Regiocontrolled addition of arylboronic acids to allenes using palladium and platinum catalysts

Article abstract of DOI:10.1021/ol8021484

(Chemical Equation Presented) Studies about the regiocontrolled addition of arylboronic acids to allenes using a palladium or a platinum catalyst have been described. The selectivity of the reaction can be altered by the choice of the metal reagent and base. Contrary to the formation of endo-olefinic products in the reactions using hydroxopalladium complex, predominant production of exo-olefinic products was observed by the reaction with hydroxoplatinum complex.

Full text of DOI:10.1021/ol8021484

ORGANIC
LETTERS
2008
Vol. 10, No. 22
5183-5186
Regiocontrolled Addition of Arylboronic
Acids to Allenes Using Palladium and
Platinum Catalysts
Masahiro Yoshida,*,† Kennosuke Matsuda,† Yasunobu Shoji,† Takahiro Gotou,‡
Masataka Ihara,§ and Kozo Shishido†
Graduate School of Pharmaceutical Sciences, The UniVersity of Tokushima,
1-78-1 Sho-machi, Tokushima 770-8505, Japan, Graduate School of Pharmaceutical
Sciences, Tohoku UniVersity, Aobayama, Sendai 980-8578, Japan, and Institute of
Medicinal Chemistry, Hoshi UniVersity, 2-4-41 Ebara, Shinagawa, 142-8501 Japan
yoshida@ph.tokushima-u.ac.jp
Received September 16, 2008
ABSTRACT
Studies about the regiocontrolled addition of arylboronic acids to allenes using a palladium or a platinum catalyst have been described. The
selectivity of the reaction can be altered by the choice of the metal reagent and base. Contrary to the formation of endo-olefinic products in
the reactions using hydroxopalladium complex, predominant production of exo-olefinic products was observed by the reaction with
hydroxoplatinum complex.
The unique reactivity of allenes which derives from the
existence of two orthogonal π-bonds makes them impor-
tant compounds in organic and synthetic organic chem-
istry. As a result, the reactions of allenes have been the
object of extensive examination.1 Moreover, during the
last two decades, the reactivity of allenes, particularly in
the presence of transition-metal catalysts, has attracted
increasing interest.1,2 Oh and Ma reported the palladium-
catalyzed addition of organoboronic acids to allenes under
acidic conditions, in which tri- and tetrasubstituted alkenes
were stereoselectively produced.3,4 During the course of our
studies on transition-metal-catalyzed reactions of allenes,5
we found that similar reactions occurred using a hydroxo-
(3) (a) Oh, C. H.; Ahn, T. W.; Reddy, V. R. Chem. Commun. 2003,
2622. (b) Ma, S.; Jiao, N.; Ye, L. Chem.sEur. J. 2003, 9, 6049. (c) Ma,
S.; Guo, H.; Yu, F. J. Org. Chem. 2006, 71, 6634. (d) Guo, H.; Ma, S.
Synthesis 2007, 2731
.
(4) Other examples of transition-metal-catalyzed reactions of allenes with
organoboron compounds include the following: (a) Yamamoto, Y.; Fujika-
wa, R.; Yamada, A.; Miyaura, N. Chem. Lett. 1999, 1069. (b) Onozawa,
S.; Hatanaka, Y.; Tanaka, M. Chem. Commun. 1999, 1863. (c) Suginome,
M.; Ohmori, Y.; Ito, Y. J. Organomet. Chem. 2000, 611, 403. (d) Hopkins,
C. D.; Guan, L.; Malinakova, H. C. J. Org. Chem. 2005, 70, 6848. (e)
Takahashi, G.; Shirakawa, E.; Tsuchimoto, T.; Kawakami, Y. AdV. Synth.
Catal. 2006, 348, 837. (f) Nishimura, T.; Hirabayashi, S.; Yasuhara, Y.;
Hayashi, T. J. Am. Chem. Soc. 2006, 128, 2556. (g) Tsukamoto, H.;
† The University of Tokushima.
‡ Tohoku University.
§ Hoshi University.
(1) (a) Schuster, H. F.; Coppola, G. M. Allenes in Organic Synthesis;
John Wiley & Sons: New York, 1988. (b) Krause, N.; Hashmi, A. S. K.
Modern Allene Chemistry; VCH Verlagsgesellschaft Mbh: Weinheim, 2004.
(c) Hashmi, A. S. K. Angew. Chem., Int. Ed. 2000, 39, 3590. (d) Brummond,
K. M.; DeForrest, J. E. Synthesis 2007, 795.
Matsumoto, T.; Kondo, Y. Org. Lett. 2008, 10, 1047.
(5) (a) Nemoto, H.; Yoshida, M.; Fukumoto, K. J. Org. Chem. 1997,
62, 6450. (b) Yoshida, M.; Sugimoto, K.; Ihara, M. Tetrahedron Lett. 2000,
41, 5089. (c) Yoshida, M.; Sugimoto, K.; Ihara, M. Tetrahedron Lett. 2001,
42, 3877. (d) Yoshida, M.; Sugimoto, K.; Ihara, M. Tetrahedron 2002, 58,
7839. (e) Yoshida, M.; Gotou, T.; Ihara, M. Tetrahedron Lett. 2003, 44,
7151. (f) Yoshida, M.; Gotou, T.; Ihara, M. Chem. Commun. 2004, 1124.
(2) Zimmer, R.; Dinesh, C. U.; Nadanan, E.; Khan, F. A. Chem. ReV.
2000, 100, 3067
.
10.1021/ol8021484 CCC: $40.75
Published on Web 10/22/2008
2008 American Chemical Society

