An efficient approach to chiral allyloxyamines by stereospecific allylation of nitrosoarenes with chiral allylboronates

Article abstract of DOI:10.1002/anie.201410188

A novel and efficient approach to allyloxyamines by the allylation of nitrosoarenes with α-chiral allylboronates is described. C-O bond formation occurs with high stereospecificity and the product allyloxyamines are easily transformed into valuable chiral

Full text of DOI:10.1002/anie.201410188

Angewandte
Chemie
DOI: 10.1002/anie.201410188
Nitrosoarenes
An Efficient Approach to Chiral Allyloxyamines by Stereospecific
Allylation of Nitrosoarenes with Chiral Allylboronates**
Yuanming Li, Shyamal Chakrabarty, and Armido Studer*
Abstract: A novel and efficient approach to allyloxyamines by
the allylation of nitrosoarenes with a-chiral allylboronates is
ꢀ
described. C O bond formation occurs with high stereospeci-
ficity and the product allyloxyamines are easily transformed
into valuable chiral building blocks such as isoxazolidines and
allylic alcohols. The reaction features complete regioselectivity
(O-selectivity), high E/Z selectivity, and excellent chirality
transfer.
N
itrosobenzene was first prepared by Baeyer at the end of
the 19th century.[1] Since then, nitrosoarenes and other nitroso
derivatives have become highly useful building blocks in
organic synthesis.[2] Nitroso compounds are reactive sub-
strates which undergo various valuable reactions such as
[3,3] sigmatropic rearrangements,[3] ene reactions,[4] cycload-
dition reactions,[2d,5] and aldol-type reactions.[6] In recent
years, a growing number of reports have presented nitro-
sobenzene as a versatile electrophile in catalytic enantiose-
lective aldol-type reactions. Nitrosoarenes can either react at
the oxygen or at the nitrogen atom depending on the
activation mode, but commonly products derived from N-
attack are obtained.[7] Momiyama and Yamamoto first
reported O-selective nucleophilic attack of silyl enol ethers
to nitrosobenzene with Lewis acids as catalysts (Scheme
1a).[8] Along these lines, the research groups of MacMillan,[9]
Zhong,[10] Hayashi,[11] and Yamamoto[12] independently dis-
closed the enantioselective O-nitroso aldol reactions of
various aldehydes with nitrosobenzene by using proline as
a catalyst (Scheme 1b). Although much progress has been
made on stereo- and O-regioselective reactions with nitro-
soarenes as electrophiles, transformations are restricted to
nitroso aldol reactions.
Scheme 1. Stereoselective O-nitroso aldol reaction and O-nitroso allyl-
boration.
unselectively,[14a] the corresponding allylation with allylboro-
nates was reported to give allylic alcohols derived from O-
ꢀ
allylation and subsequent N O bond cleavage as major
products.[14b] In both cases stereoselective nitrosoarene ally-
lations using a-chiral allyl boron compounds were not
addressed. The following questions have to be answered:
a) Can the reaction be controlled to provide products of O-
ꢀ
allylation regioselectively while keeping the N O bond intact,
b) does the reaction occur stereospecifically, and c) can the
geometry of the newly formed double bond (E/Z) be
controlled?
The direct allylation of aldehydes and imines has evolved
into a highly efficient method for the enantioselective
[15]
ꢀ
formation of a C C bond. Of the allyl metal species, allyl
boronates have found particular attention.[16] Encouraged by
the many successful examples on the use of a-chiral allylbor-
onates in stereospecific allylations,[17] we studied the reaction
of readily prepared racemic 2a with nitrosopyridine 1a to
afford allyloxyamine 3a (Table 1).
As a continuation of our studies on the exploration of the
reactivity of nitrosoarenes towards allylic nucleophiles[13] we
investigated the stereoselective O-allylboration of nitrosoar-
enes to give allyloxyamines (Scheme 1c). These products
should be valuable building blocks for organic synthesis. To
date only two reports on this type of reaction have
appeared.[14] Whereas triallylborane reacts at N and O
The reaction was conducted by using a slight excess of 2a
(1.2 equiv) in THF at room temperature for 2 h, and 3a was
obtained in 35% yield (entry 1). Azoxy compound 4a and 2-
methyl-3-nonen-5-ol derived from the reaction of nitrosopyr-
idine 1a with 3a were formed in this and all following
transformations as side products.[14b] 4a was the major product
in the presence of NaOH (entry 2). The addition of acetic acid
or H2O led to an improvement of the yield (entries 3 and 4).
