A sterically modified (salen)chromium(III) complex - An efficient catalyst for high-pressure asymmetric allylation of aldehydes

Article abstract of DOI:10.1055/s-2005-872662

A novel (salen)chromium(III) catalyst with a modified salen ligand was synthesised in a simple way starting from readily available precursors. High-pressure allylation reaction of aromatic and aliphatic aldehydes with allyltributyltin upon application of

Full text of DOI:10.1055/s-2005-872662

LETTER
2301
A Sterically Modified (Salen)Chromium(III) Complex – An Efficient Catalyst
for High-Pressure Asymmetric Allylation of Aldehydes
A
Sterically
M
i
odified
o
(Salen)Chro
t
mium
r
a
Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
b
Department of Chemistry, Warsaw University, Pasteura 1, 02-093 Warsaw, Poland
Fax +48(22)6326681; E-mail: jurczak@icho.edu.pl
Received 2 July 2005
for carrying out allylation have been developed,9 often
using allylstannanes in the presence of chiral Lewis acids
as catalysts. The most efficient catalysts are BINOL com-
plexes with Ti(IV),10 and Zr(IV)11 as well as BINAP/
Abstract: A novel (salen)chromium(III) catalyst with a modified
salen ligand was synthesised in a simple way starting from readily
available precursors. High-pressure allylation reaction of aromatic
and aliphatic aldehydes with allyltributyltin upon application of
1 mol% of a modified (salen)chromium(III) complex provides Ag(I)12 and chiral acyloxy boranes derived from tartaric
acid.13 Generally, most of these procedures require large
homoallylic alcohols in good yield and with high enantioselectivity
(up to 92%). The catalyst reveals higher enantioselectivity than the
classic Jacobsen’s complex.
amounts (10–20 mol%) of catalyst, as well as anhydrous,
sometimes even oxygen-free, reaction conditions.
Key words: aldehydes, allylstannane, asymmetric catalysis, high-
pressure technique, homoallylic alcohols, salen(chromium) com-
plexes
Recently, we published a communication concerning
enantioselective allylation of aldehydes with allyltributyl-
tin under high-pressure conditions catalysed by the classic
Jacobsen’s
(salen)chromium(III)
complex
(1a;
Figure 1).14 The complex afforded good yields; however,
the enantioselectivities obtained were rather moderate,
usually in the range of 55–79%. Complex 1a works well
under ambient conditions only for highly reactive alde-
hydes such as glyoxylates15 and is ineffective as a catalyst
for simple aromatic and aliphatic aldehydes due to its
relatively low Lewis acidity. The solution to this problem
is the application of a high-pressure technique, a well-
known methodology in organic synthesis,16 but asymmet-
ric synthesis under high pressure is practically restricted
to diastereoselective methods. The high-pressure tech-
nique was used for the allylation of aldehydes with allylic
stannanes at room temperature without any catalyst over
20 years ago by Yamamoto et al.,17 and, contrary to the
catalytic version reported by us,14 the non-catalysed reac-
tion proceeds via a six-membered-ring transition state and
is much slower.
Chiral metallosalen complexes are very attractive catalyst
candidates and have been shown to catalyse a large num-
ber of reactions.1 Among them are tetradentate chromium
complexes of type 1a, introduced by Jacobsen.2 These
complexes are well known as efficient catalysts for hete-
ro- and homo-Diels–Alder reactions,3 ring-opening of ep-
oxides with trimethylsilyl azide,2,4 epoxidation of
alkenes,5 alkylation of tributyltin enolates6 as well as for
allylation in the catalytic Nozaki–Hiyama–Kishi reaction
with allylic halides7 and for certain other reactions.1,8
In this communication, we focus our attention on the ap-
plication of modified chromium complexes in the asym-
metric synthesis of homoallylic alcohols via allylation of
simple aldehydes (Scheme 1). The optically active ho-
moallylic alcohols are very useful chiral building-blocks
in organic synthesis. Until now, many efficient methods
Figure 1 The classic Jacobsen's catalyst 1a and the improved, modified complex 1g
SYNLETT 2005, No. 15, pp 2301–2304
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9
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Advanced online publication: 08.08.2005
DOI: 10.1055/s-2005-872662; Art ID: G17405ST
© Georg Thieme Verlag Stuttgart · New York

