Chiral bis(oxazolinyl)phenylrhodium(III) complexes as Lewis acid catalysts for enantioselective allylation of aldehydes

Article abstract of DOI:10.1039/a807716i

The reaction of (Phebox)SnMe₃ 1 [PheboxH = 2,6-bis(oxazolinyl)benzene] and [(c-octene)RhCl]₂ in the presence of CCl₄ provided air-stable and water-tolerant (Phebox)RhCl₂(H₂O) complexes 2 which acted a

Full text of DOI:10.1039/a807716i

Chiral bis(oxazolinyl)phenylrhodium(III) complexes as Lewis acid catalysts for
enantioselective allylation of aldehydes
Yukihiro Motoyama, Hiroki Narusawa and Hisao Nishiyama*
School of Materials Science, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan.
E-mail: hnishi@tutms.tut.ac.jp
Received (in Cambridge, UK) 5th October 1998, Accepted 4th November 1998
The reaction of (Phebox)SnMe3 1 [PheboxH = 2,6-bis(ox-
[(c-octene)2RhCl]2
azolinyl)benzene] and [(c-octene)RhCl]2 in the presence of
CH2Cl2
O
O
O
O
room temp.
CCl4 provided air-stable and water-tolerant (Phe-
box)RhCl2(H2O) complexes 2 which acted as asymmetric
catalysts for the enantioselective allylation of aldehydes in
up to 80% ee.
Cl
H
N
N
N
N
then CCl4
SnMe3
Rh
O
R
R
R
R
Cl
H
The development of chiral Lewis acid catalysts, particularly for
carbon–carbon bond forming reactions, is one of the most
challenging and formidable endeavors in organic synthesis.1
Among various carbon–carbon bond forming reactions, asym-
metric allylation of carbonyl compounds is a valuable means of
constructing chiral functionalized structures, and therefore
many chiral allylmetal reagents have been designed and
synthesised.2 Although numerous reactions using a stoichio-
metric amount of chiral allylmetal reagents have been reported,3
there are only a few reports of catalytic processes using chiral
Lewis acid complexes, including chiral (acyloxy)borane (CAB)
complex and allylsilanes4 or allylstannanes,5 binaphthol-de-
rived chiral titanium complexes and allylstannanes6 or allylsi-
lanes,7 and BINAP-derived chiral silver complexes and allyl-
stannanes.8 The key to success in developing a new chiral Lewis
acid catalyst is the choice of the central metal and the design of
the chiral ligands. Traditional Lewis acids, such as boron,
aluminium, titanium and tin, are extremely moisture-sensitive,
and are known to form oligomers in solution. Furthermore,
chiral complexes are almost always generated in situ, so there
are a variety of species having different Lewis acid activity and
enantioselectivity. On the other hand, transition metal com-
plexes are generally insensitive to water and form a single well-
defined species.9,10 Here we report a catalytic enantioselective
allylation reaction of aldehydes with allyltributylstannane
1a R = Pri
2a R = Pri (67%)
b
c
d
R = Ph
R = Bn
R = But
b
c
d
R = Ph (64%)
R = Bn (50%)
R = But (24%)
Scheme 1
(S,S)-(Phebox)RhCl2(H2O)
(5 mol%)
R
H
R
+
SnBu3
O
OH
Scheme 2
reaction was strongly dependent on the solvent: the allylation
reaction proceeded smoothly in CH2Cl2, but when using THF or
an aromatic solvent such as benzene or toluene, the reaction
rates were remarkably slow although the enantioselectivities
was almost the same (entries 1–4). In the presence of 4 Å
molecular sieves this catalytic reaction was accelerated while
the enantiomeric excess of the product 3 was not changed
(Table 1, entries 4 vs. 5). The selectivity did not improve when
the reaction temperature was lowered to 0 °C (entries 5 vs. 6).
The substituent on the oxazoline rings had a significant effect on
the chemical yields and ees (entries 5, 7–9). Notably, the
enantioselectivity reaching 61% ee using Bn-Phebox-derived
complex 2c. It is worth noting that the complex 2 can be
recovered almost quantitatively from the reaction medium by
silica gel chromatography, and the recovered complex 2
catalyses the reaction with almost the same catalytic activity
and enantioselectivity (entries 5 vs. 10).
catalyzed by new air-stable and water-tolerant rhodium(III
)
complexes 2 bearing a 2,6-bis(oxazolinyl)phenyl group (Phe-
O
O
O
O
–
Cl
H
N
N
N
N
Table 1 Asymmetric catalytic allylation of benzaldehyde catalyzed with
Phebox–RhIII complexes 2a
Rh
O
R
R
R
R
Cl
Phebox
Entry Catalyst Solvent T/°C
t/h
Yield (%) Ee (%)b
H
Phebox–RhIII
2
1
2
3
4
5c
6c
7c
8c
9c
2a
2a
2a
2a
2a
2a
2b
2c
2d
toluene
benzene room temp. 24
THF room temp. 24
room temp. 24
23
64
66
79
88
46
42
88
43
88
45
45
52
52
51
49
6
61
46
49
box) as chiral ligands.11 The Phebox ligand bonds via a central
carbon–metal covalent bond and both of the oxazoline rings.
The (Phebox)RhCl2(H2O) complexes 2† were easily synthe-
sized by the transmetalation reaction of [(c-octene)2RhCl]2 and
(Phebox)SnMe3 1 in CH2Cl2 followed by treatment with CCl4.‡
These complexes are air-stable enough to be purified by silica
gel chromatography (Scheme 1).
The optimized allylation reaction conditions employed
benzaldehyde and allyltributylstannane§ in the presence of 5
mol% of the (S,S)-(Phebox)RhCl2(H2O) complex 2 as a chiral
catalyst (Scheme 2). The absolute configuration of the allylated
product was determined to be S by comparison of its optical
rotation value with literature data.3e The rate of this allylation
CH2Cl2 room temp.
CH2Cl2 room temp.
7
7
CH2Cl2
0
24
7
7
7
7
CH2Cl2 room temp.
CH2Cl2 room temp.
CH2Cl2 room temp.
CH2Cl2 room temp.
10c,d 2a
a All reactions were carried out using 0.5 mmol of benzaldehyde, 0.75 mmol
of allyltin and 0.025 mmol of chiral catalyst 2 in 2 ml of solvent.
b Determined by chiral HPLC analysis using Daicel CHIRALCEL OD. c In
the presence of 4 Å molecular sieves (250 mg). Recovered catalyst was
used.
d
Chem. Commun., 1999, 131–132
131

