Inversion versus retention of configuration for nucleophilic substitution at vinylic carbon

Shelf-Stable (E)- and (Z)-Vinyl-λ3-chlorane: A Stereospecific Hyper-vinylating Agent

We report the first stereoselective synthesis of stable (E)- and (Z)-β-chlorovinyl-λ³-chlorane via direct mesitylation of 1,2-dichloroethylene with mesityldiazonium tetrakis(pentafluorophenyl)borate under mild reaction conditions. The structure of the (E)-vinyl-λ³-chlorane was established by single-crystal X-ray analysis. Because of the enormously high leaving group ability of the aryl-λ³-chloranyl group, vinyl-λ³-chloranes undergo not only S_NVσ-type reaction with extremely weak nucleophiles such as perfluoroalkanesulfonate, iodobenzene, and aromatic hydrocarbons but also coupling with phenylcopper(I) species.

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.

Concerted nucleophilic aromatic substitutions

Nucleophilic aromatic substitution (SNAr) is one of the most widely applied reaction classes in pharmaceutical and chemical research, providing a broadly useful platform for the modification of aromatic ring scaffolds. The generally accepted mechanism for SNAr reactions involves a two-step addition-elimination sequence via a discrete, non-aromatic Meisenheimer complex. Here we use 12C/13C kinetic isotope effect (KIE) studies and computational analyses to provide evidence that prototypical SNAr reactions in fact proceed through concerted mechanisms. The KIE measurements were made possible by a new technique that leverages the high sensitivity of 19F as an NMR nucleus to quantitate the degree of isotopic fractionation. This sensitive technique permits the measurement of KIEs on 10 mg of natural abundance material in one overnight acquisition. As a result, it provides a practical tool for performing detailed mechanistic analyses of reactions that form or break C-F bonds.

Nucleophilic Substitution (SN 2): Dependence on Nucleophile, Leaving Group, Central Atom, Substituents, and Solvent

The reaction potential energy surface (PES), and thus the mechanism of bimolecular nucleophilic substitution (SN 2), depends profoundly on the nature of the nucleophile and leaving group, but also on the central, electrophilic atom, its substituents, as well as on the medium in which the reaction takes place. Here, we provide an overview of recent studies and demonstrate how changes in any one of the aforementioned factors affect the SN 2 mechanism. One of the most striking effects is the transition from a double-well to a single-well PES when the central atom is changed from a second-period (e. g. carbon) to a higher-period element (e.g, silicon, germanium). Variations in nucleophilicity, leaving group ability, and bulky substituents around a second-row element central atom can then be exploited to change the single-well PES back into a double-well. Reversely, these variations can also be used to produce a single-well PES for second-period elements, for example, a stable pentavalent carbon species.

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.

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.

Dynamic origin of the stereoselectivity of a nucleophilic substitution reaction

A nucleophilic substitution on a dichlorovinyl ketone was studied experimentally and computationally. A mixture of products is observed experimentally, but a conventional computational analysis does not account for the formation of the minor stereoisomer. Instead, the product mixture is predicted accurately from a dynamic trajectory study on a bifurcating energy surface. The dynamic origin of the stereoselectivity of the reaction is discussed.

