Aqueous Solution Chemistry of the Mo(3)PdS(4) Cube: Substitution Reactions and the Double to Single Cube Interconversion Induced by CO, Two Phosphines, Cl(-), Br(-), and NCS(-)

The kinetics of conversion of an edge-linked double cube, in this case [{Mo(3)PdS(4)(H(2)O)(9)}(2)](8+), to the corresponding single cube [Mo(3)(PdX)S(4)(H(2)O)(9)](4+), has been studied for the first time. Reaction is induced by six reagents X = CO, two water-soluble phosphines, Cl(-), Br(-), and NCS(-), which complex at the tetrahedral Pd. The first stage of reaction is fast and is accompanied by color changes, e.g. purple to dark blue in the case of Cl(-), assigned as double to single cube conversion. With X = CO and the two phosphines, when absorbance changes are intense enough for stopped-flow monitoring with reactants at Pd-SCN. On removal of e.g. Cl(-) by chromatography or addition of Ag(+), the double cube re-forms.     

Reaction of pentaammine(urea-O)cobalt(III) cation with chlorine and hypochlorous acid in aqueous solution

Summary The Co(NH₃)₅[OC(NH₂)₂]³⁺ cation in aqueous acid reacts with chlorine and hypochlorous acid, with two sequential steps observed in each case. Rate constants for both steps show a first-order dependence on [oxidant], with k₁/k₂ always <20, but varying with the choice of reactant and acid. Rate constants with Cl₂ as reactant are faster than with HOCl, possibly related to preferential attack by Cl⁺ compared with OCl^– on the bound urea. Competition by ions (HSO ₄ ^– , Cl^– or NO ₃ ^– ) measured by product analysis of reactions conducted in 1 M acid produced competition ratios R (R=[CoX]/[CoOH₂][X]) which are similar to values determined with a range of leaving groups previously, indicating a mechanistic constancy. No formation of Co(NH₃)₅Cl²⁺ was observed in reactions conducted in H₂SO₄ or HNO₃, implying that free Cl^– is not generated at the reaction site and captured by the metal ion. Electronic and vibrational spectra of the intermediate formed in the two stage reaction is indicative of a change from an O-bound to an N-bound ligand in forming that intermediate, although it cannot be a simple isomerization due to the dependence on [oxidant]. A possible mechanism is discussed.     

Thermodynamics of binding of 2-methoxy-3-isopropylpyrazine and 2-methoxy-3-isobutylpyrazine to the major urinary protein

In the present study we examine the thermodynamics of binding of two related pyrazine-derived ligands to the major urinary protein, MUP-I, using a combination of isothermal titration calorimetry (ITC), X-ray crystallography, and NMR backbone (15)N and methyl side-chain (2)H relaxation measurements. Global thermodynamics data derived from ITC indicate that binding is driven by favorable enthalpic contributions, rather than the classical entropy-driven hydrophobic effect. Unfavorable entropic contributions from the protein backbone and side-chain residues in the vicinity of the binding pocket are partially offset by favorable entropic contributions at adjacent positions, suggesting a "conformational relay" mechanism whereby increased rigidity of residues on ligand binding are accompanied by increased conformational freedom of side chains in adjacent positions. The principal driving force governing ligand affinity and specificity can be attributed to solvent-driven enthalpic effects from desolvation of the protein binding pocket.     

A Convenient Synthesis of 4-Hydroxybeno-(B)-Furan, (Karanjol)

The synthesis of karanjol from tetrahydro-(b)-furan intermediates is described.     

An Improves Method for the Cleavage of Methyl Esters

Synthetic organic chemists are continually looking for milder, more specific methods of effecting given transformations, and one such problem which recently arose in our laboratory involved ester cleavage. In particular, we sought a specific, wild, non-hydrolytic method for effecting teh ester - acid transformation under conditions which other acid or base sensitive functional groups would survive.     

Palladium-Catalyzed Coupling of L-Proline Acid Chloride with Vinylstannanes

Vinylstannanes were found to undergo a palladium-catalyzed coupling with L-N-protected proline acid chloride to produce the corresponding N-protected α'-amino-α,β-unsaturated ketones in moderate to good yield.