Communication
10 mmol, 5 equiv.), and a magnetic stir bar. The vial was then
sealed, placed in a microwave reactor, and heated at 115 °C for 1 h
in high absorption mode. The volatiles were then removed under
reduced pressure, and the crude residue was purified by automated
flash chromatography on silica gel to give 2a–p.
Table 4. Comparison of the activities of Ca(NTf2)2 (5 mol-%) and HNTf2 (2 mol-
%).[a]
Entry
Aniline
Conversion with
Ca(NTf2)2 [%][b]
Conversion with
HNTf2 [%][b]
1
2
3
4
5
6
7
8
1a
1b
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
>99
>99
>99
>99
>99
63
>99
>99
90
90
67
70
85
96
>99
80[c] (55)[d]
>99
95
Keywords: Synthetic methods · Microwave chemistry ·
Formylation · Anilines · Brønsted acids
43
97
89
90
85
57
68
80
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16
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[a] Reaction conditions: methyl formate (5 equiv.) was added to a 2 mL micro-
wave vial equipped with a magnetic stir bar and a mixture of 1a–o (2 mmol)
and Ca(NTf2)2 (5 mol-%) or HNTf2 (2 mol-%). The vial was then capped, placed
in a microwave reactor, and heated at 115 °C for 1 h. [b] Conversion was
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ment of Brønsted acid catalysis in this reaction if Ca(NTf2)2 is
used alone. Worthy of note, the use of triflic acid (1 mol-%) as
the catalyst (Table 4, entry 3; number in parentheses) was less
effective than the use of an identical loading of HNTf2, which
is reminiscent of the differences in activity observed between
calcium triflate and calcium triflimide (Table 2, entries 2 and
12).
Conclusions
In summary, we uncovered that calcium triflimide was an effi-
cient catalyst for the solvent-free microwave-assisted N-formyl-
ation of different weakly basic anilines by using a noncorrosive
and cheap formylating agent. On the other hand, calcium tri-
flate showed lower performance under the same reaction con-
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metal triflimides relative to that of their triflates counterparts.[21]
Additional control experiments showed that formylations of
weakly basic anilines could also be achieved with HNTf2. How-
ever, in practice the use of Ca(NTf2)2 as the catalyst was pre-
ferred, as its handling was much easier: no special precautions
to exclude air or moisture were required. Although an excess
amount of the formylating agent was used in this transforma-
tion, no additional (toxic) solvents were required and unreacted
methyl formate should be easily recovered together with the
methanol formed. The recycling of calcium triflimide is currently
the object of further investigations.
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Experimental Section
General Procedure for the Synthesis of Formanilides: A 0.5–2 mL
microwave vial was charged successively with Ca(NTf2)2 (60 mg,
0.1 mmol, 5 mol-%), aniline 1a–p (2 mmol), methyl formate (0.6 mL,
Eur. J. Org. Chem. 2016, 1836–1840
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