Diphenylphosphinophenolate: A ligand for the palladium-catalysed silylation of aryl halides activating simultaneously both palladium and silicon

Article abstract of DOI:10.1039/b006165o

Diphenylphosphinophenolate was found to be an effective ligand for the palladium-catalysed silylation of aryl halides, activating not only palladium but also silicon of a disilane, where aryl bromides and iodides having such substituents as methyl, methoxy, amino, ethoxycarbonyl, trifluoromethyl, formyl or phenyl are applicable to the reaction with hexamethyldisilane to give the corresponding trimethylsilylarenes.

Full text of DOI:10.1039/b006165o

Diphenylphosphinophenolate: a ligand for the palladium-catalysed silylation of
aryl halides activating simultaneously both palladium and silicon
Eiji Shirakawa,*a Takuya Kurahashi,b Hiroto Yoshidab and Tamejiro Hiyama*b
a Graduate School of Materials Science, Japan Advanced Institute of Science and Technology, Asahidai,
Tatsunokuchi, Ishikawa 923-1292, Japan. E-mail: shira@jaist.ac.jp
b Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto, 606-8501,
Japan. E-mail: thiyama@NPC05.kuic.kyoto-u.ac.jp
Received (in Cambridge, UK) 31st July 2000, Accepted 25th August 2000
First published as an Advance Article on the web 18th September 2000
Diphenylphosphinophenolate was found to be an effective
ligand for the palladium-catalysed silylation of aryl halides,
activating not only palladium but also silicon of a disilane,
where aryl bromides and iodides having such substituents as
methyl, methoxy, amino, ethoxycarbonyl, trifluoromethyl,
formyl or phenyl are applicable to the reaction with
hexamethyldisilane to give the corresponding trimethylsi-
lylarenes.
The transition metal-catalysed silylation of aryl halides with
disilanes should be one of the most straightforward and
economical ways to arylsilanes,1,2 which are versatile synthetic
precursors especially for the palladium-catalysed cross-coup-
ling reaction with organic electrophiles.3 Although there have
been many reports on the palladium-catalysed silylation using
Scheme 2
triphenylphosphine as a ligand,1 the reaction requires drastic
conditions to obtain arylsilanes in acceptable yields. Low
reaction rate is ascribed to the low reactivity of a disilane toward
a Pd(II) complex that is generated by oxidative addition of an
aryl halide to a Pd(0) complex. In order to accelerate the rate-
mixture of [PdCl(p-C3H5)]2–PO (5 mol% of Pd) and sodium
hydroxide (1.2 mol) in a 1+1 mixture of THF and H2O at 100 °C
for 4 h to give trimethylsilylbenzene (8a) in 80% yield.
Reduction of the amount of sodium hydroxide (0.1 mol, 9%; 0
mol, 0%), masking the hydroxy group of PO as 9, or use of
triphenylphosphine (10) in lieu of PO decisively reduced the
yield, implying the significance of the phenolate moiety,
whereas inefficiency of p-hydroxy derivative 11 revealed the
importance of the o-phenolate anionic functionality. Amino-
phosphine 12 also was ineffective. These results clearly
demonstrate that the catalytic cycle depicted in Scheme 1 is
working smoothly with the Pd–PO catalyst.
The applicability of the catalytic system was proved by
trimethylsilylation of various aryl bromides and iodides
(Scheme 3 and Table 1). Trimethylsilylbenzene and 4-trime-
thylsilyltoluene were obtained in high yields from bromo-
benzene and 4-bromotoluene, respectively (Table 1, entries 1
and 2). Although an electron-donating substituent like methoxy
or amino on the aryl bromide decreased the yield, as was the
case with a triphenylphosphine ligand,1a use of a 1+1 mixture of
toluene and H2O as the solvent in combination with 10 mol%
tetrabutylammonium bromide afforded the corresponding ar-
ylsilanes in yields over 90% (entries 3–6). The toluene/H2O/
Bu4NBr system was effective also for the silylation of an aryl
bromide having an ester moiety, preventing ester hydrolysis
during the reaction (entries 7 and 8). Other aryl bromides with
various substituents gave the corresponding arylsilanes in good
yields (entries 9–12). Dibromo- or tribromobenzene also
reacted smoothly to give bis(trimethylsilyl)- or tris(trimethylsi-
lyl)benzene, respectively (entries 13 and 14). Aryl iodides also
were found to be good substrates for this silylation reaction
(entries 15–17). This is noteworthy, as iodobenzene is recorded
not to react at all with a Pd–Ph3P catalyst, the low efficiency of
determining transmetalation step, two routes are possible in
terms of electronic balance: one is to make the palladium(II
)
more electron-deficient and the other is to increase the electron
density of the nucleophile. Ligands play significant roles for the
former but not for the latter.
We envisaged that a phosphine having a phenolate group
should not only serve as a bidentate ligand for palladium but
also activate directly an incoming silyl nucleophile. Thus, in the
palladium-catalysed silylation of aryl halides with disilanes, we
considered that the phenolate anion in the ligand could
coordinate to the silicon atom of a disilane and enhance its
nucleophilicity in the transmetalation step in the catalytic cycle
that we assumed to be working (Scheme 1). Here we report that
a palladium complex coordinated by 2-(diphenylphosphino)-
phenolate (PO)4 efficiently catalyses the silylation of aryl
halides using hexamethyldisilane.
We first compared the PO ligand in efficiency with other
phosphines in the palladium-catalysed reaction of bromo-
benzene (6a) with hexamethyldisilane (7) (Scheme 2). Thus, 6a
(1.0 mol) was treated with 7 (1.1 mol) in the presence of a 1+2
Scheme 1
Scheme 3
DOI: 10.1039/b006165o
Chem. Commun., 2000, 1895–1896
This journal is © The Royal Society of Chemistry 2000
1895

