Development of asymmetric reactions catalyzed by chiral organotin-alkoxide reagents

Article abstract of DOI:10.1002/tcr.201200019

Asymmetric catalysis under almost-neutral reaction conditions is key for the efficient synthesis of optically active polar molecules. We have developed catalytic enantioselective reactions of acyclic or cyclic alkenyl esters by using an (S)-BINOL-derived

Full text of DOI:10.1002/tcr.201200019

Development of Asymmetric
Reactions Catalyzed by Chiral
Organotin-Alkoxide Reagents
T H E
C H E M I C A L
R E C O R D
AKIRA YANAGISAWA* AND KAZUHIRO YOSHIDA
Department of Chemistry, Graduate School of Science, Chiba University, Inage, Chiba
263-8522 (Japan)
E-mail: ayanagi@faculty.chiba-u.jp
Received: September 11, 2012
ABSTRACT: Asymmetric catalysis under almost-neutral reaction conditions is key for the effi-
cient synthesis of optically active polar molecules. We have developed catalytic enantioselective
reactions of acyclic or cyclic alkenyl esters by using an (S)-BINOL-derived chiral tin-dibromide
reagent that possesses a bulky aryl group at the 3 or 3′ position as the chiral pre-catalyst in the
presence of a sodium alkoxide and an alcohol, in which a chiral tin alkoxide bromide is generated
in situ and recycled with the assistance of an alcohol. In this Personal Account, we describe three
types of asymmetric transformation that proceed through a chiral tin enolate: 1) The asymmetric
aldol reaction of alkenyl esters or unsaturated lactones with aldehydes or isatins; 2) the asymmetric
three-component Mannich-type reaction of alkenyl esters and related cycloaddition reactions;
and 3) the asymmetric N-nitroso aldol reaction of unsaturated lactones with nitrosoarenes. DOI
10.1002/tcr.201200019
Keywords: aldol reaction, alkoxides, asymmetric catalysis, ligand effects, tin
1. Introduction
The development of highly enantioselective methods for the
synthesis of optically active natural products and biologically
active compounds is one of the most important research topics
in modern synthetic chemistry. The asymmetric aldol reaction
and the asymmetric Mannich-type reaction, which employ
catalytic amounts of a chiral Lewis acid, are useful routes to
non-racemic b-hydroxy-carbonyl compounds[1] and non-
racemic b-amino-carbonyl compounds,[2] respectively, and they
have attracted a great deal of attention from organic chemists.
Numerous chiral Lewis acid catalysts have been designed and
prepared in an effort to develop aldol and Mannich processes
that have superior stereoselectivity and chemical yield.
However, in the case of a strong Lewis acid catalyst, there is an
intrinsic problem that the catalytic activity is decreased owing
to the coordination of the catalyst with the aldol product or the
Mannich product, which is more polar than the starting mate-
rials (i.e., product inhibition).[2a,3] Furthermore, Lewis acid
catalysts are generally moisture sensitive and, thus, cannot be
used in a solvent that contains water or alcohol. We have found
that dibutyltin dimethoxide [Bu2Sn(OMe)2] catalyzes the
aldol reaction between alkenyl trichloroacetates and aldehydes,
including aliphatic aldehydes, in the presence of MeOH
(Scheme 1).[4] A probable catalytic mechanism for this reaction
is shown in Scheme 2. First, alkenyl trichloroacetate 1 reacts
with Bu2Sn(OMe)2 to afford tin enolate 2 and methyl trichlo-
roacetate. Subsequently, aldehyde 3 is added to tin enolate 2 to
provide the tin alkoxide of aldol adduct 4. The catalytic cycle is
completed by the alcoholysis of tin alkoxide 4 with MeOH to
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T H E C H E M I C A L R E C O R D
7a (10 mol%)
MeONa (x mol%)
O
OH
*
cat. Bu2Sn(OMe)2
MeOH
OCOCCl3
R3
O
OH
OCOCCl3
+
t-BuCHO
*
R4CHO
Ph
t-Bu
+
X
Y
R1
R1
R4
Ph
10 (E/Z = 1/4, 2 eq)
MeOH (10 eq)
THF, rt, 7 h
Sn
THF
R2 R3
R2
11
x (tin catalyst)
yield, % syn/anti ee, % (syn)
7a (X = Y = Br)
8a (X = Y = OMe)
9a (X = OMe; Y = Br)
Scheme 1. Aldol reaction of alkenyl trichloroacetates with aldehydes cata-
lyzed by Bu2Sn(OMe)2.
20 (8a)
10 (9a)
<1
10
78/22
24
Scheme 3. Evaluation of BINOL-derived tin-methoxide reagents as chiral
catalysts.
R4CHO
O
OSnBu2(OMe)
R4
OSnBu2(OMe)
R3
3
R1
R1
alkenyl esters or unsaturated lactones by using the chiral tin
compound as the chiral pre-catalyst in the presence of a sodium
alkoxide and an alcohol.
