Face selection in addition and elimination in sterically unbiased systems

Pauli Exclusion by n→π* Interactions: Implications for Paleobiology

Proteins have evolved to function in an aqueous environment. Collagen, which provides the bodily scaffold for animals, has a special need to retain its integrity. This need was addressed early on, as intact collagen has been detected in dinosaur fossils, even though peptide bonds have a half-life of only ∼500 years in a neutral aqueous solution. We sought to discover the physicochemical basis for this remarkable resistance to hydrolysis. Using experimental and computational methods, we found that a main-chain acyl group can be protected from hydrolysis by an O···C=O n→π* interaction with a neighboring acyl group. These interactions engage virtually every peptide bond in a collagen triple helix. This protection, which arises from the Pauli exclusion principle, could underlie the preservation of ancient collagen.

Asymmetric Divergent Synthesis of ent-Kaurane-, ent-Atisane-, ent-Beyerane-, ent-Trachylobane-, and ent-Gibberellane-type Diterpenoids

A unified strategy toward asymmetric divergent syntheses of nine C8-ethano-bridged diterpenoids A1-A9 (candol A, powerol, sicanadiol, epi-candol A, atisirene, ent-atisan-16α-ol, 4-decarboxy-4-methyl-GA₁₂, trachinol, and ent-beyerane) has been developed based on late-stage transformations of common synthons having ent-kaurane and ent-trachylobane cores. The expeditious assembly of crucial advanced ent-kaurane- and ent-trachylobane-type building blocks is strategically explored through a regioselective and diastereoselective Fe-mediated hydrogen atom transfer (HAT) 6-exo-trig cyclization of the alkene/enone and 3-exo-trig cyclization of the alkene/ketone, showing the multi-reactivity of densely functionalized polycyclic substrates with π_C═C and π_C═O systems in HAT-initiated reactions. Following the rapid construction of five major structural skeletons (ent-kaurane-, ent-atisane-, ent-beyerane-, ent-trachylobane-, and ent-gibberellane-type), nine C8-ethano-bridged diterpenoids A1-A9 could be accessed in the longest linear 8 to 11 steps starting from readily available chiral γ-cyclogeraniol 1 and known chiral γ-substituted cyclohexenone 2, in which enantioselective total syntheses of candol A (A1, 8 steps), powerol (A2, 9 steps), sicanadiol (A3, 10 steps), epi-candol A (A4, 8 steps), ent-atisan-16α-ol (A6, 11 steps), and trachinol (A8, 10 steps) are achieved for the first time.

Electronic forces as descriptors of nucleophilic and electrophilic regioselectivity and stereoselectivity

One of the main tasks of theoretical chemistry is to rationalize computational results with chemical insights. Key concepts of such nature include nucleophilicity, electrophilicity, regioselectivity, and stereoselectivity. While computational tools are available to predict barrier heights and other reactivity properties with acceptable accuracy, a conceptual framework to appreciate above quantities is still lacking. In this work, we introduce the electronic force as the fundamental driving force of chemical processes to understand and predict molecular reactivity. It has three components but only two are independent. These forces, electrostatic and steric, can be employed as reliable descriptors for nucleophilic and electrophilic regioselectivity and stereoselectivity. The advantages of using these forces to evaluate molecular reactivity are that electrophilic and nucleophilic attacks are featured by distinct characteristics in the electrostatic force and no knowledge of quantum effects included in the kinetic and exchange-correlation energies is required. Examples are provided to highlight the validity and general applicability of these reactivity descriptors. Possible applications in ambident reactivity, σ and π holes, frustrated Lewis pairs, and stereoselective reactions are also included in this work.

Bent bonds (τ) and the antiperiplanar hypothesis, and the reactivity at the anomeric center in pyranosides

The stereoselectivity of nucleophilic addition on oxocarbenium ions derived from the bicyclic pyranoside model with or without a C₂-OR group can be understood through the use of the bent-bond and the antiperiplanar hypothesis in conjunction with the concept of hyperconjugation as an alternative interpretive model of structure and reactivity.

