1.1- Diphenylethylene, dimerization

In contrast, the radical-anion from 1,1-diphenylethylene dimerizes readily ... 

The q1-coordinated carbene complexes 421 (R = Ph)411 and 422412) are rather stable thermally. As metal-free product of thermal decomposition [421 (R = Ph) 110 °C, 422 PPh3, 105 °C], one finds the formal carbene dimer, tetraphenylethylene, in both cases. Carbene transfer from 422 onto 1,1-diphenylethylene does not occur, however. Among all isolated carbene complexes, 422 may be considered the only connecting link between stoichiometric diazoalkane reactions and catalytic decomposition [except for the somewhat different results with rhodium(III) porphyrins, see above] 422 is obtained from diazodiphenylmethane and [Rh(CO)2Cl]2, which is also known to be an efficient catalyst for cyclopropanation and S-ylide formation with diazoesters 66). 

These chemical electron-transfer reactions are in contrast with the electrode reactions. For instance, stilbene gives a mixtnre of 1,2-diphenylethylene with 1,2,3,4-tetraphenylbutane on electrolysis (Hg) in DMF at potentials about that of the first one-electron wave. This solvent has faint proton-donor properties. The stilbene anion-radical is stable under these conditions it has enough time to diffuse from the electrode into the solvent and dimerize therein (Wawzonek et al. 1965). 

Diphenylethylene radical cation forms a dimeric product (90) via a 1,4-bifunctional-distonic intermediate (89 " ") that undergoes an alternative (1,6-) cycli-zation with participation of one phenyl group. 

The second electronic transfer to the oxygen produces the diradical (C) which evolves into monomer formation. The latter possibility (IV) is a homolytlc cleavage giving another anion radical. If the process follows scheme III or IV, we must obtain monomer formation after the oxidation reaction in all cases. We have carried out the oxidation of carbanionic dimers derived from isoprene, crmethylstyrene, styrene, 1,1-diphenylethylene. 

Other olefinic substrates known to dimerize through photo-induced electron transfer sensitization include enamines (72), diarylethylenes (73-75), vinyl ethers (76), styrenes (77,78), and phenyl acetylenes (79). Alternate ring closures (besides cyclobutanes) are sometimes observed, probably via 1,4-radical cationic intermediates. For example, a tetrahydronaphthalene is formed from the radical cation of 1,1-diphenylethylene, eq. 

The reactions of poly(styryl)lithium in benzene with an excess of diphenyl-ethylene 272) and bis[4-(l-phenylethenyl)phenyl]ether158) also were found to proceed by a first order process. However, the reactions of poly(styryl)lithium with the double diphenylethylenes l,4-bis(l-phenylethenyl)benzene and 4,4 bis(l-phenyl-ethenyl)l,l biphenyl gave l58) non-linear first order plots with the gradients decreasing with time. This curvature was attributed to departure from a geometric mean relationship between the three dimerization equilibrium constants (Ka, Kb and Kab). The respective concentrations of the various unassociated, self-associated and cross-associated aggregates involved in the systems described by Equations (49) to (51) are dependent upon the relative concentrations of the two active centers and the respective rate constants which govern the association-dissociation events. 

In a process related to RAFT, BASF workers have shown that 1,1-diphenylethylene will control the molecular weight of PMMA and polystyrene, and permit block polymer synthesis [92]. They propose that radical chain ends add to the diphenylethylene to form a stable diphenylalkyl radical that does not add more monomer but can reverse to diphenylethylene and the same radical chain end for addition of more monomer. The diphenylalkyl radical cap has the additional possibility of forming a reversible dimer (Scheme 35). 

Dimerization products of alternative structure types have been observed for 1,1-diphenylethylene, where a 1,6-cyclization process occurs with participation of one phenyl group. The resulting ring-extended product (7) constitutes additional evidence for the 1,4-bifunctional intermediate [122],... 

Only the dimerization process is considered, even if it is easily seen that the scheme may be expanded to include formation of trimers, tetramers, etc., and polymers. Thus, even in a system as simple as this the formation of "MM" can occur via two pathways due to the intervention of the electron transfer reaction (Eq. (40) ). This reaction scheme has been demonstrated for the radical anion from 1,1-diphenylethylene 112 (see 14.4). 

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). 

Photochemical reactions of substituted phenylethylenes, frans-stilbene and 1,1-diphenylethylene, on silica gel, has been reported by Sigman et al. [40]. Irradiation of fraus-slilbcnc adsorbed on silica gel produced two dimers, along with ds-stilbene, phenanthrene, and a small amount of ben-zaldehyde which arose from a type II oxidation mechanism (Scheme 9). The formation of photodimers was claimed to be promoted by inhomogeneous surface loading and slow diffusion of fraus-slilbcnc on silica [40]. 

