Carbonic Anhydrase Variants Catalyze the Reduction of Dialkyl Ketones with High Enantioselectivity.

Human carbonic anhydrase II (hCAII) naturally catalyzes the reaction between two achiral molecules - water and carbon dioxide - to yield the achiral product carbonic acid through a zinc hydroxide intermediate. We have previously shown that a zinc hydride, instead of a hydroxide, can be generated in this enzyme to create a catalyst for the reduction of aryl ketones. Dialkyl ketones are more challenging to reduce, and the enantioselective reduction of dialkyl ketones with two alkyl groups that are similar in size and electronic properties, is a particularly challenging transformation to achieve with high activity and selectivity. Here, we show that hCAII, as well as a double variant of it, catalyzes the enantioselective reduction of dialkyl ketones with high yields and enantioselectivities, even when the two alkyl groups are similar in size. We also show that variants of hCAII catalyze the site-selective reduction of one ketone over the other in an unsymmetrical aliphatic diketone. Computational docking of a dialkyl ketone to the double variant containing the zinc hydride provides insights into the origins of the reactivity of various substrates and the high enantioselectivity of the transformations and show how a confined environment can control the enantioselectivity of an abiological intermediate.

Palladium-Catalyzed Amination of Aryl Halides with Aqueous Ammonia and Hydroxide Base Enabled by Ligand Development.

The conversion of aryl halides to primary arylamines with a convenient and inexpensive source of ammonia has been a long-standing synthetic challenge. Aqueous ammonia would be the most convenient and least expensive form of ammonia, but such a palladium-catalyzed amination reaction with a high concentration of water faces challenges concerning catalyst stability and competing hydroxylation, and palladium-catalyzed reactions with this practical reagent are rare. Further, most reactions with ammonia to form primary amines are conducted with tert-butoxide base, but reactions with ammonium hydroxide would contain hydroxide as base. Thus, ammonia surrogates, ammonia in organic solvents, and ammonium salts have been used under anhydrous conditions instead with varying levels of selectivity for the primary amine. We report the palladium-catalyzed amination of aryl and heteroaryl chlorides and bromides with aqueous ammonia and a hydroxide base to form the primary arylamine with high selectivity. The palladium catalyst containing a new dialkyl biheteroaryl phosphine ligand (KPhos) suppresses both the formation of aryl alcohol and diarylamine side products. Mechanistic studies with a soluble hydroxide base revealed turnover-limiting reductive elimination of the arylamine and an equilibrium between arylpalladium amido and hydroxo complexes prior to the turnover-limiting step.

Copper-Mediated Cyanodifluoromethylation of (Hetero)aryl Iodides and Activated (Hetero)aryl Bromides with TMSCF2CN.

Molecules bearing fluorine are increasingly prevalent in pharmaceuticals, agrochemicals, and functional materials. The cyanodifluoromethyl group is unique because its size is closer than that of any other substituted difluoromethyl group to the size of the trifluoromethyl group, but its electronic properties are distinct from those of the trifluoromethyl group. In addition, the presence of the cyano group provides synthetic entry to a wide range of substituted difluoromethyl groups. However, the synthesis of cyanodifluoromethyl compounds requires multiple steps, highly reactive reagents (such as DAST, NSFI, or IF5), or specialized starting materials (such as α,α-dichloroacetonitriles or α-mercaptoacetonitriles). Herein, we report a copper-mediated cyanodifluoromethylation of aryl and heteroaryl iodides and activated aryl and heteroaryl bromides with TMSCF2CN. This cyanodifluoromethylation tolerates an array of functional groups, is applicable to late-stage functionalization of complex molecules, yields analogues of FDA-approved pharmaceuticals and fine chemicals, and enables the synthesis of a range of complex molecules bearing a difluoromethylene unit by transformations of the electron-poor CN unit. Calculations of selected steps of the reaction mechanism by Density Functional Theory indicate that the barriers for both the oxidative addition of iodobenzene to [(DMF)CuCF2CN] and the reductive elimination of the fluoroalkyl product from the fluoroalkyl copper intermediate lie in between those of [(DMF)CuCF3] and [(DMF)CuCF2C(O)NMe2].

