P-protonations

Figure B 1.16.9 shows background-free, pseudo-steady-state CIDNP spectra of the photoreaction of triethylamine with (a) anthroquinone as sensitizer and (b) and (c) xanthone as sensitizer. Details of the pseudo-steady-state CIDNP method are given elsewhere [22]. In trace (a), no signals from the p protons of products 1 (recombination) or 2 (escape) are observed, indicating that the products observed result from the radical ion pair. Traces (b) and (c) illustrate a usefiil feature of pulsed CIDNP net and multiplet effects may be separated on the basis of their radiofrequency (RF) pulse tip angle dependence [21]. Net effects are shown in trace (b) while multiplet effects can...

Four multiplets between Sh = 7.46 and 9.18 indicate monosubstitution of the pyridine ring, either in the 2- or 3-position but not in the 4-position, since for a 4-substituted pyridine ring an AA XX system would occur. The position of the substituents follows from the eoupling eonstants of the threefold doublet at Sh = 7.46, whose shift is appropriate for a p-proton on the pyridine ring (A). 

In the El cb mechanism, the direction of elimination is governed by the kinetic acidity of the individual p protons, which, in turn, is determined by the polar and resonance effects of nearby substituents and by the degree of steric hindrance to approach of base to the proton. Alkyl substituents will tend to retard proton abstraction both electronically and sterically. Preferential proton abstraction from less substituted positions leads to the formation of the less substituted alkene. This regiochemistry is opposite to that of the El reaction. 

The leaving group also affects the amount of internal versus terminal alkene that is formed. The poorer the leaving group, the more El cb-like is the transition state. This trend is illustrated for the case of the 2-butyl system by the data in Table 6.6. Positively charged leaving groups, such as in dimethylsulfonium and trimethylammonium salts, may favor a more El cb-like transition state because their inductive and field effects increase the acidity of the p protons. 

The EPR spectrum of the ethyl radical presented in Fig. 12.2b is readily interpreted, and the results are relevant to the distribution of unpaired electron density in the molecule. The 12-line spectrum is a triplet of quartets resulting from unequal coupling of the electron spin to the a and P protons. The two coupling constants are = 22.38 G and Op — 26.87 G and imply extensive delocalization of spin density through the a bonds Note that EPR spectra, unlike NMR and IR spectra, are displayed as the derivative of absorption rather than as absorption. 

The El mechanism is a two-step process in which the rate-determining step is ionization of the substrate to give a carbocation that rapidly loses a P proton to a base,... 

Figure 2. DQFCOSY spectrum of RpII in D2O. Cross peaks corresponding to a-P protons of various spin systems are labeled. The valine spin systems are shown explicitly.

Homoenolate Protonation The p-protonation of homoenolates has been observed by Scheidt and co-workers, resulting in a redox transformation of enals to afford saturated esters 48. This process is catalysed by the NHC derived from imidazolium salt 46 and utilises phenol as a proton source [14]. A range of primary and secondary alcohols, and phenol itself, are competent nucleophiles with which to trap the acylazolium intermediate 47 generated by protonation (Scheme 12.8). 

Figure 14-6. Two-dimensional free energy surfaces for in-line monoanionic mechanisms for the (A) un-catalytic model reaction in solution, and the catalytic (B) O p and (C) O2p pathways in the hairpin ribozyme. fi is defined as Rp-o5, R02,-P> and 2 is R02,-H2, rOnb-H2, for fl < 0.0 A and r05,-H2, rOnb-H2, f°r 1 > 0.0 A, where Opjg is for the G p proton transfer in (B), and for the O2p proton transfer in (A) and (C), respectively. The units for free energies and distances are kcal/mol and A, respectively...

Use of CW ENDOR techniques to detect P-proton hyperfine couplings and matrix nuclei Pulsed ENDOR techniques to detect P-proton hyperfine couplings and matrix nuclei HYSCORE techniques to detect a-proton anisotropic coupling tensors... 

