Ortho-hydrogen Atoms Research Articles

Особенности строения полифункциональных карбоксилатов тетраарилсурьмы

From ferrocenedicarboxylic acid, sulfosalicylic acid and pentaarylantimony Ar5Sb (Ar = Ph, p-Tol) in benzene, fully substituted organoantimony derivatives of ferrocenedicarboxylic and sulfosalicylic acids 1–3 were obtained, the structure of which after recrystallization from a benzene-octane mixture was proven by X-ray diffraction analysis. Structures 1–3 were established by X-ray diffraction. Crystals 1 [C68H64FeO4Sb2, M 1244.54; monoclinic syngony, symmetry group C21/c; cell parameters: a = 16.9983(2), b = 10.94570(10), c = 30.3224(3) Å; β = 99.8220(10); V = 5559.04(10) Å3; Z = 4; calc = 1.487 g/cm3; 2 5.28138.76 degrees; total reflections 32707; independent reflections 10281; number of specified parameters 684; Rint = 0.0496; GOOF 1.062; R1 = 0.0345, wR2 = 0.0853; residual electron density (max/min) 0.76/0.90 e/Å3], 2 [C60H48FeO4Sb2, M 1132.33; monoclinic syngony, symmetry group P21/n; cell parameters: a = 12.40720(10), b = 10.30830(10), c = 36.7290(4) Å; β = 94.1110(10) deg., V = 4685.45(8) Å3, Z = 4; calc = 1.605 g/cm3; 2 4.82138.56 degrees; total reflections 27748; independent reflections 8680; number of specified parameters 604; Rint = 0.0367; GOOF 1.033; R1 = 0.0277, wR2 = 0.0709; residual electron density (max/min) 0.86/0.81 e/Å3], 3 [C63H62O7SSb2, M 1206.78; monoclinic syngony, symmetry group P21/c; cell parameters: a = 20.9246(3), b = 13.5375(2), c = 21.4222(3) Å; β = 111.284(2) deg., V = 5654.30(16) Å3, Z = 4; calc = 1.4175 g/cm3; 2 4.54160 degrees; total reflections 48463; independent reflections 11892; number of specified parameters 699; Rint = 0.0443; GOOF 1.024; R1 = 0.0327, wR2 = 0.0907; residual electron density (max/min) 0.93/0.85 e/Å3]. The antimony atoms in molecules 1, 2 have a distorted octahedral configuration, taking into account the coordination of the carbonyl oxygen atoms to the metal atom. Structure 3 contains tetrahedral tetraaryl stibonium cations, the antimony atoms in which are coordinated with the oxygen atoms of sulfo groups (Sb∙∙∙O(5)=S 2.601 Å), and the same oxygen atom is coordinated with the ortho-hydrogen atom of one of the tolyl groups (H(46)∙∙∙O(5)=S 2.60 Å). Water molecules provide a structure to crystal 3 (H(7)∙∙∙O(4)=S 1.98 Å, H(7)∙∙∙O(5)=S 2.60 Å). (CCDC 2320709 for 1, 2320711 for 2, 2320717 for 3; deposit@ccdc.cam.ac.uk; http://www.ccdc.cam.ac.uk).

Oxidative Cycloaddition Reactions of Arylboron Reagents via a One-pot Formal Dehydroboration Sequence.

Arylboron compounds are widely available and synthetically useful reagents in which the boron group is typically substituted. Herein, we show that the boron group and ortho-hydrogen atom are substituted in a formal cycloaddition reaction. This transformation is enabled by a one-pot sequence involving diaryliodonium and aryne intermediates. The scope of arylboron reagents and arynophiles is demonstrated, and the method is applied to the formal synthesis of an investigational drug candidate.

