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Nitration proton sponges

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. [Pg.97]

Occasional formation of such salts in a small amount on nitration of compound 1, e.g. with N2O4 in 1,2-dichloroethane, indicates that the process could be initiated with common oxidants47. However, a question arises here as to why a similar cyclization is not observed for acenaphthene sponges (cf. Scheme 22) The answer may be that the N N distance in these sponges is remarkably longer (Table 4) and thus the cyclization becomes unfavourable. That the double proton sponge 35 with an N N distance yet shorter than 1 (Table 4) is very prone to cyclize to dihydroperimidinium salts44,64,137, confirms this explanation (Section IV.B.8). [Pg.983]

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... [Pg.987]

The naphthalene ring activation with the NMe2 groups is so high that the nitration of l,8-bis(dimethylamino)-3,6-dinitronaphthalene (43) proceeds further, enabling one to isolate the hexanitro proton sponge 168 in a good yield (equation 14)37. The second... [Pg.988]

A distinct feature of the acenaphthene proton sponge 40 is that the para-positions are already occupied and the nitration may proceed at either the sterically hindered ortho-positions or the poorly activated meta-positions. As noted above (Schemes 22 and 23), the ortho-nitration is complicated by demethylation of one of the NMe2 groups. Still, selective mono-ortao-nitration without demethylation under comparatively mild conditions (tetranitromethane/CCl4/0 °C) has been reported to proceed in a good yield47. Even more important is the possibility to perform a regioselective meta-nitration of compound 40 in sulfuric acid to the derivatives 172-174 (Scheme 29). Obviously, the reaction occurs in this case via the cation 40 H+ 37. [Pg.989]


See other pages where Nitration proton sponges is mentioned: [Pg.48]    [Pg.54]    [Pg.983]    [Pg.986]    [Pg.987]    [Pg.989]    [Pg.989]    [Pg.996]    [Pg.997]    [Pg.44]   
See also in sourсe #XX -- [ Pg.987 , Pg.988 ]




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