Artificial Metalloenzymes for Asymmetric Allylic Alkylation on the Basis of the Biotin–Avidin Technology

Abstract

Dedicated to Professor Dieter Seebach on the occasion of his 70th birthdayPalladium-catalyzed CÀC bond-forming reactions play a prominent role in the construction of complex organic molecules. [1][2][3][4] As this versatile precious metal is absent in natural enzymes, nature has devised very different strategies to create C À C bonds for the construction of complex natural products. [5][6][7] The palladium-catalyzed asymmetric allylic alkylation (AAA) is a CÀC bond-forming reaction that has attracted much attention. [4,[8][9][10][11][12][13] In this transformation, the enantiodiscrimination event occurs through the external attack of a soft nucleophile on a palladium-bound h 3 -allyl moiety (Scheme 1 a). [4,12] The AAA thus bears resemblance to enzymatic reactions, in which the substrate(s) need not necessarily bind to the active site of the enzyme for the reaction to proceed with high stereoselectivity. [7] In recent years, there has been increasing interest in the creation of artificial metalloenzymes for enantioselective catalysis. For this purpose, an active-catalyst precursor is anchored in a biomolecule, which provides the chiral environment. [14][15][16][17][18][19][20][21][22][23] The enantioselective reactions implemented thus far include ester hydrolysis, [24] dihydroxylation, [25] epoxidation, [26,27] sulfoxidation, [28][29][30][31][32] hydrogenation, [33][34][35][36][37][38][39][40][41][42][43] transfer hydrogenation, [44,45] and Diels-Alder reactions. [46][47][48][49][50] Inspired by the early studies of Wilson and Whitesides, [33] several research groups [34,35,41] have exploited the biotinavidin technology to ensure the localization of a biotinylated catalytic moiety within (strept)avidin (either avidin or streptavidin). This methodology has allowed the creation of artificial metalloenzymes for enantioselective hydrogenation and transfer-hydrogenation reactions. [33][34][35][36][37][38][39][40][41][42][43][44][45] Herein we describe our efforts to create artificial metalloenzymes for the AAA. We show that a combination of chemical and genetic optimization allows the identification of [Pd(h 3 -allyl)(biotin-spacer-ligand)] + &(strept)avidin catalysts that afford both R and S alkylation products (in up to 90 and 82 % ee, respectively; Scheme 1).With the aim of identifying the most promising chelating ligand, we evaluated five different biotinylated scaffolds in the presence of (strept)avidin (Scheme 2). Little, if any, conversion was detected (Table 1, entry 1): Most of the starting 1,3-diphenylallylacetate was hydrolyzed to the corresponding 1,3-diphenylallyl alcohol. We therefore screened various surfactants that are commonly used in aqueous Scheme 1. The postulated enantiodiscrimination event in the AAA in a) a homogeneous catalytic system and b) an artificial metalloenzyme. The host protein (streptavidin, brown) displays high affinity for the anchor (biotin, green triangle). The introduction of an amino acid spacer (red star) combined with a chelating ligand (purple) allows the chemical optimizat...

Bioorganic and Bioinorganic Chemistry

“…This insight led to an expansion of Ward's research ranging from asymmetric transfer hydrogenation of prochiral ketones and imines, catalyzed by d 6 piano-stool complexes (Scheme 2); [12] and palladiumcatalyzed asymmetric allylic alkylation (Scheme 3) [13] towards oxidation reactions including the vanadyl-catalyzed sulfoxidation of prochiral sulfides (Scheme 4) [14] and the dihydroxylation of olefins.…”

A Kinetic and Structural Investigation of DNA‐Based Asymmetric Catalysis Using First‐Generation Ligands

“…Prior to our studies it was demonstrated that the chirality of DNA can be transferred in a stoichiometric DNA-templated reaction. [21][22][23] Furthermore, a DNAzyme capable of enantioselection of an RNA substrate [24] and an enantioselective Diels-Alderase RNAzyme have been reported. [25] In our approach to DNA-based enantioselective catalysis, [26][27][28] we have demonstrated that the chirality of DNA can be transferred directly to a metal-catalyzed reaction.…”

Artificial Metalloenzymes for Enantioselective Catalysis Based on the Biotin-Avidin Technology

“…[9][10][11][12][13] For the localization of an organometallic moiety within a macromolecular host, three anchoring strategies can be envisaged: covalent, dative or supramolecular. [5] Using such approaches, enantioselective artificial metalloenzymes have been created for hydrolysis, [12] hydrogenation, [14][15][16][17] transfer hydrogenation, [18] allylic alkylation, [19] sulfoxidation, [20][21][22][23][24] epoxidation, [25,26] dihydroxylation, [27] Diels-Alder, [28,29] trans-amination, [30] Michael addition [31] and fluorination. [32] Comparatively, supramolecular anchoring appears as the most appealing, since it allows separate variation of both biological and chemical components, followed by straightforward combination of the organometallic moiety and the macromolecular host.…”