One-dimensional stacking array of 10,20-diphenyl-5,15-diazaporphyrin metal complexes

We prepared a free-base 10,20-diphenyl-5,15-diazaporphyrin and its Ni(II), Cu(II), Pd(II), and Pt(II) complexes. These molecules uniformly form an infinite one-dimensional [Formula: see text]-stacking array in the solid state. The four-fold intermolecular hydrogen bonding interactions between the meso-nitrogen atom of the diazaporphyrin core and the ortho-hydrogen atom of the phenyl substituent dominate the crystal packing arrangement. The present study revealed that the meso-nitrogen atom of diazaporphyrins would be used as an inherent hydrogen bonding acceptor to construct supramolecular assemblies.

Block deformation analysis: Density matrix blocks as intramolecular deformation density.

Block deformation analysis as deformation density of atomic orbitals is introduced to analyze intramolecular interactions. In this respect, density matrix blocks in terms of natural atomic orbitals are employed to find interacting and noninteracting multicenter subsystem and extract the corresponding deformation density. Eigenanalysis of this deformation density is performed to result eigenvalues and eigenorbitals as displaced charge due to the intramolecular interaction and orbital space responsible for charge reorganization, respectively, that possesses advantages of other methods, simultaneously. It is applied to several small molecules, different types of carbon allotropes including zero-, one-, and two-dimensional nanostructures, and challenging systems such as ortho-hydrogen atoms in planar biphenyl. Results highly correlate with delocalization and Wiberg bond indices and show that eigenvalues of block deformation analysis deserved to be considered as bonding index.

Changes in Electron Structure of the Triple Bond in Substituted Acetylene and Diacetylene Derivatives

The substituent effect is usually considered by means of various Hammett-like substituent constants and is most often related to aromatic systems. Unlike this, we present results of our research on the influence of 27 substituents spanning a wide range of electronic properties, from strongly electron-withdrawing to strongly electron-donating, on the electron structure of X-substituted acetylenes and diacetylenes - thus the systems which until now have practically not been subject of any deeper studies. It is shown that the interaction through triple bond(s) is associated with a significant advantage of resonance effects and that the substituent effect transmitted by the C≡C-C≡C unit is about half of that transmitted by the C≡C unit alone. Substituent X mainly affects the closest carbon atom by means of proximity effect, hence changes of charge on this atom do not follow any substituent constants. The effect on further carbon atoms is much smaller. The presence of the C≡C-C≡C unit withdraws more charge from X than a triple bond alone, and hinders communication between X and the terminal H atom. Comparison of substituent effects to those present in X-substituted benzene derivatives shows that the electronic properties of the terminal hydrogen atom in acetylenes and diacetylenes are most similar to the electronic properties of ortho and para hydrogen atoms in X-substituted benzene derivatives.

Enhanced piezoresponse and nonlinear optical properties of fluorinated self-assembled peptide nanotubes

Self-assembled L,L-diphenylalanine (FF) nanostructures offer an attractive platform for photonics and nonlinear optics. The nonlinear optical (NLO) coefficients of FF nanotubes depend on the diameter of the tube [S. Khanra et al. Phys. Chem. Chem. Phys. 19(4), 3084–3093 (2017)]. To further enhance the NLO properties of FF, we search for structural modifications. Here, we report on the synthesis of fluorinated FF dipeptides by replacing one ortho-hydrogen atom in each of the phenyl groups of FF by a fluorine atom. Density-functional theoretical calculations yield insights into minimum energy conformers of fluorinated FF (Fl-FF). Fl-FF self-assembles akin to FF into micron-length tubes. The effects of fluorination are evaluated on the piezoelectric response and nonlinear optical properties. The piezoelectric d15 coefficient of Fl-FF is found to be more than 10 times higher than that of FF nanotubes, and the intensity of second harmonic generation (SHG) polarimetry from individual Fl-FF nanotubes is more than 20 times that of individual FF nanotubes. Furthermore, we obtain SHG images to compare the intensities of FF and Fl-FF tubes. This work demonstrates the potential of fluorine substitution in other self-assembled biomimetic peptides for enhancing nonlinear optical response and piezoelectricity.

