嵩高い置換基を持つルイス酸とルイス塩基の同士は、その立体障害ゆえにお互い錯形成を行うことができない。このような酸-塩基コンビネーションを、フラストレイティド・ルイスペア(frustrated Lewis pair;FLP)と呼ぶ[1]。
酸・塩基双方の性質を打ち消し合うことが無く、両者の協同作用を期待した反応性・化学系の設計がFLPでは実現されうる。
分子状水素をヘテロリティクに開裂させることも可能である[2]。他にも、極めて安定な物質たる二酸化炭素、亜酸化窒素などを活性化可能なことが示されている。
この特性ゆえ、水素貯蔵材料、環境対策に向けての新規な化学変換プロセス、斬新な有機分子触媒などの次世代機能性材料開発に有効なベーシック・コンセプトとみなされており、近年大きな注目を集める概念の一つである。
関連論文
[1] review: (a) ““Frustrated Lewis pairs”: a concept for new reactivity and catalysis”Stephan, D. W. Org. Biomol. Chem. 2008, 6, 1535. doi:10.1039/b802575b
The concept of “frustrated Lewis pairs” is described and shown to result in molecular systems capable of unique reactivity as well as applications in catalysis.
(b) “Tuning Lewis acidity using the reactivity of “frustrated Lewis pairs”: facile formation of phosphine-boranes and cationic phosphonium-boranes”
Welch, G .C.; Cabrera, L.; Chase, P. A.; Hllink, E.; Masujda, J. D.; Wei, P.; Stephan, D. W. Dalton Trans. 2007, 3407. DOI: 10.1039/B704417H
The concept of “frustrated Lewis pairs” involves donor and acceptor sites in which steric congestion precludes Lewis acid–base adduct formation. In the case of sterically demandingphosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon parato B followed by fluoride transfer affording zwitterionic phosphonium borates [R3P(C6F4)BF(C6F5)2] and [R2PH(C6F4)BF(C6F5)2]. These can be easily transformed into the cationic phosphonium-boranes [R3P(C6F4)B(C6F5)2]+ and [R2PH(C6F4)B(C6F5)2]+ or into the neutral phosphino-boranes R2P(C6F4)B(C6F5)2. This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann–Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C6F5)3, while the acidity of the phosphine–boranes is diminished.
(c) “Frustrated Lewis Pairs: Metal-free Hydrogen Activation and More”
Stephane, D. W.; Erker, G. Angew. Chem. Int. Ed. 2010, 49, 46. DOI: 10.1002/anie.200903708
Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form “classical” Lewis acid/Lewis base adducts. Rather, both the unquenched Lewis acidity and basicity of such sterically “frustrated Lewis pairs (FLPs)” is available to carry out unusual reactions. Typical examples of frustrated Lewis pairs are inter- or intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C6F5)2 components. Many examples of such frustrated Lewis pairs are able to cleave dihydrogen heterolytically. The resulting H+/H− pairs (stabilized for example, in the form of the respective phosphonium cation/hydridoborate anion salts) serve as active metal-free catalysts for the hydrogenation of, for example, bulky imines, enamines, or enol ethers. Frustrated Lewis pairs also react with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three-component reactions, offering new strategies for synthetic chemistry.
(d) “A Perspective on Frustrated Lewis Pairs”
Stephan, D. W. J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b06794
[2] “Reversible, Metal-Free Hydrogen Activation”
Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124. doi:10.1126/science.1134230
Although reversible covalent activation of molecular hydrogen (H2) is a common reaction at transition metal centers, it has proven elusive in compounds of the lighter elements. We report that the compound (C6H2Me3)2PH(C6F4)BH(C6F5)2 (Me, methyl), which we derived through an unusual reaction involving dimesitylphosphine substitution at a para carbon of tris(pentafluorophenyl) borane, cleanly loses H2 at temperatures above 100°C. Preliminary kinetic studies reveal this process to be first order. Remarkably, the dehydrogenated product (C6H2Me3)2P(C6F4)B(C6F5)2 is stable and reacts with 1 atmosphere of H2 at 25°C to reform the starting complex. Deuteration studies were also carried out to probe the mechanism.
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