R2 R3
2
R2
4
MeOCOCCl3
MeOH
2. Synthesis of Chiral Organotin(IV) Dibromides
OCOCCl3
O
OH
R4
R3
Bu2Sn(OMe)2
R1
R1
As the chiral tin catalyst, we chose a binaphthol-derived orga-
notin(IV) compound because a number of chiral catalysts that
contain the binaphthyl motif have been found to be effica-
cious in diverse asymmetric transformations.[5] First, we
attempted to synthesize chiral tin dimethoxide 8a from (S)-
BINOL (6). According to the literature,[6] chiral tin dibro-
mide 7a can be prepared from compound 6 in 20% overall
yield; however, we failed to obtain targeted tin dimethoxide
8a by treating tin dibromide 7a with two equivalents of
MeONa in MeOH, owing to the instability of compound 8a
(Scheme 3).[7]
Consequently, we attempted to generate compound 8a in
situ from a 1:2 mixture of compound 7a and MeONa and to
apply it as a chiral catalyst to the aldol reaction of alkenyl
trichloroacetate 10 with pivalaldehyde. However, no target
aldol adduct (11) was obtained at all (Scheme 3). In contrast,
chiral tin bromide methoxide 9a, which was prepared from a
1:1 mixture of compound 7a and MeONa in MeOH, dis-
played definite reactivity and the reaction actually furnished
R2 R3
R2
1
5
Scheme 2. Proposed catalytic cycle for the aldol reaction of alkenyl trichlo-
roacetates with aldehydes catalyzed by Bu2Sn(OMe)2.
yield the regenerated Bu2Sn(OMe)2 and the desired aldol
product (5). The slow protonation of tin enolate 2 with
MeOH, compared to the rate of reaction of tin enolate 2
with aldehyde 3, is key to the success of the catalytic cycle.
We envisaged that, if a chiral organotin dimethoxide,
R*2Sn(OMe)2, were applied as a chiral catalyst to this catalytic
system, an asymmetric version of the catalytic aldol reaction
would be possible in which an optically active aldol product
would be obtained through a chiral tin enolate. In this Personal
Account, we describe the synthesis of binaphthol-based chiral
organotin(IV) dibromides and the asymmetric reactions of
Kazuhiro Yoshida received his PhD
from Kyoto University in 2003 under
the supervision of Professor Tamio
Hayashi. He then became a postdoc-
toral fellow at the Scripps Research
Institute under the guidance of Profes-
sor Kim. D. Janda (2003–2004). In
2004, he became an Assistant Professor
at Chiba University, where he was
promoted to Associate Professor in 2009. His main research
interests lie in the field of transition-metal-catalyzed
reactions.
Akira Yanagisawa received his PhD
from Nagoya University. He became an
Assistant Professor at Nagoya Univer-
sity and, later, an Associate Professor
there. In 2001, he moved to take up the
position of Professor at Chiba Univer-
sity. His research interests focus on the
development of new synthetic methods
by using allylic organometallic reagents,
such as allylic barium reagents, and asymmetric synthesis,
including BINAP-silver(I)-catalyzed asymmetric carbon-
carbon bond-forming reactions and the asymmetric protona-
tion of metal enolates.
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A s y m m e t r i c R e a c t i o n s C a t a l y z e d b y C h i r a l O r g a n o t i n - A l k o x i d e R e a g e n t s
OMe
OTf
Ar
14b: Ar = Ph
14c: Ar = 4-t-BuC6H4
14d: Ar = 3,5-(t-Bu)2C6H3
14e: Ar = 3,5-(t-BuC6H4)2C6H3
14f: Ar = 1-naphthyl
14g: Ar = 4-MeOC6H4
14h: Ar = 4-CF3C6H4
14i: Ar = 2,4-(CF3)2C6H3
OMOM
OMOM
Me
Me
OH
OH
Me
Me
a-c
Ar
d-h
i
OMe
OTf
Ar
(S)-BINOL (6)
12
13
14
Ar
7b: Ar = Ph
15b: Ar = Ph
15c: Ar = 4-t-BuC6H4
15d: Ar = 3,5-(t-Bu)2C6H3
15e: Ar = 3,5-(t-BuC6H4)2C6H3
15f: Ar = 1-naphthyl
15g: Ar = 4-MeOC6H4
15h: Ar = 4-CF3C6H4
7c: Ar = 4-t-BuC6H4
7d: Ar = 3,5-(t-Bu)2C6H3
7e: Ar = 3,5-(t-BuC6H4)2C6H3
7f: Ar = 1-naphthyl
7g: Ar = 4-MeOC6H4
7h: Ar = 4-CF3C6H4
7i: Ar = 2,4-(CF3)2C6H3
Br
Br
Br
j
k
Sn
Br
Ar
Ar
15i: Ar = 2,4-(CF3)2C6H3
15
7
(a) i) NaH, THF, 0 oC ~ rt, ii) MOMCl, THF, 0 oC ~ rt; (b) i) BuLi, THF, rt, ii) B(OMe)3, THF, -78 oC ~ rt, iii) H2O2 aq., CHCl3,
reflux; (c) MeI, K2CO3, acetone, reflux; (d) conc. HCl aq., dioxane, 50 oC; (e) Tf2O, Et3N, CHCl3, 0 oC ~ rt; (f) MeMgBr,
Ni(dppp)Cl2, Et2O, reflux; (g) BBr3, CHCl3, 0 oC; (h) Tf2O, Et3N, CHCl3, 0 oC ~ rt; (i) ArB(OH)2, Pd(OAc)2, PPh3, K3PO4,
THF/H2O, reflux; (j) NBS, BPO, CCl4, reflux; (k) Sn, H2O, toluene, reflux.
