Aminyl radicals metal complexes

Biittner T, Geier J, Prison G et al (2005) A stable aminyl radical metal complex. Science 307 235-238... 

The electronic nature of a nitrogen centered radical, dictated by reaction conditions and/or the radical precursor employed, is crucial to the mode of reaction, to the ability to undergo efficient intramolecular cyclizations or intermolecular additions, and to the products isolated from the radical reaction. The types of radicals discussed in this review include neutral aminyl radicals, protonated aminyl radicals (aminium cation radicals), metal complexed aminyl radicals, and amidyl radicals. Sulfonamidyl and urethanyl radicals are known (71S1 78T3241), but they are not within the scope of this chapter. 

Cyclic chain termination with aromatic amines also occurs in the oxidation of tertiary aliphatic amines (see Table 16.1). To explain this fact, a mechanism of the conversion of the aminyl radical into AmH involving the (3-C—H bonds was suggested [30]. However, its realization is hampered because this reaction due to high triplet repulsion should have high activation energy and low rate constant. Since tertiary amines have low ionization potentials and readily participate in electron transfer reactions, the cyclic mechanism in systems of this type is realized apparently as a sequence of such reactions, similar to that occurring in the systems containing transition metal complexes (see below). 

However the existence of the bromine co-product is speculative and has not been experimentally verified. The reaction may be regarded as a one electron oxidation by the S i. Me , )2 aminyl radical, and it is interesting to note that aminyl radical complexes of transition metals such as [Rh(Ntrop2)(bipy)][OTf]5 (trop = 5-H-dibenzo[a,d]cycloheptene) have been isolated. The manganese and cobalt compounds were the first examples of three-coordinate Mn(lll) and Co(lll) complexes to have been structurally characterized. 

To facilitate the cyclizations of nitrogen-centered radicals, one need only increase the electronegativity at nitrogen. This makes the radical more electrophilic (which accelerates the cyclization), and strengthens the forming bonds (which shifts the equilibrium to the cyclic product). Amminium radical cations are more well-behaved in cyclizations than aminyl radicals, and these radical cations can either be formed directly from ammonium salts, or by protonation of aminyl radicals (which are relatively weak bases and require strong acids for protonation). Metal-complexed aminyl radicals are highly reactive, and amidyl radicals are also useful.178... 

Neutral aminyl radicals (I), also referred to as amino radicals, can be considered to be nucleophilic species whereas aminium cation radicals (II), metal complexed aminyl radicals (III), and amidyl radicals (IV) are electrophilic in nature. Greater utility has been observed with electrophilic nitrogen radicals than with neutral aminyl radicals (71 SI). Aminyl radicals are easily protonated with Br0nsted acids to give aminium cation radicals and readily complex with Lewis acids to form radicals III therefore, control of the reaction conditions is critical to ensure that reactions of interest are occurring from only one species. 

Neutral aminyl radicals bearing electron-withdrawing tV-substituents (for example, amidyl radicals), or aminium radicals (protonated aminyl radicals) and metal ion complexed aminyl radicals are more reactive due to the lower nonbonding electron density on nitrogen, and consequently the diminished repulsive interaction with the it electrons of the alkene4,6. 

Recentiy, it was demonstrated that aminyl radicals are substantially stabilized by coordination to Rh(I). Deprotonation of cationic [Rh (trop2N/f)(bpy)]+ 5-H-dibenzo-[fl,h]cycloheptene-5-yl-trop) species generates a stable (under inert atmosphere) neutral amido complex [Rh trop2N )(bpy)]. Remarkably, one-electron oxidation of the latter with [Fc]+ does not result in a Rh(ll) species, but yields a stable aminyl radical complex [Rh trop2N )(bpy)]. This is the first example of a stable aminyl radical coordinated to a transition metal ever reported (66). Both detailed EPR spectroscopic investigations (gn = 2.0822, g22 = 2.0467, g33 = 2.0247, = 45 MHz), and DFT calculations revealed that... 

Thus, the types of reactions favored by amino radicals depend to a significant degree upon the extent with which the lone electron pair is associated with a proton, with a Lewis acid, or with an electron-withdrawing group. Clearly, aminium radical cations 2 (Scheme 1), metal-complexed aminyl radicals 3, amidyl radicals 4, sulfo-namidyl radicals 5 and cyanamidyl radicals 6 are electrophilic in nature. On the other hand, dialkylaminyl radicals 1 have been shown to display nucleophilic character... 

In contrast, aminyl radicals complexed to metal ions, such as neutral aminyl radicals, do not undergo Hofmann-Loffler reaction. The Hofmann-Loffler reaction is unable to compete with intramolecular aromatic amination, with intramolecular addition to olefins, and with intermolecular addition to dienes [7],... 

The jV-chloro-compounds were the first to be employed as radical cyclization precursors in the synthesis of pyrrolidines and piperidines, as well as fused and bridged heterocyclic skeletons [7], Aminyl and amidyl radicals were thus generated and used in intramolecular additions. Higher yields and selectivities are obtained with the metal-complexed species. Some selected examples are reported in Table 4. Generally, a typical radical chain mechanism is involved (with chlorine atom transfer from 7V-chloro-compound). In the particular case of copper-cornplexed aminyl radical cyclization, a redox chain process operates (with fast chlorine ligand transfer from cupric chloride)... 

As already discussed in the Cy5/Cy6 case, products resulting from the (Cy 5) radical are generally formed exclusively. In the Cy5/Cy4 case no cyclized products have been observed from aminium radicals but a low yield (13%) of 3-chloropyrrolidine, i.e., a (Cy5) product, is obtained with metal-complexed aminyl radicals. ... 

In the Cy6/Cy7 case, intramolecular addition of aminyl radicals is a facile process. In the aminium form or metal-complexed form only products resulting from the (Cy 6) radical have been observed. In the latter case the yields are fairly high (up to 72%) when compared with the other Cy6/Cy7 cases discussed in previous sections. With neutral aminyl radicals, a low yield (14%) of cyclized products is observed as a mixture of Cy6/Cy7 products in a ratio 75 25 at —5°C and 50 50 at 45°C the main products are dimers. ... 

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