Synthesis of Pyridylimido Complexes of Tantalum and Niobium by Reductive Cleavage of the N═N Bond of 2,2'-Azopyridine: Precursors for Early-Late Heterobimetallic Complexes.

We report the syntheses of 2-pyridylimido complexes of tantalum and niobium by N═N bond cleavage of 2,2'-azopyridine. Reaction of MCl5 (M = Ta and Nb) with 2,2'-azopyridine in the presence of 0.5 equiv of 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (abbreviated Si-Me-CHD) afforded a dark red solution (for Ta) and a dark blue solution (for Nb) with some insoluble precipitates. After removing the solids, another 0.5 equiv of Si-Me-CHD was added to each solution, giving [M(═Npy)Cl3]n (1a: M = Ta; 1b: M = Nb) through reductive cleavage of the N═N bond of 2,2'-azopyridine. The initial products of the above reactions were determined to be 2,2'-azopyridine-bridged dinuclear complexes, [(MCl4)2(μ-pyNNpy)] (2a: M = Ta; 2b: M = Nb), which were isolated by treating MCl5 with 2,2'-azopyridine and Si-Me-CHD in a 2:1:1 molar ratio. In 2a and 2b, the N═N bond was reduced to a single bond via two-electron reduction. Further reduction of complexes 2a and 2b with 1 equiv of Si-Me-CHD afforded complexes 1a and 1b. An anionic doubly μ-imido-bridged ditantalum complex, [nBu4N][Ta2(μ-Npy)2Cl7] (3a), was generated upon addition of nBu4NCl to complex 1a, while addition of nBu4NCl to niobium complex 1b gave a polymeric terminal imido complex, [nBu4N]n/2[{Nb(═Npy)Cl3}2(μ-Cl)]n/2 (3b). Complexations of 1a and 1b with 1 equiv of 2,2'-bipyridine resulted in the formation of mononuclear 2-pyridylimido complexes, M(═Npy)Cl3(bipy) (4a: M = Ta; 4b: M = Nb), whose main structural feature is intramolecular hydrogen bonding between the ortho hydrogen atom of 2,2'-bipyridine and the nitrogen atom of the pyridyl group on the imido ligand. Isolated 2-pyridylimido complexes 4a and 4b reacted with [RhCl(cod)]2 to produce the corresponding early-late heterobimetallic complexes, (bipy)MCl3(μ-Npy)RhCl(cod) (5a: M = Ta; 5b: M = Nb).

C–Hortho⋯Pd interactions in palladium complexes derived from tetrasulfur-difluorinated ligands. Experimental, computational and catalytic studies

The symmetric ligands [Ar-S-CH2-Ph-S-(CH2)-]2, L1 (Ar = o-FPh) and L2 (Ar = p-FPh), and their PdII complexes (1 and 2) were synthesized and structurally characterized. Molecular structures of complexes 1 and 2 display the L1 and L2 ligands coordinating palladium in a κ2-SS bidentate fashion. The metal center exhibits a square planar geometry involving the two sulfur atoms and two chloride atoms which are oriented in cis disposition. Interestingly, the two ortho-hydrogen atoms from the phenyl rings in both complexes are engaged in Pd⋯Hortho interactions with distances from 2.976 to 3.280 Å. These distances can suffer an increase or decrease in their longitude due to the presence of intramolecular C–H⋯π interactions. Additionally, the presence of Pd⋯H interactions in palladium complexes (1 and 2) were identified by an NCI analysis. The complexes 1, 2 and a similar complex 3 that also displays Pd⋯Hortho interactions have been evaluated in the Suzuki-Miyaura cross-coupling reactions.

