Ni-Catalyzed 1,2-iminoacylation of alkenes via a reductive strategy

Lin Wang and Chuan Wang*
Department of Chemistry, Center for Excellence in Molecular Synthesis, Hefei National Laboratory for Physical Science at the Microscale, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 20237, P. R. China. E-mail: chuanw@ustc.edu.cn

Received 27th September 2018 , Accepted 23rd October 2018

First published on 25th October 2018


In this protocol, we developed a reductive strategy for 1,2-iminoacylation of alkenes. Under the catalysis of the Ni-biquinoline system, various oxime esters incorporating a pendant terminal olefinic unit were successfully reacted with acid chlorides or anhydrides as electrophilic acylating reagents in the presence of Zn as a reductant, furnishing a series of pyrrolines in moderate to excellent yields. This reaction is distinguished by safe and mild reaction conditions that avoid the use of CO gas as a carbonyl source, pregenerated organometallics and strong bases as reaction additives.


Introduction

Difunctionalizations of inactivated C–C double bonds are among the most important organic transformations, as diverse bifunctional compounds can be accessed from simple olefins. Tremendous progress has been achieved in this field using transition metal catalysis based on a redox-neutral1 or oxidative strategy,2 in which a nucleophile and an electrophile or two nucleophiles are directly installed onto the unsaturated substrates, such as dicarbonation,1a–g carboamination,1h–o,2f–i carboalkoxylation,1p–r,2j–n aminooxygenation,1s–x,2o–t diamination,2a–e dialkoxylation,2u,v etc. In contrast, although cross-electrophile-coupling has proved to be a reliable and powerful tool in organic synthesis,3,4 the reactions involving introduction of two electrophiles into an olefinic unit with the assistance of a reducing agent are much less developed and only a limited number of successful examples have been reported to date, which all focus on dicarbofunctionalizations utilizing aryl or alkyl halides as precursors.5 To the best of our knowledge, the implementation of this conceptually new reductive strategy to achieve heteroatom-carbofunctionalization of alkenes still remains elusive. Our attention was drawn to 1,2-iminoacyclation reaction, in which synthetically useful imino and carbonyl groups can be introduced across the C–C double bonds in one single step. Recently, Bower et al. reported a redox-neutral Pd-catalyzed 1,2-iminoacylation reaction of diverse alkenes incorporating an oxime ester with carbon monoxide and organoborons providing multi-substituted pyrrolines with high efficiency (Scheme 1A).6 However, the use of toxic carbon monoxide gas and organoborons requiring prefunctionalization from the corresponding halides is still less desirable from the viewpoint of safety and step-economy. Therefore, we envisaged a Ni-catalyzed 1,2-iminoacylation of oxime ester-tethered alkenes via a reductive strategy using commercially available acid chlorides or anhydrides as the source of acyl moiety with the assistance of an inexpensive reductant under base-free conditions (Scheme 1B).
image file: c8qo01044g-s1.tif
Scheme 1 Pd-Catalyzed redox-neutral 1,2-iminoacylation (eqn (A)) and Ni-catalyzed reductive 1,2-iminoacylation (eqn (B)).

Results and discussion

For optimization of the reaction conditions, we used the benzoyl oxime ester 1a-1 tethered with a terminal olefinic unit and benzoyl chloride (2a-1) as standard substrates (Table 1). Systematic screening of Ni-salts, ligands, reducing agents, temperature and solvents provided the optimum conditions, under which the desired product 3a was obtained in 77% yield (entry 1). The use of other Ni-sources including Ni(COD)2, Ni(dme)Br2 and Ni(acac)2 gave rise to lower yields (entries 2–4). Ligand screening indicated that all the reactions employing pyridine-based ligands L2–L4 could afford the product 3a in moderate yields (entries 5–7), while in the case of the bis(oxazoline) ligand L5 only traces of product were formed (entry 8). Performing the reaction in polar solvents including DMF and DMPU led to decreased yields (entries 9 and 10), whereas the reaction was completely shut down using dipropyl ether as the solvent (entry 11). Replacing the reducing agent Zn by Mn did not improve the yield (entry 12). In the absence of the reductant, no reaction occurred at all (entry 13). When the reaction was conducted in the dark, it still proceeded smoothly (entry 14). Moreover, raising or lowering the reaction temperature resulted in diminished efficiency (entries 15 and 16). In addition, the influence of the leaving group LG of the oxime ester substrates was also investigated (entries 17–19). In the case of OPiv as the leaving group, the desired product was also obtained in a moderately good yield (entry 19). Notably, in the case of benzoic acid anhydride (2a-2) as the acyl source instead of benzoyl chloride (2a-1), the reaction furnished the product in an excellent yield (entry 20).
Table 1 Variation of the reaction parametersa

