Mechanism reaction with anthracene

The close association between metal ions and p-benzoquinones catalyzes their Diels-Alder reactions with anthracenes. The efficiency of the metal cations correlates with their Lewis acidity171. A mechanism proceeding via radical-anions for a [3,3] sigmatropic rearrangement was established172. 

The coordination chemistry of this anionic, cyclometallated precatalyst and its derivatives, along with its stoichiometric reactions with anthracene, have been reported by Halpem. On the basis of these studies, a plausible mechanism can be proposed for the hydrogenation of polycyclic arenes by this catalyst. Although kinetic studies of the complete catalytic cycle have not been reported, Halpem s investigation of the stoichiometric reactions of this system provides strong evidence that the reaction occurs by a series of steps involving soluble complexes. 

The results are consistent with the rate-determining step being addition of the aryl radical to the aromatic ring, Eq. (9). Support for this mechanism is derived from the results of three other studies (a) When A -nitrosoacetanilide is decomposed in pyridine, the benzene formed by abstraction of hydrogen from pyridine by phenyl radical accounts for only 1 part in 120 of the reaction leading to phenyl-pyridines. (b) 9,9, 10,lCK-Tetrahydro-10,10 -diphenyl-9,9 -bianthryl is formed in the reaction between phenyl radicals and anthracene, probably by the addition mechanism in Eq. (11). Adducts are also formed in the reactions of benzyl radicals with anthracene- and acridine. ... 

Luche and coworkers [34] investigated the mechanistic aspects of Diels-Alder reactions of anthracene with either 1,4-benzoquinone or maleic anhydride. The cycloaddition of anthracene with maleic anhydride in DCM is slow under US irradiation in the presence or absence of 5% tris (p-bromophenyl) aminium hexachloroantimonate (the classical Bauld monoelectronic oxidant, TBPA), whereas the Diels Alder reaction of 1,4-benzoquinone with anthracene in DCM under US irradiation at 80 °C is slow in the absence of 5 % TBPA but proceeds very quickly and with high yield at 25 °C in the presence of TBPA. This last cycloaddition is also strongly accelerated when carried out under stirring solely at 0°C with 1% FeCh. The US-promoted Diels Alder reaction in the presence of TBPA has been justified by hypothesizing a mechanism via radical-cation of diene, which is operative if the electronic affinity of dienophile is not too weak. 

The mechanism of cycloaddition reaction of maleic anhydride with anthracene promoted by US irradiation has been the subject of many controversies [32, 37]. Recent work of Da Cunha and Garrigues [35] shows that the reaction proceeds in toluene solution in the 60 85 °C temperature range in 6 3 h. 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

In the reaction of cyclopentadiene with maleic-D2 anhydride206 an inverse experimental KIE of 8% (KIE = 0.92) was found at 298 K. The reaction between butadiene-D4, D2C=CHCH=CD2 and maleic anhydride gave a large inverse D4-KIE of 0.76. The two reactions between anthracene and maleic anhydride presented below also favour the concerted rather than the stepwise mechanism which requires 3-6% KIE in the normal direction (i.e. >1). 

Analysis of the transition state in terms of energy is certainly a key aspect of the S 2-ET problem. Entropy considerations may, however, bring about additional information, possibly helping us to conceive better the transition between the two mechanisms. It was observed in this connection that, whereas the entropy of activation of both the anthracene anion radical and of the ETIOPFe(O) porphyrin (pp. 99, 100) (which have about the same standard potential) is close to zero in their reaction with s- and t-butyl bromides a definitely negative value, ca. — 20 eu is obtained for the reaction of the porphyrin with n-butyl bromide (Lexa et al., 1988). The same was found for the reaction of two other iron porphyrins, TPPFe(o) and OEP-Fe(i). These activation entropies were estimated from (153), where Z is... 

The above considerations should bear some relationship with the stereochemistry of the reaction. As indicated earlier (8ection 2 Hebert et ai, 1985), in the reaction of anthracene anion radicals with optically active 2-octyl bromide, racemization is mostly observed together with a small but distinct amount (ca. 10%) of inversion. In the context of the ET-8n2 mixed mechanism sketched above, this can be rationalized in terms of a minor contribution of the latter pathway that would not detectably affect the overall rate constant of the reaction. The weakness of the bonded interactions in the transition state derives from the relatively poor affinity of the alkyl radical for the aromatic hydrocarbon. This is consistent with the fact that in those of the radical-anthracene pairs that were not favourably oriented for the 8, 2 reaction to occur, the alkyl radical escapes from the... 

The peroxyoxalate system is the only intermolecular chemiluminescent reaction presumably involving the (71EEL sequence (Scheme 44), which shows high singlet excitation yields (4>s), as confirmed independently by several authors Moreover, Stevani and coworkers reported a correlation between the singlet quantum yields, extrapolated to infinite activator concentrations (4> ), and the free energy involved in back electron-transfer (AG bet), as well as between the catalytic electron-transfer/deactivation rate constants ratio, ln( cAx( i3), and E j2° (see Section V). A linear correlation of ln( cAx( i3) and E /2° was obtained for the peroxyoxalate reaction with TCPO and H2O2 catalyzed by imidazole and for the imidazole-catalyzed reaction of 57, both in the presence of five activators commonly used in CIEEL studies (anthracene, DPA, PPO, perylene and rubrene). A further confirmation of the validity of the CIEEL mechanism in the excitation step of... 

