Unusual palladium-catalyzed silaboration of allenes using organic iodides as initiators: Mechanism and application

Article abstract of DOI:10.1021/ja044662q

A highly regio- and stereoselective method for the synthesis of various 2-silylallylboronates 7 from allenes 1 and 2-(dimethylphenylsilanyl)-4,4,5,5- tetramethyl[1,3,2]dioxaborolane (5) catalyzed by palladium complexes and initiated by organic iodides is

Full text of DOI:10.1021/ja044662q

Published on Web 12/03/2004
Unusual Palladium-Catalyzed Silaboration of Allenes Using
Organic Iodides as Initiators: Mechanism and Application
Kuo-Jui Chang, Dinesh Kumar Rayabarapu, Feng-Yu Yang, and
Chien-Hong Cheng*
Contribution from the Department of Chemistry, Tsing Hua UniVersity, Hsinchu, 30013 Taiwan
Received September 3, 2004; E-mail: chcheng@mx.nthu.edu.tw
Abstract: A highly regio- and stereoselective method for the synthesis of various 2-silylallylboronates 7
from allenes 1 and 2-(dimethylphenylsilanyl)-4,4,5,5-tetramethyl[1,3,2]dioxaborolane (5) catalyzed by
palladium complexes and initiated by organic iodides is described. Treatment of monosubstituted aryl and
alkylallenes RCHdCdCH2 (1a-m) and 1,1-dimethylallene (1n) with borylsilane 5 in the presence of Pd-
(dba)2 (5 mol %) and organic iodide 3a (10 mol %) afforded the corresponding silaboration products 7a-n
in moderate to excellent yields. This catalytic silaboration is totally regioselective with the silyl group of 5
adding to the central carbon and the boryl group to the unsubstituted terminal carbon of allene. Furthermore,
the reactions show very high E stereoselectivity with the Z/E ratios lying in the range from 1/99 to 7/93. In
the absence of an organic iodide, silaboration of 1 with 5 still proceeds, but gives products having completely
different regiochemistry as that of 7. The silaboration chemistry can be applied to the synthesis of homoallylic
alcohols. Treatment of allenes (1) with borylsilane 5 and aldehydes 14 in the presence of Pd(dba)2 (5 mol
%) and 3a (10 mol %) at 80 °C in ethyl acetate for 5 h afforded homoallylic alcohols 15a-p in one pot in
good to excellent yields, with exceedingly high syn selectivity (>93%). Mechanistic pathways involving an
unusual palladium-catalyzed three-component assembling reaction of dimethylphenylsilyl iodide, allene 1,
and borylsilane 5 were proposed to account for these catalytic reactions.
species.8 While stable oxidative adducts of diboron to Pt species
are reported, no oxidative addition of diboron to Pd are known.9
In a preliminary communication, we showed an unusual organic
1. Introduction
Addition of metal-metal bonds of the main group elements
to alkenes and alkynes catalyzed by transition-metal complexes
has attracted great attention in recent years.1 These reactions
provide a convenient route for synthesizing organometallic
compounds having vinylic and allylic metal moieties that are
useful intermediates for organic synthesis.2 In most cases, the
catalytic reaction proceeds via oxidative addition of the bimetal-
lic substrate to the transition-metal catalyst as a key step.1
Transition-metal-catalyzed addition of diboron to unsaturated
carbon-carbon bonds3,4 provides an efficient route to diboronic
compounds, which are versatile intermediates in organic syn-
thesis.5 Platinum complexes are known to readily catalyze the
addition of diboron to unsaturated carbon-carbon bonds to
provide diboronic compounds, whereas the palladium com-
plexes, which are the best catalysts for silyl-6 and stannylation,7
are not active for the diboration of alkynes and alkenes. The
difference in the reactivity of Pt and Pd lies in the ability to
undergo oxidative addition of the B-B bond to these metal (0)
(3) For a Pt-catalyzed diboration reaction, see: (a) Ishiyama, T.; Matsuda, N.;
Miyaura, N.; Suzuki, A. J. Am. Chem. Soc. 1993, 115, 11018. (b) Ishiyama,
T.; Yamamoto, M.; Miyaura, N. Chem. Commun. 1997, 689. (c) Ishiyama,
T.; Kitano, T.; Miyaura, N. Tetrahedron Lett. 1998, 39, 2357. (d) Iverson,
C. N.; Smith, M. R., III. Organometallics 1997, 16, 2757-2759. (e)
Ishiyama, T.; Yamamoto, M.; Miyaura, N. Chem. Commun. 1996, 2073.
(f) Iverson, C. N.; Smith, M. R., III. J. Am. Chem. Soc. 1995, 117, 4403-
4404. (g) Mann, G.; John, K. D.; Baker, R. T. Org. Lett. 2000, 2, 2105.
(h) Ishiyama, T.; Matsuda, N.; Murata, M.; Ozawa, F.; Suzuki, A.; Miyaura,
N. Organometallics 1996, 15, 713. (i) Marder, T. B.; Norman, N.; Rice,
C. R. Tetrahedron Lett. 1998, 39, 155. (j) Clegg, T. M.; Johann, T. R. F.;
Marder, T. B.; Norman, N. C.; Orpen, A. G.; Peakman, T. M.; Quayle, M.
