Generation and Application of Homoenolate Equivalents Utilizing [1,2]-Phospha-Brook Rearrangement under Br?nsted Base Catalysis

Article abstract of DOI:10.1002/chem.201605673

A new method for catalytic generation of a homoenolate equivalent and its application to carbon?carbon bond formation was developed by utilizing the [1,2]-phospha-Brook rearrangement under Br?nsted base catalysis. The α-oxygenated allyl anions, which can

Full text of DOI:10.1002/chem.201605673

A Journal of
Accepted Article
Title: Generation and Application of Homoenolate Equivalents Utilizing
[1,2]-Phospha-Brook Rearrangement under Brønsted Base
Catalysis
Authors: Azusa Kondoh, Takuma Aoki, and Masahiro Terada
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Generation and Application of Homoenolate Equivalents Utilizing
[1,2]-Phospha-Brook Rearrangement under Brønsted Base
Catalysis
Azusa Kondoh,[a] Takuma Aoki,[b] and Masahiro Terada*[a,b]
Abstract: A novel method for catalytic generation of a homoenolate
equivalent and its application to carbon-carbon bond formation was
developed by utilizing the [1,2]-phospha-Brook rearrangement under
Brønsted base catalysis. The α-oxygenated allyl anions, which can
serve as homoenolate equivalents, were catalytically generated in
situ by treating readily available chalcones with diethyl phosphite or
the pre-formed 1,2-adducts of diethyl phosphite with chalcones in
the presence of a catalytic amount of a phosphazene base, P2-tBu.
The resulting allyl anions were subsequently trapped by various
electrophiles, including Michael acceptors, imines, and aldehydes,
providing the corresponding adducts in good yields with moderate to
good diastereoselectivities.
homoenolate equivalents, from readily available compounds
under mild reaction conditions would pave the way to new useful
synthetic reactions exploiting the umpolung reactivity. In this
context, we envisioned the generation of α-oxygenated allyl
anions, which can serve as homoenolate equivalents, utilizing
the [1,2]-phospha-Brook rearrangement[6,7] under Brønsted base
catalysis, and its application to carbon-carbon bond formation.
Our proposed reaction system is shown in Scheme 1.
Carbon-carbon bond forming reactions based on inversion of the
inherent polarity of functional groups, so called umpolung, are a
privileged class of reactions in organic synthesis.[1] Specifically,
the reactions of homoenolates are powerful tools for developing
new strategies toward the synthesis of complex molecules.
Homoenolates enable the introduction of an electrophile to a
carbon at the β-position of a carbonyl group where nucleophiles
are generally introduced by conjugated addition with α,β-
unsaturated carbonyl compounds.[2] Therefore, intensive studies
have focused on the development of new methodologies for the
generation of homoenolates, and the design of synthetic
equivalents of homoenolates. Recent dramatic evolution of
catalytic reactions utilizing homoenolates generated from α,β-
unsaturated aldehydes using N-heterocyclic carbene catalysts
emphasizes the importance of molecular transformations based
on such umpolung reactivity, particularly in a catalytic fashion.[3,4]
Meanwhile, allyl anions possessing a heteroatom-substituent at
the α-position are useful synthetic homoenolate equivalents.[5]
Various types of such allyl anions, including those having a
chiral auxiliary, have been developed and utilized in organic
synthesis. However, the generation of allyl anions generally
requires tedious pre-synthesis of the corresponding allyl
compounds and subsequent deprotonation at the allylic position
by using a stoichiometric amount of a strong Brønsted base,
such as nBuLi, due to the low acidity at the allylic position. Thus,
the development of a novel methodology for the catalytic
generation of α-heteroatom-substituted allyl anions, i.e.,
Scheme 1. Proposed Reaction System.
