Iridium catalyzed fragmentation/cyclization of N-butynyl 4,4-dimethylisoxazolidine-3,5-diones: a unique access to multiply substituted pyrroles

Abstract

A unique protocol for the synthesis of substituted and fused pyrroles has been developed through the iridium catalyzed fragmentation/cyclization of related N-butynyl 4,4-dimethylisoxazolidine-3,5-diones in hot ethanol. Both the substitution pattern and electronic properties can affect the reaction rate and yield.

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The oxidative N–O single bond in hydroxylamine makes this unique moiety a powerful and convenient handle for a variety of organic transformations. The heterolytic cleavage of this bond serves as the driving force for the prestigious Beckmann rearrangement;¹ it also plays a critical role in Bode's decarboxylative condensation of N-alkylhydroxylamines and α-ketoacids (KAHA ligation).² Recently, N,O-dicarbonylated reagents A have been applied by Xu's group to achieve iron catalyzed amino-oxygenation/amino-halogenation of olefins where the N–O bond breaking event is the original power for the oxidizing process.³ Installation of an electron-withdrawing group on the O atom further polarizes the N–O bond making B a class of feasible electrophilic amination agents which have been applied in the amination of various carbon nucleophiles.⁴ The N–O bond of hydroxylamine is also activated when incorporated in a three-membered oxaziridine ring⁵ and reagents of type C have been successfully used in olefin aminohydroxylation,⁶ C–H amination⁷ and amination of arylmetals.⁸ Recently, cyclic motifs D (1,4,2-dioxazol-5-one, 1,3,2,4-dioxa-thiazole-2-oxide, 5,5-dimethyl-1,4,2-dioxazole and 1,2,4-oxadiazol-5-one) with a labile N–O bond have been used for metal catalyzed aryl C–H amidations⁹ and the synthesis of azaromatic amines.¹⁰ This progress in the chemistry of hydroxylamine derivatives aroused our interest in the related structure E (4,4-dimethylisoxazolidine-3,5-dione) featuring a cyclic N,O-diacylated hydroxylamine and we disclosed herein a unique protocol for the synthesis of substituted pyrroles (Fig. 1).

Fig. 1 Reagents bearing the N–O component.

At the outset, several N-alkyl-N,O-biacylated hydroxylamines E (R = alkyl group), which were prepared conveniently from the reaction of the corresponding aminohydroxide with 2,2-dimethylmalonyl dichloride, were subjected to extensive testing and the preliminary results indicated that these cyclic structures are exceptionally robust under both thermal and catalytic transition-metal conditions. Then our attention was turned to substrate 1a with a pending alkyl motif, and substantial conditions screening was carried out, i.e.1a was reacted in the presence of various catalytic transition metal complexes. The complexes of copper, iron, silver, nickel, manganese, rhodium, and ruthenium all failed to effect any change on the substrate (entries 1–19, Table S1†) in hot toluene; while several palladium and gold complexes led to complete decomposition of 1a (entries 20–23, Table S1†) to mess. Eventually, to our delight, when a solution of 1a in toluene was heated in the presence of 2 mol% [CpIrCl₂]₂ for 15 hours in a sealed tube, partial conversion of the starting materials was observed and the newly formed compound was isolated and determined as 1-isobutyryl-2-phenyl pyrrole 2a (entry 1, Table 1). This acylated pyrrole could be viewed as a product of the fragmentation/cyclization cascade of the starting materials. A higher yield was obtained with [Ir(COD)Cl]₂ in place of [CpIrCl₂]₂ as the catalyst under the same conditions (entry 2). The best yield of 49% was obtained at 140 °C during the course of temperature screening for the oil bath from 110 to 150 °C (entries 3–6). The reaction didn't occur when 1,4-dioxane, acetonitrile and dimethylformide were used as reaction media (entries 7–9); halogenated solvents dichloroethane and chloroform were inferior to toluene with regard to the yield (entries 10 and 11). Interestingly, alcohols were also feasible reaction solvents and ethanol proved to be the best as the highest yield of 62% was achieved in this solvent (entries 12–15). The transformation didn't occur in water (entry 16). Neither halving nor doubling the catalyst loading of [Ir(COD)Cl]₂ could improve the reaction outcome (entries 17 and 18). Substituting the Cl ligand with the OMe group gave rise to a complex [Ir(COD)OMe]₂ which showed comparable catalytic performance (entry 19).

