Cu(II)-catalyzed enantioselective aldol condensation between malonic acid hemithioesters and aldehydes

A mild, decarboxylative, aldol-type addition of malonic acid hemithioesters to aldehydes has been shown to occur with up to 39% enantioselectivity when the reaction was carried out in the presence of catalytic amounts of a Cu(II) salt, an enantiopure, tartaric acid-derived bis-benzimidazole and an achiral base.     

Polar substituent effects in the bicyclo[1.1.1]pentane ring system: acidities of 3-substituted bicyclo[1.1.1]pentane-1-carboxylic acids

Experimental gas-phase acidities are reported for a series of 3-substituted (X) bicyclo [1.1.1]pent-1-yl carboxylic acids (1, Y = COOH). A comparison with available calculated data (MP2/6-311++G**// B3LYP/6-311+G**) reveals good agreement. The relative substituent effects are shown to be adequately described by a much lower level of theory (B3LYP/6-31+G*). Various correlations are presented which clearly point to polar field effects as being the origin of the relative acidities.     

Structure factors calculated from molecular wavefunctions. II. Analysis of the molecular charge distribution in diborane

The electron density distribution in the crystal of diborane has been determined using structure factors based on molecular densities. The libration-correction and third-cumulant terms have also been included in the expression for the temperature factor. The equilibrium r_e values for BH bond lengths obtained by this treatment are about 0.02Åshorter than the spectroscopic ones. A total librational motion of the diborane molecule is determined: the root-mean-square oscillations about inertial axis are 11.0° (8), 9.6° (5) and 7.2° (5), respectively. The dynamic theoretical deformation density shows a three-center BHB bond picture for the bridge structure.     

Another aromaticity vs non-aromaticity dilemma: The structure of anti-1,6:8,13-biscarbonyl [14]annulene

The X-ray crystal structure determination of anti-1,6:8,13-bis-carbonyl [14] annulene (1) is described. The structure has been solved by direct methods and refined to an R of 0.046 for 1074 reflections with I > 2σ(I). The molecular structure of 1 appears to be aromatic, at least from geometrical parameters. This is in contrast with the great tendency of the compound to polymerize and also with the structure of the anti-bridged [14]annulenes previously studied, having polyenic character. It is possible that the geometry of the compound appears to be ‘aromatic’ due to disorder in the crystal.     

Hindered inversion of chiral ion-dipole pairs.

O-Protonated S-(-)-1-phenyl-1-methoxyethane (IS) has been generated in the gas phase by CH3(2)Cl+ methylation of S-(-)-1-phenylethanol (1S). Detailed information on the reorganization dynamics of the intimate ion-dipole pair (IIS), arising from IS by C-O bond dissociation, is inferred from the kinetic study of the intramolecular inversion of configuration of IS vs its dissociation to alpha-methylbenzyl cation (III) and CH(3)OH. The behavior of IIS in the gas phase is compared to that observed in aqueous solutions, where the loss of optical activity of IS is prevented by exchange of the leaving CH3OH with the solvent shell. Hindered inversion of IS in solution is attributed to the operation of attractive interactions between the moving CH3OH moiety and the solvent cage which inhibit internal return in the intimate ion-dipole pair IIS. Similar interactions do not operate in the solvolysis of 18O-labeled 1S in aqueous acids, whose loss of optical activity efficiently competes with exchange of the leaving H2(18)O with the solvent shell.     

The deprotonation of benzyl alcohol radical cations: a mechanistic dichotomy in the gas phase as in solution

The gas-phase acidity of ionized benzyl alcohol and of some of its derivatives with selected reference bases has been studied by Fourier Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry. The aim was to relate the gas-phase reactivity to the behavior in aqueous solution of the radical cations of benzyl alcohols bearing methoxy substituent(s) on the phenyl ring which are known to undergo deprotonation at both the CH2 and OH groups. The dual reactivity behavior is confirmed in the gas phase, in which the prototypical ion, C6H5CH2OH*+, is deprotonated at both the CH2 and OH groups, whereas the ring hydrogens are not involved. An increasing extent of O-deprotonation is shown as the strength of the base increases. Appropriate methyl substitution, as in the radical cations of C6H5C(Me)2OH and C6H5CH2OMe, allows only O- or C-acidity. The two processes are characterized by comparable thermodynamic features with a Gas-phase Basicity (GB) value of 852 kJ mol(-1) for the cumyloxyl radical and 850 kJ mol(-1) for the alpha-methoxybenzyl radical. The possible origin of the observed mechanistic dichotomy is discussed.     