Deciphering Selectivity in Organic Reactions: A Multifaceted Problem

Computational chemistry has made a sustained contribution to the understanding of chemical reactions. In earlier times, half a century ago, the goal was to distinguish allowed from forbidden reactions (e.g., Woodward-Hoffmann rules), that is, reactions with low or high to very high activation barriers. A great achievement of computational chemistry was also to contribute to the determination of structures with the bonus of proposing a rationalization (e.g., anomeric effect, isolobal analogy, Gillespie valence shell pair electron repulsion rules and counter examples, Wade-Mingos rules for molecular clusters). With the development of new methods and the constant increase in computing power, computational chemists move to more challenging problems, close to the daily concerns of the experimental chemists, in determining the factors that make a reaction both efficient and selective: a key issue in organic synthesis. For this purpose, experimental chemists use advanced synthetic and analytical techniques to which computational chemists added other ways of determining reaction pathways. The transition states and intermediates contributing to the transformation of reactants into the desired and undesired products can now be determined, including their geometries, energies, charges, spin densities, spectroscopy properties, etc. Such studies remain challenging due to the large number of chemical species commonly present in the reactive media whose role may have to be determined. Calculating chemical systems as they are in the experiment is not always possible, bringing its own share of complexity through the large number of atoms and the associated large number of conformers to consider. Modeling the chemical species with smaller systems is an alternative that historically led to artifacts. Another important topic is the choice of the computational method. While DFT is widely used, the vast diversity of functionals available is both an opportunity and a challenge. Though chemical knowledge helps, the relevant computational method is best chosen in conjunction with the nature of the experimental systems and many studies have been concerned with this topic. We will not address this aspect but give references in the text. Usually, a computational study starts with the validation of the method by means of benchmark calculations vs accurate experimental data or state-of-the-art calculations. Finally, computational chemists can bring more than the sole determination of the reaction pathways through the analysis of the electronic structure. In our case, we have privileged the NBO analysis, which has the advantage of describing interactions on the basis of terms and concepts that are shared within the chemical community. In this Account, we have chosen to select representative reactions from our own work to highlight the diversity of situations than can be addressed nowadays. These include selective activation of C(sp(3))-H bonds, selective reactions with low energy barriers, involving closed shell or radical species, the role of noncovalent interactions, and the importance of considering side reactions.

Fluorine-directed 1,2-trans glycosylation of rare sugars

To reconcile the urgent need to access well defined β-configured 2,6-di-deoxypyranose analogues for chemical biology, with the intrinsic α-selectivity of the native system, the directing role of fluorine at C2 has been explored. Localised partial charge inversion (C-H(δ+)→ C-F(δ-)) elicits a reversal of the substrate-based α-stereoselectivity, irrespective of the protecting group electronics.

Introduction of C(5/6) side chains onto 2-azabicyclo[2.1.1]hexanes via a 6-anti-bromo-5-anti-hydroxy derivative

Oxidation of the title bromoalcohol provided the strained ketone, 5-bromo-6-oxo-2-azabicyclo[2.1.1]hexane. Additions of nucleophiles to either this or the debrominated ketone have been used to introduce 5(6)-syn-alkyl and aryl groups, 5(6)-alkylidene linkages, and 5(6)-anti-alkyl and acyl substituents. Facial selectivity is for additions to the 6-bromo-5-ketone and 5-alkylidene azabicycles to occur from the face syn to the nitrogen atom. The bromine atom of the title alcohol has also been replaced by a 6-anti-(1-hydroxyethyl) substituent using a directed radical addition process. The stereoselective functionalization reactions expand the range of available methano-bridged pyrrolidines.

