Substituent cross-interaction effects on the electronic character of the C=N bridging group in substituted benzylidene anilines--models for molecular cores of mesogenic compounds. A 13C NMR study and comparison with theoretical results

13C NMR chemical shifts delta(C)(C=N) were measured in CDCl3 for a wide set of mesogenic molecule model compounds, viz. the substituted benzylidene anilines p-X-C6H4CH=NC6H4-p-Y (X = NO2, CN, CF3, F, Cl, H, Me, MeO, or NMe2; Y = NO2, CN, F, Cl, H, Me, MeO, or NMe2). The substituent dependence of delta(C)(C=N) was used as a tool to study electronic substituent effects on the azomethine unit. The benzylidene substituents X have a reverse effect on delta(C)(C=N): electron-withdrawing substituents cause shielding, while electron-donating ones behave oppositely, the inductive effects clearly predominating over the resonance effects. In contrast, the aniline substituents Y exert normal effects: electron-withdrawing substituents cause deshielding, while electron-donating ones cause shielding of the C=N carbon, the strengths of the inductive and resonance effects being closely similar. Additionally, the presence of a specific cross-interaction between X and Y could be verified. The electronic effects of the neighboring aromatic ring substituents systematically modify the sensitivity of the C=N group to the electronic effects of the benzylidene or aniline ring substituents. Electron-withdrawing substituents on the aniline ring decrease the sensitivity of delta(C)(C=N) to the substitution on the benzylidine ring, while electron-donating substituents have the opposite effect. In contrast, electron-withdrawing substituents on the benzylidene ring increase the sensitivity of delta(C)(C=N) to the substituent on the aniline ring, while electron-donating substituents act in the opposite way. These results can be rationalized in terms of the substituent-sensitive balance of the electron delocalization (mesomeric effects). The present NMR characteristics are discussed as regards the computational literature data. Valuable information has been obtained on the effects of the substituents on the molecular core of the mesogenic model compounds.     

Observation of inductive effects that cause a change in the rate-determining step for the conversion of rhenium azides to imido complexes

The cationic oxorhenium(V) complex [Re(O)(hoz)(2)(CH(3)CN)][B(C(6)F(5))(4)] [1; Hhoz = 2-(2'-hydroxyphenyl)-2-oxazoline] reacts with aryl azides (N(3)Ar) to give cationic cis-rhenium(VII) oxoimido complexes of the general formula [Re(O)(NAr)(hoz)(2)][B(C(6)F(5))(4)] [2a-2f; Ar = 4-methoxyphenyl, 4-methylphenyl, phenyl, 3-methoxyphenyl, 4-chlorophenyl, and 4-(trifluoromethyl)phenyl]. The kinetics of formation of 2 in CH(3)CN are first-order in both azide (N(3)Ar) and oxorhenium(V) complex 1, with second-order rate constants ranging from 3.5 × 10(-2) to 1.7 × 10(-1) M(-1) s(-1). A strong inductive effect is observed for electron-withdrawing substituents, leading to a negative Hammett reaction constant ρ = -1.3. However, electron-donating substituents on phenyl azide deviate significantly from this trend. Enthalpic barriers (ΔH(?)) determined by the Eyring-Polanyi equation are in the range 14-19 kcal mol(-1) for all aryl azides studied. However, electron-donating 4-methoxyphenyl azide exhibits a large negative entropy of activation, ΔS(?) = -21 cal mol(-1) K(-1), which is in sharp contrast to the near zero ΔS(?) observed for phenyl azide and 4-(trifluoromethyl)phenyl azide. The Hammett linear free-energy relationship and the activation parameters support a change in the mechanism between electron-withdrawing and electron-donating aryl azides. Density functional theory predicts that the aryl azides coordinate via N(α) and extrude N(2) directly. For the electron-withdrawing substituents, N(2) extrusion is rate-determining, while for the electron-donating substituents, the rate-determining step becomes the initial attack of the azide. The barriers for these two steps are inverted in their order with respect to the Hammett σ values; thus, the Hammett plot appears with a break in its slope.     

