A New Dirhodium Catalyst with Hemilabile Tropolonato Ligands for CH Bond Functionalization

Abstract

Most organic synthetic methods are based on functional groups and involve the transformation of a bond either at the groups themselves or in their vicinity. Methods for the functionalization of unactivated C À H bonds render complementary synthetic strategies possible, since these bonds are inert to the reaction conditions of standard transformations. [1] Metal-bound carbenes, [2] nitrenes, [3] or oxygen [4] have been shown to be efficient catalytically generated species for the functionalization of CÀH bonds. Catalysts derived) are particularly effective in the decomposition of diazo compounds to form carbenes. [5] The most successful catalysts comprise proline-derived systems from the groups of McKervey [6] and Davies, [7] tert-leucine-derived systems of Hashimoto and Ikegami, [8] carboxamidates of Doyle [9] and tethered carboxylates of DuBois et al. [10] Yet many problems regarding the inherent reactivity of Rh carbenoids, particularly in synthetically most relevant intermolecular CÀH insertion reactions, remain unsolved: reactions are often found to be highly substrate specific leading to low yields in unfavorable cases, high catalyst loadings of 6 mol % rhodium (i.e., 3 mol % [Rh 2 (L) 4 ]) are typically required and high enantiomeric excesses can be accompanied by poor or no diastereoselectivity at all. These limitations can be attributed to a lack of structural diversity in the currently applied catalysts. With only a few exceptions, [11] the development of new dimeric Rh-catalysts focuses on structures bearing four bridging ligands.We report here on 1) the preparation of a novel dirhodium catalyst (2 Scheme 1), in which two bridging carboxylates are replaced by chelating tropolonato ligands, and its X-ray crystal structure; 2) investigations on its solution behavior that demonstrate the stability of the Rh 2 4 + core and the hemilabile nature of the tropolonato ligands, and 3) examples of intra-and intermolecular C À H insertion reactions, which document the good catalytic activity of our new catalyst system.Our approach to expand the chemical space of dirhodium catalysts was guided by the idea that four bridging ligands, as in the currently applied dirhodium catalysts, might not be necessary to ensure the stability and catalytic activity of the Rh 2 4 + core. Dirhodium compounds containing only two bridging ligands have been described in the literature. [12] We wanted to use this concept to alter the reactivity of dirhodium catalysts more fundamentally than through exploiting substituent effects on acetate derivatives and to put the currently accepted mechanisms of rhodium carbenoid chemistry to a test. In particular, we wanted to scrutinize the mechanistic paradigm that carbenoid transformations with dimeric rhodium catalysts exclusively involve substrate binding to the axial positions. [13] These positions can be readily occupied by weak donor species, for example, solvent molecules, [14] whereas the stronger equatorial rhodium spectatorligand bonds are considered to remain intact. This implies ...

Effect of Tethered, Axial Thioether Coordination on Rhodium(II)-Catalyzed Silyl-Hydrogen Insertion

“…When a strong withdrawing group is present on the phenyl ring, 1 has lower activity when compared to 3. However, as the donating ability of the diazo became stronger, complex 1 began to perform more effectively (5f-h, Table 2, entries [13][14][15][16][17][18]. There was also a larger change in yield when compared to changes in yield for complex 3.…”

“…In addition, the chelate effect allows weaker LBs to be incorporated to interrogate the effects of hemilabile ligands, of which there are few examples on the Rh paddlewheel scaffold. [17][18] We hypothesized that the flexibility of a hemilabile system will allow for a sufficiently reactive Rh(II) complex while providing stability to the transient rhodium carbene. We recently reported that the incorporation of thioethercontaining ligands into the rhodium paddlewheel motif improved yields in cyclopropanation reactions when compared to rhodium acetate.…”