J.; Rice, C. R.; Scott, A. J. J. Chem. Soc., Dalton Trans. 1998, 1431. (k)
Anderson, K. M.; Lesley, M. J. G.; Norman, N. C.; Orpen, A. G.; Starbuck,
J. Chem. Commun. 1999, 1053. (l) Marder, T. B.; Norman, N. C. Top.
Catal. 1998, 5, 63. (m) Lawson, Y. G.; Lesley, M. J. G.; Marder, T. B.;
Norman, N. C.; Rice, C. R. Chem. Commun. 1997, 2051. (n) Maderna, A.;
Pritzkow, H.; Siebert, W. Angew. Chem., Int. Ed. Engl. 1996, 35, 1501.
(o) Lesley, G.; Nguyen, P.; Taylor, N. J.; Marder, T. B.; Scott, A. J.; Clegg,
W.; Norman, N. C. Organometallics 1996, 15, 5137. (p) Iverson, C. N.;
Smith, M. R., III. Organometallics 1996, 15, 5155.
(4) For Rh- or Au-catalyzed diboration reactions, see: (a) Baker, R. T.; Nguyen,
P.; Marder, T. B.; Westcott, S. A. Angew. Chem., Int. Ed. Engl. 1995, 34,
1336. (b) Cameron, T. M.; Baker, R. T.; Westcott, S. A. Chem. Commun.
1998, 2395. (c) Dai, C.; Robins, E. G.; Scott, A. J.; Clegg, W.; Yufit, D.
S.; Howard, J. A. K.; Marder, T. B. Chem. Commun. 1998, 1983.
(5) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (b) Trost, B. M.;
Spagnol, M. D. J. Chem. Soc., Perkin Trans. 1 1995, 2083-2096. (c)
Brown, H. C.; Singaram, B. Pure Appl. Chem. 1987, 59, 894. (d) Pelter,
A.; Smith, K.; Brown, H. C. Borane Reagents; Academic Press: New York,
1988.
(1) Zimmmer, R.; Dinesh, C. U.; Nandanan, E.; Khan, F. A. Chem. ReV. 2000,
100, 3067. (b) Beletskaya, I.; Moberg, C. Chem. ReV. 1999, 99, 3435. For
recent metal-mediated allene chemistry, see: (c) Tsuji, J. Organic Synthesis
with Palladium Compounds; Springer-Verlag: New York, 1980. (d)
Hegedus, L. S. Transition Metals in the Synthesis of Complex Organic
Molecules; University Science Books: Mill Valley, CA, 1994. (e) Yama-
moto, Y.; Radhakrishnanan, U. Chem. Soc. ReV. 1999, 28, 199. (f) Hashmi,
A. S. K. Angew. Chem., Int. Ed. 2000, 39, 3590.
(2) ComprehensiVe Organometallic Chemistry II; Abel, E. W., Stone, F. G.
A., Wilkinsion, G., Eds.; Pergamon: Oxford, 1995; Vol. 11. (b) Miyaura,
N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483. (c) Pelter, A.; Smith,
K.; Brown, H. C. Borane Reagents; Academic Press: New York, 1988.
(6) Sakurai, H.; Kamiyama, Y.; Nakadaira, Y. J. Am. Chem. Soc. 1975, 97,
931. (b) Tamao, K.; Hayashi, T.; Kumada, M. J. Organomet. Chem. 1976,
114, C19-C21. (c) Okinoshima, H.; Yamamoto, K.; Kumada, M. J.
Organomet. Chem. 1975, 86, C27-C30. (d) Ozawa, F.; Sugawara, M.;
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Y.; Tanaka, M. Chem. Commun. 1999, 1863.
9
126
J. AM. CHEM. SOC. 2005, 127, 126-131
10.1021/ja044662q CCC: $30.25 © 2005 American Chemical Society

Palladium-Catalyzed Silaboration of Allenes
A R T I C L E S
Table 1. Results of Addition of Borylsilane 5 to Allenes 1a
iodide-initiated and palladium-catalyzed diboration of allenes
(Scheme 1).10 This catalytic reaction is completely regioselec-
Scheme 1
1
2
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
Ph
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
7a
7b
7c
7d
7e
7f
7g
7h
7i
7j
7k
7l
7m
7n
<1/99
<1/99
<1/99
<1/99
<1/99
<1/99
<1/99
<1/99
<1/99
<1/99
5/95
82 (87)
79 (85)
93 (99)
86 (92)
44 (50)
81 (87)
79 (85)
76 (83)
85 (89)
84 (88)
84 (89)
82 (87)
70 (75)
62 (70)
1-naphthyl
p-MeOC6H4
m-MeOC6H4
o-MeOC6H4
p-ClC6H4
p-BrC6H4
m-BrC6H4
p-MeOCC6H4
p-EtO2CC6H4
n-Bu
3c
4c
5c
6d
7d
8d
9
tive, with one boryl group adding to the middle carbon and the
other to the nonsubstituted terminal carbon of the allene moiety.
A new mechanistic pathway involving a three-component
assembling reaction11,12 was proposed to account for this
diboration.
