A large accelerating effect of tri(tert-butyl)phosphine in the rhodium- catalyzed addition of arylboronic acids to aldehydes

Full text of DOI:10.1021/jo000187c

4450
J . Org. Chem. 2000, 65, 4450-4452
Sch em e 1. Ad d ition to Ald eh yd es
A La r ge Acceler a tin g Effect of
Tr i(ter t-bu tyl)p h osp h in e in th e
Rh od iu m -Ca ta lyzed Ad d ition of
Ar ylbor on ic Acid s to Ald eh yd es
Ta ble 1. Effect of P h osp h in e Liga n d sa
Masato Ueda and Norio Miyaura*
entry
ligand (equiv)
yield/% at 50 °C
yield/% at 80 °C
Division of Molecular Chemistry,
Graduate School of Engineering, Hokkaido University,
Sapporo 060-8628, J apan
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
dppmb (1)
dppec (1)
30
6
dpppd (1)
dppbe (1)
71
60
85
73
0
31
48
12
86
89
Received February 10, 2000
dppff (1)
20
DPEphosg (1)
Xantphosh (1)
DBFphosi(1)
PPh3 (1)
The addition of a metal-carbon bond to the carbon-
heteroatom double bond is a very popular reaction in
main group metal reagents of lithium and magnesium,
but less attention has been paid to the corresponding
reaction of transition-metal compounds.1 However, the
metal-catalyzed addition reactions are of interest due to
their potential application to asymmetric synthesis. We
previously demonstrated the efficiency of transmetalation
from boron to palladium in the cross-coupling reaction
of organoboron compounds with organic electrophiles.2
The transmetalation to rhodium provided another C-C
bond-forming reaction via the 1,4-addition of aryl or
1-alkenylboronic acids to R,â-unsaturated ketones,3 es-
ters,4 and amides4 and the 1,2-addition to aldehydes5 or
N-sulfonylaldimines.6 The efficiency of the protocol was
recently demonstrated in similar addition reactions of
potassium 1-alkenyl- and aryltrifluoroborates.7
33
24
88
Me3P (1)
i-Pr3P (1)
n-Bu3P (1)
i-Bu3P (1)
Cy3Pj (1)
t-Bu3P (1)
t-Bu3P (2)
t-Bu3P (3)
75
50
83
79
77
99 (99)k
67
57
a
A mixture of a 4-methoxybenzaldehyde (1 mmol), PhB(OH)2
(2 mmol), Rh(acac)(coe)2, (0.03 mmol) and a ligand (0.03-0.09
mmol) in DME/H2O (3/2, 5 mL) was stirred for 16 h at 50
or 80 °C. Bis(diphenylphosphino)methane. c 1,2-Bis(diphenyl-
b
phosphino)ethane. 1,3-Bis(diphenylphosphino)propane. e 1,4-Bis-
d
f
(diphenylphosphino)butane. 1,1′-Bis(diphenylphosphino)fer-
rocene. Bis(2-(diphenylphosphino)phenyl)ether. 9,9-Dimethyl-
g
h
i
4,6-bis(diphenylphosphino)xanthene. 1,8-Bis(diphenylphos-
phino)dibenzofuran. j Tri(cyclohexyl)phosphine. k At room temper-
ature for 16 h.
Here, we report the significant effect of tri(tert-butyl)-
phosphine in accelerating the addition of aryl- and
1-alkenylboronic acids to aldehydes even at room tem-
perature (Scheme 1).
a large P-Rh-P angle9 such as dppf, but the monophos-
phine complexes result in significantly low yields when
using 3 equiv of phosphine to the rhodium metal. How-
ever, a reinvestigation of the catalysts revealed a new
correlation, namely, that the catalyst activity is highly
dependent on both the basicity10 and the stoichiometry
of the phosphine ligands. The effect of bidentate phos-
phine was the same as that previously observed, sug-
gesting the superiority of dppf (entry 5), but the same
reaction was remarkably accelerated by bulky and donat-
ing trialkylphosphines such as tri(isopropyl)phosphine
(entry 11) and tri(tert-butyl)phosphine (entry 15) when
using 1 equiv of phosphine to the rhodium metal. The
use of tri(tert-butyl)phosphine allowed quantitative con-
version even at room temperature (entry 15). Although
the addition of excess of tert-butylphosphine dropped the
yields proportionally (entries 15-17), this effect of stoi-
chiometry was significant in small trialkylphosphines be-
cause the presence of 3 equiv of Et3P completely stopped
A series of rhodium(I)/phosphine complexes in situ
prepared from Rh(acac)(coe)2 (coe ) cyclooctene)8 and the
representative phosphines revealed the effects of biden-
tate phosphines (entries 1-8), basicity of monophos-
phines (entries 9-16), and stoichiometry of ligand on the
rhodium metal (entries 15-17) (Table 1).
