Copper-catalyzed synthesis of 1,3-butadienylphosphine oxides and chemoselective phosphoryl reduction to stereodefined 1,3-butadienylphosphines

Article abstract of DOI:10.1002/adsc.201300618

Copper-catalyzed CP cross-coupling reactions between diphenylphosphine oxide and various 1-bromo-1,3-butadienes readily provided a set of functionalized 1,3-butadienyldiphenylphosphine oxides. Application of the latter to the synthesis of stereodefined 1,3-butadienyldiphenylphosphines was successfully achieved via chemoselective reduction of their phosphoryl moiety.

Full text of DOI:10.1002/adsc.201300618

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DOI: 10.1002/adsc.201300618
Copper-Catalyzed Synthesis of 1,3-Butadienylphosphine Oxides
and Chemoselective Phosphoryl Reduction to Stereodefined
1,3-Butadienylphosphines
Jꢀrꢀmie Gatignol,a Carole Alayrac,a Jean-FranÅois Lohier,a Jorge Ballester,b
Marc Taillefer,b,* and Annie-Claude Gaumonta,*
a
Laboratoire de Chimie Molꢀculaire et Thioorganique, UMR CNRS 6507, INC3M FR3038, ENSICAEN & Universitꢀ de
Caen Basse-Normandie, 14050 Caen, France
Fax : (+33)-2-3145-2877; phone: (+33)-2-3145-2873; e-mail: annie-claude.gaumont@ensicaen.fr
Institut Charles Gerhardt Montpellier, UMR 5253 CNRS, ENSCM, 8 Rue de l’Ecole Normale, 34296 Montpellier Cedex
b
5, France
Fax : (+33)-4-6714-4319; e-mail: marc.taillefer@enscm.fr
Received: June 16, 2013; Published online: October 9, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300618.
À
Abstract: Copper-catalyzed C P cross-coupling re-
actions between diphenylphosphine oxide and vari-
ous 1-bromo-1,3-butadienes readily provided a set
of functionalized 1,3-butadienyldiphenylphosphine
oxides. Application of the latter to the synthesis of
and diphenylphosphine oxide (1). The targeted prod-
ucts are relevant molecules for applications in organic
synthesis,[8] as well as homogeneous catalysis.[9] They
could also be seen as potential precursors of the chal-
lenging butadienylphosphines if the chemoselective
reduction of the phosphoryl moiety was accessible.
Worthy of note is that the few reported syntheses of
1,3-butadienyldiphenylphosphine oxides are limited in
scope or poor-yielding.[10]
stereodefined
1,3-butadienyldiphenylphosphines
was successfully achieved via chemoselective reduc-
tion of their phosphoryl moiety.
Keywords: butadienylphosphine oxides; butadienyl-
phosphines; copper catalysis; C P coupling
Thus our goal was to develop a general and effi-
cient catalytic access to these promising versatile mol-
ecules and to apply them to the preparation of 1,3-bu-
tadienyldiphenylphosphines 4 (Scheme 1), valuable
new ligands in homogeneous catalysis,[11] through
developing a chemoselective reduction of the phos-
À
Copper-catalyzed carbon-heteroatom bond forming
ACHTUNGTRENNUNG
reactions have recently emerged as a useful and
cheaper alternative to the analogous palladium-cata- the cross-coupling reaction between the commercially
lyzed transformations, but studies so far have mostly available diphenylphosphine oxide (1) and readily
targeted the N, O, and S series.[1] In comparison C P available 1-bromo-4-phenyl-1,3-butadiene (2a) (E,E/
À
bond forming reactions have received only little at- Z,E=66:34),[12] under various catalytic conditions.
tention, and with a main focus on arylphosphorus The reactions were performed in toluene – a non-che-
compounds.[2–5] Regarding the vinyl series, most de- lating solvent – at 1108C in the presence of K2CO3
scribed procedures involve H-phosphonates as cou- (1.5 equiv.) as the base. A small excess (1.5 equiv.) of
pling partners,[3b,4a,6] while phosphines (2 reported ex-
amples of alkenylphosphines)[4a,5] or phosphine oxides
(1 reported procedure towards alkenylphosphine oxi-
des),[6a] are scarce and recent. No example with dienyl
derivatives was described in the literature.