In the proposed reaction system, an easily-prepared α,β-
unsaturated ketone 1 is employed as the precursor of a
homoenolate equivalent. Treatment of 1 with diethyl phosphite
(2) in the presence of a Brønsted base catalyst would result in
the deprotonation of 2 followed by chemoselective 1,2-addition
to provide alkoxide A. Subsequently, the migration of the
diethoxyphosphoryl moiety from carbon to oxygen, i.e., the [1,2]-
phospha-Brook rearrangement, would proceed to form α-
oxygenated allyl anion B. Finally, the addition of B to an
electrophile (“E”) at the γ-position followed by protonation by the
conjugated acid of the Brønsted base catalyst or 2 would afford
the adduct 3 along with regeneration of the Brønsted base
catalyst or the anion of 2, completing the catalytic cycle.
Hydrolysis of the alkenyl phosphate moiety of 3 would provide β-
substituted ketone 4, which is the formal adduct of the
homoenolate with the electrophile. In order to establish the
proposed reaction system, some issues must be overcome. First,
for the generation of the key allyl anion B, the addition of diethyl
phosphite to the α,β-unsaturated ketone 1 must proceed in the
1,2-fashion over the 1,4-fashion. Second, the resulting allyl
anion B should react with the electrophile selectively at the γ-
position, not at the α-position, to serve as the homoenolate
equivalent. Finally, anion B should react with the electrophile
preferentially over protonation, which is a potential side reaction
arising from the high basicity of B. Once the corresponding allyl
compound is formed by the protonation of B, it cannot re-enter
the catalytic cycle via deprotonation due to its low acidity. We
successfully managed to solve these issues and report herein a
[a]
[b]
Dr. A. Kondoh, Prof. Dr. M. Terada
Research and Analytical Center for Giant Molecules
Graduate School of Science, Tohoku University
Aramaki, Aoba-ku, Sendai 980-8578 (Japan)
E-mail: mterada@m.tohoku.ac.jp
T. Aoki, Prof. Dr. M. Terada
Department of Chemistry, Graduate School of Science
Tohoku University
Aramaki, Aoba-ku, Sendai 980-8578 (Japan)
Supporting information for this article is given via a link at the end of
the document.
novel method for catalytic generation of
a homoenolate
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10.1002/chem.201605673
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equivalent and its application to carbon-carbon bond formation
under Brønsted base catalysis.
Table 1. Optimization of Reaction Conditions[a]
To ascertain the viability of the proposed reaction system, we
began our investigation by evaluating the catalytic generation of
the α-oxygenated allyl anion from chalcone (1a) with diethyl
phosphite (2). As an initial experiment, 2 equivalents of 1a were
treated with 1 equivalent of 2 in the presence of 10 mol% P2-tBu
in THF at room temperature for 6 h in order to homo-dimerize 1a,
i.e., trap the allyl anion generated in situ with another chalcone.
As a result, the dimerized product 3aa was obtained as the
major product in fairly good yield along with 6a which is the 1,4-
adduct of 2 with 1a (Table 1, entry 1). 3aa would be formed
through the generation of the α-oxygenated allyl anion, as
expected, followed by trapping with another chalcone at the γ-
position in 1,4-fashion, which clearly demonstrated the proposed
reaction system. Furthermore, undesired products derived from
the allyl anion intermediate, such as α-adducts and protonated
products, were not observed at all. This result prompted us to
step into the optimization of the conditions for the homo-
dimerization of chalcones (Table 1). Screening of Brønsted
bases revealed that the choice of Brønsted base was critical for
generation of the homoenolate equivalents. Phosphazenes
having strong basicity, such as P2-tBu (pKBH+ = 21.5 in DMSO)[8]
and P4-tBu (pKBH+ = 30.3), provided 3aa in good yield (entries 1
yield [%][b]
entry
base
solvent
temp.
