Dedicated to Professor Anders KjΓ±r on the occasion of his 80th birthday
The selective introduction of side chains to a specific glycine residue in a peptide strand represents a challenging task for the preparation of unnatural peptides. Instead of a stepwise approach where commercially available or synthetic amino acids are incorporated by a traditional peptide synthesis, application of a direct peptide modification step is justified by the numerous analogues which may be quickly synthesized from a single and intact peptide. As reactive intermediates, glycine enolates [1Β±4] and radicals [5Β±7] as well as complexes can be generalized to U III and can be coupled to the U III redox chemistry to do multielectron reductions. This also shows that the C 8 H 8 2Γ /U IV components of uranocene can exist in forms other than the very stable parallel ring structure of [U(C 8 H 8 ) 2 ]. This reaction suggests a rich reductive chemistry can be accessed by coupling steric crowding and lower f-element oxidation states.
Experimental Section
All manipulations were performed under an N 2 or Ar atmosphere by glove box or Schlenk techniques.
1: In a THF-free glovebox, 1,3,5,7-C 8 H 8 (24 mg, 0.23 mmol) was added to a brown solution of [(C 5 Me 5 ) 3 U] (100 mg, 0.155 mmol) in toluene (10 mL). After the brown solution was stirred at room temperature for 4 d, the solvent was removed by rotary evaporation. The gummy brown solid was washed with hexanes and dried under reduced pressure to afford 1 as a dark brown powder (70 mg, 85 %).