Dimeric-Dianions

These reactions are usehil for the preparation of homogeneous difunctional initiators from a-methylstyrene in polar solvents such as tetrahydrofuran. Because of the low ceiling temperature of a-methylstyrene (T = 61° C) (26), dimers or tetramers can be formed depending on the alkaU metal system, temperature, and concentration. Thus the reduction of a-methylstyrene by sodium potassium alloy produces the dimeric dianionic initiators in THF (27), while the reduction with sodium metal forms the tetrameric dianions as the main products (28). The stmctures of the dimer and tetramer correspond to initial tail-to-tail addition to form the most stable dianion as shown in equations 6 and 7 (28). 

By contrast to the metaphosphimates, the metaphosphate analogues in which two chalcogens are replaced by imido groups form dimeric, dianionic ligands (24). There is only one example of a metal complex of the oxo system (24, E = O). An N,0-chelated bis(dimethylalumimun) complex is prepared by the reaction of two equivalents of trimethylaliuninum with ds-[ BuNH(0)P(//-N Bu)2P-(0)NH Bu] [20]. The structure of this complex is comparable to that of (15). 

For instance, in the cases of crystalline salts containing ti-[TCNE] dimers (TCNE=tetracyanoethylene) the existence of exceptionally long (>2.9 A) C-C bonding interactions involving rt-electrons has been explained on the basis of the existence in the crystals of dimer dianions stabilized by cation [TCNE]( ) interactions, which provide the electrostatic stabilization necessary to overcome the intradimer electrostatic repulsion [33a]. 

Pentahalonitrobenzenes (15) undergo electrochemical coupling to the corresponding octahalobiphenyls (16, equation 3)35. There is an interesting mechanistic dichotomy between the fluorine and chlorine compounds (15a and 15b, respectively). The radical anion of 15a couples, then the resulting dimeric dianion ejects two fluoride ions to afford 16 in contrast, the radical anion of 15b ejects chloride ion to afford a neutral radical, which then dimerizes to 16. 

Radical anion EGBs derived from aromatic carbonyl compounds are expected to be relatively weak bases but since the radical anions undergo dimerization, the more basic dimer dianions may be active as EGBs for substrates with pK values in the range 20 to 23. Aromatic carbonyl compounds have primarily been used as PBs in catalytic reactions in which the PB also functions as an electrophile (cf. Sect. 14.9.2). 

Typical acceptors in Michael additions are reducible and their radical anions often undergo dimerization (hydrodimerizations). Either the radical anions or more likely the dimer dianions can act as EGBs toward the donor in the Michael addition. Since the reaction is catalytic in base when the product anion is more basic than the donor anion, the Michael addition can take place by reduction of a small fraction (2-10%) of the acceptor [129]. The reaction takes place in 20 to 77% yield... 

A number of electrocatalytic reactions have been reported in which the EGB is derived by initial reduction of an aldehyde or a ketone that at the same time functions as the electrophile in a coupling reaction [136-139]. It is Kkely that the actual EGB is a dimer dianion of the carbonyl compound or a dianion of the carbonyl compound formed by disproportionation. The general principle is outlined in Scheme 38. The reactions become catalytic when the product anion, P , is protonated by the weak acid, NuH, whereby the nucleophile, Nu , is regenerated. 

The results summarised in Table 4 show clearly that good yields of the 1 1 Michael adducts can be obtained in cathodically initiated reactions. It is not clear, however, why the reaction should fail for acrylonitrile even if dimerisation of the initially formed radical-anion is much faster thaq,protonation the resulting dimeric dianion should still be sufficiently basic to deprotonate dialkylmalonate esters. A feature of especial significance is the usefulness of esters of ethenetetracarboxylic acid. Apart from their use in EGB catalysed reactions they have been much used in stoichiometric amount. 

The dimer cation was supposed to have a sandwich structure in which the orbitals of one molecule overlapped with those of the other molecule. The band at 450 nm (B) is due to the bonded dimer cation (St—St T) the formation of this species corresponds to the initiation step of the polymerization. The bonded dimer cation may be formed by the opening of the vinyl double-bonds. Egusa et al. proposed that the structure was a linked head-to-head type I or II, by the analogy of the dimeric dianions of styrene and a-methylstyrene. Table 1 summarizes the assignment of absorption bands observed in pulse radiolysis of 1,1-diphenylethylene in dichloromethane, which is a compound suitable for studying monomeric and dimeric cations [28],... 

