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Letter
Intermolecular [2+2] Cycloaddition/isomerization of Allenyl Imides and Unactivated Imines for the Synthesis of 1Azadienes Catalyzed by a Ni(ClO4)2·6H2O Lewis Acid Shuai Pang, Xing Yang, Ze-Hun Cao, Yu-Long Zhang, Yan Zhao, and Yi-Yong Huang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01454 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018
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ACS Catalysis
Intermolecular [2+2] Cycloaddition/isomerization of Allenyl Imides and Unactivated Imines for the Synthesis of 1-Azadienes Catalyzed by a Ni(ClO4)2·6H2O Lewis Acid Shuai Pang,a,‡ Xing Yang,a,‡ Ze-Hun Cao,a Yu-Long Zhang,a Yan Zhaob,* and Yi-Yong Huanga,* a
Department of Chemistry, School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, Wuhan 430070, China. b State Key Laboratory of Silicate Materials for Architectures, International School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, China. KEYWORDS: [2+2] cycloaddition, isomerization, allene, imine, azadiene, proton transfer, DFT calculations, γ,δunsaturated β-ketoester Supporting Information Placeholder ABSTRACT: The intermolecular [2+2] cycloaddition/isomerization between allenyl imides and N-(2methoxyphenyl) aldimine counterparts catalyzed by a Ni(ClO4)2·6H2O Lewis acid at room temperature was discovered, providing a facile access to 1-azadiene derivatives with high atom economy. The incorporation of an 2-oxazolidinone group into allene amides resulted in unusual reactivity for the imine-metathesis and synthetic application to a chiral γ,δunsaturated β-ketoimide. Mechanistic experiment of using density functional theory (DFT) computation in CH2Cl2 with the B3LYP functional rationalized the proposed catalytic pathway involving initial stepwise [2+2] cycloaddition to provide an azetidine species, two-time proton transfer to form a 2azetine intermediate, and final conrotatory ring opening for trans-1-azadiene based substances.
quired all electron rich aryl-substitutions in all substrates.6 As a matter of fact, such a process seems more challenging, not only because of the highest thermodynamic driving force to form a 2azetine intermediate comparing with other (hetero)butadiene intermediates based on the simple calculations (Scheme 1), but also because of the readily reversible conversion between the formed 1-azadienes and corresponding α,β-unsaturated carbonyls in the presence of metal-based Lewis acid catalysts. On the other hand, given its potential application in the construction of complex nitrogen-containing derivatives otherwise difficult to reach, it was worthy of further exploration of imine-metathesis.
Introduction [2+2] cycloaddition/isomerization refers a tandem reaction of [2+2] cycloaddition and rearrangement, which provides synthetic diversity and utility from simple reactants. 1,2-Metathesis of C=X (X = C, O and N) double-bond materials by alkynes via reactive (hetero)butadiene intermediates represents typical and valuable for conjugated substances. In the last two decades, a great deal of noble metal-catalyzed enyne cycloaddition/isomerizations involving olefin-metathesis have been established for 1,3-dienes and total synthesis of natural products (Scheme 1).1 In the case of alkyne/carbonyl metathesis reactions for the access to enones,2 Mikami et al. demonstrated a PdII-Lewis acid catalysis protocol regarding an oxetene intermediate in 2011.3 Furthermore, Sun et al. applied the catalytic [2+2] cycloaddition/isomerization of siloxy alkynes and aldehydes, or oxocarbenium intermediates into the strategic synthesis of medium and large ring lactones.4 On the other hand, [2+2] cycloaddition/isomerization based on alkyne/imine metathesis also has been well-known, particularly for electron-rich alkynes.5 In most cases, the outcome of α,βunsaturated amides was described. On the contrary, such strategy used for 1-azadienes is far less advanced. The only one example reported by the Bergman group had a limited scope, which re-