2302
P. Kwiatkowski et al.
LETTER
Table 1 Results of High-Pressure Allylation of Furfural (2a) with
Allyltributyltin Catalysed by Various (salen)CrBF4 Complexesa
Scheme 1 Allylation of aldehydes
In continuation of our previous studies14 we attempted to
optimise this catalytic system via modification of the
salen ligand. Application of known chiral C2-diamines
other than 1,2-diaminocyclohexane lowered the enantio-
selectivity. Also simple modifications to the substituents
on the salicylidene moiety usually resulted in lowering
(for a substituent smaller than t-Bu at the 3-position) or
keeping (t-Bu at the 3-position) the enantioselectivity
level. Therefore, we decided to modify the salen structure
by introducing a more sterically demanding substituent at
the 3-position. Modification of the salicylidene moiety at
this position was widely investigated by Katsuki at al.18,19
They synthesised a catalyst bearing additional elements of
chirality at the 3-position e.g. 1-phenylprop-1-yl substitu-
ent,18 which has similar steric properties to those intro-
duced by us (e.g. in catalysts 1d and 1g). The best results
they achieved were with so-called second generation
catalysts bearing binaphthyl units with axial chirality,19
however, synthesis of this type of ligand is difficult and
expensive.
Entry
Catalyst
1a
R
Yield (%)b ee (%)c
1
2
t-Bu
91
81
67
57
1b
3
1c
82
76
4
5
6
7
1d
1e
1f
87
89
82
86
81
82
72
91
At the beginning of our work, we applied a ligand having
a 1-methylcyclohex-1-yl substituent at the 3-position. The
resulting complex 1b displayed a lower enantioselectivity
(Table 1, entry 2) compared to the classic catalyst
(Table 1, entry 1). These results are analogous to the re-
sults reported by Jacobsen for the epoxidation of olefins
with manganese complexes.20
1g
a The reactions were carried out in a 2-mL Teflon ampoule using fur-
fural (2a; 1 mmol), (salen)chromium complex (2 mol%), allylSnBu3
(1.1 mmol), CH2Cl2, 10 kbar, 20 °C, 24 h.
b The yield was determined by GC.
c The ee was determined by GC on a capillary chiral b-dex 120
column.
When the cyclohexyl moiety was retained and methyl was
replaced with phenyl (catalyst 1c), we observed some im-
provement in the enantioselectivity (increased to 76%;
Table 1, entry 3). A better induction was achieved when
cyclohexyl was replaced with isopropyl and the phenyl
substituent was retained (catalyst 1d; Table 1, entry 4). In
the case of the model reaction with furfural, the induction
observed was slightly above 80% ee, whereas the classic
catalyst gave 67% ee under analogous reaction conditions.
This known ligand was employed for the synthesis of the
manganese complex and used for the olefin epoxidation
reaction, where it gave a slightly increased (by about
10%) asymmetric induction compared to the classic
system.21
position decreases the enantioselectivity slightly (Table 1,
entry 6). Until now, the best results (ee >90%) were ob-
served for catalyst 1g bearing the 3-(3-phenylpentyl)
group at the 3-position, a sterically demanding substituent
(Table 1, entry 7); the synthesis of 1g is shown in
Scheme 2.
The advantage of catalyst 1g is that phenol 5, aldehyde 6
and ligand 7 are readily accessible from inexpensive ma-
terials, are easily isolated and purified. Phenol 5 was ob-
tained from olefin 4 (readily available from 3-pentanone
and bromobenzene) and 4-tert-butylphenol in the pres-
ence of MeSO3H; 5 was isolated in 25% yield by crystal-
lisation from ethanol (this reaction was not optimised).
Formylation22 to aldehyde 6 and synthesis of ligand 723,24
proceed in good yields, 70% and 80%, respectively, after
crystallisation. Solid chromium complexes 825 and 1g
were synthesised according to known procedures by
Jacobsen.2,3a
As one can see from the examples, the presence of the cy-
clohexyl substituent decreases the enantioselectivity com-
pared to the catalysts having the isopropyl substituent
(Table 1, cf. entries 2 and 3 vs. 1 and 4). The change of the
electronic properties of the phenyl group resulting from
adding an electron-donor group (e.g., methoxy) has prac-
tically no influence on the asymmetric induction (Table 1,
entry 5), whereas the presence of a chlorine atom at this
Synlett 2005, No. 15, 2301–2304 © Thieme Stuttgart · New York