Preliminary results5 indicated that the reaction is
accelerated by the bidentate phosphine complexes having
(1) Cornils, B.; Herrmann, W. A. Applied Homogeneous Catalysis
with Organometallic Compounds; VCH: New York, 1996. Beller, M.;
Bolm, C. Transition Metals for Organic Synthesis-Building Blocks and
Fine Chemicals; Wiley-VCH: New York, 1998.
(2) (a) Miyaura, N.; Suzuki, A. Chem. Rev, 1995, 95, 2457-2483.
(b) Suzuki, A. Metal-Catalyzed Cross-Coupling Reactions; Diederich,
F., Stang, P. J ., Eds.; Wiley-VCH: Weinheim, Germany, 1998; Chap-
ter 2.
(3) Sakai, M.; Hayashi, H.; Miyaura, N. Organometallics 1997, 16,
4229-4230. An asymmetric 1,4-addition of aryl- or 1-alkenylboronic
acids to enones: Takaya, Y.; Ogasawara, M.; Hayashi, T.; Sakai, M.;
Miyaura, N. J . Am. Chem. Soc. 1998, 120, 5579-5580.
(4) Sakai, M.; Sakuma, S.; Miyaura, N. The 45th Symposium on
Organometallic Chemistry, J apan, 1999.
(9) The P-Rh-P angles increase in the order of dppe < dppp <
dppb < dppf <DPEphos < Xantphos < DBFphos. (a) Angermund,
K.; Baumann, W.; Dinjus, E.; Fornika, R.; Go¨rls, H.; Kessler, M.;
Kru¨ger, K.; Leitner, W.; Lutz, F. Chem. Eur. J . 1997, 3, 755-764. The
effect on hydroformylation of alkenes: (b) Casey, C. P.; Whiteker, G.
T.; Melville, M. G.; Petrovich, L. M.; Gavney, J . A.; Powell, J r., D. R.
J . Am. Chem. Soc. 1992, 114, 5535-5543. (c) Kranenburg, M.; van
der Burgt, Y. E. M.; Kamer, P. C. J .; van Leeuwen, P. W. N. M.;
Goubitz, K.; Fraanje, J . Organometallics 1995, 14, 3081-3089. The
effect on hydrogenation of CO2: (d) Fornika, R.; Go¨rls, H.; Seemann,
B.; Leitner, W. J . Chem. Soc., Chem. Commun. 1995, 1479-1481. See
also ref 15.
(5) Sakai, M.; Ueda, M.; Miyaura, N. Angew. Chem., Int. Ed. Engl.
1998, 37, 3279-3281.
(6) Ueda, M.; Miyaura, N. J . Organomet. Chem. 2000, 595, 31-35.
(7) Batey, R. A.; Thadani, A. N.; Smil, D. V. Org. Lett. 1999, 1, 1683-
1686.
(8) Rh(acac)(coe)2 was synthesized from [RhCl(coe)2]2 and Na(acac).
A private communication from T. Marder, University of Durham,
16
England. A mixture of [RhCl(coe)2]2 (800 mg, 1.62 mmol) and Na-
(acac) (400 mg, 3.28 mmol) was charged with toluene (30 mL) under
argon. After being stirred for
4 h at 50 °C and 12 h at room
temperature, the precipitate was filtered off, washed with toluene, and
stripped to dryness in vacuo. The resulting solid was extracted with
warm heptane to give 1.01 g (99%) of yellow solid.
(10) The basicity of monophosphines. Rahman, M. M.; Liu, H. Y.;
Eriks, K.; Prock, A.; Giering, W. P. Organometallics, 1989, 8, 1-7
10.1021/jo000187c CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/27/2000

Notes
J . Org. Chem., Vol. 65, No. 14, 2000 4451
Ta ble 2. Ad d ition of Ar ylbor on ic Acid s to Ald eh yd esa
yielda/%
yieldb/%
entry
ArB(OH)2
PhB(OH)2
aldehyde
Rh(I)-tBu3P/rt
Rh(I)-dppf/80 °C
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
3-O2NC6H4CHO
4-O2NC6H4CHO
4-F3CC6H4CHO
4-NCC6H4CHO
4-MeCOC6H4CHO
4-MeO2CC6H4CHO
4-BrC6H4CHO
4-MeC6H4CHO
2-MeC6H4CHO
4-MeOC6H4CHO
4-MeOC6H4CHO
4-MeOC6H4CHO
PhCHO
94
94
98
94
98
99
97
93
88
99
91
68
98
0
97
97
93
82
88
48 (76)c
83 (99)
4-MeOC6H4B(OH)2
4-FC6H4B(OH)2
4-MeCOC6H4B(OH)2
4-NCC6H4CHO
4-MeOC6H4CHO
4-F3CC6H4CHO
C5H11CHO
>1
80
2-MeC6H4B(OH)2
2,4,6-Me3C6H2B(OH)2
PhB(OH)2
99
trace
35 (84)d
28 (90)d
69
cyclo-C6H11CHO
45 (95)c
a
A mixture of an aldehyde (1 mmol), ArB(OH)2 (2 mmol), Rh(acac)(coe)2 (0.03 mmol), and t-Bu3P (0.03 mmol) in DME/H2O (3/2, 5 mL)
was stirred for 16 h at room temperature. At 80 °C for 16 h in the presence of Rh(acac)(CO)2/dppf (3 mol %). See ref 5. c At 95 °C for 16
b
d
h. At 60 °C for 16 h in dioxane/H2O (3/2, 5 mL) in the presence of Rh(acac)(coe)2/i-Pr3P (0.03 mmol).