Deconvolution and Analysis of the 1H NMR Spectra of Crude Reaction Mixtures.

Nuclear magnetic resonance (NMR) spectroscopy is an important analytical technique in synthetic organic chemistry, but its integration into high-throughput experimentation workflows has been limited by the necessity of manually analyzing the NMR spectra of new chemical entities. Current efforts to automate the analysis of NMR spectra rely on comparisons to databases of reported spectra for known compounds and, therefore, are incompatible with the exploration of new chemical space. By reframing the NMR spectrum of a reaction mixture as a joint probability distribution, we have used Hamiltonian Monte Carlo Markov Chain and density functional theory to fit the predicted NMR spectra to those of crude reaction mixtures. This approach enables the deconvolution and analysis of the spectra of mixtures of compounds without relying on reported spectra. The utility of our approach to analyze crude reaction mixtures is demonstrated with the experimental spectra of reactions that generate a mixture of isomers, such as Wittig olefination and C-H functionalization reactions. The correct identification of compounds in a reaction mixture and their relative concentrations is achieved with a mean absolute error as low as 1%.

Polymers from Plant Oils Linked by Siloxane Bonds for Programmed Depolymerization.

The increased production of plastics is leading to the accumulation of plastic waste and depletion of limited fossil fuel resources. In this context, we report a strategy to create polymers that can undergo controlled depolymerization by linking renewable feedstocks with siloxane bonds. α,ω-Diesters and α,ω-diols containing siloxane bonds were synthesized from an alkenoic ester derived from castor oil and then polymerized with varied monomers, including related biobased monomers. In addition, cyclic monomers derived from this alkenoic ester and hydrosiloxanes were prepared and cyclized to form a 26-membered macrolactone containing a siloxane unit. Sequential ring-opening polymerization of this macrolactone and lactide afforded an ABA triblock copolymer. This set of polymers containing siloxanes underwent programmed depolymerization into monomers in protic solvents or with hexamethyldisiloxane and an acid catalyst. Monomers afforded by the depolymerization of polyesters containing siloxane linkages were repolymerized to demonstrate circularity in select polymers. Evaluation of the environmental stability of these polymers toward enzymatic degradation showed that they undergo enzymatic hydrolysis by a fungal cutinase from Fusarium solani. Evaluation of soil microbial metabolism of monomers selectively labeled with 13C revealed differential metabolism of the main chain and side chain organic groups by soil microbes.

2-Aminophenanthroline Ligands Enable Mild, Undirected, Iridium-Catalyzed Borylation of Alkyl C-H Bonds.

The catalytic, undirected borylation of alkyl C-H bonds typically occurs at high reaction temperatures or with excess substrate, or both, because of the low reactivity of alkyl C-H bonds. Here we report a new iridium system comprising 2-anilino-1,10-phenanthroline as the ligand that catalyzes the borylation of alkyl C-H bonds with little to no induction period and with high reaction rates. This superior activation and reactivity profile of 2-aminophenanthroline-ligated catalysts leads to broader reaction scope, including reactions of sensitive substrates, such as epoxides and glycosidic acetals, enhanced diastereoselectivity, and higher yields of borylated products. These catalysts also enable the borylation of alkanes, amines, and ethers at room temperature for the first time. Mechanistic studies imply that facile N-borylation occurs under the reaction conditions and that iridium complexes containing N-boryl aminophenanthrolines are competent precatalysts for the reaction.

Mapping the mechanisms of oxidative addition in cross-coupling reactions catalysed by phosphine-ligated Ni(0).