LETTER
A Sterically Modified (Salen)Chromium(III) Complex
2303
Scheme 2 Synthesis of the catalyst 1g.2,3a,22,23
Table 2 Asymmetric Allylation of Aldehydes Catalysed by the Complexes (1R,2R)-1a, (1R,2R)-1d and (1R,2R)-1ga
Catalyst
Entry
1a (2 mol%)
1d (2 mol%)
1g (1 mol%)
Aldehyde
Yield (%)b
ee (%)c
67 (R)
55 (R)
60
Yield (%)b
ee (%)c
81 (R)
67 (R)
77
Yield (%)b
ee (%)c
1
2
3
4
5
6
2a
2b
2c
2d
2e
2f
R = 2-Furyl
89
82
81
83
82
84
85
87
80
90
90
94
84
89
90
92
90
92
90 (R)
86 (R)
87
R = Ph
R = p-ClC6H4
R = p-NO2C6H4
R = Cyclohexy
R = PhCH=CH
68
80
83
79 (R)
65 (R)
84 (R)
85 (R)
92 (R)
92 (R)
a Reaction conditions: aldehyde (1 mmol), (1R,2R)-chromium complex (1 or 2 mol%), allylSnBu3 (1.1 mmol) in CH2Cl2; 2 mL Teflon ampoule,
10 kbar at 20 °C for 24 h.26
b Isolated yield.
c The ee was determined by GC on a capillary chiral b-dex 120 column.27
Encouraged by the results of the reaction with furfural, we Summing up, we have synthesised a novel salen ligand
applied this reaction to a range of aldehydes (Table 2). and found its chromium complex to be a highly selective
The type of catalyst used had no significant influence on catalyst for the allylation of aldehydes under high pres-
the yield; the yields are good, usually above 80%. With re- sure. It resulted in good yields (80%) and asymmetric
gard to the ee values, they rise in the following order: 1a inductions (up to 92% ee). Its significant advantage is a
(55–79% ee), 1d (67–85% ee), and 1g (above 80% ee, up relatively simple synthesis and effectiveness at low
to 92% ee, even if only 1 mol% of the catalyst is present). concentrations (ca. 1 mol%) with no need for anhydrous
Quite a wide range of aldehydes were used; the reaction is solvents or an inert atmosphere.
effective for aromatic aldehydes 2a–d, simple aliphatic
aldehyde 2e, as well as a,b-unsaturated aldehyde 2f.
Synlett 2005, No. 15, 2301–2304 © Thieme Stuttgart · New York

