Recent advances in our mechanistic understanding of S(N)V reactions

Stereospecific alkenylidene homologation of organoboronates by SNV reaction

Concerted nucleophilic substitution, known as SN2 reaction, is a fundamental organic transformation used in synthesis to introduce new functional groups and construct carbon-carbon and carbon-heteroatom bonds1. SN2 reactions typically involve backside attack of a nucleophile to the σ* orbital of a C(sp3)-X bond (X = halogen or other leaving group), resulting in complete inversion of a stereocentre2. By contrast, the corresponding stereoinvertive nucleophilic substitution on electronically unbiased sp2 vinyl electrophiles, namely concerted SNV(σ) reaction, is much rarer, and so far limited to carefully designed substrates mostly in ring-forming processes3,4. Here we show that concerted SNV reactions can be accelerated by a proposed strain-release mechanism in metallated complexes, leading to the development of a general and stereospecific alkenylidene homologation of diverse organoboronates. This method enables the iterative incorporation of multiple alkenylidene units, giving cross-conjugated polyenes that are challenging to prepare otherwise. Further application to the synthesis of bioactive compounds containing multi-substituted alkenes is also demonstrated. Computational studies suggest an unusual SN2-like concerted pathway promoted by diminishing steric strain in the square planar transition state, which explains the high efficiency and stereoinversive feature of this metallate SNV reaction.

Triflic Acid-Promoted 1,2-Amino Migration Reactions in α-Arylaminoacrylamides: Access to Substituted β-Aminoamides

A straightforward synthesis of substituted β-aminoamides from α-arylamino-β-hydroxyacrylamides, α-arylamino-β-oxoamides, or their tautomeric mixture has been described. The (E)-enol triflate intermediates are readily generated in situ from these substrates in the presence of triflic anhydride (Tf2O) and triethylamine (Et3N) in a chemoselective manner and undergo triflic acid (TfOH)-promoted cyclization and ring-opening reactions with alcohols to deliver the desired products. The one-pot two-step synthetic protocol features the use of readily available starting materials, mild reaction conditions, high chemoselectivity, operational simplicity, and a wide range of synthetic potential of the products.

Palladium-Catalyzed Defluorinative Alkylation of gem-Difluoroalkenes with Cyclopropanols: Stereoselective Synthesis of γ-Fluorinated γ,δ-Unsaturated Ketones

A palladium-catalyzed defluorinative alkylation of gem-difluoroalkenes with cyclopropyl alcohols was developed. A range of γ-fluorinated γ,δ-unsaturated ketones were constructed in good yields with excellent stereoselectivities. In addition, by base-mediated intramolecular nucleophilic vinylic substitution (SNV), the products could be further transformed to 2,5-dimethylenetetrahydrofurans and analogues with excellent stereoselectivities.

A formal vinylic substitution reaction for the synthesis of α,β-unsaturated enol esters and their anticancer potential

Arylsulfonyl group-bearing α,β-unsaturated enol esters were readily assembled via the Cs2CO3-mediated union of 2-bromoallyl sulfones and cinnamic acids. The overall transformation is equivalent to an sp2 carbon-oxygen coupling reaction, and therefore constitutes a formal vinylic substitution. Several of the products display promising levels of antiproliferative activities higher than that of the anticancer drug carboplatin. Thiophenol reacted with 2-bromoallyl sulfones under identical conditions to afford α-thiophenyl-α'-tosyl acetone via an apparent aerial oxidation.

Enantioselective Nucleophilic Vinylic Substitution (SNV) toward 3-Alkenyl-bisoxindoles Facilitated by C6' Steric Bulk of Cinchona Alkaloid

A C6' bulky substituted quinine-catalyzed SNV reaction between 3-substituted oxindole and (E)-3-(nitromethylene)-oxindole was developed. This enantioselective C(sp3)-C(sp2) coupling furnished bisoxindole scaffolds featuring a vinyl-substituted all-carbon quaternary stereocenter with high stereoselectivities. In addition, the gram-scale synthesis and synthetic post-transformations were conducted to demonstrate the potential synthetic usefulness.

