Isolation of the transmetalation step in the hiyama cross-coupling reaction of organosilanes

Article abstract of DOI:10.1021/om970085l

The oxidative addition of o-(Me₂RSiCH₂O)C₆H₄I (R = Me, Ph, F) to palladium(0) complexes leads to the corresponding complexes o-(Me₂RCH₂O)C₆H₄Pd(L₂)I (L = PP

Full text of DOI:10.1021/om970085l

© Copyright 1997
Volume 16, Number 10, May 13, 1997
American Chemical Society
Communications
Isola tion of th e Tr a n sm eta la tion Step in th e Hiya m a
Cr oss-Cou p lin g Rea ction of Or ga n osila n es
Cristina Mateo, Carolina Ferna´ndez-Rivas, Antonio M. Echavarren,* and
Diego J . Ca´rdenas
Departamento de Qu´ımica Orga´nica, Universidad Auto´noma de Madrid, Cantoblanco,
28049 Madrid, Spain
Received February 4, 1997X
Summary: The oxidative addition of o-(Me2RSiCH2O)-
ladium-catalyzed coupling of alkenyl or aryl halides with
alkenyl-, allyl-, and alkynylsilanes (Hiyama reaction)
is only possible in the presence of stoichiometric amounts
of a fluoride salt as the activating agent.5 The best
results are usually obtained by using less readily
available fluorosilanes as the nucleophilic partners,1,5
although the presence of the fluoride ion (TASF or
TBAF)6 as a promoter is still necessary. Probably, the
acceleration observed in the presence of fluoride is a
consequence of the formation of a pentacoordinated
fluorosilane intermediate, more nucleophilic toward the
organopalladium in the rate-determining transmetala-
tion step.1,5,7,8 Fluoride salts have also been found to
enhance the reactivity of alkenyl- and arylboronic acids
in their cross-coupling with organic electrophiles (Su-
zuki reaction) by the likely formation of a reactive
fluoroborate.9,10 Alternatively, fluoride also promotes
C6H4I (R ) Me, Ph, F) to palladium(0) complexes leads
to the corresponding complexes o-(Me2RCH2O)C6H4Pd-
(L2)I (L ) PPh3, AsPh3; L2 ) dppf). The intramolecular
Pd/ Si transmetalation proceeds smoothly with fluorides
and carbonates as the promoters.
Transmetalation of organosilanes with organopalla-
dium(II) complexes is a key step in the synthetically
useful Hiyama cross-coupling reaction (eq 1),1 an at-
tractive alternative to the Stille coupling reaction of
organostannanes.2 However, although organosilanes
[Pd]
F–
R1–X + R2–SiY3
R1–R2 + SiXY3
(1)
are more convenient reagents than organostannanes in
organic synthesis from the point of view of their toxicity
and ecological impact, silanes are considerably less
reactive toward transmetalation with the in situ gener-
ated organopalladium complexes.3,4 In practice, pal-
(5) For additional references, not included in ref 1, see: (a) Taka-
hashi, K.; Minami, T.; Ohara, Y.; Hiyama, T. Bull. Chem. Soc. J pn.
1995, 68, 2649. (b) Gouda, K.; Hagiwara, E.; Hatanaka, Y.; Hiyama,
T. J . Org. Chem. 1996, 61, 7232. (c) Hiyama, T.; Matsuhashi, H.; Fujita,
A.; Tanaka, M.; Hirabayashi, K.; Shimizu, M.; Mori, A. Organometallics
1996, 15, 5762.
(6) (a) TASF ) tris(dimethylamino)sulfonium difluorotrimethylsili-
cate. (b) TBAF ) tetrabutylammonium fluoride. (c) dba ) dibenzylide-
neacetone. (d) dppf ) 1,1′-bis(diphenylphosphino)ethane.
(7) The stereochemistry of the transmetalation step has been
examined by using optically active benzylic trifluorosilanes leading to
retention or inversion of configuration as a function of temperature
and solvent polarity, see: Hatanaka, Y.; Hiyama, T. J . Am. Chem. Soc.
1990, 112, 7793.
X Abstract published in Advance ACS Abstracts, April 15, 1997.
(1) Reviews: (a) Hatanaka, Y.; Hiyama, T. Synlett 1991, 845. (b)
Hiyama, T.; Hatanaka, Y. Pure Appl. Chem. 1994, 66, 1471.
(2) Stille, J . K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508.
(3) (a) For
a review on the transmetalation of allyl-, alkenyl-,
alkynyl-, and arylsilanes with Pd(II) salts, see: Akhrem, I. S.;
Chistovalova, N. M.; Vol’pin, M. E. Russ. Chem. Rev. 1983, 52, 542.