As part of our research program related to the de-
[7]
À
velopment of catalytic C P bond forming reactions,
we report here a new and straightforward entry to
variously substituted 1,3-butadienyldiphenylphosphine
oxides 3 (Scheme 1) via copper(I)-catalyzed cross-
Scheme 1. Proposed strategy towards stereodefined 1,3-buta-
coupling reactions between 1-bromo-1,3-butadienes 2 dienylphosphines and their oxides.
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Copper-Catalyzed Synthesis of 1,3-Butadienylphosphine Oxides
Table 1. Influence of the nature of the copper source and
the structure of the ligand on the cross coupling reaction of
1 with bromodiene 2a.[a]
Table 2. Scope of the copper-catalyzed cross coupling reac-
tion of diphenylphosphine oxide (1) with 1-bromo-1,3-buta-
dienes 2.[a]
Entry Cu Source Ligand
Yield[b] [%]
1
2
3
CuI
CuI
CuI
DMEDA
proline
ethyl 2-cyclohexanone- 36
carboxylate
88 (88)
58
4
5
6
7
8
CuI
–
–
46
0
86 (72)
68
48
DMEDA
DMEDA
DMEDA
DMEDA
CuTC[c]
CuBr
CuBr2
[a]
Reaction conditions: 1 (303 mg, 1.5 mmol, 1.5 equiv.), 2a
(1 mmol, 1 equiv., E,E/Z,E=66:34), CuI (5 mol%),
ligand (20 mol%), K2CO3 (1.5 equiv.), toluene (4 mL),
1108C, 13 h.
[b]
[c]
Calculated from the crude product 1H NMR spectrum,
value in parentheses is isolated yield.
CuTC=copper thiophenecarboxylate.
[a]
Reaction conditions: 2a–f (1 mmol, 1 equiv.), 1 (303 mg,
1 was used to ensure a full conversion of 2a. Copper
iodide (5 mol%) was first selected as the copper
source and a set of well-established ligands in copper-
catalyzed reactions was screened with a 1:4 metal/
ligand ratio (Table 1).
1.5 mmol,
1.5 equiv.),
CuI
(5 mol%),
DMEDA
(20 mol%), K2CO3 (1.5 equiv.), toluene (4 mL), 1108C,
13 h.
Isomer ratio determined by 1H NMR.
Isolated yield.
[b]
[c]
A poor or modest reactivity of the catalytic systems
involving proline, or ethyl 2-cyclohexanonecarboxy-
late was observed (Table 1, entries 2 and 3). By con-
Electron-rich aryl-substituted bromodienes 2b and
trast
DMEDA
(N,N’-dimethylethylenediamine) 2c bearing an ortho- and para-anisyl group respective-
proved to be a very efficient ligand for this transfor- ly, were readily coupled with 1 in good to high yields
mation, and provided the targeted dienylphosphine (73 to 87%) (Table 2, entries 3 and 4). Similarly elec-
oxide 3a[13] in 88% yield as a mixture of stereoisomers tron-poor aryl-substituted bromodienes 2d and 2e
(Table 1, entry 1).
bearing a para-chlorophenyl and an ortho-nitrophenyl
À
In the absence of ligand the yield was much lower group respectively underwent efficient C P bond for-
(entry 4) and no reaction was observed if the copper mation. The corresponding 1,3-butadienylphosphine
catalyst was omitted (entry 5). Various combinations oxides 3d and 3e were isolated in 75 and 66% yields,
of the best ligand DMEDA with copper sources other respectively (Table 2, entries 5 and 6). Not only aryl-
than CuI were next evaluated but none of them was but also an alkyl-substituted bromodiene proved to
superior to the CuI/DMEDA catalytic system be a suitable coupling partner of 1 under the defined
(Table 1, entry 1 versus entries 6-8). After screening reaction conditions. Indeed bromodiene 2f was readi-
various catalyst loadings and metal/ligand ratios, the ly converted into butadienylphosphine oxide 3f,
catalytic conditions with CuI (5 mol%) and DMEDA which was isolated in 78% yield (Table 2, entry 7).