An unusual endo-selective C-H hydroarylationof norbornene by the Rh(I)-catalyzed reactionof benzamides

Hydroarylation is an environmentally attractive strategy which incorporates all of the atoms contained in the substrates into the desired products. Almost all the hydroarylations of norbornene reported to date involve an exo-selective reaction. Here we show the endo-selective hydroarylation of norbornene in the Rh(I)-catalyzed reaction of aromatic amides. The addition of sterically bulky carboxylic acids enhances the endo-selectivity of the reaction. The results of deuterium-labeling experiments show that both the ortho-carbon and the ortho-hydrogen atoms of aromatic amides were attached to the same carbon atom of the norbornane skeleton in the hydroarylation product. These results clearly suggest that hydrometalation or carbometalation, which are commonly accepted mechanisms for the catalytic hydroarylation of C–H bonds, are not involved as the key step in the present reaction, and suggest that the reaction involves a rhodium carbene complex generated from norbornene as the key intermediate.

Control of Reactivity and Regioselectivity for On-Surface Dehydrogenative Aryl-Aryl Bond Formation.

Regioselectivity is of fundamental importance in chemical synthesis. Although many concepts for site-selective reactions are well established for solution chemistry, it is not a priori clear whether they can easily be transferred to reactions taking place on a metal surface. A metal will fix the chemical potential of the electrons and perturb the electronic states of the reactants because of hybridization. Additionally, techniques to characterize chemical reactions in solution are generally not applicable to on-surface reactions. Only recent developments in resolving chemical structures by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) paved the way for identifying individual reaction products on surfaces. Here we exploit a combined STM/AFM technique to demonstrate the on-surface formation of complex molecular architectures built up from a heteroaromatic precursor, the tetracyclic pyrazino[2,3-f][4,7]phenanthroline (pap) molecule. Selective intermolecular aryl-aryl coupling via dehydrogenative C-H activation occurs on Au(111) upon thermal annealing under ultrahigh vacuum (UHV) conditions. A full atomistic description of the different reaction products based on an unambiguous discrimination between pyrazine and pyridine moieties is presented. Our work not only elucidates that ortho-hydrogen atoms of the pyrazine rings are preferentially activated over their pyridine equivalents, but also sheds new light onto the participation of substrate atoms in metal-organic coordination bonding during covalent C-C bond formation.

Charge Localisation in Heavy Alkali Metal Ion Complexes of 4,4'-Biphenyldicarboxylate

Crystal structure determinations on the isomorphous RbI and CsI complexes of 4,4′-biphenyldicarboxylate have shown the carboxylate entities to be coordinated in an unusual fashion where both oxygen atoms are in a tetrahedral environment indicative of negative charge localisation on each. The metal ions also show a highly irregular form of six-coordination, while the biphenyl units are planar, seemingly as a result of attractive interactions between the ortho hydrogen atoms.

Synthesis, crystal structures and conducting properties of heteroleptic nickel(II) 1,1-dithiolate-bpy/dppe ligand complexes

New heteroleptic complexes, [Ni(LL′)] (L=2-(methylene-1,1′-dithiolato)-5,5′-dimethylcyclohexane-1,3-dione (L1), L′=2,2′-bipyridyl (bpy) 1; L=2-(methylene-1,1′-dithiolato)-1,3-indandione (L2), L′=1,2-bis(diphenylphosphino)ethane (dppe) 2) have been synthesized and characterized by elemental analysis, IR, NMR and UV–Vis spectroscopy, and their structures have been determined crystallographically. In the two structures, the Ni atom lies at the centre of a distorted square plane. In 1, the ligand L1 is uniquely bonded to the Ni atom in an adjacent molecule in a μ2, κ2 S,S-chelating–bridging manner, forming a weak dimer with a Ni…S distance of 2.96Å, which is shorter than the sum of the van der Waal’s radii of the Ni and S atoms. In 2 the steric restrictions of the bulky dppe ligand hindered any dimerization, but result in the formation of a rare intramolecular C–H…Ni anagostic interaction between the ortho hydrogen atom of the phenyl ring and the nickel centre, which has been assessed by theoretical calculations. The supramolecular structures of 1 and 2 are sustained by π(chelate)…π(bpy), C–H…O, C–H…π(chelate) and C–H…π(Ar) interactions. The two compounds are weakly conducting (σrt=10−7–10−5Scm−1), but show semiconducting behavior in the 303–363K temperature range.

