A catalytic asymmetric borono variant of Hosomi-Sakurai reactions with N,O-aminals

Article abstract of DOI:10.1002/anie.201105182

Aminal attraction: The combination of indium(I) chloride and a chiral silver binol phosphate provides an excellent catalyst for asymmetric Hosomi-Sakurai reactions between N,O-aminals and boronates (see scheme; PG=protecting group, pin=pinacolato). The su

Full text of DOI:10.1002/anie.201105182

DOI: 10.1002/anie.201105182
Hosomi–Sakurai Reaction
A Catalytic Asymmetric Borono Variant of Hosomi–Sakurai Reactions
with N,O-Aminals**
Yi-Yong Huang, Ananya Chakrabarti, Naohide Morita, Uwe Schneider, and Shu¯ Kobayashi*
Catalytic intermolecular cross-couplings between Csp3 centers,
such as O,O-acetals and N,O-aminals, and allyl species are
challenging, but provide convenient access to various impor-
tant substance classes such as homoallyl ethers and amines.[1]
For these transformations, Hosomi–Sakurai reactions using
allyl silicon-based reagents are generally employed.[2,3] These
carbon–carbon bond formations proceed through either
Lewis acid or Brønsted acid activation of the electrophile to
generate a stabilized carbenium ion intermediate that can
react with a silicon-based nucleophile. However, catalytic
asymmetric Hosomi–Sakurai allylations of Csp3 centers have
Scheme 1. Asymmetric borono variant of Hosomi–Sakurai reactions?
B(pin)=pinacolatoboron, PG=protecting group.
proved to be challenging.[2f,j,4]
Allyl boronates are typically employed for additions to
Csp2 centers,[5] although a few Csp3 Csp3 cross-couplings have
À
been reported.[6,7] These nontoxic reagents are intrinsically
less nucleophilic than silicon-based compounds and have
been neglected in the context of allylation of Csp3 centers.
However, allyl boronates may offer significant advantages
such as superior stability and unique reactivity and selectivity.
During a project initially aimed at the catalytic activation of
allyl boronates for selective C–C coupling with more complex
electrophiles, we observed a peculiar reactivity with Csp3
intermediates such as N,O-aminals. These electrophiles are
abundant in natural products[8a] and play an important role in
organic synthesis.[2f,8b–e] Thus, the development of catalytic
asymmetric carbon–carbon bond formations with N,O-ami-
nals is worthwhile.[1] Intrigued by the unexpected reactivity,
we started more detailed investigations with a view towards
asymmetric catalysis. We report herein an approach to
address the challenge of catalytic asymmetric Hosomi–
Sakurai reactions involving Csp3 centers by employing boro-
nates instead of silicon-based reagents.
Hosomi–Sakurai allylations with silicon-based reagents 4a
and 4b barely proceeded,[9] which stands in sharp contrast to
our earlier study.[6a] The substantially higher reactivity of 2
over 4 under mildly Lewis acidic conditions constitutes a
prerequisite for asymmetric catalysis. We postulated a dual
catalytic activation[6a] of rac-1a and 2 to generate iminium ion
and allyl indium(I) intermediates (Scheme 1), thus we
screened potential indium(I) catalysts bearing chiral counter-
anions[9] rather than chiral ligands.[10] In these experiments the
combination of indium(I) chloride and chiral silver binol
phosphate (R)-5a-Ag[11] was found to be the most promising
chiral catalyst system for the formation of product (R)-3a
(e.r. = 88:12).[9] Also, allyl silane 4a proved to be substantially
less effective than allyl boronate 2 in terms of both reactivity
and selectivity.[9] Thus, these results demonstrate the viability
of our originally envisaged asymmetric concept, in which we
proposed the use of boronates instead of silanes. At this stage,
we explain the success of 2 based on its higher propensity to
undergo transmetalation, thereby forming a more reactive
allyl indium species.
Next, we further optimized the reaction conditions
(Table 1). A screen of silver binol phosphates identified (R)-
5b-Ag as the best chiral source (Table 1, entries 1–5). The use
of an apolar cosolvent (cyclopentylmethyl ether) and a slight
excess of the chiral silver salt further improved the asym-
metric induction even at a lower catalyst loading (Table 1,
entries 6–8). We then conducted several control experiments.
