Showing papers on "Steric effects published in 2009"

Frustrated Lewis pairs: a new strategy to small molecule activation and hydrogenation catalysis

Abstract: The combination of Lewis acids and bases that are sterically precluded from forming Lewis acid–base adducts, termed Frustrated Lewis pairs provide a unique route to the activation of small molecules and applications in catalysis.

Halogen bonds as orthogonal molecular interactions to hydrogen bonds

Abstract: Halogen bonds (X-bonds) are shown to be geometrically perpendicular to and energetically independent of hydrogen bonds (H-bonds) that share a common carbonyl oxygen acceptor. This orthogonal relationship is accommodated by the in-plane and out-of-plane electronegative potentials of the oxygen, which are differentially populated by H- and X-bonds. Furthermore, the local conformation of a peptide helps to define the geometry of the H-bond and thus the oxygen surface that is accessible for X-bonding. These electrostatic and steric forces conspire to impose a strong preference for the orthogonal geometry of X- and H-bonds. Thus, the optimum geometry of an X-bond can be predicted from the pattern of H-bonds in a folded protein, enabling X-bonds to be introduced to improve ligand affinities without disrupting these structurally important interactions. This concept of orthogonal molecular interactions can be exploited for the rational design of halogenated ligands as inhibitors and drugs, and in biomolecular engineering.

Discovering New Reactions with N-Heterocyclic Carbene Catalysis.

Abstract: Inspired by advances in our understanding of biological processes, new reactions employing organic molecules as catalysts have grown significantly over the last two decades1 In the broadest of terms, the most successful of these catalysts can be classified either as Bronsted acids,2,3 hydrogen-bond donors,4–7 or Lewis bases8–11 Each of these catalytic manifolds activates substrates in biological settings and provides an inspiring blueprint to create smaller, synthetic versions of these impressive biocatalysts Lewis base catalysis is presently an exciting area of research and encompasses a wide variety of strategies to initiate both established and new chemical processes12,13 An elegant and key biological transformation utilizes the cofactor thiamine, a coenzyme of vitamin B1, to transform α-keto acids into acetyl CoA, a major building block for polyketide synthesis In this process, a normally electron-deficient molecule (eg, pyruvic acid) is converted into an intermediate that possesses electron density on the carbon atom that was initially part of the carbonyl system These carbonyl or acyl anions are unusual since they have “umpolung” (reversed polarity) when compared to the initial keto acids In 1954, Mizuhara and Handler proposed that the active catalytic species of thiamine-dependent enzymatic reactions is a highly unusual divalent carbon-containing species,14 later on referred to as an N-heterocyclic carbene (NHC) An alternative description of the active thiamine cofactor employs the term zwitterion, which can be viewed as a resonance form of the carbene description This unique cofactor accomplishes fascinating Lewis base catalyzed transformations by utilizing the lone pair of electrons at C-2 In the early 1960s, Wanzlick and co-workers realized that the stability of carbenes could be dramatically enhanced by the presence of amino substituents, and they attempted to prepare a carbene center at C-2 of the imidazole ring15,16 However, only the dimeric electron-rich olefin was isolated Later on, Wanzlick's group demonstrated that potassium tert-butoxide can deprotonate imidazolium salts to afford imidazol-2-ylidenes, which can be trapped with phenyl isothiocyanate and mercury salts17–19 However, Wanzlick's group never reported the isolation of the free carbene Following these results, Arduengo et al isolated in 1991 a stable crystalline N-heterocyclic carbene by the deprotonation of 1,3-di(1-adamantyl)imidazolium chloride with sodium or potassium hydride in the presence of a catalytic amount of either potassium tert-butoxide or dimethyl sulfoxide20 The structure was unequivocally established by single-crystal X-ray analysis, and the carbene was found to be thermally stable, which stimulated extensive research in this field The nature of the stabilization is ascribed to the steric and electronic effects of the substituents: The two adamantyl substituents hinder reactions of the carbene center with external reagents as well as prevent dimerization In 1992, Arduengo et al expanded this carbene class by successfully isolating the carbene from 1,3,4,5-tetramethylimidazolium chloride by treating the latter with sodium hydride and catalytic amounts of potassium tert-butoxide in tetrahydrofuran21 The successful isolation of carbenes with less bulky substituents demonstrates that electronic factors may have greater impact on the stability of the carbene than steric ones Such electronic factors operate in both the π and σ frameworks, resulting in a “push-pull” synergistic effect to stabilize the carbene π donation into the carbene from the out-of-the-plane π orbital of the heteroatoms adjacent to C-2 stabilizes the typical electrophilic reactivity of carbenes The electronegative heteroatoms adjacent to C-2 provide additional stability through the framework of σ bonds, resulting in a moderation of the nucleophilic reactivity of the carbene (Figure 1)22 The combination of these two effects serves to increase the singlet–triplet gap and stabilize the singlet-state carbene over the more reactive triplet-state one Figure 1 Proposed Stabilization of N-Heterocyclic Carbenes through σ and π Electronic Effects (Ref 22) The electronic properties of NHCs are a key determinant of the unique reactivity of these catalysts Lewis bases are normally considered as single electron-pair donors However, the singlet carbenes of NHCs are distinct Lewis bases that have both σ basicity and π acidity characteristics These attributes allow for the generation of a second nucleophile in the flask Nucleophilic addition of the carbene to an aldehyde results in the formation of a new nucleophile The “doubly” nucleophilic aspect is unique to the carbenes The combination of these characteristics allows NHCs to react as powerful nucleophiles, which has driven the development of a distinct class of catalytic processes during the last decade This review highlights our work in this young and promising field