Alkynoxy-Directed C–H Functionalizations: Palladium(0)-Catalyzed Annulations of Alkynyl Aryl Ethers with Alkynes

Abstract Palladium(0)-catalyzed insertion/annulation sequence between aryl silylethynyl ethers and internal alkynes was found to proceed through activation of ortho-C–H bonds assisted by alkynoxy groups and gave stereoselectively (Z)-2-silylmethylenechromenes. These products could be easily converted into 2,2-alkylated 2H-chromene derivatives, an important structural motif in medicinal chemistry and materials science. Various aryl silylethynyl ethers and alkynes can be transformed under the reaction conditions, and a wide range of chromenes is thus accessible. When unsymmetric alkynes are employed, a regioselective annulation takes place, especially those containing aryl and/or bulky substituents. Catalytic systems based on palladium(0), such as Pd(OAc)2/PCy3/Zn, [Pd(dba)2]/PCy3, or Pd(PCy3)2 exhibit excellent catalytic activity, and the best performance is observed for Pd(PCy3)2 in combination with Zn(OAc)2 as an additive. Substituents on the aryl group in the alkynyl aryl ethers rarely affect the reaction rate. Deuterium-labeling experiments suggest that the ortho-hydrogen atom migrates to the 2-methylene position in the chromene products. The cleavage of the C–H bond is considered to be the rate-determining step in these reactions.

Experimental and theoretical studies of C–H⋯M interactions in palladium and platinum complexes derived from 1,2-bis-(2-hydroxymethylphenylthio)ethane

The molecular structures of several related cis-[κ2-SS-(L)MHal2] compounds 1, 1·DMSO, 2, 2·H2O, 3·DMSO, 4 and 5 where L=HOCH2Ph-S-CH2-CH2-S-PhCH2OH, M=Pd or Pt, and Hal=Cl or Br or I, were analyzed by single crystal X-ray diffraction analyses. In all complexes, the presence of intramolecular C–H⋯M (M=Pd or Pt) interactions with distance values of 2.687–2.914Å was established. In solid state both syn- or anti-conformers exist; this suggests that one or two ortho-hydrogen atoms on the phenyl rings are close to the metal. Furthermore, the existence of these weak interactions induce a significant effect on the geometry adopted by the metal center. The crystal structures show a wide variety of intermolecular noncovalent bonding such as hydrogen bonds, M⋯M contacts, π-π, C–H⋯π and Hal⋯π interactions. For all the complexes, the structural analysis performed by Density Functional Theory (DFT) is in good agreement with parametric data obtained from X-ray measurements. Topological analyses of the electron density function [ρ(r)], which have been performed according to Bader’s Atoms in Molecules theory, reveal bond critical points relating the metal atom with aromatic ortho-hydrogen atoms. Laplacian ∇2ρ(r) and density ρ(r) values indicate formation of the weak anagostic C–H⋯M interactions.

Hydrogen-hydrogen interaction in planar biphenyl: a theoretical study based on the interacting quantum atoms and Hirshfeld atomic energy partitioning methods.