Table 2 Asymmetric catalytic allylation of aldehydes catalyzed with
Phebox–RhIII complex 2ca
§ Other substituted allylstannane reagents gave lower yields and enantiose-
lectivities when catalyzed by 2a: allyltrimethylstannane (48%, 35% ee),
allyltriphenylstannane (7%, 27% ee).
Entry Aldehyde
Yield (%) Ee (%)b Configurationc Ref
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1
2
3
4
5
6
7
4-BrC6H4CHO
PhCHO
4-MeOC6H4CHO 99
2-MeC6H4CHO
2-furyl-CHO
PhCH2CH2CHO
94
88
43d
61f
80
Se
S
S
Se
Se
R
S
8(a)
3(e)
3(n)
8(a)
8(a)
3(n)
3(e)
98
94
84
53g
58d
63
(E)-PhCHNCHCHO 98
77
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a All reactions were carried out using 0.5 mmol of aldehyde, 0.75 mmol of
allyltributyltin and 0.025 mmol of chiral catalyst 2c in 2 ml of CH2Cl2 in the
presence of 4 Å molecular sieves (250 mg) at room temperature for 7 h.
b Determined by chiral HPLC analysis using Daicel CHIRALCEL OD-H.
c Assignment by comparison of the sign of optically rotation with reported
d
value. Determined by chiral HPLC analysis using Daicel CHIRALCEL
e
OJ.
By analogy to the other case that is known unambiguously.
f Determined by chiral HPLC analysis using Daicel CHIRALCEL OD.
g Determined by chiral HPLC analysis using Daicel CHIRALPAK AD.
O
O
Cl
N
N
Rh
O
Cl
H
R
si-face
SnBu3
4 K. Furuta, M. Mouri and H. Yamamoto, Synlett, 1991, 561; K. Ishihara,
M. Mouri, Q. Gao, T. Maruyama, K. Furuta and H. Yamamoto, J. Am.
Chem. Soc., 1993, 115, 11490.
Fig. 1
5 J. A. Marshall and Y. Tang, Synlett, 1992, 653; J. A. Marshall and M. R.
Palovich, J. Org. Chem., 1998, 63, 4381.
Table 2 summarizes the results obtained for the allylation
reaction of a variety of aldehydes catalysed with the complex
2c. The characteristic features of the results are as follows: (i) an
electron-donating substituents at the para-position of benzalde-
hyde increases the enantioselectivity; (ii) a methyl group at the
ortho-position of benzaldehyde decreases the selectivity; (iii)
all reactions result in high yields and comparable enantiose-
lectivities with both aromatic and aliphatic aldehydes; (iv) in the
reaction with enals, 1,2-addition reaction occurs exclusively;
(v) in all of the cases, allyltributylstannane attacks to the si-face
of the aldehyde’s CNO plane.
6 S. Aoki, K. Mikami, M. Terada and T. Nakai, Tetrahedron, 1993, 49,
1783; A. L. Costa, M. G. Piazza, E. Tagliavini, C. Trombini and A.
Umani-Ronchi, J. Am. Chem. Soc., 1993, 115, 7001; G. E. Keck and D.
Krishnamurthy, J. Org. Chem., 1993, 58, 6543; G. E. Keck, K. H. Tarbet
and L. S. Geraci, J. Am. Chem. Soc., 1993, 115, 8467; G. E. Keck and
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Rrückner, Chem. Eur. J., 1996, 2, 1077.
7 D. R. Gauthier, Jr. and E. M. Carreira, Angew. Chem., Int. Ed. Engl.,
1996, 35, 2363.
8 (a) A. Yanagisawa, H. Nakashima, A. Ishiba and H. Yamamoto, J. Am.
