Acid-Catalyzed [3,3] Sigmatropic Rearrangement of N-Cbz-Diaryl Hydrazide for the Synthesis of Mono-N-Cbz-1,1′-biaryl-2,2′-diamine

Selective 3,3-Rearrangement of Azobenzenes upon Complexation by a Frustrated Lewis Pair

The reaction of the oxygen-bridged frustrated Lewis pairs (FLPs) tBu₂ P-O-Si(C₂ F₅ )₃ (1) and tBu₂ P-O-AlBis₂ (2) with azobenzene, promoted by UV irradiation, led to a selective complexation of the cis-isomer. The addition product of 2 is stable, while the adduct of 1 isomerizes in solution in an ortho-benzidine-like [3,3]-rearrangement by cleavage of the N-N bond, saturation of the nitrogen atoms with hydrogen atoms and formation of a new bond between two phenyl ortho-carbon atoms. Similar rearrangements take place with different para-substituted azobenzenes (R=Me, OMe, Cl) and di(2-naphthyl)diazene, while ortho-methylated azo compounds do not form adducts with 1. All adducts were characterized by multinuclear NMR spectroscopy and elemental analyses and the mechanism of the rearrangement was explored by quantum-chemical calculations.

Reagent- and Metal-Free Anodic C-C Cross-Coupling of Aniline Derivatives

The dehydrogenative cross-coupling of aniline derivatives to 2,2'-diaminobiaryls is reported. The oxidation is carried out electrochemically, which avoids the use of metals and reagents. A large variety of biphenyldiamines were thus prepared. The best results were obtained when glassy carbon was used as the anode material. The electrosynthetic reaction is easily performed in an undivided cell at slightly elevated temperature. In addition, common amine protecting groups based on carboxylic acids were employed that can be selectively removed under mild conditions after the cross-coupling, which provides quick and efficient access to important building blocks featuring free amine moieties.

The Acid-Catalysed Benzidine Rearrangements May Proceed via Cation Radicals Formed by Electron Transfer to a Proton from a Hydrazyl Nitrogen

The acid-catalysed benzidine rearrangements are proposed to proceed via cation radical intermediates which arise by rapid electron transfer from a neutral nitrogen of the hydrazyl moiety to a proton. Consequently, both the concerted rearrangements, Table 1, benzidines, 4-methoxy- p-semidine formation, Eqns (2), (3) and o,o‘-biaryl-linked products, and non-concerted rearrangements, Table 2, diphenyline, o-semidine, Eqns (5), (6) and perhaps 4-chloro- p-semidine, may proceed via cation radical structures involving radical C – C bond formations and homolytic N – N bond cleavages. This is contrary to the current view of the Polar Transition State theory which postulates heterolytic bond formations and cleavages, Schemes 1, 3 and 4. Mono-substituted and 4,4'-disubstituted hydrazobenzenes are proposed to undergo oxidation of the most basic nitrogen as inferred from the estimated p Ka s as determined by the SPARC program. This can explain why of the two possible o-semidine rearrangement products, 2,N’ and N,2'-linked, the observed major o-semidine has the substituent para to the amino group, e.g. Eqn (5). However, if one of the para substituents is a halogen, it is the nitrogen para to the halogen which undergoes SET and oxidation. In these second-order acid reactions, this can be explained by protonation of the most basic nitrogen followed by SET from the hydrazyl nitrogen para to the halogen. In the case of 4,4'-disubstitution, the hydrazobenzene may not lose its substituents readily and undergoes a rapid one-electron reduction and consequent disproportionation reaction as shown in Eqn (8). Thus the kinetics, first-order in hydrazoarene and the same kinetic order in acid for both disproportionation and rearrangement, are explained.