A One-Pot Domino Reaction Providing Fluorinated 5,6-Dihydro-1,2-thiazine 1-Oxides from Sulfoximines and 1-Trifluoromethylstyrenes

N-Trifluoroacetylated (N-TFA) sulfoximines react with 1-trifluoromethylstyrenes in a one-pot domino reaction to give fluorinated 5,6-dihydro-1,2-thiazine 1-oxides in good to high yields. The process involves three sequential reaction steps that can be characterized as (1) nucleophilic allylic substitution (S_N2'), (2) hydrolysis, and (3) intramolecular nucleophilic vinylic substitution (S_NV). The products can further be modified by defluorination. The molecular structure of the resulting product was confirmed by X-ray crystallographic analysis.

Preparation of Vinylphosphonates from Ketones Promoted by Tf2O

An efficient triflic anhydride promoted phosphorylation of ketone was disclosed, and vinylphosphorus compounds were prepared under solvent- and metal-free conditions. Both aryl and alkyl ketones could perform smoothly to give vinyl phosphonates in high to excellent yields. In addition, the reaction was easy to carry out and easy to scale up. Mechanistic studies suggested that this transformation might involve nucleophilic vinylic substitution or a nucleophilic addition-elimination mechanism.

Nucleophilic Vinylic Substitution (SNV) of Trisubstituted Monofluoroalkenes for the Synthesis of Stereodefined Trisubstituted Alkenes and Divinyl Ethers

We herein describe a nucleophilic vinylic substitution (S_NV) of trisubstituted monofluoroalkenes with excellent stereocontrol (d.r. > 99:1). Starting from (E)-β-monofluoroacrylates, various trisubstituted (E)-alkenes containing O/N/S-substituent groups at the vinylic position can be obtained under simple conditions. Furthermore, (E,E)-divinyl ethers can be generated through dimerization of the monofluoroalkenes, triggered by adventitious water in the reaction mixture.

Insights into the nucleophilic substitution of pyridine at an unsaturated carbon center

Bimolecular nucleophilic substitution (S_N2) is a fundamental reaction that has been widely studied. So far, the nucleophiles are mainly anionic species in S_N2 reactions. In this study, we use density functional theory calculations to assess the mechanisms of substitution of carbonyl, imidoyl, and vinyl compounds with a neutral nucleophile, pyridine. Charge decomposition analysis is performed to explore the main components of the transition state's LUMO. For reactions of imidoyl or carbonyl compounds with pyridine or Cl⁻, the LUMOs of the transition states are composed of mixed orbitals originating from the nucleophile and the substrate. Considering the unique mixed nature of the orbitals, the reaction mode is termed S_Nm (m means mix). Moreover, the main components of the transition state's LUMO are pure σ*_C–Cl MO in the reactions of H₂C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHCl with pyridine or Cl⁻. Computations were also performed for RYCHX substrates with different X and Y groups (X= Cl⁻, Br⁻, or F⁻; Y = O, N, or C).

The nucleophilic substitution of carbonyl, imidoyl, and vinyl carbon centers with pyridine or halides is investigated in this paper.

Nucleophilic vinylic substitution in bicyclic methyleneaziridines: SNVπ or SNVσ?

A stereoselective synthesis of the monodeuterated methyleneaziridine shown allowed the stereochemical course of formal S_NV-mode ring-opening with copper-based organometallics to be assigned.

Recent Advances in Catalytic Enantioselective Synthesis of Pyrazolones with a Tetrasubstituted Stereogenic Center at the 4-Position

Pyrazolone [2,4-dihydro-3H-pyrazol-4-one] represents one of the most important five-membered nitrogen heterocycles which is present in numerous pharmaceutical drugs and molecules with biological activity. Recently, many catalytic methodologies for the asymmetric synthesis of chiral pyrazolones have been established with great success, specially, for the synthesis of pyrazolones bearing a tetrasubstituted stereocenter at C-4. This review summarizes these excellent research studies since 2018, including representative examples and some mechanistic pathways explaining the observed stereochemistry.