In the absence of indium(I) chloride, (R)-5b-Ag displayed
low reactivity and low asymmetric induction (Table 1,
entry 10). The use of chiral Brønsted acid (R)-5b-H, which
may be generated in situ under the present catalysis con-
ditions, did not lead to any reaction (Table 1, entry 11).[4g,8c]
The combination of indium(I) chloride and (R)-5b-H pro-
vided low asymmetric induction (Table 1, entry 12), thereby
demonstrating that an achiral metal salt and a chiral Brønsted
In an initial screen of various Lewis and Brønsted acids for
the reaction of N,O-aminal rac-1a with allyl boronate 2
indium(I) triflate[6a] was identified as the best catalyst for the
formation of homoallyl amide rac-3a (Scheme 1; R = phenyl,
PG = benzoyl, R’ = methyl).[9] In contrast, the corresponding
[*] Dr. Y.-Y. Huang, Dr. A. Chakrabarti, N. Morita, Dr. U. Schneider,
Prof. Dr. S. Kobayashi
Department of Chemistry
School of Science and Graduate School of Pharmaceutical Sciences
The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp
[**] This work was partially supported by a Grant-in-Aid for Scientific
Research from the Japan Society for the Promotion of Science
(JSPS), ERATO (JST), NEDO, and GCOE. We thank Mr. Takeshi
Yamakawa for the preparation of several precursors of chiral silver
binol phosphates (R)-5a–e.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201105182.
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Communications
Table 1: Optimization study and control experiments .
Table 2: Mechanistic control experiment.
Entry
InICl
[mol%]
5 [mol%]
Cosolvent Yield[a]
[%]
e.r.[b]
1[c]
10
10
10
10
10
10
10
5
5
–
–
5
(R)-5a-Ag (10)
(R)-5b-Ag (10)
(R)-5c-Ag (10)
(R)-5d-Ag (10)
(R)-5e-Ag (10)
(R)-5b-Ag (10)
(R)-5b-Ag (13)
(R)-5b-Ag (6.5)
–
–
–
–
–
81
96
90
91
94
96
98
96
1
88:12
2[c]
94.5:5.5
44.5:55.5
49.5:50.5
49.5:50.5
95.5:4.5
98.5:1.5
97.5:2.5
–
t [min]
3a
1a
3[c]
Yield [%][a]
e.r.[b]
Yield [%][a]
e.r.[b]
4[c]
15
60
1
50:50
50:50
50:50
50:50
50:50
50:50
50:50
95
82
67
57
43
16
5
97.5:2.5
91:9
80:20
68:32
56.5:43.5
50:50
5[c]
–
15
28
39
52
80
93
6[c]
CPME
CPME
CPME
CPME
CPME
CPME
CPME
120
180
300
480
640
7[c]
8[c,d]
9[d]
10[d]
11[d]
12[c,d]
(R)-5b-Ag (6.5)
(R)-5b-H (6.5)
(R)-5b-H (6.5)
5
57:43
–
62.5:37.5
50:50
NR[e]
88
[a] Yields of isolated rac-3a and 1a after purification by preparative TLC
on silica gel. [b] Enantiomeric ratios were determined by HPLC on a
chiral stationary phase. Bz=benzoyl.
[a] Yields of isolated (R)-3a after purification by preparative TLC on silica
gel. [b] Enantiomeric ratios were determined by HPLC on a chiral
staionary phase. [c] The chiral catalyst was preformed in toluene at RT.
1
[d] Reaction time: 18 h. [e] NR=no reaction (detected by H NMR
spectroscopy). CPME=Cyclopentyl methyl ether.
À
provide proof that the catalytic asymmetric C C bond
formation (see, Table 1) proceeds by the postulated SN1
mechanism with an iminium ion species as a key intermediate,
thus confirming the critical role of the chiral counteranion
acid are ineffective.[12,13] Importantly, we confirmed that redox
disproportionation of indium(I), which would generate
indium(0) and indium(III) in situ,[14] did not occur in the
present catalysis.[15] Thus, the combination of indium(I)
chloride and (R)-5b-Ag was shown to be crucial for the
highly enantioselective formation of homoallyl amide (R)-3a
under mild reaction conditions. The results of our control
experiments (Table 1, entries 9–12)[13,15] suggest the in situ
generation of a chiral low-oxidation-state indium species as
the active catalyst.
Next, we carried out a control experiment to investigate
the reaction mechanism (Table 2). We employed the optically
enriched aminal (R)-1a (e.r. => 99.9:0.1) and allyl boronate 2
under standard reaction conditions using indium(I) chloride
(10 mol%) combined with racemic silver phosphate rac-5 f-
Ag (13 mol%) as the catalyst system. This experiment was
carefully analyzed over time by determining yields and
enantiomeric ratios for both the generated product 3a and
the recovered substrate 1a (15–640 min). Although the
starting aminal (R)-1a was optically pure, the product 3a
proved to be racemic at all stages of the reaction. At the same
time, the racemization of (R)-1a proceeded relatively slowly
under mildly Lewis acidic conditions. For example, we
isolated product 3a in 15% yield as a racemate (e.r. =
50:50) after a reaction time of 60 min, while substrate (R)-
1a was recovered in 82% yield with high optical purity (e.r. =
91:9). These results strongly indicate an iminium ion inter-
mediate for this reaction (SN1 pathway). In turn, these data
(see, Scheme 1).[10,16,17] Overall, the present C C bond-form-
À
ing method relies on the generation of a chirally modified
electrophile (acyclic transition state), and represents there-
fore an orthogonal approach compared with our related
earlier study, in which we proposed a chirally modified
nucleophile as a key intermediate (cyclic transition state).[10]
We then examined the scope of this catalytic asymmetric
transformation (Table 3). Under the optimized reaction
conditions the reactions between substituted aromatic or
heteroaromatic aminals rac-1a–k and allyl boronate 2 pro-
ceeded smoothly to provide the desired products (R)-3a–k
with high asymmetric induction (Table 3, entries 1–12). In
addition, even the challenging aliphatic aminals rac-1l and
rac-1m proved to be good substrates for this new catalytic
asymmetric method. Product (R)-3l was formed with excel-
lent asymmetric induction (Table 3, entry 13), which demon-
strates a dramatic improvement compared with our earlier
related study.[10] Product (S)-3m was obtained in high yield
albeit with lower asymmetric induction (Table 3, entry 14).
