Enantioselective Allylation of Aldehydes with (Dialkoxyallyl)chromium(III) Complexes

Full text of DOI:10.1021/jo970069y

2
322
J . Org. Chem. 1997, 62, 2322-2323
Ta ble 1. Asym m etr ic Allyla tion of Ben za ld eh yd e Usin g
En a n tioselective Allyla tion of Ald eh yd es
w ith (Dia lk oxya llyl)ch r om iu m (III)
Com p lexes
(
Dia lk oxya llyl)- or [Bis(a r yloxy)a llyl]ch r om iu m (III)
Com p ou n d s
Kazuyuki Sugimoto, Sakae Aoyagi, and
Chihiro Kibayashi*
School of Pharmacy, Tokyo University of Pharmacy & Life
Science, Horinouchi, Hachioji, Tokyo 192-03, J apan
type of
allylchromium(III)
hydroxy
compd
yielda of
17 (%)
% eeb
(confign)
Received J anuary 14, 1997 (Revised Manuscript Received
March 3, 1997)
c
entry
1
2
3
4
5
6
7
8
9
A
A
A
B
B
C
C
C
C
C
C
C
C
4
5
6
7
8
27
53
52
49
56
47
34
64
40
32
77
72
62
20 (R)
14 (S)
10 (S)
11 (R)
46 (S)
37 (R)
21 (R)
42 (R)
65 (S)
39 (R)
44 (R)
30 (R)
82 (R)
The chromium(II) chloride-mediated addition of allylic
bromides to aldehydes referred to as the Nozaki-Hiyama
1
reaction has proved to be a valuable tool for C-C bond
formation in virtue of its high chemo- and stereoselec-
tivity and ease of the reaction under mild conditions.
9
2
10
11
12
13
14
15
16
Despite its synthetic utility, as far as we are aware, only
one example of earlier work aimed at asymmetric addi-
tion of allylchromium reagents to aldehydes has been
1
0
11
12
3
reported; in chromium(II) chloride-mediated allylation
of pentanal, the best asymmetric induction was observed
when lithium ephedrinate was used as a chiral ligand,
but the enantiomeric excess of the resulting homoallylic
alcohol was only 29% with poor yield (18%). Quite
1
3
a Isolated yield after chromatographic purification. b Deter-
mined by HPLC analysis (Chiralcel OB column, Daicel Chemical
4
Industries, Ltd.). c Assigned by comparison of the sign of optical
recently, Kishi et al. first reported the Nozaki-Hiyama-
rotation with reported value.9
type coupling between benzaldehyde and allyl bromide
using a dipyridyl chiral ligand with a significant level of
asymmetric induction in 74% ee with 51% yield. These
observations indicate that the use of allylic chromium
reagents, in which the metal is ligated by such chiral
modifiers, can lead to asymmetric allyl coupling, but with
less satisfactory results. In our investigations aimed at
developing the asymmetric induction for chromium-based
allylation,5 we envisioned that the allylic chromium
dialkoxide reagents, in which the metal is covalently
bound with chiral hydroxy compounds, can be success-
fully employed to induce high stereoselectivity. Many
homoallylic alcohol 3 with asymmetric induction. In this
paper, we report a new type of highly enantioselective
allyl coupling with aldehydes according to eq 1, which
involves the chirality-inducing process utilizing the al-
lylchromium(III) reagents with covalently bound chiral
alkoxy or aryloxy auxiliaries.
chromium alkoxides have been prepared by alcoholysis
6
of [Cr[N(SiMe
3
)
2
](THF)
2
] or from chromocene, but an
earlier report7 suggested that a convenient practical
approach might be applicable for the formation of diva-
lent transition metal alkoxides by using lithium alcoho-
lates and anhydrous transition metal halides. We thus
speculated as summarized in eq 1 that the chiral chro-
mium(II) dialkoxide 1 would be in situ prepared from
According to the above mechanistic speculation out-
lined in eq 1, we first attempted asymmetric addition to
benzaldehyde by employing the chiral dialkoxychromium-
(
III) reagents modified by the chiral alcohols 4-6 as
illustrated by type A in Figure 1. Thus, treatment of
CrCl
and the lithium salts of the chiral alcohols in THF
2
CrCl and the optically active lithium alcoholates, and
subsequent oxidative addition of allyl bromide to 1 would
rapidly occur to generate the chirally modified allylchro-
mium(III) reagent 2, which would be allowed to react
with the coexisting prochiral aldehyde to yield the
2
at -30 °C resulted in in situ formation of the chiral
chromium(II) dialkoxides (1 in eq 1), which were then
allowed to react with allyl bromide, providing the allyl-
chromium(III) reagents (2 in eq 1). Subsequent coupling
of these type-A allylic chromium(III) reagents with
benzaldehyde at -30 °C afforded 1-phenyl-3-butenol (17)
(
1) (a) Okude, Y.; Hirano, S.; Hiyama, T.; Nozaki, H. J . Am. Chem.