3aa
76
dr of 3aa[c]
79:13:8:<1
67:22:9:2
-
6a
24
18
<1
76
78
17
14
13
31
1
1
P2-tBu
P4-tBu
P1-tBu
tBuOK
KHMDS
P2-tBu
P2-tBu
P2-tBu
P2-tBu
P2-tBu
P2-tBu
KHMDS
KHMDS
THF
rt
2
THF
rt
82
3
THF
rt
<1
4
THF
rt
24
52:33:8:7
52:33:8:7
64:30:6:<1
71:23:6:<1
75:18:7:<1
77:14:9:<1
92:5:3:<1
93:5:2:<1
-
5
THF
rt
22
6
DMF
CH3CN
CH2Cl2
toluene
THF
rt
72
7
rt
85
8
rt
87
+
and 2), while the use of P1-tBu (pKBH = 15.7) resulted in no
9
rt
69
reaction (entry 3). Furthermore, inorganic bases having strong
basicity, such as tBuOK and KHMDS, were less effective than
phosphazenes, and 1,4-adduct 6a was obtained as the major
product (entries 4 and 5). Screening of solvents was then carried
out (entries 6-9), and THF was found to be the best solvent in
terms of diastereoselectivity. Decreasing the reaction
temperature was beneficial for improving the yield and
diastereoselectivity (entry 10). At −78 °C, the competing 1,4-
addition of 2 to 1a was suppressed, and thus the yield of 3aa
was increased. Finally, the use of 1.1 equivalents of 2 further
improved the yield, and 3aa was quantitatively obtained with
high diastereoselectivity (entry 11). As a control experiment, the
reaction was performed with KHMDS at −78 °C. Even though
the formation of 1,4-adduct 6a was suppressed as in the case
with P2-tBu, allylic alcohol 5a was obtained in 74% yield instead
of 3aa (entry 12). The difference between P2-tBu and KHMDS
(entries 10 and 12) was attributed to the effect of the
countercation of the anionic intermediate. Phosphazenes are
10
11[d]
12
13[f]
−78 °C
−78 °C
−78 °C
−78 °C
90
THF
99 (90)
2[e]
46[g]
<1
<1
<1
THF
THF
80:7:13:<1
[a] Conditions: 1a (0.20 mmol), 2 (0.10 mmol), base (0.010 mmol), solvent
(1.0 mL). [b] Yields were determined by 1H and 31P NMR analyses of the
crude mixtures. Trimethyl phosphate was used as the internal standard. The
isolated yield of the major diastereomer is shown in parentheses. [c]
Diastereomeric ratio was determined by 1H NMR analysis of the crude
mixture. [d] 1.1 equivalents of 2 (0.11 mmol) were used. [e] 5a was obtained
in 74% yield. [f] 10 mol% 18-crown-6 (0.010 mmol) was used. [g] 5a was
obtained in 54% yield.
known to generate
a
“naked” anion possessing high
nucleophilicity due to delocalization of the positive charge on the
bulky conjugated acid.[9] Therefore, P2-tBu would facilitate the
[1,2]-phospha-Brook rearrangement by generating a highly
active alkoxide intermediate, resulting in the formation of allyl
anion intermediate B. In contrast, potassium alkoxide generated
by KHMDS would not be active enough to accelerate the [1,2]-
phospha-Brook rearrangement at −78 °C, and thus 5a was
obtained. Increasing the reactivity of potassium alkoxide by
using 18-crown-6 as an additive resulted in the formation of 3aa
in moderate yield, which would support our hypothesis (entry 13).
The optimum conditions for the homo-dimerization were
essential for generating the α-oxygenated allyl anion. Substrates
having an alkyl substituent on the keto moiety or at the β-
position did not undergo the [1,2]-phospha-Brook rearrangement,
and 1,2-adduct 5 and/or 1,4-adduct 6 were obtained. The
relative configuration of the major diastereomer of 3 was
determined by single crystal X-ray diffraction analysis to have
the syn-configuration of substituents at the C-3 and C-4
positions, and Z configuration of the alkene moiety.[12]
With this methodology in hand, we turned our attention to the
investigation of the cross-addition reaction of homoenolate
equivalents with other electrophiles. However, the generation of
homoenolate equivalents from chalcones and diethyl phosphite
in the presence of other electrophiles did not seem feasible
applicable to
a
variety of chalcone derivatives having
substituents on the aryl groups on the keto moiety and at the β-
position (Table S1).[10,11] It should be mentioned that both of the
aryl groups on the keto moiety and at the β-position were
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because the direct addition of diethyl phosphite to the
electrophiles would compete against the 1,2-addition to
chalcones. Indeed, treatment of 1a with 2 in the presence of
other electrophiles, such as different chalcone derivatives,
resulted in the formation of complex mixtures of products.
