Preparation of α-oxygenated ketones by the dioxygenation of alkenyl boronic acids

Article abstract of DOI:10.1002/anie.201202704

Two in two: Dioxygenation of alkenyl boronic acids has been achieved with N-hydroxyphthalimide. The two-step process involves etherification of an alkenyl boronic acid with N-hydroxyphthalimide followed by a [3,3] rearrangement. The dioxygenated product can then be hydrolyzed to form either the corresponding α-hydroxy ketone or the α-benzoyloxy ketone. Copyright

Full text of DOI:10.1002/anie.201202704

Angewandte
Chemie
DOI: 10.1002/anie.201202704
Pericyclic Rearrangement
Preparation of a-Oxygenated Ketones by the Dioxygenation of
Alkenyl Boronic Acids**
Aditi S. Patil, Dong-Liang Mo, Heng-Yen Wang, Daniel S. Mueller, and Laura L. Anderson*
The ubiquitous use of aryl, alkenyl, and alkyl boronic acids for
mediated etherification of N-hydroxyphthalimide followed by
a [3,3] rearrangement to provide a-hydroxy or a-benzoyloxy
ketones in two high-yielding steps from simple starting
materials.
À
À
À
the formation of new C C, C N, and C O bonds in cross-
coupling reactions is indicative of the importance of these
compounds in organic synthesis.[1] Although the use of
boronic acids for the preparation of a variety of new bonds
is well established, we were interested in testing if the
reactivity of alkenyl boronic acids could be further diversified
to include dioxygenation and the synthesis of a-oxygenated
ketones. The conversion of alkenyl boronic acids to
a-oxygenated ketones would provide a unique retrosynthetic
disconnection for the preparation of complicated targets
containing these challenging motifs.[2–8] Towards the goal of
alkenyl boronic acid dioxygenation, we hypothesized that
etherification of an alkenyl boronic acid with N-hydroxy-
phthalimide would form an N-enoxyphthalimide poised to
undergo a [3,3] rearrangement to give an a-oxygenated
ketone (Scheme 1). This method would avoid the use of
Our efforts towards achieving the dioxygenation of
alkenyl boronic acids began with the optimization of con-
ditions for the cross-coupling of alkenyl boronic acids and
N-hydroxyphthalimide to form N-enoxyphthalimides.
Although the copper-mediated arylation of N-hydroxyphtha-
limide with aryl boronic acids is known, to the best of our
knowledge, the corresponding process for vinylation has not
yet been reported.[10–12] Mixtures of copper salts, bases, and
dessicants, as well as equivalents of reagents, were screened
for their effectiveness in promoting the desired coupling of
1 and 2a. As shown in entries 1–4 of Table 1, the use of
2 equiv of boronic acid 2a provided a higher yield of 3a for
both copper-mediated and copper-catalyzed transformations,
although the difference in reaction efficiency was more
striking for the catalytic process.[13] The greater sensitivity of
the catalytic reaction to changes in reaction conditions was
consistent throughout the optimization process and guided
our inquiry. Cu(OAc)2 (OAc = acetate) was shown to be the
Table 1: Optimization of the etherification of N-hydroxyphthalimide with
2-butenyl boronic acid.
Scheme 1. Dioxygenation of alkenyl boronic acids. OAc=acetate,
PhthN=phthalimide.
highly reactive electrophilic oxygenation reagents, not
require the preparation of a-halogenated precursors, and
allow access to linear a-oxygenated ketones from internal
alkynes.[4,5d,7–9] Moreover, the nature of the transition state of
the pericyclic reaction would allow for potential diastereose-
lective construction of the a-oxygenated stereocenter.
