Asymmetric Simmons-Smith reaction of allylic alcohols with Al Lewis acid/N Lewis base bifunctional Al(salalen) catalyst

Article abstract of DOI:10.1002/anie.200705641

(Chemical Presented) Three angles: A highly enantioselective Simmons-Smith reaction of trans-disubstituted allylic alcohols was achieved by using a catalytic amount of an Al(salalen) complex at room temperature (see scheme).

Full text of DOI:10.1002/anie.200705641

Communications
DOI: 10.1002/anie.200705641
Asymmetric Catalysis
Asymmetric Simmons–Smith Reaction of Allylic Alcohols with
Al Lewis Acid/N Lewis Base Bifunctional Al(Salalen) Catalyst**
Hiroaki Shitama and Tsutomu Katsuki*
Optically active trans- and cis-disubstituted cyclopropylme-
thanol or cyclopropylester derivatives are useful building
blocks for organic synthesis.[1,2] Asymmetric cyclopropanation
of terminal olefins with an a-diazoacetate is an efficient
method for synthesizing such esters, and many diastereo- and
enantioselective reactions have been developed.[2] However,
Figure 1. Formal classification of asymmetric Simmons–Smith reac-
reactions with a satisfactory level (greater than 99%) of trans-
or cis-selectivity are limited.[2,3] Another useful method is the
stereospecific asymmetric Simmons–Smith reaction of allylic
alcohols.[2a,b] In 1992, Ukaji et al. reported a highly enantio-
selective method by using diethyl tartrate as a stoichiometric
chiral auxiliary.[4a,c,5] Denmark and Edwards reported an
efficient method that included a chiral amino alcohol.[4b]
tions of allylic alcohols.
use of a C2-symmetric auxiliary like diethyl tartrate[4a,c] should
be important for obtaining high enantioselectivity if the two
species are captured by two identical groups), and c) capture
of the two species by a Lewis acid/base bifunctional cata-
lyst.[12] Charette et al. proposed that the dioxaborolane having
a Lewis acidic boron site and Lewis basic sites (the amide
carbonyl group and the oxygen atom of the coordinated allylic
oxide)[13] serves as a bifunctional catalyst, albeit under
stoichiometric conditions.[6] It was expected that the efficiency
of a bifunctional catalyst should be enhanced by strengthen-
ing the interaction between the iodomethylzinc species and a
Lewis base site and the interaction between the alcohol (or
alkoxyzinc species) and a Lewis acid site, respectively. We
recently discovered that metal(salalen) complexes (salalen =
salen/salan hybrid; salen = N,N’-bis(salicylidene)ethylenedi-
Subsequently,
2-butyl-1,3-dioxa-2-borolane-4,5-dicarboxa-
mide[6] and 1,1’-bi-2-naphthol-3,3’-dicarboxamide[7] were
reported to be efficient auxiliaries. Kobayashi and co-workers
reported the first catalytic and satisfactorily enantioselective
Simmons–Smith reaction by using a chiral disulfonamide/
Et2Zn/CH2I2 system at low temperatures.[8] Charette et al.