10d
11
12
13
14c
cyclopentyl
cyclooctyl
Me
7/93
7/93
Herein, we wish to report the extension of this chemistry to
the addition of borylsilane to allenes to give 2-silylallylbor-
onates. This new silaboration shows remarkably high regio- and
stereoselectivity, with the regiochemistry of the silaboration
product being totally different from that reported previously.13
The products generated in situ can further be used as allylation
reagents for aldehydes. This method offers a mild and conve-
nient route for the synthesis of homoallylic alcohols14 in one
pot from allenes, borylsilane, and aldehydes, with excellent
regio- and syn selectivity in good to excellent yields.
a Unless stated otherwise, all reactions were carried out using alkenyl
iodide 3a (0.0500 mmol, 10 mol %), Pd(dba)2 (0.0250 mmol, 5 mol %),
borylsilane 5 (0.500 mmol), and allene 1 (1.00 mmol) in ethyl acetate (0.50
mL) at 80 °C under nitrogen for 5 h. b Isolated yields; yields in parentheses
were determined by 1H NMR integration method using DMF as an internal
standard. c Reaction was carried out at room temperature. d CH3CN is used
as the solvent.
boryl group attached to the central carbon of the allene moiety
(Scheme 2).13 The prior unusual organic iodide-initiated and
Scheme 2
2. Results and Discussion
2.1. Palladium-Catalyzed Silaboration of Allenes. Sila-
boration of unsaturated carbon-carbon bonds via the oxidative
addition of a Si-B bond to transition-metal catalysts as a key
step is known.13,15 The silaboration of allenes catalyzed by
palladium complexes gave two regioisomers, both having the
(7) Mitchell, T. N.; Amamria, A.; Killing, H.; Rutschow, D. J. Organomet.
Chem. 1983, 241, C45-C47. (b) Piers, E.; Skerlj, R. T. J. Chem. Soc.,
Chem. Commun. 1986, 626. (c) Mitchell, T. N.; Wickenkamp, R.; Amamria,
A.; Dicke, R.; Schneider, U. J. Org. Chem. 1987, 52, 4868.
palladium-catalyzed diboration of allenes prompted us to
investigate the addition of borylsilane to allenes under similar
reaction conditions.
In the presence of Pd(dba)2 (5.0 mol %) and 3-iodo-2-methyl-
2-cyclohexen-1-one (3a) (10 mol %), phenylallene (1a) reacts
with borylsilane 516 at 80 °C in ethyl acetate to give 2-silyl-
allylboronate 7a in 87% yield (Scheme 3 and Table 1). This
(8) Cui, Q.; Musaev, D. G.; Morokuma, K. Organometallics 1998, 17, 1383.
(b) Cui, Q.; Musaev, D. G.; Morokuma, K. Organometallics 1998, 17, 742.
(c) Cui, Q.; Musaev, D. G.; Morokuma, K. Organometallics 1997, 16, 1355.
(9) Irvine, G. J.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C. R.;
Robins, E. G.; Roper, W. R.; Whittell, C. R.; Wright, L. J. Chem. ReV.
1998, 98, 2685.
(10) For a preliminary communication, see: Yang, F.-Y.; Cheng, C.-H. J. Am.
Chem. Soc. 2001, 123, 761.
(11) For recent reports on three-component assembly of allenes from our
laboratory, see: (a) Yang, F.-Y.; Shanmugasundaram, M.; Chuang, S. Y.;
Ku, P. J.; Wu, M.-Y.; Cheng, C.-H. J. Am. Chem. Soc. 2003, 125, 12576.
(b) Wu, M.-Y.; Rayabarapu, D. K.; Cheng, C.-H. J. Am. Chem. Soc. 2003,
125, 12426. (c) Yang, F.-Y.; Wu, M.-Y.; Cheng, C.-H. J. Am. Chem. Soc.
2000, 122, 7122. (d) Wu, M.-Y.; Yang, F.-Y.; Cheng, C.-H. J. Org. Chem.
1999, 64, 2471. (e) Yang, F.-Y.; Wu, M.-Y.; Cheng, C.-H. Tetrahedron
Lett. 1999, 40, 6055. (f) Chang, H.-M.; Cheng, C.-H. J. Org. Chem. 2000,
65, 1767. (g) Chang, H.-M.; Cheng, C.-H. Org. Lett. 2000, 2, 3439. (h)
Hung, T.-H.; Chang, H.-M.; Wu, M.-Y.; Cheng, C.-H. J. Org. Chem. 2002,
67, 99. (i) Jeganmohan, M.; Shanmugasundaram, M.; Cheng, C.-H. Chem.
Commun. 2003, 1746.
Scheme 3
(12) Chatani, N.; Amishiro, N.; Murai, S. J. Am. Chem. Soc. 1991, 113, 7778.
(b) Nakamura, H.; Shim, J. G.; Yamamoto, Y. J. Am. Chem. Soc. 1997,
119, 8113. (c) Wang, Z.; Lu, X.; Lei, A.; Zhang, Z. J. Org. Chem. 1998,
63, 3806. (d) Ikeda, S. I.; Cui, D. M.; Sato, Y. J. Am. Chem. Soc. 1999,
121, 4712. (e) Jeganmohan, M.; Shanmugasundaram, M.; Cheng, C.-H.
Org. Lett. 2003, 5, 881.
