Kinetics and mechanism of the gas-phase reaction of O(3P) atoms with acetone

The kinetics of the reaction of O(³P) atoms with acetone were investigated using fast flow methods. The reaction was studied over a temperature range of 298 to 478°K. The specific rate constant obtained was (1.9 ± 0.4) × 10¹² exp(—5040 ± 180/1.987 T) cm³/mol·sec. The observation of a sizable primary H/D kinetic isotope effect in comparing rates of CH₃COCH₃ and CD₃COCD₃ led to the conclusion that the major reaction channel involves H atom abstraction, namely, The rather low Arrhenius preexponential factor obtained in this reaction is compared and contrasted with those reported for other reactions of O(³P) with low molecular weight compounds.     

Novel approach to multifunctionalized homoallylic alcohols via regioselective ring opening of aryl oxiranes with 3-iodo allenoates

[reaction: see text] 3-Iodo allenoates were generated in situ and utilized, for the first time, in the ring opening of oxiranes in a regioselective fashion. This simple one-pot three-component reaction protocol provides easy access to highly functionalized homoallylic alcohols in good yields and moderate to very good (Z/E) selectivity. The two functional groups (ester and halogen) can be further subjected to many synthetic transformations.     

Enzyme-catalyzed synthesis of furanosyl nucleotides

A bacterial alpha-d-glucopyranosyl-1-phosphate thymidylyltransferase was found to couple four hexofuranosyl-1-phosphates, as well as a pentofuranosyl-1-phosphate, with deoxythymidine 5'-triphosphate, providing access to furanosyl nucleotides. The enzymatic reaction mixtures were analyzed by electrospray ionization mass spectrometry and NMR spectroscopy to determine the anomeric stereochemistry of furanosyl nucleotide products. This is the first demonstration of a nucleotidylyltransferase discriminating between diastereomeric mixtures of sugar-1-phosphates to produce stereopure, biologically relevant furanosyl nucleotides.     

Species and density of implant surface chemistry affect the extent of foreign body reactions

Implant-associated fibrotic capsule formation presents a major challenge for the development of long-term drug release microspheres and implantable sensors. Since material properties have been shown to affect in vitro cellular responses and also to influence short-term in vivo tissue responses, we have thus assumed that the type and density of surface chemical groups would affect the degree of tissue responses to microsphere implants. To test this hypothesis, polypropylene particles with different surface densities of -OH and -COOH groups, along with the polypropylene control (-CH2 groups) were utilized. The influence of functional groups and their surface densities on fibrotic reactions were analyzed using a mice subcutaneous implantation model. Our comparative studies included determination and correlation of the extents of fibrotic capsule formation, cell infiltration into the particles, and recruitment of CD11b+ inflammatory cells for all of the substrates employed. We have observed major differences among microspheres coated with different surface functionalities. Surfaces with -OH surface groups trigger the strongest responses, while -COOH-rich surfaces prompt the least tissue reactions. However, variation of the surface density of either functional group has a relatively minor influence on the extent of fibrotic tissue reactions. The present results show that surface functionality can be used as a powerful tool to alter implant-associated fibrotic reactions and, potentially, to improve the efficacy and function of drug-delivery microspheres, implantable sensors, and tissue-engineering scaffolds.     

Facilitating access to the most easily ionized molecule: an improved synthesis of the key intermediate, W2(hpp)4Cl2, and related compounds

A far superior synthesis is reported for W(2)(hpp)(4)Cl(2), a key intermediate in the synthesis of the most easily ionized closed-shell molecule W(2)(hpp)(4) (hpp = the anion of the bicyclic guanidine compound 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). At 200 degrees C, the one-pot reaction of the air-stable and commercially available compounds W(CO)(6) and Hhpp in o-dichlorobenzene produces W(2)(hpp)(4)Cl(2) in multigram quantities with isolated yields of over 90%. At lower temperatures, the reaction can lead to other compounds such as W(Hhpp)(2)(CO)(4) or W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2), which are isolable in good purity depending upon the specific conditions employed. These compounds provide insight into the reaction pathway to W(2)(hpp)(4)Cl(2) and W(2)(hpp)(4). Two additional derivatives, W(2)(hpp)(4)X(2) where X is PF(6)(-) or the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), have also been synthesized and structurally characterized. A comparison of the electrode potentials of W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2) and the di-p-anisylformamidinate analogue shows that oxidation of the hpp compound is significantly displaced (1.12 V) and shows that the bicyclic guanidinate ligand is considerably better than the formamidinate anion at stabilizing high oxidation states. A differential pulse voltammogram of W(2)(hpp)(4)(TFPB)(2) in THF shows two reduction processes with an E(1/2) of -0.97 V for the first and -1.81 V (vs Ag/AgCl) for the second. DFT calculations on the W(2)(hpp)(4)(2+) units in W(2)(hpp)(4)X(2) compounds show that the metal-metal bonding orbitals are destabilized by the axial ligands, which accounts for significant variations in the W-W distances. The low-energy gas-phase ionizations of W(2)(hpp)(4) are also reported and discussed.     

The combination of 2-NsNH2/NCS and MeCN as the nitrogen sources for the regio- and stereoselective formation of imidazolines from α,β-unsaturated ketones

A new nitrogen source combination was found for the regio- and stereoselective diamination of α,β-unsaturated ketones. This combination employs the readily available and inexpensive combination of NCS and 2-NsNH₂ as the electrophilic nitrogen source, and acetonitrile as the nucleophilic nitrogen source, respectively. The reaction is easily performed by mixing olefin, 2-NsNH₂, NCS and 4 Å molecular sieves in freshly distilled acetonitrile at room temperature. The reaction is chemoselective without the formation of any haloamine side products. A new aziridinium ion formed from enones and 2-NsNHCl is suggested to exist and to react with nitrile via a [2+3] cycloaddition mechanism, which is responsible for the excellent regio-, stereoselectivity of the resulting diamination products.     

Direct electrophilic diamination of functionalized alkenes without the use of any metal catalysts

A new direct electrophilic diamination reaction of alpha,beta-unsaturated ketones and esters has been established without the use of any metal catalysts. Three types of nitriles (CH(3)CN, CH(3)CH(2)CN, and CH(3)CH(2)CH(2)CN) were employed as nucleophilic nitrogen sources. A new mechanism has also been proposed to explain the resulting regio- and stereoselectivity.     

Preparations and applications of synthetic linked azamacrocycle ligands and complexes

Covalently linked azamacrocycles have been known for several decades, but only a modest number of these ligands and their complexes had been described prior to 2000. Since that time, a number of new synthetic methods for their preparation have been discovered, yielding a growing collection of these interesting ligands. Additionally, the number of uses to which these ligands and their metal complexes have been applied has expanded. In this review, the important synthetic methods yielding linked azamacrocycles will be outlined, and their applications will be discussed.