N-Acylsulfonamide Assisted Tandem C-H Olefination/Annulation: Synthesis of Isoindolinones

Abstract

A tandem C-H olefination/annulation sequence directed by N-acylsulfonamides affords a variety of isoindolinones. This transformation is compatible with aliphatic alkenes as well as conjugated alkenes. Notably, molecular oxygen can be used as the sole, eco-friendly oxidant.

Pd(II)-mediated aromatic C(sp2)-H olefinations (Fujiwara-Moritani reaction) have emerged as a powerful tool for C-C bond formation.^{1, 2} Indeed, the utility and unique characteristics of this reaction have been cogently demonstrated in the total synthesis of polyfunctional natural products of current interest and for the efficient construction of bioactive scaffolds.³ Generally, nitrogen- or oxygen-containing directing groups are required for acceptable reactivity and selectivity using the Fujiwara-Moritani reaction.^{4, 5} Hence, it is apparent that both synthetic efficiency and atom economy would be greatly extended if the heteroatom served as both the directing group and as the precursor to a C-X bond (X=N, O). An early example of this concept was provided by Miura and co-workers who revealed benzoic acid could direct tandem C-H olefination and C-O formation, albeit in unsatisfying yield;⁶ more recently, the Yu group demonstrated that a triflamide or tertiary hydroxyl were competent directing groups for the cascade process of C-H olefination/C-X bond formation resulting in six-membered heterocycles.^7,8 However, in those cases, the annulation was strictly limited to electron-deficient olefins. Herein, we disclose tosyl amide as a new directing group that affords the isoindolinone motif via a sequence of C-H olefination followed by C-N bond formation. Another notable feature is the use of molecular oxygen as the sole oxidant and shows the preference for the Pd^II/Pd⁰ redox cycle. Importantly, this transformation is broadly compatible with aliphatic alkenes as well as activated alkenes; the specific type of adduct formed is dictated by the electronic properties of the alkene (eq 1).

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For the initial survey of reaction conditions and optimization, N-tosylbenzamide and tert-butyl acrylate were selected as the model substrates since they were commercially available and amenable to deprotection and/or manipulation following annulation. Subsequent screening campaigns established some important reaction parameters:⁹ (i) Pd(OAc)₂ performed as well or better than other commercial Pd(II)-catalysts, (ii) bathophenanthroline was much superior to other intermediaries of the catalytic Pd^II/Pd⁰ redox cycle, and (iii) toluene furnished better yields than THF, CH₃CN, DMSO, and other common solvents. While benzoquinone (BQ) is one of the most commonly used oxidant for Pd(II) catalyzed C-H activations, and also works in our reaction, we were delighted to discover eco-friendly molecular oxygen had a higher turnover number than BQ and could function as the sole oxidant in the absence of other co-oxidants.¹⁰ Indeed, the anticipated annulation adduct 3a was isolated in good chemical yield when the optimized conditions were employed (eq 2).

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Encouraged by this result, we applied the method to a variety of substituted N-tosylbenzamides (Scheme 1). The tandem process readily provided isoindolinones regardless of the electronic properties of the aryl substituent(s). For instance, the electron-donating methoxy (3c) and methyl group (3d) as well as fluoride (3f) all furnished their respective adducts in high yields. The chemoselective formation of 3g, without competitive Heck reaction by the aryl bromide is noteworthy, as the latter provides functionality for subsequent Pd(0)-mediated cross coupling reactions. Acceptable yields of adduct when strong electron–withdrawing groups were present, e.g., CF₃, generally required prolonged reaction times (3e). The facile annulation of 3h compared with the somewhat sluggish closure of the regioisomeric 3i is also of interest and can be attributed to the torsional hinderance from the ortho-methoxy in 3i, which prevents the amide from achieving optimum overlap with the aromatic ring. To confirm this hypothesis, a comparable experiment was carried out by replacing the bulky ortho-methoxy with an ortho-fluoro group. As might be anticipated from the foregoing discussion, the annulation of 3j proceeded with an obviously better reaction rate and in higher yield than 3i.

Scheme 1.

Reaction Scope using Various N-tosylbenzamides^a,b

^a1 (0.1 mmol), alkene (0.12 mmol), Pd(OAc)₂ (0.01 mmol), ligand (0.012 mmol) and O₂ (1 atm). ^bIsolated yield. ^c72 h. ^dPerformed for 80 h using 20 mol % Pd(OAc)₂ and 24 mol % bathophenanthroline.

The range of acceptable alkenes was also investigated. When subjected to the preceding reaction conditions, electron-deficient alkenes, e.g., a conjugated ester (4a), ketone (4b) and amide (4c), smoothly evolved adducts 4 (Scheme 2). Even 2,4-pentadienoate and the α-branched olefin methacrylate were suitable, affording 4d and 4e, respectively.

Scheme 2.