Just as in NMR, a multiplet pattern gives an important clue to the identity of a radical. For example, in the naphthalene anion radical, there are four a (positions 1, 4, 5, 8) and four p protons (positions 2, 3, 6, 7). Each proton splits the electronic energy levels in two. Since the a protons are equivalent, for example, the splitting is the same for each proton. Thus, as shown on the right side of Figure 2.1, five equally spaced energy level values result. 

FIGURE 10.2 The spectrum of the DMPO 011 adduct. The rapidly tumbling adduct affords an isotropic spectrum split by 14N (I = 1) in three lines, each of which is split by the P proton (I = 1/2) in two lines. Overlap of lines, due to AN AH, gives a 1 2 2 1 intensity pattern. 

Like electrons, some nuclei also have spin. Protons and neutrons also have spins of Thus, if a nucleus consists of p protons and n neutrons its total spin will be a vector sum of p + n spins of Each isotope will have its own spin value, but the laws governing the vector addition of nuclear spins are not known, and at the moment, nuclear spins are known only in terms of some empirical rules. 

Skinner, J. L. and Trommsdorf, H. P. Proton transfer in benzoic acid crystals A chemical spin-boson problem. Theoretical analysis of nuclear magnetic resonance, neutron scattering, and optical experiments, J.Chem.Phys., 89 (1988), 897-907... 

In some of their publications Higashimura s group, and others using the same terminology, are close to our view when they write about the modifiers reducing the reactivity of the carbocation . However, since in our view there is no carbenium ion to be stabilised, we see these donors as reducing the polarity of the ester bond and the reactivity of the 0-protons, and they obstruct physically the transfer of a P-proton to the monomer or to any other base. 

The living polymerisations differ from the transfer-dominated ones in that the latter are mostly initiated by oxo-acids only, whereas to achieve living character, the ester needs to form a donor-acceptor complex with a third component, the modifier . One function of the modifiers is to prevent the chain-breaking transfer of a P-proton from the growing end to a base, such as the monomer or solvent. 

Carbocations have similar electronic structures to carbenes. The P-protonated derivative of phosphinine should also be similar to 23. Indeed, while investigating the proton affinity of 3. the most preferred protonation site was phosphorus and not carbon, whereby the cyclic jt system would be interrupted. ... 

The relative energies apparently exhibit a rather weak basis set dependence the equilibrium between the N- and p-protonated forms is driven by electron correlation, while the ortho-para equilibrium is apparently quite well reproduced even at the HartreeFock level. We may safely argue that the Wlc results are converged with respect to the basis set. 

Let us now turn to the density functional methods. All of them correctly predict the para-ortho ordering, but considering that this is the case even for a Hartree-Fock treatment this is a somewhat hollow victory. Without exception, all functionals wrongly predict p-protonation. Arguably, this small energy difference falls within the error margin of any type of calibration for (semi-) empirical DFT functionals. 

In 2007, Scheldt and co-workers reported the intramolecular desynunetrization of 1,3-diketones utilizing triazolinm pre-catalyst 249 (Scheme 39) [129], Generation of a homoenolate is followed by P-protonation and aldol reaction. In accordance with the proposed mechanism by Nair (Scheme 37), acylation occurs followed by loss of carbon dioxide. Cyclopentenes are formed in enantioselectivities up to 94% ee. The scope of this reaction is limited to aryl substitution of the diketone and alkyl substitution of R. 

Related to the pyridine studies are the results of base-catalyzed hydrogen exchange in cyclobutabenzene derivatives, which suggest that cyclobutyl annelation increases both the kinetic and thermodynamic acidity at the a-position. The most significant study is the thermodynamic deprotonation/carboxylation reaction of cyclobutabenzene with amyl sodium/COj, in which only the a-carboxy isomer is formed (Figure 6). This is consistent with the value for the a-proton being several p units lower than that for the P-proton in cyclobutabenzene (38). 

Transfer of a P-proton from the propagating carbocation is the most important chain-breaking reaction. It occurs readily because much of the positive charge of the cationic propagating center resides not on carbon, but on the P-hydrogens because of hyperconjugation. Monomer, counterion or any other basic species in the reaction mixture can abstract a P-proton. Chain transfer to monomer involves transfer of a P-proton to monomer with the formation of terminal unsaturation in the polymer. 

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