Chiral Bismuth‐Rhodium Paddlewheel Complexes Empowered by London Dispersion: The C−H Functionalization Nexus

Abstract

Heterobimetallic [BiRh] tetracarboxylate catalysts endowed with 1,3‐disilylated phenylglycine paddlewheels benefit from interligand London dispersion. They were originally designed for asymmetric cyclopropanation but are now shown to perform very well in asymmetric C−H functionalization reactions too. Because of the confined ligand sphere about the derived donor/acceptor carbenes, insertions into unhindered methyl groups are kinetically favored, although methylene units also react with excellent levels of asymmetric induction; even gaseous ethane is a suitable substrate. Moreover, many functional groups in both partners are tolerated. The resulting products are synthetically equivalent to the outcome of traditional asymmetric ester alkylation, allylation, benzylation, propargylation and aldol reactions and therefore constitute a valuable nexus to more conventional chemical logic.

Taming of Furfurylidenes by Chiral Bismuth‐Rhodium Paddlewheel Catalysts. Preparation and Functionalization of Optically Active 1,1‐Disubstituted (Trifluoromethyl)cyclopropanes

“…[56] All available [BiRh] paddlewheel complexes 7 proved adequate, but the diiodo variant 7 c gave the best result. [33] Under the optimized conditions (entry 7), product 3 a comprising a trifluoromethylated allcarbon quarternary center was obtained in 84 % yield with 92 % ee, virtually as a single diastereomer (dr > 20 : 1, trans). [57] Variously substituted styrene derivatives gave similarly good results, largely independent of whether they carry electron-donating or -withdrawing substituents at the paraposition of the phenyl ring; the corresponding cyclopropanes 3 a-f were obtained with > 90 % ee (Figure 2).…”

“…[25] Provided such a compound can be made and decomposed in a controlled manner with the aid of a chiral transition metal complex, the resulting reactive intermediate falls into the category of donor/acceptor carbene complexes, which are usually well behaved in [2+1] cycloaddition reactions and often lead to high levels of asymmetric induction [26,27,28,29,30] . Amongst the many catalyst systems that potentially qualify for this purpose, [31] the heterobimetallic bismuth‐rhodium paddlewheel complexes recently developed in our laboratory seem particularly adequate [32,33,34,35] . They owe their excellent performance to the following factors: the Bi(+2) site itself is incapable of converting diazo derivatives into carbenes but properly tunes the electronic character of the rhodium atom it is coordinated to [36,37] .…”

“…The use of toluene, C 6 F 6 or pentane as the solvent resulted in significantly lower yields, while the ee of the product remained largely unaffected [56] . All available [BiRh] paddlewheel complexes 7 proved adequate, but the diiodo variant 7 c gave the best result [33] . Under the optimized conditions (entry 7), product 3 a comprising a trifluoromethylated all‐carbon quarternary center was obtained in 84 % yield with 92 % ee, virtually as a single diastereomer (dr >20 : 1, trans ) [57]…”

“…[26,27,28,29,30] Amongst the many catalyst systems that potentially qualify for this purpose, [31] the heterobimetallic bismuth-rhodium paddlewheel complexes recently developed in our laboratory seem particularly adequate. [32,33,34,35] They owe their excellent performance to the following factors: the Bi(+ 2) site itself is incapable of converting diazo derivatives into carbenes but properly tunes the electronic character of the rhodium atom it is coordinated to. [36,37] At the same time, a multitude of stabilizing interligand London dispersion interactions [38,39] chiefly fostered by the peripheral silyl substituents on the phenylalanine-derived ligands craft a narrow, conformationally well-defined and highly ordered chiral binding site about the reactive rhodium center.…”

Diruthenium Tetracarboxylate-Catalyzed Enantioselective Cyclopropanation with Aryldiazoacetates

“…The chiral rhodium–bismuth lantern complexes are capable of cyclopropanation reactions with donor/acceptor carbenes and achieve similar levels of asymmetric induction as the corresponding dirhodium complexes, but they react about 1000 times slower . Recently, rhodium–bismuth complexes have been shown to be effective catalysts for site-selective and enantioselective C–H functionalization at activated sites . A chiral diruthenium paddlewheel complex is able to perform the cyclopropanation of styrene derivatives using acceptor/acceptor carbenes derived from iodonium ylides with high yield and enantioselectivity .…”