Complexation of nitrous oxide by frustrated Lewis pairs.

Abstract: Frustrated Lewis pairs comprised of a basic yet sterically encumbered phosphine with boron Lewis acids bind nitrous oxide to give intact PNNOB linkages. The synthesis, structure, and bonding of these species are described.

Statistical and energetic analysis of side-chain conformations in oligopeptides.

Abstract: The distributions of side-chain conformations in 258 crystal structures of oligopeptides have been analyzed. The sample contains 321 residues having side chains that extend beyond the C beta atom. Statistically observed preferences of side-chain dihedral angles are summarized and correlated with stereochemical and energetic constraints. The distributions are compared with observed distributions in proteins of known X-ray structures and with computed minimum-energy conformations of amino acid derivatives. The distributions are similar in all three sets of data, and they appear to be governed primarily by intraresidue interactions. In side chains with no beta-branching, the most important interactions that determine chi 1 are those between the C gamma H2 group and atoms of the neighboring peptide groups. As a result, the g- conformation (chi 1 congruent to -60 degrees) occurs most frequently for rotation around the C alpha-C beta bond in oligopeptides, followed by the t conformation (chi 1 congruent to 180 degrees), while the g+ conformation (chi 1 congruent to 60 degrees) is least favored. In residues with beta-branching, steric repulsions between the C gamma H2 or C gamma H3 groups and backbone atoms govern the distribution of chi 1. The extended (t) conformation is highly favored for rotation around the C beta-C gamma and C gamma-C delta bonds in unbranched side chains, because the t conformer has a lower energy than the g+ and g- conformers in hydrocarbon chains. This study of the observed side-chain conformations has led to a refinement of one of the energy parameters used in empirical conformational energy computations.

A systematic investigation of factors influencing the decarboxylation of imidazolium carboxylates.

