Protonation external

There are also examples of [18]annulene systems constructed around a saturated central core, such as in compound 4. In this compound, the internal protons are at very high field (—6 to —8 ppm), whereas the external protons are far downfield ( 9.5ppm). 

Spectacular differences in absorption/excitation spectra are often observed for the dyes that exist in protonation-deprotonation equilibria. Their straightforward application is for pH sensing and also for designing the reporters, in which the shifting of such equilibrium by external proton donor or acceptor group is involved in sensing event. 

That the [19]annulenone 136 is diatropic is evident from the high field resonance of H13 compared to the adjacent external protons Ht2 and H14, and to all the olefinic protons of the homoannulene 137. Further evidence is the significantly lower field resonance of H3, H4, H17, and Hlg when compared with the similar protons H3 and H4 of the atropic ketones 143,144, and 145 (see below). 

Information about the kinetics of interconversion of the species in Scheme 12 has been obtained (Smith et al., 1981). The values of the rate coefficients for external protonation of ii to give io+ and o+o+ are probably close to the diffusion-controlled limit. However, the rate of internal monoprotonation of ii to ii+ is quite low and the reaction can be followed by observing the change in nmr signals with time. At pH 1 and 25°C the half-life is 7 min. Under these conditions, insertion of the second proton into the cavity takes several weeks to reach completion, but can be observed in convenient times at higher... 

As usual we start consideration of the contributions of order Za)Ep with the infrared divergent integral (9.9) corresponding to the two-photon skeleton diagram in Fig. 9.2. Insertion of factors GE —k ) — 1 or Gm(— )/(l + k) — 1 in one of the external proton legs corresponds to the presence of a nontrivial proton form factor. ... 

Later, Huisgen et al. 1 put forward a scheme which was similar to that of Dolfini. Their work indicated that both cis and trans adducts arose from the same intermediate, giving the cis adduct by internal proton shift, and both adducts, but mostly the trans compound, through the agency of external proton donors. Dolfini had shown that these adducts do not isomerize spontaneously below 80° and that the stability of the adducts to acid is high. In the NMR spectrometer the larger coupling constant for trans protons over cis protons provides a simple method for structure and product ratio determination. 

Again the limiting step corresponds to the transfer of the external proton once this is achieved, the rest of the TS and the intermediate are below this barrier indicating that the proton will be transferred freely generating a barrierless racemi-zation along the chain in a wavelike expansion [37] (Scheme 3.20). 

The bismuth nitrate-catalyzed Michael reaction of enones with amino compounds is a very simple and efficient method. This method is totally independent of solvent or external proton source. The reaction of many structurally diverse indoles with enones under Bi(N03)3-catalyzed conditions afforded products at the 3-position of the indole ring in high yield when this position was unoccupied. When the 3-position was occupied by a substituent, the reaction took place at the 2-position of the indole ring (Scheme 17) [83,84]. 

For PIGE measurements, transverse bone sections are cut with a diamond saw and polished with SiC paper, and then placed directly in front of the external proton beam. It is not necessary to coat the sample surface with a conductive layer as the charges are dissipated in air and helium. Step width of the concentration profiles is determined by precisely recorded sample translation in front of the beam. The above experimental conditions were used for F analysis in archaeological bone materials in the applications described in this chapter. 

CT interaction 132). In the case of substituted benzenes with benzylic hydrogens, proton transfer yields radicals. The main evidence for this CT photoreduction mechanism is the lack of any isotope effect on kt when toluene- -d3 is substituted for toluene-hg. In the absence of any labile protons on the aromatic half of the CT complex, such as with benzene itself, the CT interaction is primarily a quenching one, unless an external proton source is present, in which case the complex is apparently protonated 183>. 

The alternative method shown in equation 18, using a chiral alcohol, like (S,S)-97, as the chiral Brpnsted acid, affords a high level of facial selectivity for the enantioselective protonation of lithium enolate 95 to give the required 2-methyltetralone (5)-93 in yields and enantiomeric purity similar to those of equation 17 °. The concept of internal versus external proton delivery has been probed ° . The use of disulfonamide (R,R)-9S as chiral Brpnsted acid leads to the (R)-93 enantiomer (equation 19), whereas using as chiral scaffold (R,R)-99, the (V,(V -dilithio salt of (R,R)-99 and acetic acid (as external proton source) gives (S)-93 (equation however, in poorer yield and enantiomeric... 

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