B(C6F5)3-catalyzed silylation versus reduction of phosphonic and phosphinic esters with hydrosilanes

Article abstract of DOI:10.1016/S0040-4039(02)01161-9

HSiR₃/cat-B(C₆F)₃ induced dealkylation or reduction of esters of phosphorus at 20°C. A specific conversion to silylesters occurred by reaction with tertiary silanes. In contrast, free phosphines were observed in the reaction with mono- or disubstituted silanes. A mechanism was proposed to rationalize these results.

Full text of DOI:10.1016/S0040-4039(02)01161-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 5569–5571
B(C F ) -catalyzed silylation versus reduction of phosphonic and
6
5 3
phosphinic esters with hydrosilanes
a,
a
b
b
Jean-Marc Denis, * Henrietta Forintos, Helga Szelke and Gy o¨ rgy Keglevich
a
Universit e´ de Rennes I, CNRS-UMR 6510, Campus de Beaulieu, F35042 Rennes, France
b
Department of Organic Chemical Technology, Budapest University of Technology and Economics, 1521 Budapest, Hungary
Received 10 May 2002; accepted 13 June 2002
Abstract—HSiR /cat-B(C F) induced dealkylation or reduction of esters of phosphorus at 20°C. A specific conversion to
3
6
3
P-silylesters occurred by reaction with tertiary silanes. In contrast, free phosphines were observed in the reaction with mono- or
disubstituted silanes. A mechanism was proposed to rationalize these results. © 2002 Elsevier Science Ltd. All rights reserved.
9
Tris(pentafluorophenyl)borane is a commercially avail-
in organophosphorus chemistry. Unfunctionalized
phosphines have been obtained by reduction of the
able Lewis acid of comparable strength to BF , but
3
10,11
without the problems resulting from the B-F reactivity.
Application of this water-tolerant reagent in organic
corresponding acids or esters by silanes,
LiAlH . When a functional group was present in posi-
tion a to the phosphorus, alane and dichloroalane were
found to be the most selective reducing agents. The
protocol involving hydrosilanes remained the most sim-
ple method to deoxygenate the PꢀO bond. However,
the elevated temperature required (>120°C) limited
their application to the preparation of stable and non-
or by
4
1
synthesis is rapidly growing. The most comprehensive
1
2
results concerned the use of the anhydrous B(C F ) as
6
5 3
a catalyst associated with various HSiR reagents. The
3
B(C F ) /HSiR reagent combination was used in the
6
5 3
3
selective hydrosilation of the carbon–oxygen double
2
bond, in the synthesis of silylethers from the corre-
3
sponding alcohol, during the cleavage of aryl and alkyl
volatile phosphines. We assumed that the strong SiꢀH
4
2,4
ethers and in the course of direct reduction of
bond activation induced by B(C F )
should decrease
6
5 3
aliphatic- aldehyde-, ester- and carboxylic functions
the reduction temperature thus allowing access to
volatile and unstabilized phosphines. In this paper, it is
shown that the chemoselectivity of the reduction pro-
5
into a methyl group. Also stable organophosphoryl
adducts of B(C F ) with ligands such as phosphine-
6
5 3
6
†
oxides, phosphonate and phosphinate esters and
organophosphine adducts have been recently synthe-
sized and characterized.
cess involving the HSiR /cat.-B(C F ) system was
3
6 5 3
7,8
strongly dependent on the structure of hydrosilane:
dealkylation of phosphonic and phosphinic esters was
observed with trisubstituted silanes while reduction to
free phosphines was observed with mono- or disubsti-
To our knowledge, chemical transformations of
organophosphorus derivatives involving B(C F ) have
‡
6
5 3
tuted silanes. A mechanism was proposed to explain
never been described previously. We decided to evalu-
ate the potential of this Lewis acid as a catalyst in
organophosphorus chemistry. This paper shows the
preparation of some primary and secondary free phos-
phines from the corresponding phosphonic and phos-
phinic esters. Free phosphines are very sensitive
towards oxidation. They are usually stored as air-stable
phosphine/borane complexes which are useful reagents
these results.
Dealkylation process. A stable CH PO(OMe) /B(C F )
3
2
6 5 3
8
complex was formed by the addition of 2 mol% of
B(C F ) in solution to methylphosphonate 1a in tolu-
6
5 3
†
‡
The use of anhydrous grade B(C F ) was critical (sublimation at
6
5 3
1
05°C under vacuum (0.02 mbar). All the manipulations should be
carried out under a neutral gas in dry solvents and reagents.
Esters 1, 3 or 6 were added dropwise at ambient temperature to a
mixture containing 1–5 mol% B(C6F5)3 and the corresponding
silane (1–4 equiv.) in toluene solution. Reaction time (1–3 days) was
Keywords: homogeneous catalysis; transesterification; reduction;
hydrosilane; Lewis acidity; arylborane.
*
Corresponding author. Tel.: +33 23 23 62 79; e-mail:
3
1
jean-marc.denis@univ-rennes1.fr
monitored by P NMR.
0
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01161-9

