Periodic Reporting for period 4 - FunctionalP4 (Metal-Mediated Methods for the Functionalization of White Phosphorus (P4))
Reporting period: 2023-03-01 to 2023-08-31
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 explored 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 were 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, was introduced for this purpose. Reactions of the phosphorus fragments in these new, reactive complexes with electrophiles produced 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 was a key objective of this project.
By providing novel synthetically useful and even catalytic procedures for converting P4 into organophosphorus compounds, this project contributed to the development of phosphorus chemistry and more sustainable synthesis methods.
The project FunctionalP4 explored 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 were 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, was introduced for this purpose. Reactions of the phosphorus fragments in these new, reactive complexes with electrophiles produced 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 was a key objective of this project.
By providing novel synthetically useful and even catalytic procedures for converting P4 into organophosphorus compounds, this project contributed to the development of phosphorus chemistry and more sustainable synthesis methods.
A part of the project dealt with the development of new methods for activating white phosphorus using transition metal (complexes. These investigatoins led to unique polyphosphorus complexes of iron, cobalt and nickel (e.g. Angew. Chem. Int. Ed. 2019, 58, 18931-18936; Angew. Chem. Int. Ed. 2020, 59, 14148-14153; Chem. Sci., 2021, 12, 11225-11235). Some of these complexes were used to access previously inaccessible phosphorus compounds, such as the unusual cyanodiphosphanide anions (Angew. Chem. Int. Ed., 2019, 58, 18931-18936, Figure 2). Two review articles on the topic of transition-metal mediated P4 functionalization were published (Chem. Eur. J. 2021, 27, 1886-1902; Coord. Chem. Rev. 2021, 441, 213927).
The cooperative interaction of two different metal atoms with P4 was pursued as a method to produce metal complexes with reactive phosphorus fragments. The use of a d-block (e.g. Co and Fe) in combination with a p-block elements (e.g. Ga, Si, and Sn) created 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; Dalton Trans. 2021, 50, 13985–13992; J. Am. Chem. Soc. 2022, 144, 44, 20434-20441). Even complexes of pure polyphosphorus ligands can be accessed, such as the unusual [Co(P5)(P3)]- anion (manuscript in preparation).
As a "spin-off" from investigations into the coordination chemistry of P4, the first diphosphatetrahedrane was successfully synthesized and isolated (Angew. Chem. Int. Ed. 2019, 58, 16918–16922; Figure 3). This species is a close carbon analogue of P4. Initial reactivity studies have already revealed intriguing parallels, but also differences between diphosphatetrahedrane and P4 (Angew. Chem. Int. Ed. 2020, 59, 14148–14153; Chem. Commun. 2021, 57, 2356–2359). Furthermore, the project has contributed to the development of fundamental aspects of organometallic chemistry, which is the basis for cutting-edge transition-metal-mediated P4 functionalization (Inorg. Chem. 2020, 59, 16035-16052; Organometallics 2021, 40, 1042–1052; Organometallics 2022, 41, 776−784). The chemistry of low-valent transition metalate anions has been summarized in a comprehensive review artile which will be published shortly (Chem. Rev. DOI 10.1021/acs.chemrev.3c00121).
The ultimate and most ambitious objective of the FunctionalP4 project was the development of catalytic methods for converting white phosphorus into phosphorus-based chemicals. In order to reach this very challenging goal, several reaction steps needed to be integrated into a catalytic cycle, and the reactivity of all reagents must be finely balanced. 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). Subsequent investigations improved the scope of the method and elucidated the reaction mechanism (e.g. Chem. Eur. J. 2020, 26, 16374-16382; Angew. Chem. Int. Ed. 2021, 60, 24650-24658). The recent development of photoinduced P4 arylation by aryl chlorides and bromides is another major step forward (Chem. Commun., 2022, 58, 1100-1103 and unpublished results).
