A catalytic enantioselective allylation reaction of aldehydes in an aqueous medium

Article abstract of DOI:10.1016/S0040-4039(00)00801-7

A modified Yamamoto-Yanagisawa's catalyst (S)-Tol-BINAP·AgNO₃ was successfully applied to a catalytic enantioselective allylation reaction of aldehydes in an aqueous system. The reactions with aromatic aldehydes afforded the desired products in

Full text of DOI:10.1016/S0040-4039(00)00801-7

Tetrahedron Letters 41 (2000) 5261±5264
A catalytic enantioselective allylation reaction of aldehydes in
an aqueous medium
Teck-Peng Loh* and Jian-Rong Zhou
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
Received 13 March 2000; accepted 16 May 2000
Abstract
.
A modi®ed Yamamoto±Yanagisawa's catalyst (S)-Tol-BINAP AgNO3 was successfully applied to a
catalytic enantioselective allylation reaction of aldehydes in an aqueous system. The reactions with aromatic
aldehydes a€orded the desired products in high yields with good stereoselectivities. # 2000 Published by
Elsevier Science Ltd.
Keywords: enantioselectivity; catalyst; allylation; aldehyde; aqueous.
Catalytic enantioselective allylation reactions of aldehydes are valuable C±C bond-formation
methods for preparing enantiomerically enriched secondary alcohols. To date several catalysts
have been devised to serve this purpose.1,2 Another research focus among today's synthetic
community is performing organic transformations in aqueous media,3 which entails extra-
ordinary chemical stability and reactivity. Furthermore, extension of catalytic enantioselective
transformations to the aqueous media poses additional challenges in catalyst tolerance towards
water. Several such examples using catalytic amounts of chemzymes have been reported,4 but not
with the allylation reactions between allylmetal reagents and carbonyl functionalities. Herein, we
report our recent progress in this area employing a modi®ed Yamamoto±Yanagisawa's catalyst
.
(S)-Tol-BINAP AgNO3 (Scheme 1).
First, we started with the reaction of benzaldehyde with allyltributyltin in the presence of (S)-
2
.
.
BINAP AgOTf, developed by Yamamoto and Yanagisawa in dry THF. The (S)-BINAP AgOTf
complex was generated by stirring (S)-BINAP and silver(I) tri¯ate in a 1:9 water:ethanol mixed
solvent at 25ꢀC for 30 min in the dark. Treatment of the catalyst (5 mol%) with equimolar
amounts of benzaldehyde and allyltributyltin and subsequent stirring at 25ꢀC for 14 h in the dark
a€orded the (S)-enriched product in 61% yield with 54% ee. Investigation on the solvent e€ect
(Table 1, entries 1±4) showed that the 1:9 water:ethanol was a superior choice. Other chiral
diphosphines 2±6 (Scheme 1) were also screened under the same conditions and the highest 66%
* Corresponding author. Fax: 65-7791691; e-mail: chmlohtp@nus.edu.sg
0040-4039/00/$ - see front matter # 2000 Published by Elsevier Science Ltd.
PII: S0040-4039(00)00801-7

5262
Scheme 1.
Table 1
Enantioselective allylation of benzaldehyde
ee was achieved with 2, (S)-Tol-BINAP (entry 5). Lowering the temperature to ^40ꢀC further
increased the selectivity to 75% ee with a higher isolated yield (entry 12). On the other hand, no
reaction was observed at ^60ꢀC.
Next we surveyed the catalytic activities of other transition metal salts (Table 2). The experiments
were carried out by stirring benzaldehyde, allyltributyltin and 5 mol% pre-formed complexes with
(S)-Tol-BINAP at ^40ꢀC for 14 h. The results are summarised in Table 2. Only silver(I) salts were
found to be e€ective in the catalysis. Furthermore, the reactions with the silver(I) salts exhibited
essentially no counterion e€ect, which is in sharp contrast to those observed in organic solvents.2
This is probably due to complete ionic dissociation in the aqueous medium. Additionally, increasing

5263
Table 2
Catalytic activities of transition metal salts
.
the amount of (S)-Tol-BINAP AgNO3 to 10 mol% (entry 9) led to a little improvement in the
selectivity (79% ee).
With the optimised conditions, (S)-Tol-BINAP AgNO3 was applied to other aldehydes (Table 3).
.
.
The (S)-Tol-BINAP AgNO3 complex (0.1 mmol) was generated in 10 mL of 1:9 H2O:EtOH
mixed solvent as described above. The colourless catalyst solution was cooled to ^40ꢀC and
treated with an aldehyde (1 mmol) and allyltributyltin (1 mmol, 310 mL). The reaction mixture
was stirred in the dark for several hours as indicated in Table 3. In general, aromatic aldehydes
a€orded the desired products with good stereocontrol. It is noteworthy that 71% ee was obtained
with 3-hydroxybenzaldehyde without protection of its phenolic group. However, only low to
moderate stereoselectivity was observed with aliphatic, ole®nic and acetylenic aldehydes (entries
8±10).
Table 3
Enantioselective allylation of aldehydes

5264
In conclusion, the modi®ed Yamamoto±Yanagisawa's catalyst was successfully applied to the
enantioselective allylation reaction of aldehydes in an aqueous system. The reactions with aromatic
aldehydes usually a€orded desired products in high yields with good stereoselectivities. The
experiment was easy to perform without the need to exclude moisture or air. In addition, com-
pounds carrying sensitive functional groups need no protection under the reaction conditions.
Ligand modi®cation is still underway in our laboratory.
Acknowledgements
This research was assisted ®nancially by grants from the National University of Singapore.
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