The dimerization of the anion radicals was studied in NAH/THF and LAH/THF by following the decay of the absorption maxima of the styrene, a-methylstyrene, and 1,1-diphenylethylene radical-anions. The dimerization followed second order kinetics with the rate constants shown in Table 2. The absorption spectrum with a peak at 320 nm, which grew concomitantly with the decay of the ion-pair MST, Na+(or Li+) was assigned to the dimeric dianion MSMS in the form of the solvent-separated ion pair (Fig. 5). 

In principle, dimerization is reversible and the equilibrium is rapidly established. The equilibrium concentration of radical ions derived from vinyl monomers is so small that it cannot be detected by ESR (for monomer concentrations 10-1 mol dm-3 it is < 10-7 mol dm-3). The rate constant of dissociation of the dimeric dianion of a-methylstyrene (aMeS)... 

The dimerization of radical anions derived from 9-X-substituted anthracenes (Scheme 4), where X is an electron-withdrawing substituent, is related to electrohydrodimerization and might be expected to be less complex since proton donors are not involved in the formation of the products (Hammerich and Parker, 1981b). The reactions, where X is NOj, CHO, or CN, were studied by LSV and DCV. Primarily on the basis of the near independence of the reaction rates on temperature, the simple dimerization mechanism was excluded. It was proposed that the overall reaction consists of two reversible steps (i) formation of a radical anion dimer complex in which the two anthracene moieties are not bonded at the 10 positions and (//) the rearrangement of the complex to the stable dimeric dianions. The rate of the reaction was found to be independent of the water concentration in DMF. The radical... 

Quinazoline and 2-phenylquinazoline both form dimeric dianions, prolonation of which produces the respective 4,4 -bis(3,4-dihydroquinazolinyI) derivatives 17 (R = H) and 17 (R = Ph). Alkylation of the dimeric dianion of 2-phenylquinazoline with iodomethane and acylation of the same with ethyl chloroformate occurs at N3 giving the corresponding 4,4 -bis(3,4-dihydroquinazolinyl) derivatives 18 (R = Me) and 18 (R = CO Et), respectively. " ... 

The dimerization of anthracene " has been studied extensively [243, 244]. With strongly electron-withdrawing substituents at position 9 the radical anions undergo reversible dimerization in aprotic solvents such as DMF, MeCN, propylene carbonate, DMSO etc. followed by rate determining a bond formation to furnish the stable dimer dianion [245]. In DMF k m were found to decrease in the... 

In general, reductive generation furnishes a ketyl radical anion which undergoes radical-radical coupling to form the dimer dianion [251]. This dimerization was found to be faster than the reaction of the radical anion with the parent molecule (e.g. for of p-cyanobenzaldehyde 28.6 m" s" compared with 1.45 M" s" ) [252]. 

In imine reductions rapid equilibration of dimeric dianions and the precursor radical anions leads to the thermodynamically more stable isomer [257]. In the reductive coupling of salicylideneanilines, however, rate-determining C-C bond formation is preceded by intramolecular H-bridging [258]. 

Alternatively, the radical anions of pyridine and other azines are obtained on reduction by metals or other reducing agents, e.g. lithium diethylamide. Under suitable conditions the salts of the radical anions can be obtained as crystalline materials (Section 6.2.3). Alternatively, dimerization follows as an example, treatment of pyridine and other azines with 1 equiv. sodium in HMPA gives the well characterized radical anion, whereas in tetrahydrofuran the dimeric dianion is formed [125]. A later study with pyridine showed that treatment with sodium leads to a tetrahydro-4,4-bipyridine dianion (57), which is rearomatized to yield 58 in the presence of excess sodium (Scheme 40) [126], Treatment with LiNEt2 gives 2,2 -bipyridine, however, possibly because of stronger coordination with the lithium cation [127]. 2,2 -Biquinoline and 1,1 -biisoquinoline are similarly obtained [128]. 

For radical cations this situation is typically observed when deprotonation of the dimer dication is slow and for radical anions under conditions that are free from electrophiles, for example, acids, that otherwise would react with the dimer dianion. Most often, this type of process has been observed for radical anions derived from aromatic hydrocarbons carrying a substituent that is strongly electron withdrawing, most notably and well documented for 9-substituted anthracenes [112,113] (see also Chapter 21). Examples from the radical cation chemistry include the dimerization of the 1,5-dithiacyclooctane radical cations [114] and of the radical cations derived from a number of conjugated polyenes [115,116]. 