(b) Weber, W. P.; Felix, R. A.; Willard, A. K.; Koenig, K. E. Tetrahedron
Lett. 1971, 4701. (c) Yamamoto, K.; Shinohara, K.; Ohuchi, T.; Kumada,
M. Tetrahedron Lett. 1974, 1153.
(4) Allylsilanes are several orders of magnitude less reactive than
allylstannanes toward electrophiles, see: Mayr, H.; Patz, M. Angew.
Chem., Int. Ed. Engl. 1994, 33, 938.
(8) See also: (a) Chuit, C.; Corriu, R. J . P.; Reye, C.; Young, J . C.
Chem. Rev. 1993, 93, 1371. (b) The driving force is the formation of a
strong Si-F bond, see: Walsh; R. In The Chemistry of Organic Silicon
Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989.
S0276-7333(97)00085-X CCC: $14.00 © 1997 American Chemical Society

1998 Organometallics, Vol. 16, No. 10, 1997
Communications
Sch em e 1
the reduction of phosphine palladium(II) complexes to
palladium(0), probably by nucleophilic attack on the
coordinated phosphine.11
We have recently reported that iodoaryl stannane 1
reacts with Pd(PPh3)4 to afford palladacycle 2, as a
result of an intramolecular Pd/Sn transmetalation of the
intermediate oxidative addition arylpalladium(II) com-
plex (eq 2).12 We have now found that by using silanes
(2)
instead of stannanes the intermediate complexes can
be obtained as stable compounds, allowing one to
systematically study the transmetalation step of an
important cross-coupling reaction, isolated from the
oxidative-addition and reductive-elimination steps. Im-
portantly, the Pd/Si transmetalation can be promoted
by the formation of a transient Pd-O bond.
Silanes 3, 4, and 513 reacted smoothly with Pd(PPh3)4
6c,14
Ta ble 1. Tr a n sm eta la tion of Com p lexes
Ar -P d (L)2-I (6-12) to P a lla d a cycles 2, 13-15
(toluene, 40 °C) or Pd(dba)(AsPh3)2
(acetone, 23 °C)
to give the oxidative-addition complexes 6-10 (Scheme
1). Similarly, reaction of 3 and 5 with Pd(dba)(dppf)6d,15
(toluene, 23 °C) afforded 11 and 12, respectively.16
Palladium(II) complexes 6-12 are stable compounds
and did not suffer intramolecular Pd/Si transmetalation
after being heated at 50 °C for 17 h (CDCl3 or CD3CN
solutions). Furthermore, trimethylsilyl derivatives 6
and 11 failed to undergo transmetalation in the pres-
ence of fluoride anion as the promoter. We envisioned
that the desired transmetalation could be achieved by
using the less strongly coordinating ligand triphenyl-
arsine,17 which could facilitate the formation of a
coordinatively unsaturated 14-electron complex, which
would be more reactive in the transmetalation reac-
tion.18 This was shown to be the case. Thus, once TBAF
was added to a solution of complex 7 in acetonitrile at
entry
T
reaction
yield
(%)b
no. Ar-Pd-I reagenta (°C) time (h) palladacycle
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
6
6
TBAFc
Ag2CO3 50
23
d
d
7
7
TASF
TBAF
23
23
51
51
13
13
(22)
(80)
95 e
100
d
(67)
(100)
(20)
(100)
(100)
(25)
70 (100)
(27)
94
7
7
8
8
8
8
9
9
9
9
9
10
11
12
Ag2CO3 23
13 + 14
K2CO3
TASF
TBAF
50
23
23
5
13
51
51
44
1
1
51
2
13
13
13
13
13
13
13
13
2
Ag2CO3 23
K2CO3
TASF
TBAF
KF
50
23
23
23
Ag2CO3 23
K2CO3 23
Ag2CO3 23
Ag2CO3 50
Ag2CO3 23
1
1
d
15
100
(9) Wright, S. W.; Hageman, D. L.; McClure, L. D. J . Org. Chem.
1994, 59, 6095.
(10) For a recent review, see: Miyaura, N.; Suzuki, A. Chem. Rev.
1995, 95, 2457.
a
Two molar equivalents of additive was used. Unless otherwise
b
stated, the reactions were carried out in MeCN. Isolated yields.
(11) Mason, M. R.; Verkade, J . G. Organometallics 1992, 11, 2212.