(20 mol%) were found to be optimal for the forma-
Regarding the stereoselectivity of the reaction, only
tion of butadienylphosphine oxide 3a. The reaction a slight isomerization of the double bond in favour of
conditions being optimized, we evaluated the scope the E-isomer was observed during the coupling reac-
and limitations of this reaction. For this purpose vari- tion (see Table 2). The use of stereochemically en-
ously substituted 1-bromo-1,3-butadienes 2b–f were riched (Z,E)-bromodiene 2a (2:98 isomer ratio) as
readily prepared,[12] and tested as cross-coupling part- substrate,[14] provided the corresponding butadienyl-
ners of diphenylphosphine oxide (1). The results are phosphine oxide 3a with a (E,E)/ACTHNUGRTNEUNG(Z,E) ratio of 10:90
collected in Table 2.
(Table 2, entry 2).
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Figure 1. X-ray structures of (Z,E)-1,3-butadienylphosphine oxides 3c (left) and 3f (right).
Worthy of note is that the (E,E) and (Z,E) isomers by increasing the oxophilic properties of the silane re-
of all butadienylphosphine oxides 3 could be separat- agent, probably accounts for its higher reactivity.
ed by column chromatography on silica gel. Single These mild reduction conditions were tested on
crystals of the (Z,E) isomers of 3c and 3f were ob- (E,E)-dienylphosphine oxide 3a using 1.5 equiv. of
tained by slow evaporation of solvent and their struc- PhSiH3 and 0.5 equiv. of PhSiCl3. The reaction proved
tures were unambiguously confirmed by X-ray dif- to be sluggish at 308C, but when the reaction temper-
fraction analysis (Figure 1).
ature was raised to 608C, a conversion superior to
With a set of variously functionalized and isomeri- 98% could be reached after 16 h, and no side-product
cally pure 1,3-butadienyldiphenylphosphine oxides in was formed. The resulting (E,E)-dienylphosphine 4a
hand, we next investigated the reduction of their was isolated in 92% yield after filtration on alumina
phosphoryl moiety. This would give access to 1,3-bu- (Table 3, entry 1).[21] The defined reaction conditions
tadienylphosphines, a relevant class of ligands for ho- were applied to isomerically pure dienylphosphine
mogeneous catalysis.[11,15,16] With the exception of the oxides 3a–f and proved to be general and chemoselec-
reduction product of 3a (R=Ph),[11,17] all other target- tive. Indeed whatever the substitution pattern of sub-
ed dienylphosphines are unprecedented.
strates 3, the phosphoryl reduction was successfully
Phosphoryl reduction is usually achieved under achieved without affecting the conjugated diene
rather harsh conditions involving hydride or silane re- moiety. The targeted stereopure aryl- or alkyl-dienyl-
agents.[18] Thus the chemoselective reduction of phos- diphenylphosphines 4a–f were delivered in high yields
phine oxides into phosphines in the presence of other ranging from 67 to 93% yield (Table 3) whatever the
reducible functions is highly challenging.[19] The diffi- substituents. This two-step procedure – copper(I)-cat-
À
culty is even greater when the phosphorus atom is di- alyzed C P cross-coupling followed by chemoselec-
rectly linked to the unsaturation like in alkenyl deriv- tive reduction of the phosphoryl moiety – thus offers
atives because the presence of the P=O function will an efficient and general access to stereodefined 1,3-
enhance the double bond reactivity towards reduc- butadienylphosphines.
