Practical Synthesis of Dirhodium(II) Tetrakis[N-phthaloyl-(S)-tert-leucinate]

Abstract

An efficient and reliable procedure for the preparation of dirhodium(II) tetrakis[N-phthaloyl-(S)-tert-leucinate], Rh 2 (S-PTTL) 4 , a universally effective catalyst for a range of enantioselective carbene transformations, is described. The N-phthaloylation of (S)-tert-leucine by the method of Bose with essentially no racemization is a key to this process.

Chiral Dirhodium(II) Carboxylates: New Insights into the Effect of Ligand Stereo-Purity on Catalyst Structure and Enantioselectivity

“…It was clear that accessing ligands with minimal racemization can be readily accomplished by using toluene/TEA as a reaction solvent. This result is in agreement with the previous report by Hashimoto et al for the preparation of the PTTL ligand [53]. By the application of this reaction conditions, the racemization degree in the prepared N-protected amino acids is restricted and kept to minimal.…”

“…The green residue was then purified by means of silica gel column chromatography using ethyl acetate: n-hexane as mobile phase. Spectroscopic data of all prepared complexes were consistent with the previously published literature [36,53].…”

“…This is because, the inclusion of a ligand with an opposite absolute configuration will result in the interruption of the symmetry of the chiral pocket. As a consequence, Hashimoto et al explored the racemization free N-phthaloylation of tertleucine [53]. They reported that minimal racemization can be achieved in refluxing toluene in the presence of triethylamine (TEA).…”

“…As a consequence, Hashimoto et al explored the racemization free N-phthaloylation of tert-leucine [53]. They reported that minimal racemization can be achieved in refluxing toluene in the presence of triethylamine (TEA).…”

On the Structure of Chiral Dirhodium(II) Carboxylate Catalysts: Stereoselectivity Relevance and Insights

“…In 1993, Hashimoto, Ikegami and co-workers first reported their famous complex, Rh 2 (S-PTTL) 4 [76,77], as a catalyst that manifested (along with its variants) high enantioselectivity levels in wide range of organic transformations [2,54,[78][79][80][81][82][83][84]. For example, in enantioselective [2,3]-sigmatropic rearrangement of the cyclic propargylic oxonium ylide shown in Scheme 5a, Rh 2 (S-PTTL) 4 provided the best enantioselectivity of 79% ee of the corresponding allenic bearing benzofuran-3-one product (Scheme 5a) [85].…”