Catalytic asymmetric conjugate addition of dialkylzinc reagents to β-aryl-α,β-unsaturated N-2,4,6- triisopropylphenylsulfonylaldimines with use of N-Boc-L-Val-connected amidophosphane-copper(I) Catalyst

Article abstract of DOI:10.1021/jo0484225

(Chemical Equation Presented). Asymmetric conjugate addition of dialkylzinc to α,β-unsaturated N-2,4,6-triisopropylphenylsulfonylaldimines 9 was catalyzed by 5 mol % N-Boc-L-valine-connected amidophosphane 5a-Cu-(MeCN) ₄BF₄ in the presence of 4 A MS in toluene to afford, after hydrolysis of an imine to an aldehyde through a short alumina column and reduction with sodium borohydride, the corresponding β-alkylated alkanols 10 with 67-91% ee in reasonably high yields.

Full text of DOI:10.1021/jo0484225

Catalytic Asymmetric Conjugate Addition of Dialkylzinc Reagents
to â-Aryl-r,â-unsaturated
N-2,4,6-Triisopropylphenylsulfonylaldimines with Use of
N-Boc-L-Val-Connected Amidophosphane-Copper(I) Catalyst
Takahiro Soeta, Masami Kuriyama, and Kiyoshi Tomioka*
Graduate School of Pharmaceutical Sciences, Kyoto University,
Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
tomioka@pharm.kyoto-u.ac.jp
Received September 8, 2004
Asymmetric conjugate addition of dialkylzinc to R,â-unsaturated N-2,4,6-triisopropylphenylsulfon-
ylaldimines 9 was catalyzed by 5 mol % N-Boc-L-valine-connected amidophosphane 5a-Cu-
(MeCN)4BF4 in the presence of 4 Å MS in toluene to afford, after hydrolysis of an imine to an
aldehyde through a short alumina column and reduction with sodium borohydride, the corresponding
â-alkylated alkanols 10 with 67-91% ee in reasonably high yields.
Introduction
be effective for the asymmetric conjugate addition of
Grignard reagents and organolithiums. We and others
have already developed three types of enantioselective
1,2-addition to aldimines; the chiral ether-mediated
addition of organolithium reagents to 4-methoxyphen-
ylimines,6 copper(I)-chiral amidophosphane-catalyzed ad-
dition of dialkylzinc reagents to N-sulfonylimines,7,8 and
Conjugate addition reaction of organometallic reagents
with R,â-unsaturated aldehydes has maintained a unique
position because these acceptors are prone to be more
susceptible to 1,2-addtion than R,â-unsaturated ketones,
esters, amides, and equivalents.1,2 Complexation of an
aldehyde carbonyl group with a bulky aluminum agent
has been proven to block a carbonyl group from 1,2-
addition permitting the 1,4-conjugate addition of orga-
nolithium and Grignard reagents.3 Chemical modification
to chiral4 and achiral5 aldimines has been also proven to
(4) Kogen, H.; Tomioka, K.; Hashimoto, S.; Koga, K. Tetrahedron
1981, 37, 3951-3956.
(5) (a) Tomioka, K.; Shindo, M.; Koga, K. J. Am. Chem. Soc. 1989,
111, 8266-8268. (b) Shindo, M.; Koga, K.; Tomioka, K. J. Am. Chem.
Soc. 1992, 114, 8732-8733. (c) Shindo, M.; Koga, K.; Tomioka, K. J.
Org. Chem. 1998, 63, 9351-9357. (d) Tomioka, K.; Shioya, Y.; Nagaoka,
Y.; Yamada, K. J. Org. Chem. 2001, 66, 7051-7054.
(6) (a) Tomioka, K. Synthesis 1990, 541-549. (b) Tomioka, K.; Inoue,
I.; Shindo, M.; Koga, K. Tetrahedron Lett. 1990, 31, 6681-6684. (c)
Fujieda, H.; Kanai, M.; Kambara, T.; Iida, A.; Tomioka, K. J. Am.
