Diastereo- and enantioselective synthesis of compounds with a trifluoromethyl- and fluoro-substituted carbon centre

Amine adducts of triallylborane as highly reactive allylborating agents for Cu(I)-catalyzed allylation of chiral sulfinylimines

The implementation of selective catalytic processes with highly active reagents is an attractive strategy that meets the modern principles of sustainable development of chemistry. In the current study, we for the first time describe the method and general principles of Cu(I)-catalyzed allylation of imines with amine adducts of allylic triorganoboranes. Triallylborane is an extremely reactive compound and cannot be used for the catalytic allylation of imines, whereas its amine adducts are ideal substrates for catalysis. The structure of the amine fragment successfully balances the safety, selectivity and stability of the allylboron reagent, allowing it to demonstrate high activity in catalytic allylation reactions, exceeding many times any known allylboranes. The obtained results are supported by quantitative kinetics data and DFT calculations. The catalytic efficacy of the system was demonstrated on model sulfinylimines (23 examples). High diastereoselectivity up to >99% was achieved, including for the gram-scale synthesis of 2-hydroxyphenyl-derivatives. Taking into account the high reactivity and unsurpassed atom-economy of amine adducts of triallylborane (AAT), they can be considered as prospective allylation reagents with Cu(I) and other appropriate metallocatalysts.

Stereoselective Nucleophilic Halogenation at CF3-Substituted Nonclassical Carbocation

CF3-substituted cyclopropyl carbinol derivatives undergo regioselective and diastereoselective nucleophilic halogenation at the quaternary carbon center to provide acyclic products as a single diastereomer. The selectivity of the substitution is rationalized by the formation of a nonclassical cyclopropylcarbinyl cation intermediate, reacting at the most-substituted carbon center. Tertiary alkyl chlorides, bromides, and fluorides adjacent to a stereogenic C-CF3-motif are diastereomerically pure and can be obtained in few catalytic steps from commercially available alkynes.

Pd/IPrBIDEA-Catalyzed Hydrodefluorination of gem-Difluorocyclopropanes: Regioselective Synthesis of Terminal Fluoroalkenes

Developing new strategies to enable chemo- and regioselective reductions is an important topic in chemical research. Herein, an efficient and regioselective Pd/IPrBIDEA-catalyzed ring-opening hydrodefluorination of gem-difluorocyclopropanes to access terminal fluoroalkenes is developed. The success of this transformation was attributed to the use of 3,3-dimethylallyl Bpin as a novel hydride donor. DFT calculations suggest that a direct 3,4'-hydride transfer via a 9-membered cyclic transition state is more favorable, which combined with the irreversibility of the reaction enables the unusual selectivity for the less thermodynamically stable terminal alkene isomer. This reaction mode is also applicable to a variety of regioselective allylic and propargyl reductions.

Site-selective chemical reactions by on-water surface sequential assembly

Controlling site-selectivity and reactivity in chemical reactions continues to be a key challenge in modern synthetic chemistry. Here, we demonstrate the discovery of site-selective chemical reactions on the water surface via a sequential assembly approach. A negatively charged surfactant monolayer on the water surface guides the electrostatically driven, epitaxial, and aligned assembly of reagent amino-substituted porphyrin molecules, resulting in a well-defined J-aggregated structure. This constrained geometry of the porphyrin molecules prompts the subsequent directional alignment of the perylenetetracarboxylic dianhydride reagent, enabling the selective formation of a one-sided imide bond between porphyrin and reagent. Surface-specific in-situ spectroscopies reveal the underlying mechanism of the dynamic interface that promotes multilayer growth of the site-selective imide product. The site-selective reaction on the water surface is further demonstrated by three reversible and irreversible chemical reactions, such as imide-, imine-, and 1, 3-diazole (imidazole)- bonds involving porphyrin molecules. This unique sequential assembly approach enables site-selective chemical reactions that can bring on-water surface synthesis to the forefront of modern organic chemistry.

