[7308-67-0] · C10H8Li · Lithium Naphthalenide · (MW 134.12)
Alternate Name: LN.
Physical Data: no data on the isolated material; only available in solution.
Solubility: sol ether, benzene, THF; reacts with protic solvents and THF at elevated temperatures.2
Preparative Methods: made by addition of freshly cut Lithium metal to a solution of naphthalene in THF. Preparation can be accelerated by ultrasonication.3
Analysis of Reagent Purity: two titration methods have been described;4 the more convenient4b involves conversion of 1,1-diphenylethylene by lithium naphthalenide to an intensely red-colored dianion, which is then titrated against s-butanol.
Handling, Storage, and Precautions: can be stored in solution up to several days; must be protected from air and moisture; can be used to ambient temperature.
The powerful reductive nature of this reagent makes it an important tool for lithium-heteroatom exchange reactions. Thus, it was established early on that (phenylthio)alkanes can be converted into their requisite alkyllithium species.5 This has become the method of choice over generation by lithium metal alone. The resultant alkyllithium species can either be quenched with a proton source (eq 1),6 or intercepted with an electrophile. This has subsequently evolved into a powerful technique, since the reaction is general for all chalcogens (eq 2)7 and halides (eq 3).8
Metal-heteroatom exchange can also be persuaded to occur with a variety of other systems, resulting in allyl-9 and vinyllithium10 species, as well as a-lithio ethers (eq 3),1c,8,11 a-lithio thioethers,1c,12 a-lithio amines,11b and a-lithio silanes.1c,12b,13 The latter class provides useful intermediates for the Peterson alkenation reaction (eq 4).13a
In some cases, however, it may be advantageous to proceed via either Lithium 1-(Dimethylamino)naphthalenide (LDMAN), or Lithium 4,4´-Di-t-butylbiphenylide (LDBB).1c With the former reagent, the byproduct formed, (dimethylamino)naphthalene, is more easily removed than is naphthalene from product mixtures. In the latter case, the greater reduction potential of di-t-butylbiphenyl appears to lead to more efficient halogen-lithium exchange.8b
Lithium naphthalenide efficiently deprotonates b-alkynyloxy14 and carboxylate anions (eq 5).15 In addition, the previously mentioned phenomenon of reductive metalation has been exploited to access dianions from halohydrins,16 b-halo carboxylic acids,17 and b-halo carboxamides,18 and even trianions from b,o´-dihalo alcohols.19 A major pathway for the polyanionic species is b-elimination (eq 6);16a,19 when such processes can be avoided, the polyanions react according to Hauser's rule (eq 7).16a,20
Since lithium naphthalenide is a particularly effective initiator for halogen-metal exchange, it has found widespread use for the conversion of dihalides to unsaturated species. Thus, 1,2-dichlorodisilanes have been converted to silenes (eq 8)21 and diphosphiranes to phosphacumulenes.22 In a related field, silicon cages23 have been constructed from trichlorodisilanes.
Lithium naphthalenide is a convenient reducing agent for a variety of metals, and shows great promise in the synthetic area. Thus, CuI complexes have been reduced to Cu0; the resultant highly reactive species adds in an oxidative fashion across the carbon-halogen bond.24 As a consequence, the well known organocuprate addition chemistry can be carried out in one step from halocarbons, without having to initially prepare the organolithium species (eq 9). In addition to the reduction of CuI, lithium naphthalenide reduces SiIV to SiII,25 SnIV to SnII (eq 10),26 and various lanthanide compounds.27
Lithium naphthalenide has long been a convenient initiator for anionic
living polymerization reactions.28 Thus, styrenes, acrylates, dienes, and other monomers have been polymerized using lithium naphthalenide as an anionic initiator. In some circumstances, however, oligomerization can be controlled to furnish only dimers (eq 11).29 Also, in the presence of a secondary amine, 1,3-dienes can be persuaded to react in a formal 1,4-fashion to produce allyl amines (eq 12).30
Interesting approaches toward functional polymers have recently been detailed,31 wherein previously described chemistry involving lithium naphthalenide is conducted on suitably substituted polystyrene derivatives (eq 13).
Copper(I) Iodide-Triethylphosphine-Lithium Naphthalenide;
Kevin M. Short
Wayne State University, Detroit, MI, USA