Amination of pyridylketenes: experimental and computational studies of strong amide enol stabilization by the 2-pyridyl group

Laser flash photolyses of 2-, 3-, and 4-diazoacetylpyridines 8 give the corresponding pyridylketenes 7 formed by Wolff rearrangements, as observed by time-resolved infrared spectroscopy, with ketenyl absorptions at 2127, 2125, and 2128 cm(-1), respectively. Photolysis of 2-, 3-, and 4-8 in CH(3)CN containing n-BuNH(2) results in the formation of two transients in each case, as observed by time-resolved IR and UV spectroscopy. The initial transients are assigned as the ketenes 7, and this is confirmed by IR measurements of the decay of the ketenyl absorbance. The ketenes then form the amide enols 12, whose growth and decay are monitored by UV. Similar photolysis of diazoacetophenone leads to phenylketene (5), which forms the amide enol 17. For 3- and 4-pyridylketenes and for phenylketene, the ratios of rate constants for amination of the ketene and for conversion of the amide enol to the amide are 3.1, 7.7, and 22, respectively, while for the 2-isomer the same ratio is 1.8 x 10(7). The stability of the amide enol from 2-7 is attributed to a strong intramolecular hydrogen bond to the pyridyl nitrogen, and this is supported by the DFT calculated structures of the intermediates, which indicate this enol amide is stabilized by 12.8 kcal/mol relative to the corresponding amide enol from phenylketene. Calculations of the transition states indicate a 10.9 kcal/mol higher barrier for conversion of the 2-pyridyl amide enol to the amide as compared to that from phenylketene.     

Hydration and dissociation of hydrogen fluoric acid (HF)

The hydration and dissociation phenomena of HF(H(2)O)(n)() (n < or = 10) clusters have been studied by using both the density functional theory with the 6-311++G[sp] basis set and the M?ller-Plesset second-order perturbation theory with the aug-cc-pVDZ+(2s2p/2s) basis set. The structures for n > or = 8 are first reported here. The dissociated form of the hydrogen-fluoric acid in HF(H(2)O)(n) clusters is found to be less stable at 0 K than the undissociated form until n = 10. HF may not be dissociated at 0 K solely by water molecules because the HF H bond is stronger than the OH H bond, against the expectation that the dissociated HF(H(2)O)(n) would be more stable than the undissociated one in the presence of a number of water molecules. The dissociation would be possible for only a fraction of a number of hydrated HF clusters by the Boltzmann distribution at finite temperatures. This is in sharp contrast to other hydrogen halide acids (HCl, HBr, HI) showing the dissociation phenomena at 0 K for n > or = 4. The IR spectra of dissociated and undissociated structures of HF(H(2)O)(n) are compared. The structures and binding energies of HF(H(2)O)(n) are found to be similar to those of (H(2)O)(n+1). It is interesting that HF(H(2)O)(n=5,6,10) are slightly less stable compared with other sizes of clusters, just like the fact that (H(2)O)(n=6,7,11) are slightly less stable. The present study would be useful for the experimental/spectroscopic investigation of not only the dissociation phenomena of HF but also the similarity of the HF-water clusters to the water clusters.     

Studies in pyrimidine-annulated heterocycles: unusual formation of spiroheterocyclic compounds from acid catalyzed reaction of enol ether

A number of spiroheterocycles are synthesized in moderate yield by treating 5-aryloxymethylene-6,7,8-trihydropyrido[2,3-d]pyrimidines-2,4(1H,3H)diones with H₂SO₄ in chloroform-methanol-water. These spiroheterocycles are also regioselectively synthesized by tri-n-butyltin hydride induced radical cyclization of 5-(o-bromoaryloxymethylene)-6,7,8-trihydropyrido[2,3-d]pyrimidines-2,4(1H,3H)diones.     

