Organic Letters
Letter
build a new type of complicated ester 6 in 58% yield (Scheme
5D). Moreover, when 5a was treated with POCl3 reagent, the
4-chloropyrrolo[1,2-a]quinoxaline 7 was accessed in 80% yield
(Scheme 5E). Next, the C−Cl bond could be further
functionalized to furnish product 8 by Suzuki−Miyaura
cross-coupling (Scheme 5F).
To gain more insights into the mechanistic rationality,
additional experiments were undertaken. In view of the verified
fact that the dioxazolones could readily decompose into CO2,
we introduced CO2 gas, another typical CO resource, into the
reaction system in the absence or catalytic amount of 2d,
respectively. Both of these cases failed to generate the titled
product smoothly, which unequivocally obviated the involve-
ment of CO2 as the carbonylation source (Scheme 6A). The
which product 3a could be generated properly (Scheme 6D).
This result supported that the urea species were involved in
this cyclization procedure.
On the basis of these findings, the reaction mechanism was
then put forward (Scheme 6E). The thermal decomposition of
dioxazolones initially takes place to generate isocyanate
accompanied by dislodging the CO2 molecule,13,16 followed
by reacting with 2-alkenylaniline via nucleophilic addition to
access the key intermediate urea 10b. At this stage, two
pathways might be employed, involving a direct cyclization
(path a) or sequential forming isocyanate species/Prins-type
reaction (path b). To further probe the exact reaction path, the
Me-protected 2-alkenylaniline 11 or its corresponding urea 10c
was used to undergo this issue. Notably, the envisioned
quinolinone 12 failed to occur (Scheme 6F), which proved
that the urea species was futile to enable this reaction by one-
step cyclization. Alternatively, to affirm the feasibility with
respect to isocynate, the reaction between substrate 1a with
bis(trichloromethyl) carbonate agent, which is able to in situ
generate the relevant isocyanate smoothly, was carried out and
indeed released quinolione product 3a (Scheme 6G), so the
path b was rational.
Scheme 6. Mechanistic Studies
In summary, we discovered a class of new carbonylating
agents resulting from the dioxazolones, by which the metal-free
[5 + 1] carbonylation of 2-alkenylanilines or 2-pyrrolylanilines
with dioxazolones is accomplished, leading to a variety of
quinolinones (23 examples) and pyrrolyl-fused quinoxalinones
(16 examples) in good yields with excellent functionality
tolerance. This study represents an unprecedented chemistry
reactivity with dioxazolones via simultaneously constructing
new N−C(O) and C(O)−C bonds. Mechanistic investigations
clearly illustrate that the generated double isocyanate species
are pivotal in this carbonylation conversion. Extensive studies
into the scope of the carbonylation reaction generated from
dioxazolones are currently ongoing in our laboratory and will
be presented in due course.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
Experimental procedures and spectral data (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Jiang Nan − Shaanxi Key Laboratory of Chemical Additives
for Industry, College of Chemistry and Chemical Engineering,
Shaanxi University of Science and Technology, Xi’an 710021,
Authors
Pu Chen − Shaanxi Key Laboratory of Chemical Additives for
Industry, College of Chemistry and Chemical Engineering,
Shaanxi University of Science and Technology, Xi’an 710021,
China
Xue Gong − College of Bioresources Chemical and Materials
Engineering, Shaanxi University of Science and Technology,
Xi’an 710021, China
reaction with 2m still delivered the wanted 3a, but no 4-
phenylquinoline-2(1H)-thione 9 was made (Scheme 6B).
Furthermore, the isotope-labeling experiment with diaxazolone
18O-2a led to the formation of product 18O-3a in 72% yield
(Scheme 6C). These findings provided solid evidence for the
viewpoint that the carbonyl group appended to the product
arose from the C3−O4 fragment. Subsequently, urea
compound 10a was synthesized to execute the reaction, in
Yan Hu − Shaanxi Key Laboratory of Chemical Additives for
Industry, College of Chemistry and Chemical Engineering,
3764
Org. Lett. 2021, 23, 3761−3766