On the hydrolysis step in osmium catalyzed asymmetric dihydroxylations

In order to obtain information about the most important features that affect the efficiency of osmium catalyzed asymmetric dihydroxylation, a series of substituted styrenes have been studied by using a Hammett type approach as well as solvent kinetic isotope effects. A concave shaped Hammett plot with a minimum at X=H revealed a change in the mechanism going from electron-donating to electron-withdrawing substituents for both NaClO2 and K3[Fe(CN)6] asymmetric dihydroxylations. The Hammett plot together with solvent isotope effect results indicates that osmium (mono)glycolates of styrenes with electron-withdrawing substituents are hydrolyzed by a stepwise attack of the nucleophile to the electrophilic osmium-center and subsequent protonation of the alkaline intermediate. Osmium (mono)glycolates in dihydroxylation, using NaClO2 as the stoichiometric oxidant of styrenes with electron-donating substituents, are hydrolyzed by specific acid catalysis. The rate-limiting step is an A1 type process. Differences in the rho values in the Hammett plots for NaClO2 and K3[Fe(CN)6] asymmetric dihydroxylations indicate that in dihydroxylations with NaClO2 as the secondary oxidant, the reactive osmium(VI) mono(glycolate) is oxidized to osmium(VIII) mono(glycolate) prior to hydrolysis. The reaction rate was found to have an effect on the enantioselectivity in asymmetric dihydroxylation. If the hydrolysis step is slow enough, a competitive bis(glycolation) deteriorates the enantioselectivity in K3[Fe(CN)6] asymmetric dihydroxylations and even more so in NaClO2 asymmetric dihydroxylations.     

cyclo-CH2[Sn(CI2)CH2Si(Me2)]2O]: synthesis and complexation behaviour of a novel, cyclic, bidentate Lewis acid and its conversion into a tin-containing fluorosilane with intermolecular Si-F...Sn bridges

Acid-catalysed hydrolysis of [CH2[(Sn(Ph2)CH2Si(OiPr)Me2]2] followed by subsequent reaction with mercuric chloride in acetone afforded the novel silicon- and tin-containing eight-membered ring [cyclo-CH2[Sn(Cl2)CH2Si(Me2)]2O] in good yield, the crystal structure of which is reported. 119Sn NMR and X-ray studies indicate that [cyclo-CH2[Sn(Cl2)CH2Si(Me2)]2O] acts as a bidentate Lewis acid towards chloride ions exclusively forming the 1:1 complex [(Ph3P)2N]+[cyclo-CH2[Sn(Cl2)CH2Si(Me2)]2OCl]- upon addition of [(Ph3P)2N]+Cl- . Also reported are the synthesis and structure of [K(dibenzo[18]crown-6)]+[cyclo-CH2(Sn(Cl2)CH2Si(Me2)]2OF]-, the first completely characterised organostannate with a C2SnCl2F- substituent pattern. No ring-opening polymerisation could be achieved for [cyclo-CH2[Sn(Cl2)CH2Si(Me2)]2O] or for its perphenylated derivative [cyclo-CH2[Sn(Ph2)CH2Si(Me2)]2O]. The reaction of [cyclo-CH2[Sn(Cl2)CH2Si(Me2)]2O] with Me3O+BF4- gave the tin-containing fluorosilane [CH2[Sn(Cl2)CH2Si(F)Me2]2], in which the Si-F bond is activated by intermolecular Si-F...Sn interactions in the solid state.     

Synthesis of polycyclic aromatic iodides via ICl-induced intramolecular cyclization

The reaction of 2-(arylethynyl)biphenyls with ICl at -78 degrees C affords substituted polycyclic aromatic iodides in good to excellent yields. The aryl substituents can be either electron-donating or electron-withdrawing groups such as OMe, Me, CHO, CO(2)Et or NO(2) groups. This chemistry has been successfully extended to systems containing a variety of polycyclic and heterocyclic rings. [reaction: see text]     

Synthesis, structures, and stability of N-donor-stabilized N-silylphosphoranimine cations

