Bismuth Compounds in Radical Reactions: Fundamental Aspects and Synthetic Applications

Project Information

Bismuth Goes Radical

Grant agreement ID: 946184

DOI

10.3030/946184

EC signature date 30 October 2020

Start date 1 May 2021

End date 30 April 2027

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 470 753,00

EU contribution

€ 1 470 753,00

Coordinated by

PHILIPPS UNIVERSITAET MARBURG
Germany

Periodic Reporting for period 3 - Bismuth Goes Radical (Bismuth Compounds in Radical Reactions: Fundamental Aspects and Synthetic Applications)

Reporting period: 2023-07-01 to 2024-12-31

Summary of the context and overall objectives of the project

Many of the classical approaches in synthetic chemistry are based on starting materials from fossil feedstocks. The reactions may be energy-intensive (e.g. not using catalytic methods) and oftentimes contain reactants that are not environmentally benign. The vast majority of these approaches utilizes polar reaction pathways. So reactions of compounds with unpaired electrons (so-called radicals), that were classically thought to be very difficult to control, are underrepresented.
One of the major challenges in modern synthetic chemistry is to realize the transformation towards reactions and processes that are safer, environmentally more benign, and more sustainable compared to existing approaches. This includes the utilization of elements that are currently available in sufficient amounts, at low costs, and mined under conditions that are unproblematic from environmental and social viewpoints. In addition, a transformation towards more sustainable approaches also makes the utilization of catalysts desirable in order to lower the energetic demand of chemical processes. When turning towards non-fossil resources the utilization of compounds that tolerate polar functional groups will be of growing importance.
Finding novel approaches to controlled radical reactions would help to unlock the tremendous potential of this class of chemical transformations, including reactions with a high tolerance towards polar functional groups and catalytic protocols.
Bismuth has been coined a “green element” based on its good availability, low costs, as well as the low toxicity and the low environmental concerns linked to its compounds. In view of the positioning of bismuth in the periodic table of the elements, it shows remarkably low homolytic bond dissociation energies, i. e. the generation of radical species from well-defined bismuth compounds should be possible at relatively low energetic costs. However, the exploitation of these inherent characteristics has been hampered by limitations in the identification of species that combine a good synthetic accessibility with a promising reactivity and by limitations in the synthetic methodologies to generate certain bismuth-containing functional groups.
The project “Bismuth Goes Radical” aims to unlock the potential of well-defined bismuth compounds in stoichiometric and catalyzed reactions involving radical species. This includes to generate access to new types of bismuth compounds, to liberate radicals under mild reaction conditions, to exploit bismuth-centered radical reactivity, radical equilibria, and photochemical procedures, to find alternative approaches to known reactions and to develop new types of chemical transformations.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The project “Bismuth Goes Radical” deals with the controlled, stimulus-triggered generation of radical species under mild conditions, with the generation of unusual low-valent bismuth-based building blocks, and with di- or multinuclear bismuth compounds. These topics are investigated in the context of new approaches to controlled radical reactions.

Well-defined molecular bismuth compounds with known and novel bonding motifs have been synthesized, characterized, and evaluated with respect to their potential to engage in homolytic bond dissociation reactions, according to the general reaction equation R2Bi–X ⇌ R2Bi• + X•. Promising candidates have been subjected to stoichiometric and catalytic reactivity studies in thermally and in photochemically initiated reactions. In this context, N–N, P–P, As–As, Si–O, C–S, and C–C bond formation events have been realized. These investigations include comparative approaches, aiming at the evaluation of the role of the central atom (i.e. does the bismuth atom play a decisive role in the approach).

Molecular bismuth compounds have been designed and synthesized as precursors for the generation of short-lived, reactive low-valent bismuth compounds of the type Bi–R (R = simple, monoanionic ligand). These species evolve from a combination of two homolytic bond cleavage events. Due to their reactive nature, the conditions for their generation are decisive in order to realize selective follow-up reactions. Strategies that have been investigated involve the release of ring strain and the exploitation of energy gain due to re-aromatization events.

Dinuclear bismuth complexes have been conceptualized and synthesized, aiming to investigate unprecedented structural motifs in the context of strained compounds and organometallic species exhibiting electronic communication between two or more bismuth centers. Synthetic pathways involving radical reactions have been evaluated for species connecting two bismuth centers through different types of organic linkers. Following initial findings in this part of the project, the formation of multinuclear species has been investigated and efforts have been undertaken to elucidate the role of details in the course of these reactions, including concentration effects, solvent effects, subtle substituent effects, and templating strategies.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Significant progress in “Bismuth Goes Radical” has been realized in all three parts of the project.

Aminyl radicals are important species in biological processes and represent valuable reactive intermediates in chemical reactions, but their generation and synthetic exploitation under mild reaction conditions remains challenging. In the project “Bismuth Goes Radical”, the synthesis of well-defined bismuth compounds with the ability to release aminyl radicals under mild reaction conditions has been achieved. This allows for facile and highly selective N–N bond formation reactions via radical pathways. The general viability of this approach has been demonstrated in a proof-of-principle study, which allowed for the highly selective P–P and As–As bond formation reactions in operationally simple protocols, without the necessity to isolate the respective bismuth compounds.

Low-valent bismuth compounds Bi–X without any stabilizing effects such as steric protection or interactions with additional Lewis bases have been coined “non-stabilized bismuthinidenes” and represent highly reactive species, likely with biradical character (as demonstrated for Bi–Me). The controlled generation of these entities in common organic solvents has been an open challenge and consequently, their reactivity can be described as uncharted territory. In the project “Bismuth Goes Radical”, precursors for the controlled release of non-stabilized bismuthinidenes have been designed. The successful strategy combines Lewis acidic properties of the precursor with its light-responsiveness. This allows for precoordination of the substrate followed by the photochemically induced release and transfer of a Bi–X moiety. Overall, this corresponds to a highly selective, photochemically-induced bismuthinidene transfer.

The design of dinuclear bismuth compounds featuring weak Bi–C bonds has allowed to bring molecular bismuth species onto the stage of dynamic covalent chemistry. Sequences of the consecutive dynamic cleavage and formation of more than 10 Bi–C bonds involving radical processes can be induced and deliberately steered towards product formation. This grants access to unprecedented types of macrocyclic and cage-like structural motifs that can be interconverted upon exposure to external stimuli, opening up perspectives for promising applications in fields such as host-guest chemistry and switchable catalysis.