reported a titanium(taddolate) complex that was an excellent
catalyst, albeit under substoichiometric conditions.[9] Never-
theless, conducting asymmetric Simmons–Smith reactions of
allylic alcohols in a catalytic and highly enantioselective
manner at room temperature remains a challenge.[10,11]
Asymmetric Simmons–Smith reactions have been pro-
posed to proceed through an in situ generated intermediate
derived from an iodomethylzinc species and a chiral auxil-
iary.[2b,4,9] When the substrate is an allylic alcohol, the alcohol
or the resulting alkoxyzinc species forms an aggregate with
the iodomethylzinc species and the chiral auxiliary, and
subsequently undergoes an asymmetric Simmons–Smith
reaction. The aggregate can occur in one of three different
modes (Figure 1a–c): a) capture of an allyloxy(iodomethyl)-
zinc species by a Lewis acid catalyst derived from the chiral
auxiliary,[8a,11c] b) capture of an allyloxyzinc and iodomethyl-
zinc species by a bifunctional chiral auxiliary (in this case, the
amine);
salan = N,N’-bis(o-hydroxybenzyl)-1,2-diamino-
ethane) show unique asymmetric catalysis.[14] Metal(salalen)
complexes have an amine donor atom[15] and a Lewis acidic
metal center and it is known that zinc ions and amines form
stable complexes. Thus, we were intrigued by the bifunctional
catalysis of metal(salalen) complexes. Taking into consider-
ation the high oxophilicity of the aluminum ion, we expected
an Al(salalen) complex to be a promising catalyst for the
asymmetric Simmons–Smith reaction.[16,17]
We first examined the cyclopropanation of cinnamyl
alcohol (1), a widely used substrate for asymmetric Simmons–
Smith reactions, in dichloromethane for 1 hour with 2 equiv-
alents of Et2Zn and 3 equivalents of CH2I2 in the presence of
10 mol% Al complex prepared in situ from a salalen ligand
and DIBAL (Table 1). To our delight, the reaction with ligand
2[18] was complete within 1 hour at room temperature and
gave the product in quantitative yield with 91% ee (Table 1,
entry 1). The diastereomeric complex prepared in situ from 4
was a far less efficient catalyst (Table 1, entry 3), and the
complex derived from 6 was a poor catalyst (Table 1, entry 5).
Notably, both N-methylated complexes 3 and 5, in which the
amine group does not have a proton that can be abstracted
and cannot serve as Lewis base, were poor asymmetric
catalysts irrespective of their stereochemistry (Table 1,
entries 2 and 4). This suggested that the NH group plays an
[*] H. Shitama, Prof. T. Katsuki
Department of Chemistry, Faculty of Science
Graduate School, Kyushu University
Hakozaki, Higashi-ku, Fukuoka 812-8581 (Japan)
Fax: (+81)92-642-2607
E-mail: katsuscc@mbox.nc.kyushu-u.ac.jp
[**] This study was supported by Grant-in-Aid for Scientific Research
(Specially Promoted Research 18002011) and the Global COE
Program (Science for Future Molecular Systems) from the Ministry
of Education, Science, and Culture (Japan). salalen= salen/salan
hybrid; salen=N,N’-bis(salicylidene)ethylenediamine); sal-
an=N,N’-bis(o-hydroxybenzyl)-1,2-diaminoethane.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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dichloromethane, chloroform, dichloroethane, 1,1,2,2-tetra-
chloroethane, and toluene, proceeded with high enantiose-
lectivity (90–95% ee), the best results were obtained in
dichloromethane or 1,1,2,2-tetrachloroethane (Table 1,
entries 9 and 10). The reaction in THF was slow and the
product had
a significantly reduced enantioselectivity
(Table 1, entry 11). When the catalyst loading was reduced
to 5 mol% the reaction proceeded smoothly, but the enan-
tioselectivity was diminished to 93% ee (Table 1, entry 12).
The scope of the reaction was examined under the
optimized conditions (Table 1, entry 9). The reactions of p-
substituted (E)-cinnamyl alcohols 7 and 8 proceeded with
high enantioselectivities, irrespective of the electronic nature
of the substituents (Table 2, entries 1 and 2), whereas the
important role in the asymmetric induction by the
salalen complex. Moreover, the less Lewis acidic
zinc complex of 2 (not reported) showed poor
enantioselectivity (8% ee). Thus, we examined
several Al(salalen) complexes derived from 2 and
different Al sources, and found that the complex
prepared from diethylaluminum chloride was the
Table 1: Asymmetric Simmons–Smith reactions of 1 with Al(salalen)
Table 2: Asymmetric Simmons–Smith reaction of allylic alcohols with 2.
