Multicomponent assembly of anionic and neutral decanuclear copper(II) phosphonate cages

A multicomponent synthetic strategy involving copper(II) ions, tert-butylphosphonic acid (t-BuPO(3)H(2)) and 3-substituted pyrazole ligands has been adopted for the synthesis of soluble molecular copper(II) phosphonates. The use of six different 3-substituted pyrazoles, 3-R-PzH [R = H, Me, CF(3), Ph, 2-pyridyl (2-Py), and 2-methoxyphenyl (2-MeO-C(6)H(4))] as ancillary ligands afforded nine different decanuclear cages, [Cu(5)(μ(3)-OH)(2)(O(3)P-t-Bu)(3)(3-R-Pz)(2)(X)(2)](2)·(Y) where R = H, X = t-BuPO(3)H, and Y = (Et(3)NH(+))(4)(solvent) (1); R = Me, X = 3-MePzH, and Y = solvent (2); R = Me, X = t-BuPO(3)H, and Y = (Et(3)NH(+))(4)(solvent) (3); R = CF(3), X = t-BuPO(3)H, and Y = (Et(3)NH(+))(4)(solvent) (4); R = Ph, X = 3-PhPzH, and Y = solvent (5); R = 2-Py, X = 0.5 MeOH, and Y = solvent (6); R = 2-Py, X = none, and Y = solvent (7); R = 2-Py, X = H(2)O, and Y = (Et(3)NH(+)·PF(6)(-))(2)(solvent) (8); R = 2-MeO-C(6)H(4), X = MeOH or 0.5:0.5 MeOH/H(2)O, and Y = solvent (9). Compounds 1-6, 8, and 9 were isolated using a direct synthetic method which involves the reaction of copper(II) salts and the ligands, while 7 was obtained from an indirect route involving the reaction of preformed copper-pyridylpyrazolate precursor complexes and t-BuPO(3)H(2). The decametallic compounds 1-9 possess a butterfly shaped core. The core of the cages 1, 3, and 4 are tetraanionic and contain more phosphonates than pyrazole ligands, while the other cages are neutral and contain more pyrazoles than phosphonate ligands. Compounds 1-6 have been studied by electrospray ionization-high-resolution mass spectrometry (ESI-HRMS). The decanuclear cage 6 was shown to be a good plasmid modifier.     

Capillary zone electrophoresis of proteins in semidilute polymer solutions: inter- and intra-polymer predictability of size-dependent retardation.

The retardation of three "spherical" proteins with Stokes' radii of 2.0, 2.4, and 3.0 nm (35-104 kDa) was studied in capillary zone electrophoresis (CZE), using semidilute solutions of polyethylene glycol (PEG), linear polyacrylamide (PA), and polyvinyl alcohol (PVA). The purpose was to test the models predicting that the ratio of particle radius, R, to the mesh size of polymer network (the correlation or screening length of a semidilute polymer solution), xi, directly governs the size-dependent retardation in the form: mu/muo = exp (-R/xi). Here xi = kc-0.75, where c is polymer concentration and the numerical factor kcan be calculated based on polymer molecular weight. In application to polymers in a "good solvent" (PA and PEG in the aqueous buffer) and to proteins of 2.4 and 3.0 nm radius, that relation between relative mobility and R/xi was found to be obeyed for PA, while for PEG the value of k derived from retardation experiments significantly exceeded that which was theoretically calculated. Thus, the retardation appears to be polymer-specific, rather than universal, even for polymers in a "good solvent". It is suggested that, in that case, retardation of proteins of R > 2 nm be quantitatively described in the form mu/muo = exp[-p(R/xi], where p is a parameter depending on monomer type and/or polymer polydispersity. For PVA, the logarithm of mu/muo was found to be linearly related to c (in line with the prediction that the aqueous buffer is a "poor solvent" for this polymer) and to be near-independent of R.     

