Efficient, enantioselective synthesis of a β,β-disubstituted carboxylic acid by Ru-XylPhanePhos-catalyzed asymmetric hydrogenation

Enantioselective preparation of a key α_vβ₃ integrin antagonist intermediate was accomplished via catalytic asymmetric hydrogenation of the corresponding β,β-disubstituted α,β-unsaturated carboxylic acid bearing a 3-quinolinyl moiety. The successful application of a Ru-(R)-XylPhanePhos catalyst to this type of substrate is unprecedented. In situ NMR experiments of pre-catalyst formation/activation by CH₃CO₂H, and reaction parameter modification, revealed that [Ru(COD)(CF₃CO₂)₂]₂/(R)-XylPhanePhos is a highly active and efficient catalytic system.     

Hexanuclear cobalt carbonyl carbide clusters: the interplay between octahedral and trigonal prismatic structures

The six-vertex cobalt carbonyl clusters [Co6C(CO)n](2-) (n = 12, 13, 14, 15, 16) with an interstitial carbon atom have been studied by density functional theory (DFT). These DFT studies indicate that the experimentally known structure of [Co6C(CO)15](2-) consisting of a Co6 trigonal prism with each of its edges bridged by carbonyl groups is a particularly stable structure lying more than 20 kcal/mol below any other [Co6C(CO)15](2-) structure. Addition of a CO group to this [Co6C(CO)15](2-) structure gives the lowest energy [Co6C(CO)16](2-) structure, also a Co6 trigonal prism with one of the vertical edges bridged by two CO groups and the remaining eight edges each bridged by a single CO group. However, this [Co6C(CO)16](2-) structure is thermodynamically unstable with respect to CO loss reverting to the stable trigonal prismatic [Co6C(CO)15](2-). This suggests that 15 carbonyl groups is the maximum that can be attached to a Co6C skeleton in a stable compound. The lowest energy structure of [Co6C(CO)14](2-) has a highly distorted octahedral Co6 skeleton and is thermodynamically unstable with respect to disproportionation to [Co6C(CO)15](2-) and [Co6C(CO)13](2-). The lowest energy [Co6C(CO)13](2-) structure is very similar to a known stable structure with an octahedral Co6 skeleton. The lowest energy [Co6C(CO)12](2-) structure is a relatively symmetrical D3d structure containing a carbon-centered Co6 puckered hexagon in the chair form.     

Cyclic 1,3-dipoles or acyclic phosphonium ylides? Electronic characterization of "Montréalones"

The structural features of a recently introduced class of 1,3-dipolar reagents have been computed by density functional theory and ab initio methods. The reagents are formally derived from Münchnones by replacement of the C O group with a PR3 unit. The parent species (PR3 = PH3) shows a long P...O interaction (2.55 A at the B3LYP/6-31+G(d) level), together with a nonplanar ring, and is best described as a weakly chelated acylamino-phosphonium ylide. The corresponding acyclic form, in which the P...O interaction is absent, is predicted to be 2-3 kcal mol-1 higher in enthalpy. Variation of the phosphorus substituents exerts a marked effect on the P...O distance, with electron-withdrawing groups favoring a covalent interaction [P...O 1.97 A for PR3 = PPh(catechyl)] and electron-donating groups favoring a weak interaction [P...O 3.92 A for PR3 = PPh3]. However, this variation has little effect on the relative energies of the cyclic and acyclic forms. The barriers for concerted cycloadditions with ethylene are 22.8 kcal mol-1 (PH3), 31.7 kcal mol-1 (PPh3), and 16.2 kcal mol-1 [PPh(catechyl) with axial O], which correspond with experimental observations and follow the same trend as the energies required to distort the dipole to the TS geometry.     