A series of DMAP-stabilized (DMAP=4-dimethylaminopyridine) N-silylphosphoranimine cations [DMAPPR(2)==NSiMe(3)](+), bearing R=Cl ([8](+)), Me ([10 a](+)), Me/Ph ([10 b](+)), Ph ([10 c](+)), and OCH(2)CF(3) ([10 d](+)) substituents, have been synthesized from the reactions of the parent phosphoranimines Cl(3)P==NSiMe(3) (3) and XR(2)P==NSiMe(3) (X=Cl (9), Br (11); R=Me (9 a and 11 a), Me/Ph (9 b and 11 b), Ph (9 c and 11 c), and OCH(2)CF(3) (9 d and 11 d)) with DMAP and silver salts as halide abstractors. Reactions in the absence of silver salts yield the corresponding cations, with halide counterions. The stability of the salts is highly dependent on the phosphoranimine substituent and the nature of the counteranion, such that electron-withdrawing substituents and non-coordinating anions yield the most stable salts. X-ray structural determination of the cations reveal extremely short phosphoranimine P--N bond lengths for the cations [8](+) and [10 d](+) (1.47-1.49 A) in which electron-withdrawing substituents are present and a longer phosphoranimine P--N length for the cation [10 a](+) (1.53 A) in which electron-donating substituents are present. Very wide bond angles at nitrogen are observed for the salts containing the cation [10 d](+) (158-166 degrees ) and indicate significant sp hybridization at the nitrogen centre.     

Studies on the polarographic reduction of the azomethine bond in the products of beta-keto-esters coupled with aryldiazonium chlorides

The products of coupling beta-keto-esters with aryldiazonium chlorides have been studied polarographically and give a single well-defined 4-electron diffusion-controlled irreversible wave in the pH range 2.0-11.0. The effect of electron-donating and electron-withdrawing substituents and the correlation between the half-wave potential and Hammett substituent constant have been studied.     

Molybdenocene trihydride complexes: influence of a [Me2Si] ansa bridge on classical versus nonclassical nature, stability with respect to elimination of dihydrogen, and acidity

Experimental and computational studies on a series of cationic molybdenocene trihydride complexes, namely [Cp(2)MoH(3)]+, [(Cp(Bu)t)(2)MoH(3)]+, [Cp(2)MoH(3)]+, and ([Me(2)Si(C(5)Me(4))(2)]MoH(3))+, demonstrate that the most stable form for the ansa molybdenocene derivative is a nonclassical dihydrogen-hydride isomer, ([Me(2)Si(C(5)Me(4))(2)]Mo(eta(2)-H(2))(H))+, whereas the stable forms for the non-ansa complexes are classical trihydrides, [Cp(2)Mo(H)(3)]+, [(Cp(Bu)t)(2)Mo(H)(3)]+, and [Cp(2)Mo(H)(3)]+. In addition to altering the classical versus nonclassical nature of [Cp(2)MoH(3)]+ and ([Me(2)Si(C(5)Me(4))(2)]Mo(eta(2)-H(2))(H))+, the [Me(2)Si] ansa bridge also markedly influences the stability of the complex with respect to elimination of H(2) and dissociation of H+. Finally, computational studies on ([H(2)Si(C(5)H(4))(2)]MoH(2)D)+ and ([H(2)Si(C(5)H(4))(2)]MoHD(2))+ establish that deuterium exhibits a greater preference than hydrogen to occupy dihydrogen versus hydride sites.     

Substituent effect on the acid-promoted hydrolysis of 2-aryloxazolin-5-one: normal vs reverse

Computational investigations on the acid-promoted hydrolysis of 2-aryl-4,4-dimethyloxazolin-5-one (AMO) and its seven para- and meta-substituted derivatives (with electron-donating groups R = OH, OCH(3), CH(3) and electron-withdrawing groups R = Cl, m-Cl, CF(3), NO(2)) were presented by the density functional theory (B3LYP) method. Two types of reaction mechanism, N-path and O-path, are taken into account, in which the attacks by water molecules at the C2 and C5 are accelerated after the protonation on N3 and carbonyl oxygen atoms, respectively. Our computational results clearly manifest that the hydrolysis of AMOs has an obvious substituent effect at the para and meta positions of the benzene ring by correlating the barrier heights with the Hammett constants of substituents. Furthermore, the N-path shows a normal substituent effect, while the favorable O-path shows a reverse substituent effect. The observed reverse substituent effect in experiment actually springs from the reverse substituent effect of the O-path, not the N-path. The substituent effect of the N-path and O-path can be explained by the electrostatic potential at nuclei (EPN) values and Fukui function, respectively. Our theoretical data provided will allow for a better understanding of the acid-promoted hydrolysis mechanism and the observed reverse substituent effect of the AMOs, in nice agreement with the available experimental conclusion.     

