The energy-transfer-enabled disulfide–ene reaction: discovery, mechanistic elucidation, biocompatibility
For members of the Glorius group discovery lies at the heart of the scientific process. Our goal is the development of novel, straightforward reaction methodology. The omnipresence of methylthioether scaffolds in biological systems and the challenges associated with their chemoselective synthesis prompted us to rationally design a photocatalytic methylthiolation strategy.
Acknowledging that S–S bond dissociation energies of dialkyl disulfides are around 65 kcal/mol, we wondered whether a successful interaction between the excited state photocatalyst and the aforementioned disulfide can lead to homolytic S–S bond cleavage and the formation of the desired methanethiyl radicals. The simplest method to verify this hypothesis was our “catalyst speed dating” screening approach. By monitoring the photocatalyst luminescence in the presence of diverse dialkyl disulfides, we observed desired interactions due to decreasing luminescence. With our screening data, we could successfully develop an unprecedented disulfide–ene reaction, allowing the hydro-alkyl/aryl thiolation of diverse alkenes and alkynes.
Since we had some initial hints towards an energy transfer activation of the disulfide species, we set-out to fully determine the underlying mechanism with the aid of advanced spectroscopic techniques, such as transient absorption. We quickly identified the group of Prof. Dr. Guldi from the University of Erlangen-Nuremberg as ideal collaboration partner and experts, owing to their long-standing expertise in advanced photon management. Considering the distance between Münster and Erlangen, we were surprised how efficient “external” collaborations can ideally be. After sending our chemicals, the required supporting data was carefully measured and analyzed within a couple of weeks.
A struggle, however, was the performance of some control experiments using other photosensitizers (e.g. Michler’s ketone) required as final evidence for proving the energy transfer activation. Technical issues dictated that the available instrument time in the Guldi group was limited, allowing us to consider further applications of our designed energy-transfer enabled disulfide–ene reaction in the meantime.
Motivated by the importance of methylthioether scaffolds in biological processes, we wondered whether the disulfide–ene reaction might be suitable for applications in biological systems, such as labeling of biomolecules. We therefore teamed up with the group of Prof. Dr. Rentmeister at the University of Münster, recognizing their outstanding know-how in labeling of biomolecules.
In 2013, we had introduced a simple screening-method to investigate the functional group tolerance and scope of a new reaction methodology – the “robustness-screen”. This additive-based screening approach requires carrying out the reaction of interest in the presence of a range of additives, which contain functionalities commonly encountered in potential substrates. Moieties that affect the outcome of the reaction are easily identified using this approach, potentially directing modification of the reaction conditions. This strategy has been widely adapted by both academic and industrial groups – as recently elegantly demonstrated in the context of reaction optimization.
We envisioned that the addition of diverse biomolecules to the reaction of interest would successfully probe the biocompatibility of the transformation. Amino acids, saccharides, nucleosides, thiols, single stranded DNA, RNA (short RNA and total RNA) and human cell lysate – most closely mimicking the physiological environment – were among the commercially available biomolecules tested. By adding these biomolecules one-by-one to the standard reaction, a reliable judgement of the biocompatibility of the methodology can be made:
- The reaction product formation (quantified by NMR or GC) is analyzed to examine the chemical yield of the reaction in the presence of bio-additives. The chosen biomolecules mimic the physiological environment for in vitro & in vivo applications of the reaction methodology. The chemoselectivity of the reaction in the presence of bio-additives exhibiting diverse functional groups is also monitored in this approach.
- The qualitative analysis of the biomolecule by UPLC-MS or gel electrophoresis allows a judgement of whether the reaction conditions affect its stability, e.g. by modification or degradation. The integrity of the biomolecules throughout the reaction signals the tolerance of the physiological environment towards the reaction conditions.
In contrast to the “classical” robustness screen, biocompatibility screening determines the suitability of a (novel) chemical methodology for biochemical applications. This protocol allows the facile and highly efficient examination of the biocompatibility of a transformation of interest and will thus stimulate the development of new biocompatible reactions. We are convinced that biochemists, medicinal chemists and material chemists will use this strategy to develop biocompatible methodologies useful for diverse biological applications, e.g. structure-activity relationship (SAR) studies or metabolic labeling.
Curious about further details on the disulfide–ene reaction, the biocompatibility screening approach or the exhaustive mechanistic studies?
The full paper can be found here.