On the Structure of Chiral Dirhodium(II) Carboxylate Catalysts: Stereoselectivity Relevance and Insights

Frady Adly
2017 Catalysts  
Modern experiments have offered alternative interpretations on the symmetry of chiral dirhodium(II) carboxylate complexes and its relationship to their level of enantioselectivity. So, this contribution is to provide an insight on how the knowledge around the structure of these catalysts has evolved with a particular emphasis on the impact of this knowledge on enantioselectivity prediction and catalyst design. Initially, Hansen and Davies suggested that catalysts having two different
more » ... mation sites should not lead to high enantiocontrol [53] . In other words, complexes with α,α,β,β-and α,β,α,β-arrangements (having C 2 -and D 2 -symmetry, respectively) are the only effective catalysts Catalysts 2017, 7, 347 3 of 19 in terms of enantioselectivity as they possess two equivalent catalyst faces. While complexes with α,α,α,α-and α,α,α,β-conformations, which have non-equivalent catalyst faces, are likely to induce low or no enantioinduction as their more accessible and kinetically active face is achiral. This previous proposal from Davies et al. originates from their discovery of Rh 2 (S-DOSP) 4 catalyst (Scheme 2) [36] , which offered extraordinary enantioselectivity in a wide range of chemical transformations (up to 99% ee) [7, 13, 36, 40, 42, 44, 47, 48, [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] . This exceptional enantioselectivity of Rh 2 (S-DOSP) 4 was proposed to originate from a favored α,β,α,β-arrangement of ligands adapted during catalysis. The arrangement that leads to a catalyst with two equivalent rhodium active sites and adequate sterically overburden groups that can limit the trajectories approaching the axial carbene ligand (Scheme 2) [7, 13, 36, 53] . For example, low temperature Rh 2 (S-DOSP) 4 -catalyzed cyclopropanation of styrene with methyl styryldiazoacetate resulted in the generation of the corresponding cyclopropane product in more than 50:1 E/Z diastereomeric ratio and 98% ee (Scheme 2a) [13, 69] . A general model was then proposed to explain the observed stereochemical outcome [13, 36, 68, 70] . In this model, Rh 2 (S-DOSP) 4 is assumed to favor the α,β,α,β-conformation during catalysis at which, the si-face of the metal-carbene complex is protected by a ligand blocking group. The substrate will approach from the re-face to generate the product in the observed absolute configuration (Scheme 2a).
doi:10.3390/catal7110347 fatcat:rekrzkhxt5aavbnpp2jwz7gq5q