Periodic Reporting for period 3 - FunctionalP4 (Metal-Mediated Methods for the Functionalization of White Phosphorus (P4))
Période du rapport: 2021-09-01 au 2023-02-28
Organophosphorus compounds are an important and industrially relevant class of molecules with numerous uses, e.g. as reagents in organic synthesis, ligands in catalytically active metal complexes, and in pest control. State-of-the-art synthesis methods for most of these valuable and useful compounds rely on an atom inefficient and hazardous multi-step procedure involving the oxidation of white phosphorus (P4) with toxic chlorine gas (Figure 1). Less wasteful and more environmentally benign methods are highly desirable, but transformations of white phosphorus directly into organophosphorus compounds are hardly developed.
The project FunctionalP4 explores new methods for the activation and functionalization of white phosphorus. The metal-mediated stepwise transformation of P4 into organophosphorus compounds is a key objective. Novel transition metal compounds are designed and synthesized, which can generate reactive phosphorus units. The concept of heterobimetallic P4 activation, where two electronically different metal complexes interact with P4 cooperatively, is introduced for this purpose. Reactions of the phosphorus fragments in these new, reactive complexes with electrophiles will produce novel, fundamentally interesting organophosphorus compounds avoiding chlorinated intermediates. Catalytic methods for P4 functionalization are currently unknown, and developing such methods using transition metal and photoredox catalysts is an additional objective of this proposal.
By providing novel synthetically useful and even catalytic procedures for converting P4 into organophosphorus compounds, this project will significantly contribute to the development of phosphorus chemistry and more sustainable synthesis methods.
The project FunctionalP4 explores new methods for the activation and functionalization of white phosphorus. The metal-mediated stepwise transformation of P4 into organophosphorus compounds is a key objective. Novel transition metal compounds are designed and synthesized, which can generate reactive phosphorus units. The concept of heterobimetallic P4 activation, where two electronically different metal complexes interact with P4 cooperatively, is introduced for this purpose. Reactions of the phosphorus fragments in these new, reactive complexes with electrophiles will produce novel, fundamentally interesting organophosphorus compounds avoiding chlorinated intermediates. Catalytic methods for P4 functionalization are currently unknown, and developing such methods using transition metal and photoredox catalysts is an additional objective of this proposal.
By providing novel synthetically useful and even catalytic procedures for converting P4 into organophosphorus compounds, this project will significantly contribute to the development of phosphorus chemistry and more sustainable synthesis methods.
One part of the project deals with the synthesis of novel transition metal complexes and their utilization for the synthesis of useful (poly)phosphorus compounds. To that end, new transition metalate anions have been synthesized and successfully employed to selective activate P4. The resulting complexes can be further transformed and used to generate previously inaccessible phosphorus compounds. An example for this approach is the synthesis of rare cyanodiphosphanide anions in a sequence of reactions starting with P4 and a low-valent cobaltate anion (Angew. Chem. Int. Ed., 2019, 58, 18931-18936, Figure 2). In addition, new molecular nickel phosphide have been prepared by reaction of organometallic nickel(0) complexes with P4 (Angew. Chem. Int. Ed., 2020, 59, 14148-14153). These compounds have potential as single-source precursors for catalytically active materials. Further investigations of the coordination chemistry of P4 are on-going. Furthermore, a review article on the topic of transition-metal mediated P4 functionalization was published (Chem. Eur. J. 2021, 27, 1886-1902).
The cooperative interaction of two different metal atoms with P4 is pursued as a method to produce metal complexes with reactive phosphorus fragments. Substantial progress has been made through the use of a d-block (e.g. Co and Fe) in combination with a p-block elements (e.g. Ga, Si, and Sn) to create compounds showing unusual P4 activation modes (J. Am. Chem. Soc. 2018, 140, 13195−13199; Chem. Sci. 2019, 10, 1302–1308; Z. Anorg. Allg. Chem. 2020, 646, 552-557; Chem. Commun. 2020, 56, 14074-14074).
As a "spin-off" from investigations into the coordination chemistry of P4, the first diphosphatetrahedrane (Figure 3) was successfully synthesized and isolated (Chem. Int. Ed. 2019, 58, 16918–16922). This species is a close carbon analogue of P4. Initial reactivity studies have already revealed intriguing parallels, but also differences between diphosphatetrahedrane and the isolobal P4 molecule (Angew. Chem. Int. Ed. 2020, 59, 14148–14153; Chem. Commun. 2021, 57, 2356–2359).
The ultimate and most ambitious aim of the FunctionalP4 project undoubtedly is the development of catalytic methods for converting white phosphorus into phosphorus-based chemicals. Such catalytic methods are of extremely high academic and industrial significance. In order to reach this very challenging goal, several reaction steps need to be integrated into a catalytic cycle, and the reactivity of all reagents must be finely balanced in order to be able to close such a cycle. The ability of white phosphorus to effectively trap organyl and p-block element radicals was identified as a promising concept. Meanwhile, photoredox catalysis was proposed as a means to generate the desired radicals in homogeneous solution. Based on these considerations, an unprecedented photocatalytic method was developed that converts P4 directly into valuable aryl phosphines and phosphonium salts (Nat. Catal. 2019, 2, 1101–1106,Figure 4). This method presents a solution to the hitherto unsolved challenge of catalytic P4 functionalization. Current investigations aim at an improved efficiency and a broader substrate scope of the method (Chem. Eur. J. 2020, 26, 16374-16382). An extension to alternative photocatalytic manifolds is planned.