Chem. Soc. 1997, 119, 2060-2061. (d) Kambara, T.; Hussein, M. A.;
Fujieda, H.; Iida, A.; Tomioka, K. Tetrahedron Lett. 1998, 39, 9055-
9058. (e) Hussein, M. A.; Iida, A.; Tomioka, K. Tetrahedron 1999, 55,
11219-11228. (f) Kambara, T.; Tomioka, K. J. Org. Chem. 1999, 64,
9282-9285. (g) Tomioka, K.; Fujieda, H.; Hayashi, S.; Hussein, M. A.;
Kambara, T.; Nomura, Y.; Kanai, M.; Koga, K. Chem. Commun. 1999,
715-716. (h) Taniyama, D.; Hasegawa, M.: Tomioka, K. Tetrahedron
Lett. 2000, 41, 5533-5536. (i) Hata, S.; Iguchi, M.; Iwasawa, T.;
Yamada, K.; Tomioka, K. Org. Lett. 2004, 6, 1721-1724. (j) Hata, S.;
Iwasawa, T.; Iguchi, M.; Yamada, K.; Tomioka, K. Synthesis 2004,
1471-1475.
(1) For reviews on enantioselective conjugate addition, see: (a)
Tomioka, K.; Nagaoka, Y. In Comprehensive Asymmetric Catalysis;
Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer: New York,
1999; Vol. III, Chapter 31. (b) Tomioka, K. In Modern Carbonyl
Chemistry; Otera, J., Ed.; Wiley-VCH: Weinheim, 2000; Chapter 12.
(c) Feringa, B. L. Acc. Chem. Res. 2000, 33, 346-353. (d) Sibi, M. P.;
Manyem, S. Tetrahedron 2000, 56, 8033-8061. (e) Tomioka, K. In
Comprehensive Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A.,
Yamamoto, H., Eds.; Springer: New York, 2004; Supplement to
Chapter 31.1, pp 109-124.
(2) For selected works on the covalently bound chiral auxiliary-
controlled asymmetric conjugate addition of organometals, see: (a)
Tomioka, K.; Suenaga, T.; Koga, K. Tetrahedron Lett. 1986, 27, 369-
372 1986. (b) Oppolzer, W.; Poli, F.; Kingma, A. J.; Starkemann, C.;
Bernardinelli, G. Helv. Chim. Acta 1987, 70, 2201-2214. (c) Basil, L.
F.; Meyers, A. I.; Hassner, A. Tetrahedron 2002, 58, 207-213. (d)
Dambacher, J.; Anness, R.; Pollock, P.; Bergdahl, M. Tetrahedron 2004,
60, 2097-2110.
(7) (a) Fujihara, H.; Nagai, K.; Tomioka, K. J. Am. Chem. Soc. 2000,
122, 12055-12056. (b) Nagai, K.; Fujihara, H.; Kuriyama, M.; Yamada,
K.; Tomioka, K. Chem. Lett. 2002, 8-9. (c) Soeta, T.; Nagai, K.;
Fujihara, H.; Kuriyama, M.; Tomioka, K. J. Org. Chem. 2003, 68,
9723-9727.
(3) Maruoka, K.; Imoto, H.; Saito, S.; Yamamoto, H. J. Am. Chem.
Soc. 1994, 116, 4131-4132.
10.1021/jo0484225 CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/20/2004
J. Org. Chem. 2005, 70, 297-300
297

Soeta et al.
SCHEME 1. Copper-Chiral Phosphane-Catalyzed
Asymmetric Ethylation of Cinnamaldehyde 1a and
Its Aldimine 1b with Diethylzinc Giving 2 and 3
obtained in 55% combined yield after sodium borohydride
reduction of an aldehyde (Table 1, entry 1). We then
examined the reaction of the corresponding aldimines 1
with respect to regio- and enantioselectivity. N-Meth-
anesulfonylimine 1b15 was treated with diethylzinc under
the same catalysis conditions as the reaction of 1a,
followed by imine-hydrolysis with 10% HCl and then
sodium borohydride reduction, to give a mixture of
comparable amounts of 2 with 32% ee in 41% yield and
3b with 68% ee in 32% yield (entry 2). The source of
copper was found to be influential to the 1,4-/1,2-
selectivity as well as enantioselectivity. Copper(I) triflate
benzene complex gave the slightly improved level of
selectivity with copper(II) triflate (entry 3). Although
naphthenate,16 acetate, bromide and iodide dimethyl
sulfide complexes were 1,4-selective copper sources ex-
cepting cyanide and free iodide,17 enantioselectivity was
generally unsatisfactory (entries 4-9).
Copper(I)-L-Val-Connected Phosphane-Catalyzed
Reaction with Sterically Tuned N-Sulfonylimine.