Scheme 1. [2+2] Cycloaddition/isomerization via (hetero)butadiene intermediates 1-Azadienes bearing carbon-carbon double bond and conjugated imino functional groups are synthetically versatile building blocks participating in a substantial classes of (asymmetric) transformations. For example, 1,4-addition of C-,7 N-,8 B-,9 and P-10 nucleophiles, hetero Diels-Alder reaction11, annulation,12 and miscellaneous type of reactions13 have been well documented. In this regard, the development of efficient and practical synthetic strategies to generate 1-azadienes represents a significant task for organic chemists. The common approach to assemble 1-azadienes
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is mainly considered by means of condensation of α,β-unsaturated carbonyls and primary amines. This method requires the preformation of α,β-unsaturated carbonyls, and usually suffers from the undesired aza-Michael addition reaction, especially with α,βunsaturated ketones. The alternative synthetic strategies relying on the olefination reaction of aldehydes with α-phosphorated imines or enamines,14 or aza-Wittig reaction of phosphazenes and β,γ-unsaturated α-ketoesters,15 produce a large amount of phosphorus-containing waste products. The existing alkyne/imine metathesis strategy suffered from very limited scope and unusual catalyst system.6 Therefore, a simple reaction protocol with high atom economy under mild condition remains lacking and highly desirable. In an effort to address the issue as mentioned above and create a conceptually unprecedented paradigm of catalytic [2+2] cycloaddition/isomerization of imines for 1-azadienes, we turned our attention to investigate the reaction by using allenes and imines. Allenes possessing two cumulative unsaturated bonds are useful reactive components with rich regiochemistry for constructing various heterocycles and building blocks.16 In fact, [2+2] cycloaddition reactions of only activated imines with electron-rich allenes under Lewis acid catalysis condition,17 or with allenoates in the presence of Lewis base catalyst18 have already been realized by several groups. In these established protocols, fourmembered N-heterocycles containing at least one electronwithdrawing group were stable enough to isolate, and no example of rearrangement after [2+2] cycloaddition was reported. It should be noted that the utilization of unactivated imines in such mode of reactions remains essentially unexplored thus far. As a continuing research interest in allene chemistry,19 we reasoned that allenes bearing an electron-withdrawing group may be involved in the catalyzed [2+2] or [4+2] cycloaddition of unactivated imines based on their Pull-Push reactivity nature. If four-membered Nheterocycle ring without any electron-withdrawing groups is generated, it is possible to undergo reorganization under Lewis acid condition. With this in mind, allenoate 1a was selected at the outset, but no reaction was observed with either N-tosyl activated imine 2a or unactivated imines 2b and 2c in CH2Cl2 in the presence of a Ni(ClO4)2·6H2O catalyst (Table 1, entry 1). In view of the electron-withdrawing ability of the carbonyl group rendering Csp atom of allene more electron deficient than simple allenoate, allenyl imide 1b having an 2-oxazolidinone group for the convenience of being bound and activated by Lewis acids was synthesized.20 Imine 2a revealed type-mismatched in the model reaction (entry 2). Gratifyingly, when N-phenyl imine 2b was subjected, unanticipated imine-metathesis adduct 3bb featuring a 1-azadiene unit rather than cycloadduct was formed in 60% yield (entry 3). This outcome is totally in contrast to the observations made in the past. More electron rich imine 2c with a 2-methoxyphenyl (2OMePh) N-protecting group was proved to be more suitable with furnishing 1-azadiene 3bc in 85% yield after 12 h (entry 4). The structures of 3bb and 3bc were further confirmed by X-ray crystallographic analysis.21 A β-ketoimide by-product 4 stemming from the competing side addition of 1b and H2O, may from the NiII-catalyst, was observed in the NMR spectra. Fortunately, the removal of crystal H2O from the catalyst by using 5 Å MS before adding substrates resulted in a higher yield (92%, entry 5). The use of 4 Å MS additive gave inferior result (entry 6). Screening of other transition-metal Lewis acids was unable to offer better yields entries 7-9). Notably, an alkaline earth metal Lewis acid of Mg(ClO4)2 was also a good catalyst in this reaction (entry 10). CH2Cl2 solvent was beneficial to the highest outcome by comparing with THF and toluene (entries 11-12).
