* Trp arylation
Multiple methods have been reported to achieve tryptophan C–H arylation, where diverse electrophiles such as aryl halides and aryl boronic acids (an example shown below) have been used to transfer the aryl groups.
However, current tryptophan C–H arylation reaction conditions remain relatively harsh, requiring organic solvents, low pH and/or high temperatures.
* Cys arylation
Free thiols has been considered unfavorable for Pd-mediated reactions due to Pd-catalyst decomposition. However, Pd oxidative addition complexes (OACs) supported by dialkylbiaryl phosphine ligands have shown to work efficiently towards cysteine S-arylation.
The first example is the use of Pd OAC with RuPhos: The Pd complex resulting from the oxidative addition of aryl halides or trifluoromethanesulfonates and using RuPhos as the ligand could chemoselectively modify cysteines in various buffer with 5% organic co-solvent under neutral pH. This method has been shown to modify peptides and proteins, achieve peptide macrocyclization (by using bis-palladium reagent and peptides with two unprotected cysteines) and synthesizing antibody-drug conjugates (ADCs). Changing the ligand to sSPhos supports the Pd complex to be sufficiently water soluble to achieve cysteine S-arylation under cosolvent-free aqueous conditions.
There are other applications of this method where the Pd complexes were generated as Pd-peptide OACs by introducing 4-halophenylalanine into peptides during SPPS to achieve peptide-peptide or peptide-protein ligation.
Alternate to directly oxidative addition to the peptide, the Pd OACs could also be transferred to the protein through amine-selective acylation reaction via NHS ester. The latter has been applied to selectively label surface lysine residues of a protein (forming Pd-protein OACs) and oligonucleotides (forming Pd-oligonucleotide OACs), which could then be linked to cysteine-containing peptides or proteins.
Another example of protein-protein cross-coupling is achieved through converting cysteine residues into an electrophilic S-aryl–Pd–X OAC by utilizing an intramolecular oxidative addition strategy.
* Lys arylation
Similar to cysteine, lysine N-arylation could be achieved through Pd OACs with different dialkylbiaryl phosphine ligands. Due to weaker nucleophilicity and slower reductive elimination rate compared to cysteine, the selection of supporting ligands is shown to be critical. The bulky BrettPhos and t-BuBrettPhos ligands in conjunction with mildly basic sodium phenoxide have been used as the strategy to functionalize lysines on peptide substrates. The reaction happens in mild conditions and is selective over most other nucleophilic amino acid residues.