(π-Allyl)palladium complexes bearing diphosphinidenecyclobutene ligands (DPCB): Highly active catalysts for direct conversion of allylic alcohols

Article abstract of DOI:10.1021/ja0274406

The (π-allyl)palladium complex bearing an sp²-hybridized phosphorus ligand (DPCB-OMe: 1,2-bis(4-methoxyphenyl)-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene) efficiently catalyzes direct conversion of allylic alcohols in the absence of activating agents of alcohols such as Lewis acids. N-Allylation of aniline proceeds at room temperature to afford monoallylated anilines in 90-97% yields. C-Allylation of active methylene compounds is also successful at 50 °C using a catalytic amount of pyridine as a base, giving monoallylation products in 85-95% yields. The catalytic mechanism involving hydrido- and (π-allyl)palladium intermediates has been proposed on the basis of stoichiometric examinations using model compounds of presumed intermediates. Copyright

Full text of DOI:10.1021/ja0274406

Published on Web 08/20/2002
(π-Allyl)palladium Complexes Bearing Diphosphinidenecyclobutene Ligands
(DPCB): Highly Active Catalysts for Direct Conversion of Allylic Alcohols
Fumiyuki Ozawa,*,† Hideyuki Okamoto,† Seiji Kawagishi,† Shogo Yamamoto,† Tatsuya Minami,† and
Masaaki Yoshifuji‡
Department of Applied Chemistry, Graduate School of Engineering, Osaka City UniVersity, Osaka 558-8585, Japan,
and Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-8578, Japan
Received June 24, 2002
Table 1. Catalytic Allylation of Aniline with Allylic Alcoholsa
Palladium-catalyzed allylation is a reliable and widely used
method for constructing C-C, C-N, and C-O bonds in organic
synthesis.1 This catalysis is generally performed with allylic
carboxylates, carbonates, phosphates, and related compounds as
substrates. Because these substrates are synthesized from the
corresponding allylic alcohols, palladium catalysts that enable the
conversion of allylic alcohols directly into allylation products are
highly desirable, especially from the viewpoint of the atom
economy.2 Although several attempts have been reported in this
connection,3 most of them require rather severe reaction conditions,
and the design of really effective catalysts for direct conversion of
allylic alcohols still remains a major objective in catalytic allylation.
This is apparently due to the poor leaving ability of the OH group.
Accordingly, catalytic conversion of allylic alcohols has been
examined in most cases through in situ activation of the OH group
with the aid of Lewis acids (e.g., Ti(OPri)4, BEt3, BPh3, SnCl2)4 or
by converting it into the esters of inorganic acids (e.g., As2O3, B2O3,
CO2).5 We herein report that (π-allyl)palladium complexes (1)
bearing sp2-hybridized phosphorus ligands (diphosphinidenecy-
a Reaction conditions: 1.0 mmol (allyl)OH, 2.0 mmol PhNH2, 0.1 mol
clobutene: DPCB-Y)6,7 effectively catalyze the direct conversion
of allylic alcohols in the absence of activating agents (eq 1).
% 1a, 1 mL of toluene, 0.25 g of MgSO4, room temperature. b Monoallyl-
ation products in runs 4-7 were obtained as a mixture of stereo- and/or
regioisomers, whose ratio was determined by GLC. c 1b was used in place
of 1a. d 1c was used in place of 1a. e 1a was used in 2 mol %.
Table 2. Catalytic Allylation of Active Methylene Compoundsa
a Reaction conditions: 2.0 mmol (allyl)OH, 4.0 mmol CH2Z2, 2 mol %
1a, 10 mol % pyridine, 0.25 g of MgSO4, 50 °C.
Tables 1 and 2 list the representative results. It has been reported
that, in the presence of Pd(OAc)2/4PPh3 (1 mol %) as a catalyst
and Ti(OPri)4 (25 mol %) as an activating agent, catalytic allylation
of aniline derivatives with allylic alcohols takes place under heated
conditions (50-80 °C).4b On the other hand, similar reactions
between aniline and six kinds of allylic alcohols (2a-f) proceeded
only with 0.1 mol % 1a at room temperature (Table 1, runs 1 and
4-8). Complexes 1b and 1c exhibited comparable catalytic activity,
although the amount of diallylation product was higher than that
for 1a (runs 2 and 3). Catalysts 1a-c are fairly stable toward
oxidation, and the catalytic reactions could be carried out in the
air.8 The two regioisomers of butenyl alcohol (2b and 2c) were
converted into N-(2-butenyl)aniline and N-(1-methyl-2-propenyl)-
* To whom correspondence should be addressed. E-mail: ozawa@a-chem.en-
g.osaka-cu.ac.jp.