(13) For the addition of Si-B to allenes, see: (a) Suginome, M.; Ohmori, Y.;
Ito, Y. Synlett 1999, 1567. (b) Onozawa, S.-Y.; Hatanaka, Y.; Tanaka, M.
Chem. Commun. 1999, 1863. (c) Suginome, M.; Ohmori, Y.; Ito, Y. J.
Organomet. Chem. 2000, 611, 403.
(14) Hoffmann, R. W. Angew. Chem., Int. Ed. Engl. 1982, 21, 555. (b) Matteson,
D. S. Synthesis 1986, 973. (c) Hoffmann, R. W.; Neil, G.; Schlapbach, A.
Pure Appl. Chem. 1990, 62, 1993. (d) Roush, W. R. In ComprehensiVe
Organic Synthesis; Trost, B. M., Heathcock, C. M., Eds.; Pergamon Press:
Oxford, 1991; Vol. 2, p 1. (e) Brown, H. C.; Ramachandran, P. V. Pure
Appl. Chem. 1991, 63, 307. (f) Vaultier, M.; Carboni, B. ComprehensiVe
Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G.,
Eds.; Pergamon Press: Oxford, 1995; Vol. 11, p 191.
catalytic silaboration is completely regioselective, with the silyl
group of 5 adding to the central carbon and the boryl group to
(15) For the addition of Si-B to alkynes, see: (a) Suginome, M.; Nakamura,
H.; Ito, Y. Chem. Commun. 1996, 2777. (b) Suginome, M.; Matsuda, T.;
Nakamura, H.; Ito, Y. Tetrahedron 1999, 55, 8787. (c) Onozawa, S.-y.;
Hatanaka, Y.; Tanaka, M. Chem. Commun. 1997, 1229. (d) Suginome, M.;
Matsuda, T.; Ito, Y. Organometallics 1998, 17, 5233. For alkenes, see:
(e) Suginome, M.; Nakamura, H.; Ito, Y. Angew. Chem., Int. Ed. Engl.
1997, 36, 2516. For 1,3-dienes, see: (f) Suginome, M.; Matsuda, T.;
Yoshimoto, T.; Ito, Y. Org. Lett. 1999, 1, 1567. (g) Suginome, M.;
Nakamura, H.; Matsuda, T.; Ito, Y. J. Am. Chem. Soc. 1998, 120, 4248.
9
J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005 127

A R T I C L E S
Chang et al.
the unsubstituted terminal carbon of allene 1a. Furthermore, the
reaction shows completely stereoselectivity giving only the
corresponding E isomer within the NMR detecting limit. Product
7a was characterized by its 1H and 13C NMR and mass spectral
data.
presence of a broad methylene carbon signal at 14-17 ppm in
their 13C NMR spectra due to the interaction of the methylene
carbon and boron nuclei in products 7a-n.15f,g The stereochem-
istry of the products derived from monosubstituted allenes 1
was determined by typical 1H NMR NOE experiments. For aryl
allenes, the corresponding E isomers are the sole products
observed (entries 1-10). For alkyl-substituted allenes, the
reactions gave E isomers as the major species with Z/E ratios
between 7/93 and 5/95 (entries 11-13). It is noteworthy that
the regiochemistry of products 7a-m is entirely different from
that observed previously using Pd(acac)2/2,6-xylyl isocyanide13a
or Pd2(dba)3/etpo13b as the catalyst systems (Scheme 2).
No silaboration product was formed in the absence of Pd-
(dba)2. However, without alkenyl iodide 3a, silaboration of
phenylallene 1a by 5 still proceeded to give products 6a and
6b in a 3:1 ratio and in a combined yield of 40% (Scheme 2).13
It is noteworthy that these two products with the boryl group
attached to the central carbon of the allene moiety are completely
different in regiochemistry from 7a. The use of 2-4 mol % 3a
and 5.0 % Pd(dba)2 relative to borylsilane 5 led to the formation
of a mixture of products 6a, 6b, and 7a. Products 6a and 6b
were completely inhibited as 3a employed in the reaction
solution was increased to 8%. Further increase of 3a did not
significantly change the yield of 7a, but a substantial amount
of 3a was left unreacted in the solution.
The reaction of 1a with 5 in the presence of 3a depends
greatly on the catalyst used. In addition to Pd(dba)2, other
phosphine-free Pd complexes, Pd(OAc)2 and Pd(CH3CN)2Cl2,
tested also show activities for the addition reaction, but with
lower yields (72 and 66%) of product 7a. The reaction using
Pd(dba)2 as the catalyst is strongly inhibited by the phosphine
present in the solution. Addition of 1, 2, and 4 equiv of PPh3 to
the Pd(dba)2-catalyzed reaction solution reduces the yield of
7a to 53, 23, and 0% yields, respectively. In agreement with
the foregoing observation, Pd(PPh3)2Cl2 and Pd(dppe)Cl2 show
low catalytic activity, giving 7a in 5 and 46% yields, respec-
tively. The selection of solvent is crucial for achieving a high
yield of product 7a. Ethyl acetate and acetonitrile appear to be
the best solvents tested, giving 7a in 87 and 85% yields,
respectively. Toluene and DMF are less effective, affording 7a
in 67 and 10% yields, respectively. The observed low yield of
7a in DMF is due to incomplete reaction, probably caused by
the decomposition of the palladium complex that gave black
precipitate in the reaction.
