Antiperiplanar Effect Research Articles

Regioselectivity in Wacker oxidations of internal alkenes: antiperiplanar effects?

Computational chemistry, with considerable effort, was used to elucidate the reason for regioselectivity (12:1 distal:proximal product distribution) in a published Wacker oxidation of internal alkenes with a homoallylic lactam ring. Such a distribution is reproduced by a non-chelating reaction pathway; chelated intermediates are found to be too high in energy. Despite much speculation of chelation effects in the literature, this particular result is most probably an antiperiplanar field effect with no chelation. Partial charge data are provided to support the proposed antiperiplanar effect. The great care needed in modelling Wacker oxidations is emphasized, but optimism is offered that its various product mysteries can be elucidated on a case-by-case basis.

Orbital theory for diastereoselectivity in electrophilic addition

Electrophilic attack to a double bond is often observed anti to the most electron-donating σ-bond at the α-position (hereafter, we refer to this as the extended anomeric effect). This preference is believed to result from the antiperiplanar effect between the bond that is formed between the double bond and the electrophilic reagent, and the donating vicinal σ-bond which is located on the substituent at the α-position. From an orbital viewpoint, however, it is still unclear why the approach of the electrophile anti to the substituent results in stabilization or why the frontier molecular orbital (FMO) deforms, expanding toward the reagent with this antiperiplanar interaction. We demonstrate here that cyclic orbital interaction including geminal bond participation plays an important role in the diastereoselectivity in electrophilic addition. We examined our idea using the electrophilic addition of chlorine to 3-substituted propenes as a model reaction. Our bond model approach should contribute to a better understanding of orbital mixing in FMO.

1,2-Metallobridging and the Antiperiplanar Effect. Thermodynamic Preference for the Z-Configuration in Imidoyl(Alkali Metals) and their Phosphorus Analogs

We showed that imidoyl- and phosphaethenyl(alkali metals) would thermodynamically prefer the Z-configuration. The bond model analysis of the electronic structures showed that the Z-preference should originate from 1,2-metallobridging by the delocalization of lone pairs on N or P to vacant p-orbitals of the alkali metals and from the antiperiplanar effect of the delocalization from σ C—M to σ* N(P)—R 2 and from n N(P) to the C—R1. The Z-preference increases by more electron-withdrawing groups at the carbon atom of the double bond. However, substitution at the nitrogen/phosphorus results in E-preference because of 1,4-chelation of the lone pair of the substituents to alkali metals. Most of halogen derivatives were not stable and eliminate metal halides.

Natural bond orbital analysis of hyperconjugative stabilization effects in the transition states of cyclohexanone reduction with LiAlH4

Natural bond orbital (NBO) analysis of the transition states of cyclohexanone reduction with LiAlH4 located at the B3LYP/6-31+G(d) level along the intrinsic reaction coordinate (IRC) strongly indicate that the antiperiplanar effect involving the incipient bond may not be important as a controlling factor of π-facial selection in carbonyl reduction.

Reactions of Charged Substrates. 7. The Methoxymethyl Carbenium Ion Problem. 2. A Semiempirical Study of the Kinetic and Thermodynamic Stabilities of Linear and Cyclic Oxo- and Thiocarbenium Ions Generated from Aldehyde Hydrates, Hemiacetals, Acetals, and Methyl Ribosides and Glucosides.

Factors affecting the cleavage of the carbon-oxygen bond in linear and cyclic aldehyde hydrates, heimacetals, acetals, and methyl ribosides and glucosides have been investigated using semiempirical calculations (AM1 and PM3). (For some systems, low- and high-level ab initio energies are available for comparison with the semiempirical results. With one exception, the results obtained by the two methods show excellent agreement in relative energies and trends in reactivity.) The effects on reactivity and stability caused by substituting a sulfur for the alpha oxygen in the oxocarbenium ion were also studied. In general, systems that can have an antiperiplanar alignment of lone pairs on the leaving group and potential oxocarbenium ion oxygens undergo spontanteous cleavage. An examination of various conformers of the leaving group relative to the potential oxocarbenium oxygen shows, however, that lone pair repulsion and steric factors for MeOH as the leaving group are more important than the antiperiplanar effect for bond cleavage. All compounds in which the alpha-oxygen in the potential carbenium ion is replaced by sulfur undergo spontaneous cleavage regardless of the leaving group or structure of the compound. Energy profiles, DeltaH(), and DeltaH(R) values show that linear and cyclic thiocarbenium ions are much more stable than the corresponding oxocarbenium ions. Comparison of results for methyl ribosides and glucosides with results for corresponding pyridinium substrates suggests that both should hydrolyze through an A-1 mechanism. General-acid catalysis with hydronium as the acid was studied. With solution results, the computations suggest that substrates with either a good leaving group or stable oxocarbenium ion react with rate-limiting proton transfer from the acid to the leaving group but that substrates with both a good leaving group and stable carbenium ion react with concerted proton transfer and bond cleavage.

Experimental and theoretical studies of acid-catalyzed oxygen-18 exchange rates of conformationally rigid ketones. Is the antiperiplanar effect important?

The relative rates for the acid-catalyzed addition of H 2 18 O to the carbonyl group of the conformationally fixed bridged biaryl ketone, 1, and several derivatives bearing methyl, chloro, and methoxy substituents in axial-like and equatorial-like orientations have been measured. The effects of axial- versus diaxiallike methyl and chloro substituents produce rate decreases which are consistentwith the antiperiplanar (hyperconjugative) interaction proposed by Cieplak; however, the negligible influence of a methoxy group when antiperiplanar is not consistent

ChemInform Abstract: The Antiperiplanar Effect in 1,2‐Asymmetric Induction.

