Azaphosphatranes in confined space for CO2 valorization – AZAP-CO2

Our synthetic schemes are highly modular, and thus one can easily probe the effects of changes in a) the catalytic site structure, b) the supramolecular cage, c) morphology of the mesopore cavity) the philicity of the oxide surface and e) inclusions in the oxide walls. The synthetic technologies for each of these parameters are mature, economical and well mastered by our groups. Operational objectives of the study include:
1) A full investigation of the potential and limits of the use of azaphosphatranes as tunable replacements for ammonium and phosphonium. This implies varying substitution around the reactive site and the probe of structure-activity effects.
2) The synthesis and comparison of various multiscale and nested nanosized reaction chambers. The current proposal is novel in that the same catalytically active species will be studied in well-defined reaction spaces that vary in size by orders of magnitude: which confinement scale has the most positive effects on catalysis? This also presents a unique opportunity to study nested systems (a cage within a cage).
3) Determine which of the multiple catalytic induction structural factors available is the most efficient to produce enantioselective catalytic nanoreactors. The catalysts proposed comport several different manners to effectuate enantioselection during the catalytic process: we wish to determine the role and relative effect of changes in different factors and rationally combine the most effective factors to work cooperatively to achieve the highest degree of enantioselectivity.
4) The computational modeling of the multiscale systems. The use of computational calculations as proposed in the project will provide the theoretical foundation to a better understanding of the structure (space shape)/activity correlations and of the different factors which can influence the enantioselection in a confined environment.

Survey of the main results (more details are provided in the report T + 18)
Task 1
- Varying the nature of the halide anion in the AZAP structure (X = Cl, Br, I).
- Synthesis of two new chiral AZAP compounds where the stereogenic centers have been introduced on the equatorial nitrogens of the tren unit.
- Preparation of enantiopure caged AZAP at the hundred milligrams scale.
- Synthesis of an AZAP entity with propargyl groups for further reaction with silica matrices.
Task 2
- Synthesis and full characterization of AZAP containing mesoporous SBA-15 silica via click chemistry.
- Preparation of chiral silicas via molecular imprinting of D- and L-proline as well as corresponding D,L-proline material.
Task 3
- Implementation of the catalytic tests under homogeneous and heterogeneous conditions.
- Evaluation of the role of the halide counteranion in the AZAP structure on catalytic performance under homogeneous conditions.
- Evaluation of the catalytic reactivity of the AZAP@SBA-15 material in the coupling of styrene oxide and CO2.
Task 4
- Preliminary study to determine the choice of the calculation level for the molecular systems.

Task 1
- Varying the nature of the halide anion in the supramolecular AZAP structure (X = Cl, Br, I).
- Introduction of trialkoxysilane incipient linkers in the AZAP and caged AZAP to avoid the pre-functionalization of the silica surface and leave the surface silanols free.
- Synthetic effort will also concentrate on the production of chiral and graftable AZAP and caged AZAP.
Task 2
- We plan to evaluate the effect of the support morphology and textural properties through the immobilization of AZAP onto non-porous silica and ultra-large pore SBA-15 silica with long and short channels.
- Another molecular imprinting approach towards chiral silica is also envisaged via the preparation of chiral structure directing agents; in the same vein, we plan to synthesize chiral organic molecules bearing one or two polycondensable groups in order to introduce the chirality whether in the pores or within the walls of the inorganic silica framework.
- Preparation of Lewis acidic supports (Zn, Ti, Al) for use with AZAP and caged AZAP.
- Preparation of the first generation of chiral materials by grafting achiral AZAP and caged AZAP onto chiral silicas.
Task 3
- Evaluation of all the different catalysts prepared in task 1 and 2.
- Characterization of the polymers obtained under heterogeneous catalytic conditions
Task 4
- The theoretical task will also take off after the hiring of the 2nd post-doc.

No dessimination for the moment

The “Azaphosphatranes in confined space for CO2 valorization” project (AZAP-CO2) brings together researchers from organic chemistry, materials science and theoretical chemistry to produce highly engineered molecular and supramolecular environments to catalyze the enantioselective addition of carbon dioxide to substituted epoxides. The study represents a significant departure from current technology for catalytic site and nanoreactors design. The catalytic reaction, the cycloaddition of carbon dioxide to epoxides, was chosen for its green chemical aspects (use of catalysis, renewable feedstock) as well as for the potential development of high-value added enantioselective reaction pathways for fine chemical synthesis.

The principal characteristics of the catalytically active molecules and hybrid materials are as follows. The catalytically active site is based on the azaphosphatrane unit (AZAP), the conjugate acid of the Verkade’s superbase, which has potential as a stable, tunable replacement for ammonium and phosphonium in metal-free organocatalysis. This cationic species is to be augmented with a hemicryptophane supramolecular cage and/or incorporated into various mesostructured silica hybrid materials. The synthetic schemes to be used are highly modular, and thus one can easily probe the effects of changes in a) the catalytic site structure , b) the supramolecular cage, c) morphology of the mesopore cavity) the philicity of the oxide surface and e) inclusions in the oxide walls. Several molecular and surface science characterization techniques, both well mastered by the partners, will be used at each major step of elaboration to fully elucidate the molecular state of the inclusions as well as the organic-inorganic interface in the hybrid material.

Several series of fundamental studies will be performed. The baseline reactivity of the azaphosphatrane unit for the target reaction has already been established in a previous report; Herein, we propose to fully investigate the potential and limits of the use of azaphosphatranes, as well as the substituent effects (auxiliaries on the amino groups around the cationic P-H core) on catalyst stability and selectivity. Of great interest will be the study of confinement effects at the radically different nanoreactors scales we are proposing: 0.6-0.8 nm in diameter for supramolecules vs. 8-15 nm for mesoporous silica. Some cocatalysts will be studied in the form of Lewis acids which can be associated either as free molecules or as inclusions embedded into the walls of the silica framework of the hybrid materials.
The ultimate goal of the project is the production of enantioselective catalysts. The core technology, the integrated design of supramolecular catalysts and hybrid catalytic materials, comports several different manners to effectuate enantio-selection during the catalytic process. One can synthesize chiral N-substituted AZAP, modify the oxide surface with chiral auxiliaries, or include chirality in the mesoporous organosilica matrix during the synthesis of the solid.

The AZAP-CO2 project implicates multiple scientific fields in chemistry and materials science: organic and supramolecular chemistry, mesoporous hybrid materials, catalysis and theoretical chemistry. The project consortium reunites teams from two laboratories based at CPE Lyon (Dufaud group, materials science and heterogeneous catalysis) and the ENS Lyon (Martinez/Dutasta group, supramolecular and organic chemistry, homogeneous catalysis and theoretical chemistry). Each group is internationally recognized in its core domain and together provide all of the competencies necessary to the success of the AZAP-CO2 project.