The complexes of first-row transition metals can undergo elementary reactions by multiple pathways due to their propensity to undergo both one- and two-electron redox steps. Classic and recent studies of the oxidative addition of aryl halides to Ni(0)-a common step in widely practised cross-coupling processes-have yielded contradictory conclusions about stepwise, radical versus concerted mechanisms, but such information is crucial to the design of catalysts based on earth-abundant metals. Here we show that the oxidative addition of aryl halides to Ni(0) ligated by monophosphines occurs by both mechanisms and delineate how the branching of radical and non-radical pathways depends on the electronic properties of both the ligand and reactant arene as well as the identity of the halide. The one-electron pathway occurs by outer-sphere electron transfer to form an aryl radical rather than the often-proposed halogen atom transfer.

Mechanistic Insights into the Origins of Selectivity in a Cu-Catalyzed C-H Amidation Reaction.

The catalytic transformation of C-H to C-N bonds offers rapid access to fine chemicals and high-performance materials, but achieving high selectivity from undirected aminations of unactivated C(sp3)-H bonds remains an outstanding challenge. We report the origins of the reactivity and selectivity of a Cu-catalyzed C-H amidation of simple alkanes. Using a combination of experimental and computational mechanistic studies and energy decomposition techniques, we uncover a switch in mechanism from inner-sphere to outer-sphere coupling between alkyl radicals and the active Cu(II) catalyst with increasing substitution of the alkyl radical. The combination of computational predictions and detailed experimental validation shows that simultaneous minimization of both Cu-C covalency and alkyl radical size increases the rate of reductive elimination and that both strongly electron-donating and electron-withdrawing substituents on the catalyst accelerate the selectivity-determining C-N bond formation process as a result of a change in mechanism. These findings offer design principles for the development of improved catalyst scaffolds for radical C-H functionalization reactions.

Enantio- and Diastereodivergent Cyclopropanation of Allenes by Directed Evolution of an Iridium-Containing Cytochrome.

Alkylidene cyclopropanes (ACPs) are valuable synthetic intermediates because of their constrained structure and opportunities for further diversification. Although routes to ACPs are known, preparations of ACPs with control of both the configuration of the cyclopropyl (R vs S) group and the geometry of the alkene (E vs Z) are unknown. We describe enzymatic cyclopropanation of allenes with ethyl diazoacetate (EDA) catalyzed by an iridium-containing cytochrome (Ir(Me)-CYP119) that controls both stereochemical elements. Two mutants of Ir(Me)-CYP119 identified by 6-codon (6c, VILAFG) saturation mutagenesis catalyze the formation of (E)-ACPs with -93% to >99% ee and >99:1 E/Z ratio with just three rounds of 96 mutants. By four additional rounds of mutagenesis, an enzyme variant was identified that forms (Z)-ACPs with up to 94% ee and a 28:72 E/Z ratio. Computational studies show that the orientation of the carbene unit dictated by the mutated positions accounts for the stereoselectivity.

Oxidative addition of an alkyl halide to form a stable Cu(III) product.

The step that cleaves the carbon-halogen bond in copper-catalyzed cross-coupling reactions remains ill defined because of the multiple redox manifolds available to copper and the instability of the high-valent copper product formed. We report the oxidative addition of α-haloacetonitrile to ionic and neutral copper(I) complexes to form previously elusive but here fully characterized copper(III) complexes. The stability of these complexes stems from the strong Cu-CF3 bond and the high barrier for C(CF3)-C(CH2CN) bond-forming reductive elimination. The mechanistic studies we performed suggest that oxidative addition to ionic and neutral copper(I) complexes proceeds by means of two different pathways: an SN2-type substitution to the ionic complex and a halogen-atom transfer to the neutral complex. We observed a pronounced ligand acceleration of the oxidative addition, which correlates with that observed in the copper-catalyzed couplings of azoles, amines, or alkynes with alkyl electrophiles.

Cross-coupling by a noncanonical mechanism involving the addition of aryl halide to Cu(II).

