Catalytic asymmetric alkylation of aldehydes with Grignard reagents

Article abstract of DOI:10.1002/anie.200704963

(Chemical Equation Presented) Added benefits: Grignard reagents can be employed in the asymmetric alkylation of aldehydes by using a titanium(IV) catalyst derived from binol in the presence of excess titanium tetraisopropoxide (see scheme). The reaction proceeds with a low catalyst loading (2 mol%) and exhibits high enantioselectivity for both aromatic and unsaturated aldehydes and for both alkyl and aryl Grignard reagents.

Full text of DOI:10.1002/anie.200704963

Communications
DOI: 10.1002/anie.200704963
Asymmetric Catalysis
Catalytic Asymmetric Alkylation of Aldehydes with Grignard
Reagents**
Yusuke Muramatsu and Toshiro Harada*
The catalytic asymmetric addition of organometallic reagents
to carbonyl compounds is a reaction of fundamental impor-
tance in modern synthetic organic chemistry.[1] Less reactive
reagents, typically diorganozinc derivatives,[2,3] are used to
achieve this transformation because a direct, noncatalyzed
reaction would deminish the enantioselectivity.[4–6] The reac-
tion of Grignard reagents with carbonyl compounds is one of
the most reliable methods for forming carbon–carbon bonds.
However, Grignard reagents have been recognized to be
unsuitable for asymmetric alkylation, because to their high
reactivity. Indeed, in previously reported asymmetric alkyl-
ations with Grignard reagents, more than a stoichiometric
amount of a chiral modifier was required to obtain high
enantioselectivity.[7]
A
taddolate–titanium(IV)-catalyzed
reaction (taddol = a,a,a’,a’-tetraaryl-2,2-dimethyl-1,3-dioxo-
lan-4,5-dimethanol) of dialkylzinc derivatives, generated
in situ from Grignard reagents and ZnCl2 with strict exclusion
of magnesium salts, have also been reported.[8,9]
We recently reported[10] that a titanium(IV) complex
derived from (R)-3-(3,5-diphenylphenyl)-2,2’-dihydroxy-1,1’-
binaphthyl (DPP-binol) exhibits a remarkably enhanced
catalytic activity in the asymmetric alkylation of aldehydes
with diethylzinc.[11,12] This observation prompted us to exam-
ine the catalyst derived from DPP-binol in the reaction with
Grignard reagents. Herein, we report that the asymmetric
alkylation of aldehydes can be achieved by using Grignard
reagents in the presence of a catalytic amount of DPP-binol
(2 mol%) and excess titanium tetraisopropoxide [Eq. (1)].
The general procedure is straightforward: A Grignard
reagent (R2MgX; X = Cl, Br, 2–3m in Et2O or THF;
2.2 equiv) is treated with titanium tetraisopropoxide
(4.4 equiv) in CH2Cl2 at À788C. The resulting mixture,
containing titanate [R2Ti(OiPr)4]MgX,[13] is slowly added
(over a period of 2 h) to a solution of an aldehyde,
(R)-DPP-binol (2 mol%), and titanium tetraisopropoxide
(1.4 equiv) in CH2Cl2 at 08C and is stirred for a further 1 h.
Subsequent aqueous workup followed by Kugelrohr distilla-
tion or flash chromatography on silica gel then affords the
corresponding secondary (R)-alcohol (1–5) enantioselec-
tively.
Results for the asymmetric alkylation of various alde-
hydes using Grignard reagents are summarized in Table 1.
The reaction of aromatic aldehydes, including 1- and
2-naphthaldehyde, with nBuMgCl afforded the correspond-
ing butylation products in high yield and enantioselectivity
(91–96% ee; Table 1, entries 1, 6, 8, 9, 11, 13, and 19). The
reactions with nPrMgCl (entry 4) and EtMgX (X = Cl, Br)
were also enantioselective (entries 5, 7, 10, 12, 14, and 16),
whereas the yield of the ethylation products was moderate
(see below). Chloromagnesium reagents and bromomagne-
sium reagents could be employed with comparable efficiency
and selectivity (entries 14 and 16). The solvent in which the
Grignard reagents are prepared influences the reaction
outcome. EtMgCl in Et2O gave better enantioselectivity
compared with THF (compare entries 14 and 15). The use of
unsubstituted binol in the reaction of benzaldehyde with
nBuMgCl resulted in decreased enantioselectivity (79% ee)
and yield (77%) of the butylation product (entry 3).
[*] Y. Muramatsu, Prof. Dr. T. Harada
Department ofChemistry and Materials Technology
Kyoto Institute ofTechnology
Matsugasaki, Sakyo-ku, Kyoto 606-8585 (Japan)
Fax: (+81)75-724-7514
The reaction of 1-naphthaldehyde with MeMgCl resulted
in lowenantioselectivity (entry 17). In contrast, relatively
high enantioselectivity was obtained for the phenylation with
PhMgBr (entry 18). Aromatic aldehydes as well as
a,b-unsaturated aldehydes reacted efficiently with nBuMgCl
E-mail: harada@chem.kit.ac.jp
[**] This work was partially supported by a grant from the Kyoto Institute
ofTechnology Research Fund.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
