Protonation and Isomerization

Protonation and Isomerization Unsubstituted and a-Substituted Pyrrole Complexes 

Notes Value adjusted for coupled isomerization in water. 

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On the basis of the measurement of the heats of protonation and isomerization and estimates of the heats of formation of the starting ketones, it has been shown that the additional delocalization energy in 33 as compared to 36 is 2.8 kcal mol 169. 

In the presence of Ni(dppe)Br2 the oxabicyclic alkenes 693 react with propynoic esters to yield 2H-benzo[ ]coumarins 696 (Scheme 172) <2001AGE1286>. The reaction mechanism involves cyclometallation of the propynoic ester and oxabicyclic alkene 693 to form the nickelacyclopentene intermediate 694. (3-Oxy elimination then forms the intermediate 695, which undergoes protonation and isomerization of the double bond followed by intramolecular lactonization to afford the desired 2/7-benzo[ ]coumarins 696 (Scheme 172) <2001AGE1286>. 

Figure 8. Protonation and isomerization of P-substituted ri -pyrrole complexes.

Me2NCONH , though it does not aquate, is protonated and isomerizes rapidly to the relatively stable 0-bonded form in acidic solution (81). The high basicity of many ligands makes them amenable to protonation almost invariably, protonation causes an increase in lability. 

These examples demonstrate a complicated dependence of the ratio of O-protonated and isomeric C-protonated forms arising from aromatic hydroxyderivaties and their ethers on the nature of the acid medium and temperature. The part of C-protonation usually grows with the rising medium acidity (cf. 324,340,343) ... 

Molecular heats of adsorption play a role in many catalytic reactions. Figure 6.23 illustrates this for an isomerization reaction catalyzed by a solid acid. As explained in Chapter 3, the hydroisomerization of alkanes on a zeolite-supported metal proceeds through a bifunctional reaction mechanism, in which the metal has the function of activating C-H bonds and H2 at a low reaction temperature. The alkane-alkene equilibrium is established by metal catalysis, and the alkene is protonated and isomerized by the acidic protons of the zeolite... 

Scheme 7.46) [77]. The reaction was applicable to the synthesis of a-ethenyl ke- tones possessing acidic a-protons, and isomerization to the thermodynamically stable conjugated enone was not observed. Equatorial preferences were observed in the ethenylation of cyclohexanone enolates for example, the ethenylation of a silyl enol ether derived from trans-3-decalone predominantly gave an equatorial isomer (Scheme 7.47) [78]. Silyl dienolates, synthesized by the a-ethenylation of thioesters followed by silylation, were ethenylated by this method at the a-position [79]. Ethenylation also occurred with silylated 1,3-dicarbonyl compounds, and ethenylmalonate possessing an acidic a-proton was obtained (Scheme 7.48) [80]. 

Maleic and fiimaric acids have physical properties that differ due to the cis and trans configurations about the double bond. Aqueous dissociation constants and solubiUties of the two acids show variations attributable to geometric isomer effects. X-ray diffraction results for maleic acid (16) reveal an intramolecular hydrogen bond that accounts for both the ease of removal of the first carboxyl proton and the smaller dissociation constant for maleic acid compared to fumaric acid. Maleic acid isomerizes to fumaric acid with a derived heat of isomerization of —22.7 kJ/mol (—5.43 kcal/mol) (10). The activation energy for the conversion of maleic to fumaric acid is 66.1 kJ/mol (15.8 kcal/mol) (24). 

There are four modes of radioactive decay that are common and that are exhibited by the decay of naturally occurring radionucHdes. These four are a-decay, j3 -decay, electron capture and j3 -decay, and isomeric or y-decay. In the first three of these, the atom is changed from one chemical element to another in the fourth, the atom is unchanged. In addition, there are three modes of decay that occur almost exclusively in synthetic radionucHdes. These are spontaneous fission, delayed-proton emission, and delayed-neutron emission. Lasdy, there are two exotic, and very long-Hved, decay modes. These are cluster emission and double P-decay. In all of these processes, the energy, spin and parity, nucleon number, and lepton number are conserved. Methods of measuring the associated radiations are discussed in Reference 2 specific methods for y-rays are discussed in Reference 1. 

Compounds A and B are isomeric diketones of molecular formula CgHio02. The H NMR spectrum of compound A contains two signals, both singlets, at 8 2.2 (six protons) and 2.8 (four protons). The H NMR spectrum of compound B contains two signals, one at 8 1.3 (triplet, six protons) and the other at 8 2.8 (quartet, four protons). What are the structures of compounds A and B ... 

Nuclear magnetic resonance spectra of all four parent compounds have been measured and analyzed.The powerful potentialities of NMR as a tool in the study of covalent hydration, tautomerism, or protonation have, however, as yet received no consideration for the pyridopyrimidines. NMR spectra have been used to distinguish between pyrido[3,2-d]pyrimidines. and isomeric N-bridgehead compounds such as pyrimido[l,2- ]pyrimidines and in several other structural assignments (cf. 74 and 75). 

Irradiation with UV light isomerized the azobenzene units from the trans to the cis form, while the reverse isomerization occurred thermally in the dark. The cis to trans conversion is catalyzed by both protons and hydroxyl ions. Hence, the catalyzed dark process for tethered azobenzene is greatly modified in comparison with that for free azobenzene. For the tethered azobenzene, beginning at pH 6, the cis to trans return rate sharply decreased with increasing pH up to 10, whereas the rate for free azobenzene rapidly increased in the same pH range owing to OH- catalysis. These observations can be explained by the electrostatic repulsion which lowers the local OH concentration on the polyion surface below that in the bulk aqueous phase. 

In most cases, the behaviour of sulphones at the cathodic interface (obviously rendered basic when insufficiently buffered due to the accumulation of electrogenerated bases) may be strongly modified by the presence of vicinal CH groups in the a position to the S02 group. These acidic groups may transfer protons and are often responsible for low yields of the cleavage processes or for undesirable isomerization reactions. 

The isomeric 1- and 2-fluoronaphthalenes have fluorine chemical shifts of -124 and -116 ppm, respectively. A full analysis of the proton and carbon spectra of 1-fluoronaphthalene is given in Scheme 3.56. NMR data for a number of other fluoropolycyclic aromatic compounds are available.7... 

The effects of transfer of atoms by tunneling may play an essential role in a number of phenomena involving the transfer of atoms and atomic groups in the condensed phase. One may expect that these effects may exist not only in the proton transfer reactions considered above but also in such processes as the diffusion of hydrogen atoms and other light ions (e.g., Li+) in liquids, tunnel inversion and isomerization in some molecules, quantum diffusion of defects and light atoms in the electrode at cathodic incorporation of the ions, ion transfer across the liquid/solid interface, and low-temperature chemical reactions. 

Fig. 3 Protonation states, isomerism and mesomerism of the HBI chromophore (p-hydroxybenzi-lidene-imidazolinone). The chromophore is shown in its most stable Z ( cw ) conformation, conventionally associated to a 0° value of the dihedral angle t, while the E ( trans ) conformation corresponds to t = 180°. For model compound HBDI (4 -hydroxy-benzylidene-2,3-dimethyl-imidazolinone), Ri = R2 = CH3, for chromophore in GFP, Ri, and R2 stand for the peptidic main chains toward N-terminus and C-terminus, respectively, (a) Possible protonation states of HBI (a) neutral, (b) anionic, (c) enolic, (d) cationic, and (e) zwitterionic. (b) Two resonance structures of the anionic form of HBI...

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