Orbital phase environments and stereoselectivities

Facial selections are reviewed to propose a new theory, orbital phase environment, for stereoselectivities of organic reactions. The orbital phase environment is a generalized idea of the secondary orbital interaction between the non-reacting centers and the unsymmetrization of the orbitals at the reacting centers arising from in-phase and out-of-phase overlapping with those at the neighboring non-reacting sites. In this context, the nucleophilic addition preferentially occurs on the face of the carbonyl functionality opposite to the better electron-donating orbital at the β position. In a similar manner to the carbonyl cases, the preferred reaction faces of olefins in electrophilic addition reactions are opposite to the better electron-donating orbitals at the β positions. The orbital phase environments in Diels-Alder reactions are also reviewed.

Stereoinduction by distortional asymmetry

Stereoselective chemical synthesis requires the two faces of a π bond to be differentiated. Theoretically sound qualitative models for understanding stereoinduction seem to break down in sterically unbiased cyclic systems. Presented here as the distortional asymmetry model is new insight that identifies circumstances where distortional ground state contributions are highly asymmetric and thereby contribute significantly to face selectivity. Out-of-plane distortional potential calculations, transition state calculations and molecular orbital analysis agree with experimental data that cannot otherwise be attributed to steric, torsion, polar or emergent transition state stabilizing effects. The model is readily understood in terms of reaction theory. The explanatory power of the model is also discussed.

Gas-phase facial diastereoselectivity of equatorial and axial 4-chloro-adamant-2-yl cations

The acid-catalyzed addition of CH3(18)OH to 2-methylene-adamantanes bearing a chlorine atom in the 4-equatorial (1e) or 4-axial (1a) position has been investigated in the gas phase, at 760 Torr, in the 40-120 degrees C temperature range. Two different experimental approaches were employed: (1) by adding neutral CH3(18)OH to the 2-methyl-4-Cl-adamant-2-yl cation, generated by protonation of the corresponding 2-methylene-4-Cl-adamantane (the extracomplex reaction) and (2) by reaction of 2-methylene-4-Cl-adamantane with CH3(18)OH2+, generated by methylation of H2(18)O (the intracomplex reaction). The crucial role of the nature of the noncovalent intermediates involved along the reaction coordinates emerges from the difference between the results obtained in the extracomplex and intracomplex reactions for both substrates investigated. The kinetic and stereochemical results indicate that the 4-Cl substituent plays a different role depending on its equatorial or axial orientation. Examination of the experimental results in the light of MP2/6-31G* theoretical calculations provides important information about the intrinsic factors governing the facial diastereoselectivity of trigonal carbocations. The effects due to differential face solvation phenomena emerge from the comparison of the present gas-phase results with those obtained from strictly related studies in solution.

Gas-Phase Diastereoselectivity of Secondary 5-Substituted (X)-Adamant-2-yl (X = F, Si(CH3)3) Cations

Secondary 5-X-adamant-2-yl cations IX (X = F, Si(CH3)3) have been generated in the gas phase (total pressure = 760 Torr) from protonation-induced defluorination of epimeric 2-F-5-X-adamantanes 1X and their kinetic diastereoselectivity toward CH318OH investigated in the 40-160 degrees C range. The experimental results indicate that the facial selectivity of IX is insensitive to the composition of the starting 1X epimers as well as to the presence and the concentration of a powerful base (N(C2H5)3). This kinetic picture, supported by B3LYP/6-31G* calculations, is consistent with a single stable pyramidalized structure for IX, that is, (Z)-5-F-adamant-2-yl (I(Z)F) and (E)-5-Si(CH3)3-adamant-2-yl cations (I(E)Si). The temperature dependence of the IX diastereoselectivity lends support to the intermediacy of noncovalent adducts [IX*CH318OH], characterized by a specific C2-H+...O18(H)CH3 hydrogen bonding interaction. Their conversion to the covalently bonded O-methylated (Z)- (II(Z)X) and (E)-5-X-adamantan-2-ols (II(E)X; X = F, Si(CH3)3) is governed by activation parameters, whose magnitude depends on the specific IX face accommodating CH318OH. The gas-phase diastereoselectivity of IX toward CH318OH is compared to that exhibited in related gas-phase and solution processes. The emerging picture indicates that the factors determining the diastereoselectivity of IX toward simple nucleophiles in the gaseous and condensed media are completely different.