Overall, we consider these results remarkable as the levels of
asymmetric induction exceed[4c,e,g] or equal[18b,c] even those of
the corresponding catalytic asymmetric allylations of unac-
tivated aldimines (Csp2 centers) with boronate 4 or silicon-
based reagents 2.[18]
In addition, we were pleased to find that the novel chiral
catalyst system was applicable to asymmetric allenylation
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Table 3: Scope for the catalytic asymmetric N,O-aminal allylation.
Entry[a] Product (R)-3
Yield e.r.[c]
[%][b]
1
2
(R)-3a
96
97.5:2.5
96[d]
98.5:1.5[d]
3
4
5
(R)-3b: X=OMe 98
97.5:2.5
98:2
97:3
(R)-3c: X=Me
(R)-3d: X=F
98
96
6
7
(R)-3e: X=Me
(R)-3 f: X=CF3
99
94
98:2
97:3
8
9
(R)-3g: X=Me
(R)-3h: X=F
96
98
96.5:3.5
95:5
10
11
12
(R)-3i
(R)-3j
(R)-3k
99
98
99
97.5:2.5
95:5
Scheme 2. Catalytic asymmetric allenylation and subsequent cycliza-
tion. Cbz=benzyloxycarbonyl, Cy=Cyclohexyl.
This report features several notable characteristics: 1)
Under mildly Lewis acidic conditions, boronates proved to be
dramatically more reactive and selective than classic silicon-
based reagents. 2) The described transformations represent
the first highly enantioselective Hosomi–Sakurai reactions
with Csp3 centers.[2f,j] 3) This study also constitutes the first
main-group-metal-catalyzed activation of allyl boronates for
97.5:2.5
13
14
(R)-3l
96
88
98:2
asymmetric C C bond formation with Csp3 centers.[6] 4) Chiral
À
Brønsted acid catalysis either with[12] or without[4g,8c] achiral
metal salts proved to be inefficient. 5) In the context of
(S)-3m
86:14
À
asymmetric intermolecular C C bond formation, the chemis-
try presented herein is a rare example not only of chiral-
counteranion-directed metal catalysis,[10,12] but also of
dynamic kinetic resolution.[22] Current investigations include
elucidation of the catalyst structure and application to other
reactions.
[a] Reaction conditions: rac-1a–m (1 equiv), InICl (5 mol%), (R)-5b-Ag
(6.5 mol%), 2 (1.2 equiv), toluene/CPME (0.2m), 238C, 18 h. [b] Yields
of isolated (R)-3a–l and (S)-3m after purification by preparative TLC on
silica gel. [c] Enantiomeric ratios were determined by HPLC on a chiral
stationary phase. [d] Modified reaction conditions: InICl (10 mol%), (R)-
5b-Ag (13 mol%), 12 h.
Received: July 23, 2011
Published online: October 4, 2011
Keywords: asymmetric catalysis · boron · chiral counteranion ·
Hosomi–Sakurai reaction · indium
(Scheme 2). The reactions of aromatic and aliphatic aminals
rac-1a’ and rac-1l’ with allenyl boronate 6 afforded the
homoallenyl carbamates (R)-8a and (R)-8l in 71% and 75%
yields, respectively, with high asymmetric induction (e.r. =
93:7 and 94:6, respectively). The homopropargyl carbamates
7a and 7l were obtained as the minor regioisomers, which
were separated from (R)-8a and (R)-8l by chromatography.
The observed regioselectivity is unprecedented for the use of
allenyl boronate 6 in asymmetric catalysis. Thus, our work is
clearly distinct from related studies.[6a,19] The utility of highly
functionalized compounds of type 8 was demonstrated by a
catalytic intramolecular hydroamination with (R)-8a[20] to
generate azaheterocycle (R)-9[21] (Scheme 2). This 5-endo-trig
cyclization occurred smoothly without loss of optical purity
(e.r. = 93:7), and constitutes a straightforward method to
access optically enriched 2-substituted 2,5-dihydropyrroles.
.
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Communications
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À
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[16] Indeed, our attempts employing indium(I) catalysts bearing
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À
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À
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[9] For details, see the Supporting Information.
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