palladium complex [Pd2(OH)2(PPh3)4][BF4]2 (1)6 under aque-
ous basic conditions. Furthermore, it was revealed that using
the hydroxoplatinum complex [Pt2(OH)2(PPh3)4][BF4]2 (2)
was key to producing one regioisomer selectively. Herein,
we describe our results.
Table 2. Reactions of 3a with Various Arylboronic Acids
4b-ga
In our previous study involving the palladium-catalyzed
direct coupling reaction of allenic alcohol 3a with
phenylboronic acid (4a),5f it was shown that adduct 6aa
having an endo olefin was obtained predominately in 73%
yield along with 8% of diene 5aa when the reaction was
conducted under aqueous conditions (dioxane/H2O )20/
1) (entry 1 in Table 1). Although the reaction in the
products (yield, %)
entry
Ar
conditions
5
6
7
1
2
3
4
5
6
7
8
9
2-methylphenyl (4b)
2-methylphenyl (4b)
4-methylphenyl (4c)
4-methylphenyl (4c)
2-methoxyphenyl (4d)
2-methoxyphenyl (4d)
4-methoxyphenyl (4e)
4-methoxyphenyl (4e)
1-naphthalene (4f)
A
B
A
B
A
B
A
B
A
B
A
B
6ab (91) trace
7ab (70)
6ac (80) trace
5ab (18)
5ac (19) 6ac (6) 7ac (59)
6ad (88) trace
5ad (18) 6ad (10) 7ad (59)
6ae (83) trace
Table 1. Palladium- and Platinum-Catalyzed Addition of
Arylboronic Acid 4a to Allenic Alcohol 3a
5ae (23) trace
7ae (61)
6af (96) trace
10 1-naphthalene (4f)
11 4-acetylphenyl (4g)
12 4-acetylphenyl (4g)
5af (38) trace
7af (58)
6ag (68) trace
5ag (18) trace
7ag (39)
a Conditions A: Reactions were carried out in the presence of 5 mol %
of 1 and 5 equiv of Et3N in dioxane/H2O (20/1) at 80 °C. Conditions B:
Reactions were carried out in the presence of 5 mol % of 2 and 5 equiv of
KOH in dioxane/H2O (20/1) at 100 °C.
yields, %
acid (4d). When the siloxyethyl-substituted allene 3b was
subjected to reactions under palladium- and platinum-
catalyzed conditions, adducts 6bd and 7bd, respectively,
were selectively obtained (entries 1 and 2). It is noteworthy
that the yield of the exo-olefinic product 7bd increased
(entry 2). The substrates 3c-f having other alkyl side
chains also reacted regioselectively with 4d to afford the
corresponding products 6cd-fd or 7cd-fd, depending on
the reaction conditions (entries 3-10). Similarly, while
the reactions of the aryl-substituted allenes 3g and 3h
using the palladium catalyst 1 provided 6gd and 6hd as
the sole products, the regioisomers 7gd and 7hd were
selectively produced in the presence of the platinum
catalyst 2 (entries 11-14).
A plausible mechanism for the reaction is shown in
Scheme 1. It is presumed that the (dihydroxo)metal
complex 1′ or 2′ exists as an active species in this reaction
and that a catalytic cycle would involve the transmetalation
with the arylboronic acid to give the arylmetal complex
8 as the initial step. Although it is not clear why the
observed regioselectivities depend on the metal species,7
it is presumed that the insertion of the allene to 8 is the
crucial step. In the reaction using 1 with Et3N, palladium
undergoes coordination with the amine to increase the
electron density of the corresponding complex Pd-8. As
a result, regio- and stereoselective insertion occurs at the
relatively electron-poor terminal double bond of the allene
9, leading to the allylpalladium 10. The complex 10 is
then subjected to hydrolysis with water to afford the endo-
olefinic product 6. In the reaction of 2 and KOH, the
resulting arylplatinum species Pt-8 is preferentially
coordinated to the electron-rich internal double bond 11
entry
reagents
5aa 6aa 7aa
1
2
3
Pd2(dba)3·CHCl3, 40 mol % PPh3
[Pd2(OH)2(PPh3)4][BF4]2 (1)
[Pd2(OH)2(PPh3)4][BF4]2 (1), Et3N
[Pt2(OH)2(PPh3)4][BF4]2 (2), Et3N
[Pt2(OH)2(PPh3)4][BF4]2 (2), KOH
8
37
3
47
20
73
6
81
16
2
4
7
66
5a
a The reaction was carried out at 100 °C.
presence of only 1 was unsuccessful (entry 2), the
reactivity was dramatically improved by the addition of
Et3N to afford 6aa in 81% yield (entry 3). When the
hydroxoplatinum complex 2 was used as the catalyst, a
small amount of the exo-olefinic adduct 7aa, the regioi-
somer of 6aa, was generated in 7% yield with the
production of 5aa and 6aa being major (entry 4).
However, the yield of 7aa increased to 66% when the
reaction was conducted in the presence of KOH (entry
5).
Results of the reactions of 3a with various arylboronic
acids 4b-g in the presence of palladium or platinum
complexes 1 or 2 are summarized in Table 2. The endo-
olefinic products 6ab-ag were regio- and stereoselectively
obtained in high yields from the reactions of 4b-g in the
presence of 1 and Et3N (conditions A, entries 1, 3, 5, 7,
9, and 11). On the other hand, predominant production of
the exo-olefinic products 7ab-ag was observed in moder-
ate to good yields along with dienes 5ab-5ag when the
reactions were carried out with KOH in the presence of 2
(conditions B, entries 2, 4, 6, 8, 10, and 12).
Table 3 shows results of the reactions of various
substituted allenes 3b-3h with 2-methoxyphenylboronic
(6) Bushnell, G. W.; Dixon, K. R.; Hunter, R. G.; McFarland, J. J. Can.
J. Chem. 1972, 50, 3694.
(7) Control of regiochemistry depending on the metal catalyst has been
observed in the hydro- and silaboration of allenes; see refs 4a and 4b.
5184
Org. Lett., Vol. 10, No. 22, 2008

