Anthracene electron-withdrawing substituents

However, there is an important subtlety in 4. While amine receptor-anthracene lumophore pairs produce excellent PET based quenching, a benzo-15-crown-5 ether receptor is demonstrably incapable of transferring an electron to an excited anthracene lumophore unless the latter carries an electron withdrawing substituent. Then, what is the secret of 4 s success First, only one rapid PET process is... 

Electron-donating substituents (e.g., methyl) generally lead to increased relative transformation rates for example, the relative reactivity of anthracene is 1200 compared to 21,000, 6500, and 1600 for 9,10-dimethyl-, 9-methyl-, and 2-methylanthracene, respectively (Table 10.29). Electron-withdrawing substituents, e.g., nitro, decreased rates. These effects are characteristic of electrophilic reactions. 

The dimerization of radical anions derived from 9-X-substituted anthracenes (Scheme 4), where X is an electron-withdrawing substituent, is related to electrohydrodimerization and might be expected to be less complex since proton donors are not involved in the formation of the products (Hammerich and Parker, 1981b). The reactions, where X is NOj, CHO, or CN, were studied by LSV and DCV. Primarily on the basis of the near independence of the reaction rates on temperature, the simple dimerization mechanism was excluded. It was proposed that the overall reaction consists of two reversible steps (i) formation of a radical anion dimer complex in which the two anthracene moieties are not bonded at the 10 positions and (//) the rearrangement of the complex to the stable dimeric dianions. The rate of the reaction was found to be independent of the water concentration in DMF. The radical... 

The dimerization of anthracene " has been studied extensively [243, 244]. With strongly electron-withdrawing substituents at position 9 the radical anions undergo reversible dimerization in aprotic solvents such as DMF, MeCN, propylene carbonate, DMSO etc. followed by rate determining a bond formation to furnish the stable dimer dianion [245]. In DMF k m were found to decrease in the... 

Reduction of 9-substituted anthracenes, (91), leads to radical anions, which, because of the electron-withdrawing substituents, are quite stable with respect to protonation and cleavage in aprotic solvents. In polar aprotic solvents the radical anions exclusively dimerize, and the reaction has been the subject of a number of studies [247-258]. The products are the tail-to-tail dimeric dianions as in Eq. (57), which are fairly stable. In CV the dimer dianions can be detected as a new oxidation peak on the reverse scan at a potential several hundred millivolts anodic relative to the potential of radical anion formation. On preparative or semipreparative scales the dimer dianion has in a single case been detected by H-NMR [249], and oxidative electrolysis of the dimer dianions in most cases restores the starting material. 

For radical cations this situation is typically observed when deprotonation of the dimer dication is slow and for radical anions under conditions that are free from electrophiles, for example, acids, that otherwise would react with the dimer dianion. Most often, this type of process has been observed for radical anions derived from aromatic hydrocarbons carrying a substituent that is strongly electron withdrawing, most notably and well documented for 9-substituted anthracenes [112,113] (see also Chapter 21). Examples from the radical cation chemistry include the dimerization of the 1,5-dithiacyclooctane radical cations [114] and of the radical cations derived from a number of conjugated polyenes [115,116]. 

A specific example will clarify the procedure. When the sigma charge descriptor is generated using the following substructure with four differently substituted 12-methyl benz(a)-anthracenes, the differences in electron withdrawing power of the 7 position substituents are reflected in the descriptor value. 

A secondary reaction has been identified in the reaction of magnesium-trimethylsilyl substituted anthracene complexes w ith benzylic halides, which is the addition of preformed Grignard reagent with the generated anthracene, equation (8.5) [44]. However, this is slow enough not to be a serious problem. The addition is favoured by the electron-withdrawing silyl substituents otherwise more forcing conditions are required [49]. 

Staggered configurations have also been observed for the tricarbonyl chromium complexes of phenanthrene 294, 295), 9,10-dihydro-phenanthrene 293, 295), anthracene 202), naphthalene 262), and 1-aminonaphthalene 58). The eclipsed configuration has been observed for the tricarbonyl complexes of anisole (57, 229), toluidine 60, 61), methylbenzoate (59), o-methoxyacetylbenzene, o-hydroxyacetylbenzene 101), 2-methoxy-[l-hydroxy-ethyl]benzene (99), and 2-methyl-[l-hydroxy-l-phenylpropyl]benzene (97). It is apparent that the orientation of the chromium tricarbonyl moiety is in many cases controlled by the substituents on the ring to which it is coordinated, and this has been attributed to mesomeric electron repulsion or withdrawal by the substituents 374). 

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