Abstract: A series of 1,3-disubstituted-2-imidazolium carboxylates, an adduct of CO2 and N-heterocyclic carbenes, were synthesized and characterized using single crystal X-ray, thermogravimetric, IR, and NMR analysis. The TGA analysis of the NHC-CO2’s shows that as steric bulk on the N-substituent increases, the ability of the NHC-CO2 to decarboxylate increases. The comparison of NHC-CO2’s with and without methyls at the 4,5-position indicate that extra electron density in the imidazolium ring enhances the stability of an NHC-CO2 thereby making it less prone to decarboxylation. Single crystal X-ray analysis shows that the torsional angle of the carboxylate group and the C−CO2 bond length with respect to the imidazolium ring is dependent on the steric bulk of the N-substituent. Rotamers in the unit cell of a single crystal of ItBuPrCO2 (2f) indicate that the C−CO2 bond length increases as the N-substituents rotate toward the carboxylate moiety, which suggests that rotation of the N-substituents through the plane of ...

Revisiting the Ramachandran plot: hard-sphere repulsion, electrostatics, and H-bonding in the alpha-helix.

Abstract: What determines the shape of the allowed regions in the Ramachandran plot? Although Ramachandran explained these regions in terms of 1-4 hard-sphere repulsions, there are discrepancies with the data where, in particular, the alphaR, alphaL, and beta-strand regions are diagonal. The alphaR-region also varies along the alpha-helix where it is constrained at the center and the amino terminus but diffuse at the carboxyl terminus. By analyzing a high-resolution database of protein structures, we find that certain 1-4 hard-sphere repulsions in the standard steric map of Ramachandran do not affect the statistical distributions. By ignoring these steric clashes (NH(i+1) and O(i-1)C), we identify a revised set of steric clashes (CbetaO, O(i-1)N(i+1), CbetaN(i+1), O(i-1)Cbeta, and O(i-1)O) that produce a better match with the data. We also find that the strictly forbidden region in the Ramachandran plot is excluded by multiple steric clashes, whereas the outlier region is excluded by only one significant steric clash. However, steric clashes alone do not account for the diagonal regions. Using electrostatics to analyze the conformational dependence of specific interatomic interactions, we find that the diagonal shape of the alphaR and alphaL-regions also depends on the optimization of the NH(i+1) and O(i-1)C interactions, and the diagonal beta-strand region is due to the alignment of the CO and NH dipoles. Finally, we reproduce the variation of the Ramachandran plot along the alpha-helix in a simple model that uses only H-bonding constraints. This allows us to rationalize the difference between the amino terminus and the carboxyl terminus of the alpha-helix in terms of backbone entropy.

Possible steric control of the relative strength of chelation enhanced fluorescence for zinc(II) compared to cadmium(II): metal ion complexing properties of tris(2-quinolylmethyl)amine, a crystallographic, UV-visible, and fluorometric study.

Abstract: The idea is examined that steric crowding in ligands can lead to diminution of the chelation enhanced fluorescence (CHEF) effect in complexes of the small Zn(II) ion as compared to the larger Cd(II) ion. Steric crowding is less severe for the larger ion and for the smaller Zn(II) ion leads to Zn-N bond length distortion, which allows some quenching of fluorescence by the photoinduced electron transfer (PET) mechanism. Some metal ion complexing properties of the ligand tris(2-quinolylmethyl)amine (TQA) are presented in support of the idea that more sterically efficient ligands, which lead to less M-N bond length distortion with the small Zn(II) ion, will lead to a greater CHEF effect with Zn(II) than Cd(II). The structures of [Zn(TQA)H(2)O](ClO(4))(2).1.5 H(2)O (1), ([Pb(TQA)(NO(3))(2)].C(2)H(5)OH) (2), ([Ag(TQA)(ClO(4))]) (3), and (TQA).C(2)H(5)OH (4) are reported. In 1, the Zn(II) is 5-coordinate, with four N-donors from the ligand and a water molecule making up the coordination sphere. The Zn-N bonds are all of normal length, showing that the level of steric crowding in 1 is not sufficient to cause significant Zn-N bond length distortion. This leads to the observation that, as expected, the CHEF effect in the Zn(II)/TQA complex is much stronger than that in the Cd(II)/TQA complex, in contrast to similar but more sterically crowded ligands, where the CHEF effect is stronger in the Cd(II) complex. The CHEF effect for TQA with the metal ions examined varies as Zn(II) >> Cd(II) >> Ni(II) > Pb(II) > Hg(II) > Cu(II). The structure of 2 shows an 8-coordinate Pb(II), with evidence of a stereochemically active lone pair, and normal Pb-N bond lengths. In 3, the Ag(I) is 5-coordinate, with four N-donors from the TQA and an oxygen from the perchlorate. The Ag(I) shows no distortion toward linear 2-coordinate geometry, and the Ag-N bonds fall slightly into the upper range for Ag-N bonds in 5-coordinate complexes. The structure of 4 shows the TQA ligand to be involved in pi-stacking between quinolyl groups from adjacent TQA molecules. Formation constants determined by UV-visible spectroscopy are reported in 0.1 M NaClO(4) at 25 degrees C for TQA with Zn(II), Cd(II), and Pb(II). When compared with other similar ligands, one sees that, as the level of steric crowding increases, the stability decreases most with the small Zn(II) ion and least with the large Pb(II) ion. This is in accordance with the idea that TQA has a moderate level of steric crowding and that steric crowding increases for TQA analogs tris(2-pyridylmethyl)amine (TPyA) < TQA < tris(6-methyl-2-pyridyl)amine (TMPyA).