Herein, we describe the development of a new method for
the dioxygenation of alkenyl boronic acids through a copper-
Entry
[Cu]
[2a]
Base
Yield [%]
of 3a[a]
1
2
3
4
5
6
7
8
Cu(OAc)2 (1 equiv)
Cu(OAc)2 (1 equiv)
Cu(OAc)2 (20 mol%)
Cu(OAc)2 (20 mol%)
CuCl (20 mol%)
1 equiv
2 equiv
1 equiv
2 equiv
2 equiv
2 equiv
2 equiv
2 equiv
2 equiv
2 equiv
2 equiv
2 equiv
2 equiv
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
NEt3
71
96
6
87
7
78
61
8
81
68
NR
NR
NR
CuI (20 mol%)
Cu(TFA)2 (20 mol%)
Cu(OTf)2 (20 mol%)
CuTC (20 mol%)
Cu(OAc)2 (20 mol%)
Cu(OAc)2 (20 mol%)
Cu(OAc)2 (20 mol%)
Cu(OAc)2 (20 mol%)
9
[*] A. S. Patil, Dr. D.-L. Mo, H.-Y. Wang, D. S. Mueller,
Prof. L. L. Anderson
10
11
12
13
DABCO
imidazole
KOtBu
Department of Chemistry, University of Illinois at Chicago
845 W. Taylor St. MC 111, Chicago, IL 60613 (USA)
E-mail: lauralin@uic.edu
[a] Yields were determined by 1H NMR spectroscopy using 1,3,5-
trimethoxybenzene as an internal standard; NR=no reaction.
DABCO=1,4-diazabicyclo[2.2.2]octane, DCE=1,2-dichloroethane,
OAc=acetate, TC=2-thiophenecarboxylate, Tf=trifluoromethanesulfo-
nate, TFA=trifluoroacetate.
[**] We acknowledge generous funding from ACS-PRF (50491-DNI) and
the University of Illinois at Chicago. We also thank Prof. T. Driver
and group for insightful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202704.
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Table 2: Scope of the etherification of N-hydroxyphthalimide with alkenyl boronic acids.
optimal catalyst when compared to
other CuI and CuII salts (entries 5–9)
and pyridine was shown to be the
optimal base when compared to
other amines and inorganic bases
(entries 10–13). Neither the copper-
mediated nor the copper-catalyzed
coupling reaction showed any con-
version to the desired product when
run in the absence of air, and both
transformations required the use of
a halogenated solvent. The cross-
coupling process was fairly insensi-
tive to the choice of desiccant; 4 ꢀ
molecular sieves and MgSO4 gave
the desired product in only slightly
attenuated yields.[14] The optimiza-
tion study concluded that treatment
of a 1:2 mixture of 1/2a in 1,2-
dichloroethane (DCE) with Cu-
(OAc)2 (1 equiv or 20 mol%), pyri-
Entry
1
Product
Yield [%]
Entry
12
Product
Yield [%]
of 3[a]
of 3[a]
98[b] (76)
82 (73)
2
3
4
81 (70)
87 (74)
88 (78)
13
14
15
83 (76)
73 (68)
91 (89)
5
6
7
81 (77)
47
16
17
18
86 (82)
84 (78)
41
dine
(3 equiv),
and
Na2SO4
(4 equiv) in air provided optimal
conversion of 2a to 3a.
With the optimal conditions for
the cross-coupling of 1 and 2a in
hand, the scope of the transforma-
tion was evaluated with a variety of
alkenyl boronic acids to determine
the tolerance for boronic acid sub-
stitution patterns. As shown in
Table 2, both copper-mediated and
copper-catalyzed conditions con-
verted 1- and 2-trans-substituted
vinyl boronic acids, Z-disubstituted
alkenyl boronic acids, and cyclic
alkenyl boronic acids to the desired
N-enoxyphthalimides 3 with reten-
tion of alkene geometry.[15] Both
alkyl- and aryl substituents were
tolerated for the boronic acid cou-
pling partner, as were common aryl
86 (77)
8
76 (67)
63 (66)
67 (55)
70 (71)
19
20
21
22
86 (80)
64
9
10
11
83
76 (52)
[a] Yield of isolated product using 1 equiv Cu(OAc)2 and (yield of isolated product using 20 mol%
Cu(OAc)2). [b] When run on a 1 mmol scale, the yield of isolated product using 1 equiv of Cu(OAc)2 was
74%. Cy=cyclohexyl, DCE=1,2-dichloroethane, PhthN=phthalimide, OAc=acetate, pyr=pyridine.
electron-withdrawing
functional
groups such as nitro, fluoro, and
trifluoromethyl, as well as common
protecting groups such as ketals. Trisubstituted alkenyl
boronic acids and alkenyl boronic acids with ortho-substituted
aryl groups currently represent a limitation of this method.
