Organic Process Research & Development
Article
Scheme 3. General Curtius Rearrangement Process toward
Table 2. Development of the Curtius Rearrangement on
Substrate 8a
residence time
(min)
temperature
equiv. of
BnOH
isolated yield
(%)
entry
(°C)
1
2
3
4
20
30
30
30
120
120
120
120
1.0
1.5
1.8
2.0
70
79
86
85
necessary for high isolated yields of >80%. The flow process
was performed successfully for prolonged periods of time (1−2
h) to generate gram quantities of the carbamate product 9a,
reaching a throughput of ca. 7 mmol/h (2.1 g/h).
Furthermore, the conditions proved to be effective in
circumventing any observation related to blockages or reactor
fouling that could have been associated with precipitation of
insoluble materials.
facilitated a simple purification under continuous flow to purge
this residual reagent. The development work on the Curtius
rearrangement subsequently focused on the conversion of 4-
(
trifluoromethyl)benzoic acid (8a) into the corresponding
Cbz-carbamate 9a (Scheme 4).
Solvent choice was a critical consideration for the develop-
ment work, with initial investigations focusing on acetonitrile
and toluene. It was observed that even traces of water in
commercial acetonitrile resulted in the formation of insoluble
urea side products. With this observation in mind, toluene was
selected because of its favorable properties of high boiling
point (i.e., low vapor pressure at elevated temperature) and
low propensity to contain significant amounts of water (ca.
The resultant crude carbamate product 9a was contaminated
with residual benzyl alcohol, which proved to be difficult to
purge during workup because of its high boiling point of ca.
205 °C. To address this, targeted development of a continuous
biocatalyzed protocol was carried out utilizing Candida
antarctica lipase B (CALB) as a robust enzyme to convert
benzyl alcohol to benzyl butyrate (11) in the presence of vinyl
butyrate (10). CALB was used in an immobilized form using a
hydrophobic carrier (acrylic resin) that provides a loading of
ca. 10 wt %. CALB has been shown to be a versatile enzyme in
chemical processing that is amenable to immobilization and
0
.033% at 25 °C). Reactor fouling due to precipitation of
products is one of the fundamental hurdles often experienced
by chemists performing reactions under flow, and careful
consideration of solubility is critical for the successful
development of a flow process.
16
Substrate 8a was only sparingly soluble in toluene, but with
the addition of triethylamine (1.0 equiv.) and generation of the
corresponding salt, 8a was completely soluble at a concen-
tration of 1 M. Limiting the stoichiometry of DPPA to 0.9
equiv. avoided contamination of the final product with residual
azide species. Considering previous reports on flow-based
protein engineering to tune the enzyme properties.
17
An Omnifit glass column (length = 100 mm; i.d. = 6.6
mm) was packed with CALB, and the reaction carried out at
ambient temperature. The initial flow setup was then modified
by mixing 10 (3 equiv. in toluene) with the crude product
stream from the Curtius rearrangement via a T-piece before it
passed through the CALB column (residence time of ca. 2−5
min) and the product solution was collected for NMR and
7
−11
Curtius rearrangement reactions,
a flow system was
investigated in which a stream containing substrate 8a (1.0
18
equiv, 1 M in toluene), NEt (1.0 equiv.), and benzyl alcohol
HPLC analysis. The robustness of CALB meant that it could
tolerate the nonpurified reaction mixture with no detrimental
effect on the performance of the enzyme as a result of spent
reagents. Further development experiments demonstrated that
3 equiv. of 10 (bp 116 °C) was necessary to achieve full
conversion of benzyl alcohol to 11, whereas with only 1−2
equiv. typically about 15% of unreacted benzyl alcohol was
present (Scheme 5).
3
(
1.0−2.0 equiv) was mixed via a T-piece (1/8″ PEEK) with a
stream of DPPA (0.9 M in toluene). The combined mixture
was then reacted in a coiled reactor (PFA, 10 mL) of a
Vapourtec E-Series flow reactor before passing a BPR (100
psi) (Scheme 4). From initial experiments it was determined
that a temperature of 120 °C and a residence time of 30 min
(
combined flow rate of 0.33 mL/min) yielded full conversion
of substrate 8a (Table 2).
A CALB column with a path length (length of the enzyme
bed) of about ∼8 cm was sufficient to observe full conversion
of benzyl alcohol. Furthermore, it was found that the enzyme
performance did not deteriorate over several runs (5 × 1 mmol
As a result of the formation of nitrogen gas, a biphasic slug
flow regime was observed within the reactor coil (10 mL). It
was noted that a slight excess of benzyl alcohol (1.8 equiv) was
Scheme 4. Flow Setup for Continuous Curtius Rearrangement toward Crude 9a
C
Org. Process Res. Dev. XXXX, XXX, XXX−XXX
Baumann, Marcus