The reaction is a chain reaction with a dimerization of the diazoalkane radical anion as the first chemical step the structure of the two kinds of intermediate dimerized dianions detected by CV is not settled [105]. 

Most—if not all—of the preparative results just presented for reduction of 12 can be rationalized within the RR mechanism when effects of hydrogen-bonding and proton transfer are taken into account. Under conditions with very low proton donor concentrations DPSCC measurements on 12a-b in DMF indicate that the initial dimerization step is reversible with a large equilibrium constant, and the rate-determining step is protonation (or cyclization) of the dimer dianion [10,11]. Assuming this mechanism, the equilibrium constants for the dimerization were determined = 109 for 12a and = 53.1... 

In dry DMF, polymers were formed in addition to the ( ) LHD (Table 7). However, in the presence of a stoichiometric concentration of metal cations (Li, Cr"" ", Mn , Fe", Co", or Zn" ), polymerization was avoided and a yield of >90% of dimers obtained [21]. With Li" the ( ) LHD was obtained exclusively, whereas with Fe or Co" the CHD was the sole product. The stereochemistry of the CHD was assigned by NMR to arise from meso coupling [21], but later studies have shown the stereochemistry to correspond to ( ) coupling by application of H NOESY NMR and x-ray crystallography [105]. The metal cations are expected to form ion-pairs with the radical anions (an anodic shift of the reduction peak was observed). Thus a templating effect is a likely explanation of the stereoselectivity, and stabilization of the dimer dianion by ion-pairing may prevent polymerization. 

In MeCN the radical anion derived from 9-anthryl styryl ketone, 20b, dimerizes ( dim = 10 s ), formmg a stable dimeric dianion, most likely via coupling in the... 

The derivatives of Meldrum s acid, 59a-b, undergo reductive dimerization in MeCN, and the mechanism has been studied [137,143]. An RS mechanism with rate-determining electron transfer has been invoked either as the only mechanism (for 59b) or as one of two parallel reaction pathways (for 59a) in order to explain the apparent reaction orders found by DCV or LSV in the presence of AcOH [137]. However, AcOH (pA (AcOH) = 12.3 in DMSO [144]) is not likely to be strong enough to protonate the dimer dianion, since the basicity of the dimer dianion is expected to be close to that of the conjugate base of Meldrum s acid (pAT(Meldrum s acid) = 7.3 in DMSO [145]). Consequently, the dimerization may be reversible, and this, in turn, may lead to anomalous apparent reaction orders, although the coupling is of the RR type [76]. 

Simple alkyl acrylates have been used as partner in mixed hydrocoupling reactions in a number of cases (Table 14). Like mixed couplings with 7, LHD formation and hydrogenation compete with formation of the MHC of alkyl acrylates. Coelectrolysis of ethyl 3,4-dimethoxycinnamate Ey z= —1.94 V) with ethyl crotonate Ey = —2.37 V) in a molar ratio of 1 7.2 in MeCN at E = —2.03 V gave none of the MHC bW only the CHD of the cinnamate. The only product derived from the crotonate was diethyl 2-ethylidene-3-methylglutarate (49%) formed by action of an EGB, most likely the dimer dianion of the cinnamate [61]. 

Reduction of 9-substituted anthracenes, (91), leads to radical anions, which, because of the electron-withdrawing substituents, are quite stable with respect to protonation and cleavage in aprotic solvents. In polar aprotic solvents the radical anions exclusively dimerize, and the reaction has been the subject of a number of studies [247-258]. The products are the tail-to-tail dimeric dianions as in Eq. (57), which are fairly stable. In CV the dimer dianions can be detected as a new oxidation peak on the reverse scan at a potential several hundred millivolts anodic relative to the potential of radical anion formation. On preparative or semipreparative scales the dimer dianion has in a single case been detected by H-NMR [249], and oxidative electrolysis of the dimer dianions in most cases restores the starting material. 

The tetrahydrobianthryl products formed by protonation of the dimeric dianions have been isolated (up to 90% yield) in some cases after addition of acids [253.255,256]. Unless the dimer dianion is trapped by acid (or in a single case by methyl iodide [259]), air oxidation of the dimer dianion to starting material takes place during work-up [249-254]. 

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