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O O
O
N •
N
PG
+
solvent, 5 Å MS rt
Me 1b
O O
O
O O
imine
1c
2a 2b or 2c
2 3 4 5d 6e 7d 8d 9d 10d 11d 12d
2a 2b 2c 2c 2c 2c 2c 2c 2c 2c 2c
product
3ba 3bb 3bc 3bc 3bc 3bc 3bc 3bc 3bc 3bc 3bc
PG
Me rac-3
Ph
O
O
N
Me Me 4
CO2Et 1a
entry
N
N
2a (PG = Ts) 2b (PG = Ph) 2c (PG = 2-OMePh)
•
Me
Lewis acid (10 mol%)
solvent
Lewis acid
t/h
yield/%b
CH2Cl2
Ni(ClO4)2·6H2O
24
none
CH2Cl2
Ni(ClO4)2·6H2O
24
none
CH2Cl2
Ni(ClO4)2·6H2O
24
60
CH2Cl2 CH2Cl2
Ni(ClO4)2·6H2O Ni(ClO4)2·6H2O
12 12
85 92
CH2Cl2 CH2Cl2
Ni(ClO4)2·6H2O Ni(OTf)2
12 12
67 40
CH2Cl2 CH2Cl2
Cu(ClO4)2·H2O Zn(ClO4)2·H2O
12 12
none 32
CH2Cl2
Mg(ClO4)2
12
78
THF toluene
Ni(ClO4)2·6H2O Ni(ClO4)2·6H2O
12 12
80 65
a
Reaction conditions: 1b (0.1 mmol), 2 (0.12 mmol), Lewis acid (10 mol%), additive (30 mg), solvent (0.25 mL). bIsolated yield. c Allenoate 1a was used. dThe mixture of Lewis acid and 5 Å MS was stirred at rt for overnight in the glovebox. e4 Å MS was used. Having identified the optimal reaction conditions of using allenyl imide 1b, Ni(ClO4)2·6H2O catalyst, CH2Cl2 solvent and 5 Å MS, the scope of differently substituted aromatic aldimines 2 was performed to delineate the importance of electronic and steric effects (Scheme 2). By placing a halogen group, such as F-, Cland Br-, at the para-position of phenyl, 1-azadienes 3bd-f were furnished in good to excellent level of yields. The incorporation of an electron-donating methyl group at the same position was not beneficial to a high yield (3bg, 53%), whereas electronwithdrawing groups including ester, nitro and trifluoromethyl were able to assist the rearranged products 3bh-j in comparable high yields. Aldimines with electron-neutral (2k), donating (2l) or withdrawing (2m) groups at the 3-position conferred 3bk-m with excellent results. In the case of 2-substituted aldimines, bromo and methyl groups were both tolerated. 1-Naphthyl aldimine 2p exhibited remarkably inferior result to the 2-naphthyl case presumably at the result of negative steric hindrance effect. 9-Anthryl azadiene 3br was isolated in 97% yield, and the heterocyclic example 2s also proved to be suitable in such type of conversion. O O
O •
N Me 1b
Table 1. Optimized reaction conditionsa
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N
+
PG
Ar 2 (PG = 2-OMePh)
O
Ni(ClO4)2 • 6H2O 5 Å MS, CH2Cl2 rt, 12 h
O
O
N
PG
N Me rac-3
Ar
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ACS Catalysis solvent with extreme exclusion of air or moisture. In light of imine 2l generated from benzyl aldehyde and 2-methoxyaniline, and the hydrolysis of 3bl to regenerate 2-methoxyaniline, it seems like a NiII-Lewis acid catalyzed formal intermolecular allene/aldehyde metathesis process in an indirect manner.24
Scheme 4. Gram scale synthesis and synthetic utility M O
O
Ph N Ph zwitterionic intermediate Int-2
O M O O
O N
•
Me Int-1
N
N
Me
Ph
M O O
Path A
O
Me Int-7
M
Ph O O
O O
O
Path B
M O
Ph
O N
N
Ph
Int-6
Int-1 ligand exchange N
Ph
O Ph
N Me cis-conformer
Figure 1. Proposed reaction pathway.