Pursuing a different strategy, an efficient one-step synthesis of monophosphines from P4 was developed (Nat. Chem. 2021, 13, 458-464, 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. Subsequent publications elaborate and further expand on these landmark results (Chem. Commun. 2022, 58, 8986-8989; Chem. Eur. J. 2022, 28, e202202456). Such methods have signficant potential for industrial application. Thus, IP protection was successfully obtained through a European patent granted to the University of Regensburg based on the results (EP3865493, inventors: D. J. Scott and R. Wolf).
The cooperative interaction of two different metal atoms with P4 was pursued as a method to produce metal complexes with reactive phosphorus fragments. The use of a d-block (e.g. Co and Fe) in combination with a p-block elements (e.g. Ga, Si, and Sn) created 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; Dalton Trans. 2021, 50, 13985–13992; J. Am. Chem. Soc. 2022, 144, 44, 20434-20441). Even complexes of pure polyphosphorus ligands can be accessed, such as the unusual [Co(P5)(P3)]- anion (manuscript in preparation).
As a "spin-off" from investigations into the coordination chemistry of P4, the first diphosphatetrahedrane was successfully synthesized and isolated (Angew. Chem. Int. Ed. 2019, 58, 16918–16922; Figure 3). This species is a close carbon analogue of P4. Initial reactivity studies have already revealed intriguing parallels, but also differences between diphosphatetrahedrane and P4 (Angew. Chem. Int. Ed. 2020, 59, 14148–14153; Chem. Commun. 2021, 57, 2356–2359). Furthermore, the project has contributed to the development of fundamental aspects of organometallic chemistry, which is the basis for cutting-edge transition-metal-mediated P4 functionalization (Inorg. Chem. 2020, 59, 16035-16052; Organometallics 2021, 40, 1042–1052; Organometallics 2022, 41, 776−784). The chemistry of low-valent transition metalate anions has been summarized in a comprehensive review artile which will be published shortly (Chem. Rev. DOI 10.1021/acs.chemrev.3c00121).
The ultimate and most ambitious objective of the FunctionalP4 project was the development of catalytic methods for converting white phosphorus into phosphorus-based chemicals. In order to reach this very challenging goal, several reaction steps needed to be integrated into a catalytic cycle, and the reactivity of all reagents must be finely balanced. 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). Subsequent investigations improved the scope of the method and elucidated the reaction mechanism (e.g. Chem. Eur. J. 2020, 26, 16374-16382; Angew. Chem. Int. Ed. 2021, 60, 24650-24658). The recent development of photoinduced P4 arylation by aryl chlorides and bromides is another major step forward (Chem. Commun., 2022, 58, 1100-1103 and unpublished results).
Pursuing a different strategy, an efficient one-step synthesis of monophosphines from P4 was developed (Nat. Chem. 2021, 13, 458-464, 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. Subsequent publications elaborate and further expand on these landmark results (Chem. Commun. 2022, 58, 8986-8989; Chem. Eur. J. 2022, 28, e202202456). Such methods have signficant potential for industrial application. Thus, IP protection was successfully obtained through a European patent granted to the University of Regensburg based on the results (EP3865493, inventors: D. J. Scott and R. Wolf).
The FunctionalP4 project has advanced the field of phosphorus chemistry in many ways. The first photocatalytic and main-group-element-catalyzed P4 functionalization procedures are a major breakthroughs, which presents solutions to the hitherto unsolved challenge of catalytic P4 functionalization. Furthermore, the project has substantially advanced our understanding of the utility of transition metal complexes for P4 functionalization. The use of heterobimetallic transition metal compounds is a promising concept that can be used for the preparation of unprecendented phosphorus compounds. The development of p-block-element compounds (e.g. tin hydrides) and organic catalysts for (catalytic) P4 functionalization was an unexpected outcome of the project, which inspires further studies. In fact, many similarities between main-group element-based and transition-metal-mediated P4 functionalization have been discovered, which will be explored in subsequent projects. Furthermore, the catalytic synthesis of the first diphosphatetrahedrane opened a whole new area in phosphorus chemistry. By developing these aspects, the project has contributed fundamentally to synthetic methodology and the expansion of chemical space.