Indium(I)-catalyzed asymmetric allylation, crotylation, and αchloroallyalation of hydrazones with rare constitutional and high configurational selectivities

Article abstract of DOI:10.1002/anie.200906308

The hydra-zone: The first example of asymmetric In catalysis had been developed. In I combined with a chiral semicorrin ligand (L*) is an effective catalyst for enantioselective allylation, crotylation and α-chloroallylation of hydrazones. In the two latter cases, C-C bond formations proceeded with high selectivity where both reactive aliphatic C-Cl and aromatic O-H bonds were tolerated. Chemical equation representation

Full text of DOI:10.1002/anie.200906308

Communications
DOI: 10.1002/anie.200906308
Asymmetric Catalysis
Indium(I)-Catalyzed Asymmetric Allylation, Crotylation, and a-
Chloroallylation of Hydrazones with Rare Constitutional and High
Configurational Selectivities**
Ananya Chakrabarti, Hideyuki Konishi, Miyuki Yamaguchi, Uwe Schneider, and
Shu¯ Kobayashi*
The main group metal indium is appealing for use in catalysis,
as indium-based compounds have low toxicity, and are
inexpensive, selective, and tolerant toward functional
groups.[1,2] Although InIII Lewis acids are commonly used in
catalysis,[3] InI compounds[4] can display amphiphilic proper-
ties depending on the ligands to which they are coordinated,
owing to two vacant p orbitals (acid) and a lone pair of
electrons (base).[5] Despite this intriguing character, synthesis
exploiting InI is rare; more than stoichiometric amounts of InI
reagents are generally required in synthetic use.[1,6] In earlier
reports,[7] our research group uncovered indium(I)-catalyzed
which display rare regioselectivity and high configurational
selectivities.
Initial investigations involved ligand screening[15] for the
reaction employing acylhydrazone 1a and allyl boronate 2 in
the presence of a catalytic amount of indium(I) iodide[7a–c]
(Table 1). Chiral bis(oxazoline) ligands 3a and 3b gave the
desired product 4a with promising asymmetric induction;
with a catalyst loading of 10 mol% in dichloromethane, 3b
proved to be more efficient than 3a (Table 1, entries 1 and 2).
Although the enantiomeric ratio (e.r.) dropped in toluene,
high asymmetric induction was restored in toluene/MeOH
À
racemic C C bond formations with allyl boronates. In the
course of our studies, NMR spectroscopic analyses revealed
the generation of reactive allyl InI species through catalytic B-
to-In transmetalation.[7b] Meanwhile, the importance of
suitable InI ligation for both structural or physical properties[8]
and chemical reactivity[8j,k] has been demonstrated. To the
best of our knowledge, however, chiral InI complexes and
Table 1: Optimization of reaction conditions for asymmetric InI catalysis.
À
their catalytic use for asymmetric C C bond formation are
unknown.
Entry InI
L* 3
Reaction
Conditions
Yield e.r.
[%][a]
Catalytic asymmetric allylation of non-activated imines
and their derivatives is challenging,[9] but can provide access
to optically enriched homoallyl amine derivatives, which have
proved to be useful chiral building blocks for pharmaceutical
and agrochemical applications. Most powerful catalytic
asymmetric methods have been developed using Sn,[10] Si,[11]
B,[12] and in situ prepared indium reagents.[13] Unfortunately,
very few catalytic methods were found to be efficient,
[mol%] (mol%)
1
2
3
4
10
10
10
10
3a (10)
3b (10)
3b (10)
3b (10)
CH2Cl2, RT
CH2Cl2, RT
toluene, RT
toluene/MeOH (16:1),
40
65
36
89
81:19[b]
86:14
62:38
96:4
RT
5
5
5
5
5
5
–
3b (5)
3c (5)
3d (5)
3d (5)
3d (5)
3d (5)
3d (5)
toluene/MeOH (16:1),
08C
toluene/MeOH (16:1),
08C
toluene/MeOH (16:1),
08C
toluene/EtOH (16:1),
08C
toluene/iPrOH (16:1),
08C
toluene/MeOH (16:1),
08C
toluene/MeOH (16:1),
08C
quant 98:2
6[c]
7[c]
8
90
99
91
97:3[b]
98:2
practical, or general. Herein we report the catalytic use of
I
À
an unprecedented chiral In complex for asymmetric C C
bond formations between hydrazones[14] and allyl boronates,
98:2
9[d]
10[c]
11[c]
quant 95:5
trace n.d.
