Dimethylamino radicals

The stabilizing role of other functional groups can also be described in resonance terms. Both electron-attracting groups such as carbonyl and cyano and electron-donating groups such as methoxy and dimethylamino have a stabilizing etfect on a radical intermediate at an adjacent carbon. The resonance structures which depict these interactions indicate delocalization of the unpaired electron onto the adjacent substituents ... 

Other electrophilic substitutions proceed with difficulty, or not at all. Nitrosation and diazo coupling require the presence of the strongly activating dimethylamino group (see Section VIII). Bromine adds, in the presence of sunlight, to give tetrabromotetrahydrobenzofuroxan (48) the initial attack is probably free-radical in nature. The product can be dehydrobrominated to form 4,7-, or a mixture of 4,5- and 4,6-dibromobenzofuroxan, depending upon the conditions. More conventional electrophilic bromination conditions have been tried in an attempt to obtain a monosubstituted product, but without success. 

The absolute rate constants for the reaction of a variety of electrophilic free radicals with 4-(dimethylamino)-l,5-dimethyl-2-phenyl-l,2-dihydro-3//-pyrazol-3-one (aminopyrine) and l,5-dimefliyl-2-phenyl-l,2-dihydro-3//-pyrazol-3-one... 

Another redox reaction leading to arenediazonium salts was described by Morkov-nik et al. (1988). They showed that the perchlorates of the cation-radicals of 4-A,A-dimethylamino- and 4-morpholinoaniline (2.63) react with gaseous nitric oxide in acetone in a closed vessel. The characteristic red coloration of these cation-radical salts (Michaelis and Granick, 1943) disappears within 20 min., and after addition of ether the diazonium perchlorate is obtained in 84% and 92% yields, respectively. This reaction (Scheme 2-39) is important in the context of the mechanism of diazotization by the classical method (see Sec. 3.1). 

This statement does not mean, however, that the mechanism of diazotization was completely elucidated with that breakthrough. More recently it was possible to test the hypothesis that, in the reaction between the nitrosyl ion and an aromatic amine, a radical cation and the nitric oxide radical (NO ) are first formed by a one-electron transfer from the amine to NO+. Stability considerations imply that such a primary step is feasible, because NO is a stable radical and an aromatic amine will form a radical cation relatively easily, especially if electron-donating substituents are present. As discussed briefly in Section 2.6, Morkovnik et al. (1988) found that the radical cations of 4-dimethylamino- and 4-7V-morpholinoaniline form the corresponding diazonium ions with the nitric oxide radical (Scheme 2-39). 

To a flame-dried, argon-purged flask, equipped with a glass-coated jstirring bar, was added THF (10 ml) and lithium ribbon (5.8 mg atom). The mixture was cooled to between -45 and -55 C (hexan-l-ol/dry ice), and l-(dimethylamino)naphthalene (5.1 mmol) was added slowly. The Idark-green colour of the radical anion appeared within 10 min, and Iformation of LDMAN was complete after 3.5 h of rapid stirring. 

Reactions between much stronger donors and acceptors belong to the electron tranter band. Such olefins do not form cyclobutanes but ion radical pairs or salts of olefins. refrato(dimethylamino)elhylene has an ionization potential as low as Na. The olefin with extraordinary strong electron-donating power is known not to undergo [2+2]cycloaddition reaction, but to give 1 2 complex with TCNE (transfer band in Schane 3) [23]. 

Experimentally, the molecular geometry has been determined by X-ray analysis for several larger radicals. These data indicate, in agreement with the theory, that bond alternation characteristic in many reduced and oxidized closed-shell forms is diminished in radical ions. Precise crystallographic data are available for 4,4 -A/s(dimethylamino)diphenylamine radical cation (87, 88), N,N -diphenyl-p-phenylenediamine radical cation (89), and Wiirster s blue (90). 

Other [AuCIL] complexes with monophosphines are those derived of o-carborane (427),2506 pyr-anoside group (428),2507 a-dimethylamino(3)ferrocenophan-2-yl,2508 calix[4]arene derived phosphites,25 or aminoxyl radical-substituted phosphines (429),2510 for which ferromagnetic interactions have been observed. 

As the final example in this section, a Li-mediated carboaddition/carbocycliza-tion process will be described. Thus, Cohen and coworkers observed a 5-e%o-trig-cy-clization by reaction of the lithium compound 2-349 and a-methyl styrene 2-350 to give 2-352 via 2-351 (Scheme 2.82). Quenching of 2-352 with methanol then led to the final product 2-353 [189]. In this process, 2-349 is obtained by a reductive lithia-tion of the corresponding phenyl thioether 2-348 with the radical anion lithium 1-(dimethylamino)naphthalenide (LDMAN) (2-354). Instead of the homoallylic substance 2-348, bishomoallylthioesters can also be used to provide substituted six-membered ring compounds. 