Therefore, in order to avoid the issue of chemoselectivity, allylic
alcohols 5, which were easily prepared from 1 by treatment with
2 in the presence of a catalytic amount of KHMDS (cf. Table 1,
entry 12) or TMSCH2MgCl,[10] were employed as the precursor of
the homoenolate equivalent, and the cross-addition reaction was
explored. Initially, the reaction of 5a with chalcone derivative 1b
was attempted under the conditions for the homo-dimerization
reaction (Table 2, entry 1). As expected, the desired adduct 3ab
was obtained in good yield with good diastereoselectivity.
Following this result, the scope of the cross-addition was
examined (Table 2 and Scheme 2). First, the screening of allylic
alcohols 5 was carried out by using 1b as an electrophile (Table
2). The allylic alcohols 5c and 5d having an electron-
withdrawing chloro group at the para position of the aryl group
on the keto moiety or at the β-position underwent the reaction
smoothly to afford the desired products in good yields with good
diastereoselectivities (entries 2 and 3). On the other hand, in the
case of 5e and 5f having an electron-donating methoxy group,
the reaction was sluggish (entries 4 and 5). In addition,
particularly in the reaction of 5e, the elimination of diethyl
yield along with a significant amount of 3bb which was the
dimerized product of 1b. The reaction of o-tolyl-substituted 5g
and 5h provided the corresponding products 3gb and 3hb in
good yields while the diastereoselectivities were moderate
(entries 6 and 7). Heteroaryl-substituted 5i and 5j were also
applicable to this reaction to provide the products 3ib and 3jb,
respectively, in good yields with good diastereoselectivities
(entries 8 and 9). Next, the scope of electrophiles was
investigated with 5a as the precursor of the homoenolate
equivalent (Scheme 2).[13] These results clearly reveal that this
methodology is applicable to a variety of Michael acceptors
including chalcone derivatives having different substituents at
the β-position, an alkyl-substituted enone, an α,β-unsaturated
acyl pyrrole, and a trifluoromethyl-substituted α,β-unsaturated
ester, to afford the corresponding products in good yields with
good diastereoselectivities. Furthermore, in addition to Michael
acceptors, imines and aldehydes were suitable electrophiles. N-
Boc imine provided the corresponding amine 10 in high yield
with good diastereoselectivity. Trapping with benzaldehyde
resulted in the formation of alcohol 11 with moderate
diastereoselectivity. In this case, a small amount of α-adduct
was observed as a by-product.
phosphite
competed
with
the
[1,2]-phospha-Brook
rearrangement, resulting in the formation of 3eb in moderate
Table 2. Scope of Allylic Alcohols 5[a]
entry
1
5
Ar1
Ar2
3
yield [%][b]
88 (74)
90 (68)
90 (82)
52[e]
dr[c]
5a
5c
5d
5e
5f
Ph
Ph
3ab
3cb
3db
3eb
3fb
3gb
3hb
3ib
3jb
90:10
91:9
2
4-Cl-C6H4
Ph
Ph
3
4-Cl-C6H4
Ph
94:6
4[d]
5[d]
6
4-MeO-C6H4
Ph
79:21
85:15
70:30
74:26
92:8
Scheme 2. Scope of Electrophiles.[a] [a] Conditions: 5a (0.10 mmol),
electrophile (0.10 mmol), P2-tBu (0.010 mmol), THF (1.0 mL), −78 °C, 6 h.
