Amino-acid containing metallomonomers copolymerized into porous organic polymers: applicability to allylic alkylation catalysis

doi:10.1016/j.ica.2004.04.017

Inorganica Chimica Acta

Volume 357, Issue 13, 1 November 2004, Pages 3854-3864

Dedicated to Prof. Tobin J. Marks on his 60th birthday

https://doi.org/10.1016/j.ica.2004.04.017 Get rights and content

Abstract

This paper reports the synthesis of a polymerizable dppe-derivative with eight pendent styrenyl units linked to the dppe core through four tyrosine residues. This diphosphine ligand was utilized in the synthesis of a series of P₂PdX₂ complexes (X₂=(R)-BINOL, (S)-BINOL, Cl₂ and π-1,3-Ph₂-allyl⁺). The compounds were used as comonomers for the synthesis of porous organic polymers (poly EDMA). Molecular imprinting effects on X₂-ligand removal were investigated. Overall, the presence of a cavity of significant size was found to be beneficial to the rate of the allylic alkylation reaction, however, chiral BINOL shaped cavities did not influence the enantio-selectivity of the reaction.

The synthesis and copolymerization/imprinting of amino-acid containing palladium complexes into highly crosslinked porous polymers are described. Removal of the non-polymerizable ligand after network formation provides catalysts imbedded within the polymer that have shaped cavities. The utility of these catalysts in allylic alkylation reactions is highlighted.

Introduction

The synthesis of artificial active sites [1] for hosting transition metal catalysts with unique reactivity profiles is an important problem, but one also fraught with technical difficulties. Our first forays into this area focused on the molecular imprinting of transition metal complexes into styrene/divinylbenzene or ethylene dimethacrylate (EDMA)-based networks for creating artificial “active sites”, wherein the metal complex had an associated chiral cavity whose topology could be influenced by the shape of a large, removable imprinting ligand on the metallomonomer (Scheme 1). When (R)-BINOL shaped cavities were created, the metal sites were capable of differentiating between (R)- and (S)-BINOL in rebinding experiments (85:15 R:S). In fact some of the sites were capable of providing up to 97:3 selectivities, though others were only 2:1 selective; the metal fragment is of course achiral [2].

Since these cavities were generally constructed of non-functionalized monomers (styrene, EDMA, etc.), we speculated that organized functionality might provide a means to improve the molecular recognition properties of the artificial active sites [3], [4]; a hypothesis grounded in the fact that metalloenzymes utilize functional groups to construct and outfit their active sites [5], [6], [7]. To this end we have synthesized a series of diphosphine complexes that contain polymerizable amino acid units [8], [9] for copolymerizing into the matrix of the host polymer, hoping that the basic carbonyl or H-bonding amide might provide the desired functionality to the active site [10]. We report herein the synthesis and characterization of several P₂PdX₂ [11] complexes, their polymerization into the matrix of highly crosslinked methacrylate polymers and their subsequent activation towards catalysis of the allylic alkylation reaction [12], [13], [14], [15], [16].

Section snippets

Synthesis of a tyrosine-containing, polymerizable diphosphine ligand [17]

Synthesis of the desired compounds began with a suitably protected reactive diphosphine reagent for coupling to an amino acid ester. The diphosphine 2 was synthesized in high yield by the addition of the Grignard generated from the 4-iodobenzoate ester 1 [18] to bis(dichlorophosphino)ethane and borane protection (Scheme 2). The tetra ester was transformed to its corresponding acid 3 by the action of TBAF and acidification. The amino-acid bearing polymerizable arm 5 was prepared in two steps

Summary

This paper reports the synthesis of a polymerizable amino acid containing diphosphine ligand along with its coordination chemistry to the PdCl₂, Pd(BINOL) and Pd(π-allyl)⁺ fragments. These metallomonomers were used as comonomers in the synthesis of permanently porous EDMA-based polymers; the polymers in turn served as catalysts for the alkylation of allylic acetates.

These studies represent one of the few approaches to the functionalization of transition metal active sites, though it appears

Compound 1

Oxalyl chloride (21.0 mL, 243 mmol) was added to a solution of 4-iodobenzoic acid (20.0 g, 80.9 mmol) in dichloromethane (150 mL) at room temperature, followed by the addition of few drops of dry DMF. The reaction mixture was then stirred overnight and dichloromethane and excess of oxalyl chloride were removed under reduced pressure. The white solid was dried en vacuo for 24 h. This residue was dissolved in dichloromethane (100 mL). 2-Trimethylsilyl ethanol (12.2 g, 101 mmol) was added and the

Acknowledgements

We thank the National Science Foundation (CHE-0315203 and CHE-0075717) for generous support of this research. M.R.G. is a Camille-Dreyfus Teacher Scholar.

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Amino-acid containing metallomonomers copolymerized into porous organic polymers: applicability to allylic alkylation catalysis

Abstract

Introduction

Section snippets

Synthesis of a tyrosine-containing, polymerizable diphosphine ligand [17]

Summary

Compound 1

Acknowledgements

J. Mater. Chem

Tetrahedron

Chem. Eur. J

Angew. Chem., Int. Ed

Chem. Rev

J. Am. Chem. Soc

Chem. Commun

Chem. Commun

J. Organometallic Chem

Chem. Soc. Rev

Chem. Rev

Chem. Eur. J

Org. Lett

Organometallics

Organometallics

J. Org. Chem

Asymmetric allylic alkylation reactions