Table 1 Palladium-catalysed silylation of aryl halides with hexamethyldi-
silane
In conclusion, we have demonstrated that a palladium
catalyst coordinated by phosphinophenolate is efficient for the
silylation of aryl halides using hexamethyldisilane. The proto-
col using a ligand that activates a transition metal and an
incoming nucleophile simultaneously should be applicable to
various transition metal-catalysed reactions. Studies on the
details of the mechanism as well as application of the protocol
are in progress in our laboratories.
Amount
of 8 (mol)
Yieldb
(%)
Entry
Aryl halide 6
Conditionsa
1c
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
PhBr
A
A
A
B
A
B
A
B
A
A
A
A
A
A
Ac
A
A
1.1
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
1.1
2.5
2.5
5.0
7.5
1.1
2.5
2.5
88
82
73
91
50
92
23
87
70
65
80
64
81
81
83
76
70
4-MeC6H4Br
4-MeOC6H4Br
4-MeOC6H4Br
4-NH2C6H4Br
4-NH2C6H4Br
4-EtOCOC6H4Br
4-EtOCOC6H4Br
4-CF3C6H4Br
4-HCOC6H4Br
4-PhC6H4Br
2-NaphthBr
C6H4Br2-1,4
C6H3Br3-1,3,5
PhI
Notes and references
1 (a) H. Matsumoto, S. Nagashima, K. Yoshihiro and Y. Nagai,
J. Organomet. Chem., 1975, 85, C1; (b) D. Azarian, S. S. Dua, C. Eaborn
and D. R. M. Walton, J. Organomet. Chem., 1976, 117, C55; (c) H.
Matsumoto, K. Yoshihiro, S. Nagashima, H. Watanabe and Y. Nagai,
J. Organomet. Chem., 1977, 128, 409; (d) C. Eaborn, R. W. Griffiths and
A. Pidcock, J. Organomet. Chem., 1982, 225, 331; (e) Y. Hatanaka and
T. Hiyama, Tetrahedron Lett., 1987, 28, 4715.
2 The palladium-catalysed decarbonylative silylation of arenecarbonyl
halides also affords arylsilanes: (a) J. D. Rich, J. Am. Chem. Soc., 1989,
111, 5886; (b) J. D. Rich and T. E. Krafft, Organometallics, 1990, 9,
2040; (c) T. E. Krafft, J. D. Rich and P. J. McDermott, J. Org. Chem.,
1990, 55, 5430.
4-MeOC6H4I
4-CF3C6H4I
a The reaction was carried out at 100 °C for 24 h using an aryl halide
(0.40 mmol), hexamethyldisilane and sodium hydroxide (0.48 mmol) in the
presence of [PdCl(p-C3H5)]2 (0.01 mmol) and PO (0.04 mmol). A:
THF(1 ml)–H2O(1 ml); B: tetrabutylammonium bromide(0.04 mmol),
toluene(1 ml)–H2O(1 ml). b Isolated yields based on aryl halide are given.
c The reaction was completed in 10 h.
3 T. Hiyama, in Metal-catalyzed Cross-coupling Reactions, ed. F.
Diederich and P. J. Stang, Wiley-VCH, Weinheim, 1998, ch. 10, pp.
421–453 and references therein.
4 Although there have been many reports on PO as a ligand for various
transition metals, no report appeared concerning direct activation of a
reaction partner for the transition metal. For Pd–PO complexes, see: H. D.
Empsall, B. L. Shaw and B. L. Turtle, J. Chem. Soc., Dalton Trans., 1976,
1500.
the catalyst being attributed to the inertness of the oxidative
adduct PhPdI(Ph3P)2.1a,5 In our case the oxidative adduct 2
lacks a halide ligand which explains the results that the Pd–PO
catalyst is free from the influence of the kind of halide.
5 Aryl iodides react with 7 in the presence of Pd(PPh3)4 and TASF
(Et2N)3S+F2Si2Me3, see ref. 1(e).
1896
Chem. Commun., 2000, 1895–1896