2104
F. A. Siqueira et al. / Tetrahedron Letters 51 (2010) 2102–2105
OMe
Cl
N2BF4
O
Cl
Cl
Pd(OAc)2 (10 mol% )
MeOH, rt, 16 h
O
16a
O
17
64% yield
16
Scheme 1. Pd-catalysed domino Heck–Matsuda-hydroalkoxylation reaction.
O
X
II
OEt
AcOH + NaBF4
N2BF4
a) NOBF4
CH3CN, -10 oC, 30 min
NH2
Pd(0)Ln
I
O
X = O or NR
NaOAc
Oxidative
addition
b) 10 mol% Pd(OAc)2/CO
CH3CN, rt, 16 h
3
OEt
X
19
18
HPd+(II)Ln
VIII
BF4
53-67% yield
III
N2Pd+(II)Ln
BF4
Scheme 2. Intramolecular Heck–Matsuda of a
a,b-unsaturated ester.
X
X
N2
VI
β - Hydride
elimination
Table 5
Cyclisation of aryl allyl methylcarbamatesa
X
IV
Pd+(II)Ln BF4
Entry
Substrate
Product
X
V
VII
O
N
O
O
O
Olefin Insertion
BF4
Pd+(II)Ln
N
isomerization
1
N2BF4 20
Figure 1. Postulated catalytic cycle for the intramolecular Heck–Matsuda reaction.
21(85%)a
O
O
O
O
nium salts with olefins can be achieved. The intramolecular Heck–
Matsuda reaction provided benzofuran derivatives which, depend-
ing on the reaction conditions employed, could be further trans-
N
N
2
3
22
24
N2BF4
O
23(57%)a
formed into carboxylic acid derivatives via
a carbonylation
O
O
O
process. The scope of this methodology was also successfully ex-
panded to include the synthesis of indole, tetrahydronaphthalene,
chromene and chroman derivatives. Further studies are underway,
including reaction optimisation and studies towards discerning the
reaction mechanism and these will be reported in due course.
N
N
25
N2BF4
(41%)a
(19%)b
O
O
N
O
O
Acknowledgements
N
4
5
27
This work was supported by Grants from the Research Support-
ing Foundation of the State of São Paulo (FAPESP) and the Brazilian
National Research Council (CNPq). The authors also thank FAPESP
and CNPq for fellowships.
26
N2BF4
(32%)a
(41%)b
O
O
N
O
O
N
Supplementary data
29
28
MeO
MeO
N2BF4
(45%)a
(77%)b
Supplementary data (experimental procedures and selected
NMR spectra) associated with this article can be found, in the on-
a
Reaction conditions: 10 mol % Pd(OAc)2, NaOAc (3 equiv), MeOH, 50 °C, 0.5 h.
5 mol % Pd(OAc)2, Mo(CO)6 (1.5 equiv), NaOAc (3 equiv), MeCN, 25 °C, 2.5 h.
b
References and notes
1. For selected reviews, see: (a) Shibasaki, M.; Vogl, E. M.; Ohshima, T. Adv. Synth.
Catal. 2004, 346, 1533; (b) Negishi, E.-i.; Copéret, C.; Ma, S.; Liou, S.; Liu, F.
Chem. Rev. 1996, 96, 365; (c) Dounai, A. B.; Overman, L. E. Chem. Rev. 2003, 103,
2945; (d) Sato, Y.; Sodeoka, M.; Shibasaki, M. J. Org. Chem. 1989, 54, 4738; (e)
Carpenter, N. E.; Kucera, D. J.; Overman, L. E. J. Org. Chem. 1989, 54, 5846; (f)
Link, J. T. Org. React. 2002, 60, 157.
2. For reports on intramolecular Heck coupling reactions of aryl bromides, see: (a)
Michael, J. P.; Chang, S. F.; Wilson, C. Tetrahedron Lett. 1993, 34, 8365; (b)
Pigeon, P.; Decroix, B. Tetrahedron Lett. 1996, 37, 7707; (c) Mamouni, A.; Daïch,
A.; Decroix, B. Synth. Commun. 1998, 28, 1839; (d) Kiewel, K.; Tallant, M.;
Sulikpwski, G. A. Tetrahedron Lett. 2001, 42, 6621; (e) Cacchi, S.; Fabrizi, G.
Chem. Rev. 2005, 105, 2873.
3. For reports on intramolecular Heck coupling reactions of aryl iodides, see: (a)
Joseph, B.; Cornec, O.; Mérour, J.-Y. Tetrahedron 1998, 54, 7765; (b) Huang, Q.;
Fazio, A.; Dai, G.; Campo, M. A.; Larock, R. C. J. Am. Chem. Soc. 2004, 126, 7460;
(c) Arnold, L. A.; Luo, W.; Guy, R. K. Org. Lett. 2004, 6, 3005; (d) Majumdar, K. C.;
Chattopadyay, B.; Nath, S. Tetrahedron Lett. 2008, 49, 1609.
um salt 28, changing the reactions’ conditions in a similar way had
a more drastic effect in significantly improving yield (entry 5).
A plausible mechanism is proposed and depicted in Figure 1.
Starting from Pd(0) I, an arylazopalladium complex III would ini-
tially form via oxidative addition but rapidly extrude nitrogen
and generate cationic complex IV. Next, olefin insertion takes place
and affords alkyl metal complex V with subsequent palladium
beta-hydride elimination to give Heck adduct VI and hydridopalla-
dium(II) complex VIII. Finally, heterocycle VI isomerises to the
lower energy isomer VII. The tetrafluoroborate counteranion is re-
placed by an acetate anion, and Pd(0) complex I is reconstituted.
In summary, using either Pd(OAc)2 in an atmosphere of CO or
Pd2dba3 as a catalyst, the intramolecular cyclisation of arenediazo-