DOTA-M8: An extremely rigid, high-affinity lanthanide chelating tag for PCS NMR spectroscopy.

Abstract: A new lanthanide chelating tag (M8) for paramagnetic labeling of biomolecules is presented, which is based on an eight-fold, stereoselectively methyl-substituted DOTA that can be covalently linked to the host molecule by a single disulfide bond. The steric overcrowding of the DOTA scaffold leads to an extremely rigid, kinetically and chemically inert lanthanide chelator. Its steric bulk restricts the motion of the tag relative to the host molecule. These properties result in very large pseudocontact shifts (>5 ppm) and residual dipolar couplings (>20 Hz) for Dy-M8 linked to ubiquitin, which are unprecedented for a small, single-point-attachment tag. Such large pseudocontact shifts should be well detectable even for larger proteins and distances beyond approximately 50 A. Due to its exceptionally high stability and lanthanide affinity M8 can be used under extreme chemical or physical conditions, such as those applied for protein denaturation, or when it is undesirable that buffer or protein react with excess lanthanide ions.

C-C bond formation via C-H bond activation using an in situ-generated ruthenium catalyst.

Abstract: We report here our full results concerning the possibility of generating in situ from a stable and readily available ruthenium(II) source a highly active ruthenium catalyst for C-H bond activation. The versatility of this catalytic system has been demonstrated, as it offers the possibility of modifying the electronic and steric properties of the catalyst by fine-tuning of the ligands, allowing functionalization of various substrates. Aromatic ketones and imines could be easily functionalized by the reaction with either vinylsilanes or styrenes, depending on the electronic and steric nature of the ligand. Moreover, variable-temperature NMR experiments and the isolation of a ruthenium intermediate complex provided some insights into the generation of the active catalytic ruthenium species in this reaction.

Theoretical insights on methylbenzene side-chain growth in ZSM-5 zeolites for methanol-to-olefin conversion.

Abstract: The key step in the conversion of methane to polyolefins is the catalytic conversion of methanol to light olefins. The most recent formulations of a reaction mechanism for this process are based on the idea of a complex hydrocarbon-pool network, in which certain organic species in the zeolite pores are methylated and from which light olefins are eliminated. Two major mechanisms have been proposed to date-the paring mechanism and the side-chain mechanism-recently joined by a third, the alkene mechanism. Recently we succeeded in simulating a full catalytic cycle for the first of these in ZSM-5, with inclusion of the zeolite framework and contents. In this paper, we will investigate crucial reaction steps of the second proposal (the side-chain route) using both small and large zeolite cluster models of ZSM-5. The deprotonation step, which forms an exocyclic double bond, depends crucially on the number and positioning of the other methyl groups but also on steric effects that are typical for the zeolite lattice. Because of steric considerations, we find exocyclic bond formation in the ortho position to the geminal methyl group to be more favourable than exocyclic bond formation in the para position. The side-chain growth proceeds relatively easily but the major bottleneck is identified as subsequent de-alkylation to produce ethene. These results suggest that the current formulation of the side-chain route in ZSM-5 may actually be a deactivating route to coke precursors rather than an active ethene-producing hydrocarbon-pool route. Other routes may be operating in alternative zeotype materials like the silico-aluminophosphate SAPO-34.

