9-Methylanthracene, radical cation

Fig. 8. Transient spectrum (33 pseconds) obtained from the charge-transfer excitation at 532 nm of a solution of 0.2 AT 9-methylanthracene and 3.4 mAf (HMB)2Fe2+. The inset shows the time-resolved spectrum of the 9-methylanthracene cation radical for comparison (123).

Similar electron-transfer intermediates are observed with other (aromatic) donors that promote deligation of Ar2Fe + acceptors. For example, charge-transfer laser excitation of the EDA complex of (HMB)2pe + with 9-methylanthracene (MeANT) generates the 9-methylanthracene cation radical with its characteristic absorption centered at 700 nm (see Figure 1 IB Eq. 38). 

Coupling at the benzylic position occurs with 9-methoxy-lO-methylanthracene. The dimer of a quinone-methide is obtained in about 50% yield. The product is thought to be formed by deprotonation of the initially formed cation radical, dimerization of the benzyl radicals, and subsequent oxidative demethylation at the methoxy group (Table 4, number 14). 

Interestingly, when the C-H bond is forced into a conformation in which it is almost collinear with the 71-system as in 1,9-ethanoanthracene (EA), a ratio for deprotonation of 28 1 is observed between the ethano group in EA + and the methyl group in 9-methylanthracene radical cation [150]. 

The mechanism discussed above for the deprotonation of alkylaromatic radical cations, involving a bimolecular reaction between the radical cation and the base (B), leading to a carbon centered neutral radical and the conjugated acid of the base (BH" ") as described in Scheme 28, has been recently questioned by Parker who provided evidence for an alternative mechanism in proton-transfer reactions between methylanthracene radical cations and pyridine bases [154] this involved reversible covalent adduct formation between the radical cation and the base followed by elimination of BH+ (Scheme 36). 

Product studies of the reaction of the 9-methylanthracene radical cation with 2,6-lutidine in acetonitrile show X to be the exclusive reaction product it results from proton transfer from the methyl substituent (Scheme 37). 

Comparison of the reactivities of both 9-methylanthracene and 9,10-dimethylanthracene radical cations with those of pyridine and 2,6-lutidine shows that in the absence of severe steric effects radical cation-nucleophile combination is kinetically favored over direct proton transfer, even though the latter process is highly exergonic. Because the addition step cannot usually be readily detected, however, experimental evidence does not usually enable distinction between addition-elimination and direct proton transfer mechanisms. 

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