Naphthalene alkali-metal reduction

The addition of alkyl halides to aromatic anion radicals, generated by alkali metal reduction in etheral solvents, was already known in the 1950s [176] and was reviewed by Garst in 1971 [177]. The first electrochemical analogue was observed by Lund et al. [178]. These authors cathodically reduced hydrocarbons such as naphthalene, anthracene, stilbene [128,129], and perylene [130] in the presence of alkyl halides and isolated hydrogenated and alkylated products. Similar reactions are observed when the halides are replaced by ammonium or sulfonium [179]. 

Similarly, the alkali-metal reduction of naphthalene in ammonia at -33 °C for extended reaction periods produces substantial amounts (20-70%) of oligomeric products (33-35). This result is due to a slow buildup of 1,2-dihydronaphthalene (22), which then adds to its monoanion to form a dimeric monoanion (Scheme XVI) the dimeric monoanion is protonated by ammonia. Hence, this process is dependent on the generation of neutral, reduced products that are formed by higher temperatures, Li over Na, and extended reaction periods. Presumably, these factors are the reasons that such products were not noted in the earlier study (32). 

A second general approach is to use an alkali metal in conjunction with an electron carrier such as naphthalene. The electron carrier is normally used in less than stoichiometric proportions, generally 5 to 10 mole percent based on the metal salt being reduced. This procedure allows reductions to be carried out at ambient temperatures or lower in contrast to the previous approach which requires refluxing. A convenient reducing metal is lithium. 

Pentachloropyridine undergoes a regio-selective dechlorination to give 2,3,5,6-tetrachloropyridine [118]. The reduction of 1- and 2-phenoxynaphthalene at the cathode or by an alkali metal afforded phenol and naphthalene. The intermediacy of the naphthalene radical has been made probable by the use of an internal radical trap [119]. 5-Aryloxy-l-phenyltetrazoles cleave reductively predominantly at the tetrazolyl-O bond. The relative rates of... 

Sodium-naphthalene reduction of organotrineopentoxyphosphonium salts led to the instantaneous loss of phosphonium ion phosphonates and phosphites were obtained748 (reaction 224). Alkali metal amalgams are efficient reagents for the reductive cleavage of both achiral and optically active phosphonium salts configuration is retained750 (Table 23). 

The cyclic peralkylsilane oligomers, (R2Si) with n = 4-6, manifested especially strong electron delocalization.5 These rings are structurally analogous to those of the cycloalkanes, since the silicon atoms form four sigma bonds. However, the electronic properties of the cyclosilanes more nearly resemble those of aromatic hydrocarbons such as benzene. One example of such behavior is their reduction to anion radicals. Aromatic hydrocarbons such as naphthalene can be reduced, electrolytically or with alkali metals, to deeply colored anion radicals in which an unpaired electron occupies the lowest unoccupied molecular orbital (LUMO) of the hydrocarbon (equation (2)). 

These results, and also the failure of the exciton coupling model in reproducing the CD spectra of (R)-(+)-56, were consistent with weak, but still significant, conjugation between naphthalene moieties. Notably, dihedral angles of 70° were found between naphthalene moieties in the X-ray structure of enantiopure (R)-(+)-56. Carbodianion (R)-562. 2M+ (M = Li, Na) was prepared via reduction of (R)-(+)-56 with alkali metals gave carbodianion (R)-562. 2M+ (M = Li, Na) no intermediate radical anion could be detected or isolated, in agreement with the cyclic voltammetric data [101]. 

Many aromatic compounds can undergo one-electron reduction by alkali metals, such as Na and Li. For example, the reaction of naphthalene with sodium in an aprotic solvent gives the naphthalene radical anion - sodium ion salt. 