Steric effects in the reaction of aryl radicals on surfaces.

Abstract: Steric effects are investigated in the reaction of aryl radicals with surfaces. The electrochemical reduction of 2-, 3-, 4-methyl, 2-methoxy, 2-ethyl, 2,6-, 2,4-, and 3,5-dimethyl, 4-tert-butyl, 3,5-bis-tert-butyl benzenediazonium, 3,5-bis(trifluoromethyl), and pentafluoro benzenediazonium tetrafluoroborates is examined in acetonitrile solutions. It leads to the formation of grafted layers only if the steric hindrance at the 2- or 2,6-position(s) is small. When the 3,5-positions are crowded with tert-butyl groups, the growth of the organic layer is limited by steric effects and a monolayer is formed. The efficiency of the grafting process is assessed by cyclic voltammetry, X-ray photoelectron spectroscopy, infrared, and ellipsometry. These experiments, together with density functional computations of bonding energies of substituted phenyl groups on a copper surface, are discussed in terms of the reactivity of aryl radicals in the electrografting reaction and in the growth of the polyaryl layer.

The effect of electrostatic interactions on conformational equilibria of multiply substituted tetrahydropyran oxocarbenium ions.

Abstract: The three-dimensional structures of dioxocarbenium ions related to glycosyl cations were determined by an analysis of spectroscopic, computational, and reactivity data. Hypothetical low-energy structures of the dioxocarbenium ions were correlated with both experimentally determined (1)H NMR coupling constants and diastereoselectivity results from nucleophilic substitution reactions. This method confirmed the pseudoaxial preference of C-3 alkoxy-substituted systems and revealed the conformational preference of the C-5 alkoxymethyl group. Although the monosubstituted C-5 alkoxymethyl substituent preferred a pseudoequatorial orientation, the C-5-C-6 bond rotation was controlled by an electrostatic effect. The preferred diaxial conformer of the trans-4,5-disubstituted tetrahydropyranyl system underscored the importance of electrostatic effects in dictating conformational equilibria. In the 2-deoxymannose system, although steric effects influenced the orientation of the C-5 alkoxymethyl substituent, the all-axial conformer was favored because of electrostatic stabilization.

Preferred conformation of the benzyloxycarbonyl-amino group in peptides.

Abstract: Structural parameters, derived from X-ray crystallographic data, have been compiled for 35 derivatives of amino acids, peptides, and related compounds, which contain the N-terminal benzyloxycarbonyl (Z) group. The geometry of the urethane moiety of this end group is closely similar to that of the tert-butoxycarbonyl (Boc) group, except for a relaxation of some bond angles because the Z group is sterically less crowded than the Boc group. For the same reason, the Z group has greater conformational flexibility. As a result, packing forces in the crystal may cause greater deformations of bond angles, resulting in larger variations of observed bond lengths and bond angles than in Boc-peptide crystals. The aromatic rings of the Z end groups tend to stack in crystals. Conformational energy calculations indicate that most conformations of Z-amino acid-N'-methylamides and of corresponding Boc derivatives have similar dihedral angles and relative energies, i.e. the nature of the N-terminal end group has little effect on the conformational preferences of the residue next to it. In particular, the computed fraction of molecules with a cis urethane (C-N) bond is similar for the two derivatives: 0.51 and 0.42 in Boc-Pro-NHCH3 and Z-Pro-NHCH3, respectively, and 0.02 in the two Ala derivatives. There exist several computed conformations of Z-Ala-NHCH3 and Z-Pro-NHCH3 in which the phenyl ring and the C-terminal methylamide group are close to each other. Because of favorable nonbonded interactions, such conformations are of low energy.