Scheme 4. Synthesis of chiral tin dibromides 7b-7i. MOM=methoxymethyl, NBS=N-bromosuccinimide,
BPO=benzoyl peroxide.
compound 11 in 10% yield, even though the enantiomeric
excess of the major syn isomer was low (Scheme 3).
aldol reaction of alkenyl trichloroacetate 10 with pivalalde-
hyde. The anticipated aldol adduct (11) was produced in 70%
yield with a syn/anti ratio of 94:6 through a reaction that
employed compound 7c as the pre-catalyst in toluene at 40 °C
for 1.5 h (Table 1, entry 1); the syn isomer was formed in 85%
ee. Then, we performed the chiral-tin-catalyzed enantioselec-
tive aldol reaction of diverse combinations of acyclic alkenyl
trichloroacetates and aldehydes under the optimized reaction
conditions; the results are shown in Table 1. The introduction
of a phenyl group at the para position of alkenyl trichloroac-
etate 10 allowed the reaction to take place smoothly at low
temperatures and, indeed, the non-racemic aldol products were
furnished syn-selectively in approximately 70% yield and
94–98% ee in the reactions with different aliphatic aldehydes,
even at room temperature (Table 1, entries 3–5). The reaction
of 2-methyl-2-phenylpropanal at 0 °C provided an almost
enantiomerically pure product (Table 1, entry 6). From these
aforementioned results, BINOL-derived chiral tin bromide
methoxides that contained bulky substituents at the 3,3′ posi-
tions were found to be effective in affording high enantioselec-
tivities in this asymmetric aldol reaction.
Because this second set of reaction conditions that we
tested were found to be promising, we tried to synthesize
various chiral tin dibromides that contained aryl groups at the
3,3′ positions, which were expected to induce high asymmetric
induction in this aldol reaction.[8] The synthesis of compounds
7b-7i from (S)-BINOL (6) is shown in Scheme 4. MOM
protection of compound 6, followed by a hydroxylation/
methylation sequence, gave 3,3′-dimethoxy BINOL derivative
12. The next five-step transformation of compound 12 by 1)
deprotection of the MOM groups, 2) trifluoromethanesulfo-
nylation of the 2,2′-OH groups, 3) Ni-catalyzed cross-coupling
with a methyl-Grignard reagent, 4) demethylation, and 5)
trifluoromethanesulfonylation of the 3,3′-OH groups provided
ditriflate 13. Then, ditriflate 13 underwent Suzuki-Miyaura
coupling with arylboronic acids to afford the corresponding
coupling products (14b-14i). Benzylic bromination[9] and sub-
sequent oxidative addition of the resulting products (15b-15i)
by tin metal completed the synthesis of the target chiral tin
dibromides (7b-7i).
Isatin derivatives could be also employed as electrophiles
in the chiral-tin-methoxide-catalyzed asymmetric aldol reac-
tion. Isatin and its derivatives are beneficial synthetic interme-
diates of various biologically active molecules.[10] To accomplish
their asymmetric transformation, numerous chiral organocata-
lysts[11] have been developed, in contrast to the few examples
of reactions that utilize chiral Lewis acid catalysts.[12] We
3. Catalytic Asymmetric Aldol Reaction of
Alkenyl Esters and Unsaturated Lactones
With chiral tin dibromides 7 in hand, we studied the catalytic
activity of their corresponding tin bromide methoxides in the
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T H E C H E M I C A L R E C O R D
Table 1. Catalytic asymmetric aldol reaction of alkenyl trichloroacetates with aldehydes by using chiral tin dibromide 7c and sodium
methoxide as pre-catalysts.