Sch em e 2. 1,4-Ad d ition ver su s 1,2-Ad d ition
the reaction. Thus, the basicity and stoichiometry exhib-
ited a more pronounced effect than that of bidentate
ligands.
The addition of arylboronic acids to the representa-
tive aromatic and aliphatic aldehydes in the presence of
Rh(acac)/t-Bu3P or Rh(acac)/dppf are summarized in
Table 2.
The previous reaction catalyzed by the Rh(acac)/dppf
complex at 80 °C was highly sensitive to electronic effects
both in aldehydes and arylboronic acids.5 The reaction
was facilitated in the presence of an electron-withdrawing
group in aromatic aldehydes and a donating group in
arylboronic acids. On the other hand, the addition to
the conjugate 1,4-addition, chemoselectively produced a
1,2-addition product (90%).
electron-rich aldehydes (entries 8, 17, and 18) and the
arylation with electron-deficient arylboronic acids (entry
14) significantly were very slow. Moreover, no reaction
was observed for 4-nitrobenzaldehyde (entry 2). However,
the tri(tert-butyl)phosphine complex quantitatively cata-
lyzed the reaction at room temperature for the represen-
tative aldehydes including nitrobezaldehydes (entries 1
and 2), electron-rich (entries 8-12) and electron-deficient
aromatic aldehydes (entries 1-7), and electron-deficient
arylboronic acid (entry 13). The additions to aliphatic
aldehydes such as hexanal and cyclohexanecarbaldehyde
were very slow at room temperature due to their lower
electrophilicity than that of aromatic aldehydes (entries
17 and 18). The t-Bu3P complex also resulted in low yields
at temperature higher than 50 °C, but the i-Pr3P complex
that was stable at 80 °C (entry 11 in Table 1) afforded
the corresponding alcohols in 84% and 90% yields,
respectively (entries 17 and 18). On the other hand, no
addition products were observed for acetophenone and
cyclohexanone at 80 °C, suggesting high chemoselectivity
for the aldehyde carbonyls.
The t-Bu3P complex yielded the1,2-addition product in
preference to the conjugate 1,4-addition, which was
demonstrated in the addition of phenylboronic acid to
cinnamaldehyde (Scheme 2). The 1,4-addition reaction
catalyzed by the cationic rhodium complex in aqueous
DME or dioxane gave a mixture of both isomers at 50
°C, but the same reaction proceeded smoothly at room
temperature in aqueous MeOH without an accompanying
a 1,2-product. The t-Bu3P complex, which did not catalyze
Many transition metal complexes catalyze the addition
reactions of main group metal reagents, but the reaction
mechanism has not yet been studied in detail. However,
the transmetalation from a main metal reagent to a
transition-metal complex has been proposed as a key step
of the catalytic cycle.11-13 The present reaction should also
involve the transmetalation between the organoboronic
acid and the rhodium(I) complex yielding an R-Rh(I)
species, the insertion of CdO into the Rh-C bond, and
finally, hydrolysis of the O-Rh(I) bond with water, as
was proposed in the our previous paper (Figure 1).5 The
insertion of aldehydes into the C-Rh(I) bond has not yet
been well studied, but the assumed pathway may be
consistent with the related addition reactions to carbo-
nyls; the addition of R-Rh(I) complexes (R)Me, Ph) to
carbon dioxide11 and other related additions of orga-
nostannanes12 or organosilanes13 to aldehydes catalyzed
by a cationic rhodium complex.
The present reaction revealed an electronic effect of
the functional groups both in the aldehydes and arylbo-
ronic acids, suggesting that an aryl group derived from
the boronic acid nucleophilically attacks the carbonyl
(11) Darensbourg, D. J .; Grotsch, G.; Wiegreffe, P.; Rheingold, H.