5
570
J.-M. Denis et al. / Tetrahedron Letters 43 (2002) 5569–5571
ene containing 4 equiv. of HSiEt at 20°C. No further
involving 6a was incomplete (40% conversion). This
3
reaction was observed even after heating this solution
at 100°C for several days. The situation dramatically
changed when 1a was slowly added to a toluene solu-
tion of HSiEt /cat.-B(C F ) ; the silylester 2a was the
methodology is easy to perform as compared to that
1
2
involving alane derivatives as the reducing agent.
3
6 5 3
only product observed (Eq. (1)).
(
(
3)
4)
(
(
1)
2)
Mechanism. Crossover experiments provided solid sup-
ports for an unusual mechanism involving SiꢀH bond
This reaction was extended to the preparation of silyl
phosphonates 2b–2d at room temperature bearing vari-
ous functional groups (Eq. (1)). A similar process was
observed with diphenylmethylsilane. Thus phosphonate
§
+
−
activation through the ate-complex [R%%Si] [HB(C F )]
3
6 5
2,4
8
as the intermediate.
In our experiments, the
silylester 10 was formed by coordination of the silice-
nium ion to the oxygen atom of POEt via the oxonium
ion 9, followed by hydrid transfer and evolution of
ethane (Scheme 1, part A). Ester 10 should be consid-
ered as the common primary product. Silylesters 2 and
3
was converted to the corresponding silylester 4 in a
nearly quantitative yield (Eq. (2)). All these compounds
were fully stable in the crude mixture at 100°C for a
few days. No traces of the free phosphine could be
observed. Silylesters are versatile intermediates in
organophosphorus chemistry and their preparation and
4
were only observed in the absence of an additional
SiꢀH bond. In contrast, formation of free phosphines 5
and 7 was consistent with an internal hydrid transfer
from the activated SiꢀH bond to the phosphorus (inter-
mediate 11a) followed by reduction of the phosphine
13
synthetic applications are well documented. This
methodology is an interesting alternative to previous
methods since the yields are excellent and the protocol
is very easy to perform.
18
oxide (Scheme 1, part B). The by-products observed
during the reduction of phosphonates 1 can result from
a competitive reduction involving the adjacent P-OEt
group (intermediate 11b). A transient cyclic silylester
formed by hydride transfer and evolution of ethane was
¶
Reduction. The reduction of phosphonates 1a–c to free
phosphines 5a–c was performed with PhSiH3 or
Ph SiH (4 equiv.) in the presence of 3–5 mol%
2
2
B(C F ) (Eq. (3)). The reaction was generally slow at
6
5 3
18
proposed tentatively (Scheme 1, part C). Selectivity
can be expected in the absence of another POEt group.
Free phosphines 7 were the main products in the reduc-
tion of phosphinates 6a–c and H-phosphinate 6d. The
incomplete reduction of ester 6c was attributed to a
2
0°C (2–4 days). No change could be observed by
increasing the temperature. Yields were generally better
with PhSiH . The formation of unidentified by-prod-
3
ucts could not, however, be avoided. Yields fell in the
range of ca. 25% (for 5a,c) to 95% (for 5b,d). On the
other hand, the reduction of phosphinates 6a–c and
H-phosphinate 6d occurred with only 1 equiv. of
competitive formation of the B(C F ) /6c complex
6 5 3
occurring when the reduction was too sluggish. The
above mechanism was in good agreement with the
experimental data observed.
1
6
phenylsilane (Eq. (4)). Phosphines 7a,b and 7d were
formed in an efficient way (85% yields) but the reaction
Scheme 1.
§
All the new products were fully characterized by 31P, 13C, and 1H NMR data, as well as HRMS.
¶
Phosphines 5 and 7 or their borane complexes were characterized on the basis of their H and 31P NMR data as compared with those of
1
authentic samples.1
4–17
Yields of free phosphines were evaluated from NMR data. The reaction involving the inflammable PH 1d was performed
3
in a sealed NMR tube.