Copper complexes are widely used in the synthesis of fine chemicals and materials to catalyze couplings of heteroatom nucleophiles with aryl halides. We show that cross-couplings catalyzed by some of the most active catalysts occur by a mechanism not previously considered. Copper(II) [Cu(II)] complexes of oxalamide ligands catalyze Ullmann coupling to form the C-O bond in aryl ethers by concerted oxidative addition of an aryl halide to Cu(II) to form a high-valent species that is stabilized by radical character on the oxalamide ligand. This mechanism diverges from those involving Cu(I) and Cu(III) intermediates that have been posited for other Ullmann-type couplings. The stability of the Cu(II) state leads to high turnover numbers, >1000 for the coupling of phenoxide with aryl chloride electrophiles, as well as an ability to run the reactions in air.

Chemical Modification of Oxidized Polyethylene Enables Access to Functional Polyethylenes with Greater Reuse.

Polyethylene is a commodity material that is widely used because of its low cost and valuable properties. However, the lack of functional groups in polyethylene limits its use in applications that include adhesives, gas barriers, and plastic blends. The inertness of polyethylene makes it difficult to install groups that would enhance its properties and enable programmed chemical decomposition. To overcome these deficiencies, the installation of pendent functional groups that imbue polyethylene with enhanced properties is an attractive strategy to overcome its inherent limitations. Here, we describe strategies to derivatize oxidized polyethylene that contains both ketones and alcohols to monofunctional variants with bulk properties superior to those of unmodified polyethylene. Iridium-catalyzed transfer dehydrogenation with acetone furnished polyethylenes with only ketones, and ruthenium-catalyzed hydrogenation with hydrogen furnished polyethylenes with only alcohols. We demonstrate that the ratio of these functional groups can be controlled by reduction with stoichiometric hydride-containing reagents. The ketones and alcohols serve as sites to introduce esters and oximes onto the polymer, thereby improving surface and bulk properties over those of polyethylene. These esters and oximes were removed by hydrolysis to regenerate the original oxygenated polyethylenes, showing how functionalization can lead to materials with circularity. Waste polyethylenes were equally amenable to oxidative functionalization and derivatization of the oxidized material, showing that this low- or negative-value feedstock can be used to prepare materials of higher value. Finally, the derivatized polymers with distinct solubilities were separated from mechanically mixed plastic blends by selective dissolution, demonstrating that functionalization can lead to novel approaches for distinguishing and separating polymers from a mixture.

Transition-Metal-Catalyzed Silylation and Borylation of C-H Bonds for the Synthesis and Functionalization of Complex Molecules.

The functionalization of C-H bonds in organic molecules containing functional groups has been one of the holy grails of catalysis. One synthetically important approach to the diverse functionalization of C-H bonds is the catalytic silylation or borylation of C-H bonds, which enables a broad array of downstream transformations to afford diverse structures. Advances in both undirected and directed methods for the transition-metal-catalyzed silylation and borylation of C-H bonds have led to their rapid adoption in early-, mid-, and late-stage of the synthesis of complex molecules. In this Review, we review the application of the transition-metal-catalyzed silylation and borylation of C-H bonds to the synthesis of bioactive molecules, organic materials, and ligands. Overall, we aim to provide a picture of the state of art of the silylation and borylation of C-H bonds as applied to the synthesis and modification of diverse architectures that will spur further application and development of these reactions.

Diverse functional polyethylenes by catalytic amination.

Functional polyethylenes possess valuable bulk and surface properties, but the limits of current synthetic methods narrow the range of accessible materials and prevent many envisioned applications. Instead, these materials are often used in composite films that are challenging to recycle. We report a Cu-catalyzed amination of polyethylenes to form mono- and bifunctional materials containing a series of polar groups and substituents. Designed catalysts with hydrophobic moieties enable the amination of linear and branched polyethylenes without chain scission or cross-linking, leading to polyethylenes with otherwise inaccessible combinations of functional groups and architectures. The resulting materials possess tunable bulk and surface properties, including toughness, adhesion to metal, paintability, and water solubility, which could unlock applications for functional polyethylenes and reduce the need for complex composites.