3. Mechanistic Consideration
3.1. Mechanism for Silaboration of Allenes. The observa-
tion that an aryl iodide, alkenyl iodide, I2, or Me3SiI was
required for the catalytic reaction and that the use of organic
iodide changes completely the regiochemistry of the catalytic
reaction is crucial to the understanding of the catalytic mech-
anism. To account for the present palladium-catalyzed and
organic halide-initiated silaboration of allenes, possible path-
ways, as depicted in Schemes 4 and 5, are proposed. The organic
Scheme 5
While the presence of 3a is essential for the catalytic reaction
(see Scheme 4) to proceed, 3a can be replaced by other organic
Scheme 4
iodide likely acts as an initiator undergoing oxidative addition
with Pd(0) to give a palladium(II) intermediate that reacts with
allene 1 and borylsilane 5 to afford product 8 and a common
product silyl iodide 9 (Scheme 4), respectively. Compound 9
then reacts with Pd(0) to begin the catalytic reaction (Scheme
5). Oxidative addition of 9 to Pd(0) species affording Pd(II)
intermediate 10 is followed by coordination and insertion of
allene 1 to give Pd-allyl species 12, with the silyl group
attached to the central carbon of the allyl group. Transmetalation
of 12 with borylsilane 5 regenerates 9 and affords Pd-allyl
species 13. Reductive elimination of 13 yields the final product
7 and regenerates the Pd(0) catalyst. The formation of 8 is
evidenced by the observation of 8b (R1 ) R2 ) Me; R )
p-MeCOC6H4) in GC-MS and 1H NMR in ca. 4% yield from
the reaction of 1,1-dimethylallene (1n), borylsilane 5, and
p-iodoacetophenone (3b). The role of silyl iodide 9 in the
catalytic cycle is also supported by the fact that Me3SiI is an
effective initiator for the present catalytic reaction. The observa-
tion that phosphine retards the catalytic reaction may be
explained in terms of the competition of phosphine and the
allene substrate for coordination to palladium intermediate 10.
iodides. Aryl iodides, such as p-iodoanisole and p-iodoacetophe-
none, are also efficient initiators for the catalysis, giving 7a in
85 and 87% yields, respectively. Interestingly, I2 and Me3SiI
also promote the silaboration of 1a successfully to afford 7a in
85 and 75% yields, respectively. On the other hand, the use of
4-bromoacetophenone gave only a trace of the desired product.
Under similar reaction conditions, various monosubstituted
aryl and alkylallenes RCHdCdCH2 (1b-m) and 1,1-dimethy-
lallene (1n) underwent silaboration with 5 in the presence of
Pd(dba)2 and 3a to give the corresponding silaboration products
7b-n in moderate to excellent yields. As revealed in Scheme
3 and Table 1, the addition reaction is compatible with various
functional groups on the allenes (entries 3-10). Again, all of
these reactions are completely regioselective. A key evidence
for the proposed regiochemistry with the boryl moiety adding
to the unsubstituted terminal carbon of the allene moiety is the
9
128 J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005

Palladium-Catalyzed Silaboration of Allenes
A R T I C L E S
Table 2. Results of Allylation of Aldhydesa
Similar behavior was found in the carbosilylation of allenes
catalyzed by palladium complexes.11a,c
The nature of the present catalytic silaboration reaction is
further unraveled from the results of silaboration of phenylallene
(1a) with borylsilane 5 in the presence of various amount of
3-iodo-2-methyl-2-cyclohexen-1-one (3a) catalyzed by Pd(dba)2
(5%). In the absence of 3a, the reaction gave products 6a and
6b (see Scheme 2), while in the presence of 2-4% 3a relative
to borylsilane 5, the silaboration gave a mixture of 6a, 6b, and
7a. Products 6a and 6b were totally inhibited as 3a employed
in the reaction solution was increased to 8%. Further increase
of 3a did not significantly change the yield of 7a, and a
substantial amount of organic iodide 3a was left unreacted in
the solution. These experiments appear to show that silaboration
of allenes via direct oxidative addition of 5 to Pd(0) (Scheme
2) is completely inhibited if the organic iodide 3a present is
greater than the Pd catalyst used. Similar results were also
observed for the silaboration of allene 1n with borylsilane 5 in
the presence of various amount of p-iodoacetophenone (3b)
catalyzed by Pd(dba)2. The above results indicate that silyl
iodide 9 is much more reactive toward Pd(0) than most organic
iodides 3. In summary, during the present catalytic reaction,
organic iodide 3, borylsilane 5, and silyl iodide 9 are competing
for the palladium(0), and it appears that 5 , 3 , 9 in the
reactivity for the oxidative addition to the palladium(0). The
initial amount of silyl iodide 9 produced was controlled by the
reaction shown in Scheme 4. The high reactivity of 9 compared
to that of 3 toward Pd(0) leads to the result that the three-
component assembling reaction in Scheme 4, mediated by the
Pd complex, is limited to about one turnover. The amount of
three-component assembling product 8 (4% for 8b (R1 ) R2 )
Me; R ) p-MeCOC6H4) is approximately equal to that of the
Pd catalyst used (5%).