L. Inorg. Chem. 1987, 26, 3827-3830 and references therein.
(12) Oi, S.; Moro, M.; Inoue, Y. Chem. Commun. 1997, 1621-1622.
(13) Moro, M.; Ito, H.; Kawanishi, T.; Oi, S.; Inoue, Y. The abstract
of the 76th Annual Meeting of J apan Chemical Society 1999; p 925,
2B714.

4452 J . Org. Chem., Vol. 65, No. 14, 2000
Notes
Dppf was obtained by a two-step synthesis from ferrocene.14
DPEphos, Xantphos, and DBFphos were obtained by deproto-
nation of the backbones with sec-butyllithium/TMEDA in ether;
the dilithiated species are then treated with chlorodiphenylphos-
phine.15 Rh(acac)(coe)2 was synthesized from [RhCl(coe)2]2 and
Na(acac) in toluene.8
Gen er a l P r oced u r e for Ta ble 2. Rh(acac)(coe)2 (0.03 mmol)
and phenylboronic acid (2.0 mmol) were added to a flask
containing a magnetic stirring bar. The flask was flushed with
argon and then charged with DME (3 mL), (t-Bu3)P (0.03 mmol),
water (2 mL), and 4-methoxybenzaldehyde (1.0 mmol). The
mixture was then stirred at room temperature (25-28 °C) for
16 h. The product was extracted with benzene, washed with
brine, and dried over MgSO4. Chromatography over silica gel
gave (4-methoxyphenyl)(phenyl)methanol in 99% yield (entry
10).
F igu r e 1. Proposed catalytic cycle.
1,4-Ad d ition to tr a n s-Cin n a m a ld eh yd e (Sch em e 2). [Rh-
(cod)(MeCN)2]BF4 (0.03 mmol) and phenylboronic acid (2.0
mmol) were added to a flask containing a magnetic stirring bar,
a septum inlet, and a reflux condenser. The flask was flushed
with argon and then charged with MeOH (3 mL), water (0.5 mL),
and trans-cinnamaldehyde (1.0 mmol). The mixture was then
stirred for 16 h at room temperature (25-28 °C). Chromatog-
raphy over silica gel (EtOAc/hexane 1/6) afforded 185 mg (88%)
of the title compound as a pale yellow solid.
carbon. The electronic effect often reduced the yields of
the Rh(I)/dppf-catalyzed reaction (entries 8 and 14),
whereas it was not significant in the Rh(I)/t-Bu3P cata-
lyst. Another dominant factor in both catalysts is that
the dppf complex is more Lewis acidic than the tert-
butylphosphine complex. Thus, the coordination of a nitro
group to the dppf complex completely retarded the
addition to nitrobenzaldehydes, though the tert-butylphos-
phine catalyst tolerates the nitro group (entries 1 and 2)
and, presumably, other polar functional groups as well.
There are two opposing factors in the metal catalyst both
accelerating the addition to carbonyl compounds. The
polarization of the C-Rh bond by the donating ligand
enhances the nucleophilicity of the organic group on the
rhodium metal, but the metal has to keep the Lewis
acidity for coordination of the carbonyl. Thus, the coor-
dinateively unsaturated rhodium(I) species having a
limited amount of the donating ligand provides the most
active catalyst since both effects can coexist at the metal
center.
1,2-Ad d ition to tr a n s-Cin n a m a ld eh yd e (Sch em e 2). The
general procedure for Table 2 gave 189 mg (90%) of the title
compound as a clear oil.
Ack n ow led gm en t. We appreciate Professor T. B.
Marder for sending us their original procedure for the
preparation of Rh(acac)(coe)2.
Su p p or tin g In for m a tion Ava ila ble: The procedures for
Tables 1 and 2 and Scheme 2 and spectral analyses of all
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O000187C
(14) Bishop, J . J .; Davison, A.; Katcher, M. L.; Lichtenberg, D. W.;
Merrill, R. E.; Smart, J . C. J . Organomet. Chem. 1971, 27, 241-249.
(15) Kranenburg, Y. E. M. M.; van der Burgt, P. C. J .; Kamer, P.
W. N. M.; van Leeuwen, K.; Goubitz, K.; Fraanje, J . Organometallics
1995, 14, 3081-3089.
Exp er im en ta l Section
Triphenylphosphine, tricyclohexylphosphine(PCy3), dppm, dppe,
dppp, and dppb were commercially available. PMe3, PEt3,
PnPr3, PiPr3, PnBu3, PiBu3, and PtBu3 were distilled before use.
(16) van der Ent, A.; Onderdelinden, A. L. Inorg. Synth. 1973, 14,
93.