Iridium-Catalyzed, Site-Selective Silylation of Secondary C(sp3)-H Bonds in Secondary Alcohols and Ketones.

We report the iridium-catalyzed, stereoselective conversion of secondary alcohols or ketones to anti-1,3-diols by the silylation of secondary C-H bonds γ to oxygen and oxidation of the resulting oxasilolane. The silylation of secondary C-H bonds in secondary silyl ethers derived from alcohols or ketones is enabled by a catalyst formed from a simple bisamidine ligand. The silylation occurs with high selectivity at a secondary C-H bond γ to oxygen over distal primary or proximal secondary C-H bonds. Initial mechanistic investigations suggest that the source of the newly achieved reactivity is a long catalyst lifetime resulting from the high binding constant of the strongly electron-donating bisamidine ligand.

Hybrid Machine Learning Approach to Predict the Site Selectivity of Iridium-Catalyzed Arene Borylation.

The borylation of aryl and heteroaryl C-H bonds is valuable for the site-selective functionalization of C-H bonds in complex molecules. Iridium catalysts ligated by bipyridine ligands catalyze the borylation of the C-H bond that is most acidic and least sterically hindered in an arene, but predicting the site of borylation in molecules containing multiple arenes is difficult. To address this challenge, we report a hybrid computational model that predicts the Site of Borylation (SoBo) in complex molecules. The SoBo model combines density functional theory, semiempirical quantum mechanics, cheminformatics, linear regression, and machine learning to predict site selectivity and to extrapolate these predictions to new chemical space. Experimental validation of SoBo showed that the model predicts the major site of borylation of pharmaceutical intermediates with higher accuracy than prior machine-learning models or human experts, demonstrating that SoBo will be useful to guide experiments for the borylation of specific C(sp2)-H bonds during pharmaceutical development.

Progression of Hydroamination Catalyzed by Late Transition-Metal Complexes from Activated to Unactivated Alkenes.

ConspectusCatalytic intermolecular hydroamination of alkenes is an atom- and step-economical method for the synthesis of amines, which have important applications as pharmaceuticals, agrochemicals, catalysts, and materials. However, hydroaminations of alkenes in high yield with high selectivity are challenging to achieve because these reactions often lack a thermodynamic driving force and often are accompanied by side reactions, such as alkene isomerization, telomerization, and oxidative amination. Consequently, early examples of hydroamination were generally limited to the additions of N-H bonds to conjugated alkenes or strained alkenes, and the catalytic hydroamination of unactivated alkenes with late transition metals has only been disclosed recently. Many classes of catalysts, including early transition metals, late transition metals, rare-earth metals, acids, and photocatalysts, have been reported for catalytic hydroamination. Among them, late transition-metal complexes possess several advantages, including their relative ease of handling and their high compatibility of substrates containing polar or sensitive functional groups.This Account describes the progression in our laboratory of hydroaminations catalyzed by late transition-metal complexes from the initial additions of N-H bonds to activated alkenes to the more recent additions to unactivated alkenes. Our developments include the Markovnikov and anti-Markovnikov hydroamination of vinylarenes with palladium, rhodium, and ruthenium, the hydroamination of dienes and trienes with nickel and palladium, the hydroanimation of bicyclic strained alkenes with neutral iridium, and the hydroamination of unactivated terminal and internal alkenes with cationic iridium and ruthenium. Enantioselective hydroaminations of these classes of alkenes to form enantioenriched, chiral amines also have been developed.Mechanistic studies have elucidated the elementary steps and the turnover-limiting steps of these catalytic reactions. The hydroamination of conjugated alkenes catalyzed by palladium, rhodium, nickel, and ruthenium occurs by turnover-limiting nucleophilic attack of the amine on a coordinated benzyl, allyl, alkene, or arene ligand. On the other hand, the hydroamination of unconjugated alkenes catalyzed by ruthenium and iridium occurs by turnover-limiting migratory insertion of the alkene into a metal-nitrogen bond. In addition, pathways for the formation of side products, including isomeric alkenes and enamines, have been identified during our studies. During studies on enantioselective hydroamination, the reversibility of the hydroamination has been shown to erode the enantiopurity of the products. Based on our mechanistic understandings, new generations of catalysts that promote catalytic hydroaminations with higher rates, chemoselectivity, and enantioselectivity have been developed. We hope that our discoveries and mechanistic insights will facilitate the further development of catalysts that promote selective, practical, and efficient hydroamination of alkenes.