tion. No successful example of selective reduction of
In conclusion, we have developed a new and
butadienylphosphine oxides into butadienylphos- straightforward catalytic synthesis of a variety of 1,3-
phines could be found in the literature. For example, butadienylphosphine oxides. It relies on the cop-
it was reported that the reduction of 1,3-butadienyldi- per(I)-catalyzed cross-coupling reaction between di-
phenylphosphine oxide with phenylsilane led to phenylphosphine oxide and readily available 1-
double bond hydrogenation.[10a] Other reducing agents bromo-1,3-butadienes. Separation of the (E,E) and
(LiAlH4, HSiCl3, Si2Cl6) also proved to be unsuitable (Z,E) stereoisomers was readily achieved through
for the chemoselective conversion into butadienyl- silica gel column chromatography. The reduction of
phosphines.[16] Previous work in our laboratories dem- the phosphoryl moiety could be chemoselectively ac-
onstrated that the use of a 3:1 mixture of phenylsilane complished for the first time and afforded a set of
and trichlorophenylsilane in THF at 308C was optimal stereopure 1,3-butadienylphosphines. Further explora-
for the phosphoryl reduction of alkynylphosphine tion of the potential of the 1,3-butadienylphosphine
oxides, another challenging substrate type.[20] The in oxides as building blocks through chemical modifica-
situ generated active reducing agent is believed to be tion of the diene moiety via Michael addition or
PhSiHCl2. The presence of chlorine atoms on silicon, Diels–Alder reactions will be the next goal of this
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Copper-Catalyzed Synthesis of 1,3-Butadienylphosphine Oxides
Table 3. Chemoselective reduction of 1,3-butadienylphos-
phine oxides 3 into 1,3-butadienylphosphines 4.[a]
drous, degassed toluene (2 mL) and DMEDA (22 mL,
20 mol%). The solution was stirred for 5 min. Then 1-
bromo-4-phenyl-1,3-butadiene (2a) (209 mg, 1 mmol,
1 equiv.) and diphenylphosphine oxide (303 mg, 1.5 equiv.)
were added at once followed by anhydrous, degassed tolu-
ene (2 mL) to rinse the tube walls. The Schlenk tube was
sealed with a Teflon valve and the reaction mixture was
stirred at 1108C for 13 h. After cooling the crude product
was purified by filtration on silica gel with ethyl acetate as
eluent to provide the desired product 3a; yield: 291 mg
(0.88 mmol, 88%); see the Supporting Information.
Typical Procedure for Phosphoryl Reduction of 1,3-
Butadienylphosphine Oxides 3 into 1,3-Buta-
dienylphosphines 4 (Synthesis of 4a as an Example,
Table 3, entry 1)
An oven-dried Schlenk tube was evacuated and refilled with
nitrogen three times and then charged with 4-phenyl-1,3-bu-
tadienylphosphine oxide (3a) (165 mg, 0.5 mmol, 1 equiv.)
followed by anhydrous, degassed THF (1 mL), phenylsilane
(92 mL, 1.5 equiv.) and phenyltrichlorosilane (40 mL,
0.5 equiv.). The solution was stirred at 608C for 16 h. The re-
action mixture was allowed to cool to room temperature
then degassed H2O was added. The organic layer was dried
on MgSO4 and after filtration and solvent evaporation, the
crude product was purified by filtration on alumina with
pentane as eluate to provide the desired product 4a; yield:
145 mg (0.46 mmol, 92%); see the Supporting Information.
CCDC 924259 and CCDC 924258 contain the supplemen-
tary crystallographic data for the (Z,E)-isomers of 3c and 3f
respectively. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
We gratefully acknowledge financial support from the
Agence Nationale de la Recherche, projet no. ANR-09-
CP2D-13-Phoscat, the Ministꢀre de la Recherche et des Nou-
velles Technologies, the CNRS (Centre National de la Recher-
che Scientifique), the Rꢁgion Basse-Normandie and the Euro-
pean Union (FEDER Funds). COST ACTION CM0802
“PHOSCINET” is also acknowledged for support.
[a]
Reaction conditions: 3a–f (0.5 mmol, 1 equiv.), PhSiH3
(92 mL,
0.75 mmol,
1.5 equiv.),
PhSiCl3
(40 mL,
0.25 mmol, 0.5 equiv.), THF (1 mL), 608C, 16 h.
Isolated yield.
[b]
work together with the use of the obtained new
family of butadienylphosphine ligands in catalysis.
References
[1] Recent reviews: a) D. S. Surry, S. L. Buchwald, Chem.
Sci. 2010, 1, 13; b) F. Monnier, M. Taillefer, Angew.