The nature of H-H interaction between ortho-hydrogen atoms in planar biphenyl is investigated by two different atomic energy partitioning methods, namely fractional occupation iterative Hirshfeld (FOHI) and interacting quantum atoms (IQA), and compared with the traditional virial-based approach of quantum theory of atoms in molecules (QTAIM). In agreement with Bader's hypothesis of H-H bonding, partitioning the atomic energy into intra-atomic and interatomic terms reveals that there is a net attractive interaction between the ortho-hydrogens in the planar biphenyl. This falsifies the classical view of steric repulsion between the hydrogens. In addition, in contrast to the traditional QTAIM energy analysis, both FOHI and IQA show that the total atomic energy of the ortho-hydrogens remains almost constant when they participate in the H-H interaction. Although, the interatomic part of atomic energy of the hydrogens plays a stabilizing role during the formation of the H-H bond, it is almost compensated by the destabilizing effects of the intra-atomic parts and consequently, the total energy of the hydrogens remains constant. The trends in the changes of intra-atomic and interatomic energy terms of ortho-hydrogens during H-H bond formation are very similar to those observed for the H2 molecule.

Proton-bound dimers of nitrogen heterocyclic molecules: Substituent effects on the structures and binding energies of homodimers of diazine, triazine, and fluoropyridine

The bonding energies of proton-bound homodimers BH(+)B were measured by ion mobility equilibrium studies and calculated at the DFT B3LYP/6-311++G** level, for a series of nitrogen heterocyclic molecules (B) with electron-withdrawing in-ring N and on-ring F substituents. The binding energies (ΔH°(dissoc)) of the proton-bound dimers (BH(+)B) vary significantly, from 29.7 to 18.1 kcal/mol, decreasing linearly with decreasing the proton affinity of the monomer (B). This trend differs significantly from the constant binding energies of most homodimers of other organic nitrogen and oxygen bases. The experimentally measured ΔH°(dissoc) for (1,3-diazine)2H(+), i.e., (pyrimidine)2H(+) and (3-F-pyridine)2H(+) are 22.7 and 23.0 kcal/mol, respectively. The measured ΔH°(dissoc) for the pyrimidine(·+)(3-F-pyridine) radical cation dimer (19.2 kcal/mol) is signifcantly lower than that of the proton-bound homodimers of pyrimidine and 3-F-pyridine, reflecting the stronger interaction in the ionic H-bond of the protonated dimers. The calculated binding energies for (1,2-diazine)2H(+), (pyridine)2H(+), (2-F-pyridine)2H(+), (3-F-pyridine)2H(+), (2,6-di-F-pyridine)2H(+), (4-F-pyridine)2H(+), (1,3-diazine)2H(+), (1,4-diazine)2H(+), (1,3,5-triazine)2H(+), and (pentafluoropyridine)2H(+) are 29.7, 24.9, 24.8, 23.3, 23.2, 23.0, 22.4, 21.9, 19.3, and 18.1 kcal/mol, respectively. The electron-withdrawing substituents form internal dipoles whose electrostatic interactions contribute to both the decreased proton affinities of (B) and the decreased binding energies of the protonated dimers BH(+)B. The bonding energies also vary with rotation about the hydrogen bond, and they decrease in rotamers where the internal dipoles of the components are aligned efficiently for inter-ring repulsion. For compounds substituted at the 3 or 4 (meta or para) positions, the lowest energy rotamers are T-shaped with the planes of the two rings rotated by 90° about the hydrogen bond, while the planar rotamers are weakened by repulsion between the ortho hydrogen atoms of the two rings. Conversely, in ortho-substituted (1,2-diazine)2H(+) and (2-F-pyridine)2H(+), attractive interactions between the ortho (C-H) hydrogen atoms of one ring and the electronegative ortho atoms (N or F) of the other ring are stabilizing, and increase the protonated dimer binding energies by up to 4 kcal/mol. In all of the dimers, rotation about the hydrogen bond can involve a 2-4 kcal/mol barrier due to the relative energies of the rotamers.