SAMPL5: 3D-RISM partition coefficient calculations with partial molar volume corrections and solute conformational sampling

Implicit solvent methods for classical molecular modeling are frequently used to provide fast, physics-based hydration free energies of macromolecules. Less commonly considered is the transferability of these methods to other solvents. The Statistical Assessment of Modeling of Proteins and Ligands 5 (SAMPL5) distribution coefficient dataset and the accompanying explicit solvent partition coefficient reference calculations provide a direct test of solvent model transferability. Here we use the 3D reference interaction site model (3D-RISM) statistical-mechanical solvation theory, with a well tested water model and a new united atom cyclohexane model, to calculate partition coefficients for the SAMPL5 dataset. The cyclohexane model performed well in training and testing (\(R=0.98\) for amino acid neutral side chain analogues) but only if a parameterized solvation free energy correction was used. In contrast, the same protocol, using single solute conformations, performed poorly on the SAMPL5 dataset, obtaining \(R=0.73\) compared to the reference partition coefficients, likely due to the much larger solute sizes. Including solute conformational sampling through molecular dynamics coupled with 3D-RISM (MD/3D-RISM) improved agreement with the reference calculation to \(R=0.93\). Since our initial calculations only considered partition coefficients and not distribution coefficients, solute sampling provided little benefit comparing against experiment, where ionized and tautomer states are more important. Applying a simple \(\hbox {p}K_{\text {a}}\) correction improved agreement with experiment from \(R=0.54\) to \(R=0.66\), despite a small number of outliers. Better agreement is possible by accounting for tautomers and improving the ionization correction.     

Synthetic and mechanistic studies of metal-free transfer hydrogenations applying polarized olefins as hydrogen acceptors and amine borane adducts as hydrogen donors

Metal-free transfer hydrogenation of polarized olefins (RR'C=CEE': R, R' = H or organyl, E, E' = CN or CO(2)Me) using amine borane adducts RR'NH-BH(3) (R = R' = H, AB; R = Me, R' = H, MAB; R = (t)Bu, R' = H, tBAB; R = R' = Me, DMAB) as hydrogen donors, were studied by means of in situ NMR spectroscopy. Deuterium kinetic isotope effects and the traced hydroboration intermediate revealed that the double H transfer process occurred regio-specifically in two steps with hydride before proton transfer characteristics. Studies on substituent effects and Hammett correlation indicated that the rate determining step of the H(N) transfer is in agreement with a concerted transition state. The very reactive intermediate [NH(2)=BH(2)] generated from AB was trapped by addition of cyclohexene into the reaction mixture forming Cy(2)BNH(2). The final product borazine (BHNH)(3) is assumed to be formed by dehydrocoupling of [NH(2)=BH(2)] or its solvent stabilized derivative [NH(2)=BH(2)]-(solvent), rather than by dehydrogenation of cyclotriborazane (BH(2)NH(2))(3) which is the trimerization product of [NH(2)=BH(2)].     

Migratory aptitudes of simple alkyl groups in the anionotropic rearrangement of quaternary chloromethyl borate species: a combined experimental and theoretical investigation

A combined experimental and theoretical investigation at the DFT and MP2 levels on the boron-to-carbon 1,2-shift in "ate species", coming from the quaternization of boranes (A) and boronate (B), is reported. To discuss the different migratory aptitudes of various alkyl groups, we have examined the migration of primary (R = Me, Et), secondary (R = i-Pr), and tertiary (R = t-Bu) alkyl groups. The effect of the counterion Li(+) and of the solvent (polarized continuous model (PCM) method) has been considered. The following results are relevant: (a) in all cases, the reaction proceeds via a concerted-type mechanism which explains the retention of configuration at the migrating group and the inversion at the migration terminus experimentally observed. (b) The trend of the migration barriers along the direction primary --> secondary --> tertiary alkyl group observed in "ate" species A is reversed in boronate species B, in agreement with the experimental evidences. (c) A simple theoretical model is proposed where the barrier trend is the result of a delicate interplay between two opposite factors: (1) a "steric effect", which favors the most sterically demanding migrating groups, and (2) a "charge effect" associated with the partial carbanionic nature of the migrating carbon atom and which favors the less substituted migrating carbons.     

