Initiation charge-transfer complexes

The first possibility envisages essentially the same mechanism as for the second-order process, but with Bt2 replacing solvent in the rate-determining conversion to an ion pair. The second mechanism pictures Bt2 attack on a reversibly formed ion-pair intermediate. The third mechanism postulates collide of a ternary complex tiiat is structurally similar to the initial charge-transfer complex but has 2 1 bromine alkene stoichiometry. There are very striking similarities between the second-order and third-order processes in terms of magnitude of p values and product distribution. In feet, there is a quantitative correlation between the rates of the two processes over a broad series of alkenes, which can be expressed as... 

Stoicescu and Dimonie103 studied the polymerization of 2-vinylfuran with iodine in methylene chloride between 20 and 50 °C. The time-conversion curves were not analysed for internal orders but external orders with respect to catalyst and monomer were both unity. Together with an overall activation energy of 2.5 kcal/mole for the polymerization process, these were the only data obtained. Observations about the low DP s of the products, their dark colour, their lack of bound iodine and the presence of furan rings in the oligomers, inferred by infrared spectra (not reported), completed the experimental evidence. The authors proposed a linear, vinylic structure for the polymer, and a true cationic mechanism for its formation and discussed the occurrence of an initial charge-transfer complex on the... 

Furan and maleic anhydride undergo the Diels-Alder reaction to form the tricycHc 1 1 adduct, 7-oxabicyclo [2.2.1]hept-5-ene-2,3-dicarboxyHc anhydride (4) in exceUent yield. Other strong dienophiles also add to furan (88). Although both endo and exo isomers are formed initially, the former rapidly isomerize to the latter in solution, even at room temperature. The existence of a charge-transfer complex in the system has been demonstrated (89,90). 

The diacyl peroxide-amine system, especially BPO-DMT or BPO-DMA, has been used and studied for a long time but still no sound initiation mechanism was proposed. Some controversy existed in the first step, i.e., whether there is formation of a charge-transfer complex of a rate-controlling step of nucleophilic displacement as Walling 1] suggested ... 

The ethylenediamine derivative [31] possesses higher promoting activities than other diamines. This phenomenon may be ascribed to the copromoting effect of the two amino groups on the decomposition of persulfate through a CCT (contact charge transfer complex) formation. So we proposed the initiation mechanism via CCT as the intimate ion pair and deprotonation via CTS (cyclic transition state) as follows ... 

Other postulated mechanisms for spontaneous initiation include electron transfer followed by proton transfer to give two monoradicals, hydrogen atom transfer between a charge-transfer complex and solvent,110 and formation of a di radical from a charge-transfer complex, JJ[Pg.111]

The D/A complexation in equation (41) is further substantiated by infrared and NMR studies. These observations suggest that an initial thermal electron transfer within the D/A charge-transfer complex generates an ion-radical pair, and a rapid methyl transfer subsequently completes the 1,4-addition (equation 42). 

Photoinduced copolymerization of donor-acceptor monomer pairs (Scheme 1) can be either self initiated by excitation of the charge transfer complex (charge transfer initiation) or by polymerization of the charge transfer complex/monomer equilibrium... 

It is, of course, possible that the charge-transfer complex between metal halide and olefin, which is well known, is an intermediary in this reaction. There is here another variation on the theme of direct initiation. The thermochemical analysis, analogous to the previous ones, goes as follows ... 

Another example concerns the initial electronic reduction of a-nitrostilbene (Todres et al. 1982, 1985, Todres and Tsvetkova 1987, Kraiya et al. 2004). The reduction develops according to direction a in Scheme 2.9 if the mercury cathode as well as cyclooctatetraene dianion are electron sources and according to direction b if the same stilbene enters the charge-transfer complexes with bis(pyridine)-tungsten tetra(carbonyl) or uranocene. For direction b, the charge-transfer bands in the electronic spectra are fixed. So the mentioned data reveal a great difference in electrochemical and chemical reduction processes a and b as they are marked in Scheme 2.9. 

The formation of charge-transfer complexes presumes that donor and acceptor molecules are held in place by some forces. Here too, an increase in temperature hinders formation of complexes because it enforces the disorder of molecules in the solution. To summarize, the initial stage of the reaction (the ion-radical conception) has, as a rule, a negative temperature coefficient or, in any event, does not increase the total activation energy of the ion-radical reaction. 

The 7i-donor behavior of 4-arylmethylene-2-phenyl-5(47/)-oxazolones 762 with the 7i-acceptor tetracyanoethylene has also been studied. The initially formed charge-transfer complex is converted via intermediate 763 to a new compound for which a 2-aryl-l-benzamido-3,3,4,4-tetracyanocyclobutanecarboxylic acid 764 has been proposed on the basis of the NMR spectral data (Scheme 7.233). Charge-transfer complexes of 2-aryl-4-arylidene-5(47/)-oxazolones with di- and trinitrobenzene as n acceptors have also been prepared. ... 

Another very instructive case concerns the alleged initiation of a cationic polymerisation by a charge-transfer complex formed by the compound chloranil (2,3,5,6-tetrachloroquinone) with the monomer N-vinyl-carb-azole. It was shown (Natsuume et al., 1969 1970) that this compound is not an initiator, but that the polymerisations were caused by a hydrolysis product, 2-hydroxy-3,5,6-trichloroquinone, which is a strong acid. One has learnt from this finding to be extremely suspicious of any claims for charge-transfer catalysis and to test one s suspicions by appropriate experiments involving progressive purification of the putative catalyst. 

---