OCOCCl3
O
OH
*
7c (10 mol%)
NaOMe (10 mol%)
*
+
R
RCHO
MeOH (10 eq)
toluene
X
X
(2 eq)
Entry
X
R
Conditions
Yield[a] [%]
syn/anti[b]
ee [%][c] (syn)
1
2
3
4
5
6
7
8
H (10, E/Z = 1:4)
H (10, E/Z = 1:4)
Ph (E/Z = 1:99)
Ph (E/Z = 1:99)
Ph (E/Z = 1:99)
Ph (E/Z = 1:99)
MeO (E/Z = 9:91)
Br (E/Z = 1:4)
t-Bu
Me2PhC
i-Pr
40 °C, 1.5 h
40 °C, 4 h
r.t., 18 h
r.t., 16 h
r.t., 14 h
0 °C, 30 h
40 °C, 4 h
r.t., 14 h
70 (11)
80 (11)
80
65
75
41
63
54
94:6
95:5
85
93
94
97
98
99
88
95
>99:1
>99:1
85:15
89:11
96:4
t-Bu
Me2PhC
Me2PhC
Me2PhC
t-Bu
99:1
[a]Yield of the isolated product. [b]Determined by 1H NMR spectroscopy. [c]Value corresponds to the syn isomer; determined by HPLC.
OMe
7c (5 mol%)
NaOMe (5 mol%)
MeOH (30 eq)
O
O
*
OCOCCl3
Br
HO
+
O
Br
toluene, 0 oC, 1.5 h
*
N
O
MeO
Bn
N
17
16 (2 eq)
Bn
18, >99% yield
dr = 82 (91% ee):18
Scheme 5. Catalytic asymmetric aldol reaction of alkenyl trichloroacetate 16 with isatin 17.
attempted to react 6-methoxy-1-tetralone-derived alkenyl
trichloroacetate 16 with N-benzyl isatin derivative 17 by using
chiral tin dibromide 7c and sodium methoxide as pre-catalysts
and, as a result, the anticipated aldol product (18) was afforded
with noticeable asymmetric induction. For example, the treat-
ment of a mixture of compounds 16 (2 equiv) and 17 (1 equiv)
with chiral tin dibromide 7c (5 mol%) and NaOMe (5 mol%)
in the presence of MeOH (30 equiv) in toluene at 0 °C for
1.5 h resulted in the formation of an 82:18 diastereomeric
mixture of compound 18 in quantitative yield (Scheme 5);[13]
the major diastereomer of compound 18 was formed in 91%
ee. The corresponding alkenyl trifluoroacetate[14] could be also
used as a substrate for the generation of a chiral tin enolate.
In addition to ketone-derived alkenyl esters, unsaturated
lactones could be also applied as enolate precursors to the
chiral-tin-bromide-methoxide-catalyzed asymmetric aldol
reaction. With these former substrates, there was the drawback
that MeOH was essential as an additive for recycling the cor-
responding chiral tin enolates, in addition to the fact that
undesired methyl trichloroacetate was produced. In contrast,
the aldol reaction of unsaturated lactones could proceed in the
absence of MeOH because the subsequent lactonization of
the generated tin alkoxide of b-hydroxy ketones that contained
an ester group regenerated the chiral tin bromide methoxide.
A plausible catalytic mechanism for the chiral-tin-catalyzed
tandem asymmetric aldol-reaction/cyclization by using g-
substituted b,g-didehydro-g-butyrolactone 19 as the enolate
precursor is shown in Scheme 6. First, chiral tin dibromide 7
reacted with an equimolar amount of sodium methoxide to
give the corresponding chiral tin bromide methoxide. The
thus-generated chiral tin bromide methoxide was subsequently
added to b,g-didehydro-g-butyrolactone 19 to form chiral tin
enolate 20. The following aldol reaction of an aldehyde with
chiral tin enolate 20 afforded the tin alkoxide of b-hydroxy
ketone 21. Lastly, tin alkoxide 21 underwent lactonization
through participation of its ester portion to give non-racemic
b,g-disubstituted g-butyrolactone 22 with regeneration of the
chiral tin bromide methoxide. The methoxycarbonyl group
of intermediates 20 and 21 played a key role in the catalytic
cycle.
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A s y m m e t r i c R e a c t i o n s C a t a l y z e d b y C h i r a l O r g a n o t i n - A l k o x i d e R e a g e n t s
To confirm of the validity of this catalytic mechanism,
various g-substituted b,g-didehydro-g-butyrolactones (19)
were transformed into their corresponding optically active
b,g-disubstituted g-butyrolactones (22).[15] For instance,
when a 1:1 mixture of 5-(4-methoxyphenyl)furan-2(3H)-one
(19a) and pivalaldehyde was treated with chiral tin dibromide
7c (10 mol%) and NaOMe (10 mol%) in toluene at
room temperature for 19 h, trans-5-(tert-butyl)-4-(4-
methoxybenzoyl)dihydrofuran-2(3H)-one (22a) was obtained
exclusively in 76% yield and >99% ee (Scheme 7). Non-
racemic g-butyrolactones 22b and 22c were also prepared from
their corresponding b,g-didehydro-g-butyrolactones (19) and
aldehydes. Enantiomerically enriched g-lactones are useful
chiral synthons for the synthesis of a variety of biologically
active natural products or synthetic drugs.[16] Numerous
methods are available to obtain such versatile synthetic inter-
mediates;[17] however, as far as we know, there are no reports of
catalytic pathways that involve this aldol approach.