Chiral α-Branched Benzylic Carbocations: Diastereoselective Intermolecular Reactions with Arene Nucleophiles and NMR Spectroscopic Studies

The chiral benzylic alcohols 1-6 were prepared and subjected to S(N)1-type displacement reactions with various arene nucleophiles in acidic medium. Under optimized conditions (HBF(4).OEt(2), CH(2)Cl(2), -78 degrees C --> r.t.) the corresponding 1,1-diarylalkanes 11-18 and 20 were obtained in good chemical yields (48-99%). The facial diastereoselectivity of the reaction is high (d.r. = 91/9-97/3) when the substrate bears a stereogenic carbon center -CHtBuMe in the alpha-position to the electrophilic carbon atom. If the starting material was enantiomerically pure, no significant racemization was observed (94% ee --> 92% ee). The reactions proceed stereoconvergently as demonstrated by the conversion of the separated diastereoisomers syn-1a and anti-1a in separate reactions to the same product syn-11 (d.r. = 97/3). Further evidence for long-lived chiral benzylic carbocations as reaction intermediates was obtained from NMR studies in superacidic medium. The chiral cation 24 was generated in SO(2)ClF as the solvent at -70 degrees C employing SbF(5) as the Lewis acid and characterized by its (1)H and (13)C NMR spectra. NOE measurements suggest a preferred conformation in which the diastereotopic faces of the cation are differentiated by the two carbon substituents R and Me at the stereogenic carbon center in the alpha-position. The hypothesis is further supported by the observation that the diastereoselectivity of the substitution reaction decreases if the bulky tert-butyl (R = tBu) substituent in the substrate 1a is replaced by a smaller ethyl group (2a, R = Et).

Novel and Efficient Method for the Allylation of Carbonyl Compounds and Imines Using Triallylaluminum

This is the first report of the use of triallylaluminum as a reagent for the allylation of carbonyl compounds and imines. The allylation of ketimines without additional metal catalyst is known so far only in the case of the Grignard reagent. Triallylaluminum is a useful alternative to provide the homoallylic amines in excellent yield upon addition to aldimines and ketimines. The significant reactivity of this reagent was confirmed by its reaction with a sterically rigid ketone such as adamantanone to provide 1-adamantyl-3-buten-1-ol in 98% yield. The chemoselectivity of triallylaluminum was demonstrated by using different ketoesters. It is noteworthy that triallylaluminum is prepared from allyl bromide and aluminum metal, and not from a Grignard reagent, and that the procedure is operationally simple, leading to good to excellent product yields.

One-Pot Synthesis of Adamantane Derivatives by Domino Michael Reactions from Ethyl 2,4-Dioxocyclohexanecarboxylate

Adamantane derivatives were constructed by the one-pot reaction of ethyl 2,4-dioxocyclohexanecarboxylate with 2-phenylethyl 2-(acetoxymethyl)acrylate or 2-(acetoxymethyl)-1-phenyl-2-propen-1-one via domino Michael reactions and a Dieckmann condensation or an aldol-type reaction (four-bond formation). This is the first one-pot construction of adamantane derivatives from cyclohexanone derivatives not involving enamines.

Importance of Entropy in the Diastereoselectivity of 5-Substituted 2-Methyladamant-2-yl Cations