(12) (a) Ca´rdenas, D. J .; Mateo, C.; Echavarren, A. M. Angew. Chem.,
Int. Ed. Engl. 1994, 33, 2445. (b) Mateo, C.; Ca´rdenas, D. J .;
Ferna´ndez-Rivas, C.; Echavarren, A. M. Chem. Eur. J . 1996, 2, 1596.
(13) (a) Silane 3 was prepared by alkylation of o-iodophenol with
(iodomethyl)trimethylsilane (92% yield). (b) Silane 4 was prepared
similarly by using (chloromethyl)dimethylphenylsilane (81% yield). (c)
Fluorosilane 5 was prepared from 4 by reaction with HBF4 in CH2Cl2
at 23 °C (82% yield) (cf. Fleming, I.; Henning, R.; Plaut, H. J . Chem.
Soc., Chem. Commun. 1984, 29).
(14) This complex was prepared in situ by reaction of Pd2(dba)3‚dba
with 2 equiv of AsPh3. For the preparation of similar complexes, see:
(a) Takahashi, Y.; Ito, T.; Sakai, S.; Ishii, Y. J . Chem. Soc., Chem.
Commun. 1970, 1065. (b) Ukai, T.; Kawazura, H.; Ishii, Y.; Bonnet, J .
J .; Ibers, J . A. J . Organomet. Chem. 1974, 65, 253. (c) Amatore, C.;
J utand, A.; Khalil, F.; M’Barki, M. A.; Mottier, L. Organometallics
1993, 12, 3168.
Numbers in parentheses are for yields determined by 1H NMR.
c MeOH as the solvent. No reaction was observed after 24-48
d
h. e A 1:1 mixture of 13 and 14.
23 °C, transmetalation ensued to give palladacycle 1312b
(Table 1, entry 4). Addition of TASF led to only 22%
conversion of 7 into 13 under these conditions (entry
3). Among the fluoride sources examined with 8, only
(15) (a) Xi, Z.; Yang, R.; J in, D. Chin. Chem. Lett. 1991, 2, 331. (b)
For the synthesis of related PdL2(dba) complexes (L2 ) dppe, dppp),
see: Herrmann, W. A.; Thiel, W. R.; Brossmer, C.; O¨ fele, K.; Priermeier,
T.; Scherer, W. J . Organomet. Chem. 1993, 461, 51.
(16) The isolation of a moderately stable stannane analogue of 12
with dppf as a ligand, which undergoes rapid transmetalation, has been
recently achieved.12b
(17) (a) Farina, V.; Krishnan, B.; Marshall, D. R.; Roth, G. J . Org.
Chem. 1993, 58, 5434. (b) Farina, V.; Krishnan, B. J . Am. Chem. Soc.
1991, 113, 9585. (c) For a review, see: Farina, V. In Comprehensive
Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson,
G., Eds.; Pergamon: Oxford, 1995; Vol. 12, Chapter 3.4.
(18) Louie, J .; Hartwig, J . F. J . Am. Chem. Soc. 1995, 117, 11598.
TBAF led to a significant conversion into 13 (entry 8).
The more favorable cleavage of the alkyl-Si bond in the
presence of a phenyl-Si bond in 8 is another example

Communications
Organometallics, Vol. 16, No. 10, 1997 1999
of endocyclic restriction19 in the intramolecular trans-
metalation reaction.12 (Fluorosilyl)palladium(II) com-
plex 9 was the most reactive, leading to 13 in almost
quantitative yield in the presence of 1 equiv of TASF
or TBAF (entries 11 and 12). Treatment of a solution
of 9 in acetonitrile with potassium fluoride, which was
a very poor promoter for the transmetalation of 7 and
8, led to 33% conversion into 13 after 51 h at 23 °C
(entry 13).20
decarboxylation. This proposal is in agreement with the
activation promoted by Ag2O. Therefore, these results
suggest that formation of an intermediate palladium-
(II) complex with a Pd-O bond is key for the success of
the Pd/Si transmetalation reaction. Interestingly, for-
mation of complexes with a Pd-O bond has been
proposed to accelerate the Pd/B transmetalation, which
occurs in the Suzuki coupling reaction.10,25,26
In summary, we have achieved the intramolecular
transmetalation of alkylsilanes with arylpalladium
complexes, the key step in the Hiyama coupling reac-
tion. The developed system allows one to study the
effect of ligands and additives on the rate of the
transmetalation step, isolated from the oxidative-addi-
tion and reductive-elimination steps. The preliminary
results shown here demonstrate that the Pd/Si trans-
metalation can be promoted by the formation of a Pd-O
bond. Additionally, the use of AsPh3 as the ligand for
palladium, which has been demonstrated to facilitate
the transmetalation in the Stille reaction,17 substan-
tially accelerates the Pd/Si transmetalation step. The
study of the effect of additional ligands and additives
on the transmetalation and the application of these
findings for the development of new coupling reactions
are in progress.
