[HCo(CO)4] -catalyzed three-component cycloaddition of epoxides, imines, and carbon monoxide: Facile construction of 1,3-oxazinan-4-ones

Article abstract of DOI:10.1002/anie.201403998

The three-component [3+2+1] cycloaddition of epoxides, imines, and carbon monoxide to produce 1,3-oxazinan-4-ones has been developed by using [HCo(CO)₄] as the catalyst. The reaction occurs for a wide variety of imines and epoxides, under 60 bar of CO pressure at 50 C, to produce 1,3-oxazinan-4-ones with different substitution patterns in high yields, and provides an efficient and atom-economic route to heterocycles from simple and readily available starting materials. A plausible mechanism involves [HCo(CO)₄]-induced ring-opening of the epoxide, followed by sequential addition of carbon monoxide and the imine, and then ring closure to form the product accompanied by regeneration of [HCo(CO)₄].

Full text of DOI:10.1002/anie.201403998

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Chemie
DOI: 10.1002/anie.201403998
Cycloaddition
[HCo(CO)4]-Catalyzed Three-component Cycloaddition of Epoxides,
Imines, and Carbon Monoxide: Facile Construction of 1,3-Oxazinan-4-
ones**
Lixia Liu and Huailin Sun*
Abstract: The three-component [3+2+1] cycloaddition of
epoxides, imines, and carbon monoxide to produce 1,3-
oxazinan-4-ones has been developed by using [HCo(CO)4]
as the catalyst. The reaction occurs for a wide variety of imines
and epoxides, under 60 bar of CO pressure at 508C, to produce
1,3-oxazinan-4-ones with different substitution patterns in high
yields, and provides an efficient and atom-economic route to
heterocycles from simple and readily available starting materi-
als. A plausible mechanism involves [HCo(CO)4]-induced
ring-opening of the epoxide, followed by sequential addition of
carbon monoxide and the imine, and then ring closure to form
the product accompanied by regeneration of [HCo(CO)4].
ricci and co-workers have also demonstrated the construction
of benzo[d]-1,3-oxazin-6-ones by similar multicomponent
reactions.[15] While these reactions are quite efficient, they
all involve elimination of small molecules as by-products. A
transition-metal-catalyzed MCC involving incorporation of
two isocyanates to form pyrimidine-2,4-diones has been
developed by Louie et al.,[16] Kondo et al.,[9b] and Murakami
et al.[17] However, the only transition-metal-catalyzed MCC
which incorporates two different heteroatom-containing sub-
strates, without losing any atoms, is reported by the group of
Coates, and it employs epoxides, isocyanates, and carbon
monoxide (CO) as starting materials to give 1,3-oxazinane-
2,4-diones.[18] To date, synthesis of other heterocycles by
sequential addition of different heteroatom-containing sub-
strates still remains a challenge for MCC methods.
Herein we report a new transition-metal-catalyzed MCC
which uses epoxides, imines, and CO as building blocks to
construct 1,3-oxazinan-4-ones (1). This heterocycle is useful
as a key intermediate in the total synthesis of natural
products,[19] or as a precursor for the synthesis of b-hydroxy
acids,[20] b-hydroxy esters,[21] b-hydroxy amides,[22] a,b-epoxy-
carboxylic acids,[23] and 1,3-amino alcohol derivatives,[24] all of
which are valuable synthetic intermediates for the synthesis of
pharmaceutically important compounds. To date there are
only two major synthetic routes to such heterocycles, includ-
ing acid-catalyzed dehydration-condensation of various alde-
hydes or ketones with appropriate hydroxy amides,[25] and
hetero-Diels–Alder cycloadditions between aldehydes and 2-
aza-3-silyloxy-1,3-butadienes.[26] While these approaches are
widely used, they often require multiple steps owning to the
fact that the reactants are not easily obtained by classical
routes, thus, making substrate variation difficult to achieve. In
contrast, the present MCC method turns out to be a much
more convenient and versatile methodology for the synthesis
of a wide variety of substituted derivatives of 1 from the
readily available starting materials.
T
ransition-metal-catalyzed multicomponent cycloadditions
(MCCs) are among the most efficient methodologies for the
synthesis of heterocycles as it has the advantages of both
cycloadditions and multicomponent reactions, namely atom
economy and multibond formation in one step.[1–3] This
strategy is especially desired for the synthesis of heterocycles,
having increased ring sizes, using relatively simple and readily
available starting materials. The key to such syntheses is the
incorporation of the heteroatom-containing substrates into
the cyclic adducts, together with those commonly used to
construct carbocycles through MCCs. To date, many types of
heterocyclic structures containing a single heteroatom, such
as oxygen- or nitrogen-containing five- and six-membered
rings,[4–9] have been synthesized by MCCs. However, synthe-
ses of heterocycles with more than one heteroatom within the
cyclic skeleton require the incorporation of more heteroatom-
containing substrates, and such methods have rarely been
reported.