Chem. Soc., 1996, 118, 4723; (b) A. Yanagisawa, A. Ishiba, H.
Nakashima and H. Yamamoto, Synlett, 1997, 88.
9 H. Nishiyama and Y. Motoyama, in Lewis Acid Chemistry: A Practical
Approach, ed. H. Yamamoto, OUP, UK, 1998, ch. 13; W. Odenkirk,
A. L. Rheingold and B. Bosnich, J. Am. Chem. Soc., 1992, 114, 6392;
T, K. Hollis, W. Odenkirk, N. P. Robinson, J. Whelan and B. Bosnich,
Tetrahedron, 1993, 49, 5415.
10 Recent representative papers; chiral iron complex: E. P. Kündich, B.
Bourdin and G. Bernardinelli, Angew. Chem., Int. Ed. Engl., 1994, 33,
1856; Chiral ruthenium complex: D. L. Davies, J. Fawcett, S. A. Garratt
and D. R. Russell, Chem. Commun., 1997, 1351; Chiral rhodium
complex: A. J. Davenport, D. L. Davies, J. Fawcett, S. A. Garratt, L. Lad
and D. R. Russell, Chem. Commun., 1997, 2347; Chiral nickel complex:
S. Kanemasa, Y. Oderaotoshi, H. Yamamoto, J. Tanaka, E. Wada and
D. P. Carran, J. Org. Chem., 1997, 62, 6454; S. Kanemasa, Y.
Oderaotoshi, S. Sakaguchi, H. Yamamoto, J. Tanaka, E. Wada and D. P.
Carran, J. Am. Chem. Soc., 1998, 120, 3074.
A transition state that accounts for the observed ster-
eoselectivities is shown in Fig. 1. The allylstannane approaches
the carbonyl si-face because the re-face is shielded by the
substituent on the oxazoline ring of the Phebox ligand.
In summary, we have demonstrated the effectiveness of
Phebox–RhIII complexes as chiral transition metal Lewis acid
catalysts for the enantioselective addition of allyltributyl-
stannane to aldehydes. Application of these complexes to other
asymmetric reactions is now under investigation.
This work was partly supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Science
and Culture, Japan.
Notes and references
† We previously reported the synthesis of 2a by the transmetalation of
RhCl3(H2O)3 and 1a in 47% yield (see ref 12). However, the chemical yield
of Ph-Phebox-derived 2b is very low (16%) by this method.
‡ We have already reported that the (dMe-Pybox)RhCl complex prepared
from the dMe-Pybox and [(c-octene)2RhCl]2 readily reacted with alkyl
chlorides to form RhIII species via an oxidative addition reaction, see ref.
13(a). And recently, Vrieze reported the same oxidative addition of carbon–
11 S. E. Denmark, R. A. Stavenger, A.-M. Faucher and J. P. Edwards,
J. Org. Chem., 1997, 62, 3375; M. A. Stark and C. J. Richards,
Tetrahedron Lett., 1997, 38, 5881.
12 Y. Motoyama, N. Makihara, Y. Mikami, K. Aoki and H. Nishiyama,
Chem. Lett., 1997, 951.
13 (a) H. Nishiyama, M. Horihata, T. Hirai, S. Wakamatsu and K. Itoh,
Organometallics, 1991, 10, 2706; (b) H. F. Haarman, J. M. Ernsting, M.
Kranenburg, H. Kooijman, N. Veldman and K. Vrieze, Organome-
tallics, 1997, 16, 887.
chloride bonds to rhodium(
I) complexes containing terdentate nitrogen
ligands [2,6-(CR1NNR2)2C2H3N], see ref. 13(b).
Communication 8/07716I
132
Chem. Commun., 1999, 131–132