Pd-catalyzed direct oxidative mono-aroyloxylation of O-aralkyl substituted acetoxime ethers

Abstract

A highly site-selective palladium-catalyzed ortho-mono-aroyloxylation of O-aralkyl substituted acetoxime ethers via direct Csp²–H bond activation has been developed with simple exo-acetoxime as a directing group. The broad scope of masked aralkylalcohols and various aromatic acid partners are compatible with this transformation, which should undergo a mechanistic pathway of six, seven, or even eight-membered exo-cyclopalladated intermediates. In addition, the acetoxime directing group can be readily removed through N–O bond selective cleavage at a late stage, providing a potential utility for the preparation of valuable functionalized aromatic alcohols.

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Introduction

Transition metal-catalyzed functionalization of C–H bonds assisted by directing groups has been rapidly developed and emerged as a powerful tool in organic synthesis.¹ This strategy achieves highly site-selective C–H transformation in a step- and atom-economical manner. Among these important reactions, a variety of nitrogen-containing directing groups such as pyridine,² quinoline,³ imine,⁴ and others⁵ are widely exploited.

In the last decade, the pioneering work by Sanford⁶ and other following work⁷ has demonstrated that the oxime methyl ether moiety possesses an excellent directing ability for C–H bond activation to construct C–O, C–N, C–X (X = halogen), and C–C bonds via a five-membered endo-cyclopalladated intermediate (π-bond of the directing group inside the metallocycle, Scheme 1A). On the other hand, the oxime ethers directed C–H bond activation/functionalization via exo-cyclopalladated intermediate still remains to be further developed, despite a few elaborate examples reported (π-bond of the directing group outside the metallocycle, Scheme 1B).^8,9

Scheme 1 endo- and exo-cyclopalladated intermediates.

To realize exo-oxime ethers as powerful directing groups, Dong⁸ wisely constructed O-aliphatic, benzyl, or secondary benzyl substituted oxime ethers to successfully attain a selective C–H transformation in O-substituted sides. The catalytic cycle should undergo a five- or six-membered exo-cyclopalladated pathway, and the endo-site selectivity could be avoided by blocking the potential reactive position of aldehyde moiety (Scheme 2A). More recently, Zhao⁹ nicely employed a simple acetoxime directing group to implement C–H transformation in O-substituted side, thus the challenging task of six-membered exo-cyclopalladation¹⁰ could be overcome (Scheme 2B).

Scheme 2 Direct C–H functionalization in O-substituted side of oxime ethers.

Aralkylalcohols belong to versatile synthetic precursors in synthetic chemistry,¹¹ and continuous advances have been acquired in transition metal-catalyzed direct C–H bond functionalization of aralkylalcohol derivatives.¹² As aralkylalcohols are convenient to be incorporated into the acetoxime ethers, and the relevant C–H bond activation is limited. Thus, new functionalization still deserves further expansion. Based on the previous works of C–H bond activation/C=O and C–O bond formation,¹³ we utilized the potential directing group to carry out a novel ortho-mono-aroyloxylation for O-aralkyl substituted acetoxime ethers. Compared to arylmethyl groups, phenethyl group is also compatible for the protocol. Notably, direct functionalization of phenylprophyl group via an eight-membered exo-cyclopalladated intermediate is firstly achieved, albeit other advances on direct C–H activation via eight-membered palladacylces.¹⁴ In this reaction, the system of undecorated catalyst Pd(OAc)₂ with a combination of cheap oxidant K₂S₂O₈, proved to be highly effective to accomplish the conversion under mild conditions. Aroyloxylation reaction still remains scarce in the C–H bond activation field.

Results and discussion

Initially, we treated model substrates O-benzyl substituted acetoxime ether 1a and benzoic acid 2a by employing Pd(OAc)₂ (10 mol%) as catalyst and K₂S₂O₈ (2.0 equiv.) as oxidant in DCE at 80 °C, in order to develop an easily accessible aroyloxylation protocol. The mono-benzoyloxylated product 3a was isolated in 67% yield, without dibenzoyloxylated product observed (Table 1, entry 1). In subsequent control experiments, other oxidants including oxone, PhI(OAc)₂, Na₂S₂O₈, TBHP and AgOAc were inspected. Oxone, PhI(OAc)₂ and Na₂S₂O₈ brought about diminished yields (entries 2–4), while AgOAc and TBHP was inefficient (entries 5 and 6). In comparison to DCM and DMSO, screening of solvents uncovered CH₃CN gave the best efficiency (entries 7–9). Remarkably, increasing the loading of oxidant K₂S₂O₈ to 3.0 or 4.0 equiv. could afford an improved yield of 76% (entries 10 and 11), still without difunctionalized product detected. Furthermore, the experiments revealed that the reaction temperature of 80 °C was more suitable for the conversion (entries 12 and 13). Meanwhile, we also attempted other Pd catalysts, and the results proved Pd(OAc)₂ to be the best. (η³-C₃H₅)₂Pd₂Cl₂ led to an inferior efficiency (entry 14), while PdCl₂ and (Ph₃P)₂PdCl₂ were inefficient (entries 15 and 16). It was found that increasing catalyst loading was unable to facilitate the reaction (entry 17), and lowering catalyst loading led to a reduced yield (entry 18). Therefore, we established entry 10 as the standard reaction conditions. To further verify the product geometrical configuration, the NMR-based structure of 3a was determined by X-ray single crystal crystallography (Fig. 1, see the ESI†).

