金属を含まず、有機化合物だけで構成される触媒を有機触媒(organocatalyst)と呼びます。
有機合成で用いられてきた触媒は、金属を活性中心とするものがほとんどでした。しかしながら金属の種類によっては、高価・有毒・廃棄困難であったり、水や酸素に不安定であったりと、困った点も少なくありません。
近年では、金属を使わずに、有機分子そのものを触媒として用いる反応開発研究が盛んです。有機分子は一般に、取り扱いや構造のチューニングが金属錯体に比べて簡単であり、安価で環境に優しいなどのメリットがあるとされています。その中には金属触媒ではどうやっても進行させられない反応を触媒するものすらあります。
とはいえ金属触媒に比べまだまだ活性が低いものが多く、実用にこぎ着けられるかどうかは今後の発展次第といえそうです。
日本人研究者では、京都大学の丸岡啓二教授が第一人者です。アミノ酸の不斉合成に用いられる、高活性な不斉相間移動触媒の開発は有名な業績です。
関連文献
[1] “Enantioselective Organocatalysis”Dalko, P. I.; Moisan, L. Angew. Chem. Int. Ed. 2001, 40, 3726. DOI: 10.1002/1521-3773(20011015)40:20<3726::AID-ANIE3726>3.0.CO;2-D
[2] “In the Golden Age of Organocatalysis”The last few years have witnessed a spectacular advancement in new catalytic methods based on metal-free organic molecules. In many cases, these small compounds give rise to extremely high enantioselectivities. Preparative advantages are notable: usually the reactions can be performed under an aerobic atmosphere with wet solvents. The catalysts are inexpensive and they are often more stable than enzymes or other bioorganic catalysts. Also, these small organic molecules can be anchored to a solid support and reused more conveniently than organometallic/bioorganic analogues, and show promising adaptability to high-throughput screening and process chemistry. Herein we focus on four different domains in which organocatalysis has made major advances: 1) The activation of the reaction based on the nucleophilic/electrophilic properties of the catalysts. This type of catalysis has much in common with conventional Lewis acid/base activation by metal complexes. 2) Transformations in which the organic catalyst forms a reactive intermediate: the chiral catalyst is consumed in the reaction and requires regeneration in a parallel catalytic cycle. 3) Phase-transfer reactions: The chiral catalyst forms a host–guest complex with the substrate and shuttles between the standard organic solvent and the second phase (i.e. a solid, aqueous, or fluorous phase in which the organic transformation takes place). 4) Molecular-cavity-accelerated asymmetric transformations: the catalyst can select between competing substrates, depending on size and structure criteria. The rate acceleration of a given reaction is similar to the Lewis acid/base activation and is the consequence of the simultaneous action of different polar functions. Herein it is shown that organocatalysis complements rather than competes with current methods. It offers something conceptually novel and opens new horizons in synthesis.
Dalko, P. I.; Moisan, L. Angew. Chem. Int. Ed. 2004, 43, 5138. DOI: 10.1002/anie.200400650
[3]”Asymmetric Organocatalysis”The term “organocatalysis” describes the acceleration of chemical reactions through the addition of a substoichiometric quantity of an organic compound. The interest in this field has increased spectacularly in the last few years as result of both the novelty of the concept and, more importantly, the fact that the efficiency and selectivity of many organocatalytic reactions meet the standards of established organic reactions. Organocatalytic reactions are becoming powerful tools in the construction of complex molecular skeletons. The diverse examples show that in recent years organocatalysis has developed within organic chemistry into its own subdiscipline, whose “Golden Age” has already dawned.
Houk, K. N.; List, B. (Eds.) Acc. Chem. Res. 2004, 37, 487. DOI: 10.1021/ar040216w
[4] “Asymmetric organocatalysis”Seayad, J.; List, B. Org. Biomol. Chem. 2005, 3, 719. DOI: 10.1039/B415217B
[5] Kohovský, P.; Malkov, A. V. (Eds.) Tetrahedron Symposia-in-print: Asymmetric Organocatalysis 2006, 62, 243. LinkThe field of asymmetric organocatalysis is rapidly developing and attracts an increasing number of research groups around the world. Here we present a brief overview of this area, guided by a mechanistic classification. Accordingly, organocatalysts are categorized as either Lewis base, Lewis acid, Brønsted base, or Brønsted acid catalysts.
関連書籍
[amazonjs asin=”4882319136″ locale=”JP” title=”有機分子触媒の新展開 (CMC Books)”][amazonjs asin=”3527332367″ locale=”JP” title=”Comprehensive Enantioselective Organocatalysis: Catalysts, Reactions, and Applications, 3 Volume Set”][amazonjs asin=”3527305173″ locale=”JP” title=”Asymmetric Organocatalysis: From Biomimetic Concepts to Applications in Asymmetric Synthesis”]関連リンク
- プリンストン大学: David W. C. MacMillan研究室
- 京都大学：丸岡啓二研究室
- Organocatalyst – Wikipedia
- 触媒 – Wikipedia
- Organocatalysis (organic-chemistry.org)
