Amino Acids and Peptides in Ball Milling - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 130

A similar approach was developed by Margeti´ and coworkers to produce
amino amides by using a mechanochemical process (Scheme 6.12). 39
Boc-Alanine was activated in the reaction media by addition of N-ethyl-N 0 -
(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC HCl) in the
presence of 2 equivalent of dimethylaminopyridine (DMAP) as the base, 20
equivalents of NaCl as a solid grinding assistant and small quantities
of nitromethane as a liquid grinding auxiliary. Subsequent reaction with
p-anisidine or 4-choroaniline provided the corresponding amino amides in
87% and 88% yield respectively.
6.3 Mechanosynthesis of Peptides
6.3.1 Synthesis of Di- and Tripeptides
In 2009, motivated by the desire to mitigate the environmental impact
of classical peptide synthesis approaches, Lamaty and coworkers envisioned
performing solvent-free peptide synthesis based on the use of the ball-
milling technology. 40 Initial investigations were carried out on the coupling
of the Boc-protected phenylalanine N-carboxyanhydride (Boc-Phe-NCA) with
alanine methyl ester hydrochloride salt (HCl H-Ala-OMe) in the presence of
NaHCO 3 in a vibrating ball-mill (Scheme 6.13).
After 1 h of vigorous agitation (30 Hz) in a vibrating ball-mill, reaction media
was recovered from the milling jar by using both EtOAc and water. Classical
treatments of the organic phase such as water washings, drying and concen-
tration furnished the pure dipeptide Boc-Phe-Ala-OMe in 79% yield without
epimerization as observed by HPLC analysis. The low environmental impact of
this reaction has to be noticed. The usual highly toxic solvents such as DMF,
CH 2 Cl 2 , NMP or THF were not required and volatile and corrosive bases such
as Et 3 N and DIPEA were replaced by innocuous NaHCO 3 . In addition, this
approach furnished non-toxic CO 2 and NaCl as the only byproducts.
As one could argue that this reaction could have taken place during re-
action media recovery with EtOAc and water, the authors performed solid-
state IR and CP/MAS 13 C NMR analysis of the reaction media before EtOAc
recovery. These analysis revealed the disappearance of the characteristic
signals of Boc-Phe-NCA (n ¼ 1817 and 1872 cm 1 in IR and d ¼ 147.6 in CP/
MAS 13 C NMR) and the emergence of the typical signals of the desired
product Boc-Phe-Ala-OMe (n ¼ 1624 and 1655 cm 1 in IR and d ¼ 156.0 and
177.8 in CP/MAS 13 C NMR). As these analytical samples were not involved in
any solubilization process, these results proved that the reaction clearly
occurred in the solid state.
Another particularity of these reaction conditions has been revealed by
studying the kinetics of the reaction. As previously described by others
on solid state reactions, 41 measuring the conversion of Boc-Phe-NCA into
Boc-Phe-Ala-OMe at different times of reaction revealed apparent zero-order
kinetics (Figure 6.2). Notably, the reaction order seems to be independent of
the frequency applied to the reaction media.