Table 1 Investigations of the reaction parameters^a

Entry	Catalyst (n1 mol%)	Oxidant (n2 equiv.)	Solvent	Yield^b (%)
a Reaction conditions: 1a (1.0 mmol), 2a (2.0 mmol), catalyst (n1 mol%), oxidant (n2 equiv.), solvent (6 mL) at 80 °C for 10 h.b Isolated yields.c 60 °C.d 100 °C. DMSO = dimethylsulfoxide; DCM = dichloromethane; DCE = 1,2-dichloroethane.
1	Pd(OAc)2 (10)	K2S2O8 (2.0)	DCE	67
2	Pd(OAc)2 (10)	Oxone (2.0)	DCE	65
3	Pd(OAc)2 (10)	PhI(OAc)2 (2.0)	DCE	61
4	Pd(OAc)2 (10)	Na2S2O8 (2.0)	DCE	59
5	Pd(OAc)2 (10)	TBHP (2.0)	DCE	0
6	Pd(OAc)2 (10)	AgOAc (2.0)	DCE	0
7	Pd(OAc)2 (10)	K2S2O8 (2.0)	CH3CN	71
8	Pd(OAc)2 (10)	K2S2O8 (2.0)	DCM	61
9	Pd(OAc)2 (10)	K2S2O8 (2.0)	DMSO	0
10	Pd(OAc)2 (10)	K2S2O8 (3.0)	CH3CN	76
11	Pd(OAc)2 (10)	K2S2O8 (4.0)	CH3CN	76
12^c	Pd(OAc)2 (10)	K2S2O8 (3.0)	CH3CN	70
13^d	Pd(OAc)2 (10)	K2S2O8 (3.0)	CH3CN	55
14	(η^3-C3H5)2Pd2Cl2 (10)	K2S2O8 (3.0)	CH3CN	31
15	PdCl2 (10)	K2S2O8 (3.0)	CH3CN	0
16	(Ph3P)2PdCl2 (10)	K2S2O8 (3.0)	CH3CN	0
17	Pd(OAc)2 (15)	K2S2O8 (3.0)	CH3CN	67
18	Pd(OAc)2 (5)	K2S2O8 (3.0)	CH3CN	65

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Fig. 1 Single crystal structure of 3a.

With the standard reaction conditions in hand, we turned our attention to evaluate the scope of acid partners. As depicted in Scheme 3, it was observed that a wide range of functional groups including Me, MeO, multi-F, Cl, NO₂ and CF₃ groups were tolerated. The aromatic acid substrates with electron-donating groups suffered from the protocol to provide good yields of 74–81% (3b, c, g, i and j). With respect to the substrates containing electron-withdrawing groups, the relatively lower yields of 51–69% were presented (3d–f, h, k and l). Partial starting materials still remained intact even prolonging reaction time to 15 h, and no byproduct was observed. Impressively, 2-thenoicacid and E-cinnamic acid were also amenable to the protocol (3m and n). However, 2-picolinic acid completely failed to give the desired product, possibly due to the strong chelation effect of the nitrogen atom in picolinic acid with Pd, which inhibited the indispensable catalyst turnover (3o). Furthermore, acetic acid and propionic acid were subjected to the reaction to deliver moderate yields of 74% and 63% (3p and 3q), respectively, illustrating that aliphatic carboxylic acids are compatible with the protocol.

Scheme 3 Substrate scope of aromatic acids. Reaction conditions: 1 (1.0 mmol), 2 (2.0 mmol), Pd(OAc)₂ (10 mol%), K₂S₂O₈ (3.0 equiv.) in CH₃CN (6 mL) at 80 °C for 10 h; [a] reaction for 15 h.

Next, various O-arylmethyl substituted acetoxime ethers were prepared to test the generality of masked aralkylalcohols with this reaction (Scheme 4). Diverse functional groups were tolerated, such as Me, MeO, F, Cl and Br groups. In particular, the reactive Cl and Br atoms, as well as the late-stage introduced aroyloxy group, were readily elaborated for further synthetic transformation. The electron-donating group substituted substrates delivered the corresponding aroyloxylated products in good yields of 74–84% (4a, d, f and k). Meanwhile, the electron-withdrawing group substituted substrates gave reasonable results, albeit with relatively lower yields (4b, c, e and g). With respect to the meta-substituted benzyl moieties, the benzoyloxylation tended to happen at sterically less hindered positions (4d and e). In a sharp contrast, as for the substrate with meta-methoxy, the reaction particularly took place at sterically higher hindered ortho-position to deliver good yield of 84%, indicating the coordination of oxygen atom in the methoxy group could potentially stabilize the arylpalladium intermediate (4k).¹⁵ Substrates derived from secondary benzyl alcohols also worked well under current conditions (4h–j). Especially, highly crowded diphenylmethanol-derived substrate also supplied the desired mono-benzoyloxylated product in moderate yield (4j). Again, diaroyloxylation reaction was not observed in the reaction system. It is probable that the sterically bulky hindrance of first aroyloxylation would suppress the possible second formation of highly crowded palladacycle oxidative additive.¹⁶