1 Introduction

2 Catalytic Enantioselective Synthesis of Chiral Pyrazolones with a Full Carbon Tetrasubstituted Stereocenter at C-4

3 Catalytic Enantioselective Synthesis of Chiral Pyrazolones with a Quaternary Carbon Stereocenter at C-4 bearing a Heteroatom

4 Catalytic Enantioselective Synthesis of Chiral Spiropyrazolones

5 Conclusion

Stereospecific Isomerization of Allylic Halides via Ion Pairs with Induced Noncovalent Chirality

A regioselective protocol for the synthesis of substituted allylic chlorides, bromides, and fluorides has been established. Remarkably, the method can be applied to the enantioselective synthesis of challenging chiral allylic chlorides. When the allylic halides are treated with the base triazabicyclodecene as the catalyst, a [1,3]-proton shift takes place, giving the corresponding vinyl halides in excellent yields with excellent Z:E ratios. Furthermore, the [1,3]-proton shift takes place with an outstanding level of chirality transfer from chiral allylic alcohols (≤98%) to give chiral trifluoromethylated vinyl chlorides.

Disrotatory Ring-Opening of Furans Gives Stereocontrol

The ring-opening of 2-methylfuran and 2,3-dihydro-5-methylfuran catalyzed by the Lewis acid catalyst tris(pentafluorophenyl)borane in the presence of hydrosilanes was studied using quantum chemical methods. In a previous study, it was suggested that the stereoselective formation of the product is due to a nucleophilic vinylic substitution (S_NV) during the reaction. Our calculations show that the pathway involving the S_NV reaction is energetically not accessible. Instead, the intramolecular C-O bond cleavage is found to be much more favorable in energy for the ring opening reaction. The experimentally observed excellent stereoselectivity toward the Z-isomer product originates from an intrinsic preference of the furan ring to couple the C-O bond cleavage with a disrotatory motion of the oxygen and carbon fragments. This stereoselective feature is naturally programmed into the furan ring manifold and should be generally exploitable for engineering stereoselective ring-opening processes of bioderived furans.

Metal-Free Catalytic Reductive Cleavage of Enol Ethers

In contrast to the well-known reductive cleavage of the alkyl-O bond, the cleavage of the alkenyl-O bond is much more challenging especially using metal-free approaches. Unexpectedly, alkenyl-O bonds were reductively cleaved when enol ethers were reacted with Et₃SiH and a catalytic amount of B(C₆F₅)₃. Supposedly, this reaction is the result of a B(C₆F₅)₃-catalyzed tandem hydrosilylation reaction and a silicon-assisted β-elimination. A mechanism for this cleavage reaction is proposed based on experiments and density functional theory (DFT) calculations.

Contrasting C- and O-Atom Reactivities of Neutral Ketone and Enolate Forms of 3-Sulfonyloxyimino-2-methyl-1-phenyl-1-butanones

The mechanisms of intramolecular cyclization of 3-sulfonyloxyimino-2-methyl-1-phenyl-1-butanones (1) under basic (DABCO and t-BuOK) and acidic (AcOH and TFA) conditions were investigated by means of experimental and computational methods. The ketone, enol, and enolate forms of 1 can afford different intramolecular cyclization products (2, 3, 4), depending on the conditions. The results of the reaction of 1 under basic conditions suggest intermediacy of neutral enol (DABCO) and anionic enolate (t-BuOK), while the results under acidic conditions (AcOH and TFA) indicate involvement of neutral ketones, which exhibit reactivities arising from both the oxygen lone-pair electrons (O atom reactivity) and carbon σ-electrons (C atom reactivity). The neutral enol in DABCO afforded 2H-azirine 4. On the other hand, the products (isoxazole 2 and oxazole 3) generated from the ketone form and from the enolate form are the same, but the reaction mechanisms are apparently different. The results demonstrate ambident-like reactivity of neutral ketone in the 3-sulfonyloxyimino-2-methyl-1-phenyl-1-butanone system.