J.-M. Denis et al. / Tetrahedron Letters 43 (2002) 5569–5571
5571
In summary, these preliminary results demonstrate well
house, M. B. J. Chem. Soc., Dalton Trans. 2001, 1768–
1772.
the potential of the HSiR /cat-B(C F) system in silyla-
3
6
3
tion reactions and in the reduction of esters of phos-
phorus to free phosphines. The efficiency and the
simplicity of the protocol for the silylation of P-esters
make this method a useful alternative to the previously
described approaches. On the other hand, the practical
and simple reduction of phosphonates and phosphi-
nates to free phosphines is also of synthetic value. In
some instances, no work-up is necessary. This process
remains, however, mainly reserved to the structures
bearing no other reducible group in proximity of the
target P-OR function. The proposed mechanism is in
good agreement with the experimental data.
7. Bradley, D. C.; Hursthouse, M. B.; Motevalli, M.; Zheng,
D. H. J. Chem. Soc., Chem. Commun. 1991, 7–8.
8. Bradley, D. C.; Harding, I. S.; Keefe, A. D.; Motevalli,
M.; Zheng, D. H. J. Chem. Soc., Dalton Trans. 1996,
3931–3936.
9. For a recent review, see Carboni, B.; Monnier, L. Tetra-
hedron 1999, 55, 1197–1248.
10. Fritzsche, H.; Hasserodt, F.; Korte, F. Chem. Ber. 1965,
98, 1681.
11. See for example Soulier, E.; Yaouanc, J.-J.; Laurent, P.;
des Abbayes, H.; Cl e´ ment, J.-C. Eur. J. Org. Chem. 2000,
3497–3503.
1
2. (a) Gaumont, A.; Denis, J.-M. Chem. Rev. 1994, 94,
413–1439; (b) Keglevich, G.; Gaumont, A.-C.; Denis,
J.-M. Heteroat. Chem. 2001, 12, 161–167.
13. Review: Wozniak, L.; Chojnowski, J. Tetrahedron 1989,
5, 2465–2524.
1
References
4
1
. Reviews: (a) Piers, W. E.; Chivers, T. Chem. Soc. Rev.
14. (a) Cabioch, J.-L.; Pellerin, B.; Denis, J.-M. Phosphorus
Sulfur Silicon 1989, 44, 27–32; (b) Guillemin, J.-C.;
Janati, T.; Guenot, P.; Savignac, P.; Denis, J.-M. Angew.
Chem., Int. Ed. Engl. 1991, 30, 196–198.
1
997, 26, 345–354; (b) Ishihara, K.; Yamamoto, H. Eur.
J. Chem. 1999, 527–538.
2
3
4
5
6
. Parks, D. J.; Blackwell, J. B.; Piers, W. E. J. Org. Chem.
2
000, 65, 3090–3098 and references cited therein.
15. Bourumeau, K.; Gaumont, A.-C.; Denis, J.-M. J.
. Blackwell, J. M.; Foster, K. L.; Beck, V. H.; Piers, W. E.
Organomet. Chem. 1997, 529, 205–213.
J. Org. Chem. 1999, 64, 4887–4892.
16. Benie, A. Ph. D. Thesis, University of Rennes, 1999.
17. Gaumont, A.-C.; Morise, X.; Denis, J.-M. J. Org. Chem.
1992, 57, 4292–4295.
. Gevorgyan, V.; Rubin, M.; Benson, S.; Lui, J.-X.;
Yamamoto, Y. J. Org. Chem. 2000, 65, 6179–6186.
. Gevorgyan, V.; Rubin, M.; Liu, J.-X.; Yamamoto, Y. J.
Org. Chem. 2001, 66, 1672–1675.
18. An analogous process was proposed for a CꢀOR bond
cleavage catalyzed by InCl ; Onishi, M. Y.; Ueba, M.;
3
. Beckett, M. A.; Brassington, D. S.; Light, M. E.; Hurst-
Miyai, T.; Baba, A. J. Org. Chem. 2001, 66, 7741–7744.