1
2
3
4
5
6
7
8
9
1a Ph
1a Ph
1a Ph
1a Ph
1a Ph
1a Ph
1a Ph
1a Ph
1a Ph
14a Ph
15a
>99/1 96
14b p-MeOC6H4
15b >99/1 55
14c p-MeOCOC6H4 15c
>99/1 93
14d p-ClC6H4
14e p-MeC6H4
14f p-MeOCC6H4
14g p-CNC6H4
14h 2-thienyl
14i n-pentyl
15d >99/1 92
15e
15f
15g
>99/1 70
>99/1 60
>99/1 85
15h >99/1 74
15i
15j
15k >99/1 86
15l
>99/1 67
>99/1 70
10 1a Ph
14j n-propyl
11 1i p-MeOCC6H4 14a Ph
12 1j p-EtO2CC6H4 14a Ph
>99/1 87
13 1c p-MeOC6H4
14a Ph
14a Ph
14a Ph
14a Ph
15m >99/1 70
14c 1k n-Bu
15n 95/5
15o 93/7
15p 93/7
92
75
85
15 1o cyclohexyl
16 1l cyclopentyl
a Unless stated otherwise, all reactions were carried out using 3a (0.0500
mmol, 10 mol %), Pd(dba)2 (0.0250 mmol, 5 mol %), borylsilane 5 (0.500
mmol), allene 1 (1.00 mmol), and aldehyde 14 (1.00 mmol) in ethyl acetate
(0.50 mL) at 80 °C for 5 h. b Isolated yields based on the borylsilane used.
c The reaction experiment was run for 48 h at room temperature instead of
80 °C for 5 h.
4. Application of Silaboration of Allenes
4.1. Stereoselective One-Pot Allylation of Aldehydes. The
allylation of aldehydes to give homoallylic alcohols is one of
the most powerful synthetic methods in modern organic chemi-
stry.17 Functionalized homoallylic alcohols are a class of val-
uable synthetic intermediates used in the construction of a wide
variety of complex natural products.18 Despite numerous
methods that are available, only a few of them offer highly
stereocontrolled routes for the allylation of aldehydes.19 Herein,
we wish to describe a new and convenient way for the prepa-
ration of homoallylic alcohols via the application of the present
silaboration chemistry.
Treatment of phenylallene (1a) (2 equiv) with borylsilane 5
and benzaldehyde (14a) (2 equiv) in the presence of Pd(dba)2
(5 mol %) and alkenyl iodide 3a (10 mol % relative to 5) at 80
°C in ethyl acetate for 5 h afforded homoallylic alcohol 15a in
96% yield (Scheme 7 and Table 2) with excellent syn selectivity
The observed regioselectivity is determined at the step of
transmetalation of 12 with 5. A completely different regiose-
lectivity for the product would result if a boryl iodide was
generated instead of silyl iodide 9. The observed stereoselectivity
may be explained on the basis of a face-selective coordination
of allenes to the Pd center (Scheme 6). For a monosubstituted
Scheme 7
Scheme 6
(>99%). Product 15a with the silyl group of 5 adding to the
central carbon and the benzaldehyde moiety adding to the
phenyl-substituted carbon of allene 1a was fully characterized
1
1
by its H and 13C NMR and mass spectral data. The H NMR
spectrum of 15a showed characteristic resonances at δ 6.13 and
(16) For the synthesis of 2-(dimethylphenylsilanyl)-4,4,5,5-tetramethyl[1,3,2]-
dioxaborolane (5), see: Suginome, M.; Matsuda, T.; Ito, Y. Organometallics
2000, 19, 4647.
(17) For a recent review on allylmetal addition, see: (a) Denmark, S. E.;
Almstead, N. G. In Modern Carbonyl Chemistry; Otera, J., Ed.; Wiley-
VCH: Weinheim, Germany, 2000; Chapter 10, pp 299-402. (b) Stereo-
selectiVe Synthesis, Methods of Organic Chemistry (Houben-Weyl), Edition
E21; Helmchen, G., Hoffmann, R., Mulzer, J., Schaumann, E., Eds.;
Thieme: Stuttgart, Germany, 1996; Vol. 3, p 1357.
(18) Chemler, S. R.; Roush, W. R. In Modern Carbonyl Chemistry; Otera, J.,
Ed.; Wiley-VCH: Weinheim, Germany, 2000; Chapter 10, pp 403-490.
(19) For a mini review on allylation reaction by addition of boron and silicon
reagents, see: Kennedy, J. W. J.; Hall, D. G. Angew. Chem., Int. Ed. 2003,
42, 4732.
allene, selective coordination of the terminal double bond to
the Pd moiety at the face opposite to the substituents R1
(structure 11a) is favorable to avoid steric congestion. This face
selection leads to the formation of Pd-π-allyl species 12, having
an anti stereochemistry, and eventually to the formation of
E-form products.
9
J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005 129

A R T I C L E S
Chang et al.
Scheme 9
5.67 ppm for the geminal methylene protons and at δ 5.14 and
3.79 ppm, both as doublets for the protons on the methine car-
bons to which the OH and R1 (Ph) groups are attached, respec-
tively. This multiple component assembling reaction provides
a unique route for the synthesis of homoallylic alcohols in one
pot in high yields with excellent regio- and stereoselectivity.