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

Nucleophilic vinylic substitution (S(N)V), in which a leaving group such as halogen is replaced by a carbon, oxygen, nitrogen, sulfur, or other nucleophile, is an important synthetic tool. It generates compounds with a carbon- or heteroatom-substituted carbon-carbon double bond, such as vinyl ethers, enamines, a variety of heterocyclic systems, and intermediates to pharmaceutically important compounds. The S(N)V reaction has many mechanistic variants, which depend on the substituents, nucleophile, leaving group, and solvent, among other factors. Among these mechanisms, the "addition-elimination" S(N)V route is the most important to synthetic chemists. S(N)V reactions are involved in several biological processes, notably (i) in the inactivation of proteases, (ii) in intermediates of herbicide metabolism, and (iii) in the formation of mutagenic intermediates by reaction of glutathione with the environmental pollutant trichloroethylene. A variant involving a tetrahedral intermediate was found in the enzymatic transfer of an enolpyruvyl group of phosphoenolpyruvate. The main S(N)V mechanism was previously analyzed in terms of a variable transition state with perpendicular nucleophilic attack. Electron-withdrawing groups Y and Y' in the beta position adjacent to the C(alpha) reaction site increase the nucleophilic attack rate; the retention of stereochemistry was mostly ascribed to formation of carbanionic intermediate 1, in which internal rotation is slower than nucleofuge expulsion (k2). As predicted, poor nucleofuges and high activation led to partial or complete stereoconvergence, and an intramolecular element effect in polyhaloethylenes gave competition ratios, kF/kBr < 1. Evidence for a zwitterionic intermediate comes from amine-catalyzed substitutions with amines. The mechanistic spectrum investigated is wide in terms of rate constants, electron-withdrawing groups, nucleophiles, leaving groups, and solvents. However, the two extremes, that is, the very slightly activated systems where in-plane invertive substitution is feasible and conversely the highly activated systems carrying poor nucleofuges where the intermediate may be observable and kinetics examined, remained almost unexplored for a long time. In this Account, we describe the progress during the last two decades in these areas. Computations on low-reactivity systems showed that the in-plane invertive single-step nucleophilic sigma attack can have a lower barrier than the pi-perpendicular retentive attack. A kBr/kCl > 1 could be deduced for the H2C=CHX (X = Cl, Br) system. Several inverted substitution-cyclizations or inverted ring openings were observed. Alkenyl iodonium salts with superb nucleofuges, showed in-plane substitutions by various nucleophiles. In parallel, we demonstrated that several highly activated systems carrying poor nucleofuges enabled a direct detection of the intermediate 1 when attacked by strong nucleophiles. Poor correlation between the equilibrium constants K1(RS) for RS- attack and pKa(CH2YY') indicates large nucleofuge steric effects (SPr > SMe > OMe >> H). Rate and equilibrium constants for RS- attack as a function of YY' also correlate poorly owing to differences in intrinsic barriers caused by different resonance effects of YY'. The expulsion of either the nucleofuge (k2) or the nucleophile (k(-1)) from 1 was analyzed with respect to several factors. Challenges still remain, including acquiring experimental data for unactivated systems and observing an intermediate carrying a good nucleofuge.

Computational studies of nucleophilic substitution at carbonyl carbon: the S(N)2 mechanism versus the tetrahedral intermediate in organic synthesis

A theoretical study specifically addresses the question of whether nucleophilic addition to the carbonyl groups of acid chlorides, esters, and anhydrides involves an addition-elimination pathway or proceeds by a concerted S(N)2-like mechanism in the absence of the generally assumed tetrahedral intermediate. Density functional calculations [B3LYP/6-31+G(d,p)] establish that chloride ion exchange reactions with both formyl and acetyl chloride proceed by a pi attack on the C=O bond. No discernible tetrahedral intermediate typical of an addition-elimination pathway was found in either case. While a tetrahedral intermediate does exist for the addition of fluoride ion to (Cl)(2)C=O, halide exchange of LiCl with both ClFC=O and (Cl)(2)C=O also proceeds by a concerted S(N)2-like pathway. The formation of a tetrahedral intermediate from the addition of methanol to acetyl chloride is slightly exothermic (4.4 kcal/mol). The ion-dipole complex of methanol weakly bonded to the carbonyl carbon of protonated acetyl chloride is stabilized by 13.8 kcal/mol but does not collapse to a tetrahedral intermediate. When four CH(3)OH molecules are H-bonded to protonated acetyl chloride, a tetrahedral intermediate is not completely formed and this solvated complex more closely resembles the precursor to an S(N)1-type ionization of Cl(-). With six H-bonding methanol molecules, a methanol adds to the carbonyl carbon and a proton relay occurs with formation of a tetrahedral-like structure that immediately loses chloride ion in an S(N)1-like solvolysis. These results corroborate earlier suggestions (Bentley et al. J. Org. Chem. 1996, 61, 7927) that the methanolysis of acetyl chloride does not proceed through the generally assumed addition-elimination pathway with a discrete tetrahedral intermediate but is consistent with ionization of Cl(-). The reaction of methoxide ion with methyl acetate proceeds via a multiple-well energy surface and involves the intermediacy of an asymmetrical species with differing C-OMe bond lengths. Models of synthetic applications of acyl transfer reactions involving anhydrides that form N-acyloxazolidinones also proceed by a concerted S(N)2-type pathway even with the carboxylate leaving group. Concerted transition states were observed for the reactions of each enantiomer of a 1,3-diphenylcycloprop-2-ene carboxylic anhydride by S-3-lithio-4-phenyloxazolidinone. Despite close structural similarities between the diastereomeric transition states, the relative energies correlated closely with the experimental results.