Table 1 Condition optimization for iridium catalyzed fragmentation/cyclization of phenylbutynyl 4,4-dimethylisoxazolidine-3,5-dione^a

Entry	Catalyst	Solvent	Temp (°C)	Yield^b
a Reaction conditions: 1a (0.2 mmol) and catalyst (2 mol%) in solvent (4 mL) were heated in a sealed tube under N2 for 15 h. b Isolated yield. c 1 mol% of [Ir(COD)Cl]2. d 4 mol% of [Ir(COD)Cl]2.
1	[CpIrCl2]2	Toluene	110	20%
2	[Ir(COD)Cl]2	Toluene	110	32%
3	[Ir(COD)Cl]2	Toluene	120	38%
4	[Ir(COD)Cl]2	Toluene	130	44%
5	[Ir(COD)Cl]2	Toluene	140	49%
6	[Ir(COD)Cl]2	Toluene	150	45%
7	[Ir(COD)Cl]2	1,4-Dioxane	140	Trace
8	[Ir(COD)Cl]2	MeCN	140	NR
9	[Ir(COD)Cl]2	DMF	140	NR
10	[Ir(COD)Cl]2	DCE	140	42%
11	[Ir(COD)Cl]2	CHCl3	140	25%
12	[Ir(COD)Cl]2	MeOH	140	32%
13	[Ir(COD)Cl]2	EtOH	140	62%
14	[Ir(COD)Cl]2	^iPrOH	140	50%
15	[Ir(COD)Cl]2	^tBuOH	140	20%
16	[Ir(COD)Cl]2	H2O	140	NR
17^c	[Ir(COD)Cl]2	EtOH	140	55%
18^d	[Ir(COD)Cl]2	EtOH	140	52%
19	[Ir(COD)OMe]2	EtOH	140	58%

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The pyrrole scaffold is widely present in natural and synthetic compounds and many of them play essential roles in biological processes.¹¹ Recently, pyrrole has become a hot structural platform for the development of therapeutic agents.¹² Therefore an efficient protocol for selective synthesis of pyrrole derivatives is highly appreciated despite many existing methods.¹³ Since the current findings presaged a promising new method for pyrrole synthesis, we were prompted to carry out further studies to reveal its potential and limitations.

For this purpose, a series of arylbutynyl dimethylisoxazolidinediones 1 have been prepared and subjected to the optimal conditions to collect more information about this reaction (Table 2). Substrates with p/m methyl or p/m methoxyl goups on the phenyl ring all gave the corresponding 2-aryl pyrroles 2b–2e in 50–60% isolated yields after treatment with these conditions. With an o-methoxyl group, the reaction of related 4,4-dimethylisoxazolidine-3,5-dione delivered the pyrrole product 2f in a lower yield (42%). Electron-withdrawing substituents such as F and Cl are beneficial for this reaction giving about 10% increment in yields (2g and 2h). The ester group was compatible with these reaction conditions very well and 2i was obtained in 63% yield. 2-Thiophenyl pyrrole 2j could also be prepared via this protocol very efficiently.

Table 2 Iridium catalyzed synthesis of 2-aryl pyrroles from 4-arylbutynyl dimethylisoxazolidinediones^a

a Reaction conditions: 1 (0.2 mmol) and [Ir(COD)Cl]₂ (2 mol%) in EtOH (4 mL) were heated at 140 °C in a sealed tube under N₂ for 15 h.