Tris(pentafluorophenyl)silyl enol ethers: synthesis and aldol reactions

Silyl enol ethers bearing three pentafluorophenyl groups at the silicon atom are described. These compounds undergo uncatalyzed aldol reactions with aliphatic, α,β-unsaturated, and aromatic aldehydes. The observed reactivity is analyzed in terms of the Lewis acidity of the silyl fragment.     

Synthesis and relaxivity of Gd(III), Fe(III) and Mn(II) complexes with dihydropyridine derivative of diethylenetriaminepentaacetic acid

A novel ligand of DTPA-dihydropyridine derivative was synthesized by reaction of DTPA-dianhydride with 4-aniline-1,4-dihydropyridine. Its complexes of gadolinium, manganese and iron were prepared. Their spin-lattice relaxivities (T₁) were investigated. The results show that the NMR T₁ relaxivitives (R₁) for complexes of Fe(III), Mn(II) are less than that of Gd(III) complex, which has a high relaxivity (R₁) on the surrounding water protons, indicating that the Gd(III) complex possesses the precondition to be contrast agents for magnetic resonance imaging.     

Synthesis and relaxivity of Gd(Ⅲ), Fe(Ⅲ) and Mn(Ⅱ)complexes with dihydropyridine derivative of diethylenetriaminepentaacetic acid

A novel ligand of DTPA-dihydropyridine derivative was synthesized by reaction of DTPA-dianhydride with 4-aniline-1,4-dihydropyridine. Its complexes of gadolinium, manganese and iron were prepared. Their spin-lattice relaxivities (T1) were investigated. The results show that the NMR T1 relaxivitives (R1) for complexes of Fe(Ⅲ), Mn(Ⅱ) are less than that of Gd(Ⅲ) complex,which has a high relaxivity (R1) on the surrounding water protons, indicating that the Gd(Ⅲ) complex possesses the precondition to be contrast agents for magnetic resonance imaging.     

Diketene-based neat four-component synthesis of the dihydropyrimidinones and dihydropyridine backbones using silica sulfuric acid(SSA)

Heterocyclic skeleton building blocks to afford dihydropyrimidinones and dihydropyridines based on neat adducts of diketene,alcohols and aldehydes via silica sulfuric acid(SSA) catalyzed ring opening of diketene in four-component Biginelli-type and Hantzsch-type reactions are presented.     

Ultrafast formation of transients in spiropyran photochromism

Transient absorption spectra in the photocolouration and photobleaching of the indoline 6-nitrosubstituted spiropyran have been investigated with picosecond spectroscopy. The cis—cisoid intermediate stereoisomer at 440 nm and some of the final coloured forms are produced in less than 10 ps in the photocolouration.     

Hydration of ions and formation of crystal hydrates

Effects of electrostatic and entropic factors on the dependence between the type of hydration of ions and their ability to form crystal hydrates from water solutions are discussed. It is shown that in the case of the single-charged ions because of the insignificant electrostatic interaction the crystal hydrates are formed mainly by the salts containing ions with different type of hydration. In the case of poly-charged ions the increase in the electrostatic interaction leads to formation of crystal hydrates mainly by the salts containing ions with the positive type of hydration. It was established also that the formation of salt crystal hydrates is influenced not only by the electrostatic and entropic factors, but also by the spatial arrangement and the electronic density distribution of ions.     

Enhanced stereocomplex formation of poly(L-lactic acid) and poly(D-lactic acid) in the presence of stereoblock poly(lactic acid)

Stereoblock poly(lactic acid) (sb-PLA) is incorporated into a 1:1 polymer blend system of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) that has a high molecular weight to study its addition effect on the stereocomplex (sc) formation of PLLA and PDLA. The ternary polymer blend films are first prepared by casting polymer solutions of sb-PLA, PLLA, and PDLA with different compositions. Upon increasing the content of sb-PLA in the blend films the sc crystallization is driven to a higher degree, while the formation of homo-chiral (hc) crystals is decreased. Lowering the molecular weight of the incorporated sb-PLA effectively increases the sc formation. Consequently, it is revealed that sb-PLA can work as a compatibilizer to improve the poor sc formation in the polymer blend of PLLA and PDLA.     