complexes.[a]
Entry
Substrate
t [h]
Yield [%][a]
ee [%][b]
Config.[c]
Entry Ligand Al source Solvent Yield [%][b] ee [%][c] Config.[d]
1
2
3
4
5
6
7
8
9
10
11
12
13
13
3
1
1
3
3
3
4
10
>99 (93)
>99 (94)
>99 (99)
>99 (95)
>99 (99)
>99 (98)
98
94
94
58
86
90
87
63
70
1S,2S
1S,2S
1R,2S
1S,2S
n.d.[d]
1S,2S
1S,2S
1S,2S
1
2
3
4
5
6
7
2
3
4
5
6
2
2
2
2
2
2
2
(iBu)2AlH CH2Cl2
(iBu)2AlH CH2Cl2
(iBu)2AlH CH2Cl2
(iBu)2AlH CH2Cl2
(iBu)2AlH CH2Cl2
>99
>99
>99
81
>99
>99
>99
>99
>99
91
7
18
8
14
91
86
95
95
95
<1
93
1S,2S
1S,2S
1S,2S
1R,2R
1S,2S
1S,2S
1S,2S
1S,2S
1S,2S
1S,2S
–
7
Me3Al
CH2Cl2
8[e]
>99 (92)
Et2AlOEt CH2Cl2
[a] Calculated from 1H NMR (400 MHz) analysis by using 1-bromonaph-
thalene as an internal standard. The values in the parentheses show the
yields of the isolated products. [b] Determined by HPLC analysis by using
chiral stationary phase column. For details, see the Supporting
Information. [c] Determined by chiroptical comparison. For details, see
the Supporting Information. [d] Not determined. [e] Reaction was carried
out at 08C.
8
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
Et2AlCl
CH2Cl2
CH2Cl2
(Cl2CH)2 >99
THF
CH2Cl2
9[e]
10[e]
11[e,f]
12[e,g,h]
61
>99
1S, 2S
[a] All reactions were carried out with 2 equiv of Et2Zn and 3 equiv of
CH2I2 for 1 h unless otherwise noted. [b] Calculated from 1H NMR
(400 MHz) analysis by using 1-bromonaphthalene as an internal
standard. [c] Determined by HPLC analysis by using a chiral stationary
À
phase column (Daicel Chiracel OD H; hexane/iPrOH=9:1). [d] Deter-
mined by chiroptical comparison (references [9b]). [e] 2 equiv of Et2Zn
and 1.2 equiv of CH2I2 were used. [f] Run for 24 h. [g] Catalyst loading
was 5 mol%. [h] Run for 3 h.
enantioselectivity of the reaction of the (Z)-cinnamyl alcohol
(9) was moderate (Table 2, entry 3). The reaction of non-
conjugated E allylic alcohols proceeded with high enantiose-
lectivities (greater than 86% ee) and in quantitative yields
(Table 2, entries 4–6). However, the reaction of trisubstituted
allylic alcohol 13, which has a Z substituent, was moderately
enantioselective (Table 2, entry 7). Lowering the reaction
temperature increased the enantioselectivity to 70% ee, but
the reaction slowed down (Table 2, entry 8).
best; it generated the product in 95% ee, the highest value
obtained by either stoichiometric or catalytic asymmetric
Simmons–Smith reactions of 1 (Table 1, entry 8). Moreover,
the same reaction was conducted by using 2 equivalents of
Et2Zn and 1.2 equivalents of CH2I2 without eroding the
enantioselectivity (Table 1, entry 9). Although the reactions
that were run in halogenated and aromatic solvents, such as
Although the mechanism of this reaction is unclear, the
experimental results show that the introduction of the N-
methyl group remarkably diminishes the enantioselectivity of
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c) C. Bonaccorsi, A. Mezzetti, Organometallics 2005, 24, 4953 –
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the reaction, and that the complex bearing a
chloride ligand at the apical position is a
better catalyst than the complex bearing an
ethoxy ligand. This suggests that the bifunc-
tional catalysis of the Al(salalen) complex
is essential for obtaining high enantioselec-
tivities (Figure 2).
In conclusion, we were able to show that
the Al(salalen) complex, an Al Lewis acid/
N Lewis base bifunctional catalyst, is a
potent catalyst for the asymmetric Sim-
mons–Smith reaction. Although good sub-
strates are limited to trans-disubstituted
allylic alcohols, the reaction proceeds with
high enantioselectivities and in quantitative
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Figure 2. A plau-
sible intermedi-
ate for Al-
(salalen)-cata-
lyzed Simmons–
Smith reactions
of allylic alco-
hols.