N-Tosylimine 1c15 was also not a good acceptor to give a
mixture of 2 (16% ee) and 3c (83% ee) in a similar product
valance with 1b (Table 2, entry 1). Fortunately, aldimine
1d bearing a bulky 2,4,6-triisopropylbenzenesulfonyl
group was found to be more 1,4-selectively converted to
2 with 47% ee in 66% yield and 3d (1% ee) in 12% yield
(entry 2). We also examined the effects of copper and
amidophosphane on the reaction of 1d with diethylzinc.
Without both copper(II) triflate and phosphane 4c, the
reaction was very sluggish at 0 °C for 19 h to give 2 in
32% yield (entry 3). Without 4c the copper-catalyzed
reaction was also sluggish, at 0 °C for 20 h, giving 2 in
43% yield and 3d in 14% yield (entry 4). These two
results apparently indicated the high potentiality of a
copper-phosphane combination as a catalyst. We then
examined the effects of chiral phosphanes 4-6 on the 1,4-
selectivity and enantioselectivity of the reaction with 1d.
Unfortunately, the less bulky amidophosphane 4a18 gave
a mixture of 2 with 20% ee in 57% yield and 3d with
67% ee in 18% yield (entry 5). Interestingly, a chiral
amidophosphane 4b7c bearing two benzyl groups on a
pyrrolidine ring efficiently suppressed the 1,2-addition
of diethylzinc to 1d at 0 °C for 2 h, giving the correspond-
ing 1,4-product 2 with 57% ee in 74% yield, after
hydrolysis of an imine to an aldehyde through a short
alumina column and subsequent reduction of an aldehyde
with sodium borohydride (entry 6). It was fortunate to
find that valine-connected amidophosphanes 5 and 6 gave
the conjugate addition product 2 as the sole product.
N-Boc-L-Valine-connected amidophosphane 5a9 gave 2
with 61% ee in 77% yield (Table 2, entry 7). However,
the use of 6 bearing two benzyl groups on the pyrrolidine
ring gave 2 with a decreased 24% ee in 72% yield (entry
9). D-Valine-connected phosphane 5b was not the better
ligand, giving 2 with 43% ee in 75% yield (entry 8).
rhodium-chiral amidophosphane-catalyzed addition of
arylboroxine reagents to N-sulfonylimines.9,10 As part of
our continuing effort to broaden the scope of the copper-
chiral amidophosphane-catalyzed addition of dialkylzinc
reagents to imines, we are interested in the possibility
of controlling 1,4- and 1,2-additions to aldimines of R,â-
unsaturated aldehydes, thereby providing an alternative
for hitherto difficult asymmetric conjugate addition to
R,â-unsaturated aldehydes.11,12 We describe herein that
the combination of Cu(MeCN)4BF4 and amidophosphane
5a catalyzes the conjugate addition of dialkylzinc to
aldimines 9 of R,â-unsaturated aldehydes to give the
corresponding adducts 10 with up to 91% ee in reason-
ably high yields.
Reaction of Diethylzinc with Cinnamaldehyde
and its N-Methanesulfonylimine. We began our stud-
ies with the reaction of cinnamaldehyde 1a with dieth-
ylzinc in the presence of 5 mol % copper(II) triflate and
6.5 mol % chiral amidophosphane 4c in toluene at 0 °C,
the conditions established in the 1,2-addition7c (Scheme
1, Figure 1). Although 1a was consumed within 5 h, a
FIGURE 1. Chiral amidophosphanes 4-6.
mixture of comparable amounts of 1,4- and 1,2-addition
alcohols (-)-213 with 7% ee and 3a14 with 13% ee was
(8) (a) Porter, J. R.; Traverse, J. F.; Hoveyda, A. H.; Snapper, M. L.
J. Am. Chem. Soc. 2001, 123, 10409-10410. (b) Boezio, A. A.;
Pytkowicz, J.; Coˆte´, A.; Charette, A. B. J. Am. Chem. Soc. 2003, 125,
14260-14261. (c) Coˆte´, A.; Boezio, A. A.; Charette, A. B. Proc. Natl.
Acad. Sci. U.S.A. 2004, 101, 5405-5410.
(9) Kuriyama, M.; Soeta, T.; Hao, X.; Chen, Q.; Tomioka, K. J. Am.
Chem. Soc. 2004, 126, 8128-8129.
