Oxidation proton sponges

Some consideration was given to appropriate bases for this oxidation system. Pyridine and sodium bicarbonate had been shown to be useful earlier and were included. Additional bases considered were a proton sponge (1,8-Bis(dimethylamino)naphthalene) and, as stated above, pyridine N-oxide. Because of the low solubilities in CXL systems of NaHCOs and the proton-sponge, pyridine was selected for these exploratory studies. 

An interesting oxidative decarboxylation process was reported for a /(-hydroxy acid. Reaction of 2-ethyl-2-(l -hydroxycyclobutyl)butanoic acid with vanadium(III) chloride in the presence of 1 equivalent of l,8-bis(dimethylamino)naphthalcne (Proton Sponge) gave 3-cyclobutylidene-pentane (20).181... 

Protection2 and activation1 of carboxylic acids. Carboxylic acids react with 1 in the presence of a 2-chloropyridinium salt, proton sponge, and DMAP to form amides (2). These amides are stable to acids and bases but deprotection is possible with oxidative hydrolysis with ceric ammonium nitrate (CAN). If the oxidation is carried out in the presence of an amine, an amide is obtained in 70-95% yield. For this purpose, the combination of copper(II) oxide and ceric pyridinium chloride is far superior to CAN.4 No racemization was observed in the benzoylation of an a-amino ester. 

A considerable number of proton sponges can be prepared by functionalizing the parent sponge 1 or its suitable derivatives. Thus, the l,F-binaphthyl sponge 36, which possesses interesting properties, is formed by treatment of 1 with some oxidants. This and many other reactions are discussed in Section IV. 

The oxidation of compound 32 occurs in a similar manner but even more easily136. Its radical cation can be stored unchanged in acetonitrile for months. The UV spectrum of 32+ (7.max = 480 nm, lg e = 3.1) is very close to that of neutral 32 itself, from which one can conclude that the naphthalene moiety does not take part in the oxidation. As found by cyclovoltammetry, the double proton sponge 35, unlike its isomer 79, is oxidized with reversible two-electron transition, composed of two superimposed one-electron steps at Eli2 —0.50 V (Table 13)45. Apparently, the driving force for this process is the formation of the resonance-stabilized dication 133 (equation 8), which was isolated in the form of black crystals with I3- anions (kmax = 643 nm, lg e = 4.02) and investigated in... 

TABLE 13. Ionization potentials (IPi) and electrochemical oxidation potentials (Ell2) for some aminonaphthalenes and proton sponges... 

Similarly, non-separable two-electron transitions were also observed for proton sponges 65, 66 and tetraamine 13454,58. As seen from Table 13, compound 35 is the strongest electron donor among all known tetrakis(dimethylamino)naphthalenes. The oxidation of 65 with four mole equivalents of iodine at low temperature also led to the formation of a black-brown salt (/.max = 723 nm, lg e = 4.31) of the dication 13558. 

It was recognized that acenaphthene 40 and acenaphthylene 107 proton sponges, unlike the pair of compounds 11 and 124, easily interconvert by oxidation and reduction, respectively38. Since in the course of this transformation, the basicity change amounts to over four plsfa units (equation 11), these compounds form a redox system with easily modified basicity. It is believed that such properties may be of use in creating molecular devices38. 

The nitration of 1 with one equivalent of HNO3 in H2SO4 gave two products the 4-nitro derivative 108 and the binaphthyl proton sponge 36 in 70 and 10% yield, respectively164,187. The reaction proceeds even at —20 °C and is completed within 5 min. These conditions are essentially milder than those for the naphthalene nitration. This is somewhat astonishing, since in such an acidic medium the diamine 1 seems to exist entirely as cation 1 H+, which should be more inert towards electrophiles than the naphthalene itself. One of the reasonable explanations of this discrepancy is that the reaction proceeds via very small equilibrium amounts of the non-protonated 1 or the non-chelated cation l H+-c. Any of them, under the action of the nitronium cation, is oxidized to the radical cation 1+ , which either dimerises or reacts with N02 to give the reaction products 36 and 108 (Scheme 27). There are several indirect pieces of evidence in favour of this. One of them... 

When l,8-bis(dimethylamino)-4-vinylnaphthalene (121) is heated with 3,6-diphenyl-s-tetrazine (DFT), a [4 + 2]-cycloaddition reaction with reverse electron demands takes place to give the 1,4-dihydropyridazine derivative 208 (Scheme 37). The latter could be oxidized with chloranil or with excess of DFT to pyridazine 209118. A similar reaction with acenaphthylene proton sponge 107 gives directly the annelated pyridazine 139, since the intermediate dihydropyridazine is readily oxidized in air. It was established that the reactivity ratio of compounds 107, 121, 5-dimethylaminoacenaphthylene and acenaphthylene in the reaction with DFT is equal to 32 17 14 1, respectively. These data are in... 

The trend that proton sponges with high thermodynamic basicity typically have a low kinetic basicity (kinetic activity in proton exchange reactions) is a serious limitation of proton sponges the captured proton does not usually take part in rapid proton exchange reactions, which would allow such neutral superbases to serve as catalysts in base-catalysed reactions. Their further limitations are moderate solubility in aprotic nonpolar solvents and stability towards auto-oxidation. 

---