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Stereoselective coordination of C5-symmetric corannulene derivatives with an enantiomerically pure [Rh(I)(nbd*)] metal complex.

Numerous stereoselective catalytic processes rely on the differentiation of internally enantiotopic faces of planar psystems by metal complexation. Chiral bowl-shaped molecules like 1,3,5,7,9-pentasubstituted corannulene, which invert rapidly on the reaction timescale, present themselves as achiral due to tautomeric equilibration of enantiomers, and thereby display externally enantiotopic p-faces. Stereoselective complexation of one such p-face shifts the tautomeric equilibrium and offers the possiblity to effect a complete dynamic resolution. Thus, such systems provide a special platform for studying chiral ligand–metal molecular recognition and metal–arene complex dynamics. The hydrogens in one bowl form of corannulene (1) are chirotopic and segregate into two internally enantiotopic sets of five homotopic hydrogens; the endo and exo faces of the bowl are diastereotopic. Bowl inversion renders all hydrogens, and the two faces, respectively homotopic. Under conditions of bowl inversion, simple metal coordination to either face of corannulene is not an event wherein stereoselection can occur (the products are homomeric). In contrast, sym-pentasubstituted corannulenes, such as 1,3,5,7,9-pentamethylcorannulene (2), oscillate between enantiomeric C5-symmetric bowl conformations (Figure 1). The hydrogens of these conformers are internally homotopic but externally enantiotopic; the faces are diastereotopic in the static bowl, and rendered enantiotopic by inversion. Simple metal complexation of 2 yields enantiomeric products, whereby enantioselection becomes possible. From this stereochemical analysis, 1) complexation of 1 with a racemic metal fragment should yield two enantiomeric products with diastereotopic hydrogen NMR signals from corannulene, 2) complexation of 2 with a metal bearing enantiotopic hydrogens should yield enantiomeric products with diastereotopic hydrogen NMR signals coming from the metal fragment, and 3) complexation of 2 with an enantiomerically pure metal fragment should lead to diastereoselective chiral complex formation that effectively resolves the bowl forms of 2 into 100 % of a single chiral diastereomer. Each of these complexes provides an unambiguous way to follow the dynamics of metal–arene rotation, migration, and transfer from a single complex. The corannulene partners in this study include 1, 2, and 1,3,5,7,9-penta-tert-butylcorannulene (3). On the metal side, the fragments include {Rh(nbd)} (nbd = bicyclo[2.2.1]hepta2,5-diene), in reagent form as [{(nbd)RhCl}2] (4), and {Rh(nbd*)} (nbd* = C2-symmetric (R,R)-2,5-dimethyl-bicyclo[2.2.1]hepta-2,5-diene), in reagent form as [{(nbd*)RhCl}2] (5). Reaction of a corannulene and a [{(nbd)RhCl}2] derivative activated by silver(I) in dichloromethane at room temperature produces the expected complexes 6–9 (Scheme 1).