Philicity, fugality, and equilibrium constants: when do rate-equilibrium relationships break down?

Linear free energy relationships, in particular relationships between rate and equilibrium constants, are the basis for our rationalization of organic reactivity. Whereas relationships between the kinetic terms nucleophilicity and nucleofugality and the thermodynamic term basicity have been in the focus of interest for many decades, much less attention has been paid to the relationships between electrophilicity, electrofugality, and Lewis acidity. By using p- and m-substituted benzhydrylium ions (Aryl2CH+) as reference electrophiles, reference electrofuges, and reference Lewis acids of widely varying electron demand and constant steric surroundings of the reaction center, we have developed comprehensive reactivity scales which can be employed for classifying polar organic reactivity and for rationally designing synthetic transformations. It is a general rule that structural variations in electron-surplus species, which increase basicities, also increase nucleophilicities and decrease nucleofugalities, and that structural variations in electron-deficient species, which increase Lewis acidities also increase electrophilicities and decrease electrofugalities. Deviations from this behavior are analyzed, and it is shown that variations in intrinsic barriers are responsible for the counterintuitive observations that structural variation in one of the reactants alters the rates of forward and backward reactions in the same direction. A spectacular example of this phenomenon is found in vinyl cation chemistry: Vinyl cations are not only generated several orders of magnitude more slowly in SN1 reactions than benzhydrylium ions of the same Lewis acidity, but also react much more slowly with nucleophiles.

Transition-metal-free chemo- and regioselective vinylation of azaallyls

Direct C(sp³)-C(sp²) bond formation under transition-metal-free conditions offers an atom-economical, inexpensive and environmentally benign alternative to traditional transition-metal-catalysed cross-coupling reactions. A new chemo- and regioselective coupling protocol between 3-aryl-substituted-1,1-diphenyl-2-azaallyl derivatives and vinyl bromides has been developed. This is the first transition-metal-free cross-coupling of azaallyls with vinyl bromide electrophiles and delivers allylic amines in excellent yields (up to 99%). This relatively simple and mild protocol offers a direct and practical strategy for the synthesis of high-value allylic amine building blocks that does not require the use of transition metals, special initiators or photoredox catalysts. Radical clock experiments, electron paramagnetic resonance studies and density functional theory calculations point to an unprecedented substrate-dependent coupling mechanism. Furthermore, an electron paramagnetic resonance signal was observed when the N-benzyl benzophenone ketimine was subjected to silylamide base, supporting the formation of radical species upon deprotonation. The unique mechanisms outlined herein could pave the way for new approaches to transition-metal-free C-C bond formations.

Borane catalysed ring opening and closing cascades of furans leading to silicon functionalized synthetic intermediates

The conversion of renewable biomass resources to synthetically valuable chemicals is highly desirable, but remains a formidable challenge in regards to the substrate scope and reaction conditions. Here we present the development of tris(pentafluorophenyl)borane–catalysed conversion of furans via ring-opening and closing cascade processes to afford silicon-functionalized synthetic chemicals under transition metal-free conditions. The furan ring-opening with hydrosilanes is highly efficient (TON up to 2,000) and atom-economical without forming any byproduct to give rise to α-silyloxy-(Z)-alkenyl silanes. Additional equivalents of silane smoothly induce a subsequent B(C₆F₅)₃-catalysed cyclization of initially formed olefinic silane compounds to produce anti-(2-alkyl)cyclopropyl silanes, another versatile synthon being potentially applicable in the synthesis of natural products and pharmacophores.

Furans are attractive staring materials in organic chemistry, due to the ease of functionalisation and sourcing from renewable feedstocks. Here the authors show the ring opening of furans to silane intermediates, followed by further conversion to cyclopropanes and other high value targets.