Photocatalytic redox-neutral selective single C(sp3)-F bond activation of perfluoroalkyl iminosulfides with alkenes and water

Visible-light-promoted site-selective and direct C-F bond functionalization of polyfluorinated iminosulfides was accomplished with alkenes and water under redox-neutral conditions, affording a diverse array of γ-lactams with a fluoro- and perfluoroalkyl-substituted carbon centre. A variety of perfluoroalkyl units, including C2F5, C3F7, C4F9, and C5F11 underwent site-selective defluorofunctionalization. This protocol allows high chemoselectivity control and shows excellent functional group tolerance. Mechanistic studies reveal that the remarkable changes of the electron geometries during the defluorination widen the redox window between the substrates and the products and ensure the chemoselectivity of single C(sp3)-F bond cleavage.

Sugar-Assisted Kinetic Resolutions in Metal/Chiral Amine Co-Catalyzed α-Allylations and [4+2] Cycloadditions: Highly Enantioselective Synthesis of Sugar and Chromane Derivatives

Functionalized triose-, furanose and chromane-derivatives were synthesized by the titled reactions. The sugar-assisted kinetic resolution/C-C bond-forming cascade processes generate a functionalized sugar derivative with a quaternary stereocenter in a highly enantioselective fashion (up to >99 % ee) by using a simple combination of metal and chiral amine co-catalysts. Notably, the interplay between the chiral sugar substrate and the chiral amino acid derivative allowed for the construction of a functionalized sugar product with high enantioselectivity (up to 99 %) also when using a combination of racemic amine catalyst (0 % ee) and metal catalyst.

Taking Olefin Metathesis to the Limit: Stereocontrolled Synthesis of Trisubstituted Alkenes

ConspectusIn this Account, we share the story of the development of catalytic olefin metathesis processes that efficiently deliver a wide range of acyclic and macrocyclic E- or Z-trisubstituted alkenes. The tale starts with us unveiling, in collaboration with Richard Schrock and his team, the blueprint in 2009 for the design of kinetically controlled Z-selective olefin metathesis reactions. This paved the way for the development of Mo-, W-, and Ru-based catalysts and strategies for synthesizing countless linear and macrocyclic Z-olefins. Six years later, in 2015, we found that abundant Z-alkene feedstocks, such as oleic acid, can be directly transformed to high-value and more difficult-to-access alkenes through a cross-metathesis reaction promoted by a Ru-catechothiolate complex that we had developed; the approach, later coined stereoretentive olefin metathesis, was extended to the synthesis of E-alkenes.It was all about disubstituted alkenes until when in 2017 we addressed the challenge of accessing stereodefined Z- and E-trisubstituted alkenes, key to medicine and materials research. These transformations can be most effectively catalyzed by Mo monoaryloxides pyrrolide (MAP) and chloride (MAC) complexes. A central aspect of the advance is the merging of olefin metathesis, which delivered trisubstituted alkenyl fluorides, chlorides, and bromides with cross-coupling. These catalytic and stereoretentive transformations can be used in various combinations, thereby enabling access to assorted Z- or E-trisubstituted alkene. Ensuing work led to the emergence of other transformations involving substrates that can be purchased with high stereoisomeric purity, notably E- and Z-trihalo alkenes. Trisubstituted olefins, Z or E, bearing a chemoselectively and stereoretentively alterable F,Cl-terminus or B(pin),Cl-terminus may, thus, be easily and reliably synthesized. Methods for stereoretentive preparation of other alkenyl bromide regioisomers and α,β-unsaturated carboxylic and thiol esters, nitriles, and acid fluorides followed, along with stereoretentive ring-closing metathesis reactions that afford macrocyclic trisubstituted olefins. Z- and E-Macrocyclic trisubstituted olefins, including those that contain little or no entropic support for cyclization (minimally functionalized) and/or are disfavored under substrate-controlled conditions, can now be synthesized. The utility of this latest chapter in the history of olefin metathesis has been highlighted by applications to the synthesis of several biologically active compounds, as well as their analogues, such as those marked by one or more site-specifically incorporated fluorine atoms or more active but higher energy and otherwise unobtainable conformers.The investigations discussed here, which represent every stereoretentive method that has been reported thus far for preparing a trisubstituted olefin, underscore the inimitable power of Mo-based catalysts. This Account also showcases a variety of mechanistic attributes─some for the first time, and each instrumental in solving a problem. Extensive knowledge of mechanistic nuances will be needed if we are to address successfully the next challenging problem, namely, the development of catalysts and strategies that may be used to synthesize a wide range of tetrasubstituted alkenes, especially those that are readily modifiable, with high stereoisomeric purity.