Soc. 1977, 99, 3179. (b) Buse, C. T.; Heathcock, C. H. Tetrahedron Lett.
978, 1685.
2) For recent reviews on the use of organochromium compounds
in carbon-carbon bond-forming reactions, see: (a) Cintas, P. Synthesis
992, 248. (b) Saccomamo, N. A. In Comprehensive Organic Synthesis;
1
(
(
Table 1, entries 1-3). As seen in Table 1, however, these
1
reactions showed low chemical yields and their enantio-
Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 2,
selectivities were only 10-20% ee.
pp 173-209. (c) Hodgson, D. M. J . Organomet. Chem. 1994, 476, 1.
(3) Cazes, B.; Verniere, C.; Gor e´ , J . Synth. Commun. 1983, 13, 73.
(4) Chen, C.; Tagami, K.; Kishi, Y. J . Org. Chem. 1995, 60, 5386.
(5) For asymmetric allylation by other metals, see: Yanagisawa, A.;
Nakashima, H.; Ishiba, A.; Yamamoto, H. J . Am. Chem. Soc. 1996,
18, 4723 and references cited therein.
6) O’Brien, P. In Comprehensive Coordination Chemistry; Wilkin-
1
(
son, G., Gillard, R. D., McCleverty, J . A., Eds.; Pergamon Press:
Oxford, 1987; Vol. 3, p 737.
(
7) Adams, R. W.; Bishop, E.; Martin, R. L.; Winter, G. Aust. J .
F igu r e 1. Type-A (dialkyloxyallyl)chromium(III) reagents.
Chem. 1966, 19, 207. Also see: Horvath, B.; Horvath, E. G. Z. Anorg.
Allg. Chem. 1979, 457, 51.
We next employed the chiral dihydroxy compounds 7
and 8 to form type-B cyclic bis(aryloxy)- and dialkoxy-
chromium(III) reagents (Figure 2), and the asymmetric
(8) (a) Denmark, S. E.; Coe, D. M.; Pratt, N. E.; Griedel, B. D. J .
Org. Chem. 1994, 59, 6161. (b) Basavaiah, D.; Dharma Rao, P. Synth.
Commun. 1994, 24, 925.
S0022-3263(97)00069-8 CCC: $14.00 © 1997 American Chemical Society

Communications
J . Org. Chem., Vol. 62, No. 8, 1997 2323
Ta ble 2. Asym m etr ic Allyla tion of Ald eh yd es Usin g
(
Dia lk oxya llyl)ch r om iu m (III) Com p lex 18
F igu r e 2. Type-B (dialkoxyallyl)- and bis(aryloxy)allyl]chro-
mium(III) reagents.
yielda of
19 (%)
[R]28
% eeb
(confign)
D
c
entry
aldehyde (R)
(benzene)
1
2
3
4
5
4-MeOC6H4
4-ClC6H4
4-ClC6H4
1-naphthyl
Ph(CH2)2
84
72
47
43
60
+30.5 (c 0.45)
+23.5 (c 0.53)
+26.4 (c 0.38)
+64.3 (c 0.19)
+10.7 (c 0.48)
49 (R)
88 (R)
98 (R)
80 (R)
61 (S)
d
a
Isolated yield after chromatographic purification. b Deter-
mined by HPLC analysis (Chiralcel OB or OD column, Daicel
c
Chemical Industries, Ltd.). Assigned by comparison of the sign
of optical rotation with reported value.8
d
Preparation of the
chromium(II) dialkoxide using 16 was carried out at room tem-
perature.
F igu r e 3. Type-C (dialkoxyallyl)- and [bis(aryloxy)allyl]-
chromium(III) reagents.
addition to benzaldehyde leading to the homoallylic
alcohol 17 was conducted following the same procedure
as described above for the type-A reagents. As sum-
marized in Table 1, while the use of the (R)-1,1′-bi-2-
naphthoxychromium(III) reagent resulted in only low
enantioselectivity (10% ee, Table 1, entry 4), in the case
where the chromium(III) reagent installing the chiral diol
The low chemical yield (32%), observed when the
L-prolinol derivative 13 was employed (Table 1, entry 10),
may be accounted for by the bulky naphthyl groups
extruding to above and below the plane of the chromium-
(II) complex that sterically block further oxidative addi-
tion of allyl bromide, thus preventing the formation of
the type-C allylic chromium(III) reagent.
