Lithium naphthalene

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, are efficient difunctional initiators (eqs. 6,7) (3,20,64). However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization, since the unique abiUty of lithium to provide high 1,4-polydiene microstmcture is lost in polar media (1,33,34,57,63,64). Consequentiy, a significant research challenge has been to discover a hydrocarbon-soluble dilithium initiator which would initiate the polymerization of styrene and diene monomers to form monomodal a, CO-dianionic polymers at rates which are faster or comparable to the rates of polymerization, ie, to form narrow molecular weight distribution polymers (61,65,66). 

This approach frequently leads to the most active metals as the relatively short reduction times at low temperatures leads to reduced sintering of the metal particles and hence higher reactivity. Fujita, et aL(62) have recently shown that lithium naphthalide in tqluepe can be prepared by sonicating lithium, naphthalene, and N, N, N, N-tetramethylethylene-diamine (TMEDA) in toluene. This allows reductions of metal salts in hydrocarbon solvents. This proved to be especially beneficial with cadmium(49). An extension of this approach is to use the solid dilithium salt of the dianion of naphthalene. Use of this reducing agent in a hydrocarbon solvent is essential in the preparation of highly reactive uranium(54). This will be discussed in detail below. 

Reactions of Mg Generated by Lithium/Naphthalene. Although we have not exhaustively duplicated the previous reactions with the newer method of reduction, examination of Table I shows comparable reactivity for the reagents tested. Instead, we hav branched out to look at other facets of the chemistry accessible with Mg. ... 

In order to eliminate competing reaction with the solvent, a method for generating active uranium in hydrocarbon solvents was desired. Thus a hydrocarbon soluble reducing agent [(TMEDA)Li ]9 [Nap] (Nap=naphthalene) was prepared. This complex has previously been maae from 1,4-dihydro-naphthalene(llO). We have prepared this complex from lithium, naphthalene and TMEDA in a convenient reaction which is amenable to large scale synthesis. 

Styrene, benzene, and tetrahydrofuran were purified as described previously (8,11). Solutions of ec-butyllithium (Lithium Corporation of America, 12.0 wt % in cyclohexane) and methyllithium (Alfa, 1.45 M in ether) and lithium naphthalene were analyzed using the double titration procedure with 1,2-dibromoethane (12). Lithium naphthalene was prepared in tetrahydrofuran from lithium metal and a 25 mole % excess of sublimed naphthalene at -25°C using standard high vacuum procedures. Sealed ampoules of lithium naphthalene were stored in liquid nitrogen. 

The lithium naphthalene was prepared in THF from lithium metal and a 25 mole % excess of naphthalene to minimize formation of the dilithium naphthalene dianion (18,19). A sample of a,w-dilithiumpolystyrene was quenched with methanol for molecular weight characterization (see Table II). 

Surprisingly, Screttas et al. (1996) have found that the reaction of the lithium naphthalene anion-radical with methanol in THF follows the 2 1 stoichiometry and leads to the CjqHj—C oHjo mixture in the 95 5 ratio. The authors proposed the following alternative ... 

The presence of organolithium compounds in etheric solvents at temperatures above 0°C may lead to extensive decomposition of the solvent and solute a slow electron transfer side reaction of lithium naphthalene or sodium naphthalene with the THE solvent (equation 5) has been reported . The three isomeric forms of BuLi were shown to induce extensive decomposition of THE. The main path for this process is metallation at position 2 of THE, leading to ring opening and elimination of ethylene. An alternative path is proton abstraction at position 3, followed by ring opening. The presence of additives such as (—)-sparteine (24), DMPU (25), TMEDA and especially HMPA does not prevent decomposition but strongly affects the reaction path. ... 

The dynamic behavior of various solid organolithium complexes with TMEDA was investigated by variable-temperature and CP/MAS and Li MAS NMR spectroscopies. Detailed analysis of the spectra of the complexes led to proposals of various dynamic processes, such as inversion of the five-membered TMEDA-Li rings and complete rotation of the TMEDA ligands in their complex with the PhLi dimer (81), fast rotation of the ligands in the complex with cyclopentadienyllithium (82) and 180° ring flips in the complex with dilithium naphthalene (83) °. The significance of the structure of lithium naphthalene, dilithium naphthalene and their TMEDA solvation coiMlexes, in the function of naphthalene as catalyst for lithiation reactions, was discussed . ... 

A detailed analysis of the reaction mechanism of f-BuCl with lithium naphthalene in THE solution raises some interesting points. Although the overall stoichiometry of the... 

The classical preparation of alkyllithium compounds by reductive cleavage of alkyl phenyl sulfides with lithium naphthalene (stoichiometric version) was also carried out with the same electron carrier but under catalytic conditions (1-8%). When secondary alkyl phenyl sulfides 73 were allowed to react with lithium and a catalytic amount of naphthalene (8%) in THF at —40°C, secondary alkyllithium intermediates 74 were formed, which finally reacted successively with carbon dioxide and water, giving the expected carboxylic acids 75 (Scheme 30) °. 

Also in this case, the use of the chloro thioether 479 allowed the introduction of two different electrophiles in a sequential process. Using lithium naphthalene (the stoichiometric version of the arene-promoted lithiation) in THF at — 78°C, only a chlorine-lithium exchange occurred, so the first electrophile R R CO was introduced (—78 to —50°C). Then the second lithiation (sulfur-lithium exchange) takes place under catalytic conditions (naphthalene) and the second electrophile R R CO was introduced. After final hydrolysis, differently substituted 1,5-diols 476 were isolated (Scheme 134) °. 

Smith (29) showed that the polymerization of styrene by sodium ketyls with excess sodium produced low yields of isotactic polystyrene. Smith also believed that sodium ketyls initiated the styrene polymerization in the same way as the anionic alfin catalyst. Das, Feld and Szwarc (30) proposed that the lithium naphthalene polymerization of styrene occured through an anionic propagating species arising from the dissociation of the alkyllithium into ion pairs. These could arise from the dimeric styryllithium as a dialkyllithium anion and a lithium cation... 

Aromatic radical anions, such as lithium naphthalene or sodium naphthalene, arc efficient difunctionai initiators, However, the necessity of using polar solvents for their formation and use limits their utility for diene polymerization. 

By oxidising the sulfide to a sulfone, the synthetic versatility of this class of compounds is further increased. Deprotonation of either or both diastereoisomers of 98 leads, under thermodynamic control, to the equatorial organolithium 101 in which a destabilising interaction between the oxygen lone pair and the lithio substituent is avoided. However, lithium-naphthalene reduction of 102 to the organolithium 103 is axially selective because of the stabilisation afforded to the intermediate radical by the axial lone pair. Protonation of the product gives 104.88... 

Bissulfones have been reductively converted to organolithiums (lithiated sulfones) such as 113 and 114, using lithium - naphthalene.92 Two successive lithiations lead to the pheromone 115 of the lesser tea tartrix. 

Rieke magnesium by reduction of magnesium chloride with lithium naphthalene [7,8]. Lithium (224 mg, 3.3 mmol), anhydrous magnesium chloride (1.57 g, 16.5 mmol)1, naphthalene (436 mg, 3.4 mmol) and dry... 

---