AbstractThe kinetics of the acid‐catalyzed isotope 18O exchange in the nucleophilic addition of H218O to the ketones (I) and (II) show an antiperiplanar effect for the C‐C bond, which is at least 1.9 kcal/mol greater than for the n‐σ* interaction with the C‐H bond.

Synthetic studies towards gelsemine, I The importance of the antiperiplanar effect in the highly regioselective reduction of non-symmetrical cis-hexahydrophthalimides

During studies aimed at the total synthesis of gelsemine an exceptional example of regioselectivity has been discovered. cis-Hexahydrophtnalimides, which are non-symmetrical through the presence of one alkyl group (see Figure 1), are reduced by sodium borohydride into the corresponding hydroxy lactams with very high regioselectivity. The corresponding cis-tetrahydrophthalimides exhibit much lower selectivity. These findings are explained on the basis of the conformational preference of the imide molecule and the antiperiplanar effect.

鎖状系における高ジアステレオ選択的アミノ化法とアミノ糖合成

We have found prominent 1, 2- and 1, 3-asymmetric induction in the intramolecular conjugate additions of γ- and δ-carbamoyloxy- α, β-unsaturated esters. They provide a good way to achieve diastereoselective amination of acyclic olefinic systems, since complementary diastereoface-selection is accomplished by changing the site of carbamoyloxy group between γ- and δ-positions, as summarized in Fig. 15; almost complete stereocontrol is achieved by using Z-olefins. We demonstrated the synthetic utility of this method by the stereoselective syntheses of all four possible diastereomers of racemic N- acyl 3-amino-2, 3, 6-trideoxyhexose. Furthermore, N-acyl derivatives, 5, 7 and 8, of three naturally occurring sugars were synthesized in a stereodivergent manner starting from L-lactate. The antiperiplanar effect was revealed to play a major role in the reactions of homoallyic carbamates with allylic chiral center.

Reversal of diastereofacial selectivity in the intramolecular michael addition of -α-carbamoyloxy-α,β-unsaturated esters. Synthesis of N-benzoyl- [formula omitted], [formula omitted]-daunosamine

Contrary to the precedents, 1,3-anti stereoselection was found in the intramolecular Michael addition of ethyl threo ,5-carbamoyloxy-4-trialkylsilyloxy-2-hexenoate to culminate in a synthesis of N-benzoyl- D , L -daunosomine. The antiperiplanar effect due to the group at 4-position was revealed to play a major role in the stereoselection in this type of reactions. N-benzoyl- D , L -3-epidaunosamine was also synthesized by 1,2-syn asymmetric induction.

Changing regioselectivity patterns in metal hydride reductions of unsymmetrically substituted cyclic anhydrides

A study of L- and K-selectride reductions of unsymmetrical cyclic anhydrides attached to six-membered rings and to bridged six-membered systems sheds a new light on the effect of the conformation of the substrate molecule on the regioselectivity of metal hydride reductions. Thus, in addition to intrinsic reactivity of the carbonyl group, the antiperiplanar effect, and steric congestion, the conformation of the parent molecule should be considered in predicting regioselectivity of nucleophilic additions to cyclic anhydrides.

Diastereoselection in Intermolecular Nitrile Oxide Cycloaddition (NOC) Reactions: Confirmation of the "Anti-Periplanar Effect" through a Simple Synthesis of 2-Deoxy-d-ribose†.

Diastereoselection in Intermolecular Nitrile Oxide Cycloaddition (NOC) Reactions: Confirmation of the "Anti-Periplanar Effect" through a Simple Synthesis of 2-Deoxy-d-ribose†.

Metal hydride reductions of unsymmetrically substituted cyclic anhydrides attached to strained ring systems

A dramatic reversal in regioselectivity is observed in the metal hydride reduction of unsymmetrical cyclic anhydrides such as 2,3, and 4 compared to cyclic anhydrides attached to bridged ring systems (e.g. 1). The synthesis of model cyclic anhydrides attached to strained rings is described and the ratios of isomeric lactones obtained upon reduction with metal hydride are reported. On the basis of theoretical calculations and, taking into account the intrinsic reactivity of the carbonyl group, the antiperiplanar effect, and steric congestion, an explanation is offered for the regioselectivity observed in the reduction of these compounds.

Metal hydride reductions of unsymmetrical cyclic anhydrides. The importance of the antiperiplanar effect on the regioselectivity of these reactions

A quantum mechanical study by the SCF abinitio method of the interaction of H− with methylsuccinic and 2,2-dimethylsuccinic anhydrides (naked and in the presence of a cation) suggests that nonperpendicular rearside attack cannot be the factor responsible for the regioselectivity of hydride transfer to the more sterically hindered carbonyl group. In this model, the nucleophilic attack at the less hindered carbonyl group is calculated to be of lower energy (with or without cation). Deformation of the planar succinic anhydride ring to the quasi-chair conformation is energetically favoured as it allows the nucleophile to attack both carbonyl functions antiperiplanar to a quasi axial C—H or C—C bond. The attack antiperiplanar to the C—CH3 bond is lower in energy than the attack antiperiplanar to the C—H bond suggesting that the reduction will occur at the sterically more hindered carbonyl group which is in agreement with the experimental findings.

Origin of .pi.-facial stereoselectivity in additions to .pi.-bonds: generality of the anti-periplanar effect

Origin of .pi.-facial stereoselectivity in additions to .pi.-bonds: generality of the anti-periplanar effect