Synthesis and Photocatalytic Reactivity of Vinylsulfonium Ylides

Although sulfur ylides are textbook reagents in organic synthesis, surprisingly little variation of substituents on sulfur is usually observed. In particular, vinylsulfonium ylides have been neglected so far. Herein, we present a study on their synthesis and reactivity, including interesting behavior under photocatalytic conditions.

Carbonylmetallates--A Special Family of Nucleophiles in Aromatic and Vinylic Substitution Reactions

Carbonylmetallates, [M(CO)(n)L](-), anionic transition-metal carbonyl complexes, represent a large family of metal-centered nucleophiles, and studying carbonylmetallates allows us to understand the differences in the behavior of the metal-centered complexes versus heteroatom-based nucleophiles. The mechanisms of carbonylmetallate reactions with aryl- and alkenyl halides have been examined by employing radical and, especially, carbanion trapping techniques. Carbonylmetallates show a marked preference for halogenophilic attack, and nucleophilic substitution with carbonylmetallates is often not a direct process, but proceeds through the initial attack at halogen with subsequent coupling of carbanion and HalM(CO)(n)L intermediates. Factors governing the competition between the halogenophilic and more common "carbophilic" reaction pathways, as well as the means of predicting the actual course of reaction are discussed. The review also considers other aspects of carbonylmetallate reactivity, including ion-pairing effects, radical-mediated nucleophilic substitution pathways, and the carbonylmetallate nucleophilicity scale in the reactions with π-electrophiles.

Improving selectivity in catalytic hydrodefluorination by limiting SNV reactivity

Competition of HM, SBM and S_NV in hydrodefluorination can lead to low selectivity which can be improved via solvent change.

Nucleophilic substitution in ionizable Fischer thiocarbene complexes: steric effect of the alkyl substituent on the heteroatom

A detailed kinetic study has been carried out for the aminolysis of ionizable Fischer thiocarbene complexes (CO)5M[double bond, length as m-dash]C(SR)CH3 (M = Cr, W; R = iPr, nBu, cHex, tBu) with five primary amines and one secondary amine in aqueous acetonitrile solutions (50% MeCN-50% water (v/v)). The observed rate constants for the reaction with primary amines showed a first-order dependence on the amine concentration, while with morpholine, the rate constant has second-order dependence. The general base catalysis process was confirmed by the variation of the rate constants with the concentration of an external catalyst and the pH. The results agree with a stepwise mechanism where the nucleophilic addition to the carbene carbon to produce a tetrahedral intermediate (T±) is the first step, followed by a rapid deprotonation of to form the anion T- which leads to the products by general-acid catalysed leaving group (-SR) expulsion. In general, it was found that the chromium complexes are less reactive than the tungsten analogues. The obtained Brønsted parameters for the nucleophilic addition (βnuc) indicate that C-N bond formation has made little progress at the transition state. By using Charton's correlation, the role that the steric factor plays throughout the mechanism has been unraveled. The nucleophilic addition to the thiocarbenes is less sensitive to steric effects than the alkoxycarbenes regardless of the nature of the metal centre. Conversely, the steric effects on the general-base catalysis can be strong depending on the volume of the catalyst and the metal centre. On the basis of the structure-reactivity coefficients β and ψ and comparison with alkoxycarbene complexes, esters and thiolesters, insights into the main factors ruling the reactivity in terms of transition state imbalances are discussed.

Vinyl sulfone building blocks in covalently reversible reactions with thiols

A combination of quantum-chemical calculations, Hirshfeld surface analyses and reactivity studies predicts how to turn vinyl sulfones into electrophiles that react covalently but reversibly with thiols.

An approach towards expanding the scope of masked o-benzoquinone dimers to generate diverse compound libraries

In this report, a simple and rapid approach for the synthesis of masked o-benzoquinone (MOB) heterodimers is presented.