Theoretical study of the molecular and electronic structure of methanol on a TiO 2 (110) surface

Abstract: We present density-functional-theory calculations of the molecular and electronic structure of methanol adsorption on stoichiometric TiO2(110) surface. We have investigated 11 different molecular and dissociated adsorption structures of CH3OH at 1 monolayer coverage. The relative stabilities of different structures depend on the chemisorption-induced charge transfer, the relative strengths of different types of hydrogen bonds, the steric hindrance between methyl groups and the surface stress. We found the intermolecular hydrogen bonding to play an important role in stabilizing the overlayer. We also investigated the occupied and unoccupied surface electronic structure, and the adsorbate-induced surface dipole moment and work-function changes. The electronic structures show that the highest-occupied molecular orbital of CH3OH is near the valance-band maximum, which reflects the character of CH3OH as a hole scavenger on TiO2 surfaces. The unoccupied partially solvated or “wet” electron states for CH3OH on TiO2 are primarily distributed on H atoms of methyl groups. Despite many different structural motifs, the wet-electron-state energy primarily correlates with the surface dipole moment.

On the electronic impact of abnormal C4-bonding in N-heterocyclic carbene complexes.

Abstract: Sterically similar palladium dicarbene complexes have been synthesized that comprise permethylated dicarbene ligands which bind the metal center either in a normal coordination mode via C2 or abnormally via C4. Due to the strong structural analogy of the complexes, differences in reactivity patterns may be attributed to the distinct electronic impact of normal versus abnormal carbene bonding, while stereoelectronic effects are negligible. Unique reactivity patterns have been identified for the abnormal carbene complexes, specifically upon reaction with Lewis acids and in oxidative addition-reductive elimination sequences. These reactivities as well as analytical investigations using X-ray diffraction and X-ray photoelectron spectroscopy indicate that the C4 bonding mode increases the electron density at the metal center substantially, classifying such C4-bound carbene ligands amongst the most basic neutral donors known thus far. A direct application of this enhanced electron density at the metal center is demonstrated by the catalytic H 2 activation with abnormal carbene complexes under mild conditions, leading to a catalytic process for the hydrogenation of olefins.

Unusual structures and reactivity of mixed metal cluster complexes containing the palladium/platinum tri-t-butylphosphine grouping.

Abstract: Polynuclear metal carbonyl complexes have a range of applications in chemical research: for example, they can serve as surface models to probe features of heterogeneous catalysis and can perform novel transformations of organic molecules in solutions. Mixed metal complexes can demonstrate bimetallic cooperativity and synergism and can also serve as precursors to multimetallic heterogeneous catalysts that have superior activities and selectivities. This Account describes the results of our recent comprehensive study of the chemistry of mixed metal cluster complexes containing the sterically encumbered M(PBut3), M = Pd or Pt, group. This grouping readily adds to the metal−metal bonds of metal carbonyl cluster complexes and modifies their reactivity. We have prepared new, highly electronically unsaturated mixed metal complexes that exhibit unusually high reactivity toward hydrogen. The platinum atom of the Pt(PBut3) grouping can bond to as many as five metal atoms, and it can interconvert, sometimes rapidly,...

Synthesis and Photovoltaic Properties of Two Benzo[1,2-b:3,4-b′]dithiophene-Based Conjugated Polymers

Abstract: Side chains of conjugated polymers play an important role in their properties. To investigate the influence of side chains on photovoltaic properties, two conjugated polymers with benzo[1,2-b:3,4-b]dithiophene (BDT) and 4,7-dithiophen-2,1,3-benzothiadiazole (DTBT) units, Z3 and Z4, were designed and synthesized. These two polymers have identical conjugated main chains and very similar side chains. In polymer Z3, hexyl groups are linked to the thiophene units directly, and in polymer Z4, hexyloxy groups are linked to the thiophene units. Since the hexyloxy group exhibits a smaller steric hindrance and a stronger electron donating effect than the hexyl group, properties of Z3 and Z4 are very different. The hole mobility of the Z3:PCBM blend film is 1.84 × 10−7 cm2/(V s), which is 2 orders lower than that of the Z4:PCBM blend, and photovoltaic results demonstrate that the fill factor (FF) of the Z3-based device is 0.37, while that of the Z4-based polymer solar cell (PSC) device reached 0.61. These results re...