Table 1: Catalytic asymmetric alkylation ofaldehydes with Grignard
Experimental Section
reagents.[a]
Typical procedure: (R)-1-Phenylpentan-1-ol (1a; Table 1, entries 1
and 2): nBuMgCl (2m in Et2O; 1.10 mL, 2.2 mmol) was added to a
solution of titanium tetraisopropoxide (1.30 mL, 4.4 mmol) in dry
CH2Cl2 (16 mL) at À788C in an argon atmosphere. After stirring the
reaction for 10 min at this temperature, it was slowly added (over a
period of 2 h by using a syringe pump) to a solution of (R)-DPP-binol
(10.3 mg, 0.020 mmol), benzaldehyde (0.106 g, 1.0 mmol), and tita-
nium tetraisopropoxide (0.41 mL, 1.4 mmol) in CH2Cl2 (4 mL) at 08C
in an argon atmosphere and stirred for a further 1 h. The reaction
mixture was then quenched by the addition of aqueous 1n HCl and
extracted three times with Et2O. The organic layers were washed
successively with aqueous 5% NaHCO3 and brine, dried (MgSO4),
and concentrated in vacuo. Kugelrohr distillation (1508C/5 mmHg)
gave 1a (0.141 g, 86% yield, 93% ee). The distillation residue was
purified by flash chromatography on silica gel (10–50% ethyl acetate
in n-hexane) and gave (R)-DPP-binol (6.1 mg, 59% recovery) and
1,2-diphenylethane-1,2-diol (4.2 mg, 4% yield), where meso:dl =
1.3:1. The ee value was determined by HPLC analysis using a
Chiralcel OD column (1.0 mLminÀ1, 2% iPrOH in n-hexane);
retention times: 11.8 min (major R enantiomer) and 14.7 min
(minor S enantiomer). The absolute configuration of the product
was determined based on the reported retention times:[16] 12.9 min
(R enantiomer) and 16.4 min (S enantiomer).
Entry Aldehyde
RMgX[b]
Alkylation Yield [%][c] ee [%]
product
1
PhCHO
PhCHO
PhCHO
PhCHO
nBuMgCl 1a
nBuMgCl 1a
nBuMgCl 1a
nPrMgCl 1b
EtMgCl
nBuMgCl 1d
EtMgCl 1e
86
94
77
82
57
82
41
62
86
40
79
46
90
56
39
63
89
94
92
60
49
36
93
92
79
94
90
95
88
95
95
95
94
80
96
94
84
93
28
86
91
91
84
92
2[d]
3[e]
4
5
PhCHO
1c
6
7
8
9
p-MeC6H4CHO
p-MeC6H4CHO
p-CF3C6H4CHO
m-MeOC6H4CHO
m-MeOC6H4CHO
o-ClC6H4CHO
o-ClC6H4CHO
1-naphCHO
1-naphCHO
1-naphCHO
1-naphCHO
1-naphCHO
1-naphCHO
2-naphCHO
nBuMgCl 1 f
nBuMgCl 1g
10
11
12
13
14
15
16
17
18
19
20
21
22
EtMgCl
nBuMgCl 1i
EtMgCl 1j
nBuMgCl 2a
EtMgCl 2b
EtMgCl[f] 2b
EtMgBr 2b
MeMgCl[f] 2c
PhMgBr
nBuMgCl
nBuMgCl 4a
nBuMgCl 4b
EtMgCl
1h
2d
3
=
PhCH CHCHO
The above reaction was repeated on a 10.5-mmol scale by
following the same procedure except that a dropping funnel was used
for the slowaddition over 2 h. Kugelrohr distillation (120–150 8C/
5 mmHg) of the crude products gave (R)-1a (1.62 g, 94% yield,
92% ee) ([a]D25 = 35.4 degcm3 gÀ1 dmÀ1 (c = 0.0303 gcmÀ3, C6H6)). The
distillation residue was purified by flash chromatography on silica gel
(10–50% ethyl acetate in hexane) to give (R)-DPP-binol (101.2 mg,
98% recovery) and 1,2-diphenylethane-1,2-diol (76.2 mg, 7% yield),
where meso:dl = 1.4:1.
=
CH2 C(CH3)CHO
PhCH2CH2CHO
5
[a] Unless otherwise noted, reactions were carried out on a 1-mmol scale
with DPP-binol (2 mol%) according to the procedure described in the
Experimental Section. [b] Unless otherwise noted, a commercial solution
in Et2O (1.6–3m) was used. [c] Yields ofisolated product. [d] The
reaction was performed on a 10-mmol scale. [e] Binol (2 mol%) was
used. [f] THF solution. naph =naphthalene.
Received: October 26, 2007
Published online: January 4, 2008
to give the corresponding allylic alcohols 4a and 4b enantio-
selectively (entries 20 and 21). Although sluggish, the reac-
tion of an aliphatic aldehyde was also enantioselective
(entry 22).
The pinacol coupling of aldehydes was observed as a
major side reaction when using the alkyl Grignard
reagents.[14,15] In the ethylation reaction, the corresponding
pinacol by-product (R1CH(OH)CH(OH)R1) was formed in
22–44% yield. The side reaction was less dominant for
nPrMgCl and nBuMgCl (< 10%) and not observed for
MeMgCl and PhMgX (X = Cl, Br).
To demonstrate the preparative utility of our asymmetric
alkylation procedure, the reaction of benzaldehyde with
nBuMgCl was carried out on a 10.5-mmol scale. Kugelrohr
distillation of the crude product gave 1a (1.62 g, 94% yield) in
92% ee (entry 2), and DPP-binol was recovered quantita-
tively by flash chromatography of the distillation residue.
In summary, we have demonstrated that Grignard
reagents can be effective in asymmetric alkylation of alde-
hydes by using a catalyst derived from binol–titanium(IV)
derivative in the presence of excess titanium tetraisoprop-
oxide. The reaction proceeds with a low catalyst loading
(2 mol%) and exhibits high enantioselectivity for aromatic
and unsaturated aldehydes as well as for both alkyl and aryl
Grignard reagents.
Keywords: alkylation · asymmetric catalysis ·
asymmetric synthesis · Grignard reaction · titanium
.
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