Bordwell compares the alkoxy effect with the C—H BDE for dimethyl ether as reported by McMillen and Golden (1982). The latter is lower by 12 kcal mol" than that of methane, and from the comparable magnitude of the effect in the a-alkoxy acetophenonyl radical it is concluded that an additive substituent effect exists. Similar arguments hold for the dimethylamino-substituted radical. It is stated that additivity is more than expected for bis-... 

In a systematic study of the addition of cyclohexyl radicals to a-substi-tuted methyl acrylates, Giese (1983) has shown that the captodative-substituted example fits the linear correlation line of log with o-values as perfectly as the other cases studied. Thus, no special character of the captodative-substituted olefin is displayed. More recently, arylthiyl radicals have been added to disubstituted olefins in order to uncover a captodative effect in the rate data (Ito et aL, 1988). Even though a-A, A -dimethyl-aminoacrylonitrile reacts fastest in these additions, this observation cannot per se be interpreted as the manifestation of a captodative effect. Owing to the lack of rate data for the corresponding dicaptor- and didonor-substituted olefins, it is not possible to postulate a special captodative effect. The result confirms only that the A, A -dimethylamino-group, as expected from its a, -value, enhances the addition rate. In the sequence a-alkoxy-, a-chloro-, a-acetoxy- and a-methyl-substituted acrylonitriles, it reacts fastest. 

Oxidation of enaminone 1 is initiated by electron loss from the dimethylamino moiety leading to radical cation, RH". The following chemical reaction would be an intramolecular cyclization through addition of a hydroxy group on the radical cation site yielding a cyclic radical cation, cRH ". This step is most likely the rate-determining step. The cyclic radical cation then dimerizes... 

In comparison with hydrocarbons, aromatic amines easily transform into cation radicals. Structures of these cation radicals are well documented on the basis of their ESR spectra and MO calculations (see, e.g., Grampp et al. 2005). The stable cation radical of A/,A,A, A -tetramethyl-p-phenylenediamine (the so-called Wuerster s blue) was one of the first ion radicals that was studied by ESR spectroscopy (Weissmann et al. 1953). The use of this cation radical as a spin-containing unit for high-spin molecules has been reported (Ito et al. 1999). Chemical oxidation of N,N -bis [4-(dimethylamino)-phenyl-A/,A -dimethyl-l,3-phenylenediamine with thianthrenium perchlorate in -butyronitrile in the presence of trifluoroacetic acid at 78°C led to the formation of the dication diradical depicted in Scheme 3.58. 

In terms of nitration, the system (NaN02 + CF3SO3H) is of no interest. At the same time, dimerization in this system can be attractive. For the last direction, CF3SO3H (or FSO3H) is necessary to produce binaphthyl derivatives more preferentially than nitro compounds (Tanaka et al. 1996). This work was preceded by the observation that the reaction of NO+AICI4 with 1-methyl-, 1,2-dimethyl-, 1,3-dimethyl, or 1,8-dimethylnaphthalenes in liquid SO2 leads to a partial a,a-dimerization (see Borodkin et al. 1993). Ozeryanskii et al. (1998) published the dimerization of l,8-Af,Af-bis(d imethylamino)naphthalene by the action of NOj in CHCI3. This reaction is accompanied by the formation of 4-nitro-l,8-A,A-bis(dimethylamino)naphthalene. Both gronps of anthors consider cation-radicals of the initial substrates as intermediate species. 

An useful alternative to the already known retropinacol reactions is presented by Liu and co-workers [7], This works demonstrates that pinacols bearing (dimethylamino)phenyl substiments can be subjected to fast oxidative fragmentation via photoinduced electron transfer with chloroform as the electron acceptor in yields up to 80%. The extremely fast dechlorination of the chloroform radical anion inhibits back-electron transfer and thus leads to effective fragmentation of the pinacol radical cation (Scheme 8). 

The subsequent reactions of the alkyl radical formed in reaction (80a) and the dimethylamino radical in (80b) are expected to be as follows ... 

It is noteworthy that the dimethylamino radical reaction with 02 is about a factor of 106-107 slower than its reactions with NO and N02. For example, Lindley et al. (1979) measured the ratio of rate constants /c81//c82 = 1.5 X 10-6 and /c81//c83a = 3.9 X 10-7. Thus, at 10 ppb NOx, reactions of the nitrogen-centered radical with NO and N02, in addition to 02, become important. This is perhaps not surprising, given that NH2 radicals also react extremely slowly with 02, with an upper limit of 6 X 10-21 cm3 molecule-1 s-1 (Tyndall et al., 1991). 

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