[7] a) H. Kitajima, Y. Aoki, K. Ito, T. Katsuki, Chem. Lett. 1995,
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yields at room temperature. The present study provides a new
approach to the development of catalytic asymmetric Sim-
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[8] a) H. Takahashi, M. Yoshioka, M. Ohno, S. Kobayashi, Tetrahe-
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Experimental Section
Typical example of an asymmetric Simmons–Smith reaction with 2: A
solution of Et2AlCl (0.92m, 54 mL, 0.05 mmol) in hexanes at 08C
under a nitrogen atmosphere was added to a solution of salalen ligand
2 (41.5 mg, 0.05 mmol) in anhydrous dichloromethane (5 mL). The
reaction mixture was warmed to room temperature and stirred for
30 min. Then 1 (64 mL, 0.5 mmol), a solution of Et2Zn (1.0m, 1.0 mL,
1.0 mmol) in hexanes, and CH2I2 (48 mL, 0.6 mmol) were successively
added to the reaction mixture, which was then stirred for 1 h at room
temperature. The mixture was quenched with an aqueous NaOH
solution (2n) and the organic layer was separated. The aqueous layer
was extracted with dichloromethane, and then the organic layers were
combined and dried over anhydrous Na2SO4, and concentrated under
reduced pressure. The residue was chromatographed on silica gel
(hexane/diethyl ether= 2:1) to give the desired cyclopropylalcohol
(71.0 mg, 96%) as a colorless oil. The enantiomeric excess of the
product was determined to be 95% by HPLC analysis with a Daicel
Chiralcel OD-H column (hexane/iPrOH = 9:1).
[9] a) A. B. Charette, C. Brochu, J. Am. Chem. Soc. 1995, 117,
11367 – 11368; b) A. B. Charette, C. Molinaro, C. Brochu, J. Am.
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[10] For highly selective asymmetric Simmons–Smith reactions of
various enol ethers and ethers of allylic alcohols, see: a) J. Long,
Y. Yuan, Y. Shi, J. Am. Chem. Soc. 2003, 125, 13632 – 13633; b) J.
Long, H. Du, K. Li, Y. Shi, Tetrahedron Lett. 2005, 46, 2737 –
2740; c) M.-C. Lacasse, C. Poulard, A. B. Charette, J. Am. Chem.
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[11] Other catalytic asymmetric Simmons–Smith reactions: a) S. E.
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2003, 5, 4417 – 4420; g) H. Y. Kim, A. E. Lurain, P. G. -Garcꢁa,
P. J. Carroll, P. J. Walsh, J. Am. Chem. Soc. 2005, 127, 13138 –
13139.
Received: December 10, 2007
Published online: February 20, 2008
Keywords: allylic compounds · aluminum ·
.
bifunctional catalysts · homogeneous catalysis · Simmons–
Smith reaction
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[13] Although the allyloxy group is not a part of the dioxaborolane, it
has been proposed that the oxygen atom of the coordinated
allylic oxide serves as a Lewis base synergetically with the amide
carbonyl group.[6]
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[15] A metal-bound amine is not a base, but the pKa value of the
amine proton is small. Under the conditions with diethylzinc, it
should be deprotonated and the resultant amide should serve as
a base.
[16] We recently found that Al(salalen) complexes showed unique
catalysis for asymmetric oxidation of sulfides: a) T. Yamaguchi,
K. Matsumoto, B. Saito, T. Katsuki, Angew. Chem. 2007, 119,
4813 – 4815; Angew. Chem. Int. Ed. 2007, 46, 4729 – 4731; b) K.
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Chem. Asian J. 2008, 3, 351 – 358..
[17] Kobayashi et al. have reported that Al– and Zn– disulfonamide
complexes show similar asymmetric Simmons–Smith catalysis.[8b]
Analogous intermediates generated by aggregation such as
shown in Figure 1a have been proposed to participate in these
reactions.
[18] Salalen ligands used in this study were prepared according to the
reported procedures (references [14b,16b]).
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