The diastereofacial selectivity of 2-methyl-5-X-adamant-2-yl cations IX (X = CN, Cl, Br, CH3O, COOCH3, C6H5, CH3, and (CH3)3Sn) toward methanol has been investigated in the gas phase at 750 Torr and in the 40-120 degrees C temperature range and compared with that of IF (X = F) and ISi (X = (CH3)3Si) measured previously under similar conditions. Detailed analysis of the energy surface of the IMe (X = CH3) ion reveals that the activation barrier of its syn addition to methanol is significantly lower than that of the anti attack. In the 40-100 degrees C range, such a difference is strongly reduced by adverse entropic factors which are large enough to invert the IMe diastereoselectivity from syn to anti at T > 69 degrees C. The behavior of IMe diverges markedly from that of IF and ISi. Large adverse entropic factors account for the predominant syn diastereoselectivity observed in the reaction with IF (X = F), notwithstanding the anti enthalpy barrier is lower than the syn one. Adverse entropy plays a minor role in the reaction with ISi (X = (CH3)3Si) which instead exhibits a preferred anti diastereoselectivity governed by the activation enthalpies. Depending on the electronic properties of X, the kinetic behavior of the other IX ions obeys one of the above models. The gas-phase diastereoselectivity of IX ions responds to a subtle interplay between the sigma-hyperconjugative/electrostatic effects of the X substituent and the activation entropy terms. sigma-Hyperconjugation/field effects determine the pyramidal structure and the relative stability of the syn and anti conformers of IX as well as the relative stability of their addition transition structures and their position along the reaction coordinate. The diastereoselectivity of IX in the gas phase is compared with that measured in solution and with theoretical predictions.

Reversal of diastereoselectivities in intra- and intermolecular reactions of 2-adamantanylidenes primarily caused by electron-donating and electron- withdrawing substituents on C5

[reaction: see text] A reversal of diastereoselectivity was observed for novel 5-(trimethylsilyl)adamantan-2-ylidene (1c) with regard to 5-hydroxyadamantan-2-ylidene (1a). Ostensibly in intermolecular reactions, 5-substituted 2-adamantanylidenes (1) are sterically unbiased. However, inductive effects originating from the pendant group bend the divalent carbon bridge of 1 either toward (ERG's, e.g., -Si(CH(3))(3)) or away from (EWG's, e.g., -OH) the gamma-position. Hence, the more exposed side is more susceptible to intermolecular reaction and the other side concomitantly undergoes intramolecular 1,3-CH insertions more readily.

An ab initio study of conformations and IR spectra of 5-substituted 1,3-cyclopentadienes

A computational study of 5-substituted cyclopentadienes is presented. The substituents considered are the group 14–17 elements of the second through fifth periods, saturated by hydrogens as needed to fulfill normal valence requirements. The conformational characteristics are examined and rationalized using bond–antibond interactions and steric arguments. Trends in vibrational frequencies are discussed and compared with experiment where possible.Key words:cyclopentadiene, ab initio, spectra.

Stereoelectronic and conformational effects on the stereochemical course of reduction of bicyclo[3.3.1]nonane 1,3-diketones

The stereoselectivity of the reduction of bicyclo[3.3.1]nonane 1,3-diketones was studied. The experimental data of π-facial stereoselection of the reduction of the carbonyl group were successfully rationalized by the application of the exterior frontier orbital extension (EFOE) model. The observed facial diastereoselectivity of the carbonyl reduction of bicyclo[3.3.1]nonane diketones could be reasonably explained by the ground-state facial anisotropy of the frontier orbital extension, steric effects, and the intrinsic reactivity of carbonyl groups. Although the EFOE density and PDAS values predicted the enhanced reactivity at the C-2 carbonyl compared to C-9 carbonyl, transition-state calculations at the B3LYP/6-31+G(d,p) level showed that the reactivity of C-9 and C-2 in a hydride addition to both carbonyl groups should be nearly the same. The EFOE analysis also strongly indicated that in the second hydride reduction step the corresponding oxobicyclononanolates are most likely to be involved in the reduction as the major species rather than the free hydroxyketones. The antiperiplanar effects in the transition states of the LiAlH4 reduction as measured by both the elongation and the natural bond population of the antiperiplanar bonds clearly indicated they should not be an essential factor of the facial diastereoselection of studied diketones.Key words: bicyclo[3.3.1]nonanedione, EFOE model, reduction, stereoselectivity, transition state.

