Preparation aromatic alkali metal anions

Polymetallic anions, prepared by dissolution of alloys of the alkali and post-transition metals in amine solvents (often with a complexand for the alkali metal cation), have been characterized in crystalline and solution phases. Clusters TlSng3, Ge92 (with 20 skeletal bonding electrons), Sn93- (21 skeletal e) and Bi95+ (22 skeletal e) possess a tricapped trigonal prismatic structure, symmetry D3A, with variations of dimensional detail which correlate with the electron population.291 292 This structure is a ctoso-deltahedron, and with 20 (2h + 2) skeletal electrons can be construed to be three-dimensionally aromatic.292 The 22e clusters M94 (M = Ge, Sn, Pb) occur as the C4v monocapped square antiprism, a nido polyhedron. 

Pentamethylcyclopentadiene is a useful aromatic building block for the preparation of other compounds. It can be converted to many salts of its conjugate base with alkali metals or strong bases such as butyl lithium.4 These pentamethylcyclopentadienyl anion salts as well as the diene itself can be transformed into n -pentamethylcyclopentadienyl ligands of organotransition metal complexes by many known methods.4... 

The formation of molecular radical ions by electron transfer reactions between alkali metals and a wide variety of aromatic and other organic compounds in polar solvents is well established. A very large number of radical anions have been prepared by this method and extensive studies of their e.s.r. and optical spectra have been made (Bowers, 1965 Gerson, 1967 Kaiser and Kevan, 1968). In solution the electron transfer reaction will be facilitated by the subsequent solvation of the two ions (or ion pair) by the polar solvent molecules. However, we have observed that similar electron transfer reactions occur readily when alkali metal atoms are deposited on a variety of relatively non polar substances at 77°K in the rotating cryostat. In most cases the parent compound acts as the matrix, though for some radical ions an inert matrix of a non-polar hydrocarbon has been used successfully. It is perhaps surprising that the reactions occur so readily as the energy of solvation of the ions must be quite small in most of these systems as compared with that in the polar liquids. 

Bifunctional Initiation. The bifunctional initiators like alkali metal complexes of polycyclic aromatic compounds can be used to produce ABA triblock copolymers even when the A anion is not sufficiently basic to initiate polymerization of B monomers. In these cases polymerization would be started with monomer B to produce a polymeric dianion which could initiate polymerization of the A monomer which is added later. These initiators can be prepared only in aliphatic ethers, however. This precludes their use for the synthesis of useful styrene-diene ABA copolymers because polydienes made anionically in such solvents have low 1,4 contents and are not good rubbers. 

A few comments concerning the crystallization of carbanions are in order. These comments are based upon the personal experience developed in our own laboratory and also upon observations noted in the literature in the course of crystallizing enolate anions. Although alkali metal enolate anions are relatively unstable compounds, they have been prepared in the solid state, isolated, and characterized by IR and UV spectroscopy in the 1970s. Thus the ot-lithiated esters of a number of simple esters of isobutyric acid are prepared by metallation of the esters with lithium diisopropylamide in benzene or toluene solution. The soluble lithiated esters are quite stable at room temperature in aliphatic or aromatic hydrocarbon solvents and are crystallized out of solution at low temperature (e.g. -70 °C.). Alternatively the less soluble enolates tend to precipitate out of solution and are isolated by centrifugation and subsequent removal of the solvent. Recrystallization from a suitable solvent can then be attempted. The thermal stability of the lithiated ester enolates is dramatically decreased in the presence of a solvent with a donor atom such as tetrahydrofuran. 

The polycyclic anions were first prepared by metal reduction in 1914 by Schlenk et al.5 a) and studied later by Schlenk and Bergmann 5 b). This class of conjugated anions opened a new era in carbanion chemistry by pointing out the electron transfer process as a source for charged species. The mechanism of the metal reduction of polycyclic hydrocarbons has been investigated and is well established 1,215 18-68>. The addition of two electrons to the fully conjugated (4n + 2)n-molecules yields 4mt paratropic systems 2°. 137 139>. The chemistry of this reaction is simple, with electrons initially on the alkali metal going to 7t-molecular orbitals associated with the aromatic hydrocarbon molecule (Eq. 13). 

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... 

A variety of ammonia solvated alkali metal phosphides has recently been prepared in the Korber group. Examples of these peculiar compounds are Cs2P4-3NH3 with the Gtt aromatic P4 anion and CssPn-SNHs. ... 

The reactions of monomers with aromatic radical anions or directly with alkali metals can be used to prepare oligomeric dianionic initiators from monomers such as a-methylstyrene which have accessible ceiling temperatures (T = 61 °C) as shown in Scheme 7.5 (R = CH3) [54], Dimers or tetramers can be formed depending on the alkali metal system, temperature, and concentration. 

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