† Osaka City University.
‡ Tohoku University.
9
10968
J. AM. CHEM. SOC. 2002, 124, 10968-10969
10.1021/ja0274406 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
Scheme 3
Scheme 2
negatively in the ground state and the acidity develops when a
special driving force that causes deprotonation exists.11 In the
present case, we may consider the unique coordination property of
DPCB to serve as a powerful driving force. Thus, DPCB as a low-
coordinate phosphorus compound bears an extremely low-lying π*
orbital, mainly located around the sp2 phosphorus atoms, and has
a marked tendency to engage in metal-to-phosphorus π back-
bonding.12 Consequently, DPCB may effectively stabilize 10 as a
Pd(0) species, rather than 9 having a cationic Pd(II) center. This
situation should facilitate the conversion of 9 to 10 via proton
transfer and results in the C-O bond cleavage of allylic alcohols.
Acknowledgment. This paper is dedicated to Prof. Robert H.
Grubbs (Caltech) on the occasion of his 60th birthday. We thank
Dr. Kunihiko Murata (Kanto Chemicals) for the gift of 2g.
Supporting Information Available: Experimental details (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
aniline in almost the same regio- and stereoselectivity (runs 4 and
5). Similarly, the reactions of the (E)- and (Z)-isomers of 2-hexenyl
alcohol (2d and 2e) gave a nearly identical distribution of products
(runs 6 and 7). These results are consistent with a catalytic
mechanism involving rapid interconversion between the syn- and
anti-isomers of the π-allyl intermediate. Optically active alcohol
2g (98.5% ee) was converted into the corresponding monoallylated
aniline without a notable loss of optical purity (run 9). Catalytic
allylation of active methylene compounds (CH2Z2; 3a-c) was also
successful (Table 2); the reactions proceeded at 50 °C in the
presence of 2 mol % 1a and 10 mol % pyridine9 to give
monoallylation products in 85-95% yields.
Scheme 1 shows our proposed catalytic mechanism. The key to
direct conversion of allylic alcohol is the C-O bond cleavage giving
(π-allyl)palladium intermediate 1. On the basis of the stoichiometric
observations summarized in Scheme 2, we assumed hydride
complex 4 to be responsible for this process. First of all, platinum
hydride 5 was prepared as a model of 4. After several attempts,
this complex was synthesized by the reaction of methyl complex 7
with HSiMe2Ph in wet CH2Cl2 and was isolated as a hydrido-
bridged dimer (5′) in 52% yield. Complex 5′ reacted with
2-propenyl alcohol (2a) at 50 °C to give (π-allyl)platinum 8, which
was isolated in 67% yield. A similar set of experiments was carried
out with methylpalladium 6 instead of 7. Unlike the platinum
system, the hydride complex 4 could not be detected, while π-allyl
complex 1b was successfully prepared from 2-propenyl alcohol
(2a). Thus, the treatment of 6 with HSiMe2Ph in wet CD2Cl2 in
the presence of 2a at room temperature led to instant formation of
1b in quantitative yield as confirmed by NMR spectroscopy; the
complex was isolated in 62% yield.10
References
(1) (a) Tsuji, J. Transition Metal Reagents and Catalysts; Wiley: New York,
2000. (b) Davis, J. A. In ComprehensiVe Organometallic Chemistry II;
Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press:
Oxford, 1995; Vol. 9, p 291. (c) Godleski, S. A. In ComprehensiVe
Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: New
York, 1991; Vol. 3, p 585.
(2) (a) Trost, B. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 259. (b) Trost,
B. M. Science 1991, 254, 1471.
(3) (a) Bricout, H.; Carpentier, J.-F.; Mortreux, A. J. Mol. Catal. A 1998,
136, 243. (b) Sakakibara, M.; Ogawa, A. Tetrahedron Lett. 1994, 35, 8013.