Scheme 4 Substrate scope of masked aralkylalcohols. Reaction conditions: 1 (1.0 mmol), 2 (2.0 mmol), Pd(OAc)₂ (10 mol%), K₂S₂O₈ (3.0 equiv.) in CH₃CN (6 mL) at 80 °C for 10 h.

Particularly, O-phenylacetoxime failed to provide any desired product (Scheme 5, 5a), thus illustrating that the semi-rigid five-membered exo-cyclopalladated intermediate with strong intramolecular torsion could hardly be generated. Compared to the product 3a resulting from six-membered exo-cyclopalladated intermediate, the O-phenethyl substituted acetoxime afforded a good yield of 69% via a rare seven-membered exo-cyclopalladated intermediate (5c).^9a To our delight, O-phenylprophyl moiety could also be directly modified at ortho-position in a synthetically useful yield of 42%, through a rarer eight-membered exo-cyclopalladated intermediate (5d). To our knowledge, the remote C–O bond formation of O-phenylprophyl group at ortho-position is firstly achieved.

Scheme 5 Substrate scope of extended aralkylalcohols. Reaction conditions: 1 (1.0 mmol), 2 (2.0 mmol), Pd(OAc)₂ (10 mol%), K₂S₂O₈ (3.0 equiv.) in CH₃CN (6 mL) at 80 °C for 10 h.

Reasonably, we demonstrated that the strategy could be adopted to expediently remove the acetoxime directing group (Scheme 6). Treatment of 3a with Mo(CO)₆ in CH₃CN/H₂O (4 : 1) under nitrogen atmosphere, in which the N–O bond was selectively cleaved,¹⁷ furnished the versatile functionalized benzyl alcohol 6 in good yield, thus achieving a synthetically valuable transformation.

Scheme 6 Selectively removal of the acetoxime directing group.

To gain insight into the reaction mechanism, deuterium labeling kinetic isotope experiment was conducted in the Pd(OAc)₂-catalyzed C–H aroyloxylation. As shown in Scheme 7, the intermolecular competition reactions between 1a and 1a′ (1a-d₇) with benzoic acid in one vessel demonstrated a notable primary kinetic isotopic effect (k_H/k_D = 3.44; see the ESI†). The result suggests that the Csp²–H bond cleavage might be involved in the rate-limiting step.^13c,d

Scheme 7 Kinetic isotope effect study.

On the basis of experimental investigations and the well-documented reports,^8,9,18 a plausible mechanism involving a Pd(II)/Pd(IV) catalytic cycle is depicted for the mono-aroyloxylation of the O-aralkyl substituted oxime compounds (Scheme 8). Firstly, Pd(II) coordinates with nitrogen atom of the substrate 1a, thus inducing a formation of the cyclopalladated intermediate I by chelate-directed C–H activation. This Pd(II) intermediate is oxidized by K₂S₂O₈ in the presence of benzoic acid to afford a hexacoordinated Pd(IV) adduct II, which proceeds with a subsequent reductive elimination to furnish the final ortho-aroyloxylated product 3a and regenerate the Pd(II) catalyst. Here, the generated Pd(IV) adduct II would deactivate the arene ring for further substitution and disfavor overoxidation, and accordingly make the protocol regioselectively.¹⁶ According to the well- characterized exo-cyclopalladated active intermediates^8b,9b and the reported functionalization of 3,5-diphenylisoxazoles,¹⁹ the coordination of palladium occurs at the nitrogen atom of substrate rather than the oxygen atom.

Scheme 8 Proposed mechanism for the aroyloxylation of the O-arylalkyl substituted oxime compounds.

Conclusions

In conclusion, we have developed a Pd-catalyzed direct mono-aroyloxylation of O-aralkyl substituted oxime ethers with acetoxime as a powerful directing group. The exo-acetoxime directing strategy turned out to be efficient for aromatic Csp²–H bond oxidative functionalization. With the wide range of aralkyl patterns and aromatic acid partners, a variety of 2-aroyloxy aromatic alcohol derivatives can be synthesized in moderate to good yields. This transformation should go through a pathway of six-, seven-, or even eight-membered exo-cyclopalladated intermediates. Additionally, the acetoxime directing group can be readily removed to offer synthetically valuable functionalized aralkylalcohols.

Acknowledgements

We gratefully thank the National Natural Science Foundation of China (Project No. 21176074 and 21476074) and Research Fund for the Doctoral Program of Higher Education of China (Project No. 20130074110009) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1486709. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra16105g

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