available and comprehensively screened, we set out to
prepare electron-deficient BINAP ligands containing CF3
groups attached to the phenyl rings. To take advantage of
the facile chiral resolution of the BINAP(O)2 system (2,2′-
bis(diphenylphospinyl)-1,1′-binaphthyl), we prepared the
electron-deficient BINAP(O)2 1a by quantitatively oxidizing
2a which was prepared by coupling diarylbromophosphine
with binaphthylmagnesium bromide in 70% isolated yield.6
Considerable difficulties were met in the reduction of the
electron-deficient BINAP(O)2 compound (1a) involving the
most commonly used reagent HSiCl3/Et3N. After several
failed attempts with other reduction methods [LiAlH4/NaBH4/
CeCl3, MeOTf/LiAlH4/DME, SmI2/THF, and (EtO)3SiH/
Ti(OiPr)4],4 the HSiCl3/Et3N system was investigated in great
detail. The monoxide (3a) was the only product isolated
(40%) along with the starting material (50%) under various
conditions. When pure monoxide 3a was further subjected
to the reducing reagent under anerobic conditions a mixture
of dioxide (1a) and BINAP (2a) was obtained with 70% of
monoxide (3a) being recovered (Scheme 1).
reduction reaction using HSiCl3/PPh3 (2 equiv) in the absence
of Et3N. To our delight the dioxide 1a was reduced to the
desired product 2a in 90% yield to achieve the first practical
preparation of this type of electron-withdrawing BINAP
ligands (Scheme 2).
Scheme 2
An 18O labeling study was carried out to investigate
in detail the oxygen transfer mechanism. 18O-labeled
BINAP(O)2 was prepared via coupling Grignard reagent with
18O-diarylphosphinyl chloride which was obtained from using
18O2. The reduction reaction was then performed using the
18O-labeled substrate and triphenylphosphine as the oxygen
acceptor. The reaction was stopped after 2 days since at the
end of the reaction virtually all of the triphenylphosphine
oxide generated was reduced under these conditions by the
excess silane. In fact, this probably helps to drive the oxygen
transfer to completion by pulling the equilibrium over in
favor of the formation of sacrificial triphenylphosphine oxide.
Consistent with the proposed oxygen transfer mechanism,
the 18O-labeled triphenylphosphine oxide was isolated and
characterized by mass spectrometry. To the best of our
knowledge, this is the first observation of an oxygen transfer
reaction between phosphorus atoms mediated by silane
(Scheme 3).8
Scheme 1. HSiCl3-Mediated Oxygen Transfer between
Phosphines
A control experiment in the absence of HSiCl3 was also
carried out, and no formation of the dioxide was observed.
These combined results are consistent with a proposal that
oxygen transfer takes place between the phosphine oxide and
phosphine under the influence of HSiCl3. This observation
suggested that if a sacrificial phosphine was added to the
reaction mixture it could alter the equilibrium to afford a
better yield of the reduced ligand. Therefore, triphenylphos-
phine was introduced into the reaction, and interestingly, the
dioxide 1a was reduced to a mixture of 2a (30%) and 3a
(60%). Two well-documented mechanisms could explain this
reactivity: either an intramolecular hydride transfer in the
absence of Et3N leading to retention of configuration or an
intermolecular hydride transfer promoted by Et3N with
inversion of configuration.7 Since an oxygen transfer pathway
can be conceived within the framework of the intramolecular
hydride transfer mechanism (Scheme 3), we carried out the
Scheme 3. Proposed Mechanism for HSiCl3-Mediated
Oxygen-Transfer Reaction
(5) (a) Mendez, N. Q.; Seyler, J. W.; Arif, A. M.; Gladysz, J. A. J. Am.
Chem. Soc. 1993, 115, 2323. (b) RajanBabu, T. V.; Ayers, T. A.;
Casalnuovo, A. L. J. Am. Chem. Soc. 1994, 116, 4101. (c) Casalnuovo, A.
L.; RajanBabu, T. V.; Ayers, T. A.; Warren, T. H. J. Am. Chem. Soc. 1994,
116, 9869. (d) Faller, J. W.; Nguyen, J. T.; Ellis, W.; Mazzieri, M. R.
Organometallics 1993, 12, 1434.
(6) The coupling of aryl Grignard with the commonly used phosphinyl
chloride gave poor yields under the optimized conditions.
(7) (a) Horner, L.; Balzer, W. D. Tetrahedron Lett. 1965, 6, 1157. (b)
Naumann, K.; Zon, G.; Mislow, K. J. Am. Chem. Soc. 1969, 91, 7012. (c)
Marsi, K. L. J. Org. Chem. 1974, 39, 265.
Electron-deficient triarylphosphine [C6H3(CF3)2]3P was
also tested as an oxygen acceptor but was found to be much
(8) An interesting example of intramolecular oxygen transfer reaction
between phosphine and arsine was reported previously: Cook, V. C.; Willis,
A. C.; Zank, J.; Wild, S. B. Inorg. Chem. 2002, 41, 1897. For examples of
oxygen transfer from epoxide to triphenyl phosphine-iodine complex,
see: Paryzek, Z.; Wydra, R. Tetrahedron Lett. 1984, 25, 2601. Sonnet, P.
E. Synthesis 1980, 828.
4676
Org. Lett., Vol. 6, No. 25, 2004