The hexacarbonylniobate(—1) and hexacarbonyltanta-late(—1) anions are well established. The [Nb(CO)6] anion can be prepared at atmospheric pressure of CO from NbCls by using a prereduction step with alkali metal/naphthalene at low temperature, followed by carbonylation of the intermediate at low temperature in DME as solvent. Another method of preparation, also operating at atmospheric pressure of CO, uses the reduction of NbCls with Mg/Zn/pyridine at room temperature and gives yields as high as 48% of recrystallized Na(THF)[Nb(CO)6]. The alkali metal derivatives of [M(CO)6] , M = Nb, Ta, are rapidly oxidized by air the PPN derivatives are stable in air for short periods of time. PPN[Nb(CO)6] and PPN[Ta(CO)6] are isostructural the hexacarbonyl anions possess an almost exact octahedral geometry with the following bond distances Nb-CO, 2.089A Ta-CO, 2.083 A. 

Naphthalene is reduced to 1,4-dihydronaphthalene by sodium and alcohol. Isomerization of this product to 3,4-dihydronaphthalene occurs with sodamide in liquid ammonia. Tetrahydronaphthalene (tetralin) is formed from naphthalene by sodium in amyl alcohol or by reduction with nickel-aluminum alloy and aqueous alkali. Catalytic hydrogenation of naphthalene can be stopped at the tetralin stage over copper chromite, Raney nickel, or alkali metal catalysts. cis-Decahydronaphthalene is produced by high-pressure hydrogenation of tetralin over Adams catalyst, whereas a mixture of cis- and trans-decalins is obtained from naphthalene under the same conditions. ... 

The Sternberg reductive alkylation procedure was carried out as described in the literature (i, 2j, employing up to a 40 1 ratio of alkali metal to naphthalene (molar basis). The model reactions involved mixtures containing 5 g (39 mmol of naphthalene, 2 g (87 mmol) of sodium, and 25 mL of tetrahydrofuran. Tetra-hydrofuran was obtained from both Fisher and Aldrich and was either distilled from lithium-aluminum hydride or dried over sodium. No differences in the decomposition behavior was noted for the two drying procedures furthermore, the presence or absence of the inhibitor butylated hydroxy toluene did not affect the decomposition. 

Reduction of NbCl4(thf)2 or Ta2Cli0 with alkali metal arenides in ether solvents at — 60 °C provided, after work-up, salts of the homoleptic arene complexes [M IL3] ((271) L = -naphthalene, 1 -4- 74-anthracene).663 664... 

The process now known as reductive alkylation of rc-conjugated anions (quenching of anions) is as old as the preparation of the ions themselves5). The highly colored solutions obtained by the addition of alkali metals to solutions of aromatic hydrocarbons in ether were reacted with electrohpiles such as protons or alkyl halides (Scheme 2). The products of such a process are reduced hydrocarbons. The Birch reduction is one example oT such a process, reaction of an anion with an alkyl halide leading to an alkylated reduced hydrocarbon is another example 165). The complexity of the quenching experiments is demonstrated by the naphthalene radical anion 150-1581... 

The reduction of [Cr(>f6-naphthalene)2]+ by lithium naphthalide was thought to give [Cr(>f6-naphthalene)2] (410), but this is doubtful since the formal potential of the naphthalene-naphthalide couple is at least several hundred millivolts positive of that of the complex (416). A similar reaction between [Cr(f/-C6H6)2]+ and alkali metals in thf or 1,2-dimethoxyethane yielded the radical [Cr(f/-C6H6)(f/-C6H5)] (36) ESR spectroscopy showed an intramolecular, interannular hydrogen exchange reaction (Scheme 32) with a rate of 107 sec"1 (430). 

Unpaired electrons can be present in ions as well as in the neutral systems that have been considered up to this point. There are many such radical cations and radical anions, and we consider some representative examples in this section. Various aromatic and conjugated polyunsaturated hydrocarbons undergo one-electron reduction by alkali metals. Benzene and naphthalene are examples. The ESR spectrum of the benzene radical anion was shown earlier in Figure 11.2a. These reductions must be carried out in aprotic solvents, and ethers are usually used for that purpose. The ease of formation of the radical anion increases as the number of fused rings increases. The electrochemical reduction potentials of some representative compounds are given in... 

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