Carbanions alkali metal cations

Mixed carbanion/alkoxide complexes were formed from combining n,sBu2Mg, TuOM (M = Na or K), and TMEDA to form the dimeric CIPs [(Bu)2(tBuO)MMg(TMEDA)]2 (M = Na and K) 437, 438.442 While the two structures are identical in their atom connectivity, they are not precisely isostructural in that K shows a bias toward C-coordination while Na is inclined toward N. The alkali metal cations are both formally five-coordinate bonding with two a-C(Bu) atoms, two N(TMEDA) atoms, and a single 0(cBu) atom. The dominant feature in both structures though is the [(Bu)2Mg(/x-tBuO)2MgBu2]2 dianion. 

The preferential -configuration of the enol esters, derived from p-dicarbonyl compounds under phase-transfer conditions, contrasts with the formation of the Z-enol esters when the reaction is carried out by classical procedures using alkali metal alkoxides. In the latter case, the U form of the intermediate enolate anion is stabilized by chelation with the alkali metal cation, thereby promoting the exclusive formation of the Z-enol ester (9) (Scheme 3.5), whereas the formation of the ion-pair with the quaternary ammonium cation allows the carbanion to adopt the thermodynamically more stable sickle or W forms, (7) and (8), which lead to the E-enol esters (10) [54],... 

In carbanions, for instance enolates, in which delocalization of the negative charge to a more electronegative atom (N, O, S) is possible, the cation is usually bound to this atom if the cation is hard (e.g. alkali metal cations, Mg2+ [530], Zn2+) in this case the nucleophilic carbon atom becomes a planar sp2 hybrid. If the cation is soft (e.g. the cations of late transition metals) it will often be bound to the depro-tonated carbon atom these organometallic compounds are, however, usually only weak nucleophiles and will not be treated here. 

Various modes of termination of anionic polymerization can be visualized. The growing chain end could split out a hydride ion to leave a residual double bond. This is, however, a high activation energy process and has not as yet been reported in the cases where alkali metal cations are present. It is important in systems involving Al—C bonds, however (73). A second possibility is termination through isomerization of the carbanion to an inactive anion. Proton transfer from solvent, polymer, or monomer would also cause termination of the growing chain. Lastly, the carbanion could undergo an irreversible reaction with solvent or monomer. The latter three types have been shown or postulated as termination or transfer reactions. 

This discussion of aliphatic carbanion structures has included mainly organolithium compounds simply because the structures of most aliphatic caibanions incorporate lithium as the counterion and also because this alkali metal cation is the most widely used by synthetic organic chemists. For comparison the entire series of Group la methyl carbanion structures, i.e. MeNa, MeK, MeRb and MeCs, have been determined. Methylsodium was prepared by reaction of methyllithium with sodium r-butoxide. Depending upon the reaction conditions, the products obtained by this procedure contain variable amounts of methyllithium and methylsodium (Na Li atom ratios from 36 1 to 3 1). Hie crystal structure of these methylsodium preparations resembles the cubic tetramer (38) obtained for methyllithium with the Na— Na distances of 3.12 and 3.19 A and Na—C distances of 2.58 and 2.64 A. 

In general, these anions are associated with a counterion, typically an alkali metal cation. The exact nature of the anion can be quite varied depending on the structure of the anion, counterion, solvent, and temperature [3-5]. The range of possible propagating species in anionic polymerization is depicted in terms of a Winstein spectrum of structures as shown in Equation 7.2 for a carbanionic chain end (R ) [3, 6]. In addition to the aggregated (associated) (I) and unaggregated (unassociated) (2) species, it is necessary to consider the intervention of free ions (5), contact... 

Greenacre, G. C., and Young, R. N. "Ion-Pairing of Substituted 1,3-Diphenylallyl Carbanions With Alkali-Metal Cations." /. Chem. Soc. Perkin II Trans., 1661 (1975). 

