A New Ligand Design Based on London Dispersion Empowers Chiral Bismuth-Rhodium Paddlewheel Catalysts.

Santanu Singha,Giovanni Bistoni,Alois Fürstner,Richard Goddard,Michael Buchsteiner

doi:10.1021/jacs.1c01972

Santanu Singha, Giovanni Bistoni + Show 3 more

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https://doi.org/10.1021/jacs.1c01972

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Abstract

Heterobimetallic bismuth–rhodium paddlewheel complexes with phenylglycine ligands carrying TIPS-groups at the meta-positions of the aromatic ring exhibit outstanding levels of selectivity in reactions of donor/acceptor and donor/donor carbenes; at the same time, the reaction rates are much faster and the substrate scope is considerably wider than those of previous generations of chiral [BiRh] catalysts. As shown by a combined experimental, crystallographic, and computational study, the new catalysts draw their excellent application profile largely from the stabilization of the chiral ligand sphere by London dispersion (LD) interactions of the peripheral silyl substituents.

Highlights

The four N-phthalimido substituents on the tertleucine ligands were found to adopt an α,α,α,α-conformation about the carbene ligand occupying an axial site on the dirhodium core; this chiral calyx is maintained in solution.[1,27,28] Under the premise that the enantiodetermining transition state features a similar overall structure, the data allowed the sense of induction in the cyclopropanation of styrene to be explained;[1] the moderate level of induction
Bcai(ta+ly2s)t.3d4−es4i6gnF,otrheitsRhla2rcgeenr terardoiuf s1,athwiass formally replaced by main group element imparts a conical shape onto the ligand sphere, which, in turn, tightens the chiral pocket at the Rh-site
It is increasingly clear that multiple intramolecular contacts can outweigh steric repulsion and be structure-determining;[51,52] this effect might be more prevalent in catalysis than commonly appreciated,[51,53−56] examples of deliberate use of London dispersion (LD) as a key design principle for the development of new catalysts with improved application profiles are exceedingly rare.[57−59] Outlined below we present such a case

Summary

In a first foray to translate these insights into an improved

Bcai(ta+ly2s)t.3d4−es4i6gnF,otrheitsRhla2rcgeenr terardoiuf s1,athwiass formally replaced by main group element imparts a conical shape onto the ligand sphere, which, in turn, tightens the chiral pocket at the Rh-site. The fact that 3a and 3b both perform very well but 3b is the better of the two is in excellent accord with the conclusions drawn from the DFT calculations: the peripheral −TIPS groups make the larger contribution to the stabilization of the chiral ligand sphere, but the tBu− substituents on the phthalimide residues pay an additional dividend Α-Aryl-α-diazo esters comprising a strong donor substituent perform so well with the parent complex [BiRh(S-PTTL)4] (2a) that there was no real need to resort to 3a,b (entries 1− 4).[34] diazoesters comprising less electron-rich aryl groups strongly benefitted from their use: in all cases investigated, 3b led to significantly higher levels of induction, with ee’s often approaching 99% (entries 5−21) This striking invariance of the results upon substantial modulation of the diazo compound is deemed another favorable distinguishing feature of the new catalyst.[80].

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