The yield was further improved upon switching to alcohols as
the solvents, with MeOH affording the best result (63%,
entries 5–7). This is likely due to in situ hydrolysis of the
allylboration product to 3a in alcoholic solvents. 3a reacts
slower than the intermediate allylborate with 1a to afford
4a.[18] The addition of 1 equiv of H2O afforded the same result
(entry 8). Other aprotic solvents such as CH2Cl2, toluene, or
CH3CN provided worse results (entries 9–11). The formation
[*] Y. Li, S. Chakrabarty, Prof. Dr. A. Studer
Organisch-Chemisches Institut
Westfꢀlische Wilhelms-Universitꢀt
Corrensstrasse 40, 48149 Mꢁnster (Germany)
E-mail: studer@uni-muenster.de
[**] This work was financially supported by the Chinese Scholarship
Council (stipend to Y.L.) and the Deutsche Forschungsgemein-
schaft.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201410188.
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Table 1: Reaction of 1a with 2a under different conditions.[a]
Table 2: Stereospecificity and variation of the nitrosoarene.[a]
Entry
Aryl-NO
Product
E/Z[b]
Yield
[%][c]
ee
Entry
Solvent
Additive
(1 equiv)
3a
4a
[%][d]
yield [%][b]
yield [%][b]
1
2
3
4
5
6
7
8
THF
THF
THF
THF
–
35
<5
45
45
58
63
51
57
48
22[c]
34
17
20
15
16[d]
16
21
15
21
24
12
6
1
2
3
4
5
6
7
3a
3b
3c
3d
3e
3 f
3g
>20:1
>20:1
>20:1
–
82
60
73
n.d.
71
74
46
93
92
93
–
NaOH
AcOH
H2O
–
–
–
H2O
–
–
–
–
–
–
nPrOH
MeOH
iPrOH
MeOH
CH2Cl2
toluene
CH3CN
MeOH
MeOH
MeOH
9
10
11
12[e]
13[f]
14
21
27
>20:1
6:1
92
93
93
75
86[g]
<5
82[h]
[a] Reaction conditions: 1a (0.2 mmol) and 2a (0.24 mmol) in the given
solvent (3.0 mL) at room temperature for 2 h. [b] Yield determined by
1H NMR spectroscopy. [c] 2-Methyl-3-nonen-5-ol formed in 18% yield.
[d] 2-Methyl-3-nonen-5-ol formed in 10% yield. [e] Reaction conducted at
08C for 6 h. [f] Reaction conducted at ꢀ108C for 16 h. [g] Yield of
isolated product, E/Z= >20:1. [h] Run with 1a (0.4 mmol) and 2a
(0.2 mmol). 2-Methyl-3-nonen-5-ol isolated in 30% yield.
10:1
8
3h
3i
>20:1
3:1
–
55
90
90
–
9
51
of 4a was largely suppressed at lower temperatures
(entries 12 and 13). The yield of isolated 3a was 86% when
the reaction was carried out at ꢀ108C for 16 h (entry 13). The
use of 2 equiv 1a with respect to 2a led to 4a being obtained
in 82% yield, with no 3a identified in the crude reaction
mixture (entry 14).
10
11
12
3j
n.d.
26
3k
3l
17:1
–
91
–
n.d.
We tested the stereospecificity of the allylboration of
1 with 2a and also studied the effect of the aryl substituent of
the nitrososoarene on the reaction outcome under the
optimized reaction conditions (Table 2). Enantioenriched 2a
was prepared according to a literature procedure.[17e] Pleas-
ingly, the reaction of (R)-2a provided (S)-3a with excellent
stereospecificity (entry 1).[19] Excellent chirality transfer was
also obtained with the 6-alkyl-substituted nitrosopyridines 1b
and 1c, although the yield was slightly decreased (3b, 3c,
entries 2 and 3). Installing a methyl group at the ortho-
position of the nitroso functionality (see 1d) fully suppressed
the allylboration, likely for steric reasons (entry 4). Moving
the N atom of the pyridine ring to the para position (see 1e)
gave 3e in good yield (71%) and excellent stereoselectivity
(entry 5). Nitrosobenzene 1 f provided 3 f in good yield,
excellent enantiospecificity, but significantly lower E/Z selec-
tivity (entry 6), thus indicating the importance of the pyridine
N atom in the nitroso component. High E/Z selectivity was
restored upon introducing an electron-withdrawing para
substituent. The p-bromo and p-nitro congeners 1g and 1h
reacted with high stereoselectivity but moderate yield
(entries 7 and 8). It seems that the E/Z selectivity is
influenced by the electronics of the nitrosoarene. Indeed, p-
methylnitrosobenzene 1i provided 3i with low E/Z selectivity
[a] Reaction conditions: 1 (0.2 mmol) and 2 (0.24 mmol) in MeOH
(3.0 mL) at ꢀ108C for 16 h. [b] Ratio determined by 1H NMR analysis of
the isolated products. [c] Yield of isolated product (E+Z). [d] ee value of
2a and products determined by HPLC or GC analysis (see the Supporting
Information). n.d.=not detected.
