Synthesis of hetero- and carbocycles by nucleophilic substitution at sp carbon

Vinylic halides having alcohol, sulfonamide, active methine, and thiol moieties as nucleophiles cyclize to hetero- and carbocycles by intramolecular nucleophilic substitution at the sp² carbon centers. The density functional theory calculations suggest that the cyclization proceeds through S_N2-type substitution (the in-plane vinylic nucleophilic substitution, S_NVσ), when vinyl halides are substituted with oxygen, nitrogen, and carbon nucleophiles. The substitution with sulfur nucleophiles, in contrast, proceeds through both routes of S_NVσ and out-of-plane vinylic nucleophilic substitution (S_NVπ).     

Carbon-based leaving group in substitution reactions: functionalization of sp3-hybridized quaternary and tertiary benzylic carbon centers

Lewis acid promoted substitution reactions employing Meldrum's acid and 5-methyl Meldrum's acid as carbon-based leaving groups are described which transform unstrained quaternary and tertiary benzylic C(sp(3))-C(sp(3)) bonds into C(sp(3))-X bonds (X = C, H, N). Importantly, this reaction has a broad scope in terms of both suitable substrates and nucleophiles with good to excellent yields obtained (typically >90%).     

Polymorphic phases of sp3-hybridized carbon under cold compression

It is well established that graphite can be transformed into superhard carbons under cold compression (Mao et al. Science 2003, 302, 425). However, structure of the superhard carbon is yet to be determined experimentally. We have performed an extensive structural search for the high-pressure crystalline phases of carbon using the evolutionary algorithm. Nine low-energy polymorphic structures of sp(3)-hybridized carbon result from the unbiased search. These new polymorphic carbon structures together with previously reported low-energy sp(3)-hybridized carbon structures (e.g., M-carbon, W-carbon, and Cco-C(8) or Z-carbon) can be classified into three groups on the basis of different ways of stacking two (or more) out of five (A-E) types of buckled graphene layers. Such a classification scheme points out a simple way to construct a variety of sp(3)-hybridized carbon allotropes via stacking buckled graphene layers in different combinations of the A-E types by design. Density-functional theory calculations indicate that, among the nine low-energy crystalline structures, seven are energetically more favorable than the previously reported most stable crystalline structure (i.e., Cco-C(8) or Z-carbon) in the pressure range 0-25 GPa. Moreover, several newly predicted polymorphic sp(3)-hybridized carbon structures possess elastic moduli and hardness close to those of the cubic diamond. In particular, Z-carbon-4 possesses the highest hardness (93.4) among all the low-energy sp(3)-hybridized carbon structures predicted today. The calculated electronic structures suggest that most polymorphic carbon structures are optically transparent. The simulated X-ray diffraction (XRD) spectra of a few polymorphic structures are in good agreement with the experimental spectrum, suggesting that samples from the cold-compressed graphite experiments may consist of multiple polymorphic phases of sp(3)-hybridized carbon.     

On some carbon clusters containing sp2- and sp3-hybridized atoms

A new class of thread-like carbon clusters consisting of dodecahedra and containing sp²- and sp³-hybridized atoms is described. Molecular and electronic structures of the simplest representative of this class (the C₁₀₂ cluster) and of the polyhedral C₉₀H₁₂ hydrocarbon molecule, whose carbon framework is identical to the toroidal fragment of the C₁₀₂ cluster, have been calculated by the MNDO and MNDO-PM3 methods.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 3, pp. 551–553, March, 1996.     

Intramolecular nucleophilic substitution at an sp carbon: synthesis of substituted thiazoles and imidazole-2-thiones

The nucleophilic substitution reactions of vinylic bromides with intramolecular thioamide or thiourea moieties proceed to give a series of substituted thiazoles and imidazole-2-thiones.     

Visualization method to predict the nucleophilic asymmetric induction of prochiral electrophiles

[reaction: see text] This work focuses on the development of a simple technique to accurately predict and visualize the diastereoselectivity of ketone, aldehyde, and allyl chloride reductions by mapping electrostatic potential onto the frontier molecular orbital involved in the reduction. A distinct difference of electrostatic potential on the faces of the carbonyl can be used to predict the face of nucleophilic attack with a high level of accuracy.     

Inversion versus retention of configuration for nucleophilic substitution at vinylic carbon

A high-level computational study using CCSD, CCSD(T), and G2(+) levels of theory has shown that unactivated vinyl substrates such as vinyl chloride would afford gas phase, single-step halide exchange by a pure in-plane sigma-approach of the nucleophile to the backside of the C--Cl sigma bond. Geometry optimization by CCSD/6-31+G* and CCSD(T)/6-31+G* confirms the earlier findings of Glukhovtsev, Pross, and Radom that the S(N)2 reaction of Cl(-) with unactivated vinyl chloride in the gas phase occurs by a sigma attack. Complexation of vinyl chloride with Na(+) does not alter this in-plane sigma preference. However, moderately activated dihaloethylenes such as 1-chloro-1-fluoroethylene undergo gas-phase S(N)2 attack by the accepted pi-route where the nucleophile approaches perpendicular to the plane of the C==C. In the latter case a single-step pi pathway is preferred for the Cl(-) + H(2)C==CFCl reaction. This is the first definitive example at a high level of theory where a single-step pi nucleophilic vinylic substitution is preferred over a multistep mechanism in the gas phase. The activation barriers for these gas-phase single-step sigma- and pi-processes involving both naked anions and Na(+) complexes are, however, prohibitively high. Solvation and the presence of a counterion must play a dominant role in nucleophilic vinylic substitution reactions that proceed so readily in the condensed phase. In solution, nucleophilic vinylic substitution reactions involving electron-withdrawing groups on the carbon--carbon double bond (e.g., -CN, -CHO, and -NO(2)) would almost certainly proceed via a free discrete carbanionic intermediate in accord with experiment.     

