F species. The β‐fluorine elimination is facilitated by Lewis acidic F?Bpin, which is generated in situ during the defluorosilylation.     

Ligand‐Enabled Triple CH Activation Reactions: One‐Pot Synthesis of Diverse 4‐Aryl‐2‐quinolinones from Propionamides

Diverse 4‐aryl‐2‐quinolinones are prepared from propionamides in one pot by ligand‐promoted triple sequential C? H activation reactions and a stereospecific Heck reaction. In these cascade reactions, three new C? C bonds and one C? N bond are formed to rapidly build molecular complexity from propionic acid.     

Ligand‐Enabled Triple C?H Activation Reactions: One‐Pot Synthesis of Diverse 4‐Aryl‐2‐quinolinones from Propionamides

Diverse 4‐aryl‐2‐quinolinones are prepared from propionamides in one pot by ligand‐promoted triple sequential C H activation reactions and a stereospecific Heck reaction. In these cascade reactions, three new C C bonds and one C N bond are formed to rapidly build molecular complexity from propionic acid.