钴卟啉的合成及其催化性能的研究

本文报导了钴5,15-二(对-取代苯基)八烷基卟啉的合成,并讨论了卟环上不同对苯取代基对叶琳与金属钴配位反应速度的影响规律:推电子基团使反应速度减慢,即 配位反应变的速率方程均为:(-d[H₂P])/dt=k表观[H₂P],取代基的影响效应符合Hammatt方程的线性关系。此外,不同取代基卟啉对中心金属钴催化性能的影响是:推电子基增加催化活性,即 。     

Remote substituent effects on allylic and benzylic bond dissociation energies. Effects on stabilization of parent molecules and radicals

The effect of remote substituents on bond dissociation energies (BDE) is examined by investigating allylic C-F and C-H BDE, as influenced by Y substituents in trans-YCH=CHCH2-F and trans-YCH=CHCH2-H. Theoretical calculations at the full G3 level model chemistry are reported. The interplay of stabilization energies of the parent molecules (MSE) and of the radicals formed by homolytic bond cleavage (RSE) and their effect on BDE are established. MSE values of allyl fluorides yield an excellent linear free energy relationship with the electron-donating or -withdrawing ability of Y and decrease by 4.2 kcal mol-1 from Y = (CH3)2N to O2N. RSE values do not follow a consistent pattern and are of the order of 1-2 kcal mol-1. A decrease of 4.1 kcal mol-1 is found in BDE[C-F] from Y = CH3O to NC. BDE[YCH=CHCH2-H] generally increases with decreasing electron-donating ability of Y for electron-donating groups and does not follow a consistent pattern with electron-withdrawing groups, the largest change being an increase of 3.6 kcal mol-1 from Y = (CH3)2N to CF3. The G3 results are an indicator of benzylic BDE in p-YC6H4CH2-F and p-YC6H4CH2-H, via the principle of vinylogy, demonstrated by correlating MSE of the allylic compounds with physical properties of their benzylic analogues.     

Vicinal stereocontrol during nucleophilic addition to arene chromium tricarbonyl complexes: formal synthesis of (+/-)-erythro Juvabione

[reaction: see text] Vicinal stereocontrol during nucleophilic addition of tert-butyl lithiopropionate to eta(6)-anisole chromium tricarbonyl complexes with differing para substituents has been studied. Excellent vicinal double stereoinduction (>99:1) was observed when the para substituent was Si(CH(3))(3), and this has been applied to a stereoselective formal synthesis of (+/-)-erythro Juvabione. Asymmetric synthesis by chiral auxiliary directed nucleophilic addition is also discussed.     

Amide conformational switching induced by protonation of aromatic substituent

[reaction: see text] Introduction of an electron-withdrawing group on the aromatic ring of N-methylacetanilide decreased the ratio of the cis conformer, and the ratio correlates well with the Hammett sigma values of the substituents. These steric properties can be applied to achieve amide conformational switching by protonation at the aromatic substituent of 4-[bis(dimethylamino)]-N-methylacetanilide or N-[p-(dimethylamino)phenyl]-N-phenylacetamide.     

Efficient modulation of hydrogen-bonding interactions by remote substituents

[structure: see text] A series of tetralactam macrocycles having different substituents were prepared, and their binding affinities for an adipamide guest were investigated in CDCl3 by 1H NMR titrations. The association constants strongly depend on the substituents, varying up to DeltaDeltaG = 3.4 kcal/mol; electron-donating substituents (OMe, NMe2) decrease the binding affinity, while electron-withdrawing groups (Cl, NO2) increase it. These large substituent effects have been rationalized by secondary repulsions and partial perturbations of intramolecular hydrogen bonds.     