8
was used (Table 1, entry 5), the enantioselectivity was
Encouraged by the above result with the type-C (di-
alkoxyallyl)chromium(III) reagent incorporated with the
N-benzoyl-L-prolinol 16, i.e., 18, we further explored the
asymmetric coupling with aldehydes utilizing 18. As
summarized in Table 2, these reactions were found to
found to be appreciably improved; however, it still
remains at a low level of 46% ee.
We next attempted to employ the chiral monohydroxy
compounds with nitrogen functions, i.e., 9-16, for gen-
eration of the chiral allylic chromium(III) complexes
represented by type C (Figure 3), which would be in a
chelated trans planar structure rigidifying the array of
atoms and serve to differentiate the enantiotopic faces
during the coupling. Thus, allyl coupling with benzal-
dehyde was carried out using these allyl chromic re-
agents, and the results are listed in Table 1. Among
these chiral allylchromium(III) compounds of type C, the
reagent prepared from the N-benzoyl-L-prolinol deriva-
tive 16 proved to be the most efficacious in chiral
induction, affording (R)-17 with the highest enantiomeric
excess of 82% (Table 1, entry 13). In this case, the
intermediary chromium(II) dialkoxide complex is con-
sidered to constitute preferential coordination between
chromium and the nitrogen atom rather than the oxygen
atom of the amide group to form the five-membered
chelate ring, although chromium coordination can occur
with both the nitrogen and oxygen atoms.9 Thereby,
bidentate coordination by the two ligands should have
constructed efficient chiral circumstance around the
metal center.
proceed to form the corresponding homoallylic alcohols
10
1
9 with moderate to excellent enantioselectivities. For
the allyl coupling with p-chlorobenzaldehyde, when the
chromium(II) dialkoxide species was formed from CrCl
2
and the chiral ligand 16 at room temperature (Table 2,
entry 3) instead of -30 °C (Table 2, entry 2), the
selectivity was enhanced from 88% ee up to 98% ee. In
this case, a lowering of the chemical yield (47%) observed
indicates that the chromium(II) dialkoxide intermediates
have a tendency to undergo decomposition at room
temperature.
In summary, we have demonstrated that the (di-
alkoxyallyl)chromium(III) complex 18 chirally modified
by the N-benzoyl-L-prolinol derivative 16, being suggested
to constitute a rigid chelated trans plannar structure,
leads to high induction in asymmetric preparation of
homoallylic alcohols by allyl coupling with aldehydes.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and characterization data are provided for compounds
1
0, 13, 15, and 16 (4 pages).
It should be noted that using the imino phenols 9 and
0 as chiral ligands (Table 1, entries 6 and 7) leads to
J O970069Y
1
much shorter reaction time (1-2 h), albeit with unsat-
isfactory asymmetric induction, compared with the reac-
tion time in the allylation reactions using a series of the
L-prolinol ligands 11-16, which require 4-12 h (Tables
(
10) General procedure for the asymmetric allylation: To a stirred
solution of CrCl (2.0 mmol) in THF (5 mL) at -30 °C under an argon
atmosphere was cannulated
2
a solution of the lithium alkoxide,
prepared by the addition of a 1.57 M (1.46 mL) solution of BuLi (2.3
mmol) in hexane to a solution of the hydroxy compound (4.0 mmol, 2
mmol for 7 and 8) in THF (7 mL) at 0 °C. The mixture was stirred for
1 h at -30 °C, and allyl bromide (1.0 mmol) followed by the aldehyde
1
, entries 8-13). From these observations, it is expected
that type-C (diphenoxyallyl)chromium(III) complexes,
through their structural modifications, serve as potential
chiral inductors for asymmetric allylation.
(
0.5 mmol) were added. After being stirred for 1-12 h at -30 °C, the
reaction was quenched by addition of water. The insoluble material
was removed by filtration through a Celite pad, and the filtrate was
extracted with ether. The extracts were washed with brine, dried over
(
9) (a) Palenik, G. J .; Wester, D. W.; Rychlewska, U.; Palenik, R. C.
4
MgSO , and concentrated. The crude product was purified by column
chromatography on silica gel to give the homoallylic alcohol with the
chemical and optical yields reported in Tables 1 and 2.
Inorg. Chem. 1976, 15, 1814. (b) Collins, T. J .; Santarsiero, B. D.; Spies,
G. H. J . Chem. Soc., Chem. Commun. 1983, 681.