Chem. 2009, 121, 7088; Angew. Chem. Int. Ed. 2009, 48,
6954; c) F. Monnier, M. Taillefer, Angew. Chem. 2008,
120, 3140; Angew. Chem. Int. Ed. 2008, 47, 3096; d) G.
Evano, N. Blanchard, M. Toumi, Chem. Rev. 2008, 108,
3054.
Experimental Section
Typical Procedure for the Cross-Coupling Reaction
of Diphenylphosphine Oxide (1) with 1-Bromo-1,3-
butadienes 2 (Synthesis of 3a as an Example, Table 2,
entry 1)
À
[2] For a recent review on transition metal-catalyzed C P
cross-couplings, see: F. M. J. Tappe, V. T. Trepohl, M.
Oestreich, Synthesis 2010, 3037.
An oven-dried Schlenk tube was evacuated and refilled with
nitrogen 3 times and then charged with CuI (9.5 mg,
5 mol%) and K2CO3 (207 mg, 1.5 equiv.) followed by anhy-
[3] For selected examples of phosphine oxides or phos-
phite derivatives as coupling partners in the aryl series,
see: a) M. Stankevic, A. Wlodarczyk, Tetrahedron 2013,
Adv. Synth. Catal. 2013, 355, 2822 – 2826
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asc.wiley-vch.de
2825

Jꢀrꢀmie Gatignol et al.
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69, 73; b) R. Zhuang, J. Xu, Z. Cai, G. Tang, M. Fang,
Y. Zhao, Org. Lett. 2011, 13, 2110; c) M. Font, T. Parel-
la, M. Costas, X. Ribas, Organometallics 2012, 31, 7976;
d) N. B. Karlstedt, I. P. Beletskaya, Russ. J. Org. Chem.
2011, 47, 1011; e) N. T. McDougal, J. Streuff, H. Mu-
kherjee, S. C. Virgil, B. M. Stoltz, Tetrahedron Lett.
2010, 51, 5550; f) C. Huang, X. Tang, H. Fu, Y. Jiang,
Y. Zhao, J. Org. Chem. 2006, 71, 5020.
Chem. Commun. 2000, 1419; c) C. Abu-Gnim, I. Amer,
J. Organomet. Chem. 1996, 516, 235.
[10] a) F. Mathey, G. Muller, Can. J. Chem. 1978, 56, 2486,
and references cited therein; b) K. M. Pietrusiewicz, M.
Kuznikowski, M. Koprowski, Tetrahedron: Asymmetry
1993, 4, 2143; c) L.-B. Han, C.-Q. Zhao, M. Tanaka, J.
Org. Chem. 2001, 66, 5929; d) C. L. Ma, X. Y. Lu, Y. X.
Ma, Chin. Chem. Lett. 1995, 6, 747.
[11] H. Kaddouri, V. Vicente, A. Ouali, F. Ouazzani, M.
Taillefer, Angew. Chem. 2009, 121, 339; Angew. Chem.
Int. Ed. 2009, 48, 333.
[12] For the preparation of (E,E)-1-bromo-1,3-butadienes,
see: a) E. J. Corey, P. L. Fuchs, Tetrahedron Lett. 1972,
13, 3769; b) S. Abbas, C. J. Hayes, S. Worden, Tetrahe-
dron Lett. 2000, 41, 3215.
[4] For phosphines as coupling partners, see: a) D.
Gelman, L. Jiang, S. L. Buchwald, Org. Lett. 2003, 5,
2315; b) D. Van Allen, D. Venkataraman, J. Org. Chem.
2003, 68, 4590; c) K. Tani, D. C. Behenna, R. M.
McFadden, B. M. Stoltz, Org. Lett. 2007, 9, 2529.
À
[5] For tandem phosphine oxide reduction and C P cross-
coupling, see: Y. Li, S. Das, S. Zhou, K. Junge, M.
[13] M. I. Kabachnik, T. Y. Medved, E. I. Matrosov, Dokl.
Beller, J. Am. Chem. Soc. 2012, 134, 9727.
Akad. Nauk SSSR 1965, 162, 339.