Scheme 1
Table 3. Reactions of Various Allenes 3b-h with 4da
the deuterium was incorporated at the allylic position to
give 6ab-D. Similarly, the corresponding deuterated
Scheme 2
a Conditions A: Reactions were carried out in the presence of 5 mol %
of 1 and 5 equiv of Et3N in dioxane/H2O (20/1) at 80 °C. Conditions B:
Reactions were carried out in the presence of 5 mol % of 2 and 5 equiv of
KOH in dioxane/H2O (20/1) at 100 °C. b The ratios were determined by
1H NMR integration of the olefinic proton signals. c The ratio was
determined by the isolation of each product.
product 7ab-D was obtained when the platinum-catalyzed
reaction was carried out (62% deuterated). These results
support the hypothesis that the reactions proceeds via the
formation of the allylmetal intermediates 10 and 12,
respectively.
due to the comparatively low electron density of Pt-8
and furnishes predominantly the exo-olefinic product 7 via
the allylplatinum intermediate 12. The diene 5, which is
a byproduct in the platinum-catalyzed reaction of the
allenic alcohol, results from the ꢀ-OH elimination of the
corresponding allylplatinum intermediate 12′.
Scheme 3
Another pathway, in which the initially formed exo-
olefinic product 7 isomerizes to the thermodynamically
more stable 6, is possible. We subjected the isolated 7cd
to the palladium-catalyzed reaction with 4d for the
detection of isomerization (Scheme 2). However, no
reaction occurred, which implies that the endo-olefinic
product is produced directly via the allylpalladium.
Additional information on the reaction mechanism was
gained when the reaction of the allenyl alcohol 3a with
2-methylphenylboronic acid (4b) was conducted in D2O
(Scheme 3). In the palladium-catalyzed reaction, 70% of
In summary, the studies described above have resulted in
the regiocontrolled addition of arylboronic acids to allenes
Org. Lett., Vol. 10, No. 22, 2008
5185

using a palladium or a platinum catalyst. It is noteworthy
that the selectivity of the reaction can be altered by the choice
of the metal reagent and base. Continuing studies probing
the scope and synthetic applications of this reaction are now
in progress.
from the Japan Society for the Promotion of Science (JSPS)
and the Research Foundation for Pharmaceutical Sciences.
Supporting Information Available: Experimental pro-
cedures and compound characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. This study was supported in part by a
OL8021484
Grant-in-Aid for the Encouragement of Young Scientists (B)
5186
Org. Lett., Vol. 10, No. 22, 2008