4. Catalytic Asymmetric Mannich-Type
Reaction of Alkenyl Esters and Related
Cycloaddition Reaction
The asymmetric Mannich-type reaction is a convenient route
to optically active b-amino-carbonyl compounds, which can be
further converted into useful chiral organic molecules, such as
b-lactams.[2] A three-component procedure that employs an
enolate, an amine, and an aldehyde is much more preferable
than a two-component one that uses an enolate and an imine
because the former method does not require the prior prepa-
ration of imines and can be applied to less-stable aliphatic
imines.[18] We found that dibutyltin dimethoxide behaves
as a catalyst in the three-component Mannich-type reac-
tion between 1-trichloroacetoxycyclohexene, aldehydes, and
anilines, thereby yielding racemic b-amino ketones.[19] Notably,
the tin dimethoxide exhibits remarkable catalytic activity, even
in the presence of water or an alcohol. Thus, we explored the
possibility of developing an asymmetric version of this three-
component Mannich-type reaction by using an in situ gener-
ated chiral tin alkoxide that possesses a binaphthyl structure as
the chiral catalyst and we found that the desired non-racemic
b-amino ketones were produced in high yields with significant
enantioselectivities.[20]
O
OSnR*2Br
O
OSnR*2Br
R2
H
R2
R1
CO2Me
R1
20
First, we optimized the reaction conditions for chiral tin
catalyst 7, sodium alkoxide, alcohol, and additives. The com-
bination of chiral tin dibromide 7h and NaOEt/EtOH in the
presence of MS4A was found to be superior to 7c/NaOMe/
MeOH in terms of reactivity and enantioselectivity. Moreover,
the syn/anti selectivity and the enantiomeric excess of the syn
product were improved when a catalytic amount of NaI was
added. With the optimal reaction conditions in hand, we
examined the catalytic asymmetric three-component Mannich-
type reaction of 1-trichloroacetoxycyclohexene, ethyl glyox-
alate, and various anilines; selected examples are summarized in
Table 2. One point worth noting is that the presence of a
phenolic hydroxy, an amino, or an amide substituent on the
CO2Me
21
O
O
O
O
R*2SnBr(OMe)
NaBr
R1
R1
19
R2
O
NaOMe
R*2SnBr2 (7)
22
Scheme 6. Plausible catalytic cycle for the tandem asymmetric aldol-reaction/
cyclization by using unsaturated lactone 19.
O
O
7c (10 mol%)
NaOMe (10 mol%)
O
C
O
+
O
H
R2
R1
toluene, r.t.
19 R1
R1
4-MeOC6H4 (19a) t-Bu
4-FC6H4 (19b) Me2CH
R2
O
22
R2
Time, h Product Yield [%] trans/cis
19
23
22a
22b
22c
76
76
87
>99 (99% ee)/1
93 (94% ee)/7
>99 (94% ee)/1
2-naphthyl (19c) Me2PhC 22
Scheme 7. Tandem catalytic asymmetric aldol-reaction/cyclization with g-substituted b,g-didehydro-g-
butyrolactones (19 ) and aldehydes.
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T H E C H E M I C A L R E C O R D
Table 2. Catalytic asymmetric three-component Mannich-type reaction of 1-trichloroacetoxycyclohexene, ethyl glyoxalate, and various anilines.
chiral tin 7h (5 mol%)
R
NaOEt (5 mol%)
NaI (10 mol%)
EtOH (10 eq), MS4A
OCOCCl3
+
R
O
O
HN
*
+
THF, 60 oC
H
CO2Et
*
H2N
CO2Et
(2 eq)
Entry
R
Time, h
Yield[a] [%]
syn/anti[b]
ee [%][c] (syn)
1
2
3
4
5
OMe
OH
NMe2
NHPh
NHCOCH3
0.5
1
1.5
2
>99
>99
98
>99
>99
74/26
77/23
72/28
76/24
75/25
83
84
93
81
67
1.5
6
1
86
67/33
86
N
O
[a]Yield of the isolated product. [b]Determined by 1H NMR spectroscopy. [c]Value corresponds to the syn isomer; determined by HPLC.