image file: c8qo01044g-u1.tif

Entry Variation from the optimum conditions Yieldb (%)
a Unless otherwise specified, reactions were performed on a 0.20 mmol scale of the oxime ester 1a using 2.0 equiv. benzoyl chloride (2a-1), 10 mol% Ni(dme)Cl2, 10 mol% ligand L1 and 3.0 equiv. Zn at 40 °C for 6 h in 1 mL solvent (i-Pr2O[thin space (1/6-em)]:[thin space (1/6-em)]DMF = 7[thin space (1/6-em)]:[thin space (1/6-em)]3).b Yields were determined by H-NMR using 1,3,5-trimethoxybenzene as an internal standard.c Yields of the isolated product.d Bz5F: pentafluorobenzoyl.
1 None 81 (77c)
2 Ni(COD)2 instead of Ni(dme)Cl2 58
3 Ni(dme)Br2 instead of Ni(dme)Cl2 64
4 Ni(acac)2 instead of Ni(dme)Cl2 27
5 L2 instead of L1 32
6 L3 instead of L1 5
7 L4 instead of L1 58
8 L5 instead of L1 Traces
9 DMF instead of i-Pr2O[thin space (1/6-em)]:[thin space (1/6-em)]DMF (7[thin space (1/6-em)]:[thin space (1/6-em)]3) 74
10 DMPU instead of i-Pr2O[thin space (1/6-em)]:[thin space (1/6-em)]DMF (7[thin space (1/6-em)]:[thin space (1/6-em)]3) 43
11 i-Pr2O instead of i-Pr2O[thin space (1/6-em)]:[thin space (1/6-em)]DMF (7[thin space (1/6-em)]:[thin space (1/6-em)]3) 0
12 Mn instead of Zn 10
13 Without Zn 0
14 Excluding light 76
15 R.T. instead of 40 °C 52
16 60 °C instead of 40 °C 53
17 LG: OPiv (1a-2) instead of LG: OBz 68
18d LG: OBz5F (1a-3) instead of LG: OBz 17
19 LG: OAc (1a-4) instead of LG: OBz 40
20 Bz2O (2a-2) instead of BzCl (2a-1) 90


After establishing the best reaction conditions we started to evaluate the substrate spectrum of this reaction. We first reacted diverse aromatic and heteroaromatic oxime esters tethering a terminal olefinic unit with benzoic acid anhydride (2a-2) (Table 2). Generally, all the reactions proceeded smoothly under the optimum reaction conditions, providing the cyclization products 3a–m in moderate to excellent yields. Notably, simple oxime esters without geminal disubstitution were also suitable substrates for this Ni-catalyzed reaction (3j, 3k and 3m), albeit in lower yields. Furthermore, the desired cyclization products 3l and 3m were also obtained starting from monosubstituted alkenes, which are prone to undergo β-H elimination to yield the aza-Heck-products.7 In addition, benzoyl chloride (2a-1) also turned out to be a pertinent acylating reagent for all the Ni-catalyzed reactions mentioned above. In most cases, the reactions using benzoyl chloride (2a-1) provided the products in relatively low yields compared to the ones with benzoic acid anhydride (2a-2). Exceptions were observed in the case of substrates without geminal disubstitution, for which benzoyl chloride proved to be more suitable (3j, 3k and 3m). The limitation of this reaction was observed in the case of reactions employing internal alkenes, which failed to deliver the products either in an analytically pure form or at a synthetically useful level.

Table 2 Evaluation of the substrate scope of the Ni-catalyzed 1,2-iminoacylation by varying the structure of the oxime estersa,b
a Unless otherwise specified, reactions were performed on a 0.25 mmol scale of the oxime esters 1 using 2.0 equiv. benzoyl chloride (2a-1) or benzoic acid anhydride (2a-2), 10 mol% Ni(dme)Cl2, 10 mol% ligand L1 and 3.0 equiv. Zn at 40 °C for 6 h in 1 mL solvent (i-Pr2O[thin space (1/6-em)]:[thin space (1/6-em)]DMF = 7[thin space (1/6-em)]:[thin space (1/6-em)]3).b Yields of the isolated products.c Yields for the reactions using benzoic acid anhydride (2a-2).d Yields for the reactions using benzoyl chloride (2a-1)
image file: c8qo01044g-u2.tif