Origin of the surprising enhancement of electrostatic energies by electron-donating substituents in substituted sandwich benzene dimers

A recent study of substituted face-to-face benzene dimers by Lewis and co-workers [J. Am. Chem. Soc. 2011, 133, 3854-3862] indicated a surprising enhancement of electrostatic interactions for both electron-withdrawing and electron-donating substituents. Here we demonstrate that charge penetration (an attractive electrostatic interaction arising from the overlap of the electron densities on the two monomers) is the cause of this counterintuitive effect. These charge penetration effects are significant at typical π-π interaction distances, and they are not easily described by multipole models. A simple measure of a substituent's electron-donating or electron-withdrawing character, such as the Hammett parameter σ(m), is unlikely to capture subtle charge penetration effects. Indeed, correlation of the relative total energies or relative electrostatic energies with ∑σ(m) breaks down for multiply substituted face-to-face benzene dimers.     

Dissociation dynamics of asymmetric alkynyl(aryl)iodonium radicals: an ab initio DRC approach to predict the surface functionalization selectivity

The dissociation process of neutral open-shell [4-F-(C(6)H(4))-I-C≡C-(CH(2))(4)-Cl] and [4-NO(2)-(C(6)H(4))-I-C≡C-(CH(2))(4)-Cl] asymmetric iodonium radicals was studied theoretically. Vertical electron affinities and DRC (dynamic reaction coordinate) results were obtained and compared with experimental evidence. In particular, the fluorine and nitro substituent groups were selected because of (i) their opposite electron-withdrawing/electron-donating effects and (ii) experimental evidence that the grafting ability, in terms of alkynyl/aryl grafting ratio, increases with decreasing electron-withdrawing nature of the para-position substituent on the phenyl ring. DRC results show that the dissociation dynamics of the iodine-alkynyl carbon bond, for the nitro-substituted iodonium, occurs on a longer time scale than that of the fluorine-substituted iodonium. This finding is in agreement with the overall experimental results.     

Multi-electron reduction from alkyl/hydride ligand combinations in U4+ complexes

The U4+ mixed alkyl hydride complex (C5Me5)U[mu-C5Me3(CH2)2](mu-H)2U(C5Me5)2, 1, which contains a cyclopentadienyl ligand with two metalated methylene substituents, can effect four, six, and eight-electron reductions in which the combination of the two H1- ligands and the [C5Me3(CH2)2]3- moiety delivers four electrons and forms (C5Me5)1-. The reaction is formally equivalent to an alkyl hydride reductive elimination, a transformation common with transition metals not previously observed with f element compounds. This type of alkyl hydride reduction reactivity is also observed with a combination of U4+ alkyl and hydride complexes, (C5Me5)2UMe2/[(C5Me5)2UH2]2, which reduces benzene to make [(C5Me5)2U]2(C6H6), a U3+ complex formally containing a (C6H6)2- ligand.     

Neighbouring group processes in the deamination of protonated phenylalanine derivatives

The gas-phase fragmentation of protonated phenylalanine and a series of its derivatives (tyrosine, 4-methylphenylalanine, 4-aminophenylalanine, 4-methoxyphenylalanine, 4-tert-butylphenylalanine, 4-fluorophenylalanine, 4-chlorophenylalanine, 4-bromophenylalanine, 4-iodophenylalanine, 4-cyanophenylalanine, 4-nitrophenylalanine, 3-fluorophenylalanine, and 3,4-dichlorophenylalanine) were examined using a combination of low energy CID in a quadrupole ion trap mass spectrometer as well as DFT calculations and RRKM modelling. In particular, the relationship between the electron-donating ability of the substituent and the competitive losses of H2O + CO and NH3 were explored through the application of the Hammett equation. It was found that electron-donating substituents promote the loss of NH3, while electron-withdrawing substituents suppress the loss of NH3 and favour the H2O + CO loss fragmentation channel instead. These observations are consistent with a neighbouring group pathway operating for the loss of NH3. Molecular orbital calculation (at the B3LYP/6-31+G(d,p) level of theory) were also performed for a range of derivatives to compare the relative transition state energy barriers for three competing mechanisms: (i) the combined loss of H2O + CO, which is triggered by an initial intramolecular proton transfer from the ammonium group to hydroxyl OH, followed by the combined loss of H2O and CO to form an immonium ion; (ii) loss of NH3 via an aryl assisted neighbouring group pathway to yield a phenonium ion; (iii) loss of NH3 via a 1,2-hydride migration process, which results in the formation of a benzyl cation. The relative energy barriers for H2O + CO loss remain nearly constant, while that for both NH3 pathways increase as the substituent moves from electron-donating to electron-withdrawing. The relative transition state energy for loss of NH3 via the aryl assisted neighbouring group pathway is always lower than that of the 1,2-hydride migration process. RRKM modelling of the DFT predicted barrier heights suggest that the rate constants for H2O + CO loss are insensitive to the substituent on the ring, while the NH3 loss channels are greatly affected by the substituent. These theoretical results are consistent with the experimental observation of the relative yields of the competing fragmentation channels. Finally, comparisons with published gas phase and condensed phase studies on related systems are made.     