The present allylation of benzaldehyde (14a) also proceeded if
14a was added after the silaboration of phenylallene by boryl-
silane 5 was complete, but the yield of product 15a is only 72%.
To examine the scope and limitation of this Pd-catalyzed and
organic iodide-initiated reaction, different allenes and aldehydes
were tested, and the results are summarized in Table 2. Under
reaction conditions similar to those for the formation of 15a,
various aromatic and aliphatic aldehydes 14b-j react effectively
with phenylallene (1a) and borylsilane 5 to afford the corre-
sponding homoallylic products 15b-j, respectively, in good to
excellent yields with very high syn selectivity (entries 2-10).
Even thiophene-2-carbaldehyde reacts with phenylallene and
borylsilane to give the expected homoallylic alcohol 15h in 74%
yield (entry 8). In addition to phenylallene (1a), substituted
phenylallenes also reacted smoothly with benzaldehyde 14a and
5 to furnish the corresponding allylation products 15k-m in
70-87% yields, with extremely high syn selectivity. Similarly,
the assembling reaction of aliphatic allenes, such as n-butylallene
1k, cyclopentylallene 1l, and cyclohexylallene 1o, with 5 and
14a furnishes the corresponding homoallylic alcohols 15n-p
in 75-92% yields. The reaction tolerates a variety of function-
alities, such as alkyl, cyano, chloro, acetyl, methoxy, and ester
groups, on the aromatic ring of aldehydes and of allenes. In all
cases, syn homoallylic alcohols are formed predominately. The
syn/anti ratios are greater than 99/1 for aromatic allenes (entries
1-13), but the ratios drop slightly to ∼93/7 for aliphatic allenes
(entries 14-16). The syn stereochemistry of product 15o was
further confirmed by the desilylation of this compound by TBAF
((n-Bu)4N+F-) in THF at room temperature (Scheme 8). The
There are several appealing features of the present reaction.
First, the reaction proceeds via multiple bond formation and
breaking steps, and yet the final homoallylic alcohols are
obtained in excellent yields and stereoselectivity, indicating that
each step in the reaction is highly efficient and selective. Second,
in contrast to most allylation reactions of aldehydes using
allylboronates, the formation of allylic alcohol 15 does not
require an external Lewis acid, such as AlCl3, BF3‚OEt2, or
Sc(OTf)3.21,22 Third, unlike other reactions known for the
synthesis of allylic alcohols, the present allylation of aldehydes
proceeds via a one-pot multicomponent assembling reaction of
allenes, borylsilanes, and aldehydes. Finally, these homoallylic
alcohols consist of a vinylsilyl group,23 providing the possibility
for further transformation.
4.2. Mechanism for Allylation of Aldehydes. The formation
of homoallylic alcohols from this palladium-catalyzed reaction
of allenes, silaborane, and aldehydes is intriguing in view of
the extensive bond formation and breaking processes required.
The catalytic reaction probably occurs via two separated parts.
A mechanistic pathway similar to that for the silaboration of
allenes, as shown in Schemes 4 and 5, to give the corresponding
2-silylallylboronates 7 likely occurs first. The 2-silylallylbor-
onate product then undergoes allylation with aldehydes 14 in a
highly selective fashion to give the final homoallylic alcohols
15. A key evidence for this two-part mechanism comes from
the observation that silaboration product 7a reacts with ben-
zaldehyde (14a) directly at room temperature for 24 h to afford
the expected homoallylic product 15a in 60% yield. To account
for the exceedingly high syn selectivity of the reaction, we
proposed that the allylation of aldehydes by 2-silylallylboronates
7 proceeds via a six-membered chairform cyclic transition state
A, with the silyl and R3 group from the aldehyde occupying
the equatorial positions and the R1 from allene occupying the
axial position of A, as depicted in Scheme 10. Similar cyclic
Scheme 8
1H NMR data of desilylated product 16o are identical to those
of a compound reported previously that has the same structure
and stereochemistry as shown in 16o.20 The scope of the present
palladium-catalyzed multiple component reaction is limited to
aldehydes and monosubstituted allenes. Attempts to use ketones
and disubstituted allenes, such as 1n, as substrates for the
reaction failed to give any expected products.
Scheme 10
The excellent E selectivity of the silaboration product formed
in situ is responsible for the formation of homoallylic alcohols
with excellent syn selectivity. The product is formed with the
preserVation of diastereospecificity, and the results are in
agreement with the literature methods for the addition of
allylboranes to aldehydes.19 For example, it is known that the
(E)- or (Z)-crotylboron pinacolates react with benzaldehyde in
the presence of Sc(OTf)3 to give homoallylic products with
exclusive anti and syn selectivity, respectively (Scheme 9).21
transition states have been employed for the allylation of
aldehydes by various allylmetal reagents.19
(21) Ishiyama, T.; Ahiko, T. A.; Miyaura, N. J. Am. Chem. Soc, 2002, 124,
12414.
(22) Lachance, H.; Lu, X.; Gravel, M.; Hall, D. J. Am. Chem. Soc. 2003, 125,
10160 and references therein.
