Amino Acids and Peptides in Ball Milling - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 140

Indeed, in a typical experiment involving a 10 mL jar filled with one 10
mm diameter metallic ball, Boc-Phe-NCA, HCl H-Leu-OMe and NaHCO 3 ,up
to a milling load of 5.9 mg mL 1 , gave a satisfying conversion of 59%
reached after 10 min of agitation (Scheme 6.19, Figure 6.4). Unfortunately,
when the milling load was increased up to 22.5 mg mL 1 , conversion into
Boc-Phe-Leu-OMe dropped to 17% (Figure 6.4), which was attributed by the
authors to the ''highly viscous and sticky reaction media''. This observation
was hypothesized to be related to mass transfer limitations resulting in a low
speed of formation of Boc-Phe-Leu-OMe. To improve mass transfer of the
reactants in the jar, the authors envisioned the use of small amounts of a
liquid as a grinding assistant. Thus, adding 1.4 mL of EtOAc per mg of
reactants to the reaction mixture (Z ¼ 1.4 mLmg 1 ; Z stands for the ratio of
added liquid volume to the mass of solid reactants) 46 led to 91% conversion
into Boc-Phe-Leu-OMe after the same grinding time. One could argue that
adding EtOAc to the solid reactants and placing this reaction mixture under
classical agitation could be enough to overcome the mass transfer limi-
tations. The latter possibility was rejected by the fact that when the same
reaction mixture was placed under classical magnetic agitation, the speed of
the reaction was far slower than under ball-milling agitation. Indeed, when
placed under classical agitation, the conversion into Boc-Phe-Leu-OMe
reached 12% after 40 min (Figure 6.4). The latter experiment proved un-
ambiguously that it is the combination of ball-milling with the presence
of a grinding assistant that is responsible for the fast conversion into
the dipeptide. Besides, switching from EtOAc to a predominant solvent
used in peptide synthesis such as DMF did not match the performance of
the ball-mill approach as 37% of conversion was reached after 40 min of
agitation.
Another positive effect of using a liquid grinding assistant is the influence
on Boc-Phe-NCA hydrolysis. Indeed, while conversion of Boc-Phe-NCA into
Boc-Phe-OH reached 23% after 20 min of grinding with a milling load of
22.5 mg mL 1 , it declined to 2% after the same amount of time if EtOAc was
present in the reaction media (Figure 6.5).
These conditions were then successfully applied to the synthesis of a
wide range of di- to pentapeptides (Table 6.2). First of all, when previously
optimized conditions were applied to the production of Boc-Phe-Leu-OMe,
the latter could be isolated with 95% yield (Table 6.2, entry 1). No

O

O

NaHCO 3 (1.0 equiv)
Grinding assistant

O

BocN

BocHN

O

BocHN

OMe OH

H

+

OMe

O

HCl· H 2 N

Agitation, Time

O

O

Boc-Phe-NCA HCl·H-Leu-OMe Boc-Phe-Leu-OMe Boc-Phe-OH

Scheme 6.19 Liquid-assisted mechanosynthesis of Boc-Phe-Leu-OMe.