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Ph
Ph Me
1b and 2b
Intriguingly, when allenyl imide 1c without methyl substituent and N-phenyl imine 2b were applied under above standard conditions, 1,3-dienyl amine 3cb assumably isomerized from 1azadiene intermediate was preferentially constituted in 68% (Scheme 3). In order to evaluate the practicality of this [2+2] cycloaddition/isomerization, a scale-up reaction with 1b (3.0 mmol) and 2l was carried out, and 1.03 g of 3bl was collected in 86% yield. Based on the importance of γ,δ-unsaturated β-ketoesters in many catalytic asymmetric transformations,22 γ,δ-unsaturated βketoimide rac-5 was thereby prepared upon a simple hydrolysis procedure (Scheme 4). Even more interesting, enantioriched 5 could be recovered with up to 78:22 er in an undried benzene solvent without exclusion of air or moisture during the process, in the presence of a chiral phosphoric acid catalyst 6 or the combination of catalyst 6 and Hantzsch Ester 7.23 It suggests a dynamic kinetic asymmetric hydrolysis of rac-3bl along with the imineenamine isomerization (see supporting information). Unfortunately, no imine-reduction of rac-3bl was observed in a dried benzene
1,3-H shift
Me Int-5
N
Scheme 3. [2+2] Cycloaddition/isomerization of 1c and 2b
Ph N
Ph
N
O
H
N
O
N
H
Me Int-3
Ph H M = Ni(ClO4)2 H M Ph 1b and 3bb O 2b O N ligand exchange Ph O N Me Int-4 M Ph N O O
Scheme 2. Scope of aldimines 2
O
O
O
N
Ph
N Me Ph 3bb (tr ans)
ACS Catalysis Path B: with Ni(ClO4) 2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
TS3-5, 41.7
M = Ni(ClO4)2
Path A: without Ni(ClO4) 2
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36.6
34.4
34.2
25.8 22.5
TS1-2 21.6 9.4
1b+2b +M 0
O
Int-1
TS4-5 10.0
N
N Ph
N
Ph
TS5-6 4.1 0.2
Ph
Int-5 -13.3
Int-2
• Me
Int-4 1.7
O
Me
O N
TS3-4 3.6
M O
M
O
TS2-3 12.2
Int-2 11.2
Int-1 4.2 O
18.4
14.8
-15.8
Ph
-23.6
Int-3 -19.6
-26.4 M O O
M O O
O
N
Ph
N Int-6
N
Ph
Me In-7
Ph Me
O N
Int-6 -38.1
Ph
Int-7 -38.7
Figure 2. Gibbs free energy change profiles of the catalyzed and uncatalyzed reaction pathways through DFT calculations. The plausible reaction pathway of such Ni(ClO4)2-catalyzed [2+2] cycloaddition/isomerization reaction was depicted in Figure 1. On the basis of known information that 4-electrocyclic 2azetine ring without any electron-withdrawing groups readily fragments to 1-azadiene,25 the reactive intermediate Int-5 containing a 2-azetine ring is accordingly proposed. Through retrosynthetic analysis of forming Int-5, it probably involves a sequence of [2+2] cycloaddition and proton shift. In order to provide theoretical insight in support of the preferred reaction pathway, DFT computational study of enthalpy and Gibbs free energy changes in CH2Cl2 was implemented in the Gaussian 16 suite of programs (see supporting information). For comparison purposes, the Gibbs free energy change profiles of the favorable catalyzed and uncatalyzed pathways were both identified (Figure 2). To simplify the calculation, imine 2b was used as one model substrate combining with allenyl imide 1b. Stepwise [2+2] cycloaddition pathway is 20.5 kcal/mol lower in free energy barrier as compared to the concerted version (∆G⧧ = 46.3 kcal/mol, see supporting information), and apparently more reasonable. Firstly, the Ni(ClO4)2 Lewis acid activation of allenyl imide (∆Go = 4.2 kcal/mol) is essential to trigger the nucleophilic attack of the imine nitrogen lone pair on the sp-hybridized carbon atom with evolving a zwitterionic intermediate Int-2 (∆G⧧ = 17.4 kcal/mol). Then facile cycloaddition with a low free energy barrier (TS2-3, ∆G⧧ = 1.0 kcal/mol) is occurred to provide azetidine Int-3 containing an enamine substructure (∆Go = -19.6 kcal/mol). A proton shift from the azetidine ring to the close carbonyl oxygen in the 2-oxazolidinone moiety, rather than another carbonyl based on the 3D model, takes place to generate the NiII-catalyst stabilized Int-4. Subsequently, the second proton shift from Int-4 to Int-5 is achieved with a relatively lower level of free energy barrier (∆G⧧ = 8.3 kcal/mol) to the previous one (∆G⧧ = 23.2 kcal/mol) (Path A). As shown in the computational results, the NiII-catalyst appears to be in close contact (1.80-1.84 Å) to the carbonyl oxygen that is not involved in the proton transfer process, and this configuration helps to stabilize the negative charge of Int-4. However, Path B regarding as a direct 1,3-proton shift to generate Int-5 seems unlikely for the remarkably higher free energy barrier (∆G⧧ = 61.3 kcal/mol). Thus, the 2-oxazolidinone group plays a vital bridge role in the two-time proton shift transformations. Under the Lewis acid condition, Int-5 with all electron-rich groups is prone to undergo conrotatory ring opening for