[*] Dr. A. Chakrabarti, H. Konishi, Dr. M. Yamaguchi, Dr. U. Schneider,
Prof. Dr. S. Kobayashi
Department of Chemistry, School of Science
The University of Tokyo, The HFRE Division
ERATO (Japan) Science and Technology Agency (JST)
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp
5[e]
55
89:11
[a] Yield of isolated 4a after purification on silica gel (PTLC). [b] Use of L*
3a or 3c: (S)-4a was obtained as the major enantiomer. [c] Reaction time
was 12 h. [d] Reaction time was 48 h. [e] Use of InI3 instead of InI. n.d.=
not determined, pin=pinacol.
[**] This work was supported by a Grant-in-Aid for Scientific Research
from the Japan Society for the Promotion of Science (JSPS) and the
Global COE Program (Chemistry Innovation through Cooperation
of Science and Engineering), the University of Tokyo, MEXT, Japan.
H.K. gratefully acknowledges the JSPS for a Research Fellowship for
Young Scientists.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200906308.
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Chemie
(16:1; Table 1, entries 3 and 4). Further experiments revealed
that the use of chiral semicorrin ligands[16] 3c and 3d showed a
significant rate enhancement compared with 3b, while main-
taining an excellent level of asymmetric induction (e.r. = 98:2;
Table 1, entries 5–7). The use of other alcohols did not lead to
further improvement (Table 1, entries 8 and 9). Notably, the
allylation essentially did not proceed in the absence of InI,
and InI3 proved to be substantially less effective (Table 1,
cyclohexyl) proved to be less efficient in terms of asymmetric
induction.
We then examined indium(I)-catalyzed asymmetric cro-
tylation (Table 2). Initial trials with hydrazone 1a and a-
methylallyl boronate rac-5 under our reported conditions,[7c]
Table 2: Asymmetric indium(I)-catalyzed crotylation of 1 with rac-5.
entries 10 and 11).[17]
I
À
We consider this asymmetric In catalysis for C C bond
formation remarkable for a number of reasons: 1) Contrary
to reported reactions of InI halides with Lewis bases
(ligands),[4,5,8] we did not observe disproportionation of
InI.[18] 2) To the best of our knowledge, an enantiomeric
ratio of 98:2 for product 4a is to date the best result for metal-
catalyzed asymmetric allylation of aromatic imine deriva-
tives.[19] 3) These findings reveal the critical role of chiral
ligands (L*) 3a–d in stabilizing the labile InI[4,5,8] and in
creating excellent environments for asymmetric induction.
Entry
1
a/g
a-Adduct 6: R
Yield anti-6/syn-6 e.r.
[%][c]
(anti-6)
1
1a >99:1 6a: Ph
1b >99:1 6b: 4-HOC6H4
85
98
19:1
11:1
19:1
7:1
8:1
11:1
97:3
94:6
96.5:3.5
94.5:5.5
96:4
96.5:3.5
94:6
92:8
2[a]
3
1c >99:1 6c: 4-MeOC6H4 86
1d >99:1 6d: 3-MeOC6H4 90
1h >99:1 6h: 4-MeC6H4
1k >99:1 6k: 3-furyl
1l >99:1 6l: 2-thienyl
1p >99:1 6p: 4-ClC6H4
Next, we investigated the scope of hydrazones
1
4
(Scheme 1). Various aromatic substrates bearing functional-
ities such as free hydroxy, methoxy, tertiary amino, and nitro
groups were allylated in high yields with enantiomeric ratios
of up to 98:2. Furthermore, O-, S-, and N-containing hetero-
cycles proved to be excellent substrates with enantiomeric
ratios of up to 97.5:2.5. Aliphatic hydrazones such as 1o (R =
5[b]
6[b]
7
quant
98
quant 15:1
83 17:1
8
[a] Use of L* 3c: the opposite enantiomer was obtained as the major
enantiomer. [b] Reaction temperature: À208C. [c] Yield of isolated a-
adduct 6 after purification on silica gel (PTLC).