Modelling neurotransmitter functions: a laser spectroscopic study of (1S,2S)-N-methyl pseudoephedrine and its complexes with achiral and chiral molecules

Wavelength and mass resolved resonance-enhanced two photon ionization (R2PI) excitation spectra of (1S,2S)-N-methyl pseudoephedrine (MPE) and its complexes with several achiral and chiral solvent molecules, including water (W), methyl (R)-lactate (L(R)), methyl (S)-lactate (L(S)), (R)-2-butanol (B(R)), and (S)-2-butanol (B(S)), have been recorded after a supersonic molecular beam expansion and examined in the light of ab initio calculations. The spectral patterns of the selected complexes have been interpreted in terms of the specific hydrogen-bond interactions operating in the diastereomeric complexes, whose nature in turn depends on the structure and the configuration of the solvent molecule. The obtained results confirm the view that a representative neurotransmitter molecule, like MPE, "communicates" with the enantiomers of a chiral substrate through different, specific interactions. These findings can be regarded as a further contribution to modelling neurotransmitter functions in biological systems.     

Pyridine N-alkylation by lithium, magnesium, and zinc alkyl reagents: synthetic, structural, and mechanistic studies on the bis(imino)pyridine system

The 2,6-bis(alpha-iminoalkyl)pyridines 2,6-[ArNC(CR(3))](2)C(5)H(3)N [R = H, D; Ar = 2,6-i-Pr(2)C(6)H(3) (DIPP), 2,6-Me(2)C(6)H(3) (DMP)] react with MeLi in Et(2)O to give a binary mixture of products: the pyridine N-methylated species 2,6-[ArNC(CR(3))](2)C(5)H(3)N(Me)Li(OEt(2)) and the deprotonated/dedeuterated species 2-[ArNC(CR(3))],6-[ArNC(=CR(2))]C(5)H(3)NLi(OEt(2)). For R = D, the product ratio is 2:1 in favor of the N-methylated product, while, for R = H, the deprotonated product is favored by 5:1, increasing to 8:1 in toluene solvent. Warming solutions of the N-methylated species leads to clean conversion to the thermodynamically preferred deprotonated species. Crossover experiments show that MeLi is re-formed and dissociates from the terdentate ligand before deprotonating the ketimine methyl unit. For MgR(2) (R = Et, i-Pr) and ZnR(2) (R = Et) reagents, N-alkylation products are formed exclusively, but derivatives containing bulky aryl substituents are found to undergo further rearrangement to 2-alkylated species, arising by migration of the alkyl group of the N-alkyl moiety to the adjacent ring carbon atom. The reversibility of the N-alkylation process has been probed using deuterio-labeled Mg alkyl reagents and mixed alkyl zinc species. A cationic zinc derivative is shown to undergo "reverse" alkyl migration, from the heterocycle nitrogen atom to the zinc center. EPR spectroscopy reveals a paramagnetic intermediate in which the unpaired electron is delocalized over the heterocycle and di-imine moieties of the ligand, indicating that the N-alkylation reactions proceed via single electron-transfer processes.     

Acyl iodides in organic synthesis: X. Reactions with triorganylsilanes and triphenylgermane

Reaction of acyl iodides RCOI (R = Me, Ph) with triorganylsilanes R′₂R″SiH in toluene gives 50–60% of the corresponding triorganyliodosilanes R′₂R″SiI. Triethylsilane reacts with the same acyl iodides under solvent-free conditions to afford the corresponding aldehyde and triethyliodosilane as primary products. Triethyliodosilane undergoes subsequent transformations into hexaethyldisiloxane and triethyl(acyloxy)silane Et₃SiOCOR (R = Me, Ph). Reactions of acyl iodides RCOI (R = Me, Ph) with triphenylgermane in the absence of a solvent lead to formation of iodo(triphenyl)germane in more than 90% yield.     

Reaction of primary aromatic amines with alkyl carbonates over NaY faujasite: a convenient and selective access to mono-N-alkyl anilines.

At atmospheric pressure and at 130-160 degrees C, primary aromatic amines (p-XC6H4NH2, X = H, Cl, NO2) are mono-N-alkylated in a single step, with symmetrical and asymmetrical dialkyl carbonates [ROCOOR', R = Me, R' = MeO(CH2)2O(CH2)2; R = R' = Et; R = R' = benzyl; R = R' = allyl; R = Et, R' = MeO(CH2)2O(CH2)2], in the presence of a commercially available NaY faujasite. No solvents are required. Mono-N-alkyl anilines are obtained with a very high selectivity (90-97%), in good to excellent yields (68-94%), on a preparative scale. In the presence of triglyme as a solvent, the mono-N-alkyl selectivity is independent of concentration and polarity factors. The reaction probably takes place within the polar zeolite cavities, and through the combined effect of the dual acid-base properties of the catalyst.     