[6] a) J. Hu, N. Zhao, B. Yang, G. Wang, L.-N. Guo, Y.-M.
Liang, S.-D. Yang, Chem. Eur. J. 2011, 17, 5516; b) G.
Evano, K. Tadiparthi, F. Couty, Chem. Commun. 2011,
47, 179; c) S. Thielges, P. Bisseret, J. Eustache, Org.
Lett. 2005, 7, 681; d) H. Xu, S. Gu, W. Chen, D. Li, J.
Dou, J. Org. Chem. 2011, 76, 2448.
[7] For Cu- or Pd-catalyzed cross coupling reactions re-
spectively, see for example: a) E. Bernoud, C. Alayrac,
O. Delacroix, A.-C. Gaumont, Chem. Commun. 2011,
47, 3239; b) D. Julienne, J.-F. Lohier, O. Delacroix, A.-
C. Gaumont, J. Org. Chem. 2007, 72, 2247; for catalytic
hydrophosphinations, see for example: c) B. Join, D.
Mimeau, O. Delacroix, A.-C. Gaumont, Chem.
Commun. 2006, 3249.
[8] For nucleophilic additions onto vinylphosphine oxides,
see for example: a) M. Stankevic, M. Jaklinska, K. M.
Pietrusiewicz, J. Org. Chem. 2012, 77, 1991; b) A. M.
Gonzalez-Nogal, P. Cuadrado, M. A. Sarmentero, Tet-
rahedron 2010, 66, 9610; c) B. Bartels, J. Clayden, C.
Gonzalez Martin, A. Nelson, M. G. Russell, S. Warren,
J. Chem. Soc. Perkin Trans. 1 1999, 1807; d) D. J. Fox,
J. A. Medlock, R. Vosser, S. Warren, J. Chem. Soc.
Perkin Trans. 1 2001, 2240; e) J. Clayden, A. Nelson, S.
Warren, Tetrahedron Lett. 1997, 38, 3471.
[14] For the preparation of (Z,E)-2a, see: J. Uenishi, R. Ka-
wahama, O. Yonemitsu, J. Tsuji, J. Org. Chem. 1998,
63, 8965.
[15] a) D. Julienne, F. Toulgoat, O. Delacroix, A.-C. Gau-
mont, Curr. Org. Chem. 2010, 14, 1195; b) Homogene-
ous Catalysis, Understanding the Art, (Ed.: P. W. N. M.
van Leeuwen), Kluwer Academic Publishers, Dor-
drecht, 2004; c) S. Ortial, D. A. Thompson, J.-L. Mon-
tchamp, J. Org. Chem. 2010, 75, 8166; d) S. Ortial, J.-L.
Montchamp, Org. Lett. 2011, 13, 3134.
[16] F. Mathey, F. Mercier, C. Santini, Inorg. Chem. 1980,
19, 1813.
[17] D. Gloyna, H. G. Henning, Angew. Chem. 1966, 78,
907; Angew. Chem. Int. Ed. Engl. 1966, 5, 847.
[18] For a comprehensive literature survey, see: K. V. Ra-
jendran, D. G. Gilheany, Chem. Commun. 2012, 48,
817, and references cited therein.
[19] Y. Li, L.-Q. Lu, S. Das, S. Pisiewicz, K. Junge, M.
Beller, J. Am. Chem. Soc. 2012, 134, 18325.
[20] J.-F. Pilard, G. Baba, A.-C. Gaumont, J.-M. Denis, Syn-
lett 1995, 1168.
[21] The reduction conditions reported during the course of
this work and involving tetramethyldisiloxane (TMDS)
as mild reducing agent (3 equiv.) in the presence of
a catalytic amount of copper(II) triflate (10 mol%) in
toluene at 1008C as described in ref.[5] were tested on
(E,E)-3a and afforded (E,E)-4a in 65% yield.
[9] For examples of hemilabile phosphine oxide ligands,
see: a) V. V. Grushin, Chem. Rev. 2004, 104, 1629; b) R.
Weber, U. Englert, B. Ganter, W. Keim, M. Mçthrath,
2826
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