(10) (a) Review: Hayashi, T.; Yamasaki, K. Chem. Rev. 2003, 103,
2829-2844. (b) Hayashi, T.; Ishigetani, M. J. Am. Chem. Soc. 2000,
122, 976-977. (c) Hayashi, T.; Ishigetani, M. Tetrahedron 2001, 57,
2589-2595. (d) Hermanns, N.; Dahmen, S.; Bolm, C.; Bra¨se, S. Angew.
Chem., Int. Ed. 2002, 41, 3692-3694.
(11) Rhodium-catalyzed conjugate addition and reduction of cinnam-
aldehyde with arylboronic acid have been reported. (a) Itooka, R.;
Iguchi, Y.; Miyaura, N. J. Org. Chem. 2003, 68, 6000-6004. (b) Wang,
Z.; Zou, G.; Tang, J. Chem. Commun. 2004, 1192-1193.
(12) Asymmetric Michael reactions to enals have been reported. (a)
Brown, S. P.; Goodwin, N. C.; MacMillan, D. W. C. J. Am. Chem. Soc.
2003, 125, 1192-1194. (b) Ooi, T.; Doda, K.; Maruoka, K. J. Am. Chem.
Soc. 2003, 125, 9022-9023.
The source of copper was also influential on the
enantioselectivity of the reaction of 1d with a copper-5a
(13) Absolute configuration of (-)-2 was determined by comparison
of the specific rotation with the reported value. Pridgen, N.; Mokhal-
lalati, M. K.; Wu, M.-J. J. Org. Chem. 1992, 57, 1237-1241. The ee
was determined by chiral stationary phase HPLC (Daicel Chiralcel
OD-H, hexane/i-PrOH ) 50/1, 1.0 mL/min, 254 nm, major 16.0 min
and minor 19.4 min).
(15) Georg, G. L.; Harriman, G. C. B.; Peterson, S. A. J. Org. Chem.
1995, 60, 7366-7368.
(16) Alexakis, A.; Benhaim, C.; Rosset, S.; Humam, M. J. Am. Chem.
Soc. 2002, 124, 5262-5263.
(17) Kanai, M.; Nakagawa, Y.; Tomioka, K. Tetrahedron 1999, 55,
3843-3854.
(14) Okamoto, T.; Oka, S. J. Org. Chem. 1983, 49, 1589-1594.
(18) Kanai, M.; Tomioka, K. Tetrahedron Lett. 1995, 36, 4275-4278.
298 J. Org. Chem., Vol. 70, No. 1, 2005

Catalytic Asymmetric Conjugate Addition of Dialkylzinc Reagents
TABLE 1. Copper-Amidophosphane 4c-Catalyzed Asymmetric Reaction of Cinnamaldehyde 1a and Its
N-Methanesulfonylimine 1b with Diethylzinc
entry
1
CuX
time (h)
yield of 2 (%)
ee of 2 (%)a
yield of 3 (%)
ee of 3 (%)a
1
2
3
4
5
6
7
8
9
1a
1b
1b
1b
1b
1b
1b
1b
1b
Cu(OTf)2
5
1
1
1
1
6
1
2
5
25
41
54
68
60
1
62
58
7
7
30
32
21
4
13
68
63
3
Cu(OTf)2
32
34
4
(CuOTf)2-PhH
Cu(naphthenate)2
Cu(OAc)2-H2O
CuCN
CuBr-SMe2
CuI-SMe2
CuI
1
nd
4
10
1
2
8
1
nd
3
22
nd
3
2
1
a nd: Not determined.
TABLE 2. CuX-Amidophosphane-Catalyzed Asymmetric Reaction of N-Arylsulfonylimine with Diethylzinc
entry
1
CuX
ligand
4 Å MS
time (h)
yield of 2 (%)
ee of 2 (%)
yield of 3 (%)
ee of 3 (%)b
1a
2
1c
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1c
Cu(OTf)2
4c
none
none
none
none
none
none
none
none
none
none
4 Å MS
4 Å MS
none
5
3
40
60
32
43
57
74
77
75
73
79
80
77
65
16
47
0
32
14
0
83
1
Cu(OTf)2
4c
3
none
none
none
4a
4b
5a
5b
6
5a
5a
5a
4c
19
20
3
0
4
Cu(OTf)2
0
14
18
0
0
5
Cu(OTf)2
20
57
61
43
24
65
68
80
5
67
nd
nd
nd
nd
nd
nd
nd
56
6
Cu(OTf)2
2
7
Cu(OTf)2
3
0
8
Cu(OTf)2
3
0
9
Cu(OTf)2
3
0
10
11
12a
13
Cu(MeCN)4BF4
Cu(MeCN)4BF4
Cu(MeCN)4BF4
Cu(MeCN)4BF4
3
0
3
0
5
0
5
18
a Conducted at -40 °C. b nd: Not determined.
