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Alkaline earth metal catalysts for asymmetric reactions.
Acc Chem Res. 2011 Jan 18; 44(1):58-71.AC

Abstract

The group 2 alkaline earth metals calcium (Ca), strontium (Sr), and barium (Ba) are among the most common elements on Earth, abundant in both the sea and the Earth's crust. Although they are familiar in our daily lives, their application to organic synthesis has, so far, been limited. Some particularly useful properties of these elements include (i) low electronegativity, (ii) a stable oxidation state of +2, meaning that they can potentially form two covalent bonds with anions, and (iii) the ability to occupy a variety of coordination sites due to their large ionic radius. Furthermore, the alkaline earth metals, found between the group 1 and group 3 elements, show mild but significant Lewis acidity, which can be harnessed to control coordinative molecules via a Lewis acid-base interaction. Taken together, these characteristics make the metals Ca, Sr, and Ba very promising components of highly functionalized acid-base catalysts. In this Account, we describe the development of chiral alkaline earth metal catalysts for asymmetric carbon-carbon bond-forming reactions. Recently prepared chiral alkaline earth metal complexes have shown high diastereo- and enantioselectivities in fundamental and important chemical transformations. We chose chiral bisoxazoline (Box) derivatives bearing a methylene tether as a ligand for chiral modification. These molecules are very useful because they can covalently coordinate to alkaline earth metals in a bidentate fashion through deprotonation of the tether portion. It was found that chiral calcium-Box complexes could successfully promote catalytic asymmetric 1,4-addition and [3 + 2] cycloaddition reactions with high diastereo- and enantioselectivities. Both the calcium-Box complexes and chiral strontium-bis-sulfonamide and chiral barium-BINOLate complexes could catalyze asymmetric 1,4-addition reactions with high enantioselectivities. Furthermore, we designed a calcium-neutral coordinative ligand complex as a new type of chiral alkaline earth metal catalyst. We found that pyridinebisoxazolines (Pybox) worked well: they served as excellent ligands for calcium compounds in 1,4-addition reactions and Mannich reactions. Moreover, they were successful in 1,4-additions in concert with enantioselective protonation, affording the desired products in good to high enantioselectivities. Our results demonstrate that alkaline earth metals are very useful and attractive catalysts in organic synthesis. Moreover, their ubiquity in the environment is a distinct advantage over rare metals for large-scale processes, and their minimal toxicity is beneficial in both handling and disposal.

Authors+Show Affiliations

Department of Chemistry, School of Science, The University of Tokyo, The HFRE Division, ERATO, Japan Science Technology Agency, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. shu_kobayashi@chem.s.u-tokyo.ac.jpNo affiliation info available

Pub Type(s)

Journal Article
Research Support, Non-U.S. Gov't

Language

eng

PubMed ID

20979379

Citation

Kobayashi, Shū, and Yasuhiro Yamashita. "Alkaline Earth Metal Catalysts for Asymmetric Reactions." Accounts of Chemical Research, vol. 44, no. 1, 2011, pp. 58-71.
Kobayashi S, Yamashita Y. Alkaline earth metal catalysts for asymmetric reactions. Acc Chem Res. 2011;44(1):58-71.
Kobayashi, S., & Yamashita, Y. (2011). Alkaline earth metal catalysts for asymmetric reactions. Accounts of Chemical Research, 44(1), 58-71. https://doi.org/10.1021/ar100101b
Kobayashi S, Yamashita Y. Alkaline Earth Metal Catalysts for Asymmetric Reactions. Acc Chem Res. 2011 Jan 18;44(1):58-71. PubMed PMID: 20979379.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Alkaline earth metal catalysts for asymmetric reactions. AU - Kobayashi,Shū, AU - Yamashita,Yasuhiro, Y1 - 2010/10/27/ PY - 2010/10/29/entrez PY - 2010/10/29/pubmed PY - 2011/5/4/medline SP - 58 EP - 71 JF - Accounts of chemical research JO - Acc Chem Res VL - 44 IS - 1 N2 - The group 2 alkaline earth metals calcium (Ca), strontium (Sr), and barium (Ba) are among the most common elements on Earth, abundant in both the sea and the Earth's crust. Although they are familiar in our daily lives, their application to organic synthesis has, so far, been limited. Some particularly useful properties of these elements include (i) low electronegativity, (ii) a stable oxidation state of +2, meaning that they can potentially form two covalent bonds with anions, and (iii) the ability to occupy a variety of coordination sites due to their large ionic radius. Furthermore, the alkaline earth metals, found between the group 1 and group 3 elements, show mild but significant Lewis acidity, which can be harnessed to control coordinative molecules via a Lewis acid-base interaction. Taken together, these characteristics make the metals Ca, Sr, and Ba very promising components of highly functionalized acid-base catalysts. In this Account, we describe the development of chiral alkaline earth metal catalysts for asymmetric carbon-carbon bond-forming reactions. Recently prepared chiral alkaline earth metal complexes have shown high diastereo- and enantioselectivities in fundamental and important chemical transformations. We chose chiral bisoxazoline (Box) derivatives bearing a methylene tether as a ligand for chiral modification. These molecules are very useful because they can covalently coordinate to alkaline earth metals in a bidentate fashion through deprotonation of the tether portion. It was found that chiral calcium-Box complexes could successfully promote catalytic asymmetric 1,4-addition and [3 + 2] cycloaddition reactions with high diastereo- and enantioselectivities. Both the calcium-Box complexes and chiral strontium-bis-sulfonamide and chiral barium-BINOLate complexes could catalyze asymmetric 1,4-addition reactions with high enantioselectivities. Furthermore, we designed a calcium-neutral coordinative ligand complex as a new type of chiral alkaline earth metal catalyst. We found that pyridinebisoxazolines (Pybox) worked well: they served as excellent ligands for calcium compounds in 1,4-addition reactions and Mannich reactions. Moreover, they were successful in 1,4-additions in concert with enantioselective protonation, affording the desired products in good to high enantioselectivities. Our results demonstrate that alkaline earth metals are very useful and attractive catalysts in organic synthesis. Moreover, their ubiquity in the environment is a distinct advantage over rare metals for large-scale processes, and their minimal toxicity is beneficial in both handling and disposal. SN - 1520-4898 UR - https://www.unboundmedicine.com/medline/citation/20979379/Alkaline_earth_metal_catalysts_for_asymmetric_reactions_ L2 - https://doi.org/10.1021/ar100101b DB - PRIME DP - Unbound Medicine ER -