Interestingly, an alternative procedure for the trans-
metalation was uncovered by using Ag2CO3 as the
additive in the absence of fluoride anion. Thus, (dimeth-
ylphenylsilyl)palladium complex 8 led to 13 after being
treated with Ag2CO3 (Table 1, entry 9). The fluorosilyl
derivative 9 reacted very rapidly under these conditions
(entry 14). However, trimethylsilyl derivatives 6 and
11 with phosphines as the donor ligand for palladium
were also unreactive under these conditions. In con-
trast, (fluorosilyl)palladium complex 10 readily reacted
in the presence of Ag2CO3 to afford palladacycle 2 (entry
16). Complex 7 reacted with Ag2CO3 in acetonitrile at
50 °C to give palladacycle 13. Surprisingly, complex 13
was accompanied by an equimolar amount of the silyl-
substituted complex 14 (entry 5). Formation of the last
complex was unexpected, but it could be envisaged as
taking place by cleavage of palladacycle 13 by a trimeth-
ylsilyl electrophile followed by aromatic palladation.21
A similar result was obtained by using Ag2O as the
additive.22 Significantly, the presence of silver ion was
not essential for these reactions since the transmeta-
lation of 7 could be cleanly achieved with K2CO3 in
acetonitrile to give exclusively palladacycle 13 (entry
6). Furthermore, the use of silver salts like AgOTf,
AgBF4, or AgOTs gave negative results (either no
reaction or extensive decomposition of 7) under different
reaction conditions. Reaction of 8 with K2CO3 also led
to 13, albeit in low yield (entry 10). Fluorodimethylsilyl
complex 12 with a bidentate dppf ligand underwent
smooth transmetalation in the presence of Ag2CO3 to
give to oxapalladacycle 15 in quantitative yield (entry
18).
Ack n ow led gm en t. This work was supported by the
DGICYT (project PB94-0163). C.M. and C.F.-R acknowl-
edge the receipt of predoctoral fellowships by the
Ministerio de Educacio´n y Ciencia. We also acknowl-
edge J ohnson Matthey PLC for a generous loan of
palladium dichloride.
Su p p or tin g In for m a tion Ava ila ble: Text giving the
experimental details and characterization data for new com-
pounds (7 pages). Ordering information is given on any
current masthead page.
OM970085L
(23) For the formation of bis(triphenylphosphino)(carbonato)palla-
dium(II), see: Nyman, C. J .; Wymore, C. E.; Wilkinson, G. J . Chem.
Soc. A 1968, 561.
These experiments are in accord with the formation
of an arylpalladium carbonato complex,23,24 which may
lead to a reactive arylpalladium oxo intermediate by
(24) Reaction of Ag2CO3 with dichloro(diphosphine)platinum(II) com-
plexes leads to (diphosphine)(carbonato)platinum(II) complexes, see:
(a) Andrews, M. A.; Gould, G. L.; Voss, E. J . Inorg. Chem. 1996, 35,
5740. (b) Andrews, M. A.; Gould, G. L:, Klooster, W. T.; Koenig, K. S.;
Voss, E. J . Inorg. Chem. 1996, 35, 5478 and references cited therein.
(25) The palladium-catalyzed coupling of allyl carbonates with
alkenyl fluorosilanes proceeds in the absence of added fluoride.
Presumably, the alkoxide leaving group promotes the transmetalation
reaction, see: Matsuhashi, H.; Hatanaka, Y.; Kuroboshi, M.; Hiyama,
T. Tetrahedron Lett. 1995, 36, 1539.
(26) Very recently, NaOH has been shown to promote the cross-
coupling of organosilicon compounds with organic halides, see: Hagi-
wara, E.; Gouda, K.; Hatanaka, Y.; Hiyama, T. Tetrahedron Lett. 1997,
38, 439.
(19) Beak, P. Acc. Chem. Res. 1992, 25, 215.
(20) The reduced reactivity of KF may be due, at least in part, to
its low solubility in organic solvents. Similar results were obtained
with CsF.
(21) Alternatively, complex 14 may arise by an oxidative-addition-
reductive-elimination pathway. Studies on this subject are in progress.
(22) (a) As expected,18 the addition of excess AsPh3 slows down the
transmetalation reaction. (b) Complex 7 also afforded palladacycle 13
in low yield in the presence of NaOH or Na2HPO4 as the additives
(MeCN, 23 °C).