α‐Sila‐Dipeptides: Synthesis and Characterization

Abstract

The first two α-sila-dipeptides, 7 and cyclo-sila-dipeptide 8, were synthesized and characterized by several methods, including X-ray crystallography. Bulky t-BuMe Si substituents provide some kinetic stabilization to the synthesized molecules. 7 and 8 are the first examples of a "Si for C switch" in the central α-position of an amino acid or a peptide, in which silicon is bonded to both the amino and the carbonyl groups.

α‐Sila‐Dipeptides: Synthesis and Characterization

“…[21e] Theb ond distances of the asilicon (Si1) to other atoms;t hat is,c arbonyl group Si1ÀC13 (1.918 ), the Si1ÀN1 (1.712 )a nd Si1ÀSi2 (2.357 ), Si1À Si3 (2.361 ), are all in the range typical for related silicon compounds. [22] 13 Cc hemical shifts are in line with those observed for other carbamoyl silanes. [24] The 29 Si NMR of 7 reveals two sets of signals in the same ratio (77:23) measured for 11 and 12 indicating am inor influence of the H(11)!Br(12)!NH 2 (7)s ubstitution at the a-Si atom on the s-cis/s-trans ratio.DFT quantum-mechanical calculations carried out for am odel system of 7,( R 3 Si = Me 3 Si, Ot-Bu = OMe) at the 6-31 + (d,p) level, [25] predict that the s-trans isomer is more stable than s-cis isomer (DG 0 ) by 0.79 kcal mol À1 and the rotational barrier around the amide bond (DG°)i s2 0.15 kcal mol À1 ,c onsistent with the observed NMR data.…”

“…ORTEP of the X-ray molecularstructure of 8;H ydrogen atoms are omitted for clarity,a nd the ellipsoids are set at 25 % probability [22]. Selected bond lengths []…”