cis-1-azadiene species Int-6 with a much lower free energy than that of the cycloadduct complex Int-3 (-38.1 vs -19.6 kcal/mol). This explains why it is difficult for us to isolate the [2+2] cycloadduct intermediates. Geometric rearrangement from cis- to the corresponding trans-species is energetically favorable, irrespective of whether it is complexed or not. Therefore, the ligand exchange between Int-6 and 1b, or Int-7 and 1b, are both likely to enable the NiII-catalyst turnover, thereafter the target trans-1azadiene 3bb is achieved at last. The Gibbs free energy change profiles of the overall processes clearly reveal the significance of the Ni(ClO4)2 catalyst, and this is consistent with the mild experimental condition. A caveat to the above discussion is that the possibility of [2+2] cycloaddition of alkynyl imide 1b’ (in situ isomerized from allenyl imide 1b) and imine leading to Int-5, and subsequent collapsing to form product 3bb was also considered initially. However, alkynyl intermediate 1b’ was not observed experimentally. Furthermore, the transformation from allenyl imide 1b to alkynyl imide 1b’ is energetically disfavored based on DFT calculations (∆Go = 12.2 kcal/mol) (Scheme 5, eq 1). On the contrary, when we attempted to cleave the TMS-group from trimethylsilylacetylene 8 with tetrabutylammonium fluoride to synthesize alkynyl imide 1c’, allenyl imide 1c was exclusively formed within a short reaction time (eq 2).26 Additionally, no reaction was found to occur when another alkynyl imide 1d’ exposed to imine 2c (eq 3). So this alternative pathway was finally ruled out.
Scheme 5. Control experiments with alkynyl imides
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ACS Catalysis In summary, we have established a novel allene/imine metathesis reaction with respect to [2+2] cycloaddition/isomerization for facilitating access to a series of interesting 1-azadienes with remarkable chemo- and regioselectivity (up to 98% yield), as well as a chiral γ,δ-unsaturated β-ketoimide. Under mild condition, a single cost-effective Ni(ClO4)2·6H2O catalyst was capable of implementing several tandem courses including a stepwise [2+2] cycloaddition as the turnover-limiting step, two-time proton shift and conrotatory ring opening of 2-azetine ring, which was carefully elucidated by the DFT calculations of Gibbs free energy change in CH2Cl2. The installation of an 2-oxazolidinone group in allenes played a key role in the unique reactivity and key proton transfer process. In comparison with the reported alkyne/imine metathesis, this fundamentally different strategy allows the compatibility of a broader range of aldimines, in particular with electronwithdrawing substituents. Further study of applying this [2+2] cycloaddition/isomerization protocol in an asymmetric manner and total synthesis of chiral γ,δ-unsaturated β-hydroxy esters and β-amino esters27 is underway in our laboratory.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental procedures, analytical and spectroscopic data for new compounds, and copies of NMR and HPLC spectra (PDF) Crystallographic data for 3bb (CIF) Crystallographic data for 3bc (CIF)
AUTHOR INFORMATION Corresponding Author *
[email protected] *
[email protected] ORCID Yi-Yong Huang: 0000-0001-6209-8304
Author Contributions ‡
S.P. and X.Y. contributed equally.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT Y.H. gratefully acknowledges the financial support of this investigation by the National Natural Science Foundation of China (Nos. 21573169, 21772151), and Wuhan Morning Light Plan of Youth Science and Technology (No. 2017050304010314). Y.Z. is supported in part by the Thousand Innovative Talents Plan of Chinese Government. We thank an anonymous reviewer for his suggestion of adding the solvation effect to our DFT mechanistic study.
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