[Rh₂(MEPY)₄] and [BiRh(MEPY)₄]: Convenient Syntheses and Computational Analysis of Strikingly Dissimilar Siblings

Abstract

[Rh2(MEPY)4] is a versatile catalyst for asymmetric synthesis but its preparation requires purification by chromatography on surface‐modified silica. A higher yielding procedure based on a more convenient work‐up is presented herein. Moreover, a much improved method for the preparation of [BiRh(OTfa)4] is described, which makes this heterobimetallic complex readily available. Subsequent exchange of the trifluoroacetate ligands opens access to a so far underappreciated family of (chiral) paddlewheel complexes. While [BiRh] complexes comprising four carboxylate ligands are highly adequate for intermolecular asymmetric cyclopropanation reactions, [BiRh(MEPY)4] as the heterobimetallic cousin of [Rh2(MEPY)4] was found to be surprisingly unreactive; DFT calculations uncover the reasons for this inertia.

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

“…Specifically, complexes 7 a (R=H) and 7 b (R= t Bu) were recently shown to perform exceedingly well in asymmetric [2+1] cycloaddition reactions, furnishing diversely functionalized cyclopropanes and cyclopropenes in excellent optical purity and high yield with largely unparalleled reaction rates at low catalyst loadings [23, 24] . These powerful tools formally descend from the classical catalyst [Rh 2 (( S)‐ PTTL) 4 ] ( 1 a ) originally developed by the Hashimoto group (Figure 1B): [25] In a first step, one of the two rhodium atoms was deliberately replaced by a larger Bi(+2) center in order to impart a conical shape onto the chiral ligand sphere, [26–28] which adopts an α,α,α,α‐conformation in the stereodetermining transition state [29, 30] . As long as the narrower pore surrounds the catalytically active Rh(+2) site of [BiRh(( S)‐ PTTL) 4 ] ( 6 a ), improved asymmetric induction will ensue [31] .…”

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

“…[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] .…”

“…[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.…”

Elucidating the Electronic Nature of Rh‐based Paddlewheel Catalysts from 103Rh NMR Chemical Shifts: Insights from Quantum Mechanical Calculations

“…[24][25][26][27][28][29][30][31][32][33] Homoleptic dirhodium carboxamidates such as [Rh 2 ((S)-MEPY 4 ] are another type of paddlewheel complexes that enjoys popularity in the synthetic community. [34][35][36][37][38][39][40] However, the excellent selectivity that they endow in many cases has to be weighted against their generally lower reactivity towards diazo derivatives. While this fact is well documented in the literature, the reasons for it remain unclear.…”

“…Homoleptic dirhodium carboxamidates such as [Rh 2 (( S )‐MEPY 4 ] are another type of paddlewheel complexes that enjoys popularity in the synthetic community [34–40] . However, the excellent selectivity that they endow in many cases has to be weighted against their generally lower reactivity towards diazo derivatives.…”