(c) Bergbreiter, D. E.; Weatherford, D. A. J. Chem. Soc., Chem. Commun.
1989, 883. (d) Haudegond, J.-P.; Chauvin, Y.; Commeureuc, D. J. Org.
Chem. 1979, 44, 3063. (e) Atkins, K. E.; Walker, W. E.; Manyik, R. M.
Tetrahedron Lett. 1970, 3821. (f) Hata, G.; Takahashi, K.; Miyake, A. J.
Chem. Soc., Chem. Commun. 1970, 1392.
(4) (a) Satoh, T.; Ikeda, M.; Miura, M.; Nomura, M. J. Org. Chem. 1997, 62,
4877. (b) Yang, S.-C.; Tsai, Y.-C. Organometallics 2001, 20, 763. (c)
Kimura, M.; Horino, Y.; Mukai, R.; Tanaka, S.; Tamaru, Y. J. Am. Chem.
Soc. 2001, 123, 10401. (d) Horino, Y.; Naito, M.; Kimura, M.; Tanaka,
S.; Tamaru, Y. Tetrahedron Lett. 2001, 42, 3113. (e) Tamaru, Y.; Horino,
Y.; Araki, M.; Tanaka, S.; Kimura, M. Tetrahedron Lett. 2000, 41, 5705.
(f) Stray, I.; Stara´, I. G.; Kocovsky, P. Tetrahedron 1994, 50, 529. (g)
Masuyama, Y.; Kagawa, M,; Kurusu, Y. Chem. Lett. 1995, 1121.
(5) (a) Lu, X.; Lu, L.; Sun, J. J. Mol. Catal. 1987, 41, 245. (b) Lu, X.; Jiang,
X.; Tao, X. J. Organomet. Chem. 1988, 344, 109. (c) Sakamoto, M.;
Shimizu, I.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1996, 69, 1065.
(6) Minami, T.; Okamoto, H.; Ikeda, S.; Tanaka, R.; Ozawa, F.; Yoshifuji,
M. Angew. Chem., Int. Ed. 2001, 40, 4501.
(7) Applications of sp2-hybridized phosphorus ligands to catalysis have been
reviewed: Weber, L. Angew. Chem., Int. Ed. 2002, 41, 563.
(8) Although MgSO4 was added in an attempt to remove water concomitantly
generated with the allylation reactions, the addition appeared not to be
essential. For example, the reaction of aniline with 2a in toluene in the
presence of 1a (0.5 mol %) without MgSO4 at room temperature for 30
min gave a 96% yield of N-(2-propenyl)aniline.
We have demonstrated that DPCB-coordinated palladium com-
plexes exhibit hitherto unknown catalytic activity toward direct
conversion of allylic alcohols. We have also described the evidence
for C-O bond cleavage of 2-propenyl alcohol (2a) promoted by
4. The mechanistic details of the π-allyl complex formation may
be depicted by Scheme 3. Coordination of 2a followed by proton
transfer from Pd to OH forms intermediate 10, which undergoes
elimination of water to give 1b.
(9) Pyridine was added to enhance the nucleophilicity of active methylene
compounds.
(10) Complex 1b reacts rapidly with aniline at room temperature to give N-(2-
propenyl)aniline.6
(11) (a) Sweany, R. L.; Owens, J. W. J. Organomet. Chem. 1983, 255, 327.
(b) Kristja´nsdo´ttir, S. S.; Norton, J. R.; Moroz, A.; Sweany, R. L.;
Whittenburg, S. L. Organometallics 1991, 10, 2357 and references therein.
(12) (a) Le Floch, P.; Mathey, F. Coord. Chem. ReV. 1998, 179-180, 771. (b)
Mathey, F. Acc. Chem. Res. 1992, 25, 90. (c) Yoshifuji, M. J. Chem.
Soc., Dalton Trans. (1972-1999) 1998, 3343.
In this scheme, the hydridopalladium moiety in 9 should be
significantly acidic. On the other hand, it is known that hydrido
ligands in transition metal complexes are commonly charged
JA0274406
9
J. AM. CHEM. SOC. VOL. 124, NO. 37, 2002 10969