O
aniline derivative did not retard the reaction. In fact, each
+
ArNH2
28
H2O
reaction was completed within 2 h (Table 2, entries 2–5). In
the case of an aniline derivative that contained a Me2N group,
the optical purity of the syn product reached 93% ee (Table 2,
entry 3). These results clearly show that in situ generated chiral
tin ethoxide iodide can better tolerate polar substituents than
representative Lewis acids.[21]
H
CO2Et
Ar
H
27
N
Ar
*
CO2Et
O
NSnR*2I
CO2Et
OSnR*2I
29
R1
R1
*
R2
30
R2
26
Various alkenyl trichloroacetates can be employed in this
asymmetric Mannich-type reaction; not only cyclic ketone
derivatives but also acyclic ketone derivatives have been effi-
ciently transformed into their corresponding enantiomerically
enriched b-amino ketones in satisfactory yields with notable
enantioselectivities of up to 98% ee. In this three-component
system, the aldol reaction does not intervene and no products
are formed at all. In addition, no b-elimination of the Mannich
product occurs under the optimized reaction conditions.
The probable catalytic cycle is presented in Scheme 8.
Chiral tin ethoxide iodide 24, which is generated from chiral
tin dibromide 7 through chiral tin diiodide 23, reacts with
alkenyl trichloroacetate 25 to produce chiral tin enolate 26.
Then, enolate 26 undergoes an addition reaction with imine
29, which is formed by reacting ethyl glyoxalate (27) with
aromatic amine 28, thus providing the chiral tin amide of
Mannich product 30. The subsequent protonation of tin
amide 30 with EtOH completes the catalytic cycle by furnish-
ing non-racemic b-amino ketone 31 and regenerating chiral tin
ethoxide iodide 24.
EtOCOCCl3
EtOH
Ar
OCOCCl3
O
HN
*
R*2SnI(OEt) (24)
*
R1
R1
CO2Et
R2
R2
25
NaI
31
NaOEt
R*2SnI2 (23)
NaBr (2 eq)
NaI (2 eq)
R*2SnBr2 (7)
Scheme 8. Probable catalytic cycle for the asymmetric Mannich-type reaction
catalyzed by chiral tin ethoxide iodide.
dition reactions between alkenyl trichloroacetates 32 and
nitrones 33.[22] Initially, we envisaged that a nitrone would
undergo the aforementioned asymmetric Mannich-type reac-
tion to afford non-racemic b-hydroxyamino ketones; however,
instead, the electrophile reacted with an alkenyl trichloroac-
This chiral-tin-alkoxide-catalyzed asymmetric transforma-
tion was extended to enantio- and diastereoselective cycload-
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A s y m m e t r i c R e a c t i o n s C a t a l y z e d b y C h i r a l O r g a n o t i n - A l k o x i d e R e a g e n t s
7h (5 mol%), NaOEt (5 mol%)
NaI (10 mol%), EtOH (10 eq)
MS4A
OH
OCOCCl3
R2
R1
R2
–O
Ph
H
O
N+
R1
+
N
Ph
THF, r.t.
R3
R3
33
32 (2 eq)
34
Alkenyl ester
OCOCCl3
R3
Time, h Product Yield [%] dr
4-CF3C6H4 (33a)
2
34a
49
71 (74% ee)/29 (73% ee)
32a
OCOCCl3
c-C6H11 (33b)
2
34b
>99
>99 (95% ee)/1
32b
OCOCCl3
c-C6H11 (33b)
20
34c
43
O
>99 (84% ee)/1
32c
Ph
OH
O
HN
H2, Pd/C
MeOH, r.t., 2 h
R
N
Ph
S
4-CF3C6H4
4-CF3C6H4
35 (>99% yield, 74% ee)
34a
dr = 71 (74% ee)/29 (73% ee)
Scheme 9. Catalytic asymmetric cycloaddition reaction and subsequent reductive N-O cleavage of cycloadduct 34a.
etate to produce an unanticipated cycloadduct. For example,
tion. In the [3+2] mechanism, nitrone 36 was assumed to be
activated by a Lewis acidic chiral tin compound in transition-
state structure 37, thus generating products 39a and 39b
through cyclized intermediate 38. In the case of aromatic-
aldehyde-derived nitrones (R3 = Ar), an equilibrium between
compounds 39a and 39b was considered to occur more easily.
This is a new example of a catalytic enantioselective 1,3-dipolar
cycloaddition reaction of nitrones by using electron-rich
alkenes, such as dipolarophiles.[24]
when a mixture of cyclohexanone-derived alkenyl trichloroac-
etate 32a (2 equiv) and nitrone 33a (1 equiv) was treated with
chiral tin dibromide 7h (5 mol%), NaOEt (5 mol%), and NaI
(10 mol%) in the presence of EtOH (10 equiv) in THF at
room temperature for 2 h, a 71:29 diastereomeric mixture of
isoxazolidine 34a was obtained in 49% yield (Scheme 9); the
major diastereomer was formed in 74% ee. Aliphatic-aldehyde-
derived nitrone 33b was a more suitable 1,3-dipole for realizing
higher yields, diastereomeric ratios, and enantiomeric excess.