(entry 9). o-Methylnitrosobenzene 1j and the o-trifluoro-
methyl derivative 1l did not react with 2a, likely for steric
reasons (entries 10 and 12). With a cyano group as an ortho-
substituent, 3k was obtained in a low yield (entry 11). Based
on these studies we conclude that electron-poor nitrosoarenes
react with excellent stereoselectivity and substituents ortho to
the nitroso functionality lower the reactivity. We do not
currently understand why electron-richer nitrosoarenes react
with low E/Z selectivity. Kinetic experiments revealed that p-
nitronitrosobenzene reacts faster than nitrosobenzene, which
shows that the Lewis basicity of the N atom in the nitro-
soarene is less important for the allylboration than the
=
electrophilicity of the N O moiety (see the Supporting
Information). The best result in terms of reactivity and
selectivity was obtained with 2-nitrosopyridine.
2
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Table 3: Reaction of 1a with various a-chiral allyl boronates.[a]
Allyl
boronate
Product E/Z[b]
Yield ee
ee [%]
[%][c] [%]
1
2
91
3m
3n
1.6:1 95
90(S,E)[d]
94
4:1 95
94(S,E)
3
4
88
97
ent-3a
>20:1 76
88(R)
95(R)
3o
20:1 42
87(S,Z)
87(R,E)
5
6
87
93
3p
3q
2:1 64
9:1 14
93(R,Z)
93(S,E)
7
8
90
96
ent-3m
ent-3a
15:1 64
89(R)
94(R)
>20:1 13
[a] Reaction conditions: 1 (0.2 mmol) and 2 (0.24 mmol) in MeOH
(3.0 mL) at ꢀ108C for 16 h. [b] Ratio determined by 1H NMR analysis of
the isolated products. [c] Yield of isolated product (E+Z). [d] ee value of
the E isomer was determined after cyclization to 6m, see below.
Scheme 2. Formation of fully substituted C centers.
product 5r as the major compound (48%) along with 13% of
3r (Scheme 2). Higher yields for the N-allylation were
achieved with more-electron-rich nitroso derivatives (see 5s,
5t). However, 2-nitrosopyridine delivered only the O-allyla-
tion product 3u (65%). We do not currently understand the
reversal of the N,O regioselectivity. Along these lines, 2j
resulted in O-allylation with 2-nitrosopyridine (3v, 40%) and
mainly N-allylation with PhNO (5w, 53%). To study the
enantiospecificity we tested 2k,[20] which did not react with 2-
nitrosopyridine. However, the allylation of nitrosoarene 1g
with 2k gave the N-allylation product 5x as the major
compound (37%) with high enantiospecificity (es). An
improved yield of the N-allylation product was achieved
with nitrosobenzene (see 5y).
Based on the model to explain the stereochemistry of the
allylboration of aldehydes[15,16] and considering the stereo-
chemical outcome of the nitrosoarene allylations, we suggest
the following model to explain the observed stereochemistry
(Scheme 3). E-allylboronates, as exemplified by 2b, prefera-
bly react via a six-membered chair-type transition state TS1,
We next explored the scope by varying the a-chiral
allylboronate using 1a as the electrophile (Table 3). The size
of the a substituent R1 strongly influences the E/Z selectivity,
but the enantiospecificity is unaffected. With the methyl and
phenethyl derivatives 2b and 2c, 3m and 3n were obtained in
excellent yield and stereospecificity with E/Z selectivities of
1.6:1 and 4:1, respectively (entries 1 and 2). Enantiomeric
products can be obtained by using an enantiomeric boronate
(entry 3). A slightly lower yield was achieved with the
sterically more hindered allylboronate 2d, but the selectivity
remained high (entry 4). Boronate 2e bearing a b-methyl
substituent gave a low E/Z selectivity but excellent enantio-
specificity for both isomers (see 3p, entry 5). As expected,
E/Z selectivity increased with the larger isopropyl congener
2 f (see 3q, entry 6). The (Z)-allylic boronates 2g and 2h
afforded ent-3m and ent-3a with high E selectivity. In contrast
to the E isomer 2b, where a low E/Z selectivity was achieved
with the a-methyl-substituted boronate, the Z-allylboronate
2g reacted with high selectivity (E/Z = 15:1, entry 7). Yields
were lower for the Z-allylboronates and it is apparent that the
Z congeners deliver products of opposite absolute configu-
ration (entries 7 and 8).