Reaction Scope with Activated Alkenes^a,b

^a1 (0.1 mmol), alkene (0.12 mmol), Pd(OAc)₂ (0.01 mmol), ligand (0.012 mmol) and O₂ (1 atm). ^bIsolated yield. ^cReaction performed for 72 h using 20 mol % Pd(OAc)₂ and 24 mol % bathophenanthroline. ^ddr = 1.4:1, separated by preparative TLC.

Despite the otherwise rare participation of simple aliphatic alkenes in C-H olefinations owing to their poor reactivity, we found terminal aliphatic alkenes were readily converted into annulated adducts (Table 1). Not only simple alkene (5a) but also functionalized alkenes such as aliphatic ester (5b), aliphatic chloride (5c) and aryl substituted alkene (5d) were suitable substrates for the annulation cascade. An intriguing example of specificity was found in 1,7-octadiene wherein one terminal alkene underwent annulation while the other survived (5e). A distinguishing feature of the adducts in Table 1, as opposed to those in Scheme 2, is the retention and positional shift of the olefin (exclusively E-configuration by ¹H NMR) indicating a significant variation in the termination step. Additionally, a small amount of isoquinolinone 6 was also produced. The overall process represents an unprecedented tandem C-H olefination/annulation involving simple aliphatic alkenes.

Table 1.

Reaction Scope with Non-Conjugated Alkenes^a

entry	alkene	isoindolinone	combined yield (%)b
1		5a	68 (5a/6a 3.1/1)c
2		5b	65(5b/6b 3.0/1)
3		5c	60 (5c/6c 3.1/1)
4		5d	63 (5d/6d 2.7/1)
5		5e	38d

^a1 (0.1 mmol), alkene (0.12 mmol), Pd(OAc)₂ (0.01 mmol), ligand (0.012 mmol) and O₂ (1 atm).

^bIsolated yield.

^cRatio of 5/6 calculated from isolated yields.

^dTrace of 6e only detected by ¹H NMR.

While the mechanism of Pd(II)-mediated C(sp2)-H cleavage has attracted considerable attention in past decades, the precise details remain controversial. ¹¹ However, the data gained from the competitive reactions agree with an electrophilic palladation mechanism as originally proposed by Ryabov and coworkers (Scheme 3).^11a Specifically, tosyl amides bearing substituents with various electronic properties were subjected to parallel experiments in which the conversion ratio was determined by ¹H NMR. Substrates with electron donating groups, e.g., (1c), showed the highest reaction rates while an unsubstituted case (1a) was intermediate and the electron deficient substrate (1e) had the lowest reactivity. These results are consistence with the pathway of arenium (Wheland) intermediate of which the efficacy is highly relying on the electronic property of arene.^11a

Scheme 3.

Electronic Effects on Reaction Rate

To further study the C-H cleavage step, we prepared deuterium-labeled compound 7 and subjected it to competitive, intermolecular experiments (eqs 3 and 4). An equimolar mixture of 1a and 7 were treated with 1 equiv of electron-deficient alkene (or aliphatic alkene), the measured product isotope effect of 3.2 (or 3.0) might suggest that the deprotonation was involved as the rate-limiting step.4a,4j^,12

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To clarify the influence of the acidic N-H sulfonamide, control experiments, utilizing the standard reaction conditions, were conducted using benzamides in which the tosyl group was replaced by H, Ph, or OMe.¹³ The results cogently showed that the acid N-H was essential and could dramatically improve the reactivity (Scheme 4). Illuminated by the preceding investigations, the postulated mechanism is depicted in Scheme 5. We speculate that the increased acidity of the N-H, a consequence of the combined inductive effects of the carbonyl and tosyl, facilitates the formation of the Pd-N bond that in turn promotes the formation of palladacycle (I). Alkene insertion leads to intermediate II from which III is derived via β-H elimination. The electronic properties of the alkenes determines the ultimate fate of III. Electron-deficient alkenes likely participate by a Michael addition (path A),^6a,7a,8 while non-conjugated alkenes are annulated in a Wacker-type amination (path B).¹⁴ Confirmation was obtained by subjecting 10 (mixture of Z/E-olefins) to the standard annulation conditions (eq 5). The Wacker amination was complete within a few hours and afforded the anticipated product 5a irrespective of original olefin configuration.¹⁵

Scheme 4.

Dependency of Reactivity on N-H Acidity

Scheme 5.

Proposed Mechanism

If the parent nitrogen heterocycle is desired, the tosyl can be conveniently removed in just one-half hour using sodium naphthalenide, e.g., 3a to isoindolinone 11 (eq 6).

In summary, a tandem process of C-H olefination/annulation for heterocyclic synthesis is described. Tosyl amides were employed as a novel directing group to afford a series of isoindolinones. Impressively, this transformation is broadly compatible with electron-rich and -deficient alkenes. For more eco-friendly transformations, molecular oxygen or even atmospheric air could be used without the need of other co-oxidants. From a practical point of view, the only waste is water and thus this transformation meets the goal of green chemistry. Further applications are ongoing in our group.

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