Subsequently, we continue to explore the substrate scope of this Ni-catalyzed reaction by varying the structure of the acyl-sources. First, we chose acid chlorides as substrates, since they are more desirable to utilize than anhydrides, considering the commercial availability and the atom-economy. It turned out that the reactions using the O-benzoyl oxime 1a-1 as the substrate provided not only the desired products but also the undesired compound 3a (Scheme 2). This result indicated that the cleaved benzoate group possibly reacts with the acid chlorides in situ forming a mixed anhydride, which could in turn act as an acylating reagent in this Ni-catalyzed reaction and competes with the original aryl acid chlorides. Since compound 3a was afforded as the only product in the case of pivaloyl chloride as the precursor, it suggested that the in situ formed benzoic pivalic anhydride could transfer its benzoyl moiety to the olefinic unit with complete selectivity. Relying on this result, we reacted O-pivaloyl oxime 1a-2 instead of its benzoyl analogue 1a-1 with various acid chlorides under the optimized reaction conditions (Table 3). Generally, in the case of aryl and heteroaryl acid chlorides the products 3n-aa were obtained in moderate to good yields. Furthermore, we have also investigated the reactions using anhydrides as reactants for this iminoacylation reaction, furnishing the products in higher yields than the ones starting from the corresponding acid chlorides (entries 3–6, 8, 9, 12 and 13). However, the reactions utilizing aliphatic acid chlorides or anhydrides failed to deliver the desired products. Notably, compounds 3r, 3x and 3aa, which contain a chloride, an ester or a Bpin moiety, respectively, would be difficult to access via the known Pd-catalysed iminoacylation6 owing to either the chemoselectivity issue in the Pd-catalysis or the functionality compatibility in the preparation of the boronic acid precursors.


image file: c8qo01044g-s2.tif
Scheme 2 Ni-Catalyzed 1,2-iminoacylation using O-benzoyl oxime (1a-1).
Table 3 Evaluation of the substrate scope of the Ni-catalyzed 1,2-iminoacylation by varying the structure of the acid chlorides or anhydridesa

image file: c8qo01044g-u3.tif

Entry R Yieldb (%)
a Unless otherwise specified, reactions were performed on a 0.25 mmol scale of the oxime ester 1a-2 using 2.0 equiv. aroyl chlorides 2, 10 mol% Ni(dme)Cl2, 10 mol% ligand L1 and 3.0 equiv. Zn at 40 °C for 6 h in 1 mL solvent (i-Pr2O[thin space (1/6-em)]:[thin space (1/6-em)]DMF = 7[thin space (1/6-em)]:[thin space (1/6-em)]3).b Yields of the isolated products.c Yields for the reactions using the corresponding anhydrides instead of the acid chlorides.d O-Acyl oxime 1a-4 was used instead of 1a-2.e Pentafluorobenzoyl oxime ester 1a-3 was used instead of 1a-2.
1 4-MeC6H4 (3n) 51
2 3,5-(Me)2C6H3 (3o) 60
3 4-t-BuC6H4 (3p) 68 (93c)
4 4-FC6H4 (3q) 58 (78c)
5 4-ClC6H4 (3r) 36 (70c)
6 3-MeOC6H4 (3s) 55 (96c)
7 4-MeOC6H4 (3t) 35
8 3,5-(MeO)2C6H3 (3u) 59 (85c)
9 3,4,5-(MeO)3C6H2 (3v) 48 (81c)
10 2-Piperonyl (3w) 35
11 2-EtOC6H4 (3x) 32d
12 2-Naphthyl (3y) 55 (88c)
13 2-Furyl (3z) 35e (46c)
14 4-PinBC6H4 (3aa) 42


Considering that both acid chlorides and anhydrides are usually prepared from the carboxylic acids, we decided to study the feasibility of performing the iminoacylation reaction employing carboxylic acids as precursors, in order to enhance the step-economy (Scheme 3). Therefore, we attempted the reaction using 3,4,5-trimethoxy benzoic acid as the substrate with (Boc)2O as an additive, which would in situ form an unsymmetrical anhydride. To our delight, this reaction could provide the desired product 3v in the same yield as the one using acid chloride as the substrate.


image file: c8qo01044g-s3.tif
Scheme 3 Ni-Catalyzed 1,2-iminoacylation using a carboxylic acid as the precursor.

Since the cyclization products contain a synthetically useful imine moiety, two derivatizations involving the conversion of imine were conducted (Scheme 4). Complete chemoselectivity was achieved in the case of NaBH(OAc)3-mediated reduction of 3a furnishing two diastereomers of the β-aminoketone 4 with high efficiency, which could be easily separated through flash chromatography. Moreover, the [3 + 2] cycloaddition of 3a with N-hydroxybenzimidoyl chloride afforded a 1,2,4-oxadiazoline 5 in a high yield.


image file: c8qo01044g-s4.tif
Scheme 4 Derivatizations of the iminoacylation product 3a.