Binuclear iron carbonyl nitrosyls: bridging nitrosyls versus bridging carbonyls

The iron carbonyl nitrosyls Fe 2(NO) 2(CO) n ( n = 7, 6, 5, 4, 3) have been studied by density functional theory (DFT) using the B3LYP and BP86 methods, for comparison of their predicted structures with those of isoelectronic cobalt carbonyl derivatives. The lowest energy structures for Fe 2(NO) 2(CO) 7 and Fe 2(NO) 2(CO) 6 have two NO bridges, and the lowest energy structure for Fe 2(NO) 2(CO) 5 has a single NO bridge with metal-metal distances (BP86) of 3.161, 2.598, and 2.426 A, respectively, corresponding to the formal metal-metal bond orders of zero, one, and two, respectively, required for the favored 18-electron configuration for the iron atoms. The heptacarbonyl Fe 2(NO) 2(CO) 7 is thermodynamically unstable with respect to CO loss to give Fe 2(NO) 2(CO) 6. The favored structures for the more highly unsaturated Fe 2(NO) 2(CO) 4 and Fe 2(NO) 2(CO) 3 also have bridging NO groups but avoid iron-iron bond orders higher than two by formal donation of five electrons from bridging NO groups with relatively short Fe-O distances. The lowest energy structures of the unsaturated Fe 2(NO) 2(CO) n derivatives ( n = 5, 4, 3) are significantly different from the isoelectronic cobalt carbonyls Co 2(CO) n +2 owing to the tendency for Fe 2(NO) 2(CO) n to form structures with bridging NO groups and metal-metal formal bond orders no higher than two.     

Matrix isolation and theoretical study of the reaction of substituted phosphines with CrCl2O2

The reactions between CrO2Cl2 and a series of substituted phosphines have been investigated using matrix isolation infrared spectroscopy. For all of the phosphines except PF3, twin jet co-deposition of the two reagents into argon matrices at 14 K initially led to the formation of weak bands due to the corresponding phosphine oxide. For all of the phosphines, subsequent irradiation with light of lambda > 300 nm led to the growth of a number of intense new bands that have been assigned to the phosphine oxide complexed to CrCl2O, following an oxygen atom transfer reaction. Gas-phase, merged jet reactions prior to matrix deposition led to a significant yield of the uncomplexed phosphine oxide. Theoretical calculations at the B3LYP/6-311++g(d,2p) level were carried out in support of the experimental work, to support product band assignments and clarify the nature of the molecular complexes.     

PHOTOCHEMICAL ESTIMATION OF OXYGEN SOLUBILITY

Abstract— A photochemical technique for estimating the solubility of oxygen in a solvent has been developed and used to estimate the solubility of oxygen in chloroform. From a measurement of the change in rubrene concentration and rubrene fluorescence lifetime as a sealed rubrene solution is irradiated and from the fluorescence lifetime of rubrene in nitrogen, air and oxygen-saturated solvent the oxygen solubility constant and rate constant for oxygen quenching of rubrene fluorescence can be measured. For chloroform these values are 9.8 m M /atm and 7.9 10₉ M _-1 s_-1 respectively.     

The infrared spectra and theoretical calculations of frequencies of fac-tricarbonyl octahedral complexes of manganese(I)

The carbonyl stretching frequencies in the infrared spectra of 38 fac-tricarbonyl octahedral complexes of manganese(I) prepared in this laboratory were determined. These complexes may be grouped into three types: (a) neutral complexes of the structure (CO)(3)Mn(P-P)Z where P-P represents depe, dppe, or dppp, and Z represents various anionic functional groups bonded to the manganese; (b) ionic complexes of the structure [(CO)(3)Mn(P-P)Z](+)BF(4)(-) where Z represents various neutral molecules possessing one phosphorous, nitrogen, or oxygen atom coordinated to the manganese; (c) complexes of the structure (CO)(3)Mn(pn)Z where the chelating pn represents 1,1-diphenylphosphino-2,2-dimethylaminoethane, Ph(2)PCH(2)CH(2)NMe(2). All of these complexes show three carbonyl stretching modes (2A' + A"). The effects on the frequencies of these modes induced by both the various Z groups and the various ligands are discussed. Theoretical calculations (B3LYP/6-31G) with optimization of the full molecule make it possible to distinguish between the three stretching modes and to make unambiguous assignments of appropriate symmetry species to each.     