Stereoselection in nucleophilic substitution at an sp2 carbon

The stereochemical possibilities of the title reaction are considered in detail, taking into account both the relative rates of rotation, inversion, formation and decomposition, and the stabilities of the species involved. Briefly, a rationale for predominant retention is outlined and a set of conditions favorable to inversion are described. First it is assumed that the exchange of X and Y in eqn (1) includes the anion 1 or 2 as an intermediate. The reaction pathways can be mapped in the form of graphs: mechanisms by selected routes can now be related to the characteristics of the system or modified to accommodate special mechanistic alternatives, e.g. addition-elimination and concerted substitution. In the graph dealing with the pyramidal anion BAC?-CXYW, the favored retention route is anti formation and syn decomposition, while the inversion route is syn formation and syn decomposition.These stereospecific models arise for different reasons: in the first, certain rotomers of 1 are favored because of the trans (contra-gauche) effect; in the second, strong ion pairing predominates over other factors. This scheme has to be altered for the “pyramidal” anion derived from the Σ isomer of AN=CXW, since the anti-syn sequence may give both retention and inversion. In the graph dealing with the trigonal anion BAC?-CXYW or AN?-CXYW, retention paths are always favored. All of the stereospecific paths described hold for specified models—when constraints are removed, stereoconvergence follows.     

Palladium-catalyzed asymmetric allylic nucleophilic substitution reactions using chiral tert-butanesulfinylphosphine ligands

The asymmetric allylic alkylation of racemic 1,3-diphenyl-2-propenyl acetate 3 with dimethyl malonate proceeded smoothly in the presence of lithium acetate, BSA (N,O-bis(trimethylsilyl)acetamide), [Pd(η³-C₃H₅)Cl]₂, and chiral tert-butanesulfinylphosphine ligand 2c to give the allylic alkylation product in good yield and high enantiomeric excess (up to 93% ee), while the enantioselectivities of allylic amination of 3 with various amines were moderate (up to 76% ee).     

Rearrangement in nucleophilic substitution of bis(3-benzyloxy-2-bromopropyl)selenide

Russian Journal of General Chemistry -     

Ab initio and DFT benchmark study for nucleophilic substitution at carbon (SN2@C) and silicon (SN2@Si)

To obtain a set of consistent benchmark potential energy surfaces (PES) for the two archetypal nucleophilic substitution reactions of the chloride anion at carbon in chloromethane (S(N)2@C) and at silicon in chlorosilane (S(N)2@Si), we have explored these PESes using a hierarchical series of ab initio methods [HF, MP2, MP4SDQ, CCSD, CCSD(T)] in combination with a hierarchical series of six Gaussian-type basis sets, up to g polarization. Relative energies of stationary points are converged to within 0.01 to 0.56 kcal/mol as a function of the basis-set size. Our best estimate, at CCSD(T)/aug-cc-pVQZ, for the relative energies of the [Cl(-), CH(3)Cl] reactant complex, the [Cl-CH(3)-Cl](-) transition state and the stable [Cl-SiH(3)-Cl](-) transition complex is -10.42, +2.52, and -27.10 kcal/mol, respectively. Furthermore, we have investigated the performance for these reactions of four popular density functionals, namely, BP86, BLYP, B3LYP, and OLYP, in combination with a large doubly polarized Slater-type basis set of triple-zeta quality (TZ2P). Best overall agreement with our CCSD(T)/aug-cc-pVQZ benchmark is obtained with OLYP and B3LYP. However, OLYP performs better for the S(N)2@C overall and central barriers, which it underestimates by 2.65 and 4.05 kcal/mol, respectively. The other DFT approaches underestimate these barriers by some 4.8 (B3LYP) to 9.0 kcal/mol (BLYP).     

Stereospecific nucleophilic substitution at tertiary and quaternary stereocentres

Nucleophilic substitution reactions have always been considered as one of the most powerful reactions for the creation of carbon–carbon or carbon–heteroatom bonds in organic synthesis. In contrast to secondary carbons, the steric shielding of tertiary carbons retards a concerted, stereospecific nucleophilic substitution, and ionizing pathways often lead to nonselective substitution due to ion pair dissociation. In this minireview, we will detail pioneering contributions and more recent achievements emphasizing the feasibility of nucleophilic substitution on tertiary stereocentres under certain conditions, with inversion of configuration. The development of these transformations at tertiary centres are of remarkable added value to practitioners in the field of complex molecule synthesis. A stereoselective substitution at a quaternary carbon stereocentre with inversion of configuration is also discussed in the case of a three-membered ring.

In this minireview, we summarized the fascinating and rich area of stereoselective or stereospecific nucleophilic substitution at tertiary and quaternary carbon centres.     

Theoretical study of nucleophilic substitution at two-coordinate sulfur

A series of nucleophilic substitution reactions involving simple species (chloride, phosphide, methoxide, hydroxide, and amide) as nucleophile and leaving group in methylsulfenyl derivatives were examined at B3LYP/aug-cc-pVDZ. The reactions involving hydroxide and amide correspond to deprotonation and not substitution. The substitution reactions follow an addition-elimination pathway, possessing a triple-well potential energy surface. The intermediate along this pathway is of trigonal bipyramid geometry with the nucleophile and leaving group occupying apical positions.     

Gold(III)-catalyzed nucleophilic substitution of propargylic alcohols

Gold-catalyzed nucleophilic substitution on propargylic alcohols, with various C-, O-, and S-nucleophiles, is described under very mild conditions (room temperature, dichloromethane) in 0-97% yield.