Three-dimensional heterocycles: new uracil-based structures obtained by nucleophilic substitution at the sp2 carbon of bromoisoxazoline

The regioisomeric cycloadducts of bromonitrile oxide and N-benzoyl-2,3-oxaza-norborn-5-ene were easily prepared and elaborated into a novel class of uracil-based scaffolds. The key-synthetic step is the nucleophilic substitution at the sp2 carbon atom of the bromoisoxazoline three-dimensional heterocycles. The protocol to perform the nucleophilic substitution of uracil anions was optimized and adapted to the steric requirements of the substrates. A library of pyrimidine derivatives was prepared in very good yields and the products were fully characterized. They are proposed as nucleoside analogues and as synthons for β-turn motifs within PNA structures.

One-pot pseudo three-component reaction of nitroketene-N,S-acetals and aldehydes for synthesis of highly functionalized hexa-substituted 1,4-dihydropyridines

Pseudo three-component condensation of aliphatic/aromatic/α,β-unsaturated aldehydes and nitroketene-N,S-acetals to afford diversely functionalized hexa-substituted 1,4-dihydropyridines under 2-aminopyridine catalysis was achieved.

Dualism of Ion-Pairing Effects in Nucleophilic Vinylic Substitution with Transition-Metal Carbonyl Anions

Ion pairing with cations has a marked but nonuniform effect on the chemistry of carbonylmetalate anions and can either inhibit or facilitate their reactions. A study of the effect of cation-solvating agents ([18]crown-6, hexamethylphosphoramide) on the reactions of carbonylmetalates [Na(K)][M(CO)_n L] with vinyl halides activated with strong electron-withdrawing groups, which at the same time could serve as hard donor centers for sodium or potassium cations is presented. The addition of a cation-solvating agent to the solution in THF produced a sharp drop in the rate of the reactions of Na[Re(CO)₅ ] or K[Mn(CO)₅ ] with the vinyl chlorides studied; the effect (k_obs (THF)/k_obs (THF+[18]crown-6)) increased along the series: PhCClC(CN)₂ (4)<(Z)-PhC(CN)CHCl (17)<PhCClC(CO₂ Et)₂ (80)<(E)-ClCHCHCO₂ Me (200). This strong cation assistance is contrasted by "normal" effects observed for the reaction of K[Mn(CO)₅ ] with 1-chloro-2-methyl perfluorocyclopentene, accelerated by [2.2.2]cryptand, as well as for the other fluorine-activated vinyl halides studied previously. The characteristic increase in cation assistance effect in the order CO₂ R>CN≫CF allows us to explain assistance from cation coordination to the donor centers in these groups, which stabilizes the transition state. The effect of ion pairing is also dependent on the leaving group in the vinyl halide, almost disappearing for (Z)-PhC(CN)CHBr, with only minor cation assistance observed for (E)-PhCOCHCHI. This prompted us to examine the effect of the leaving group in the reactions of carbonylmetalates with Z and E isomers of Ph(CN)CCHHal (Hal=I, Br, Cl, or F).

Reactivity in nucleophilic vinylic substitution (S(N)V):S(N)Vπ versus S(N)Vσ mechanistic dichotomy

The intrinsic electronic factors that determine reactivity in prototypical identity nucleophilic vinylic substitution reactions, X(-) + ViX → XVi + X(-) (Vi = vinyl), have been studied by performing quantum chemical calculations (OPBE/6-311++G(d,p)). Of the two limiting reaction types envisaged--the S(N)Vπ and S(N)Vσ mechanisms--the former is preferred for most combinations of nucleophiles and substrates, except for the combination of unactivated substrates and poor nucleophiles, as seen for the much studied reactions Cl(-) + CH2CHCl and Br(-) + CH2CHBr. It was found that periodic trends for S(N)Vπ are essentially the same as those previously reported for nucleophilic aromatic substitution, S(N)Ar, while intrinsic S(N)Vσ nucleophilicity parallels aliphatic S(N)2. It is therefore concluded that S(N)V reactivity in general can be understood in terms of this mechanistic dichotomy. Furthermore, a few representative reactions were analyzed applying two complementary schemes for energy decomposition analysis.