Equilibrium studies on Schiff base complexes of copper(II) and nickel(II): Ligand substitution and formation of mixed ligand complexes

Summary Vis spectrophotometry has been used to study various ligand substitution equilibria (1) and (2) involving four-coordinate copper(II) and nickel(II) his chelate complexes in methanol, propan-2-ol and toluene. MA₂ + HB MAB+HA, K₁ (1) MAB + HB MB₂ + HA, K₂ (2) The Schiff base ligands, HA and HB, which are monobasic and bidentate, represent salicylaldimine type N,O-ligands ( HSA=NR) (1) or pyrrole-2-aldimine type N,N-ligands ( HPA=NR) (2) with different branching at the - or (3-carbon of the organic group R. For both types of ligand the relative thermodynamic stability of their copper and nickel complexes is governed mainly by the steric demands of R, which determine the degree of tetrahedral distortion. The order of stability as given by = K₁ K₂ is: t-Bu < neo-Pent < i-Pr < i-Bu < Et < n-Pr. The K₁/K₂ ratio is strongly solvent dependent in the sense that the mixed ligand species MAB is stabilised in toluene relative to methanol. Such a solvent effect is not observed for . The MAB complexes could not be isolated. The vis spectrum of the mixed ligand species Ni(SA=NiPr, SA=NEt) was calculated by computer fitting of the experimental data.     

Analyses of in-cage singlet radical-pair motions from irradiations of 1-naphthyl (R)-1-phenylethyl ether and 1-naphthyl -2-phenylpropanoate in n-alkanes

[reaction: see text] The regio- and stereochemistries of photo-Claisen reactions of 1-naphthyl (R)-1-phenylethyl ether ((R)-2), in combination with photo-Fries and photo-Claisen-type reactions of 1-naphthyl (R)-2-phenylpropanoate ((R)-1), have been investigated in n-alkanes of different viscosities and at several temperatures. Analyses of the results provide detailed information about the in-cage motions of the singlet prochiral 1-naphthoxy/1-phenylethyl radical pairs (radical-pair B) that are formed directly from (R)-2 and indirectly from (R)-1 via decarbonylation of singlet chiral 1-naphthoxy/2-phenylpropanoyl radical pairs (radical-pair A). In hexane at 23 degrees C, the photo-Claisen products from irradiations of (R)-2 retain up to 31% enantiomeric excess (ee), but the ees of the same photoproducts from (R)-1 are near 0%. This disparity is attributed to differences between the initial orientations of the constituent radicals of radical-pair B at the moment of their "birth". The regio- and stereoselectivities reach plateau values as the solvent viscosity increases, indicating that the relationships between the rates of radical-radical bond formation and either translational or tumbling motions within a solvent cage reach an asymptotic limit. Detailed analyses are presented of the various motions that are in competition within a solvent cage during the very short lifetimes of the radical pairs. The data, in toto, present interesting insights into how radical pairs move during short periods and over short distances when their solvent cages have walls of varying flexibility.     

Synthesis and Structural Characterization of a New Hydrogen-bonded Polyrotaxane of [Co(H2O)6]2+ with 1,1''-(Propane-1,3-diyl)dipyridinium-4-carboxylate

