Organic Letters
(
Figure 5). Analyses of the reaction stoichiometry with silver
Figure 5. Reaction stoichiometry analyses.
changes in the product yield after 5 h using more than 50 mol
%
of silver carbonate (Figure 5A). The yield of 2a was slightly
lower when 25 mol % of silver carbonate was employed, which
is consistent with the idea that the reaction requires a
stoichiometric amount of silver carbonate as an oxidant.
Contrastingly, with increased amounts of the copper catalyst,
an increase in the product yield was observed (Figure 5B). In
addition, the reaction using a stoichiometric amount of
a
Figure 6. Substrate scope. 100 mol % of Cu(OAc)
and 120 mol %
2
b
of 1,10-phenanthroline were used. Complex mixture.
a
Scheme 1. Functionalization of Dibenzoxaborins
Cu(OAc) without the ligand under an inert atmosphere
2
afforded only trace amounts of the product, while the reaction
9
partially proceeded in the presence of the ligand, indicating
that the optimized catalytic conditions facilitated the trans-
metalation step. These results suggest that the transmetalation
of the starting substrate with copper is likely the turnover-
limiting step, although the possibility for the reoxidation step
cannot be ruled out.
The optimized reaction conditions were applicable to the
deborylative ring contraction of a wide range of dibenzox-
aborins that were prepared according to our previously
8
reported method (Figure 6). Substrates bearing electron-
donating or -withdrawing groups at different positions of either
of their two benzene rings efficiently afforded the correspond-
ing dibenzofurans 2b−2j. The reaction of dibenzoxaborin with
substituents at both benzene rings similarly afforded
disubstituted dibenzofuran 2k. The method was also applicable
to multisubstituted substrates as demonstrated by the synthesis
of trisubstituted 2l, tetrasubstituted 2m, and more structurally
complicated naphtho[1,2-b]benzofuran 2n. Pyridine-fused
benzofuran 2o was also accessible using this method, although
stoichiometric amounts of copper catalyst were needed to
promote the reaction. Unfortunately, an attempt to prepare
dihydrobenzofuran 2p from dihydrobenzoxaborin under the
conditions was unsuccessful.
a
Key: (a) NBS (1.1 equiv), CH Cl , rt, 2 h; (b) I (1.0 equiv),
2 2 2
Ag SO (0.50 equiv), EtOH, rt, 30 min; (c) Pd(OAc) (5.0 mol %),
SPhos (10 mol %), 3 (1.1 equiv), K PO ·nH O (1.5 equiv), toluene,
6
equiv), toluene/Et N (1/1), rt, 3 h; (e) Cu(OAc) (10 mol %), 1,10-
phenanthroline (12 mol %), Ag CO (1.5 equiv), EtOH/H O (20/1),
4
2
4
2
3
4
2
5 °C, 18 h; (d) PdCl (PPh ) (5.0 mol %), CuI (10 mol %), 4 (3.0
2 3 2
3
2
2
3
2
0 °C, 15 h.
The prior functionalization of unsubstituted dibenzoxaborin
a followed by the deborylative ring contraction enabled facile
while leaving the C−B bonds untouched. All functionalized
dibenzoxaborins 1q−1t were efficiently transformed to
dibenzofurans 2q−2t by the deborylative ring contraction,
evidencing the broad scope of the method.
1
synthesis of various functionalized dibenzofurans (Scheme 1).
For example, bromination and iodination of 1a afforded the
corresponding 2-halogenated products 1q and 1r, respectively,
with high regioselectivity. The brominated position of 1q was
confirmed by X-ray crystallography and NMR analyses. The
halogenated dibenzoxaborins 1q and 1r were further
derivatized by Pd-catalyzed cross-coupling reactions to afford
the corresponding cross-coupling products such as 1s and 1t
Using the deborylative ring contraction in combination with
8
the dibenzoxaborin synthesis, regiodivergent synthesis of
dibenzofurans was achieved starting from the same o-borylated
phenol. For example, o-borylated estradiol derivative 5, easily
prepared by Ir-catalyzed o-borylation of phenols, was
converted to dibenzoxaborin 1u by the boron-selective
12
1
3
C
Org. Lett. XXXX, XXX, XXX−XXX
Sumida, Yuto