Examination of the role of Taft-type steric parameters in asymmetric catalysis.

Abstract: We report the use of Taft steric parameters to correlate the substituent size of a ligand with the enantiomeric ratio of a reaction. Linear free energy relationships can be constructed by plotting the log of enantiomeric ratio (er) versus the steric parameters reported by Taft and modified by Charton. Successful correlations were found for aldehyde and ketone allylation under NHK conditions using modular oxazoline ligands developed in our laboratory. Using these correlations, ligands were designed and evaluated for carbonyl allylation reactions. A break in the Charton plot results and is attributed to a global structural change in the catalyst. Additionally, several previously reported enantioselective reactions are analyzed resulting in excellent correlations for both catalysts and substrates. Finally, limitations and issues are presented with illustrative examples.

Catalysis by cationic oxorhenium(V): hydrolysis and alcoholysis of organic silanes

Abstract: The cationic [2-(2′-hydroxyphenyl)-2-oxazolinato(−2)]oxorhenium(V) complex 1 promotes oxidative dehydrogenation of organosilanes with water and alcohols in a catalytic manner to give excellent yields of silanols and silyl ethers, respectively. The reactions proceed conveniently under ambient and open-flask conditions with low catalyst loading (≤1 mol%). The scope of the reaction with water is quite broad and includes aliphatic, aromatic, tertiary, secondary and primary silanes. The rate of reaction depends on the catalyst and silane concentrations and kinetic isotope effect measurements demonstrate involvement of the Si–H bond in the activated complex. The most influential factor on the silane affecting reactivity is steric hindrance and a quantitative correlation with the Taft steric parameter (E) is presented. A combination of kinetic data and isotope labelling results are used to discuss plausible mechanisms for the oxidative dehydrogenation reaction pathway.

Dehydrogenation reactivity of a frustrated carbene-borane Lewis pair.

Abstract: The sterically demanding carbene 1,3-di-tert-butylimidazolidin-2-ylidene and B(C6F5)3 form a “frustrated” Lewis pair (FLP), which is able to cleave dihydrogen heterolytically. In the absence of reactants, this FLP system exhibits dehydrogenation reactivity to give a mixture of an imidazolidinium borate and an abnormal carbene–borane adduct.

Assessing the Potential of Zwitterionic NHC·CS2 Adducts for Probing the Stereoelectronic Parameters of N-Heterocyclic Carbenes

Abstract: Five imidazol(in)ium-2-dithiocarboxylates bearing cyclohexyl, mesityl, or 2,6-diisopropylphenyl substituents on their nitrogen atoms were prepared from the corresponding N-heterocyclic carbenes (NHCs) by reaction with carbon disulfide. They were characterized by IR, UV/Vis, and NMR spectroscopy, and by thermogravimetric analysis. Their molecular structures were determined by X-ray diffraction. For the sake of comparison, tricyclohexylphosphonium dithiocarboxylate was also examined. The data acquired were scrutinized to evaluate their usefulness for assessing the steric and electronic properties of NHC ligands. Because of their outstanding ability to crystallize, the five NHC·CS2 betaines were found to be highly suitable for probing the steric influence of nitrogen atom substituents on imidazolylidene-based ligand precursors via XRD analysis, while the corresponding NHC·CO2 adducts were deemed more appropriate for evaluating the σ-donating properties of carbene ligands.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Anionic halocuprate(II) complexes as catalysts for the oxaziridine-mediated aminohydroxylation of olefins.