Another differential reaction is copolymerization. An equi-molar mixture of styrene and methyl methacrylate gives copolymers of different composition depending on the initiator. The radical chains started by benzoyl peroxide are 51 % polystyrene, the cationic chains from stannic chloride or boron trifluoride etherate are 100% polystyrene, and the anionic chains from sodium or potassium are more than 99 % polymethyl methacrylate.444 The radicals attack either monomer indiscriminately, the carbanions prefer methyl methacrylate and the carbonium ions prefer styrene. As can be seen from the data of Table XIV, the reactivity of a radical varies considerably with its structure, and it is worth considering whether this variability would be enough to make a radical derived from sodium or potassium give 99 % polymethyl methacrylate.446 If so, the alkali metal intitiated polymerization would not need to be a carbanionic chain reaction. However, the polymer initiated by triphenylmethyl sodium is also about 99% polymethyl methacrylate, whereas tert-butyl peroxide and >-chlorobenzoyl peroxide give 49 to 51 % styrene in the initial polymer.445... 

The reaction involves the transfer of an electron from the alkali metal to naphthalene. The radical nature of the anion-radical has been established from electron spin resonance spectroscopy and the carbanion nature by their reaction with carbon dioxide to form the carboxylic acid derivative. The equilibrium in Eq. 5-65 depends on the electron affinity of the hydrocarbon and the donor properties of the solvent. Biphenyl is less useful than naphthalene since its equilibrium is far less toward the anion-radical than for naphthalene. Anthracene is also less useful even though it easily forms the anion-radical. The anthracene anion-radical is too stable to initiate polymerization. Polar solvents are needed to stabilize the anion-radical, primarily via solvation of the cation. Sodium naphthalene is formed quantitatively in tetrahy-drofuran (THF), but dilution with hydrocarbons results in precipitation of sodium and regeneration of naphthalene. For the less electropositive alkaline-earth metals, an even more polar solent than THF [e.g., hexamethylphosphoramide (HMPA)] is needed. 

The work described in this review shows that the organometallic chemistry of the heavier alkali metals has ceased to be an exotic backwater. A good deal of information about the interaction between metal cations and carbanionic fragments has been obtained. Much of it has come from studies on crystalline solids, and although it may be reasonable to expect that the species found in the solid are also present in solution, this has to be established experimentally in each case. Some evidence has been obtained from multinuclear or multidimensional NMR spectroscopy, but so far there have been few studies using solid-state NMR to link structures found in the solid with those in solution. Even when the dominant species in solution is established, still more work is required to determine which species react fastest with particular substrates. The active species in a reaction may be present only in low concentration. 

Pentadienyl carbanions are analogous to allyl anions with an extended delocalization of charge. Reaction of 1,3- or 1,4-pentadienes and alkali metals in THF in the presence of a base, such as NMes or TMEDA, affords crystalline pentadienylalkali metal complexes. A contact ion pair structure is predicted for these compounds by theoretical calculations and is consistent with solution structural data obtained by NMR. The pentadienyl anion usually interacts with the cation as an rj - or ) -ligand depending on the structural orientation of the backbone carbon atoms of the pentadienyl anion (W-, S-, or U-shaped skeletal structures). A contact ion pair structure having a W-shaped pentadienyl ligand is shown (16). 2,4-Disubstituted... 

In initiation by an alkali-metal alkyl, the alkyl links up with the monomer to form a carbanion, leaving the metal as a cation to compensate the negative charge. An example is the initiation of styrene polymerization by butyl lithium [70-72] ... 

Overall polymerization rates which are equated with the rate of propagation were first order in monomer and very fast. At -78 °C the rate constants fell between 1.0 and 3.4 x 1051 mol-1 s 1. Activation energies were very small, 2.2 and 5.5 kJ mol-1 for ethyl (EGA) and butyl (BCA) cyanoacrylates, respectively. Of a range of ammonium, phosphonium, and alkali metal salts only lithium bromide significantly reduced the rate of polymerization. Ogawa and Romero16 found the rate of acrylonitrile polymerization increased by the presence of an ammonium salt but reduced by lithium chloride. There may be a specific interaction between cyano substituted carbanions and the lithium cation. 

In case the anion is the polymeric species (anionic polymerization) a carbanion or an alkoxide anion forms the active chain end. Initiation is achieved by direct attack of organometaUic compounds, or by electron transfer from alkali metals, alkali metal complexes, or ionizing radiation. In case the cation is the polymeric species (cationic polymerization) a... 

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