Synthesis of Chiral Tertiary Allylic- and Propargylic Fluorides from Nonaflates of Chiral α-Fluorinated β-Keto Dicarbonyl Compounds

The acetyl group of chiral α-fluorinated dicarbonyl compounds was transformed to nonaflates through a reaction with perfluorobutanesulfonyl fluoride in the presence of DBU in 82-95% yield. These nonaflates were used in Suzuki and Sonogashira coupling reactions to afford chiral tertiary allylic fluorides bearing gem-disubstituted terminal alkenes with excellent optical purities (45-91%, ≥94% ee). In addition, chiral tertiary propargylic fluorides were obtained from the reaction of nonaflates with DBU (73-86%, ≥94% ee).

Fluorotrifluoromethyl Group Installation: Tetrasubstituted Tertiary Stereocenters Containing C-F and C-CF3 Bonds via Copper-Mediated Allylic Substitution

Tetrasubstituted tertiary centers containing a fluorotrifluoromethyl (FTF) motif were obtained from secondary 3-fluoro-3-trifluoromethyl allylic phosphates by a copper(I)-mediated allylic substitution reaction using Grignard reagents. The reaction features a broad substrate scope, high γ-regioselectivity, and excellent (E)-stereoselectivity. Two examples of chirality transfer from enantiopure secondary fluorine-substituted allylic phosphates to the corresponding scalemic products indicate the stereoselective potential of the method to install FTF stereocenters.

Electrochemically Generated Carbanions Enable Isomerizing Allylation and Allenylation of Aldehydes with Alkenes and Alkynes

The direct coupling of aldehydes with petrochemical feedstock alkenes and alkynes would represent a practical and streamlined approach for allylation and allenylation chemistry. However, conventional approaches commonly require preactivated substrates or strong bases to generate allylic or propargylic carbanions and only afford branched allylation or propargylation products. Developing a mild and selective approach to access synthetically useful linear allylation and allenylation products is highly desirable, albeit with formidable challenges. We report a strategy using hydrogen evolution reaction (HER) to generate a carbanion from weakly acidic sp³ C-H bonds (pK_a ∼ 35-40) under mild reaction conditions, obviating the use of strong bases, Schlenk techniques, and multistep procedures. The cathodically generated carbanion reverses the typical reaction selectivity to afford unconventional isomerizing allylation and allenylation products (125 examples). The generation of carbanions was monitored and identified by in situ ultraviolet-visible (UV-vis) spectroelectrochemistry. Furthermore, we extended this protocol to the generation of other carbanions and their application in coupling reactions between alcohols with carbanions. The appealing features of this approach include mild reaction conditions, excellent functional group tolerance, unconventional chemo- and regioselectivity, and the diverse utility of products, which includes offering direct access to diene luminophores and bioactive scaffolds. We also performed cyclic voltammetry, control experiments, and density functional theory (DFT) calculations to rationalize the observed reaction selectivity and mechanism.

Construction of quaternary stereocenters at carbon 2' of nucleosides

The non-natural nucleosides with a quaternary stereogenic center at C2' are crucial to drug discovery. They have become a cornerstone for the treatment of cancer and various viral infections as exemplified by gemcitabine and sofosbuvir. Major research effort has been expended to gain synthetic access to these nucleoside analogues with a significant steric bulk at C2' in the furanoside ring. The 2'-ketonucleosides and 2'-deoxy-2'-methylenenucleosides emerged as key intermediates in these synthetic strategies. For example, α-face addition of methyl lithium to the 2'-ketonucleosides followed by fluorination of resulting tertiary arabino alcohol with DAST provided 2'-fluoro-2'-C-methyluridine - a core nucleoside component of sofosbuvir. The α-face addition of HCN or HN₃ to the 2'-deoxy-2'-methylene nucleosides gave access to the synthetically versatile 2'-cyano-2'-C-methyl and 2'-azido-2'-C-methyl nucleosides. Likewise, the addition of diazomethane to the 2'-exomethylene group gave access to the 2'-spirocyclopropyl analogue. This review primarily discusses synthetic strategies which employs natural nucleosides as substrates but selected approaches involving coupling of the preelaborated sugar precursors with nucleobases are also examined.