in which the Lewis basic N atom of the nitroso functionality
[21]
ꢀ
activates the allylboronate through B N interaction. The
a substituent occupies a pseudoequatorial position, thereby
determining the E selectivity and absolute configuration. The
Z isomer (R,Z)-3m is likely formed via the boat transition
state TS2. Alternatively, the higher energy chair TS2’ might
We then tested g,g-disubstituted allylboronates. Surpris-
ingly, the allylation of 1g with 2i provided N-allylation
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Scheme 3. Suggested mechanism.
Scheme 4. Follow-up reactions.
20
also be considered, where the a substituent is axially oriented.
For larger a substituents such as iPr (see 2a) the Z products
are not formed. The Z-allylboronates exemplified by 2g likely
react via TS3 to the major product (R,E)-3m. As expected,
the E/Z selectivity for Z-allylboronates is far higher. The
severe threefold 1,3-diaxial interactions makes a chair TS
unlikely and we, therefore, suggest the boat TS4 leading to the
small amounts of (S,Z)-3m observed in the experiment.
To show the potential of the method we investigated
follow-up reactions. Since isoxazolidines are valuable hetero-
cycles, the electrophilic cyclization of allyloxyamines 3 was
studied. We found that cyclization of 3a with PhSeBr[22]
proceeded in CH3CN to give isoxazolidine 6a in good yield
and excellent diastereoselectivity (Scheme 4). Interestingly,
PhSeBr-induced cyclization of 3 f and 3m, which were used as
E/Z mixtures of isomers, afforded 6 f and 6m with very high
trans/trans selectivity in moderate to good yield. We assume
that only the E isomer undergoes the selenocyclization, as
reflected by the significantly lower yield obtained in the
transformation of 3m to 6m and the high diastereoselectivity.
Treatment of the allyloxyamine 3a with Hg(OAc)2
afforded the cyclized mercury acetate which was further
transformed directly.[23] Reduction with NaBH3CN in the
presence of acrylonitrile as a radical acceptor afforded 8 with
excellent trans/trans diastereoselectivity, whereas reduction
with NaBH4 gave isoxazoldine 9 (84%) as a single diastereo-
isomer, thereby revealing that 1,3-induction in the initial
Hg(OAc)2-mediated cyclization was complete.[24] We also
found that the intermediate mercury hydride reacts efficiently
with 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)[25] to
give TEMPO adduct 7 in good yield and complete trans/
alcohol (R)-10 (½aꢁD ¼ꢀ7.0 (c = 0.46, EtOH)). This known
alcohol was used for the assignment of the absolute config-
23
uration ((S)-10), 96% ee, ½aꢁD ¼ + 7.2 (c = 0.36, EtOH)[26]).
In summary, we have presented highly stereoselective O-
allylation of aryl nitroso compounds with chiral allylboro-
nates to provide the corresponding allyloxyamines. The
products are obtained with excellent stereospecificity and in
most cases very high E/Z selectivity. Our method further
expands the high value of readily available a-chiral allylboro-
nates for asymmetric synthesis. A model to explain the
stereochemical outcome of the nitrosoarene allylborations is
suggested and the products obtained are highly valuable
synthetic intermediates, as shown by various efficient follow-
up reactions.
Received: October 17, 2014
Revised: November 26, 2014
Published online: && &&, &&&&
Keywords: allylboronates · heterocycles · nitrosoarenes ·
stereoselective synthesis · synthetic methods
.
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Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Nitrosoarenes
Y. Li, S. Chakrabarty,
A. Studer*
&&& — &&&
An Efficient Approach to Chiral
Allyloxyamines by Stereospecific
Allylation of Nitrosoarenes with Chiral
Allylboronates
Stereospecific and regioselective! Allyl-
boration of nitrosoarenes with readily
available a-chiral allylboronates occur
with high stereoselectivity to provide
allyloxyamines. These allyloxyamines can
be readily transformed into various iso-
xazolidines and allylalcohols.
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!