Thiete sulfones show an irregular behavior pattern when involved in cycloaddition reactions. With 1,3-dienes, dienamines, enamines, ynamines, diazoalkenes, cyclopropadiene, and its substitution products, furan, and anthracene, the addition proceeds in the normal fashion. With certain Diels-Alder reagents such as tetraphenylcyclopentadienone (tetracycloneX however, the cyclic sulfones react anomalously. The Diels-Alder adducts undergo decomposition with SO 2 and CO extrusion to a seven-membered ring, the tetraphenylcycloheptatriene 223. Bicyclic octadienone is produced as well (Eq. 62). The mechanism of this unusual reaction is proposed by... 

The simple triplet-triplet quenching mechanism requires that at low rates of light absorption the intensity of delayed fluorescence should decay exponentially with a lifetime equal to one-half of that of the triplet in the same solution. Exponential decay of delayed fluorescence was, in fact, found with anthracene, naphthalene, and pyrene, but with these compounds the intensity of triplet-singlet emission in fluid solution was too weak to permit measurement of its lifetime. Preliminary measurements with ethanolic phenanthrene solutions at various temperatures indicated that the lifetime of delayed flourescence was at least approximately equal to one-half of the lifetime of the triplet-singlet emission.38 More recent measurements suggest that this rule is not obeyed under all conditions. In some solutions more rapid rates of decay of delayed fluorescence have been observed.64 Sufficient data have not been accumulated to advance a specific mechanism but it is suspected that the effect may be due to the formation of ionic species as a result of the interaction of the energetic phenanthrene triplets, and the subsequent reaction of the ions with the solvent and/or each other to produce excited singlet mole-... 

A mechanism of electron transfer, radical ion coupling and loss of HX is also proposed to account for the photoinduced reactions of tetrafluoro- and tetrachlorophthalonitriles with anthracene and phenanthrene, leading to the 9-(3,5,6-trihalogeno-l,2-dicyanophenyl)arenes501. 

As the focus of this chapter is on the synthetic utility of the rDA reaction, an overview of mechanism is beyond the scope of this review however, the subject has beoi reviewed previously. Structural and medium effects on the rate of the rDA reaction are of prime importance to their synthetic utility, and therefore warrant discussion here. A study of steric effects cm the rate of cycloreversicHi was the focus of early work by Bachmann and later by Vaughan. The effect of both diene and dioiophile substituticHi on Ae rate of the rDA reaction in anthracene cycloadducts has been reported in a study employing 45 different adducts. If both cycloaddition and cycloreversion processes are fast on the time scde of a given experiment, reversibility in the DA reaction is observed. Reversible cycloaddition reactions involving anthracenes, furans, fulvenes and cyclopentadienes are known. Herndon has shown that the well-known exception to the endo rule in tiie DA reaction of furan with maleic anhydride (equation 2) occurs not because exo addition is faster than endo addition (it is not), but because cycloreversion of the endo adduct is about 10 000 times faster than that of the exo adduct. ... 

Triptycene i.s an unusual molecule that has been prepared by reaction of benzya with anthracene. What kind of reaction is involved Show the mechanism. 

A mechanism consistent with these observations is presented in Figure 8. Excited state aromatic carbonyl molecules having N,n triplet states are known to abstract hydrogen from alcohols, ethers and other substrates. In contrast, n.n excited states (i.e., perylene and anthracene) are not observed to participate in this reaction. The addition of THF to perylene and anthracene resulted in only slight increases in polymerization rates and product yields. ( 5)... 

Electron-rich polyaromatic compounds such as anthracene, pyrene, and pery-lene [107] are suitable as photosensitizers as they give redox reactions with DPI salts through exciplex to finally yield the initiating species for photoinduced cationic polymerizations. Scheme 11.28 demonstrates the mechanism of a polymerization followed via exciplex formation through the excited sensitizer with the ground-state onium salt. 

There are several reports this year of photocycloaddition reactions between polycyclic aromatic hydrocarbons and 1,3-dienes. Anthracene (58 R = H) or 9-cyanoanthracene (58 R = CN) give [4 -I- 4] and [4 -I- 2]adducts on irradiation with buta-1,3-diene, but the different product ratios and temperature effects are used to support the previously proposed biradical mechanism for the reaction. Another report from the same group deals with anthracene-hexa-2,4-diene and 9-phenylanthracene-penta-1,3-diene or cyclohexa-1,3-diene systems in the case of 9-phenylanthracene (59) and cyclohexa-1,3-diene, two [2 -I- 2]cycloadducts involving a terminal aromatic ring are isolated, as well as the more usual [4 + 2] and [4 -f- 4]adducts involving the central ring. Irradiation of substituted anthracenes... 

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