Recently, Arndsten and co-workers reported syntheses of
Mꢀnchnones,[10] imidazoles,[11] imidazolines,[12] and imidazo-
lones[13] by palladium-catalyzed multicomponent cyclization
methods. Alper and co-workers have described formation of
benzo[e]-1,3-oxazin-4-one and 4(3H)-quinazolinone deriva-
tives through similar palladium-catalyzed reactions.[14] Pet-
In the course of our study on alternating copolymerization
of imines and CO to synthesize polypeptides,[27] we used the
silylcobalt complex [Ph3SiCo(CO)4] (2) as a precatalyst,
which upon reacting with oxirane and CO may generate an
acylcobalt species to act as the catalyst for the copolymeriza-
tion reaction. To our surprise, when oxirane (3a) and the
pivaladehyde imine 4a were treated with catalytic amounts of
2 under 60 bar of CO pressure, a large amount of a crystalline
solid, which was characterized to be 2-tert-butyl-3-methyl-1,3-
oxazinan-4-one (1a) rather than the desired polypeptide, was
obtained (Scheme 1). The structure of 1a was unambiguously
confirmed by single-crystal X-ray diffraction analysis in
[*] L. Liu, Prof. H. Sun
State Key Laboratory of Elemento-Organic Chemistry, Collaborative
Innovation Center of Chemical Science and Engineering, and
Department of Chemistry, Nankai University
Tianjin, 300071 (P.R. China)
E-mail: sunhl@nankai.edu.cn
[**] This work was supported by the NSFC (Project No. 20834002), the
Natural Science Foundation of Tianjin (Project No.
08JCZDJC21600), and the Ministry of Education of China (Project
No. 03406).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403998.
1
addition to H and 13C NMR spectroscopy, as well as IR and
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Table 1: [HCo(CO)4]-catalyzed cycloaddition of 3a, 4a, and CO for
synthesis of 1a. A 5 mol% catalyst loading was used and based on the
amount of imine used.[a]
Entry Catalyst
Solvent
T
P
t
Yield
[8C] [bar] [h][b] [%][c]
1
2
3
4
5
6
7
8
2/MeOH
[Co2(CO)8]/H2
[NaCo(CO)4]/p-TsOH 1,4-dioxane 50
1,4-dioxane 50
1,4-dioxane 50
60
60
60
60
60
30
60
60
60
60
60
21.5 68
Scheme 1. The three-component [3+2+1] cycloaddition of epoxides,
imines, and CO with [Ph3SiCo(CO)4] (2) as the catalyst.
[d]
26
24
21
9
60
65
78
58
73
80
83
83
90
80
2/MeOH
2/MeOH
2/MeOH
2/MeOH
2/MeOH
2/MeOH
2/MeOH
2/MeOH
1,4-dioxane 50
1,4-dioxane 80
1,4-dioxane 50
24
28
25
21
19
22
HRMS analyses (see the Supporting Information). Thus,
THF
CH2Cl2
PhCl
toluene
benzene
50
50
50
50
50
a
three-component [3+2+1] cycloaddition of epoxides,
imines, and CO to form 1,3-oxazinan-4-one has taken place.
This result lead us to pay attention to a report from Jia,[28]
in which it was revealed that the reaction of acetylcobalt
tetracarbonyl with phenylaldehyde imines, followed by addi-
tion of propylene oxide and CO could give rise to 2-phenyl-3-
alkyl-1,3-oxazinan-4-one, although no information about the
yield and experimental details were provided. We examined
the reaction by using phenoxyacetylcobalt tetracarbonyl to
react with 3a and 4a in the presence of CO.[29] Only a small
amount of 1a could be obtained, and the polypeptide turned
out to be the major product. This reaction demonstrated that
the acylcobalt complex could not effectively catalyze the
three-component cycloaddition of oxirane, imines and CO.
To gain insight into the details of the 2-catalyzed cyclo-
addition, in situ IR spectroscopy was used to monitor the
reaction. An initiation period of about 50 minutes was
observed before consumption of 4a, and then the reaction
proceeded smoothly to give 1a as the product. This induction
period is quite similar to the phenomena observed by Coates
and co-workers, involving ring-opening of tetrahydropyran by
2 to form an alkylcobalt, which underwent b-hydrogen
elimination to produce [HCo(CO)4], the catalyst for subse-
quent reactions.[30] Thus, it should be reasonable to hypothe-
size that 2 might also react with oxirane to generate
[HCo(CO)4] as the catalyst for the cycloaddition reaction.
This hypothesis was verified by direct application of
[HCo(CO)4], which was generated in situ from alcoholysis
of 2 with methanol, according to Coates and co-workers, and
showed that the cycloaddition proceeded at a markedly
higher reaction rate without an induction period to afford 1a
in good yield (Table 1, entry 1). Methods for generation of
[HCo(CO)4] and the reaction conditions have been opti-
mized, and the best results were obtained by using 2/MeOH
as the catalyst and toluene as solvent under 60 bar CO at
508C, as shown in Table 1.
9
10
11
[a] Oxirane was used in equal amounts (entries 1–3) or in a ratio of 1.3:1
(entries 4–11) to the imine. [b] Times needed for completion of the
reaction as determined by in situ IR. [c] Yield of isolated product.
