Cations naphthalene proton sponges

Selective displacement of one CIT3 by the CD3 group was employed to establish the structure of IF1B in the cations of non-symmetrically substituted naphthalene proton sponges by FI NMR1". This direct method is simple and does not require NMR on other nuclei such as deuterium, carbon or nitrogen. 

Recently, there has been research activity in the area of chiral proton sponges, in particular, those containing nitrogens with pairs of different substituents. The molecular asymmetry of such sponges, especially well pronounced in the cationic forms, arises as a result of the arrangement of bulkier functions at opposite sides of the naphthalene ring plane. Synthesis of these compounds, e.g. 5248, 5333 and 5449, is also achieved via alkylation of the corresponding A,A -dialkyl- or A,A,A -trialkyl-l,8-diaminonaphthalenes (Scheme 3). 

An interesting situation occurs in the dication of the doubly protonated proton sponge 35 2H+ and in the zwitterion of 4,5-dihydroxy-l,8-bis(dimethylamino)naphthalene (111). The asymmetry of both hydrogen bridges in these systems is clearly controlled by electrostatic factors in order to obtain maximal separation of the two positively charged centres in 35 2H+ and, in reverse, to gain an attraction between the cationic and anionic centres in 111. 

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... 

It was already mentioned (Section III.A.2) that 4,5-dihydroxy-l,8-bis(dimethylamino) naphthalene actually exists in the form of the internal salt 11139. Somewhat similar is the situation with 2,7-dihydroxynaphthalene 140, also behaving in solution as a betaine165. The latter, on treatment with acids, gives the cation 140-H+, while the action of alkali results in protonated dianion 141 (Scheme 15). All attempts to deprotonate 141 (even with the KOH-DMSO system at 100 °C or with LiH-DMSO) failed. This implies that the basicity of still unisolated dianion 142 lies in the limit of pKa = 25-26 (H2O scale). Thus, dianion 142 can be presently considered as the strongest base among all proton sponge and aniline derivatives. 

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... 

An example of the use of an intermolecular carbopalladation in complex molecule synthesis is the preparation of a PAF (platelet activating factor) antagonist (Scheme 11). In the key step, an intermolecular Heck reaction of 2-naphthyl triflate with 2,3-dihydrofuran 71 yields 2-naphthyl-2,3-dihydrofuran 72 in 52% yield with excellent enantioselectivity. The reaction presumably occurs via the cationic manifold and the alkene is isomerized by a hy-dropalladation/dehydropalladation reaction. The minor product 2,5-dihydrofuran 73 is obtained in 26% yield with modest enantioselectivity favoring the opposite absolute configuration at the key center. Critical to the reaction is the use of the sterically demanding and highly basic proton sponge [l,8-bis(dimethylamino)naphthalene] as the base. It is... 

Figure 14.17 labeled systems exhibiting strong intramolecular NHN hydrogen bonds. 28 deprotonated mono-anion of 6-nitro-2,3-dipyrrol-2-ylquinoxaline [59]. 29 protonated mono-cation of 1,6-dimethyl-1,6-diazacyclodecane [59] and 30 of 1,8-bis(dimethylamino)-naphthalene (OMAN, proton sponge) [60]. For comparison, the neutral molecules W,W -diphenyl-6-aminopentafulvene-1-aldimine (1, see... 

---