Dimethylamino radicals, reaction

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). 

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

In addition to these typical free radical reactions, it has also been reported that tetramethyltetrazene and hence possibly the dimethylamino radical react with aluminum alkyls and hydrides (13). 

However, this is probably an oversimplification, as the disproportionation reaction is surface-catalyzed and the two reactions must have different energies of activation. This behavior of dimethylamino radicals is in marked contrast to the iso-electronic isopropyl radicals, the disproportionation-combination ratio of which is unaltered by temperature and siuface (2, 30). The surface-promoted radical decomposition is also in marked contrast to the absence of such effects in the isopropyl radical reactions (2) and must be attributed to the presence of the nitrogen atom. Amino radicals also undergo surface reactions (28). The occurrence of TMA suggests the participation of methyl radicals in the reactions, and a possible route is provided by Reactions 28 and 34. 

Both of these suggestions are defective because of the absence of methane (route A) and the much greater quantities of TMMD produced compared with DMA (route B with Reaction 19 as the precursor of methylmethylene imine). A further route to TMMD could be provided by methylene insertion into the NN bond of TMH. This, though theoretically feasible, seems unlikely and requires the production of methylene from dimethylamino radicals by a surface reaction. The radical decomposition reactions (29 and 30) proposed by Gesser, MuUhaupt, and Griffiths (15) are not confirmed by our results. 

The product crystallized out of the reaction mixture as the zinc chloride complex. The state of the dimethylamino radical shown in brackets in the last equation is uncertain. It may or may not be coordinated with zinc chloride or it may originate from an S 2 displacement by styryl radical on the TMT ZnCl2 complex. Curiously no dimerization or disproportionation of the 8-amino-styryl radical was observed. Perhaps the coordination by zinc chloride prevents other reactions from occurring, allowing the combination with the second amino radical to occur reasonably efficiently. The reaction works well only with conjugated alkenes. Only slight reactions were observed with olefins such as 1-octene. 

Although neutral dialkyl ami no radicals generally do not add to simple alkenes it was possible to observe some addition of dimethylamino radicals to styrene and a-methylstyrene (48). Photolysis of tetramethy1-2-tetrazene (TMT) in cyclohexane, at room temperature, in the presence of a-methylstyrene resulted in the formation of small but reproducible yields of addition products. The bulk of the amino radicals were consumed in the formation of tetramethylhydrazine and dimethyl amine. The relative rate of formation of the adducts from substituted a-methylstyrenes were correlated by the Hammett equation yielding a p = +0.69 + 0.03 (corr. coeff.. 99). This result indicates that the dimethylamino radical behaves like a nucleophile in addition reactions i.e. the styrenes substituted by electron attracting substituents reacted more rapidly than the electron rich styrenes. This result is in marked contrast to that obtained for dimethylamino radicals coordinated by zinc chloride (48). These... 

One of the many radical reactions quoted in the Fischer s paper is shown in Scheme 22. NO radicals are known to be unable to dimerize in contrast to the dimethylamino radical (CH3)2N. Therefore, as a result of the continuous photolysis, the (almost) dominating reaction product is the starting compound (cross reaction). In the beginning, (CH3)2N first dimerize and then their concentration falls down to such a level that practically the only observed product is the starting dimethyinitrosamine (DMNO) the NO radicals are in large excess than (CH3)2N. Obviously, after a sufficiently long time, only NO radicals and dimers of dimethylamine would result. This prototype of CRPs has been analyzed by a number of authors. 

Lindley et al. (1979) have shown that the ratio of rate coefficients for the reaction of the dimethylamino radical with O2 to that with NO [A o2/feNO = (1-49 0.07) x 10 ]... 

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). 

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]. 

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. 

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 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 radical reductive cyclisation of diesters to acyloins (see also Section 5.9.1, p. 628) is an important method of synthesis for ring sizes from four-membered upwards. The example selected here is 2-hydroxy-3-methylcyclopent-2-enone ( corylone ) (29) (Expt 7.10), which is an important perfumery and flavouring material.53 In the first step (i), methyl acrylate is converted into its dimer with tris(cyclohexyl)phosphine in pyridine solution.5b Step (ii) is the protection of the double bond by conversion into the dimethylamino adduct. The acyloin reaction is step (iii), and the product is trapped as its bis(trimethylsilyl)ether. Finally, in step (iv), the protecting dimethylamino and trimethylsilyl groups are removed by passage down a column of silica gel. 

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