Synthetic oligosaccharides as tools to demonstrate cross-reactivity between polysaccharide antigens

Escherichia coli O148 is a nonencapsulated enterotoxigenic (ETEC) Gram negative bacterium that can cause diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome in humans. The surface-exposed O-specific polysaccharide (O-SP) of the lipopolysaccharide of this bacterium is considered both a virulence factor and a protective antigen. It is built up of the linear tetrasaccharide repeating unit [3)-α-L-Rhap-(1→2)-α-D-Glcp-(1→3)-α-D-GlcNAcp-(1→3)-α-L-Rhap-(1→] differing from that of the O-SP of Shigella dysenteriae type 1 (SD) only in that the latter contains a D-Galp residue in place of the glucose moiety of the former. The close similarity of the O-SPs of these bacteria indicated a possible cross-reactivity. To answer this question we synthesized several oligosaccharide fragments of E. coli O148 O-SP, up to a dodecasaccharide, as well as their bovine serum albumin or recombinant diphtheria toxin conjugates. Immunization of mice with these conjugates induced anti-O-SP-specific serum IgG antibody responses. The antisera reacted equally well with the LPSs of both bacteria, indicating cross-reactivity between the SD and E. coli O148 O-SPs that was further supported by Western-blot and dot-blot analyses, as well as by inhibition of binding between the antisera and the O-SPs of both bacteria.     

Importance of molecular details in predicting bacterial adhesion to hydrophobic surfaces

Electrostatic and hydrophobic forces are generally recognized as important in bacterial adhesion. Current continuum models for these forces often wrongly predict measurements of bacterial adhesion forces. The hypothesis tested here is that even qualitative guides to bacterial adhesion often require more than continuum information about hydrophobic forces; they require knowledge about molecular details of the bacteria and substrate surface. In this study, four different strains of bacteria were adsorbed to silica surfaces hydrophobized with alkylsilanes. The thickness of the lipopolysaccharide layers varied on the different bacteria, and the lengths of the alkylsilane molecules were varied from experiment to experiment. Bacterial adhesion was assessed using column experiments and atomic force microscopy (AFM) experiments. Results show that hydrophobized surfaces have higher bacterial sticking coefficients and stronger adhesion forces than bare silica surfaces, as expected. However, adhesion decreased as the solution Debye length became longer than the alkylsilane, perhaps since the silane molecules could not "reach" the bacterial surface. Similarly, those bacteria with a long o-antigen layer had decreased adhesion, perhaps since the silane molecules could not reach surface-bound proteins on the bacteria. This study reveals that macroscopic measurements such as contact angle are not able to fully describe bacterial adhesion; rather, additional details such as the molecular length are required to predict adhesion.     

Activation of CO2 by a heterobimetallic Zr/Co complex

At room temperature, the early/late heterobimetallic complex Co((i)Pr(2)PNMes)(3)Zr(THF) has been shown to oxidatively add CO(2), generating (OC)Co((i)Pr(2)PNMes)(2)(μ-O)Zr((i)Pr(2)PNMes). This compound can be further reduced under varying conditions to generate either the Zr oxoanion (THF)(3)Na-O-Zr(MesNP(i)Pr(2))(3)Co(CO) or the Zr carbonate complex (THF)(4)Na(2)(CO(3))-Zr(MesNP(i)Pr(2))(3)Co(CO). Additionally, reactivity of the CO(2)-derived product has been observed with PhSiH(3) to generate the Co-hydride/Zr-siloxide product (OC)(H)Co((i)Pr(2)PNMes)(3)ZrOSiH(2)Ph.