with D2O containing an excess amount of triethylamine (eq. 1).
The reaction gave indene 3a deuterated at the 2-position with a
deuteration ratio of 46% in 50% yield. A similar treatment of
1a with AlBr3 and B4 gave 3a with a deuteration ratio of 20%
in 34% yield. These observations clearly indicate the formation
of an inden-2-ylaluminum species. Both the yield and
deuteration ratio of 3a were higher in the former reaction,
which can be rationalized by the difference in the basicities of
B1 and B4. Thus, stronger base B1 can suppress the side
reaction(s) more efficiently than B4 to increase the recovery of
3a, while the weaker conjugate acid of B1 has a lower ability to
protonate the indenylaluminum species than that of B4,
resulting in the increase in the deuteration ratio of 3a. We next
investigated whether indene 3 is metallated with AlBr3 under
the conditions of methods I and II. The treatment of 3a with
AlBr3 under these conditions gave acid 2a in 45% (method I)
and 82% yields (method II) (eq. 2), indicating the formation of
an inden-2-ylaluminum species. However, B1 bearing bulky
tert-butyl groups near the nitrogen atom is not likely to be able
to abstract a proton from the zwitterionic species 5 because of
steric hindrance.11 Therefore, in this case, the deprotonation
seems to have occurred spontaneously in favor of the side
reaction(s) because the positive charge of 5 is highly stabilized
by the indan benzene ring.
Supporting Information
Supporting Information: Supplemental data, experimental
procedures, characterization data, and NMR spectral charts.
This material is available on http://dx.doi.org/10.1246/bcsj.***.
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Scheme 2. Feasible mechanism for the tandem
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12. See supporting Information
In
conclusion,
we
have
developed
tandem
cyclization–carboxylation of allenylbenzenes with CO2
mediated by AlBr3. The reaction required a base to trap in
situ-generated superacid HAlBr4 without deactivating AlBr3.
Highly bulky and highly basic 2,6-di-tert-butylpyridine (B1)
and less bulky and less basic 2,6-diiodopyridine (B4) were
suitable as such a base. In the presence of these bases, the
tandem reaction competed well with the decomposition and/or
polymerization of the substrates, despite the presence of strong
acids such as AlBr3 and 2,6-diiodopyridinium.
13. The pKa values of conjugate acids were calculated to be
5.5 (B1), −2.2 (B2), −2.8 (B3), and −1.9 (B4), using ACE
and
JChem
acidity
and
basicity
calculator
(https://epoch.uky.edu/ace/public/pKa.jsp).