Water-soluble interpolyelectrolyte complexes of polyisobutylene-block-poly(methacrylic acid) micelles: formation and properties

We report on interpolyelectrolyte complexes (IPECs) formed by micelles of ionic amphiphilic diblock copolymers with polyisobutylene (PIB) and poly(sodium methacrylate) (PMANa) blocks interacting with quaternized poly(4-vinylpyridine) (P4VPQ). The interpolyelectrolyte complexation was followed by turbidimetry and small angle neutron scattering (SANS). The data obtained by means of a combination of SANS, dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM) provide evidence on the core-shell-corona structure of the complex species with the shell assembled from fragments of electrostatically bound PMANa and quaternized P4VPQ fragments, original PIBx-b-PMAAy micelles apparently playing a lyophilizing part. The complex formation is followed by potentiometric titration as well. This process is initially kinetically controlled. In the second step larger aggregates rearrange in favor of smaller complexes with core-shell-corona structure, which are thermodynamically more stable. An increase in ionic strength of the solution results in dissociation of the complex species as proven by SANS and analytical ultracentrifugation (AUC). This process begins at the certain threshold ionic strength and proceeds via a salt-induced gradual release of chains of the cationic polyectrolyte from the complex species.     

Mild and efficient pentafluorophenylammonium triflate (PFPAT)-catalyzed C-acylations of enol silyl ethers or ketene silyl (Thio)acetals with acid chlorides

A pentafluorophenylammonium triflate (PFPAT) catalyst (5 mol %) successfully promoted C-acylation of enol silyl ethers with acid chloride to produce various beta-diketones (12 examples; 62-92% yield). Similarly, C-acylation of ketene silyl acetals or ketene silyl thioacetals (i.e., crossed Claisen condensation) proceeded smoothly to provide not only alpha-monoalkylated beta-keto (thio)esters but also thermodynamically unfavorable (less accessible) alpha,alpha-dialkylated beta-keto (thio)esters in good to excellent yield (38 examples; 60-92% yield).     

Synthesis of 1,3,5-tricarbonyl derivatives by condensation of 1,3-bis(silyl enol ethers) with acid chlorides

A variety of 1,3,5-tricarbonyl derivatives were prepared by reaction of 1,3-bis(silyl enol ethers) with acid chlorides under mild conditions. This includes reactions of both aromatic and aliphatic acid chlorides and bis(acid chlorides). The yields vary depending on the type of acid chloride employed.     

Hydration of Lanthanoids(III) and Actinoids(III): An Experimental/Theoretical Saga

The latest experimental and theoretical studies on structural and dynamical properties of lanthanoid(III) and actinoid(III) ions in water have been reviewed. In the last years, most of the issues about lanthanoid(III) hydration have been resolved combining X-ray absorption experiments and different theoretical methods. Since 2008 an effort has been made to treat the entire series thus obtaining coherent sets of experimental and theoretical results that were lately put together in such a way that it was possible to derive new basic properties, such as effective ionic radii, across the series. While for the hydration of lanthanoids(III) many experiments and simulations have been reported, the hydration of actinoids(III) was less investigated. There are some experiments performed by different research groups and few simulations that we discuss in this review. Currently, there are enough results that it is possible to gain some understanding of the hydration behavior of lanthanoids(III) and actinoids(III). The ultimate goal of this review is to provide clues on the analogies and differences between the two series. These aspects are connected to several issues: 1)?technological: the separation of these elements that is necessary for recycling and stocking of nuclear waste, 2)?practical: because experiments on actinoids need particular care, the definition of possible analogies will give the possibility to use the correct lanthanoid when the information on a specific actinoid is needed, 3)?fundamental: related to chemical similarities between the two series.