2304
P. Kwiatkowski et al.
LETTER
(17) Yamamoto, Y.; Maruyama, K.; Matsumoto, K. J. Chem.
Future work will focus on the application of this catalytic
system to other reactions as well as on further attempts at
modification of the substituents at the 3-position of the
salicylidene moiety, aimed at further improving the enan-
tioselectivity.
Soc., Chem. Commun. 1983, 489.
(18) (a) Irie, R.; Noda, K.; Ito, Y.; Matsumoto, N.; Katsuki, T.
Tetrahedron Lett. 1990, 31, 7345. (b) Hosoya, N.; Irie, R.;
Katsuki, T. Synlett 1993, 261.
(19) (a) Sasaki, T.; Irie, R.; Hamada, T.; Suzuki, K.; Katsuki, T.
Tetrahedron 1994, 50, 11827. (b) Ito, Y. N.; Katsuki, T.
Bull. Chem. Soc. Jpn. 1999, 72, 603.
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(24) Analytical data for the modified salen ligand (1R,2R)-7: mp
89–93 °C; [a]D29 –329.5 (c 1.0, CHCl3); IR (KBr): 2963,
2875, 1628, 1597, 1445, 1263, 699 cm–1; 1H NMR (200
MHz, CDCl3): d = 13.13 (s, OH, 2 H), 8.00 (s, CHN, 2 H),
7.45 (d, J = 1.8 Hz, 2 H), 7.25–7.09 (m, 10 H), 6.92 (d, J =
1.8 Hz, 2 H), 3.13–2.99 (m, 2 H), 2.50–2.25 (m, 4 H), 2.12–
1.94 (m, 4 H), 1.86–1.69 (m, 4 H), 1.66–1.44 (m, 2 H), 1.30
(s, 18 H), 0.60 (t, J = 7.2 Hz, 6 H), 0.53 (t, J = 7.2 Hz, 6 H);
13C NMR (50 MHz, CDCl3): d = 165.5 (2 × CHN), 157.5
(2 × C), 148.5 (2 × C), 139.2 (2 × C), 133.1 (2 × C), 129.2
(2 × CH), 127.2 (4 × CH), 127.0 (4 × CH), 125.8 (2 × CH),
124.7 (2 × CH), 117.5 (2 × C), 72.3 (2 × CH), 49.0 (2 × C),
34.0 (2 × C), 32.9 (2 × CH2), 31.5 (6 × CH3), 28.0 (2 × CH2),
27.1 (2 × CH2), 24.3 (2 × CH2), 8.7 (4 × CH3); Anal. Calcd
for C50H66N2O2: C, 82.60; H, 9.15; N, 3.85. Found: C, 82.55;
H, 9.23; N, 3.83; HRMS: [M + Na]+ calcd for
(6) Doyle, A. G.; Jacobsen, E. N. J. Am. Chem. Soc. 2005, 127,
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Paterson, I. Angew. Chem. Int. Ed. 2003, 42, 1032.
(8) For a review of applications of(salen)Cr complexes in
asymmetric catalysis see: Bandini, M.; Cozzi, P. G.; Umani-
Ronchi, A. Chem. Commun. 2002, 919.
(9) For a recent review on enantioselective allylation, see:
Denmark, S. E.; Fu, J. Chem. Rev. 2003, 103, 2763.
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Umani-Ronchi, A. J. Am. Chem. Soc. 1993, 115, 7001.
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C50H66N2O2Na: 749.5022, found: 749.5021.
(25) Analytical data for the complex (1R,2R)-8: [a]D29 –1420
(c 0.01, CHCl3); IR (KBr): 3429, 2961, 2873, 1622, 1533,
1437, 1258, 700, 546 cm–1; HRMS: [M – Cl]+ calcd for
C50H64N2O2Cr: 776.4373, found: 776.4392.
(26) General Procedure for High-Pressure Allylation: In a
2-mL Teflon ampoule were placed catalyst 1g (8.7 mg, 1
mol%), CH2Cl2 (ca. 1 mL), followed by aldehyde (1 mmol)
and allyltributyltin (1.1–1.2 equiv). Finally, the ampoule
was filled with CH2Cl2, closed and placed in a high-pressure
vessel, and the pressure was slowly increased to 10 kbar at
20 °C. After the pressure had stabilized, the reaction mixture
was kept under these conditions for 24 h. After
decompression, the reaction mixture was diluted with wet
Et2O and dried over MgSO4. After evaporation of solvents,
the residue was chromatographed on a silica gel column
(hexane–EtOAc).
(27) The enantioselectivity of homoallylic alcohols 3a–f was
determined by GC employing a capillary chiral b-dex 120
column. Alcohol 3f was analyzed directly, 3a, 3c and 3d as
their O-trimethylsilyl derivatives, 3b as an acetate and 3e as
a trifluoroacetate.
(16) (a) High Pressure Chemistry; Eldik, R.; Klarner, F.-G., Eds.;
Wiley: New York, 2002. (b) Chemistry under Extreme or
Non-Clasical Conditions; van Eldik, R.; Hubbard, C. D.,
Eds.; Wiley: New York, 1997. (c) Organic Synthesis at
High Pressure; Matsumato, K.; Acheson, R. M., Eds.;
Wiley: New York, 1991. (d) High Pressure Chemical
Synthesis; Jurczak, J.; Baranowski, B., Eds.; Elsevier: New
York, 1989.
Synlett 2005, No. 15, 2301–2304 © Thieme Stuttgart · New York