A new double betaine 1,1 '-(propane-1,3-diyl)dipyridinium-4-carboxylate L has been synthesized. Reaction of 1,1'-(propane-1,3-diyl)dipyridinium-4-carboxylate tetrahydrate 1 With Co(ClO4)2·6H2O leads to the formation of a new Co(Ⅱ) coordination compound,namely [Co(H2O)6]·2H2O·2L·2ClO4 2.The crystal structures of 1 and 2 have been determined by single-crystal X-ray diffraction method. Crystal data for 1: monoclinic, space group C2/c, a =18.945(4), b = 7.700(2), c = 11.888(2) (A), β = 101.67(3)°, V = 1698.3(6) (A)3, Z = 4, F(000) =760.0, Dc = 1.402 g/cm3, the final R = 0.0607 and wR = 0.1607 for 950 observed reflections (I ＞2σ(I)); and those for 2: monoclinic, space group P21/c, a = 17.982(1), b = 15.879 (1), c = 7.0716(5) (A), β = 100.675(1)°, V = 1984.3(3) (A)3, Z = 4, F(000) = 1010.0, Dc = 1.631 g/cm3, the final R = 0.0316 and wR = 0.0896 for 3784 observed reflections (I ＞2σ(I)). Crystal structure analysis indicates that in 1, molecules of L in a "V-shaped" conformation are linked to chains sustained by O-H…O hydrogen bonds between carboxylate groups and solvent water molecules.The chains are joined by O-H…O and C-H…O hydrogen bonds to further expand into a three-dimensional structure. For 2, molecules of L in a "Z-shaped" conformation are linked by hydrogen bonds between carboxylate groups and aqua ligands to form a chain of loops running down the b axis. The (2D→2D) polythreading in compound 2 represents the mode of parallel interpenetration of 2D sheets, having polyrotaxane character.     

十八烷基取代吲哚啉螺吡喃衍生物的合成及光谱性质

本文合成了十种新的十八烷基取代的吲哚螺吡喃衍生物,拟进一步研究这类化合物在LB膜中的光致变色性质.     

Synthesis and Crystal Structure of [K(18-Crown-6)]3P11·2en

The undecaphosphide anion in ethylenediamine/18-crown-6 solution is isolated as [K(18-crown-6)]3P11·2en 1.The crystal structure has been determined by single-crystal X-ray diffraction.The crystal is of monoclinic,space group P21 with a = 13.4731(7),b = 16.1123(6),c =16.5117(9) (A),β = 112.133(2)°,V = 3320.3(3) (A)3,Dc= 1.371 g/cm3,C40H88K3N4O18P11,Mr=1371.11,F(000) = 1444,μ = 0.532 mm-1,Z = 2,R = 0.0544 and wR = 0.1326 for 11423 observed reflections (I ＞ 2σ(I)).The "naked" P11 3- ion is stabilized by two partly sequestered potassiums.The third partly sequestered potassium does not interact with the P atom (P(6)) but with two solvent molecules.     

Solvent and structural effects on ultrafast chelation dynamics of arene chromium tricarbonyl sulfide derivatives

The chelation dynamics of three new [Cr{eta6-C6H5C(O)R}(CO)3] complexes, 1 [R = CH2(SCH3)], 2 [R = CH(SCH3)2], and 3 [R = C(SCH3)3], has been investigated on the picosecond to millisecond time scales by UV pump/IR probe transient absorption spectroscopy following photodissociation of CO in room temperature n-heptane, tetrahydrofuran (THF), and acetonitrile. In n-heptane, UV irradiation of 1, 2, or 3 dissociates CO to initially yield a Cr-S chelate (in which the pendant sulfide moiety is coordinated to the metal center) and a transient Cr-heptane solvate in approximately 1:2, 1:2, and 2:1 ratios, respectively. The Cr-heptane solvate is unstable and converts to the Cr-S chelate within 30 ns in each case. Irradiation of 2 or 3 in THF yields both the Cr-S chelate and Cr-THF solvate in approximately 1:3 and 1:1 ratios, respectively. The Cr-THF solvate converts to the Cr-S chelate on the second or longer time scale. All three complexes appear to yield the Cr-NCCH3 solvate exclusively within 50 ps following irradiation in acetonitrile. The solvent effect on chelation is in striking contrast to that previously reported for the analogous RCpMn(CO)3 derivatives, 4-6. In acetonitrile, only chelation is observed for the Mn series and only solvent coordination is observed for the Cr series, but in heptane both chelation and solvent coordination are observed in both series.     

