The heteroleptic dirhodium paddlewheel catalyst 7 with achiral carboxylate/acetamidate ligand sphere is uniquely effective in asymmetric [2+ +1] cycloadditions with a-diazo-atrimethylstannyl (silyl, germyl) acetate.O riginally discovered as at race impurity in as ample of the homoleptic parent complex [Rh 2 ((R)-TPCP) 4 ](5), it is shown that the protic acetamidate ligand is quintessential for rendering 7 highly enantioselective.The-NH group is thought to lock the ensuing metal carbene in place via interligand hydrogen bonding.The resulting stannylated cyclopropanes undergo "stereoretentive" cross coupling, whichs hows for the first time that even chiral quarternary carbon centers can be made by the Stille-Migita reaction. Scheme 1. Conceptualo utline.
A H(C)Rh triple resonance
NMR experiment makes the rapid detection
of
103
Rh chemical shifts possible, which were previously
beyond reach. It served to analyze a series of dirhodium and bismuth–rhodium
paddlewheel complexes of the utmost importance for metal–carbene
chemistry. The excellent match between the experimental and computed
103
Rh shifts in combination with a detailed analysis of the
pertinent shielding tensors forms a sound basis for a qualitative
and quantitative interpretation of these otherwise (basically) inaccessible
data. The observed trends clearly reflect the influence exerted by
the equatorial ligands (carboxylate versus carboxamidate), the axial
ligands (solvents), and the internal “metalloligand”
(Rh versus Bi) on the electronic estate of the reactive Rh(II) center.
A heteroleptic dirhodium
paddlewheel complex comprising three chiral
carboxylate ligands and one achiral acetamidate ligand has recently
been found to be uniquely effective in catalyzing the asymmetric cyclopropanation
of olefins with α-stannylated (silylated and germylated) α-diazoacetate
derivatives. A number of control experiments in combination with detailed
computational studies provide compelling evidence that an interligand
hydrogen bond between the −NH group of the amidate and the
ester carbonyl group of the reactive rhodium carbene intermediate
plays a quintessential role in the stereodetermining transition state.
The penalty for distorting this array outweighs steric arguments and
renders two of the four conceivable transitions states unviable. Based
on this mechanistic insight, the design of the parent catalyst is
revisited herein: placement of appropriate peripheral substituents
allows high levels of diastereocontrol to be imposed upon cyclopropanation,
which the original catalyst lacks. Because the new complexes allow
either trans- or cis-configured stannylated cyclopropanes to be made
selectively and in excellent optical purity, this transformation also
marks a rare case of diastereodivergent asymmetric catalysis. The
products are amenable to stereospecific cross coupling with aryl halides
or alkenyl triflates; these transformations appear to be the first
examples of the formation of stereogenic quaternary carbon centers
by the Stille reaction; carbonylative coupling is also achieved. Moreover,
tin/lithium exchange affords chiral lithium enolates, which can be
intercepted with a variety of electrophilic partners. The virtues
and inherent flexibility of this new methodology are illustrated by
an efficient synthesis of two salinilactones, extremely scarce bacterial
metabolites with signaling function involved in the self-regulatory
growth inhibition of the producing strain.
[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.
The heteroleptic dirhodium paddlewheel catalyst 7 with achiral carboxylate/acetamidate ligand sphere is uniquely effective in asymmetric [2+ +1] cycloadditions with a-diazo-atrimethylstannyl (silyl, germyl) acetate.O riginally discovered as at race impurity in as ample of the homoleptic parent complex [Rh 2 ((R)-TPCP) 4 ](5), it is shown that the protic acetamidate ligand is quintessential for rendering 7 highly enantioselective.The-NH group is thought to lock the ensuing metal carbene in place via interligand hydrogen bonding.The resulting stannylated cyclopropanes undergo "stereoretentive" cross coupling, whichs hows for the first time that even chiral quarternary carbon centers can be made by the Stille-Migita reaction. Scheme 1. Conceptualo utline.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.