Discovery of the Azaserine Biosynthetic Pathway Uncovers a Biological Route for α-Diazoester Production.

Azaserine is a bacterial metabolite containing a biologically unusual and synthetically enabling α-diazoester functional group. Herein, we report the discovery of the azaserine (aza) biosynthetic gene cluster from Glycomyces harbinensis. Discovery of related gene clusters reveals previously unappreciated azaserine producers, and heterologous expression of the aza gene cluster confirms its role in azaserine assembly. Notably, this gene cluster encodes homologues of hydrazonoacetic acid (HYAA)-producing enzymes, implicating HYAA in α-diazoester biosynthesis. Isotope feeding and biochemical experiments support this hypothesis. These discoveries indicate that a 2-electron oxidation of a hydrazonoacetyl intermediate is required for α-diazoester formation, constituting a distinct logic for diazo biosynthesis. Uncovering this biological route for α-diazoester synthesis now enables the production of a highly versatile carbene precursor in cells, facilitating approaches for engineering complete carbene-mediated biosynthetic transformations in vivo.

Complete integration of carbene-transfer chemistry into biosynthesis.

Biosynthesis is an environmentally benign and renewable approach that can be used to produce a broad range of natural and, in some cases, new-to-nature products. However, biology lacks many of the reactions that are available to synthetic chemists, resulting in a narrower scope of accessible products when using biosynthesis rather than synthetic chemistry. A prime example of such chemistry is carbene-transfer reactions1. Although it was recently shown that carbene-transfer reactions can be performed in a cell and used for biosynthesis2,3, carbene donors and unnatural cofactors needed to be added exogenously and transported into cells to effect the desired reactions, precluding cost-effective scale-up of the biosynthesis process with these reactions. Here we report the access to a diazo ester carbene precursor by cellular metabolism and a microbial platform for introducing unnatural carbene-transfer reactions into biosynthesis. The α-diazoester azaserine was produced by expressing a biosynthetic gene cluster in Streptomyces albus. The intracellularly produced azaserine was used as a carbene donor to cyclopropanate another intracellularly produced molecule-styrene. The reaction was catalysed by engineered P450 mutants containing a native cofactor with excellent diastereoselectivity and a moderate yield. Our study establishes a scalable, microbial platform for conducting intracellular abiological carbene-transfer reactions to functionalize a range of natural and new-to-nature products and expands the scope of organic products that can be produced by cellular metabolism.

Catalytic undirected borylation of tertiary C-H bonds in bicyclo[1.1.1]pentanes and bicyclo[2.1.1]hexanes.

Catalytic borylations of sp3 C-H bonds occur with high selectivities for primary C-H bonds or secondary C-H bonds that are activated by nearby electron-withdrawing substituents. Catalytic borylation at tertiary C-H bonds has not been observed. Here we describe a broadly applicable method for the synthesis of boron-substituted bicyclo[1.1.1]pentanes and (hetero)bicyclo[2.1.1]hexanes by an iridium-catalysed borylation of the bridgehead tertiary C-H bond. This reaction is highly selective for the formation of bridgehead boronic esters and is compatible with a broad range of functional groups (>35 examples). The method is applicable to the late-stage modification of pharmaceuticals containing this substructure and the synthesis of novel bicyclic building blocks. Kinetic and computational studies suggest that C-H bond cleavage occurs with a modest barrier and that the turnover-limiting step of this reaction is an isomerization that occurs prior to reductive elimination that forms the C-B bond.