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. 2018 Mar 30;3(3):3622-3630.
doi: 10.1021/acsomega.8b00127. eCollection 2018 Mar 31.

Chiral Discrimination in Rhodium(I) Catalysis by 2,5-Disubstituted 1,3 a,4,6 a-Tetrahydropenatalene Ligands-More Than Just a Twist of the Olefins?

Michaela-Christina Melcher  1 , Bianca Rolim Alves da Silva  1 , Trpimir Ivšić  1 , Daniel Strand  1
Affiliations

Chiral Discrimination in Rhodium(I) Catalysis by 2,5-Disubstituted 1,3 a,4,6 a-Tetrahydropenatalene Ligands-More Than Just a Twist of the Olefins?

Michaela-Christina Melcher et al. ACS Omega. .

Abstract

Chiral dienes are useful ligands in a number of asymmetric transition-metal-catalyzed reactions. Here, we evaluate the efficiency of 2,5-disubstituted 1,3a,4,6a-tetrahydropentalenes as ligands to rhodium(I). 2,5-Dibenzyl and diphenyl tetrahydropentalenes were synthesized in two steps and resolved, either chromatographically, or through fractional crystallization of diastereomeric rhodium(I) salts. When evaluated in a 1,4-arylation reaction, the 2,5-dibenzyl ligand gave up to 99% ee. The use of a well-defined rhodium complex as catalyst, Cs2CO3 as the base, and toluene/water as solvent was found to have a pronounced beneficial effect on the selectivity of the reaction. The homologous 2,5-diphenyl ligand on the other hand proved to be highly prone to racemization/loss of chirality during catalysis. Control experiments reveal that this rearrangement proceeds via a rhodium-mediated 1,3-hydride shift. Implications for ligand design and catalysis are discussed.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Ligands with twisted and parallel coordinating olefins. (a) Reported selectivities in the 1,4-addition of phenyl boronic acid to cyclohexenone are given. See references for details. (b) See also ref (18).
Scheme 1
Scheme 1. Synthesis of Enantiomerically Pure Catalysts 9a and 9b
(a) A 92:8 mixture of 5 and 6 was used as starting material; (b) yield given for the shown diastereomer. See Supporting Information for details; (c) thermal ellipsoids shown at 30% probability. Hydrogen atoms, noncoordinating counter ions, and solvate molecules are omitted for clarity; (d) the stereochemical descriptor refers to the configuration of the diene ligand.
Scheme 2
Scheme 2. Catalytic Performance of 9b in 1,4-Arylation of Cyclohexenone
Scheme 3
Scheme 3. Control Experiments to Elucidate the Reason for Low Selectivity in 1,4-Additions with 9b as the Catalyst
Scheme 4
Scheme 4. Proposed Mechanism for Loss of Stereochemical Information in 9b
See this image and copyright information in PMC

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