Following the elegant design of Du Bois and colleagues for intramolecular C-H amination catalyzed by dirhodium tetracharboxylate 8, Fruit and Muller constructed an enantioselective system using chiral catalysts 5 or 6.30. On treatment of sulfonamides or sulfamic acid esters with PhI(OAc)2 and MgO, the tethered benzylic C–H bond underwent amination to produce enantioenriched five- and six-membered nitrogen-containing heterocycles in good yields, albeit with only moderate ees. Similarly, Hashimoto and colleagues examined the enantioselective cyclization of sulfamate with 16 and 17, which are analogues of 3 conducive to intermolecular setup (scheme 7), and found good catalyst turnover but unfavourable asymmetric induction levels. More recently, Davies used a highly stereoscopic dirhodium tetracharboxylate catalyst 4 (Scheme 17) 15 under Lebel conditions to significantly improve the enantioselectivity of n-toluene sulfonioxyl carbamate cyclization.
As described above, chiral dirhodium tetracharboxylate complexes serve as catalysts for enantioselective c-H insertion of azene. However, formamide contributes more electrons to the rhodium center than carboxylate, so the feedback to the metal-azene bond (Rh=N) must be amplified for the former type of ligand. This effect should lead to better stabilization and higher asymmetric induction of the active substance, which should make dirhodium tetramethamide an attractive catalyst platform for enantioselective azene transfer reactions. Nevertheless, dirhodium complexes coordinated with formamide tend to be single-electron oxidized by a high-valence iodine (III) compound, resulting in a mixed valence Rh (III) -RH (II) substance with no catalytic activity. The oxidative deactivation process greatly limits the use of dirhodium tetramethamide for c-H insertion of azene. This contrasts with enantioselective cyclopropanation and azpropylation of olefin, and dirhodium tetramethaminate is a very efficient catalyst platform. 32 Du Bois and colleagues, however, made a breakthrough by constructing dirhodium tetramethamide 18, derived from a chiral caprolactam derivative. Their successful catalyst design relied heavily on intramolecular hydrogen bonds in the ligand skeleton to increase the oxidation potential of the rhodium center and thus protect 18 from the aforementioned one-electron oxidation. Therefore, enantioselective intramolecular azene insertion occurs when various sulfonamides with γ c-H bonds activated by aryl or heteraryl groups are condensated with PhIO (2 equivalent) to the corresponding iminoiodoalkane in the presence of 18 (2 mol%) and desiccant. MS 3A (Table 5). 33 Notably, six-membered heterocyclic compounds were obtained at high yields, with enantioselectivity up to 99% EE, while common functional groups remained intact. It should be noted that Lee and colleagues recently used current amination (item 1) as a key step in asymmetric synthesis of drug-significant (S) -dapoxetine (Option 18). The 10N study showed that 18 people actually produced amideated products that did indeed have an R configuration, not an S as originally reported. Catalytic systems with 18 are also suitable for the highly chemical and enantioselective intramolecular allyl amination of alkenes (see Table 7).
