Acyclic protonated

Recently, Czamik et al. have reported the use of the acyclic protonated amine host 9 as a chemosensor of pyrophosphate. Typical fluorescence sensing methods rely on the ability of a complexed anion to quench the fluorophore. The fluorescence intensity of host 9, however, is actually increased upon complexation of anions and its 2200-fold selectivity of pyrophosphate over phosphate allows for real-time assay of pyrophosphate hydrolysis by inorganic pyrophosphatase.18... 

The methine protons and ring carbons in 5-methyl or 5-protonated thiiranium ions appear at lower field ( H, 4.0-5.0 ppm 13C, 74.0-78.0 ppm) as shown in Table 22. The methine hydrogens of the ring are coupled to the S—H proton, 3Jh h — 6-8 Hz—of the same magnitude as observed in acyclic protonated sulfides. 

Thus the reactions of cyclic or acyclic enamines with acrylic esters or acrylonitrile can be directed to the exclusive formation of monoalkylated ketones (3,294-301). The corresponding enolate anion alkylations lead preferentially to di- or higher-alkylation products. However, by proper choice of reaction conditions, enamines can also be used for the preferential formation of higher alkylation products, if these are desired. Such reactions are valuable in the a substitution of aldehydes, which undergo self-condensation in base-catalyzed reactions (117,118). Monoalkylation products are favored in nonhydroxylic solvents such as benzene or dioxane, whereas dialkylation products can be obtained in hydroxylic solvents such as methanol. The difference in products can be ascribed to the differing fates of an initially formed zwitterionic intermediate. Collapse to a cyclobutane takes place in a nonprotonic solvent, whereas protonation on the newly introduced substitutent and deprotonation of the imonium salt, in alcohol, leads to a new enamine available for further substitution. 

Interconversion between two tautomeric structures can occur via discrete cationic or anionic intermediates (scheme 24, where T is an anion capable of reacting with a proton at a minimum of two distinct sites). Alternatively, interconversion can occur by simultaneous loss and gain of different protons (scheme 25, w here T has the same definition as in scheme 24). These mechanisms are well established for acyclic compounds, but they have been much less thoroughly investigated for heteroaromatic systems. The rate of interconversion of two tautomers is greatest when both of the alternative atoms to which the mobile proton can be attached arc hetero atoms, and isolation of the separate isomers is usually impossible in this case. If one of the alternative atoms involved in the tautomerization is carbon, the rate of interconversion is somewhat slower, but still fast. When both of the atoms in question are carbon, however, interconversion is... 

It is interesting to note that the acyclic analog, nitroguanidine, exists in the symmetrical form 288 rather than as 289. Structure 288 has been established by ultraviolet and proton nuclear magnetic resonance spectroscopy. X-ray crystallography, dipole moments, and ipK measurements (see reference 367 and references therein). 

Direct treatment of TIPS enol ethers of a variety of cyclic and acyclic ketones with the strong-base combination of n-BuLi/KO-t-Bu leads to /3-ketosilanes (2) after aqueous work-up. In contrast with the earlier method, this rearrangement appears to proceed through allylic, rather than vinylic, metallation, since enol ethers lacking an allylic a-proton are unreactive. 

As in the photolysis of protonated eucarvone, an acyclic intermediate is proposed in the mechanistic pathway. The protonated dienones 73 and 74 should be thermally stable, since a symmetry-allowed ring closure in the conrotatory mode is precluded in the cyclic system (Woodward and Hoffmann, 1970). Upon irradiation it can undergo a conrotatory ring opening however, to produce the acyclic cations 79 and 80 which in... 

Afanasyeva, V.A., Glinskaya, L.A., Klevtsova, R.E. and Mironov, I.V. (2005) Protonation of the acyclic tetraaza metallocomplex of gold(III) [Au (C9HigN4)] in aqueous solution. Synthesis and crystal and molecular structure of [Au(CgH2oN4)](H502) (C104)4. Joumal of Structural Chemistry, 46, 876. 

Silyltitanation of 1,3-dienes with Cp2Ti(SiMe2Ph) selectively affords 4-silylated r 3-allyl-titanocenes, which can further react with carbonyl compounds, C02, or a proton source [26]. Hydrotitanation of acyclic and cyclic 1,3-dienes functionalized at C-2 with a silyloxy group has been achieved [27]. The complexes formed undergo highly stereoselective addition with aldehydes to produce, after basic work-up, anti diastereomeric (3-hydroxy enol silanes. These compounds have proved to be versatile building blocks for stereocontrolled polypropionate synthesis. Thus, the combination of allyltitanation and Mukayiama aldol or tandem aldol-Tishchenko reactions provides a short access to five- or six-carbon polypropionate stereosequences (Scheme 13.15) [28],... 

The regiochemistry for trapping lithiooxazole depends upon the oxazole substituents as well as the nature of the electrophile. Hodges, Patt and Connolly observed that the major product of reaction between lithiated oxazole (5 + 6) and benzaldehyde was the C(4)-substituted oxazole 7, resulting from reaction of the dominant acyclic valence bond tautomer 5 via the initial aldol adduct 6 followed by proton transfer and recyclization [3]. 

In toluene, the methyl protons resonate at 2.34 8 where as a methyl group attached to an acyclic alkane appears at 1.958. This is due to the greater deshielding influence of the ring current in aromatic compound as compared to the other. 

To test this hypothesis further, simple acyclic cobaltocenium derivatives (Fig. 38) containing tertiary amide groups ([72] and [73]) were prepared and complexation with Cl- and Br anions was investigated in solution by nmr spectroscopy. No shifts of the host s proton resonances were observed, emphasizing again the importance of the amide N—H group in anion binding. 

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