6
7
addition processes, as well as in metal-mediated couplings.
8
Two recent reviews highlight the importance of protecting
secondary phosphines in the synthesis of chiral phosphine
ligands as well as the expanded range of controlled synthetic
possibilities available with phosphine-borane complexes in
comparison to the parent phosphines. Secondary phosphine-
boranes are generally obtained by one of the following
Scheme 1
procedures: reaction of the parent free phosphine with BH ‚
3
9
THF or BH
3
‚SMe
2
complexes; directly from parent phos-
in the presence
4 3
of NaBH and CeCl ; or by reacting parent phosphine
phine oxides by in situ reduction with LiAlH
4
10
oxides with a large excess of BH
3
2
‚SMe together with a small
amount of water.11 Recently, a general synthesis of phos-
phine-borane complexes from parent-free phosphines and
12
4
NaBH /acetic acid has also been reported.
In this paper, we report a very efficient and general one-
pot synthesis of symmetrical and asymmetrical secondary
phosphines and the possible in situ conversion into their
borane complexes in high yields, under mild conditions.
Importantly, the proposed procedure completely avoids the
need to handle obnoxious and air-sensitive secondary phos-
phines and involves the recycling of the byproduct 6 into
the starting reagent 1.
Recently, we reported a new procedure for synthesizing
the tertiary cyclic phosphines 3 and their sulfides 4. This
synthesis uses a reagent that we developed,14 the benzothia-
diphosphole 1, which is easily obtained by treatment of
p-methylthioanisole with PCl and AlCl (Scheme 1). In
3 3
More recently, we studied17 the reaction between reagent
and mono-Grignard reagents. The symmetric tertiary
phosphines 7 (R ) R′ and R * R′), or their corresponding
sulfides 8, were obtained in very high yield, and the
asymmetric tertiary 9, or their sulfides 10, were obtained in
13
1
4
5% yield by addition of equimolar amounts of different
Grignard reagents to a solution of 1 in two steps, as depicted
particular, the simultaneous or the sequential addition of
equimolar amounts of a bis(Grignard reagent) 2 (n ) 1, 2)
and a mono-Grignard reagent RMgBr (R ) alkyl, phenyl,
alkenyl) to 1 equiv of 1 gives tertiary cyclic phosphines 3
and, after addition of elemental sulfur, affords the sulfides
in Scheme 1.The yields obtained are very close to the
1
7
maximum value dictated by statistical factors.
These findings prompted us to examine whether a similar
one-pot procedure, employing the same phosphorus atom
donor reagent 1, could be used to synthesize symmetric and
asymmetric diaryl-, arylalkyl-, and dialkylphosphines and
their borane complexes.
4
in good yields at room temperature (Scheme 1).
We additionally reported15 a very efficient and atom-
economic new method for the one-pot preparation of
secondary cyclic phosphines 5 (five- and six-membered) in
The simultaneous addition, at room temperature, of
70-80% yields, as shown in Scheme 1.
1
equivalent amounts of the Grignard reagents R MgBr and
The reaction pathways underlying these synthetic proce-
2
R MgBr to a solution of benzothiadiphosphole (1) gave, after
dures were tentatively explained in terms of the intervention
18a
of hypervalent phosphorus intermediates.16
quenching with acidic water, symmetrical or asymmetrical
secondary phosphines 11 and compound 6 (Scheme 2). The
intermediate of this reaction might be a hypervalent phos-
phorus species such as A, as reported elsewhere.17 The
proposed synthesis affords symmetrical secondary phos-
(
6) Leautey, M.; Deliencourt, G. C.; Jubault, P.; Pannecoucke, X.;
Quirion, J. C. Tetrahedron Lett. 2002, 43, 9237-9240.
7) Al-Masum, M.; Kumaraswamy, G.; Livinghouse, T. J. Org. Chem.
000, 65, 4776-4778.
8) (a) Brunel, J. M.; Faure, B.; Maffei, M. Coord. Chem. ReV. 1998,
78-180, 665-698. (b) Ohff, M.; Holz, J.; Quirmhach, M.; Borner, A.
(
2
(
1
2
phines (R ) R ) in high yield (80-85%) and asymmetrical
1
1
2
Synthesis 1998, 1391-1415.
9) Beres, J.; Dodds, A.; Morabito, A. J.; Adams, R. M. Inorg. Chem.
971, 10, 2072-0.2074.
10) (a) Imamoto, Kusumoto, T.; T.; Suzuki, N.; Sato, K. J. Am. Chem.
secondary phosphines (R * R ) in yields close to the
(
17
maximum value of 50% imposed by statistical factors. To
overcome this statistical limit and to obtain the highest yields
of asymmetric secondary phosphines, we carried out the
1
(
Soc. 1985, 107, 5301-5303. (b) Imamoto, T.; Oshiki, T.; Onozawa, T.;
Kusumoto, T.; Sato, K. J. Am. Chem. Soc. 1990, 112, 5244-5252.
1
8a
(
(
(
11) Stankevic, M.; Pietrusiewicz, K. M. Synlett. 2003, 7, 1012-1016.
12) McNulty, J.; Zhou, Y. Tetrahedron Lett. 2004, 45, 407-409.
13) (a) Baccolini, G.; Boga, C.; Negri, U. Synlett. 2000, 1685-1687.
reaction with sequential addition of the two different
RMgBr reagents. We found that, with this procedure,
asymmetric secondary phosphines were obtained in 70-75%
yields. It is worth noting that the yields obtained using this
procedure are, to our knowledge, the best obtained to date
(
b) Baccolini, G.; Boga, C.; Buscaroli, R. A. Eur. J. Org. Chem. 2001,
3
421-3424.
(
14) (a) Baccolini, G.; Mezzina, E.; Todesco, P. E.; Foresti, E. J. Chem.
Soc., Chem. Commun. 1988, 304-305. (b) Baccolini, G.; Beghelli, M.;
Boga, C. Heteroatom Chem. 1997, 8, 551-556. (c) Gang Wu, R.;
Wasylishen, E.; Power, W. P.; Baccolini, G. Can. J. Chem. 1992, 72, 1229-
(16) The formation of hypervalent phosphorus intermediates was ob-
served by 31P NMR spectroscopy as reported previously in ref 13b.
(17) Baccolini, G.; Boga, C.; Mazzacurati, M. J. Org. Chem. 2005, 70,
4774-4777.
1
235.
(15) Baccolini, G.; Boga, C.; Galeotti, M. Angew. Chem., Int. Ed. 2004,
4
3, 3058-3060.
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Org. Lett., Vol. 8, No. 8, 2006