Fluorine Substitution Effects on Flexibility and Tunneling Pathways: The Rotational Spectrum of 2‐Fluorobenzylamine

The effect of ring fluorination on the structural and dynamical properties of the flexible model molecule 2-fluorobenzylamine has been studied by rotational spectroscopy in free-jet expansion and quantum chemical methods. The complete potential energy surface originating from the flexibility of the aminic side chain has been calculated at the B3LYP/6-311++G** level of theory and the stable geometries were also characterized with MP2/6-311++G**. The rotational spectra show the presence of two of the predicted four stable conformers: the global minimum (I), in which the side chain's dihedral angle with the phenyl plane is almost perpendicular, is stabilized by an intramolecular hydrogen bond between the fluorine atom and one hydrogen of the aminic group; and a second conformer II (EII -EI ≈5 kJ mol(-1)) in which the dihedral angle is smaller and the amino group points towards the aromatic ortho hydrogen atom. This conformation is characterized by a tunneling motion between two equivalent positions of the amino group with respect to the phenyl plane, which splits the rotational transition. The ortho fluorination increases, with respect to benzylamine, the tunneling splitting of this motion by four orders of magnitude. The motion is analyzed with a one-dimensional flexible model, which allows estimation of the energy barrier for the transition state as approximately 8.0 kJ mol(-1).

Iron‐Catalyzed ortho Monoarylation of Benzamide Derivatives

Exclusive! The iron-catalyzed reaction between a secondary benzamide derivative and a diarylzinc reagent regioselectively substitutes the ortho-hydrogen atom of the benzamide with an aryl group. This method exclusively produces an ortho-monoarylated product, and none of the diarylated product.“Ph2Zn”=2PhMgBr+ZnCl2/tetramethylethylenediamine; dtbpy=4,4′-di-tert-butyl-2,2′-bipyridyl.

Monomer versus Alcohol Activation in the 4‐Dimethylaminopyridine‐Catalyzed Ring‐Opening Polymerization of Lactide and Lactic O‐Carboxylic Anhydride

Model reactions for the 4-dimethylaminopyridine (DMAP)-catalyzed ring-opening polymerization of lactide and the corresponding lactic O-carboxylic anhydride (lacOCA) have been studied computationally at the B3LYP/6-31G(d) level of theory. The solvent effect of dichloromethane was taken into account through PCM/SCRF single-point calculations at the B3LYP/6-31G(d) level of theory. In marked contrast with that predicted for the reaction of alcohols with acetic anhydride, the mechanism in which nucleophilic activation of the monomer involving acylpyridinium intermediates was found to be energetically less favorable than the base activation of the alcohol through hydrogen bonding. The concerted pathway for the ring-opening of lactide and lacOCA was shown to compete with the traditional stepwise mechanism involving tetrahedral intermediates. Furthermore, DMAP is proposed to act as a bifunctional catalyst through its basic nitrogen center and an acidic ortho-hydrogen atom.

Hydrogen–hydrogen bonding in biphenyl revisited

The evidence for the stabilizing nature of the H–H bonding in planar biphenyl is succinctly reviewed. The stabilizing nature of the H–H bonding is revealed through a comparison of the atomic energy of every atom in planar biphenyl with the same atom in the twisted equilibrium structure. It is shown that the barrier to rotation via the planar transition state is the net resultant of a stabilisation of the four ortho-hydrogen atoms (by 8 kcal/mol each), a stabilisation of the two para-carbon atoms (by 3 kcal/mol each) and by the dominant destabilisation of the two carbon atoms joining the two rings—the two junction carbon atoms—(by 22 kcal/mol each). The energetic stabilisation of the four ortho-hydrogen atoms is further shown to be in large proportion due to the formation of the hydrogen–hydrogen interatomic surface. Furthermore, neither the “bond order” between the two junction carbon atoms nor the total electron delocalisation between the two rings exhibit a significant change in going from the planar to the twisted equilibrium geometry. These findings are in contrast with the classical view of a balance between “steric non-bonded repulsion” and better electron delocalisation as a function of the twist dihedral angle. Similar conclusions have been recently reached by Pacios and Gomez through a study of the electrostatic potential at the position of the hydrogen nuclei.