Exploring F/CF3 substituted oxocarbenium ions for the diastereoselective assembly of highly substituted tetrahydrofurans

Understanding the influence of emerging fluorinated motifs is of a crucial importance in the context of the exponentially growing exploitation of fluorine in many fields. Herein, we report on the dramatic effect of a local partial charge inversion by replacing a CHCH₃ group by a CFCF₃. This strategy allows the diastereoselective reduction of 5-membered ring oxocarbenium ions to access highly substituted tetrahydrofurans.

Catalytic Cross-Metathesis Reactions That Afford E- and Z-Trisubstituted Alkenyl Bromides: Scope, Applications, and Mechanistic Insights

Stereochemically defined trisubstituted alkenes with a bromide and a methyl group at a terminus can be readily and stereoretentively derivatized through catalytic cross-coupling, affording unsaturated fragments found in many bioactive natural products. A direct method for generating such entities would be by stereocontrolled catalytic cross-metathesis (CM). Such methods are scarce however. Here, we present a stereoretentive strategy for CM between tri-, Z- or E-di, or monosubstituted olefins and Z- or E-2-bromo-2-butene, affording an assortment of E- or Z-trisubstituted alkenyl bromides. The majority of the transformations were catalyzed by two Mo monoaryloxide pyrrolide (MAP) complexes, one purchasable and the other accessible by well-established protocols. Substrates, such as feedstock trisubstituted olefins, can be purchased; the alkenyl bromide reagents are commercially available or can be prepared in two steps in a multigram scale. The catalytic process can be used to generate products that contain polar moieties, such as an amine or an alcohol, or sterically hindered alkenes that are α- or β-branched. The utility of the approach is highlighted by a brief and stereocontrolled synthesis of an unsaturated fragment of phomactin A and a concise total synthesis of ambrein. An unexpected outcome of these investigations was the discovery of a new role for the presence of a small-molecule alkene in an olefin metathesis reaction. DFT studies indicate that this additive swiftly reacts with a short-lived Mo alkylidene and probably helps circumvent the formation of catalytically inactive square pyramidal metallacyclobutanes, enhancing the efficiency of a transformation.

Z-Trisubstituted α,β-Unsaturated Esters and Acid Fluorides through Stereocontrolled Catalytic Cross-Metathesis

Catalytic cross-metathesis (CM) reactions that can generate trisubstituted alkenes in high stereoisomeric purity are important but remain limited in scope. Here, CM reactions are introduced that generate Z-trisubstituted α-methyl, α,β-unsaturated, alkyl and aryl esters, thiol esters, and acid fluorides. Transformations are promoted by a Mo bis-aryloxide, a monoaryloxide pyrrolide, or a monoaryloxide chloride complex; air-stable and commercially available paraffin tablets containing a Mo complex may also be used. Alkyl, aryl, and silyl carboxylic esters as well as thiol esters and acid fluoride reagents are either purchasable or can be prepared in one step. Products were obtained in 55-95% yield and in 88:12->98:2 Z/E ratio (typically >95:5). The applicability of the approach is highlighted by a two-step conversion of citronellol to an isomintlactone precursor (1.7 g, 73% yield, and 97:3 Z/E) and a single-step transformation of lanosterol acetate to 3-epi-anwuweizic acid (72% yield and 94:6 Z/E). Included are the outcomes of DFT studies, regarding several initially puzzling catalyst activity trends, providing the following information: (1) it is key that a disubstituted Mo alkylidene, generated by a competing homo-metathesis (HM) pathway, can re-enter the productive CM cycle. (2) Whereas in a CM cycle the formation of a molybdacyclobutane is likely turnover-limiting, the collapse of related metallacycles in a HM cycle is probably rate-determining. It is therefore the relative energy barrier required for these steps that determines whether CM or HM is dominant with a particular complex.