Evidence of substituent-induced electronic interplay. Effect of the remote aromatic ring substituent of phenyl benzoates on the sensitivity of the carbonyl unit to electronic effects of phenyl or benzoyl ring substituents

Carbonyl carbon (13)C NMR chemical shifts delta(C)(C[double bond]O) measured in this work for a wide set of substituted phenyl benzoates p-Y-C(6)H(4)CO(2)C(6)H(4)-p-X (X = NO(2), CN, Cl, Br, H, Me, or MeO; Y = NO(2), Cl, H, Me, MeO, or NMe(2) ) have been used as a tool to study substituent effects on the carbonyl unit. The goal of the work was to study the cross-interaction between X and Y in that respect. Both the phenyl substituents X and the benzoyl substituents Y have a reverse effect on delta(C)(C[double bond]O). Electron-withdrawing substituents cause shielding while electron-donating ones have an opposite influence, with both inductive and resonance effects being significant. The presence of cross-interaction between X and Y could be clearly verified. Electronic effects of the remote aromatic ring substituents systematically modify the sensitivity of the C[double bond]O group to the electronic effects of the phenyl or benzoyl ring substituents. Electron-withdrawing substituents in one ring decrease the sensitivity of delta(C)(C[double bond]O) to the substitution of another ring, while electron-donating substituents inversely affect the sensitivity. It is suggested that the results can be explained by substituent-sensitive balance of the contributions of different resonance structures (electron delocalization, Scheme 1).     

Alkyl-eta2-alkene niobocene and tantalocene complexes with the allyldimethylsilyl-eta5-cyclopentadienyl ligand: synthesis, NMR studies and DFT calculations

Group 5 metal complexes [M(eta5-C5H5)[eta5-C5H4SiMe2(CH2-eta]2-CH=CH2)]X] (M = Nb, X = Me, CH2Ph, CH2SiMe3; M = Ta, X = Me, CH2Ph) and [Ta(eta5-C5Me5)[eta5-C5H4SiMe2(CH2-eta2-CH=CH2)]X] (X = Cl, Me, CH2Ph, CH2SiMe3) containing a chelating alkene ligand tethered to a cyclopentadienyl ring have been synthesized in high yields by reduction with Na/Hg (X = Cl) and alkylation with reductive elimination (X = alkyl) of the corresponding metal(iv) dichlorides [M(eta5-Cp)[eta5-C5H4SiMe2(CH2CH=CH2)]Cl2] (Cp = C5H5, M = Nb, Ta, Cp = C5Me5, M = Ta). These chloro- and alkyl-alkene coordinated complexes react with CO and isocyanides [CNtBu, CN(2,6-Me2C6H3)] to give the ligand-substituted metal(III) compounds [M(eta5-Cp)[eta5-C5H4SiMe2(CH2CH=CH2)]XL] (X = Cl, Me, CH2Ph, CH2SiMe3). Reaction of the chloro-alkene tantalum complex with LiNHtBu results in formation of the imido hydride derivative [Ta(eta5-C5Me5)[eta5-C5H4SiMe2(CH2CH=CH2)]H(NtBu)]. NMR studies for all of the new compounds and DFT calculations for the alkene-coordinated metal complexes are compared with those known for related group 4 metal cations.