Diphenylmethyl radicals

Howard and Ingold studied this equilibrium reaction in experiments on the oxidation of tetralin and 9,10—dihydroanthracene in the presence of specially added triphenylmethyl hydroperoxide[41]. They estimated the equilibrium constant K to be equal to 60 atm-1 (8 x 103 L mol-1, 303 K). This value is close to T=25atm-1 at 300 K (A/7=38kJ mol-1), which was found in the solid crystal lattice permeable to dioxygen [84], The reversible addition of dioxygen to the diphenylmethyl radical absorbed on MFI zeolite was evidenced and studied recently by the EPR technique [85],... 

Here the radical 1 acts as a strong terminator to prevent the formation of oligomers and polymers. On the other hand, it is expected that the substituted diphenylmethyl radicals which are less stable than 1 serve as both initiators and primary radical terminators. In fact, it was reported [84] that the apparent polymerization reactivities decreased in the following order diphenylmethyl, phenylmethyl, and triphenylmethyl radicals, which were derived from the initiator systems consisting of arylmethyl halides and silver. 

In the polymerization with tetraphenylethanes as the initiators, the polymer produced would be obtained as shown in Eq. (24) because the generated diphenylmethyl radical can function as both an initiator and a terminator. 

The tetraphenylethanes described above are symmetrical compounds used to generate the same two radicals by dissociation, while pentaphenylethane (28) is an unsymmetrical derivative, giving two different radicals, triphenylmethyl and diphenylmethyl radicals [138]. The former cannot initiate radical polymerization, but the latter is available as an initiating radical to produce the polymer 28, which can function as the polymeric iniferter [106]. 

FIGURE 2.48. Electrochemical reduction of the diphenylmethyl radical produced by the reaction of diphenylmethyl chloride by photo-injected electrons in dimethylformamide in the presence of increasing amounts of benzimidazole. Variations of the half-wave potential the concentrations of acid added, from bottom to top, 0, 0.018, 0.049, 0.11, 2.8, 6.7 mM. Solid lines, simulations for each acid concentration. Adapted from Figure 1 of reference 50b, with permission from the American Chemical Society. 

The laser flash photolysis of aromatic diisocyanate based polyurethanes in solution provides evidence for a dual mechanism for photodegradation. One of the processes, an N-C bond cleavage, is common to both TDI (toluene diisocyanate) and MDI (methylene 4,4 -diphenyldiisocyanate) based polyurethanes. The second process, exclusive to MDI based polyurethanes, involves formation of a substituted diphenylmethyl radical. The diphenylmethyl radical, which readily reacts with oxygen, is generated either by direct excitation (248 nm) or indirectly by reaction with a tert-butoxy radical produced upon excitation of tert-butyl peroxide at 351 nm. 

The results of an experiment for the laser flash photolysis (Xex=351 nm) of a 6.0 X 10 M solution of diphenylmethane in a 60/40 mixture of TBP and benzene (Figure 6) shows a distinct absorbance peak maximum at 340 nm characteristic of the unsubstituted diphenylmethyl radical. The results in Figure 6 illustrate the utility of TBP in indirect generation of diphenylmethyl radicals. 

The laser flash photolysis (Xex=351 nm) of a TBP/BP-MDI solution in benzene (Figure 7) yields a transient spectra with distinct maximum at 370 nm which can most likely be attributed to a substituted diphenylmethyl radical. (Similar results are obtained in other solvents such as DMF). No detectable transient species were generated above 350 nm by the laser flash photolysis (Xex=351 nm) of the 60/40 mixture of TBP and benzene alone. Results for the TBP/MDI-PU (7.0 X 10 2 g/dL) system in Figure 8 show, as in the case of the model BP-MDI (Figure 7), that the transient spectrum of MDI-PU obtained indirectly through tert-butoxy radicals has a maximum at 370 nm. This provides additional support for assignment of the transient species responsible for the 370 nm absorbance to a diphenylmethyl radical. 

For MDI based polyurethanes we have provided evidence for formation of a diphenylmethyl radical by direct excitation (248 nm) of the carbamate moiety as well as hydrogen abstraction by a tert-butoxy radical which is produced by excitation (351 nm) of tert-butyl peroxide. The diphenylmethyl radical readily reacts with oxygen. A proposed mechanism which accounts for the production (direct or indirect) and subsequent reaction with oxygen of the diphenylmethyl radical is shown in Scheme IV. The hydrogen peroxide product depicted in Scheme IV has been previously identified by FT-IR (7) we have simply provided a plausible mechanism for its formation. 