Yields are NMR yields. Isolated yields of major diastereomers are shown in
parentheses. Diastereomeric ratio (dr) is the ratio of major diastereomer to the
sum of the other three diastereomers. [b] Major diastereomer was not
4-MeO-C6H4
Ph
87 (56)
83 (55)
89 (47)
81[e]
5g
5h
5i
2-Me-C6H4
Ph
separable from
a small amount of impurity. [c] 1.2 equivalents of the
7
2-Me-C6H4
Ph
electrophile were used.
8
3-pyridyl
Ph
It is worth noting that this operationally simple methodology was
reliable enough to perform both homo-dimerization and cross-
addition reactions on the gram-scale, and the corresponding
products 3aa and 10 were obtained in good yields (Scheme 3).
Finally, transformation of the products was demonstrated
(Scheme 4). Acetal protection of the keto moiety of 3aa followed
by cleavage of the diethoxyphosphoryl group by treatment with
NaOEt in ethanol and deprotection of the acetal moiety under
acidic conditions provided the corresponding 1,6-dicarbonyl
compound 4aa in good overall yield (Scheme 4a). In contrast,
9
5j
3-thienyl
94 (80)
91:9
[a] Conditions: 5 (0.10 mmol), 1b (0.10 mmol), P2-tBu (0.010 mmol), THF
(1.0 mL), −78 °C, 6 h. [b] NMR yields. Isolated yields of major diastereomers
are shown in parentheses. [c] The ratio of major diastereomer to the sum of
the other three diastereomers. [d] Reaction was conducted with 20 mol% P2-
tBu. [e] Major diastereomer was not separable from a small amount of
impurity.
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available chalcones and their derivatives via the [1,2]-phospha-
Brook rearrangement using P2-tBu as a superior Brønsted base
catalyst. The resulting anion was subsequently trapped by
various electrophiles, including Michael acceptors, imines and
aldehydes, providing the corresponding adducts in good yields.
Further investigations using this newly-developed methodology
are now in progress, focusing on the development of
enantioselective reactions.
Acknowledgements
Scheme 3. Gram-Scale Reactions.
This research was partially supported by a Grant-in-Aid for
Scientific Research on Innovative Areas “Advanced Molecular
Transformations by Organocatalysts” from MEXT (Japan) and a
Grant-in-Aid for Scientific Research from the JSPS.
direct cleavage of the diethoxyphosphoryl group of 3aa resulted
in the formation of polysubstituted cyclopentene 13 as a single
diastereomer through the intramolecular aldol condensation
(Scheme 4b). The imine-adduct 10 was easily converted to the
corresponding γ-aminoketone 14 in high yield (Scheme 4c). A
transformation based on the cross-coupling reaction was also
possible (Scheme 4d). Thus, a Ni-catalyzed Negishi-type cross-
coupling reaction afforded the arylated product 15 in good yield.
This result suggests that the newly-developed methodology can
be regarded not only as the catalytic generation and addition of
homoenolate equivalents, but also as those of stereo-defined
polysubstituted allyl anions under Brønsted base catalysis.
Keywords: Brønsted base catalysis • homoenolate • umpolung •
[1,2]-phospha-Brook rearrangement • organocatalyst
[1]
[2]
For selected reviews on umpolung reactivity, see: a) B.-T. Gröbel, D.
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3511-3522; e) J. Streuff, Synthesis 2013, 45, 281-307.
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Engl. 1984, 23, 932-948; Angew. Chem. 1984, 96, 930-946; c) I.
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Kuwajima, E. Nakamura, In Comprehensive Organic Synthesis, Vol. 2,
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[3]
[4]
a) S. S. Sohn, E. L. Rosen, J. W. Bode, J. Am. Chem. Soc. 2004, 126,
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43, 6205-6208; Angew. Chem. 2004, 116, 6331-6334.
