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. 2022 Dec 2;12(53):34704-34714.
doi: 10.1039/d2ra06982b. eCollection 2022 Nov 29.

Synthesis of chiral-at-metal rhodium complexes from achiral tripodal tetradentate ligands: resolution and application to enantioselective Diels-Alder and 1,3-dipolar cycloadditions

Alvaro G Tejero  1 , María Carmona  1 , Ricardo Rodríguez  1 , Fernando Viguri  1 , Fernando J Lahoz  1 , Pilar García-Orduña  1 , Daniel Carmona  1
Affiliations

Synthesis of chiral-at-metal rhodium complexes from achiral tripodal tetradentate ligands: resolution and application to enantioselective Diels-Alder and 1,3-dipolar cycloadditions

Alvaro G Tejero et al. RSC Adv. .

Abstract

An improved synthesis of the racemic rhodium compound [RhCl2(κ4 C,N,N',P-L1)] (1) containing an achiral tripodal tetradentate ligand is reported. Their derived solvate complexes [Rh(κ4 C,N,N',P-L1)(Solv)2][SbF6]2 (Solv = NCMe, 2; H2O, 3) are resolved into their two enantiomers. Complexes 2 and 3 catalyze the Diels-Alder (DA) reaction between methacrolein and cyclopentadiene and the 1,3-dipolar cycloaddition reaction between methacrolein and the nitrone N-benzylidenphenylamine-N-oxide. When enantiopure (A Rh,R N)-2 was employed as the catalyst, enantiomeric ratios >99/1, in the R at C2 adduct, and up to 94/6, in the 3,5-endo isomer, were achieved in the DA reaction and in the 1,3-dipolar cycloaddition reaction, respectively. A plausible catalytic cycle that accounts for the origin of the observed enantioselectivity is proposed.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Scheme 1
Scheme 1. Chiral at zinc tetrahedral complex I and the achiral ligand H2L.
Scheme 2
Scheme 2. Chiral rhodium complex 2 bearing the achiral ligand L1.
Scheme 3
Scheme 3. Synthesis of the dichlorido complex [RhCl2(κ4C,N,N′,P-L1)] as described in the bibliography. (A) and as reported in this work (B).
Scheme 4
Scheme 4. Assignment of the methylene descriptors (A) and NOE pattern (B) on OC-6-35 isomers of complex 2.
Fig. 1
Fig. 1. The crystal structure of racemic cation of 2. The (SN)-OC-6-35-C enantiomer of the racemate is depicted. For clarity, hydrogen atoms have been omitted, and only ipso-carbon atoms of phenyl groups are depicted.
Fig. 2
Fig. 2. View of the cation of the complex (SN)-OC-6-35-C-2 along (A) and perpendicular to (B) the acetonitriles mean plane, showing the phenyl rings shielding.
Fig. 3
Fig. 3. 31P{1H} NMR of the starting mixture of complexes 4 (above) and the individual diastereomers (CRh,RN,SC)-4 (middle) and (ARh,SN,SC)-4 (below).
Fig. 4
Fig. 4. HPCL traces of rac-1, (ARh,RN)-1 and (CRh,SN)-1 and CD spectra of (ARh,RN)-1 (red) and (CRh,SN)-1 (blue).
Fig. 5
Fig. 5. Proposed catalytic cycle (A) and the suggested model for the asymmetric induction (B).
See this image and copyright information in PMC

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