Not only cyclic alkenyl esters 32a and 32b but also acyclic
analogue 32c could be used as the precursor of chiral tin
enolate. Cycloadduct 34a further underwent reductive N-O
cleavage by Pd/C under a hydrogen atmosphere to give anti
b-amino ketone 35 as a single diastereomer in 74% ee. This
result unequivocally confirmed that the diastereomers of
compound 34a originated from a stereogenic center at the
hemiketal portion. The absolute configuration of b-amino
ketone 35 was determined by HPLC to be 2R,3S.[23]
5. Catalytic Asymmetric N-nitroso Aldol Reaction
of Cyclic Alkenyl Esters
The asymmetric nitroso aldol reaction is a useful C-O/C-N
bond-forming reactions that affords optically active a-
aminooxy-carbonyl compounds and/or a-hydroxyamino-
carbonyl compounds.[25] A central issue in this transformation
is how to control the O/N regioselectivity. The enantioselective
O-nitroso aldol reaction (aminoxylation) can be readily carried
out by employing simple organocatalysts, including proline;
however, it is still not easy to obtain the opposite
a-hydroxyamino-carbonyl compounds with high regio- and
enantioselectivities. We previously found that dibutyltin
dimethoxide catalyzed the N-nitroso aldol reaction between
Based on the observed syn selectivity in the chiral-tin-
alkoxide-catalyzed asymmetric Mannich-type reaction, irre-
spective of the E/Z ratio in the alkenyl trichloroacetate,[20] and
the fact that opposite anti-Mannich product 35 was formed
from cycloadduct 34 (Scheme 9), a 1,3-dipolar cycloaddition
mechanism (Scheme 10) was proposed for the cyclization reac-
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www.tcr.wiley-vch.de 123

T H E C H E M I C A L R E C O R D
O
R1
R2
SnR*2I
O–
OSnR*2I
Ar
H
O–
R3
N+
Ar
+
N+
R1
R3
R2
H
36
26
37
OH
OH
OSnR*2I
O
R1
R2
R1
R2
R1
R2
O
O
EtOH
N
Ar
N Ar
N
Ar
R*2SnI(OEt)
R3
39α
R3
39β
R3
38
24
Scheme 10. A plausible reaction pathway for the asymmetric cycloaddition reaction.
alkenyl trichloroacetates and nitrosobenzene, in which a tin
enolate was generated in situ and the tin dimethoxide was
recycled with the assistance of MeOH.[26] We envisaged that, if
a chiral tin enolate were produced from a suitable chiral tin
alkoxide and the enolate could activate a nitrosoarene, the
asymmetric version of the N-nitroso aldol reaction would
occur. To this end, we chose g,d-unsaturated d-lactones as the
enolate precursors for the asymmetric N-nitroso aldol reaction
because related unsaturated lactones were found to be superior
substrates in the chiral-tin-alkoxide-catalyzed tandem asym-
metric aldol-reaction/cyclization.[15] We attempted to react
d-substituted g,d-didehydro-d-valerolactones with nitrosoare-
nes by using chiral tin dibromide 7 and a sodium alkoxide
as pre-catalysts and, consequently, the anticipated N-nitroso
aldol adducts were afforded with remarkable asymmetric
induction.[27] For example, when a mixture of 3,4-dihydro-
6-phenylpyran-2-one (40a, 2 equiv) and 1-isopropyl-2-
nitrosobenzene (41a, 1 equiv) was treated with chiral tin
dibromide 7c (10 mol%) and NaOEt (10 mol%) in toluene at
0 °C for 12 h, the desired product (42aa) was produced in
82% yield (Table 3, entry 1); the N-adduct was formed in 90%
ee and the corresponding O adduct was not observed at all.
The results of reactions that employed various com-
binations of d-substituted g,d-didehydro-d-valerolactone
and nitrosoarene are summarized in Table 3. Not only
electron-rich-aromatic-group-substituted lactones but also
electron-deficient-aromatic-group-substituted lactones showed
significant reactivity as masked enolates and the enantioselec-
tivities surpassed 90% ee in all cases (Table 3, entries 2–5).
