Protons labile

Zr and Hf form the mono me nc. 7-cooidtnate [MCI(acac))] complexes which have a distorted pentagonal bipyramidal stereocheinistry. Ah>o. providing alkali is pre. d to remove the labile proton, Zr and Hf wifi yield the telrakis complexes hi aqueous solutioii ... 

No fluorescence is observed at room temperature from TIN in non-polar solvents such as cyclohexane. In these solvents only the intramolecularly hydrogen-bonded form, which can undergo rapid ESIPT upon excitation, is present. The t-Bu-STIN derivative (see Table II) is very weakly fluorescent in all of the solvents examined. This is attributable to the protection of the intramolecular hydrogen bond from the solvent by the tertiary butyl group which is adjacent to the labile proton. 

FIGURE 3.10 Deprotection of functional groups by beta-elimination.17 (A) Removal of a labile proton beta to a good leaving group leads to release of the protector as the didehydro compound. (B) Recently developed protectors (Samukov et al., 1988) also designated untra-ditionally as 4-nitrophenyl- (C) Transformation of an O-protected serine residue into a dehydroalanine residue by hefa-elimination. 

Further studies on 1,3-dipolar addition reactions of diazophosphonates have been recorded,122 and work on 2-diazo-l-hydroxyalkylphosphonates also continues.123 The ester (155 R = H) reacts with esters of acetylenedicarboxylic acid without liberation of nitrogen to give stereoisomeric C-phosphorylated pyrazolines, which can be decomposed with both phosphorus-carbon and carbon-carbon bond fission, affording mixtures containing dimethyl acetylphosphonate, dimethyl hydrogen phosphonate, and tri(alkoxycarbonyl)pyrazolines. In the reaction between the same diazophosphonate and diazomethane, the latter conceivably acts as a basic catalyst for proton transfer in a series of steps which includes phosphonate-phosphate isomerization. The importance of a labile proton is demonstrated by the fact that the ester (155 R = Me) does not react in the manner described above. 

Do the number of observed protons correspond to the postulated structure Provided this question can be answered unambiguously within the uncertainties imposed by labile protons and spectral quality, it can on its own refute an incorrect structure. In practice, this is not the spectroscopist s usual first step, most preferring to grapple with making chemical shifts and couplings fit. 

The inclusion of the number of protons at each chemical shift proved to be key the number of false positives increased 5-7 fold (depending on how diverse the set of test compounds were) if this parameter was excluded. Various refinements were tested, but the most successful was the J Filter which disallowed a combination in which the number of couplings measured at a given chemical shift in the experimental spectrum was greater than in the predicted. This proved to be particularly valuable in the differentiation of isomers, where changes in the overall table of chemical shifts alone, could be quite subtle. The inconsistent appearance of labile protons in the experimental spectrum reduced the accuracy and it was asserted that where possible, they should be removed from both the experimental spectrum and the predicted spectrum. The mismatch level not only encoded the divergence of the experimental spectrum and the postulated structure, but also encoded... 

The saccharinic acids formed from hexoses have been especially examined because of the relationships of the a and /8 isomers (C-2 epi-mers). Structures of saccharinic acids derived from D-glucose are glu-cometasaccharinic acid (51), glucoisosaccharinic acid (52), and glucosaccharinic acid (53). The a- and /3- isomers of metasaccharinic acid can reversibly isomerize when exposed to base because of the labile proton at C-2. 

Proton-containing admixtures in a solvent or in benzaldehyde can act like water. The superoxide ion abstracts such labile proton and generates the HOO base. The base in its turn abstracts proton from a solvent, for example, AN. If benzaldehyde is present, it is converted into cinnamyl nitrile according to the following simple reactions ... 

