Phenoxy-Amidine Ligands for Stable Cyclic Ester ROP Catalysts
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Solution Overview
Problem
Schiff base ligands, such as phenoxy-imines, are sensitive to hydrolysis, reduction, and alkylation, leading to complications in catalyst activation and performance, particularly in ring-opening polymerization of cyclic esters, and the preparation of alkoxide derivatives is not straightforward due to imine moiety reactivity.
Innovation Solution
Development of phenoxy-amidine and bis(phenoxy-amidine) ligands, where the imine function is replaced by trisubstituted amidine, providing enhanced stability and robustness against protic impurities, allowing for improved catalyst performance in processes like ring-opening polymerization of cyclic esters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If Schiff base ligands (phenoxy-imines) are used, then ease of access and structural modularity are achieved, but sensitivity to hydrolysis, reduction, and alkylation occurs
Solution Approach 1:
The patent applies parameter changes by modifying the chemical structure of the ligand from an imine function (C=N) to an amidine function (C(=NR)NR2). This structural parameter change increases the basicity and nucleophilicity of the ligand, making it more resistant to hydrolysis and other deleterious reactions while maintaining ease of synthesis through similar condensation reactions between phenolic compounds and amidine precursors.
Solution Approach 2:
The patent creates composite ligand structures by combining the phenolic moiety with the amidine functionality in a single molecular framework. This composite structure integrates the beneficial properties of both components: the phenolic part provides coordination ability while the amidine part provides enhanced stability and basicity, resulting in a ligand that is both easy to manufacture and highly stable.
2Adaptability or versatility
If imine function is used in Schiff base ligands, then coordination to transition metals is achieved, but electrophilic character reinforces sensitivity to nucleophiles and complicates catalyst activation
Solution Approach 1:
The patent changes the electronic parameter of the ligand by replacing the electrophilic imine carbon with a less electrophilic amidine carbon. The amidine nitrogen atoms donate electron density through resonance, reducing the electrophilic character of the ligand and making it less susceptible to nucleophilic attack, while maintaining strong coordination ability to transition metals through the phenolic oxygen and amidine nitrogen atoms.
3Ease of operation
If classical alkylating agents are used with FI complexes, then catalyst activation is attempted, but intramolecular alkyl migration from metal to imine occurs during polymerisation
Solution Approach 1:
The patent changes the basicity parameter of the ligand by introducing the amidine nitrogen atoms, which are more basic than imine nitrogens. This increased basicity allows for effective catalyst activation with alkylating agents without causing intramolecular alkyl migration, as the amidine nitrogen is less prone to attack by alkyl groups compared to the electrophilic imine carbon.
4Productivity
If phenoxy-imine ligands are used for ROP of cyclic esters, then metal alkoxides can be prepared, but Meerwein-Ponndorf-Verley reduction from isoproxy ligand occurs
Solution Approach 1:
The patent changes the electronic parameter of the ligand by replacing the electron-deficient imine group with the electron-rich amidine group. This electronic change prevents the Meerwein-Ponndorf-Verley reduction by making the ligand less susceptible to hydride transfer from isoproxy ligands, as the amidine nitrogen atoms donate electron density that stabilizes the complex against such reduction reactions.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The phenoxy-amidine ligands and their coordination complexes demonstrate increased stability and versatility, enabling robust catalysts that withstand impurities like water and lactic acid, thus enhancing the efficiency of ring-opening polymerization reactions.
Implementation Method 1
Amidines differ from imines in that the free nitrogen pair of the amidine group NR2 is involved in resonance, making them much more basic and stable to nucleophiles than imines
Implementation Method 2
The additional NR2 group in the amidine fragment thus provides steric protection and electronic density to the otherwise exposed electrophilic imine carbon atom
Implementation Method 3
the strong σ and π-donor nature of the amidine function give additional stability to the metal ion
Implementation Method 4
the strong σ and π-donor nature of the amidine function give additional stability to the metal ion
Data Source
AI summary
The invention relates to compounds of formula (I): as well as stereoisomeric forms, mixtures of stereoisomeric forms, and salts thereof, wherein R1, R2, R3, R4 and R5 are as defined in claim 1. Compounds of formula (I) are particularly useful for preparing coordination metal or metalloid complexes for the catalysis of ring opening polymerisation of cyclic esters.