Naphthalene electrophilic substitution

Naphthalene undergoes electrophilic substitutions on the ring, resulting in its various derivatives. In addition to the usual electrophilic substitutions, naphthalene can also undergo oxidation and reduction reactions under specific conditions as outlined below. 

The first reaction was involved in a synthesis of morphine, the starting ketone being made by reduction of a substituted naphthalene O. Amer. Chem. Soc., 1950, 72, 3704). No doubt an epoxide could have been used as the electrophile. 

Fluoronaphthalene [321-38-0] is prepared from 1-naphthylamine by the Balz-Schiemaim reaction in 52% yield or by diazotization in anhydrous hydrogen fluoride in 82% yield. Electrophilic substitution occurs at the 4-position, eg, nitration with fuming nitric acid in acetic acid gave 88% yield of l-fluoro-4-nitro-naphthalene [341 -92-4]. 

Anthraquinone dyes are derived from several key compounds called dye intermediates, and the methods for preparing these key intermediates can be divided into two types (/) introduction of substituent(s) onto the anthraquinone nucleus, and (2) synthesis of an anthraquinone nucleus having the desired substituents, starting from benzene or naphthalene derivatives (nucleus synthesis). The principal reactions ate nitration and sulfonation, which are very important ia preparing a-substituted anthraquiaones by electrophilic substitution. Nucleus synthesis is important for the production of P-substituted anthraquiaones such as 2-methylanthraquiQone and 2-chloroanthraquiaone. Friedel-Crafts acylation usiag aluminum chloride is appHed for this purpose. Synthesis of quinizatia (1,4-dihydroxyanthraquiQone) is also important. 

In compounds with a fused benzene ring, electrophilic substitution on carbon usually occurs in the benzenoid ring in preference to the heterocyclic ring. Frequently the orientation of substitution in these compounds parallels that in naphthalene. Conditions are often similar to those used for benzene itself. The actual position attacked varies compare formulae (341)-(346) where the orientation is shown for nitration sulfonation is usually similar for reasons which are not well understood. 

Draw resonance structures of the intermediate carbocations in the bromillation of naphthalene, and account for the fact that naphthalene undergoes electrophilic substitution at Cl rather than C2. 

With naphthalene, electrophilic substitution (e.g. nitration) is found to take place preferentially at the 1- (a-), rather than the alternative 2- (/ -), position. This can be accounted for by the more effective delocalisation, and hence stabilisation, that can take place in the Wheland intermediate for 1 - attack (60a - 606) compared with that for 2-attack (61) ... 

The naphthalene-like, aromatic stmcture of 1,2,3-benzothiadiazole imparts stability to the system that survives exposure to 20% potassium hydroxide at 150°C or 27% sulfuric acid at 200 °C. It is not oxidized by potassium permanganate, potassium ferricyanide, chromic acid, or dilute nitric acid <1996CHEC-II(4)289>. Electrophilic substitution occurs in the benzo ring, predominantly at the 4-position. Chlorine in the 6-position is displaced by a variety of nucleophiles <1975SST670>. 

Substituted 2-phenyl 1,3-dioxolanes 410 reacted with lithium and a catalytic amount of naphthalene (4%) in THF at —40 °C to yield intermediates 411 and products 412, after successive electrophilic substitution reaction at the same temperature and final hydrolysis (Scheme 115) . 

When the benzene ring of (1) is strongly activated, the milder electrophilic substitution reactions (nitrosation, azo coupling, and thiocyanation) are readily effected in a manner analogous to the naphthalenes <70RCR923>. Nitrosation of the 4- and 5-hydroxy derivatives of (1) occurs at positions 5 and 4, leading to nitroso derivatives that exist predominantly in the quinone monoxime form <74T3839>. 