On co-adsorbing phenol and methanol, the protonation of methanol occurs on the active acid sites as the labile protons released from the phenol reacted with methanol. Thus protonated methanol became electrophilic methyl species, which undergo electrophilic substitution. The ortho position of phenol, which is close to the catalyst surface, has eventually become the substitution reaction center to form the ortho methylated products (Figure 3). This mechanism was also supported by the competitive adsorption of reactants with acidity probe pyridine [79]. A sequential adsorption of phenol and pyridine has shown the formation of phenolate anion and pyridinium ion that indicated the protonation of pyridine. 

Labile protons frequently give rise to broadened resonances in the NMR spectmm and their chemical shifts are critically dependent on the solvent, concentration, and on temperature and they do not have reliable characteristic chemical shift ranges. 

Labile protons exchange rapidly with each other and also with protons in water or with the deuterons in D2O. 

Labile protons can always be positively identified by in situ exchange with D2O. In practice, a normal H NMR spectmm is recorded then deuterium exchange of labile protons is achieved by simply adding a drop of deuterated water (D2O) to the NMR sample. Labile protons in -OH, -COOH, -NH2 and -SH groups exchange rapidly for deuterons in D2O and the H NMR is recorded again. Since deuterium is invisible in the iH NMR spectmm, labile protons disappear from the NMR spectmm and can be readily identified by comparison of the spectra before and after D2O is addition. 

I. Referring to an unstable and/or transient chemical species. 2. Referring to a substituent, atom, or group of a molecular entity that is easily removable e.g., the labile proton of the carboxyl group in trichloroacetic acid. 3. In reference to coordinator complexes, referring to ligands that can be readily replaced by other ligands. 4. On occasions, the term is also used with respect to stable, yet reactive, chemical species. 

Lycorine is an alkaloid that has attracted attention from both the synthetic community and pharmacologists. Prior synthetic approaches have included inter-and intramolecular Diels-Alder cycloaddition. Based on a similar retrosynthetic disconnection, Padwa and co-workers (106,109) chose to use a push-pull carbonyl ylide cycloaddition with a disubstituted pyrrolidinone core to generate a tricyclic substrate. The major difference for this synthetic smdy was the availability of a labile proton a to the carbonyl moiety (Scheme 4.53). 

In the early years of these studies, some controversy arose as to whether traces of water are necessary for the initiation induced by tertiary amines. The initiation by primary and secondary amines has been explained by postulating transfer of a labile proton to the monomer. Such protons are not available in a tertiary base, and therefore Coleman (42) suggested a co-catalysis by traces of water which would supply the protons. However, the elaborated and painstaking studies of Wessely s group (41) demonstrated that water is unnecessary for the pyridine... 

It is assumed that the labile proton, denoted symbolically by H+, is attached to a base associated with the positive ion, e. g. to CF if lithium chloride is the initiator or to carboxylate ion if the initiation results from the action of sodium salt of a weak carboxylic acid (54). 

LC-ESI-MS and LC-APCI-MS experiments involving HDX were carried out on a TSQ Quantum mass spectrometer. All labile protons, in DL, 6-OH-DL, 3-OH-DL, A-OH-DL, and 1-pyridine-A-oxide-DL, underwent complete deuterium exchange. C-Hydroxylated compounds (6-OH-DL, 3-OH-DL) underwent a total of three HDXs, while A-oxidc and the hydroxylamine exchanged only two protons. 

BASE CATALYSIS (nucleophile with labile proton)... 

Tolonen et al. [167] described a simple and efficient method for determination of labile protons in drug metabolites using postcolumn infusion of deuterium oxide in LC/MS experiments with ESI and TOF-MS. The number of exchangeable protons in analytes hydroxyl, amine, thiol, and carboxylic acid protons can easily be determined by comparing the increase in m/z values after H/D-exchange occurring on line between... 

Both kinetic and thermodynamic factors are important in determining protic character. Although the equilibrium concentration of solvated protons in a protic solvent such as water or ethanol may be very small, there is a low activation energy for dissociation or exchange of protons, and these labile protons can rapidly react with any species having an appreciable proton affinity. Aprotic solvents usually have a lower equilibrium concentration of solvated protons (perhaps by a factor of as much as 1010) and the activation energy for... 

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