TABLE 3. Cu(MeCN)4BF4-5a Catalyzed Asymmetric
Alkylation of 9 Giving 1025
catalyst, giving 2 with 65% ee in 79% yield by the use of
Cu(MeCN)4BF419 (Table 2, entry 10). Use of 4 Å molecular
sieves20 was beneficial to the reaction, improving the
enantioselectivity to 68% ee (entry 11).21 Finally, im-
provement in enantioselectivity was observed by con-
ducting the reaction with 5 mol % Cu(MeCN)4BF4-5a as
a catalyst in the presence of 4 Å MS at -40 °C for 4 h to
afford (-)-2 with 80% ee in 77% yield (entry 12). It is
important to note that a major factor in controlling 1,4-/
1,2-selectivity seems to be the structure of phosphane 5a,
not the source of copper, because Cu(MeCN)4BF4-4c
converted 1c to a mixture of 2 with 5% ee in 65% yield
and 3c with 56% ee in 18% yield (entry 13).
The reaction of a variety of R,â-unsaturated aldimines
9 has been examined with Cu(MeCN)4BF4-5a as a
catalyst in the presence of 4 Å MS at -30 °C in toluene
(Table 3). N-Sulfonylimines 9 were prepared in good
yields by condensing the corresponding aldehydes 722
with sulfonylamide 823 in tetraethoxysilane24 at 160 °C
time
(h)
yield
ee
entry imine
Ar
R
alcohols (%) (%)b
1a
2
3
4
5
6
7
8
9
1d
9e
9f
9g
9h
9i
9j
9k
9l
9m
9k
Ph
4
4
4
4
5
4
4
5
5
6
4
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
i-Pr
2
80
84
80
75
78
79
78
82
85
72
77
80
83
81
75
79
82
78
91
75
67c
78
2-MeC6H4
3-MeC6H4
2-MeOC6H4
4-MeOC6H4
2-ClC6H4
10e
10f
10g
10h
10i
10j
10k
10l
10m
10n
4-ClC6H4
1-naphthyl
2-naphthyl
2-furyl
10
11
1-naphthyl
a Reaction was conducted at -40 °C. Data taken from Table 2,
entry 12. b Unless otherwise noted, the ee was determined by a
chiral stationary phase HPLC. c Ee was determined by the 19F
NMR of the corresponding Mosher ester.
(19) Liang, Y.; Gao, S.; Wan, H.; Hu, Y.; Chen, H.; Zheng, Z.; Hu,
X. Tetrahedron: Asymmetry 2003, 14, 3211-3217.
(20) Molecular sieves 4 Å were crashed and heated three times for
1 min by microwave before use. Ohgushi, T.; Nagae, M. J. Porous
Mater. 2003, 10, 139-143.
(21) Reaction using 5a-Cu(OTf)2 and 4 Å MS afforded 2 with 62%
ee in 76% yield.
(22) Battistuzzi, G.; Cacchi, S.; Fabrizi, G. Org. Lett. 2003, 5, 777-
780.
(Scheme 2). The reactions of 9e and 9f, having 2- and
3-methylphenyl groups, gave 10e with 83% ee in 84%
yield and 10f with 80% ee in 81% yield (entries 2 and 3).
The reaction of 9g and 9h, having electron-donating 2-
J. Org. Chem, Vol. 70, No. 1, 2005 299

Soeta et al.
temperature. After stirring at room temperature for 10 min,
the reaction mixture was cooled to -30 °C over 0.5 h and then
stirred at -30 °C for 0.5 h. A hexane solution of diethylzinc
(1.2 mL, 1.2 mmol) was added at -30 °C, and the whole was
stirred at -30 °C for 2 h. Then, a hexane solution of diethylzinc
(1.2 mL, 1.2 mmol)26 was added and the whole was stirred at
-30 °C for an additional 3 h. The reaction was quenched with
saturated NH4Cl and stirred at room temperature for 0.5 h.
