K. Takamatsu, M. Kasai, H. Nishizawa et al.
Tetrahedron Letters xxx (xxxx) xxx
Table 2
Screening of solvents for the NHNPI (14)-catalyzed oxidation.
entry
solvent
R-CHO
R-CO
2
H
3
R-CO H
1
2
3
4
5
6
7
8
9
ether
PhCl
MeOH
100
99
99
99
64
43
35
35
33
25
0
0
1
1
1
12
7
8
31
11
52
30
34
0
0
0
0
24
50
57
34
56
23
60
66
H
2
O
Ph
CH
EtOH
CHCl
2
O
2
Cl
2
3
hexane
toluene
benzene
MeCN
1
1
1
0
1
2
0
resulted in the formation of dodecanoic peracid in a higher yield
than dodecanoic acid. Reactions in benzene and MeCN proceeded
smoothly (entries 11 and 12), consistent with Vanoya’s report
[
12]. It should be noted that the experimental conditions, workup
1
procedures, and analytical method used to H NMR all affected the
results, with the final product being obtained either by decompo-
sition of the peracid intermediate, or by rearrangement of the Crie-
gee intermediate.
Reaction conditions: NHNPI (14) (5 mol%), dodecanal
Fig. 2. NHNPI (14)-catalyzed oxidation of dodecanal in MeCN-d
monitored as the function of time using 1H-NMR. (A) Consumption of dodecanal in
MeCN-d and benzene-d . (B) Production of dodecanoic peracid and acid. Reaction
3 6
and benzene-d
(
0.5 mmol), air (1 atm), solvent (2 mL), room temperature, 24 h.
1
Yield of dodecanal and its peracid was determined by H NMR
analysis.
3
6
conditions NHPI (1) (5 mol%), dodecanal (0.1 mmol), air (1 atm), MeCN-d3 or
The oxidation of dodecanal in benzene and MeCN was sepa-
benzene-d6 (0.6 mL), room temperature.
1
rately monitored by H NMR using MeCN-d
3
and benzene d
6
. The
reaction was carried out in an NMR tube and a spectrum acquired
every 1 h (Fig. 2A). Using MeCN-d , the dodecanal was consumed
3
within 8 h and peaks corresponding to dodecanoic acid and its per-
acid observed in areas corresponding to their moderate yield
(
Fig. 2B). This is consistent with the aforementioned experiments
in MeCN (Table 1, entry 14 and Table 2, entry 12). On the other
hand, the NHNPI (14)-catalyzed oxidation in benzene d was
6
slower than that in MeCN. After the treatment of reaction mixture
for 8 h, the corresponding products yielded and the reaction com-
pleted for 18 h (Fig. 2A). It is noted that MeCN sufficiently acceler-
ates the NHNPI (14)-catalyzed oxidation and leads aldehyde to the
corresponding acid and its peracid (Fig. 2).
Transformation of the carboxylic peracid to carboxylic acid was
accomplished with a large excess of phenylmethylsulfide [22] at
room temperature. This step could also be accomplished using
water, especially on large scale, although the reaction was much
slower using water compared with phenylmethylsulfide. For
example, a full week was required for the hydrolysis of dodecanoic
peracid derivatives to the corresponding benzoic acids using water
Scheme 2. Trap of dodecanoic peracid by phenylmethylsulfide to dodecanoic acid.
tion (5 mol% NHNPI (14), MeCN, air, room temperature, 12 h),
but could be oxidized to m-cyanobenzoic acid in 90% yield (see
supporting information Fig. S3). It means that dodecanoic peracid
is the substrate for the rearrangement of Criegee intermediate in
the presence of dodecanal. Scheme 3 depicts a mechanism for
the oxidation of m-cyanobenzaldehyde that accounts for this
result. This reaction proceeds by reaction of m-cyanobenzaldehyde
with dodecanoic peracid, which decomposes to give product car-
boxylic acids, i.e. dodecanoic peracid is the substrate for the rear-
rangement of Criegee intermediate. Lehtinen reported that there
are two routes, Baeyer-Villiger (BV) and Anti-BV types, by the rear-
rangement of Creigee intermediate to give formates and carboxylic
acids. In this case, the corresponding formate adducts have not
been detected and therefore, anti-BV type reaction proceeded to
yield m-cyanobenzoic acid and dodecanoic acid (Scheme 3) [13].
With optimized reaction conditions in hand, the scope of this
method was investigated using a variety of alkyl, branched chain,
and aromatic aldehydes. In all cases, the corresponding acids were
(Scheme 2, Fig. S2).
In contrast, treatment of dodecanoic acid from dodecanal pro-
ceeded via rearrangement of the Criegee intermediate. It is obvious
that dodecanoic peracid was generally isolated on the NHNPI (14)-
oxidation of dodecanal. In other words, dodecanal is used to trap
mainly by PINO and to react with peracids as the side reaction.
As the dodecanoic peracid after the purification was treated with
dodecanal, the reaction with dodecanoic peracid could smoothly
proceed and give dodecanoic acid solely in satisfactory yields. m-
Cyanobenzaldehyde was not oxidized under the optimized condi-
3