The experimental verification of this relation indicates a rapid establishment of the deaggregation equilibrium, which is shifted far to the left (the concentration of free BuLi must be very small). The degradation intermediates are also inactive. Table 6 shows that similar dependences have also been observed with the polymerization of isoprene and the dimerization of 1,1-diphenylethylene in fluorene [150]. 

Alternatively, Lewis acids such as SbCl5 may initiate oligomerization directly by electron transfer from extremely reactive alkenes such as 1,1-diphenylethylene and 1,1 -di(p-methoxyphenyl)ethylene [28,143,144]. The dimeric tail-to-tail carbenium ion of 1,1-diphenylethylene shown in Eq. (32) was observed, and its formation explained by a radical cation intermediate. Because 1,1-diarylethylenes can not polymerize, only oligomerization was observed. 

Simpler procedures are of course available for the preparation and characterisation of carbenium ions in solution, particularly for the more stable ones. Concentrated sulphuric acid was extensively used as protogenic medium before the superacid mixtures were shown to be superior, but many of the spectroscopic assignements in those earlier studies were later proved erroneous, particularly in the case of such reactive entities as the 1-phenylethylium ion Model monomers which cannot polymerise because of steric hindrance can generate fairly stable carbenium ions by interacting with Lewis or Br nsted acids in normal cationic polymerisation conditions. Thus, 1,1-diphenylethylene and its dimer, and 1,1-diphenylpropene give rise to typical visible absorption bands from which the concentration of the corresponding diphenyl-methylium ions can be accurately calculated. As for carbenium ions capable of forming stable salts, their synthesis and characterisation is obviously easy. 

If complexation between TiCl4 and the olefin (and with arcxnatic dimers and polymers) does take place to such a degree that the catalyst complexed is not available for initiation, as suggested by Sauvet et al. for 1,1-diphenylethylene, then obviously the inverse catalyst efficiency, expressed as the number of its molecules consumed to prepare one active species, will be higher tiian three, although again the stoicheiometiy of the initiation step remains simply one to one. 

The kinetics of cyclisation of the linear unsaturated dimer of 1,1-diphenylethylene were also studied under the same conditions and the initial rate of this process was found to be directly proportional to the concentration of each of the three components, as shown in Fig. 7. We believe that in solvent tenzene the active species formed, i.e. the monomer and linear dimer carbenium ions, existed essentially as ion pairs, and that the counterion was most probably SnClJ resulting from the interaction of HClj with SnCl4 (see above). Since both the dimerisation and the cyclisation reactions were slow, equilibrium was reached in the various interactions leading to the generation of the carbenium ions, the concentration of which can therefore be expressed by the relationship... 

Evans et al. showed that the initial rate of conversion of the linear dimer of 1,1-diphenylethylene into an equilibrium mixture of monomer and dimer (D), catalised by the pair SbCl3—HCl in benzene follows the kinetic law ... 

Recent unpublished work by Sauvet has shown that trityl hexachloroantimonate can induce the dimerisation of 1,1-diphenylethylene. However, this process requires high concentrations of initiator probably because the interaction of the trityl ion with the monomer is sterically hindered. Most of the dimerisation proceeds by proton transfer once initiation has taken place. It is therefore difficult to look for evidence on the chemistry of the initial step since most of the product is the linear unsaturated dimer. 

Besides cyclobutane formation, alternative ring closures are sometimes observed. One example is the 9,10-dicyanoanthracene sensitized dimerization of 1,1-diphenylethylene (169). The six-membered ring is formed via the 1,4-radical cation, which results from the addition of the free radical cation to diphenylethylene as indicated in Scheme 56, while the 1,4-biradical generated by back electron transfer from the radical ion pair yields tetraphen-ylcyclobutane (170) (Mattes and Farid, 1983). 

Under conditions that are not strictly nonaqueous, the oxidized dimer may be trapped by water, as was observed during the oxidation of 1,1-diphenylethylene catalyzed by the radical cation of dibenzo-1,4-dioxin [91]. The dimer dication upon reaction with water undergoes a 1,2-phenyl shift, resulting finally in 1,2,4,4-tetraphenyl-3-buten-l-one [Eq. (42)], reminiscent of the 1,2-shifts observed during anodic oxidation of 1-phenyl- and 1,4-diphenylnaphthalene in acidic dime thy Iformamide (DMF) [92]. 

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