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Nair, S. Vellalath, B. P. Babu, Chem. Soc. Rev. 2008, 37, 2691-2698;
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T. Biju, V. Nair, Chem. Soc. Rev. 2015, 44, 5040-5052.
[5]
[6]
For selected reviews on homoenolate equivalents, see: a) D. Hoppe, T.
Hense, Angew. Chem. Int. Ed. Engl. 1997, 36, 2282-2316; Angew.
Chem. 1997, 109, 2376-2410; b) H. Ahlbrecht, U. Beyer, Synthesis
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For selected recent examples, see: a) M. A. Horwitz, B. P. Zavesky, J. I.
Martinez-Alvarado, J. S. Johnson, Org. Lett. 2016, 18, 36-39; b) M. A.
Horwitz, N. Tanaka, T. Yokosaka, D. Uraguchi, J. S. Johnson, T. Ooi,
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Johnson, Angew. Chem. Int. Ed. 2012, 51, 4685-4689; Angew. Chem.
2012, 124, 4763-4767; d) A. S. Demir, I. Esiringü, M. Gӧllü, Ӧ. Reis, J.
Org. Chem. 2009, 74, 2197-2199; e) A. S. Demir, S. Eymur, J. Org.
Chem. 2007, 72, 8527-8530; f) C. C. Bausch, J. S. Johnson, Adv.
Synth. Catal. 2005, 347, 1207-1211; g) A. S. Demir, Ӧ. Reis, A. Ç. İğdir,
İ. Esiringü, S. Eymur, J. Org. Chem. 2005, 70, 10584-10587; h) M.
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Grimaud, A. Dos Santos, Synlett 2005, 2335-2336.
Scheme 4. Transformation of Products.
In conclusion, a novel method for the catalytic generation of
homoenolate equivalents and its application to carbon-carbon
bond formation was developed by utilizing the [1,2]-phospha-
Brook rearrangement under Brønsted base catalysis. The α-
oxygenated allyl anion, which can serve as a homoenolate
equivalent, was catalytically generated in situ from readily
[7]
a) A. Kondoh, M. Terada, Org. Lett. 2013, 15, 4568-4571; b) A. Kondoh,
T. Aoki, M. Terada, Org. Lett. 2014, 16, 3528-3531; c) A. Kondoh, M.
Terada, Org. Chem. Front. 2015, 2, 801-805; d) A. Kondoh,T. Aoki, M.
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Terada, Chem. Eur. J. 2015, 21, 12577-12580; e) A. Kondoh, M.
Terada, Org. Biomol. Chem. 2016, 14, 4704-4711.
[10] See the Supporting Information for details.
[11] Other dialkyl phosphites, such as dimethyl phosphite, showed the
reactivity similar to that of diethyl phosphite while diaryl phosphite
resulted in no reaction.
[8]
[9]
Y. Kondo, M. Ueno, Y. Tanaka, Y. J. Synth. Org. Chem. Jpn. 2005, 63,
453-463.
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Chem. Commun. 1998, 2091-2092.
[12] CCDC No. 1503914. See the Supporting Information for details.
[13] The relative configuration of the major diastereomer of each product
was determined by derivatization and/or single crystal X-ray diffraction
analysis. See the Supporting Information for details.
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Azusa Kondoh, Takuma Aoki, Masahiro
Terada*
Page No. – Page No.
Generation and Application of
Homoenolate Equivalents Utilizing
[1,2]-Phospha-Brook Rearrangement
under Brønsted Base Catalysis
A novel method for catalytic generation of α-oxygenated allyl anions, which can
serve as homoenolate equivalents, was developed by utilizing the [1,2]-phospha-
Brook rearrangement under Brønsted base catalysis. The allyl anions generated in
situ were subsequently trapped by various electrophiles, including Michael
acceptors, imines, and aldehydes, providing the corresponding adducts in good
yields with moderate to good diastereoselectivities.
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