(23) Colvin, E. W. Silicon in Organic Synthesis; Butterworth: London, 1981;
pp 97-124. (b) Weber, W. P. Silicon Reagents for Organic Synthesis;
Springer: Berlin, 1983; pp 173-205. (c) Colvin, E. W. Silicon Reagents
in Organic Synthesis; Academic Press: London, 1988; pp 25-37. (d) The
Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.;
Wiley: Chichester, U.K., 1988; Part 2. (e) Fleming, I.; Barbero, A.; Walter,
D. Chem. ReV. 1997, 97, 2063.
(20) Chang, H.-M.; Cheng, C.-H. Org. Lett. 2000, 2, 3439. (b) Brown, H. C.;
Narla, G. Tetrahedron Lett. 1997, 38, 219. (c) Knochel, P.; Sidduri, A.;
Rozema, M. J. Org. Chem. 1993, 58, 2694.
9
130 J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005

Palladium-Catalyzed Silaboration of Allenes
A R T I C L E S
mmol, 10 mol %), Pd(dba)2 (0.0250 mmol, 5 mol %), and 2-(dimeth-
ylphenylsilanyl)-4,4,5,5-tetramethyl[1,3,2]dioxaborolane (5) (0.500 mmol).
The system was evacuated and purged with nitrogen three times. Ethyl
acetate (0.50 mL) and allene (1) (1.00 mmol) were added to the system,
and the reaction mixture was stirred at 80 °C for 5 h. After the reaction
was complete, the reaction mixture was concentrated. The residue was
distilled over a Kugelrohr oven to give the desired product. Compounds
7a-n were prepared according to this method. The product yield of
each reaction is listed in Table 1, while the spectroscopic data of these
compounds are shown in the Supporting Information.
5. Conclusions
We have demonstrated a new and efficient method for
silaboration of allenes catalyzed by palladium complexes and
initiated by organic iodides. The present methodology provides
a convenient and general route to various 2-silylallylboronates
7 in good yields with excellent regio- and stereoselectivity. The
mechanism for the silaboration of allenes is extraordinary and
proceeds via an unusual three-component assembling pathway.
The products formed are totally different in regio- and stereo-
chemistry from those reported previously using palladium
complexes as the catalysts. In addition, a new method for the
stereoselective synthesis of homoallylic alcohols from the
reaction of borylsilane, allenes, and aldehydes catalyzed by
palladium complexes and initiated by organic iodide is de-
scribed. Extensive bond formation and breaking processes are
required for the formation of homoallylic alcohols, and yet the
yield and selectivity are excellent. The methodology is compat-
ible with a wide variety of functional groups on both allenes
and aldehydes.
Procedure for the Allylation of Aldehydes. To a 10 mL sidearm
flask were added 3-iodo-2-methyl-2-cyclohexen-1-one (3a) (0.0500
mmol, 10 mol %), Pd(dba)2 (0.0250 mmol, 5 mol %), and 2-(dimeth-
ylphenylsilanyl)-4,4,5,5-tetramethyl[1,3,2]dioxaborolane (5) (0.500 mmol).
The system was evacuated and purged with nitrogen three times. Ethyl
acetate (0.5 mL), allene (1) (1.00 mmol), and aldehyde (14) (1.00 mmol)
were added to the system, and the reaction mixture was stirred at 80
°C for 5 h. After the reaction was complete, ethyl acetate (50 mL) was
added to the reaction mixture, and the mixture was washed with brine
(25 mL) three times. The organic layer was dried over anhydrous
magnesium sulfate and concentrated in vacuo. The residue was
chromatographed on a silica gel column using ethyl acetate and hexanes
as eluent to give the desired three-component assembly product 15.
Compounds 15a-p were prepared according to this method. The
product yield of each reaction is listed in Table 2, while the
spectroscopic data of these products are shown in the Supporting
Information.
6. Experimental Section
Procedure for 2-(Dimethylphenylsilanyl)-4,4,5,5-tetramethyl-
[1,3,2]dioxaborolane (5). To a stirred solution of 4,4,5,5-tetramethyl-
[1,3,2]dioxaborolane (pinacolborane) (128.0 mmol) in hexane (60 mL)
was added dimethylphenylsilyllithium (ca. 1.0 mol/L in THF, 62 mL,
62 mmol) dropwise at 0 °C over 30 min. Dimethylphenylsilyllithium
(ca. 1.0 mol/L in THF) was freshly prepared by treating chlorodim-
ethylphenylsilane with 4.0 equiv of lithium in THF. After the addition,
the cooling bath was removed. The mixture was stirred overnight at
room temperature. Evaporation of the volatile materials gave a white
residual solid, which was taken up to remove material insoluble in
hexane. After suction filtration under nitrogen, the filtrate was
concentrated in vacuo. Distillation of the residue gave silylborane 5 as
colorless liquid.16
Acknowledgment. We thank the National Science Council
of the Republic of China (NSC 92-2113-M-007-044) for the
support of this research.
Supporting Information Available: Spectroscopic data and
1H NMR spectra for compounds 7a-n, 8b, and 15a-p. NOE
experimental data of compounds 7a, 7k, and 7l. This material
is available free of charge via the Internet at http://pubs.acs.org
Procedure for the Silaboration of Allenes. To a 10 mL sidearm
flask were added 3-iodo-2-methyl-2-cyclohexen-1-one (3a) (0.0500
JA044662Q
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J. AM. CHEM. SOC. VOL. 127, NO. 1, 2005 131