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D. Albanese et al.
LETTER
In a control experiment TBAF◊3H2O was dehydrated by In conclusion we developed a simple procedure for the
gently heating for 31 hours under sonication and high vac- phosphine-free palladium catalysed homocoupling of aryl
uum.11 TBAF◊0.5H2O thus obtained generated the homo- halides. Further work is under way in order to determine
coupled product 11b in 68% yield in 20 h (entry 4). It the exact nature of the process and will be reported in due
seems likely that the slower reaction can be ascribed to a course.
partial decomposition to the less reactive TBABF (2) tak-
ing place during dehydration of the commercial salt.11 In
Acknowledgement
fact under the same reaction conditions TBABF afforded
Financial support by CNR and MURST is gratefully acknowledged.
55% of 11b in 7 hours without complete conversion of 9b
(entry 5). The use of tributylamine (12) instead of 8 or
Pd(OAc)2 as a substitute for 10 gave lower yields (entries
6-8).
References and Notes
(1) Semmelhack, M. F.; Helouit, P. M.; Jones, L. D. J. Am. Chem.
Soc. 1971, 93, 5308.
(2) Farina, V. Pure Appl. Chem. 1996, 68, 73.
(3) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
(4) Neguishi, E.; Acc. Chem. Res. 1982, 15, 340.
(5) Penalva, V.; Hassan, J.; Lavenot, L.; Gozzi, C.; Lemaire, M.
Tetrahedron Lett. 1998, 39, 2559.
A representative list of aryl halides 9a-g bearing various
functionalities were converted into the corresponding
biaryl derivatives 11a-g in moderate to good yield (Table
2). Iodo- and bromoaryls can be successfully used. 4-Bro-
moacetophenone (9f) generated 4,4’-diacetylbiphenyl
(11f) in a 75% yield, whereas the homocoupling of 2-bro-
moacetophenone (9g) did not take place indicating that
the reaction is sensitive to steric hindrance.
(6) Albanese, D.; Landini, D.; Penso, M. Tetrahedron Lett. 1995,
36, 8865.
(7) A solution of 4b (280 mg, 1 mmol) in benzene (1 ml) is added
dropwise to 2 (281 mg, 1 mmol) dissolved in 1 ml of benzene.
After stirring under nitrogen at 20°C for 20 h the solvent is
evaporated and the residue treated with hexane/benzene 3/1 (3
x 2 ml) and decanted. The solid residue is then filtered affor-
ding 406 mg (yield 85%) of 5b, mp 68-69°C (lit. ref. 7, 70°C).
The same procedure afforded 470 mg of 5a (yield 87%), mp
150°C, by direct filtration of the reaction mixture followed by
washing the solid with 1 ml of benzene.
(8) Christe, K. O.; Wilson, W. W.; Wilson, R. D.; Bau, R.; Feng,
J. J. Am. Chem. Soc. 1990, 112, 7619.
(9) Karl-Fischer titration of different batches of TBAF◊nH2O in-
dicated a hydration range n = 0.6-0.7.
(10) 2 (1.13 g, 4 mmol) is dissolved in 2 ml of THF and added
dropwise to a THF (2 ml) solution of 4c (625 mg, 4 mmol).
After stirring under nitrogen for 1h at 20°C the mixture was
evaporated affording 1.04 g of TBAF (95%). This material
was used for the homocoupling reaction. [PdCl(p-C3H5)]2 10
(3 mg, 0.0082 mmol) and the aryl halide (0.35 mmol) are ad-
ded to a DMSO (3 ml) solution of TBAF (8) (192 mg, 0.70
mmol). After stirring under nitrogen at 120°C for the time in-
dicated in Table 2, the mixture is cooled to room temperature,
treated with brine and extracted with Et2O (3 x 5 ml). The or-
ganic layer is washed with brine, dried (Na2SO4) and evapora-
ted. The residue is purified by silica-gel chromatography
affording biaryls 11a-f whose physical and spectroscopic data
match those reported in the literature.
(11) Albanese, D.; Landini, D.; Penso, M. J. Org. Chem. in press.
Synlett 1999, No. 2, 199–200 ISSN 0936-5214 © Thieme Stuttgart · New York