Synthesis and X-Ray Analysis of a Nickel Triphenyl-Cyclopropenyl Complex. A mo-interpretation of the geometry and bonding in the (C3H3)ML3-type of complexes

The reaction of [(C₃Ph₃)Ni(PPh₃)₂]ClO₄ with P(CH₂CH₂PPh₂)₃(pp₃) and NaBPh₄ yields the [(C₃Ph₃)Ni(pp₃)]BPh₄-complex. After long exposure of the solution of this compound in acetone/butanol to the air a new derivative [(C₃Ph₃)-Ni(pp₂po)]BPh₄· 0.5 C₄H₉OH, where pp₂po is (Ph₂PCH₂CH₂)₂P(CH₂ CH₂POPh₂), is obtained. Complete X-ray analysis has been carried out for the latter complex: a=18.303 (5); b=29.445 (6), c=13.305 (5) Å, β=112.70 (9)°; space group monoclinic, P2₁/a, Z=4. Disorder problems were encountered in the refinement of the structure. The best R is 0.093. One of the arms of the parent pp₃-molecule, not coordinated to the metal, undergoes oxidation. The Ni-atom, coordinated by the three remaining P-atoms of the ligand, is also linked in a roughly η³-mode to the cyclopropenium ligand. The geometry of the molecule is examined in detail. Extended HMO-calculations were performed to interpret how the variation of P? Ni? P angles affects the bonding between the NiP₃- and C₃H₃-fragments. The conclusion is that the overall energy of the complex may be lowered in spite of a weakening of the Ni-cyclopropenium linkage. Extensions are made to other systems containing a linkage between a metal and a X₃-ring (X=P,As).     

Gadolinium(III) Complexes of dota‐Derived N‐Sulfonylacetamides (H4(dota‐NHSO2R)=10‐{2‐[(R)sulfonylamino]‐2‐oxoethyl}‐1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic Acid): A New Class of Relaxation Agents for Magnetic Resonance Imaging Applications

Four new ligands for lanthanide ions based on the H₃do3a (=1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acid) structure and bearing one N‐sulfonylacetamide arm were synthesized, i.e., H₄dota‐NHSO₂R=10‐{2‐[(R)sulfonylamino]‐2‐oxoethyl}‐1,4,7,10‐tetraazacyclododecane‐1,4,7‐triacetic acids 1a – e . A ¹⁵N‐NMR study of the ¹⁵N‐labelled Eu³⁺ complex of one such ligands, 1d , showed that the coordination of the N‐sulfonylacetamide arm involves the carbonyl O‐atom rather than the N‐atom. The relaxometric properties of the corresponding Gd³⁺ complexes were investigated as a function of pH and temperature. These complexes have relaxivities in the range 4.5–5.3 mM ^?1 s^?1, at 20 MHz and 25°, and are characterized by a single H₂O molecule in their inner coordination sphere. The mean residence lifetime of this molecule is relatively long (500–700 ns) compared to other anionic complexes. The slow rate of H₂O exchange can be justified by the extensive delocalization of the negative charge on the N‐sulfonylacetamide arm. The long residence time of the coordinated H₂O allowed the observation of the effect of the prototropic exchange on the relaxivity. The study of the interaction between the complex [Gd( 1e )]‐ and HSA revealed a weak affinity constant highlighting the importance of a localized negative charge on the complex to promote a strong interaction with the protein.     

A theoretical study of the conformation of crystalline polyethylene

The conformation of polyethylene chains in crystals has been investigated using the MO SCF method in the CNDO/2 approximation for the isolated chain energy and an empirical method for the packing energy.     

Troposelective substitutions in microsolvated systems.

The mechanism and the stereochemistry of the intracomplex "solvolysis" of the proton-bound complexes I(X)() between CH(3)(18)OH and (R)-(+)-1-aryl-ethanol (1(R)()(X)(); aryl = phenyl (X = H); pentafluorophenyl (X = F)) have been investigated in the gas phase in the 25-100 degrees C temperature range. The results point to intracomplex "solvolysis" as proceeding through the intermediacy of the relevant benzyl cation III(X)() (a pure S(N)1 mechanism). "Solvolysis" of I(H)() leads to complete racemization at T > 50 degrees C, whereas at T < 50 degrees C the reaction displays a preferential retention of configuration. Predominant retention of configuration is also observed in the intracomplex "solvolysis" of I(F)(). This picture is rationalized in terms of different intracomplex interactions between the benzylic ion III(X)() and the nucleophile/leaving group pair, which govern the timing of their reorientation within the electrostatic complex. The obtained gas-phase picture is discussed in the light of related gas-phase and solution data. It is concluded that the solvolytic reactions are mostly governed by the lifetime and the dynamics of the species involved and, if occurring in solution, by the nature of the solvent cage. Their rigid subdivision into the S(N)1 and S(N)2 mechanistic categories appears inadequate, and the use of their stereochemistry as a mechanistic probe can be highly misleading.