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Next we decided to add one more aryl group at the propargylic C-2 of the butynyl moiety to prepare a number of 2,4-diarylbutynyl dimethylisoxazolidinediones (1k–1s). Interestingly, this alteration on the substrates accelerates the reaction rate remarkably as the reaction completed in 5 hours instead of completion in 15 hours for the substrates given in Table 2 and 2,4-diaryl pyrroles 2k–2s were collected in better yields (entries 1–9, Table 3). Generally speaking, the yields of 2,4-diaryl pyrroles are arguably higher than those of 2-aryl pyrroles and halogen substitutions on both aryl motifs were beneficial for this reaction which was typically illustrated by the isolated yield of 2s as high as 83% (entry 9). Incorporation of a cyclohexanyl ring onto the alkynyl segment resulted in a pair of stereoisomers 1t and 1u. Intriguingly, both cis- and trans-isomers underwent the fragmentation/cyclization sequence smoothly to give the same ring-fused pyrrole 2t in moderate yields though a longer reaction time was necessary for full conversion (entries 10 and 11, Table 3). Through this method, pyrroles with diverse aromatic rings at C2 and C3 could be prepared at will via arbitrary choice of aryl substituents in the corresponding starting materials.

Table 3 Iridium catalyzed synthesis of 2,4-diaryl pyrroles from 2,4-diarylbutynyl dimethylisoxazolidinediones^a

Entry	Substrate	Product, yield^b	Entry	Substrate	Product, yield^b
a Reaction conditions: 1 (0.2 mmol) and [Ir(COD)Cl]2 (2 mol%) in EtOH (4 mL) were heated at 140 °C in a sealed tube under N2 for 5 h. b Isolated yield. c Heated for 15 h.
1			7
2			8
3			9
4			10^c
5			11^c
6

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In the course of our study, it was found that some types of substrates (Fig. 2) didn't undergo this reaction. Pyridinyl alkyne 1v was almost recovered thoroughly, even after being heated to 150 °C for 15 hours, probably due to the strong coordinating property of the pyridine ring that might shut down the catalytic cycle. The triple bond was also totally unreactive for this reaction when a strong electron deficient nitro group was installed in 1w. A conjugated aromatic ring on the alkyne seems necessary since 1x also showed no reactivity under standard conditions. In sharp contrast to the isomeric substrates 1t/1u, a homologous pair of 1y/1z, namely both trans- and cis-stereomers, remained totally intact under exactly the same conditions. This discrepancy in reactivity might derive from the greater strain caused by the cyclopentane ring over the cyclohexane ring which impeded the cyclization step.

Fig. 2 Ineffective substrates.

To collect more information about the reaction details, deuterated substrate d₂-1k was prepared and subjected to the optimal reaction conditions, giving no deuterated product (eqn (a), Scheme 1). Meanwhile, the reaction of 1s in d₄-MeOH produced the deuterated product d₃-2s (eqn (b), Scheme 1). Interestingly, a mixture of 2s and [Ir(COD)Cl]₂ in d₄-MeOH was heated for 5 hours resulting in half deuteration on the pyrrole ring to afford d₂-2s with the α-proton of the isobutyryl group untouched (eqn (c), Scheme 1). Based on these findings, a mechanism is proposed as shown in Fig. 3. The iridium dimer I could break into monomer II through solvolysis; then ligand exchange with alkyne 1 followed by oxidative addition of the Ir(I) center into the pending N–O bond would give rise to Ir(III) complex III. Under thermal conditions, III could undergo CO₂ extrusion, alcohol bridged proton transfer and deprotonative metalation of the propargylic group to form allenyl Ir(III) complex IV. Then a 1,3-hydride shift would afford V and the subsequent reductive elimination would deliver the pyrrole product 2 and regenerate the active catalyst II for the next catalytic cycle.

Fig. 3 Possible mechanism for the pyrrole synthesis.

Scheme 1 Experiments with a deuterated substrate or solvent.

Conclusions

In summary, [Ir(COD)Cl]₂ has been identified as a capable catalyst to successfully promote the fragmentation of the otherwise robust 4,4-dimethylisoxazolidine-3,5-dione moiety. Subsequent reactions with a pendent alkyne group eventually led to the construction of the corresponding 2-aryl or 2,4-diaryl pyrroles. Thus a new method for the synthesis of multiply substituted pyrroles has been developed.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the National Natural Science Foundation of China (21402014 and 21672027), the QingLan Project of Jiangsu Province (2016) and the Six-Talent-Peaks Program of Jiangsu (2016) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The details of experiments, compound characterization and spectroscopic data. See DOI: 10.1039/c7qo00698e

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