in the presence of various chiral ligands, provided the desired
product 6a with disappointing regio-, diastereo-, and enan-
tioselectivites.[15] After extensive experimentation,[15] high a/g
and anti/syn ratios as well as high asymmetric induction could
be obtained with InI and L* 3d in EtOH (a/g = > 99:1, anti/
syn = 19:1, e.r. = 97:3; Table 2, entry 1). These optimized
reaction conditions were applicable to various hydrazones 1;
all aromatic and heteroaromatic substrates tested were
converted exclusively into a-adducts 6 in high yields with
anti/syn ratios of up to 19:1 and enantiomeric ratios of up to
96.5:3.5 (Table 2, entries 2–8). The present work constitutes
the first systematic crotylation study with a broad variety of
imine derivatives; several characteristic features of this
asymmetric InI catalysis are noteworthy: 1) The unusual
a selectivity[20] observed with rac-5 contrasts its exclusive
g selectivity in the absence of a catalyst[21a] and under Lewis or
Brønsted acid catalysis;[21b,c] this rare a selectivity suggests B-
to-In transmetalation prior to C C bond formation.[22] 2) The
À
use of 5 in racemic form provided enantiomerically enriched
anti-6[15] as the major product; in this context, it is noted that
the preparation of a-substituted allyl boronates of type 5 in
enantiomerically enriched form is not trivial.[23] 3) This
diastereoselective reaction avoids the use of geometrically
enriched or pure crotyl reagents, typically required to
selectively form products of type 6. 4) Our catalytic InI
method provides substantially higher regioselectivity and
configurational selectivities for product 6a compared with a
recently reported stoichiometric In0 Barbier procedure.[24]
Next, we employed a-chloroallyl boronate rac-7 (Table 3),
which is a rarely used nucleophile that adds in the absence of a
Scheme 1. Scope for asymmetric indium(I)-catalyzed allylation of 1
with 2. [a] Yield of (R)-4a–o isolated after purification on silica gel
(PTLC). [b] Reaction temperature was À208C.
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Communications
Table 3: Asymmetric indium(I)-catalyzed a-chloroallylation of 1 with rac-
7.
analysis of allyl boronate 2 in [D8]toluene/MeOH (16:1) at
room temperature revealed that 2 is stable in the presence of
the preformed chiral InI–L* 3d complex (50 mol%); no B-to-
In transmetalation was observed even after an extended
reaction time. However, upon addition of hydrazone 1a
À
(0.67 equiv) asymmetric C C bond formation occurred
smoothly. This observation suggests that, at least in the first
cycle, 1a may act as a Lewis base to activate 2 (B-to-In
transmetalation; Scheme 2). The chiral allyl InI-ate complex
À
formed in situ may then undergo C C bond formation with
Entry
1
a/g
a-Adduct 8: R Yield anti-8/syn-8 e.r.
[%][a]
(anti-8)
1a via a cyclic transition state to provide (R)-4a.[15,17,27]
1
2
3
1a
1j
1l
49:1 8a: Ph
99:1 8j: 2-naphthyl 71
>99:1 8l: 2-thienyl 89
84
99:1
33:1
99:1
92:8
90.5:9.5
93:7
[a] Yield of isolated a-adduct 8 after purification on silica gel (PTLC).
catalyst with g selectivity to electrophiles.[25] Remarkably, the
present asymmetric InI catalysis proved to be compatible with
À
Scheme 2. Assumed catalytic B-to-In transmetalation.
the reactive allylic C Cl bond of rac-7, and almost exclusive
a selectivity was observed;[22] a-adducts 8 were formed in
good yields with excellent anti/syn ratios[15] (up to 99:1) and
enantiomeric ratios of up to 93:7. These reactions represent
rare examples of the catalytic formation of stereogenic
centers bearing a chlorine atom with high asymmetric
induction, which may lead to new disconnection approaches
in the total synthesis of chlorinated natural products.[26] In
addition, further synthetic transformations of the unsaturated
b-chloroamine unit of 8 may be envisaged with respect to both
To the best of our knowledge, the reactions described in
this report represent the first example of asymmetric InI
catalysis, which are of fundamental importance. We have
uncovered enantioselective allylation, crotylation, and a-
chloroallylation of hydrazones with boronates by employing
an in situ generated chiral indium(I)–semicorrin catalyst.
Rare regioselectivity and high configurational selectivities
were observed with a-substituted allyl boronates; the present
InI protocol is therefore clearly distinct from previous
catalytic use of allyl boronates.[12,28] Various compounds
=
the chloro functional group and the C C double bond.
Our attention then turned to the mechanism of this
catalytic asymmetric C C bond formation. InI and the chiral
semicorrin ligand L* 3d (ratio = 1:1) were stirred in toluene/
MeOH (16:1) at room temperature for 1 h; through MALDI-
TOF MS analysis of this mixture we observed in situ
generation of a metal–ligand complex in a ratio of 1:1
(Figure 1). Preliminary NMR spectroscopic analysis of this
complex was hampered by its low solubility. Unexpectedly,
and contrary to our earlier study,[7b] NMR spectroscopic
À
À
À
bearing reactive aliphatic C Cl and aromatic O H bonds
were shown to be compatible. At this stage, we assume that
the in situ generation of reactive chiral allyl InI-ate species
takes place through B-to-In transmetalation. Further mech-
anistic studies and the extension of this concept to new
reactions involving boronates are currently underway.
Received: November 9, 2009
Revised: January 9, 2010
Published online: February 4, 2010
À
Keywords: asymmetric catalysis · boron · C C coupling ·
indium · semicorrin
.
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Chemie
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