Selektive Darstellung zweifach diorganophosphido-verbrückter Metallatetrahedrane [Re2(MPR3)2(μ-PR2)2(CO)6] mit Re2M2-Metallgerüst (M = Au,Ag)

Selective Preparation of Twofold Diorganophosphido-bridged Metallatetrahedranes [Re₂(MPR₃)₂(μ-PR₂)₂(CO)₆] with Re₂M₂ Metal Core (M = Au, Ag) The reaction of the in situ prepared salt Li[Re₂(AuPR)(μ-PR₂)(CO)₇Cl] (R = R′ = Cy ( 1 a ), R = Cy, R′ = Ph ( 1 b ), R = Ph, R′ = Cy ( 1 c ), R = Ph, R′ = Et ( 1 d ), R = Ph, R′ = Ph ( 1 e )) with one equivalent HPR in methanolic solution at room temperature yields the neutral cluster complexes [Re₂(AuPR)(μ-PR₂)(CO)₇(ax-HPR) (R = R′ = R″ = Cy ( 2 a ), Ph ( 2 b ), R = R′ = Cy, R″ = Et ( 2 c ), R = Cy, R′ = R″ = Ph ( 2 d ), R = Cy, R′ = Ph, R″ = Et ( 2 e ), R = R″ = Ph, R′ = Et ( 2 f ), R = Ph, R′ = Cy, R″ = Et (2 g)). Photochemically induced these complexes react in the presence of the organic base DBU in THF solution to give the doubly phosphido bridged anions Li[Re₂(AuPR)(μ-PR₂)(μ-PR)(CO)₆], which were characterized as salts PPh₄[Re₂(AuPR)(μ-PR₂)(μ-PR)(CO)₆] (R = R′ = R″ = Ph ( 3 a ), R = R′ = Ph, R″ = Cy ( 3 b ), R = Ph, R′ = Cy, R″ = Et ( 3 c ), R = R″ = Ph, R′ = Et ( 3 d )). These precursor complexes 3 then react with one equivalent of ClMPR (M = Au, Ag) to doubly phosphido bridged metallatetrahedranes [Re₂(MPR₃)₂(μ-PR₂)(μ-PR)(CO)₆] (M = Au, R = R′ = R″ = Ph ( 4 a ), M = Au, R′ = Et, R = R″ = Ph ( 4 b ), M = Au, R = R′ = Ph, R″ = Cy ( 4 c ), M = Au, R = Cy, R′ = Ph, R″ = Et ( 4 d ), M = Ag, R = R′ = R″ = Ph ( 4 e )). All isolated cluster complexes were characterized and identified by the following analytical methods: NMR- (¹H, ³¹P) and ν(CO) IR-spectroscopy and, additionally, complexes 2 b , 4 a and 4 e by X-ray structure analysis.     

Transfer ribonucleic acids

Transfer ribonucleic acids (tRNAs)

1 Abbreviations used according to IUPAC-IUB convention: tRNA = transfer ribonucleic acid; tRNA_yeast = mixture of tRNAs from yeast; tRNA^Phe = phenylalanine specific tRNA; Phe-tRNA = tRNA esterified (“charged”) with Phe; mRNA = messenger RNA; DNA = deoxyribonucleic acid; U = uridine; A = adenosine; C = cytidine; G = guanosine; pA = 5′-adenylic acid; Ap or A- = 3′-adenylic acid; m²′G = 2′-O-methyl guanosine; m⁷G = 7-methyl guanosine; mG = N(2)-dimethyl guanosine; other methylated nucleosides are abbreviated analogously; abbreviations of other odd nucleosides are given with Fig. 2; p or – signifies phosphate; RNase = ribonuclease; DEAE = diethylaminoethyl; fMet = N-formayl methionine.

occur in all living organisms. In biological protein synthesis they accept activated amino acids which are then transferred to growing peptide chains. With molecular weights lying between 25000 and 30000, tRNAs are easily within the reach of today's physical, chemical, and biochemical methods. The primary structures of several tRNAs as well as some relationships between structure and function have been elucidated. Three-dimensional structure, specificity, and mechanism of action are the subjects of present research efforts.     