Novel synthesis of alkenes via triethylborane-induced free-radical reactions of alkyl iodides and beta-nitrostyrenes

Reactions of (E)-beta-nitrostyrenes 1 and triethylborane 2 or tricyclohexylborane 4 in THF solution at room temperature in the presence of oxygen in the air as radical initiator generate high yields of trans-alkenes (E)-3 or (E)-5. Medium to high yields of different (E)-alkenes (E)-5, 7, 10, 12, and 14 also can be prepared when 1 reacts with different radicals, prepared from secondary alkyl iodides 6 and 8 or tertiary alkyl iodides 9, 11, and 13, in the presence of 2 and air as radical initiator. The generation of the only product (E)-alkenes can be explained by the generation of the benzylic radical A and/or B as the intermediate only and the mechanism is similar to Scheme 1. Both (E)- and (Z)-16a-c are generated when (E)- and (Z)-15a-c are used to react with adamantyl radical under similar conditions. Only (Z)-16d was observed when either (E)- or (Z)-15d was used to react with adamantyl radical. The generation of the (E)- and/or (Z)-alkenes can be explained by the free rotation of the A and/or B to generate A' and/or B' and vice versa, and the mechanism is proposed to be a free-radical reaction via NO2/alkyl substitution and is shown as Scheme 2.

Synergic effect of vicinal stereocenters in [3 + 2] cycloadditions of carbohydrate azadipolarophiles and mesoionic dipoles: origin of diastereofacial selectivity

The intermolecular [3 + 2] cycloaddition of carbohydrate-derived 1,2-diaza-1,3-butadienes and 1,3-thiazolium-4-olates provides a conceptual basis for the problem of diastereofacial preference in the acyclic series of unsaturated sugars. Experimental results employing a side chain of D-arabino configuration have shown the stereodifferentiation exerted by the first stereogenic center that renders the Re,Re face of the acyclic sugar-chain azadiene eligible for cycloaddition (J. Org. Chem. 2000, 65, 5089). The results of the present work, now utilizing an alternative framework of D-lyxo configuration, evidence the discriminating power of the second stereogenic carbon, which induces the preferential approach to the Re,Si face of the heterocyclic dipole. This scheme of face selectivity is also grounded in theoretical calculations at a semiempirical level. In addition to dihydrothiophenes, which are the expected products of the [3 + 2] cycloaddition, bicyclic systems based on dihydrothieno[2,3-c]piperidine skeleton can also be obtained.

Quantum mechanical study on the facial selectivity of dioxa and trioxa cage molecules

High facial selectivity (>99%) of nucleophilic addition to the carbonyl groups of the title compounds (1 and 2) has been achieved for the novel trioxa cage 2, but not for the dioxa 1. Similar experimental observations were made for the carbene addition to the double bonds of cage compounds, 3 and 4. Calculations were carried out for the cage compounds and their reaction transition structures, with LiH as a nucleophile and :CCl(2) as an attacking carbene. The calculated facial preference for nucleophilic and carbene addition agreed well with experimental results. The origins of facial selectivity are examined from the viewpoints of structure, frontier orbitals, and molecular electrostatic potential of the reactants, as well as strain, electrostatic, and hyperconjugation effects in the transition state. For dioxa cages, the structural facial difference around the reaction center is minor, but the electronic difference of syn and anti faces generated by the two remote oxygen atoms is clearly demonstrated via frontier orbital and MEP analyses. For trioxa cages, the close proximity of the third ether oxygen (O(s)) to the reaction center brings large structural and electronic changes around the reaction center. The calculated electrostatic and strain energy differences of syn and anti transition structures are significantly larger for trioxa cages than for the dioxa cages. Therefore, they both contribute to the enhanced facial selectivity of trioxa compounds. Finally, analysis of hyperconjugative stabilization in transition structures reveals the danger of relying solely on Cieplak or Anh models in rationalization of facial selectivity, especially when nonequivalent steric and electrostatic effects as those present in the trioxa systems are involved.