Unfavorable steric interactions also hinder the etherification
of 6-methyl cyclohexenyl boronic acid 2r; however, no similar
inhibition was observed for the fused system 2v. The broad
scope of the copper-mediated cross-coupling of N-hydroxy-
phthalimide 1 and alkenyl boronic acids 2 ultimately provided
an array of N-enoxyphthalimides 3 to screen for the [3,3]
rearrangement.
quantitative yields, as determined by comparison to an
internal standard by H NMR spectroscopy; however, imi-
1
dates 4 were unstable when subjected to silica gel chroma-
tography.[16] Isolation and purification of a-hydroxy ketones 5
was achieved in high yield after the hydrolysis of crude
samples of 4 (Table 3). An ion-exchange resin provided
optimal yields for the cleavage of phthalimide from 4, but
silica gel was similarly effective with longer reaction times. a-
Hydroxyketones 5 that were too volatile or hydrophilic to be
separated from phthalimide by extraction were protected in
solution and isolated as the corresponding a-benzoyloxy
ketones 6 (Table 3). The N-enoxyphthalimides 3b–3d, under-
went rearrangements to form a-oxygenated aldehydes 4b–4d,
which were isolated without further purification as the
Solutions of N-enoxyphthalimides 3 in C6D6 or toluene
were heated at 80–908C for 10–16 h to promote a [3,3]
rearrangement and afford dioxygenated alkenyl boronic acids
as imidates 4. These rearrangements occurred in almost
2
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Table 3: Preparation of a-hydroxy- and a-benzoyloxyketones by rearrangement and hydrolysis of
N-enoxyphthalimides 3.
known procedures, which originate
from ketone or aldehyde starting
materials and employ electrophilic
sources of oxygen.
Several aryl-substituted N-
enoxyphthalimides
exhibited
exceptions to the general thermal
reactivity patterns depicted in
Table 3 and equation 1 that sug-
gested trends in the [3,3] rearrange-
ment activity of these compounds.
N-Enoxyphthalimide 3d readily
formed 4d when heated to only
508C [Eq. (1)]. This transformation
is in contrast to 3b and 3c, which
rearranged at 908C, and 3g, which
exhibited no rearrangement reac-
tivity even when heated to 1308C.
The combination of an aryl group at
the 1-position of N-enoxyphthal-
imides 3h–3l, and an alkyl group
at the 2-position attenuated the
opposing affects observed for 3d
and 3g, and the rearrangements to
afford 4h–4l occurred at 808C
(Table 3); however, the addition of
an electron-donating group to the
aryl ring once again reduced the
rearrangement temperature to 25–
508C to give 5w [Eq. (2)]. N-
Enoxyphthalimide 3w could not
be isolated, as the copper-mediated
coupling provided a 2:1 mixture of
3w/5w. Filtration of this mixture
through silica gel to remove Cu-
(OAc)2, followed by warming to
508C for 10 h and hydrolysis, gave
5w in 57% yield over three steps. A
similarly efficient process was also
observed for the transformation of
2a to 6a in 67% yield with no
formal purification of intermedi-
ates, only the removal of Cu(OAc)2
prior to rearrangement [Eq. (3)].