Influence of the boron atom on the solvatochromic properties of 4-nitroaniline-functionalized boronate esters

Para-nitroaniline derivatives with peripheral 1,2- and 1,3-diol functionalities [O(2)N-C(6)H(4)-NR(1)-CH(2)CH(OH)CH(2)OH; O(2)N-C(6)H(4)-NR(1)-CH(CH(2)OH)(2); R(1) = -H, -CH(3)] covalently bonded to the amino group are esterified with various para-substituted phenylboronic acids [R(2)-C(6)H(4)-B(OH)(2); R(2) = -OCH(3), -CH(3), -H, -Br, -CHO, -NO(2), -B(OH)(2)], and the solvatochromic properties of these esters are investigated in 33 solvents of different polarity. To interpret the solvent effects, the established linear solvation energy (LSE) multiparameter equations of Kamlet-Taft and the improved Catalán scales are used. Although the boron atom is separated by two or three sp(3)-hybridized carbon atoms from the actual chromophore, solvation effects have a significant positive solvatochromic effect on the nitroaniline unit (R(1) = -CH(3)) as result of the solvent acting as a donor at the boron atom. The influence of the substituent R(2) on the coefficient b of the LSE relationship according to Kamlet-Taft and Catalán, which reflects the quantitative influence of the hydrogen-bonding acceptor or the electron-pair donor capacity of the solvent on the position of the UV-vis absorption maximum, can be determined via a linear Hammett relationship [b = f(σ(p))]. The interpretation of the effects is based on the electronic influence of the solvated boronic acid ester unit on the 4-nitroaniline group, predominantly through inductive interactions.     

Computational investigations on the general reaction profile and diastereoselectivity in sulfur ylide promoted aziridination

Mechanism and diastereoselectivity of sulfur ylide promoted aziridination reactions were studied by density functional theory with inclusion of solvent effects through the continuum solvation model. The general reaction pathway was modeled for the addition of substituted sulfur ylides (Me(2)S(+)CH(-)R) to an aldimine ((E)-methyl ethylidenecarbamate, MeHC=NCO(2)Me). The nature of the substituents on the ylidic carbon atom substantially affects the reaction profile. The stabilized (R=COMe) and semistabilized (R=Ph) ylides follow a cisoid addition mode leading to trans aziridines via anti betaine intermediates. The simplest model ylide (unstabilized, R=H) underwent cisoid addition in a similar fashion. In the case of stabilized ylides product diastereoselectivity is controlled by the barriers of the elimination step leading to the 2,3-trans aziridine, whereas it is decided in the addition step in the case of semistabilized ylides. The importance of steric and electronic factors in diastereoselective addition (2 and 5) and elimination (5) transition states was established. Comparison of results obtained with the gas-phase optimized geometries and with the fully optimized solvent-phase geometries reveals that the inclusion of solvent effects does not bring about any dramatic changes in the reaction profiles for all three kinds of ylides. In particular, diastereoselectivity for both kinds of ylides was found to be nearly the same in both these approaches.     

Substituent effects on carbocation stability: the pK(R) for p-quinone methide

A value of k(H) = 1.5 x 10(-)(3) M(-)(1) s(-)(1) has been determined for the generation of simple p-quinone methide by the acid-catalyzed cleavage of 4-hydroxybenzyl alcohol in water at 25 degrees C and I = 1.0 (NaClO(4)). This was combined with k(s) = 5.8 x 10(6) s(-)(1) for the reverse addition of solvent water to the 4-hydroxybenzyl carbocation [J. Am. Chem. Soc. 2002, 124, 6349-6356] to give pK(R) = -9.6 as the Lewis acidity constant of O-protonated p-quinone methide. Values of pK(R) = 2.3 for the Lewis acidity constant of neutral p-quinone methide and pK(add) = -7.6 for the overall addition of solvent water to p-quinone methide to form 4-hydroxybenzyl alcohol are also reported. The thermodynamic driving force for transfer of the elements of water from formaldehyde hydrate to p-quinone methide to form formaldehyde and p-(hydroxymethyl)phenol (4-hydroxybenzyl alcohol) is determined as 6 kcal/mol. This relatively small driving force represents the balance between the much stronger chemical bonds to oxygen at the reactant formaldehyde hydrate than at the product p-(hydroxymethyl)phenol and the large stabilization of product arising from the aromatization that accompanies solvent addition to p-quinone methide. The Marcus intrinsic barrier for nucleophilic addition of solvent water to the "extended" carbonyl group at p-quinone methide is estimated to be 4.5 kcal/mol larger than that for the addition of water to the simple carbonyl group of formaldehyde. O-Alkylation of p-quinone methide to give the 4-methoxybenzyl carbocation and of formaldehyde to give a simple oxocarbenium ion results in very little change in the relative Marcus intrinsic barriers for the addition of solvent water to these electrophiles.