Polar effects in free radical reactions. Homolytic aromatic amination by the amino radical cation, .cntdot.+NH3: reactivity and selectivity

Abstract

Small amounts of Fe(II) salt initiate redox chains with hydroxyamino-O-sulfonic acid (HSA) and aromatic compounds leading to the amination of the aromatic ring. The positional and substrate selectivity with a variety of substituted benzenes show the important role of the electrophilic character of the radical -+NH3. Hammett relationships with and + are roughly observed. The lower sensitivity to polar effects of the radical •+ 3 compared with (CH3)2+NH• is explained by the different reaction enthalpies. The different positional selectivity obtained with anisóle and the redox system +NH3OH/Ti(III) is discussed.The amination of aromatic compounds by AVV-dialkyl, R2+ÑH, and jV-monqalkyl, R+ÑH2, amino radical cations from iV-alkylchloramines in acidic medium is particularly interesting for the synthetic and theoretical involvements.1 The exceptional positional and substrate selectivity, observed with many aromatic substrates has been ascribed to the polar effects connected with the strong electrophilic character of the alkylamino radical cations and with a charge-transfer character of the transition state1 (eq 1).The behavior of the simple amino radical cation, •+ 3, for which a similar trend would be expected, has been less investigated. Unfortunately the use of the iV-chloramine, NH2C1, in acidic medium in order to have a homogeneous comparison proved to be unsuitable.Years ago two different radical sources were investigated for the aromatic amination by •+ 3, the redox couples +NH3OH/Ti(III)2 (eq 2) and +NH30S037Fe(II)3 (eq 3).

Efficiency of electrochemical amination of anisole in aqueous-organic solutions of 4 M H2SO4

“…In chemi cal amination [1,2], the addition of commercial Ti(III) solution to the emulsion or solution of anisole decreases molar ratio H 2 SO 4 : H 2 O and concentration of organic solvent and therefore increases the contribution of the reaction (5) into termination of amino radical intermedi ates. In electrochemical process, water is formed only in the reaction (3).…”

“…Second, the dependencies of the potential of mercury cathode obtained at 40 and 15°C showed that in acetic acid the decrease of the temperature promotes the decrease of the charge required for finishing the target electrochemical process (see figure). Change of the potential of the mercury cathode during the electrolysis of the system Ti(IV)-NH 2 OH-anisole in the electrolytes containing acetic acid (1, 2) and acetoni trile (3,4) at 15 (1,4) and 40°C (2,3).…”

“…In aqueous and aqueous-organic solutions of diluted sulfuric acid, cation radical amination of aro matic substrates activated toward electrophilic agents results in formation of appropriate monoamino deriv atives [1][2][3]. For chemical functionalization of above substrates with the salts of transient metals and hydroxylamine or with hydroxylamine O sulfoacid, the yield of primary amines to the source of amino radicals does not exceed 40% [1].…”

Metal-Catalyzed and Metal-Free Intermolecular Amination of Light Alkanes and Benzenes

“…For example, very low concentrations of Fe(II) ions (even less than 1 mol%) can activate hydroxylamine- O -sulfonic acid (H 2 N-OSO 3 H) in acidic media, to generate free • NH 3 + radicals (Scheme 4(c)). 32 Those attack mono-substituted benzenes with reactivity expected for electrophilic radical attack, and position selectivity that is significantly less discriminating versus that observed for the aforementioned alkylamino radical cations. This is likely due to the higher reactivity of the unsubstituted • NH 3 + radical (higher exothermicities), which gives rise to early transition states with less polar-effect sensitivity, leading to lower selectivity.…”

“…As a further illustration of the power of metal-free nitrogen-centered intermediates, several redox-active ions (Fe II , Cu I , V III , Ti III ) and solid-state (photo)catalysts are known 22,32–34 to generate diffusively-free or surface-adsorbed neutral or protonated amino radicals ( • NR 2 , • NHR 2 + ) from various sources ( N -haloamines, N / O -substituted hydroxylamines, ammonia), which can in turn aminate aliphatic and aromatic substrates at high rates. For instance, Minisci has shown 35 that catalytic or stoichiometric amounts of metal salts (Fe II , Ti III , Cu I , Cr II ) can activate N -alkylchloramines in acidic media to give rise to amino radical cations • NHR 2 + that can perform homolytic aromatic aminations (Scheme 4(a)).…”