Compounds protons

Compounds" Proton in a from the cycle Proton in 0 from the cycle ... 

Compound Proton gain Proton loss first second ... 

Reduction of substituted nitrobenzenes under alkaline conditions, usually with aqueous sodium acetate as electrolyte and a nickel cathode, is the classical method due to Elbs [45] for the formation of azo- and azoxy-compounds. Protons are used in the electrochemical reaction so that the catholyte becomes alkaline and under these conditions, phenylhydroxylamine reacts rapidly with nitrosobenzene to form azoxybenzene. Finely divided copper has long been known to catalyse the reduction of nitrobenzene to aniline in alkaline solution at the expense of azoxybenzene production [46]. Modem work confirms that whereas reduction of nitrobenzene at polycrystalline copper in alkaline solution gives mainly azoxybenzene, if the electrode is pre-oxidised in alkaline solution and then reduced just prior to the addition of nitrobenzene, high yields of aniline are obtained with good current efficiency... 

For reviews of organic compounds protonated at O, N, or S, sec Olah White O Brien Chem. Rev. 1970. 70. 561-591 Olah White O Brien, in Olah Schleycr Carbonium Ions, vol. 4 Wiley New York, 1973, pp. 1697-1781. 

The NMR spectra of heterocyclic compounds with seven or more ring members are as diverse as the shape, size and degree of unsaturation of the compounds. Proton-proton coupling constants provide a wealth of data on the shape of the molecules, while chemical shift data, heteroatom-proton coupling constants and heteronuclear spectra give information of the electronic structure. Some data on seven-membered rings are included in Table 7, Several additional examples of NMR spectroscopy for large heterocycles are discussed below. 

Using the pKa values in the table below, find, with the aid of Figure 3.1, (a) the fraction of each compound protonated in 60 percent H2S04 (b) the H2S04-water mixture required to protonate 40 percent of 4,4 -dinitrobenzophenone. 

The main source of conformational information for biopolymers are the easy-to-obtain chemical shifts that can be translated into dihedral restraints. In addition, for fully 13C labeled compounds, proton-driven spin diffusion between carbons [72] can be used to measure quantitatively distances between carbons. The CHHC experiment is the equivalent of the NOESY in solution that measures distances between protons by detecting the resonances of the attached carbons. While both techniques, proton-driven spin diffusion and CHHC experiment [73], allow for some variation in the distance as determined from cross-peak integrals, REDOR [74] experiments in selective labeled compounds measure very accurate distances by direct observation of the oscillation of a signal by the dipolar coupling. While the latter technique provides very accurate distances, it provides only one piece of information per sample. Therefore, the more powerful techniques proton-driven spin diffusion and CHHC have taken over when it comes to structure determination by ss-NMR of fully labeled ligands. 

In order to protect the proton, and thereby suppress the kinetically favoured proton transfer route, it has been found out that gas-phase addition followed by elimination can be enhanced by reacting the proton bound dimer of the carbonyl compound rather than the protonated monomer [ 134]. In cases where the carbonyl compound has a higher proton affinity than the nucleophile, proton transfer is of course no problem. Alternatively, if the nucleophile already is protonated, as in the reactions between NH] and various carbonyl compounds, proton catalysed addition/elimination is possible as demonstrated experimentally by observation of immonium ion formation [135-137]. Likewise, the hydrazo-nium ion has been found to react with formaldehyde and a wide range of other aldehydes and ketones [138]. 

These carbonyl anions are strong nucleophiles (see Nucleophile) and can be used to form a diverse range of new compounds. Protonation gives the /t-hydride (see Bridging Ligand) [(/u.-H) Mo(CO)5 ]. Reactions with other metal carbonyls lead to CO substitution and the formation of metal metal-bonded heteronuclear anions, for example, [Mo(CO)s Fe(CO)4] and [Mo(CO)5-Co(CO)4] . Reaction with main group halides is shown in Scheme 1. The dianion reacts similarly. 

The interactions of LSR with various organophosphorus substrates have been reported (460-463). Yb(fod)3 and Pr(fod), are considered to be the best LSR for organophosphorus compounds. Proton shifts are, as usual, dominated by pseudocontact interactions. shifts are predominantly pseudocontact in nature but have sizeable contact contributions for phosphine and phosphoryl compounds. In contrast P shifts have large contact components where direct phos-phoryl-oxygen or phosphorus-lanthanide interactions occur. Large pseudocontact P shifts for triethyl phosphite indicate little or no direct phosphorus-lanthanide interaction. 

Imidazolines can exist as three possible structures 2-imidazolines (171), 3-imidazolines (172) or 4-imidazolines (173) (Scheme 83). The first-named can exist as a pair of tautomers, but any proton shift in (172) will give (173) by rearrangement. In fact the hydrolysis of Af-unsubstituted 3-imidazolines to a-aminoketones occurs presumably via the 4-imidazoline. 2-Imidazolines are cyclic amidines, and as such exhibit the characteristic resonance stabilization and strongly basic natures of these compounds. Protonation occurs on the unsubstituted nitrogen to give a resonance-stabilized imidazolinium ion. Examples of p/fa... 

The parent imidazo[l,2-c]pyrimidine (243a) has been found to protonate on N-1 (65JCS2778). Compound (la), as the neutral compound, protonates and alkylates on N-1 (76JA7408). 

Compound Proton chemical shift (ppm) Extent of graphitization. a... 

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