In addition, g,d-disubstituted-g,d-didehydro-d-valerolactone
(40f) was a favorable substrate for the a-hydroxyamination
reaction (Table 3, entry 6). However, the use of tert-butyl–
substituted nitrosobenzene 41b as the electrophile was more
effective in improving the extent of the asymmetric induction,
although the electrophilicity decreased owing to the steric
bulkiness of the tert-butyl group (Table 3, entries 7–9). From
g,d-disubstituted substrate 40f, target product 42fb, which had
almost-perfect enantiomeric excess (99% ee), was obtained
(Table 3, entry 9). If compound 40f was chosen as the enolate
precursor, even relatively small nitrosoarene 41c showed
remarkable reactivity and enantioselectivity (Table 3, entry
10). The absolute configuration of a-hydroxyamino ketone
42eb was determined to be S by single-crystal X-ray analysis.[27]
The suggested catalytic cycle appears in Scheme 11. Ini-
tially, chiral tin dibromide 7 undergoes an exchange reaction
with an equimolar amount of NaOEt to afford the correspond-
ing chiral tin bromide ethoxide. Then, the as-formed chiral tin
species reacts with g,d-didehydro-d-valerolactone 43 to provide
chiral tin enolate 44. The following N-nitroso aldol reaction of
chiral tin enolate 44 with nitrosoarene 45 gives the tin alkoxide
of a-hydroxyamino ketone 46. In the end, ethanolysis
of tin alkoxide 46 produces enantiomerically enriched
a-hydroxyamino ketone 47 with regeneration of the chiral
tin-alkoxide catalyst. The rapid protonation of tin alkoxide 46
with EtOH is key to the catalytic mechanism.
6. Conclusion
Herein, we have described several new examples of asymmetric
transformations under almost-neutral conditions by using an
(S)-BINOL-derived chiral organotin(IV) dibromide that con-
tains bulky aryl groups at the 3,3′ positions as the chiral pre-
catalyst, which is then converted in situ into the corresponding
chiral tin alkoxide by treatment with sodium alkoxide.[28]
Alkenyl esters and unsaturated lactones are efficiently trans-
formed into chiral tin enolates in the presence of the chiral tin
alkoxide and the chiral tin catalyst is regenerated from the
reaction products under the influence of an alcohol. This
catalytic system is more environmentally friendly because the
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A s y m m e t r i c R e a c t i o n s C a t a l y z e d b y C h i r a l O r g a n o t i n - A l k o x i d e R e a g e n t s
Table 3. Catalytic asymmetric N-nitroso aldol reaction of g,d-unsaturated d-lactones.
7c (10 mol%)
NaOEt (10 mol%)
EtOH (30 eq)
O
OH
N
R
O
O
N
R
*
Ar
+
O
toluene, 0 oC
Ar
CO2Et
40a-40f (2 eq)
41a-41c
42aa-42fc
Entry
Ar
R
t [h]
Product
Yield [%][a]
ee [%][b]
1
2
3
4
5
Ph (40a)
4-MeC6H4 (40b)
4-MeOC6H4 (40c)
2-FC6H4 (40d)
4-BrC6H4 (40e)
O
i-C3H7 (41a)
i-C3H7 (41a)
i-C3H7 (41a)
i-C3H7 (41a)
i-C3H7 (41a)
12
12
12
18
12
42aa
42ba
42ca
42da
42ea
82
94
97
75
92
90
95
92
>99
95
O
6
i-C3H7 (41a)
16
42fa
>99
90
40f
7
8
Ph (40a)
4-BrC6H4 (40e)
t-C4H9 (41b)
t-C4H9 (41b)
12
12
42ab
42eb
37
73
99
96
O
O
9
10
t-C4H9 (41b)
Me (41c)
17
15
42fb
42fc
52
75
>99
92
40f
[a]Yield of the isolated product; [b]determined by 1H NMR spectroscopy.
O
amount of toxic organotin compounds is decreased to a cata-
lytic amount. These chiral tin catalysts have been utilized in
aldol reactions, Mannich-type reactions, cycloaddition reac-
tions, and N-nitroso aldol reactions, to name a few. Optically
active products are obtained in high yields with high ee values,
even from electrophiles that contain a polar substituent, such as
an amino group. These aforementioned reactions unambigu-
ously indicate that chiral tin alkoxides have much potential as
asymmetric Lewis acid catalysts and, in the near future, the
emergence of unprecedented reactions that are catalyzed by
these organometallic reagents is greatly anticipated.
N
45
Ar2
OSnR*2Br
O
OSnR*2Br
CO2Et
N
*
Ar1
Ar1
Ar2
46
44
CO2Et
EtOH
O
O
O
OH
N
R*2SnBr(OEt)
*
Ar1
Ar2
Ar1
47
NaBr
43
NaOEt
R*2SnBr2 (7)
CO2Et
Acknowledgements
We thank all of our co-authors, in particular Dr. A. Izumiseki
for his distinguished contribution. This work was supported by
a Grant-in-Aid for Scientific Research on Priority Areas
“Advanced Molecular Transformations of Carbon Resources”
from the Ministry of Education, Culture, Sports, Science and
Technology, Japan. We also gratefully acknowledge the finan-
Scheme 11. Plausible catalytic cycle for the asymmetric N-nitroso aldol
reaction.
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www.tcr.wiley-vch.de 125

T H E C H E M I C A L R E C O R D
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reactions described in this account do not proceed under the
standard reaction conditions because BINOL-derived chiral
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to activate alkenyl esters and/or electrophiles.
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