To shed light on the mechanism of this Ni-catalyzed reaction, we conducted a series of control experiments (Scheme 5). First, a stoichiometric reaction between Ni(COD)2 and benzoyl chloride (2a-1) was carried out in the presence of the ligand L1.4d,8 Upon completion of the reaction, 1 equiv. oxime ester 1a-1 was added to the reaction mixture and after 1 h 1a-1 was observed to be recovered (Scheme 5A). Next, we performed the stoichiometric reaction between Ni(COD)2 and the oxime ester 1a-1. After 30 min no conversion of 1a-1 was observed and treatment with 1 equiv. benzoyl chloride (2a-1) did not yield the 1,2-iminoacylation product 3a (Scheme 5B). In contrast, a full consumption of 1a-1 was achieved within 30 min in the stoichiometric reaction between 1a-1 and Ni(dme)Cl2 in the presence of 3 equiv. Zn. The following addition of 1 equiv. benzoyl chloride to the reaction mixture afforded the product 3a in a 50% yield. If the reaction mixture was quenched by using water or tBuSH before adding benzoyl chloride, a linear ketone 3a-1 was afforded as the product and no cyclized product 3a-2 was formed in either case (Scheme 5C). The aforementioned results indicate that this Ni-catalyzed iminoacylation is likely initiated through the interaction between the oxime ester moiety with an in situ generated Ni(I)- instead of Ni(0)-species and the cyclization cannot proceed in the absence of acid chlorides or anhydrides. Moreover, a radical clock reaction employing an oxime ester 1n incorporating a cyclopropyl-substituted olefinic unit was carried out under the standard reaction conditions (Scheme 5D). In this case, the formation of the cyclopropane-ring-opening product 3ab-2 was not observed, while compound 3ab-1 was obtained in a diastereomeric ratio of 2[thin space (1/6-em)]:[thin space (1/6-em)]1. This result suggests that our reaction probably does not involve an iminyl radical, which has been reported as a key intermediate in the Ni-catalyzed reactions involving oxime esters in the previous reports.9 However, the low diastereoselectivity does imply that the carbon–carbonyl-bond might be formed in a radical pathway. To explain this observation, we propose that acyl radicals might form in the reaction mixture and undergo addition to the tethered olefinic unit.


image file: c8qo01044g-s5.tif
Scheme 5 Mechanistic investigations of the Ni-catalyzed 1,2-iminoacylation.

Based on the experimental results mentioned above, we proposed a plausible mechanism for this Ni-catalyzed 1,2-iminoacylation (Scheme 6). Initially, a Ni(I)–Ln complex I is formed under the reductive reaction conditions, followed by the oxidative addition with the oxime esters 1. The resultant Ni(III) species II was then reduced by Zn to provide a Ni(I) complex III. Subsequently, acid chloride or anhydride 2 interacts with the generated Ni(I) complex III to give a cage IV consisting of a Ni(II)-species and an acyl radical, which can recombine with each other rapidly via a radical addition to the C–C double bond followed by cyclization. In the final step, the cyclic Ni-intermediate V undergoes the reductive elimination furnishing the corresponding pyrrolines 3 as products and the Ni(I) complex I for the next catalytic cycle.


image file: c8qo01044g-s6.tif
Scheme 6 Proposed mechanism for the Ni-catalyzed 1,2-imino-acylation.

Conclusions

In summary, we developed a Ni-catalyzed 1,2-iminoacylation of oxime ester-tethered alkenes with acid chlorides or anhydrides as an acylating reagent, providing a convenient and efficient entry to a variety of multi-substituted pyrrolines. Remarkably, it is the first example of reductive difunctionalization of inactivated C–C double bonds involving a carbon-heteroatom-bond formation. The successful avoidance of using pregenerated organometallics, strong basic additives and CO gas makes this new method desirable for practical use from the viewpoint of safety and step-economy. In addition, a mechanism involving an acyl radical was proposed based on experimental results.

Conflicts of interest

There is no conflict to declare.

Acknowledgements

This work was supported by “1000-Youth Talents Plan” starting up funding, the National Natural Science Foundation of China (Grant No. 21772183), and the Fundamental Research Funds for the Central Universities (WK2060190086), as well as by the University of Science and Technology of China.

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Experimental procedure, spectral data, NMR data, and HPLC data. See DOI: 10.1039/c8qo01044g

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