The reactivities of isomeric thienothiophenes calculated in n -electron approximation by the PPP method, and those calculated considering all valence electrons, show reasonable agreement. It should be noted, however, that the choice of parameters in PPP calculations is somewhat arbitrary, especially for heavy atoms (e.g., sulfur). This may lead to a discrepancy between theoretical (in 7r-electron approximation) and experimental estimation of reactivities. For example, Clark applied the semiempirical method PPP SCF MO to calculate the reactivities of different positions in thienothiophenes 1—3, thiophene, and naphthalene from the localization energy values and found the following order of decreasing reactivity for electrophilic substitution thieno[3,4-b]-thiophene (3) > thieno[2,3-Z>]thiophene (I) > thieno [3,2-b]thiophene... 

Another rhodium vinylidene-mediated reaction for the preparation of substituted naphthalenes was discovered by Dankwardt in the course of studies on 6-endo-dig cyclizations ofenynes [6]. The majority ofhis substrates (not shown), including those bearing internal alkynes, reacted via a typical cationic cycloisomerization mechanism in the presence of alkynophilic metal complexes. In the case of silylalkynes, however, the use of [Rh(CO)2Cl]2 as a catalyst unexpectedly led to the formation of predominantly 4-silyl-l-silyloxy naphthalenes (12, Scheme 9.3). Clearly, a distinct mechanism is operative. The author s proposed catalytic cycle involves the formation of Rh(I) vinylidene intermediate 14 via 1,2-silyl-migration. A nucleophilic addition reaction is thought to occur between the enol-ether and the electrophilic vinylidene a-position of 14. Subsequent H-migration would be expected to provide the observed product. Formally a 67t-electrocyclization process, this type of reaction is promoted by W(0)-and Ru(II)-catalysts (Chapters 5 and 6). 

Electrophilic substitution of naphthalene occurs preferentially at a position. 

Although naphthalene, phenanthrene, and anthracene resemble benzene in many respects, they are more reactive than benzene in both substitution and addition reactions. This increased reactivity is expected on theoretical grounds because quantum-mechanical calculations show that the net loss in stabilization energy for the first step in electrophilic substitution or addition decreases progressively from benzene to anthracene therefore the reactivity in substitution and addition reactions should increase from benzene to anthracene. 

Naphthalene can undergo electrophilic substitution reactions analogous to those of benzene. Two different products can be obtained, e.g., for nitration. 

There are principally two different approaches of correlating experimental rate data of electrophilic substitution with reactivity indices (1) Correlating the index with the rate data of a given reaction, e.g. bromination. For example, a satisfying correlation of Dewar reactivity numbers with the log of rate constants of the bromination of benzene, naphthalene (1- and 2-position), biphenyl (4-position), phenanthrene (9-position), and anthracene (9-position) has been observed [55]. In correlations of this type the reactivity index corresponds to the reactivity constant in the Hammett equation while the slope of the linear correlation corresponds to the reaction constant (see also Sect. 3) (2) correlating the index with experimental a values. 

There is neither a partial positive nor a partial negative charge on the two nonequivalent positions 1 and 2 of naphthalene, which are poised for electrophilic substitution. One might consequently predict that electrophiles react with naphthalene without regiocontrol. Furthermore, this should occur with the same reaction rate with which benzene reacts. Both predictions contradict the experimental results For example, naphthalene is brominated with a 99 1 selectivity in the 1-position in comparison to the 2-position. The bromination at Cl takes place 12,000 times faster and the bromination at C2 120 times faster than the bromination of benzene. 

Electrophilic Substitution Reactions of o/7/io-Lithiated Benzene and Naphthalene Derivatives... 

There are three rules which control the position of electrophilic substitution attack on a monosubstituted naphthalene. 

There is neither a partial positive nor a partial negative charge on the two nonequivalent positions 1 and 2 of naphthalene, which are poised for electrophilic substitution. Based on the above-mentioned incorrect mechanistic model, one would consequently predict that electrophiles attack naphthalene without regiocontrol. Furthermore, this... 

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