[d] P(H2)/P(CO)=1:3. p-TsOH=para-toluenesulfonic acid.
tioned reaction conditions, thus unexpectedly giving 1g in
moderate yield (entry 7). It was noted that steric hindrance of
N-alkyl groups have a strong influence on the reaction rates,
probably because of the attack of imines on the acylcobalt
intermediates is the rate-determining step of the reaction (see
below). Although both N-ethyl- and N-benzyl-substituted
imines could react successfully with 3a, they both required
higher reaction temperatures and longer reaction times, and
gave the products in lower yields (entries 8, 16 and 17).
Interestingly, imines derived from aryl aldehydes generally
lead to high yields of the desired products (entries 9–15). Of
particular note is that the ketimine 3r also underwent the
cycloaddition to give the product 1r in good yield (entry 18).
Next, this cycloaddition reaction was easily extended to
the use of other epoxides as substrates in place of oxirane
(Table 3). For example, propylene oxide (3b) reacted with
both the alkyl imine 4a and aryl imine 4i, to give the products
1s and 1t, respectively (entries 1 and 2). High regioselectivity
was observed, thus leading to formation of the 6-methyl-
substituted isomer as products, although in the reaction of 3b
and 4i a small amount of the 5-methyl isomer was also
obtained. This outcome is consistent with previous knowledge
on regioselective ring-opening of propylene oxide at the less
substituted carbon atom by a cobaltate anion.[31] Styrene
oxide (3c) was also successfully used to react with both 4a and
4i, and the 5-phenyl products with opposite regioselectivity,
relative to the case of propylene oxide, were always obtained,
due to the higher stability of the benzylic carbocation.[32]
Moreover, the reactions of styrene oxide have high diaste-
reoselectivity, thus giving only the trans-2,5-disubstituted
products (entries 3 and 4). This result can be rationalized by
predominant formation of the chairlike conformation of the
six-membered cyclic transition state with both substituents
located at equatorial positions. Surprisingly, even when
cyclohexane oxide (3d) was used, the reaction occured to
give the desired products 1w and 1x, albeit in lower yields
(entries 5 and 6). Details for structure determination of all the
products are provided in the Supporting Information.
To explore the scope of the reaction, various imines were
used under the optimized reaction conditions. A wide variety
of stable imines were used and gave the desired products in
good to excellent yields as shown in Table 2. For example, the
imines 4a–c from tert-alkyl aldehydes reacted nicely to give
1a–c in 75-90% yields (entries 1–3). The cyclic tert-alkyl-
substituted imines 4d–f also afforded the desired products
1d–f in similarly yields (entries 4–6). Although imines formed
from primary and secondary alkyl aldehydes are usually
unstable, because of the ease of rearrangement to alkenyl
amines which might inhibit the cycloaddition, the cyclohexyl
aldehyde imine 4g could still react under the above-men-
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Table 2: The scope with respect to the imine used for the synthesis of
Table 3: The scope with respect to the epoxides used for the synthesis of
1,3-oxazinan-4-ones.
1,3-oxazinan-4-ones.
[a] Yield of the isolated product. [b] Ratios of cis to trans isomers. [c] The
5-methyl isomer of 1t was also obtained as a minor product.
[d] Reactions run at 608C. [e] Ratios of two diastereomers. See the
Supporting Information.
[a] Yield of isolated product. [b] Reactions run at 608C.
From the above results and on the basis of existing
knowledge about cobalt-catalyzed reactions, a plausible
mechanism for the cycloaddition is proposed as shown in
Scheme 2. The first step involves ring-opening of the epoxide
by [HCo(CO)4], which is generated in situ from 2 and
methanol, to form the alkylcobalt tetracarbonyl 5.[33] Insertion
of CO leads to the acylcobalt species 6, to which a imine adds
to produce the adduct 7. Intramolecular attack of the hydroxy
group on the cationic acyliminium carbon atom results in the
ring closure to give the final product as mentioned in the
literature,[34] accompanied by regeneration of [HCo(CO)4].
Scheme 2. Possible mechanism for [HCo(CO)4]-catalyzed cycloaddition
of epoxides, imines, and CO.
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In conclusion, we have successfully developed a highly
efficient three-component [3+2+1] cycloaddition of epoxides,
imines, and CO using [HCo(CO)4] as the catalyst. The
reaction takes place for a wide variety of imines and epoxides
to produce 1,3-oxazinan-4-ones with different substitution
patterns in high yields, and provides a convenient and atom-
economic route to prepare the heterocycles from simple and
readily available starting materials. All evidence supports the
mechanism involving the ring-opening of epoxides by
[HCo(CO)4] as the initial step, with subsequent insertion of
CO. Attack of the imine on the acylcobalt species constitutes
the rate-determining step of the reaction. Further study of this
reaction will be focused on the details of the mechanism and
control of the stereochemistry of the products by asymmetric
catalysis. Related works are underway in our laboratory.[35]
Received: April 4, 2014
Published online: July 18, 2014
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Keywords: cobalt · cycloaddition · epoxides · heterocycles ·
multicomponent reactions
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[35] After preparation of the manuscript, a report appeared on an
identical reaction using [Co2(CO)8]/LiCl as the catalyst. We
thank the referee for reminding us of the important information.
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