Pursuing a different strategy, an efficient one-step synthesis of monophosphines from P4 has recently been developed (Nat. Chem. in press, DOI: 10.1038/s41557-021-00657-7 Figure 5). This method is based on the use of organotin hydrides, which are able to effectively hydrostannylate the P-P bonds of P4 under irradiation or in the presence of radical initiator. Using this method, a diverse range of industrially relevant organic and inorganic phosphorus compounds is accessible in one reaction step from P4. Importantly, the organotin reagent can be efficiently recycled and even used in a catalytic fashion. A European patent application was filed by the University of Regensburg based on the results (EP 20 157 197.3 inventors: D. J. Scott and R. Wolf).
The cooperative interaction of two different metal atoms with P4 is pursued as a method to produce metal complexes with reactive phosphorus fragments. Substantial progress has been made through the use of a d-block (e.g. Co and Fe) in combination with a p-block elements (e.g. Ga, Si, and Sn) to create compounds showing unusual P4 activation modes (J. Am. Chem. Soc. 2018, 140, 13195−13199; Chem. Sci. 2019, 10, 1302–1308; Z. Anorg. Allg. Chem. 2020, 646, 552-557; Chem. Commun. 2020, 56, 14074-14074).
As a "spin-off" from investigations into the coordination chemistry of P4, the first diphosphatetrahedrane (Figure 3) was successfully synthesized and isolated (Chem. Int. Ed. 2019, 58, 16918–16922). This species is a close carbon analogue of P4. Initial reactivity studies have already revealed intriguing parallels, but also differences between diphosphatetrahedrane and the isolobal P4 molecule (Angew. Chem. Int. Ed. 2020, 59, 14148–14153; Chem. Commun. 2021, 57, 2356–2359).
The ultimate and most ambitious aim of the FunctionalP4 project undoubtedly is the development of catalytic methods for converting white phosphorus into phosphorus-based chemicals. Such catalytic methods are of extremely high academic and industrial significance. In order to reach this very challenging goal, several reaction steps need to be integrated into a catalytic cycle, and the reactivity of all reagents must be finely balanced in order to be able to close such a cycle. The ability of white phosphorus to effectively trap organyl and p-block element radicals was identified as a promising concept. Meanwhile, photoredox catalysis was proposed as a means to generate the desired radicals in homogeneous solution. Based on these considerations, an unprecedented photocatalytic method was developed that converts P4 directly into valuable aryl phosphines and phosphonium salts (Nat. Catal. 2019, 2, 1101–1106,Figure 4). This method presents a solution to the hitherto unsolved challenge of catalytic P4 functionalization. Current investigations aim at an improved efficiency and a broader substrate scope of the method (Chem. Eur. J. 2020, 26, 16374-16382). An extension to alternative photocatalytic manifolds is planned.
Pursuing a different strategy, an efficient one-step synthesis of monophosphines from P4 has recently been developed (Nat. Chem. in press, DOI: 10.1038/s41557-021-00657-7 Figure 5). This method is based on the use of organotin hydrides, which are able to effectively hydrostannylate the P-P bonds of P4 under irradiation or in the presence of radical initiator. Using this method, a diverse range of industrially relevant organic and inorganic phosphorus compounds is accessible in one reaction step from P4. Importantly, the organotin reagent can be efficiently recycled and even used in a catalytic fashion. A European patent application was filed by the University of Regensburg based on the results (EP 20 157 197.3 inventors: D. J. Scott and R. Wolf).
Over the first 30 months of the project, substantial progress has been made toward understanding how the white phosphorus molecule can be broken up and transformed in the coordination sphere of metal ions. Stoichiometric P4 functionalization procedures have afforded novel and unusual mono- and polyphosphorus species. Landmark achievements of this project are the first catalytic procedures for converting P4 into industrially relevant monophosphorus compounds. Some unexpected discoveries were also made, including the development of a catalytic synthesis of a diphosphatetrahedrane, a carbon analogue of P4.
While several important objectives of the project have already been achieved, numerous positive and encouraging results have been obtained during the first 30 months which stimulate further investigations aiming at even more sophisticated P4 functionalization methods. These on-going and future studies will hopefully give rise to protocols with clear potential for implementation in industrial applications. In addition, on-going work within this action aims at the synthesis of further, unusual organic and organometallic phosphorus compounds using white phosphorus as a resource. It is hoped that such synthetic investigations will offer novel and fundamental insights into the fascinating chemistry of element 15.
While several important objectives of the project have already been achieved, numerous positive and encouraging results have been obtained during the first 30 months which stimulate further investigations aiming at even more sophisticated P4 functionalization methods. These on-going and future studies will hopefully give rise to protocols with clear potential for implementation in industrial applications. In addition, on-going work within this action aims at the synthesis of further, unusual organic and organometallic phosphorus compounds using white phosphorus as a resource. It is hoped that such synthetic investigations will offer novel and fundamental insights into the fascinating chemistry of element 15.