After filtration through Celite, the organic layer was separated
and the water layer was extracted with ethyl acetate. The
combined organic layers were washed with saturated NaHCO3
and brine and then dried over Na2SO4. Concentration gave a
pale yellow oil (513 mg), which was filtered through aluminum
oxide 90 standardized (Merck, 20 g) using ethyl acetate (300
mL) as an eluate. Concentration gave a pale yellow oil, which
was treated with NaBH4 (37 mg, 1.0 mmol) in methanol (4.0
mL) for 0.5 h at 0 °C. After addition of water and ethyl acetate,
the organic layer was separated. The aqueous layer was
extracted with ethyl acetate. The combined organic layers were
washed with saturated NaHCO3 and brine and then dried over
Na2SO4. Concentration and purification by silica gel column
chromatography (hexane/ethyl acetate ) 20/1) gave 10k (175
SCHEME 2. Synthesis of 9 from 7 and 8
and 4-methoxyphenyl groups, gave 10g with 75% ee in
75% yield and 10h with 79% ee in 78% yield (entries 4
and 5). The reaction of 9i and 9j, having electron-
withdrawing 2- and 4-chlorophenyl groups, gave 10i with
82% ee in 79% yield and 10j with 78% ee in 78% yield
(entries 6 and 7). The reaction of 3-(naphthalen-1-yl)-
acrylaldehyde imine 9k gave 10k with 91% ee in 82%
yield (entry 8). The reaction of 9l gave 10l with 75% ee
in 85% yield (entry 9). The reaction of 9m, having a
heterocyclic 2-furyl group, gave 10m with 67% ee in 72%
yield (entry 10). A propyl group was also introduced into
9k with dipropylzinc to give 10n with 78% ee in 87% yield
(entry 11).
mg, 82% yield) as a pale yellow oil of [R]25 -4.01 (c 1.27,
D
In conclusion, a catalytic asymmetric conjugate addi-
tion of dialkylzinc to sterically tuned, unsaturated sul-
fonylaldimines was achieved with high 1,4-selectivity and
enantioselectivity using the N-Boc-L-valine-connected
amidophosphane-copper(I) catalyst. The combination of
an appropriate amidophosphane and copper(I) is key to
the success. Further studies will be focused on the
examination of various natural amino acid-connected
amidophosphanes.
CHCl3). The ee was determined to be 91% by HPLC (Daicel
Chiralcel AD-H, hexane/i-PrOH ) 50/1, 1.0 mL/min, 254 nm,
major 19.9 min and minor 21.9 min). 1H NMR δ: 0.82 (3H, t,
J ) 7.3 Hz), 1.14 (1H, brs), 1.85 (2H, m), 2.01-2.13 (2H, m),
3.45-3.63 (3H, m), 7.41-8.23 (7H, m). 13C NMR δ: 11.8, 29.4,
38.7, 60.9, 123.2, 125.3, 125.5, 125.7, 126.4, 128.9, 132.6, 133.9,
141.3. IR (neat): 3350 cm-1. EIMS m/z: 214 (M+). HRMS (EI)
Calcd for C15H18O: 214.1357. Found: 214.1352.
Acknowledgment. This research was supported by
the 21st Century Center of Excellence Program “Knowl-
edge Information Infrastructure for Genome Science”
and a Grant-in-Aid from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. M.K.
is thankful for the receipt of a JSPS fellowship.
Experimental Section
Asymmetric Ethylation of 9k with Diethylzinc Cata-
lyzed by 5a-Copper(I) Giving 10k (Table 3, Entry 8).
Under an Ar atmosphere, a solution of amidophosphane 5a
(30.5 mg, 0.065 mmol) in 4 mL of toluene was added to a
suspension of Cu(MeCN)4BF4 (15.1 mg, 0.050 mmol) and 4 Å
MS (750 mg) in 14.5 mL of toluene. The resulting solution was
stirred for 1 h at room temperature. A solution of aldimine
10k (447 mg, 1.0 mmol) in 13 mL toluene was added at room
Supporting Information Available: Experimental pro-
cedure, characterization data, NMR spectra, and HPLC traces.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JO0484225
(23) (a) Pal, M.; Madan, M.; Padakanti, S.; Pattabiraman, V. R.;
Kalleda, S.; Vanguri, A.; Mullangi, R.; Mamidi, N. V. S. R.; Casturi, S.
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494.
(25) Absolute configuration of 10 was tentatively assigned on the
basis of the absolute configuration determined for 2 (10 Ar ) Ph).
(26) Addition of 2.0 equiv of Et2Zn in one portion gave 2 with 56%
ee, slightly lower ee compared with 57% ee (Table 2, entry 6).
300 J. Org. Chem., Vol. 70, No. 1, 2005