Stable naphthalene system

Sol 4. (i) The first step involves a comotatory 67u-electron cyclization of I to give an intermediate II, which undergoes [1,9] hydrogen shift to give a stable benzenoid system III. This is followed by a [1,3] hydrogen shift and opening of an oxazoline ring to form a stable naphthalene system V. 

Oxadiazole (1 R = R = H) is calculated to be 8.9 kcal moU less stable than its open chain tautomer (2) (85AG(E)713>. On the other hand, 1,2,3-benzoxadiazole is about 1 kcal mol more stable than 2-diazocyclohexadienone (91JST(247)135>. The position of the equilibrium is markedly affected by substituents. The benzoxadiazole (4) is more stable than its tautomer (5) by about 1.5 kcal mol but tetrafluorobenzoxadiazole is substantially less stable than the corresponding diazocarbonyl isomer. The open-chain forms are also favoured by polar solvents and by hydrogen bonding. In the naphthalene series, the stabilization of the fused oxadiazole (6) relative to its open chain tautomer can be ascribed to partial bond fixation in the naphthalene system, which disfavours... 

Like benzene, naphthalene typically undergoes electrophilic substitution this is one of the properties that entitle it to the designation of aromatic. An electrophilic reagent finds the ir cloud a source of available electrons, and attaches itself to the ring to form an intermediate carbonium ion to restore the stable aromatic system, the carbonium ion then gives up a proton. 

Absence of Termination Processes. The possibility of having carbanionic species that show a negligible rate of termination is now realized. In other words, just as the growing chain end in the alkoxide polymerization of ethylene oxide represents a"stable" salt of an alkali metal and an alcohol, the styryl sodium chain end, in the polymerization of styrene by sodium naphthalene, represents a "stable" salt of sodium and a hydrocarbon. This relationship was first noted in the particular case of the sodium naphthalene systems in which the organometallic species is stabilized by a high degree of solvation by an ether, such as tetrahydrofuran, so that no observable side reactions exist, that is, termination of chains, at least not within the time scale of the polymerization reaction. 

Effect of Reaction Conditions. The naphthalene system also provides an interesting example of how product composition may be affected by reaction conditions (27). Naphthalene, as well as its 1,4-dimethyl and 2,6-dimethyl derivatives, can be reduced smoothly (with yields greater than 90%) by lithium-NH3 at -78 °C (Scheme XII) because the final monoanion (see the preceding discussion) is stable at this low temperature, and rapid protonation produces the 1,4-dihydro product (13). However, at reflux, the... 

Naphthalene forms stable complexes with [TpRe(CO)(L) when L=f-BuNC, PMcj, py, DMAP, or Melm. In toluene and an excess of naphthalene, the direct reduction of TpRe(L)Br2 (L=py or DMAP) with Na° under a CO atmosphere will generate the py or DMAP naphthalene complex in -40% yield [16]. Generation of the Re(I) PMe3 and f-BuNC naphthalene systems is not as straightforward. They are synthesized by the oxidation of the corresponding Re(I) olefin complexes to Re(II) complexes, liberation of the olefin by heating, and reduction of the Re(II) species in the presence of naphthalene [16]. 

Site selectivity is the selectivity shown by a reagent toward one site (or more) of a polyfunctional molecule when several sites, in principle, are available. In cycloadditions, site selectivity always involves a pair of sites. For example, the Diels—Alder reaction of anthracene generally takes place across the 9,10 position than across the 1,4 or 3,9a. This may be explained by the fact that the highest coefficients in the HOMO of the anthracene are at the 9,10 position. Furthermore, addition at the 9,10 position creates two isolated benzene rings, which is a more stable system than that of a naphthalene system created by the addition at the 1,4 position. 

The other important carbocycHc ring system used in dyes is naphthalene [91-20-3]. Here the preferred position of attack is the 1-position. However, 2-substituted naphthalenes are thermodynamically more stable, and under equiUbratiag conditions the 2-isomer is formed ia preference to the 1-isomer. 

Complexation with polyaromatic systems has also been observed. For instance, Mlnaphthalenelj, M = Cr (88,183), Mo (183), V (183), or Ti (183) may be synthesized in a solution reactor with the appropriate, metal vapors at liquid-nitrogen temperature. The Cr/naphthalene complex is less stable (dec. 160°C) than CrtCsH ) (m.p. 283-284° C). In fact, the naphthalene ligand is sufficiently labile to allow reaction under mild conditions, to afford CrL (L = CO or Bu NC), or Cr(naphth)Ls [L = PFj, P(OMe)3, or PMea]. The Mo, V, and Ti species are equally reactive. Analogous 1-methylnaphthalene complexes were also isolated (183). In addition, the complexes shown in Fig. 38 were synthesized by reaction, at the temperature of liquid nitrogen, of Cr atoms with 1,4-diphenylbutane (35, 201, 202). Analogous complexes were formed with 1,5-diphenylbutane (202). 

Both peracyclene (105), which because of strain is stable only in solution, and dipleiadiene (106) are paratropic, as shown by NMR spectra. These molecules might have been expected to behave like naphthalenes with outer bridges, but instead, the outer n frameworks (12 and 16 electrons, respectively) constitute antiaromatic systems with an extra central double bond. 

In fused ring systems, the positions are not equivalent and there is usually a preferred orientation even in the unsubstituted hydrocarbon. The preferred positions may often by predicted as for benzene rings. Thus it is possible to draw more canonical forms for the arenium ion when naphthalene is attacked at the a position than when it is attacked at the p position, and the a position is the preferred site of attack,though, as previously mentioned (p. 682), the isomer formed by substitution at the p position is thermodynamically more stable and is the product if the reaction is reversible and equilibrium is reached. Because of the more extensive delocalization of charges in the corresponding arenium ions, naphthalene is more reactive than benzene and substitution is faster at both positions. Similarly, anthracene, phenanthrene, and other fused polycyclic aromatic hydrocarbons are also substituted faster than benzene. 

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