Entry
Product
Yield [%]
Entry
Product
Yield [%]
of 5[a]
of 6[a]
1
2
78[b]
90[b]
9
86
67
10
3
4
82
75
11
12
66
69
5
6
7
8
88
86
82
87
13
14
15
16
65
66
69
66
[a] Yield of isolated product. [b] Hydrolysis promoted with SiO2. Bz=benzoyl, Amberlite=Amberlite
IR120H, ion-exchange resin.
corresponding imidates to avoid polymerization of the
corresponding a-hydroxy aldehydes [Eq. (1)]. The products
shown in Table 3 and equation 1 describe the broad scope of
a-oxygenated carbonyl compounds that can be prepared from
the dioxygenation of alkenyl boronic acids with N-hydroxy-
phthalimide 1 through the rearrangement of N-enoxyphthal-
imides 3. This method provides a valuable alternative to
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Cu(OAc)2 can be present in substoichiometric amounts
during the rearrangement of 3a, but attenuated yields of 4a
are obtained. These results show that the [3,3] rearrangement
of 3 is inhibited by the presence of copper salts and facilitated
by electron-donating aryl groups at the 1-position and phenyl
substituents at the 2-position of the enol ether.
The diastereoselectivity of the [3,3] rearrangement of N-
enoxyphthalimides was tested using N-enoxyphthalimides
3q–3u, which are derived from substituted cyclohexenyl
boronic acids. Compounds 3q, 3s, 3t, and 3u underwent [3,3]
rearrangements to give 50:50 to 60:40, cis/trans diastereo-
meric mixtures of 4 and subsequent hydrolysis to give 6q, 6s,
and 5t with no significant change in the diastereomeric ratio
(Table 3). Hydrolysis and protection of 4u epimerized the a-
benzoyl group, resulting in a 75:25 mixture of cis/trans 6u
(Table 3). Surprisingly, the rearrangement of 3r strongly
favors formation of the trans diastereomer (Scheme 2). We
or 13C NMR spectroscopy. To investigate the possibility of
a radical reaction pathway, a radical clock experiment was
tested with N-enoxyphthalimide 3 f. Upon heating 3 f in either
the presence or the absence of Bu3SnH, no indication of the
formation of an a,b-unsaturated aldehyde was observed,
suggesting that the [3,3] rearrangement occurs through a two-
electron pathway.
In summary, we have shown that dioxygenation of alkenyl
boronic acids 2 with N-hydroxyphthalimide 1 can be achieved
by a two-step process involving copper-mediated etherifica-
tion to form an N-enoxyphthalimide 3 and a subsequent [3,3]
rearrangement to provide a-hydroxy ketones 5 or a-benzoyl-
oxy ketones 6, after hydrolysis of the phthalimide imidate.
This transformation provides a new retrosynthetic disconnec-
tion for the preparation of a-oxygenated carbonyl compounds
that does not require the use of a highly reactive electrophilic
oxygen source or a carbonyl compound as a starting material.
Ongoing work in our laboratory is focused on further
exploring the synthetic utility of this transformation and
exploiting the observed diastereoselectivity.
Scheme 2. Diastereoselective rearrangement of 3r to 4r.
Received: April 7, 2012
Published online: && &&, &&&&
assume that this result is due to minimization of steric
interactions as the rearrangement occurs via a chair transition
state (TS1). In contrast to 4-substituted cyclohexenyl sub-
strates 3q, 3s, and 3u, rotation to give an approach of the
carbonyl oxygen from the higher energy twist conformation
and provide the cis diastereomer is inaccessible for 3r because
of the 6-methyl substituent, which inhibits rotation of the N-
Keywords: boronic acids · ketones · oxygenation · phthalimides ·
.
rearrangement
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compounds was heated in C6D6 at 908C for 18 h, only 4b and 7
were observed and there was no evidence of crossover by 1H
4
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1
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Communications
Pericyclic Rearrangement
A. S. Patil, D.-L. Mo, H.-Y. Wang,
D. S. Mueller,
L. L. Anderson*
&&&& — &&&&
Two in two: Dioxygenation of alkenyl
boronic acids has been achieved with N-
hydroxyphthalimide. The two-step pro-
cess involves etherification of an alkenyl
boronic acid with N-hydroxyphthalimide
followed by a [3,3] rearrangement. The
dioxygenated product can then be hydro-
lyzed to form either the corresponding a-
hydroxy ketone or the a-benzoyloxy
ketone.
Preparation of a-Oxygenated Ketones by
the Dioxygenation of Alkenyl Boronic
Acids
6
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