Again, there is a resemblance between the emission of DPCs and those of the diphenylmethyl radicals. The emission maximum of DPCs is blue-shifted by 20-50 nm with respect to that of the radicals. For example, diphenylmethyl radical exhibits its emission at 535 nm. Another characteristic difference between these two species is that the lifetime of DPC is significantly shorter than that of the first excited state of the diphenylmethyl radical. 

Based on steady-state and time-resolved emission studies, Scaiano and coworkers have concluded that silicalite (a pentasil zeolite) provides at least two types of sites for guest molecules [234-236], The triplet states of several arylalkyl ketones and diaryl ketones (benzophenone, xanthone, and benzil) have been used as probes. Phosphorescence from each molecule included in silicalite was observed. With the help of time-resolved diffuse reflectance spectroscopy, it has been possible to show that these triplet decays follow complex kinetics and extend over long periods of time. Experiments with benzophenone and arylalkyl ketones demonstrate that some sites are more easily accessed by the small quencher molecule oxygen. Also, diffuse reflectance studies in Na + -X showed that diphenylmethyl radicals in various sites decay over time periods differing by seven orders of magnitude (t varies between 20/is and 30 min) [237]. 

Just as several alkyl substituents increasingly stabilize a radical center (Table 1.2), so do two phenyl substituents. The diphenylmethyl radical ( benzhydryl radical ) is therefore more stable than the benzyl radical. The triphenylmethyl radical ( trityl radical ) is even more stable because of the three phenyl substituents. They actually stabilize the trityl radical to such an extent that it forms by homolysis from the so-called Gomberg hydrocarbon even at room temperature (Figure 1.8). Although this reaction is reversible, the trityl radical is present in equilibrium quantities of about 2 mol%. 

Cyclization to form the ring-closed dihydrofluorenyl radical is a process that appears to be limited to diarylmethyl radicals that are substituted at the central carbon atom. Thus the excited triphenylmethyl, diphenylethyl, and diphenylcyclo-propylmethyl radicals all form ring-closed radicals whereas the parent diphenyl-methyl radical does not. The reason for this different behavior lies in the steric crowding produced by introducing a substituent at the central carbon atom. Whereas in the unsubstituted diphenylmethyl radical the angle between the phenyl... 

Charge transfer reactions of 132, cyano-substituted diphenylmethyl radicals (148,152), and 2-phenanthrylmethyl radical (159) with CC14 and CBr4 have also been reported [97,99,101]. A decreased rate constant for CBr4 quenching of the a-cyano-substituted diphenylmethyl radical (152) relative to the parent radical provided indirect evidence of charge transfer [97,145]. More recently, the diphenylmethyl cation was observed directly in acetonitrile solvent with added CC14 [145,149] (Scheme 25). 

In such an insertion the benzophenone is excited to its triplet state, resembling a C-0 diradical. The oxygen atom of the ketone then removes a hydrogen atom from the methylene group, leading to a carbinyl radical and the hydroxy diphenylmethyl radical formed by hydrogen atom addition to the benzophenone. This pair of radicals couples... 

In an alternate view of this reaction (Wender, 47), it can be assumed that the cation is reduced by the electron donor, Co(CO)4, to give a diphenylmethyl radical ... 

The enantioselective reaction mechanism can be explained by considering the photochemical aspects and also the molecular arrangements (Scheme 36 and Fig. 7). Hydroacridine radical species 155 and 156, which satisfy both such conditions, should be preferably produced with their higher stability. Next, decarboxylation of 154 gives the diphenylmethyl radical 150. In the chiral crystal lattice of M-150 DPA, the molecular pairs of acridine and diphenyl acetic acid stack with... 

The Application of IMOMO Schemes How Stable Are Benzyl and Diphenylmethyl Radicals ... 

TABLE 5.3 RSEs at 298.15 K of Benzyl Radical (11) and Diphenylmethyl Radical (12) (in kJ/mol) at Varions Levels of Theory... 

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