Abstract: We have discovered that the oxaziridine-mediated copper-catalyzed aminohydroxylation reaction recently discovered in our laboratories is dramatically accelerated in the presence of halide additives. The use of this more active catalyst system enables the efficient aminohydroxylation of electronically and sterically deactivated styrenes and also enables the use of nonstereogenic 3,3-dialkyl oxaziridines as terminal oxidants in the aminohydroxylation reaction. We present evidence that anionic halocuprate(II) complexes are the catalytically active species responsible for the increased reactivity under these conditions. This unexpected observation has led us to re-evaluate our mechanistic understanding of this reaction. On the basis of the results of a variety of radical trapping experiments, we propose a modified mechanism that involves a homolytic reaction of the olefin with a copper(II)-activated oxaziridine. Together, the observation that anionic additives significantly increase the oxidizing ability of oxaziridines and the recognition of the radical nature of reactions of oxaziridines under these conditions suggest that a variety of new oxidative transformations catalyzed by halocuprate(II) complexes should be possible.

Anaerobic photocleavage of DNA in red light by dicopper(II) complexes of 3,3'-dithiodipropionic acid.

Abstract: Binuclear copper(II) complexes [{(phen)CuII}2(μ-dtdp)2] (1), [{(dpq)CuII}2(μ-dtdp)2] (2), [{(phen)CuII}2(μ-az)2] (3), and [{(dpq)CuII}2(μ-az)2] (4) and a zinc(II) complex [{(phen)ZnII}2(μ-dtdp)2] (5), having 3,3′-dithiodipropionic acid (H2dtdp), azelaic acid (nonanedioic acid), 1,10-phenanthroline (phen), and dipyrido[3,2-d:2′,3′-f]quinoxaline (dpq), were prepared and characterized by physicochemical methods. Complex 1 has been structurally characterized by X-ray crystallography. The complexes have each metal center bound to a chelating phenanthroline base and two bridging carboxylate ligands giving a square-planar MN2O2 coordination geometry. The molecular structure of complex 1 shows two sterically constrained disulfide moieties of the dtdp ligands. The complexes show good binding propensity to calf thymus DNA in the major groove. The photoinduced DNA cleavage activity of the complexes has been studied using 365 nm UV light and 647.1 nm and >750 nm red light under both aerobic and anaerobic conditions. ...

The Interplay between Steric and Electronic Effects in SN2 Reactions

Abstract: Myths of steric hindrance: In contrast with current opinion, energy decomposition analysis shows that the presence of bulky substituents at carbon leads to the release of steric repulsion in the transition state shown in the graphic. It is rather the weakening of the electrostatic attraction, and in particular the loss of attractive orbital interactions, that are responsible for the activation barrier. Quantum chemical calculations for S(N)2 reactions of H(3)EX/X(-) systems, in which E=C or Si and X=F or Cl, are reported. In the case of the carbon system we also report on bulkier species in which the hydrogen atoms are substituted by methyl groups. It is shown how the variation in the individual energy terms of the Morokuma/Ziegler energy decomposition analysis (EDA) scheme along the reaction coordinate from reactants to products provides valuable insight into the essential changes that occur in the bond-breaking/bond-forming process during S(N)2 reactions. The EDA results for the prototypical S(N)2 reaction of the systems [X...R(3)E...X](-), in which the interacting fragments are [X...X](2-) and [R(3)E](+), have given rise to a new interpretation of the factors governing the reaction course. The EDA results for the carbon system (E=C) show that there is less steric repulsion and stronger electrostatic attraction in the transition structure than in the precursor complex and that the energy increase comes mainly from weaker orbital interactions. The larger barriers for systems in which R(3) is bulkier also do not arise from increased steric repulsion, which is actually released in the transition structure. It is rather the weakening of the electrostatic attraction, and in particular the loss of attractive orbital interactions, that are responsible for the activation barrier. The D(3h) energy minima of the silicon homologues [XH(3)SiX](-) is driven by the large increase in the electrostatic attractions and also of stronger orbital interactions, while the steric interactions is destabilizing.