A Cu-catalysed synthesis of substituted 3-methyleneisoindolin-1-one

Abstract

A Cu(I)-catalysed synthesis of substituted 3-methyleneisoindolin-1-ones using alkynyl acids as an alkyne source has been developed. The reaction involves the decarboxylative cross-coupling of 2-halobenzamides with aryl alkynyl acids, followed by 5-exo-dig heteroannulation. While reactions of 2-iodo benzamides proceeded without a ligand, for 2-bromo substrates, the assistance of a ligand is essential.

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Introduction

In modern synthetic organic chemistry, transition metal catalysed decarboxylative cross-coupling reactions using carboxylic acids have received considerable attention because of their ready availability and easy handling. In general, such reactions proceed via the in situ generation of organometallic species along with the extrusion of CO₂ as the side product. This reactive organometallic intermediate then couples with appropriate counterparts providing alternative routes to C–C or C–heteroatom bonds.¹ As one of the members of carboxylic acids, alkynyl carboxylic acids are potential candidates for cross-coupling reactions installing an alkynyl group into the substrates. Unlike aryl or vinyl carboxylic acids, the decarboxylative coupling of alkynyl carboxylic acids can be initiated by a single catalytic system.^1,2 This strategy has been demonstrated by the Xue group for the synthesis of diaryl alkynes via a copper catalysed decarboxylative cross-coupling of alkynyl carboxylic acids with aryl halides.^2a Not merely restricted to the synthesis of internal alkynes, the above mentioned strategy has much more to offer by the manipulation of the alkynyl group. One way to achieve this is to activate the in situ generated alkynyl group with a soft metal followed by an intramolecular nucleophilic attack resulting in a heterocycle. We envisaged that 2-halobenzamide with an alkynyl acid under an appropriate condition may result in either 3-methylene-isoindolin-1-one via a 5-exo-dig cyclisation or substituted isoquinolin-1-one via a 6-endo-dig cyclisation.³

We initiated our investigation by treating 2-bromobenzamide (1) with phenylpropiolic acid (a) (1.2 equiv.) in the presence of CuI (10 mol%), Cs₂CO₃ (1.5 equiv.) in DMF solvent at 120 °C. To our delight, the reaction resulted in the selective formation of 5-exo-dig product, 3-phenylmethyleneisoindolin-1-one (1a) in a 32% yield. Herein, we report the synthesis of 3-methyleneisoindolin-1-ones involving decarboxylative cross-coupling of alkynyl carboxylic acids and 2-halobenzamides with concurrent cyclisation.

Substituted 3-methyleneisoindolin-1-ones are medicinally important heterocyclic scaffolds prevalent in many natural products and pharmaceuticals.⁴ Apart from their significant biological profile, they also find applications in materials chemistry.⁵ Some natural products and biologically active molecules having 3-methyleneisoindolin-1-one as the core unit are shown in Fig. 1. Due to the myriad of applications of this important motif, numerous synthetic protocols for their synthesis have been developed. The traditional methods for the synthesis of 3-methyleneisoindolin-1-one include the Horner–Wadsworth–Emmons type of condensation,⁶ the addition/dehydration of phthalimides,⁷ 5-exo-dig cyclisation of preformed 2-alkynylbenzamides^3a,b,c,8 and base triggered addition to benzonitriles.⁹ However, they are associated with poor regioselectivity in case of unsymmetrical substrates or require a number of additional synthetic steps. These shortcomings stimulated the emergence of transition metal catalysed reactions such as Heck–Suzuki–Miyaura domino reactions involving ynamides (path a, Scheme 1),¹⁰ Sonogashira coupling-carbonylation–hydroamination of ortho dihalo arenes (path b, Scheme 1)¹¹ and Ullmann coupling-hydroamination of ortho halo benzamides (path c, Scheme 1).¹² Apart from the cross-coupling strategies, recently You et al. have developed a Cu-mediated approach to 3-methyleneisoindolin-1-one that proceeds via tandem oxidative cross-coupling between (Csp²–H) of arenes with terminal alkynes followed by an intramolecular annulation.¹³ In the latter two cross-coupling strategies (path b and c, Scheme 1), terminal alkynes have been utilised for the initial alkynylation purpose. However, the use of their acid derivatives to serve as alkyne surrogates via the decarboxylative C–C bond formation for the synthesis of 3-methyleneisoindolin-1-one is the first precedence of its kind.¹⁴ The use of alkynyl carboxylic acids as alternatives to terminal alkynes is advantageous because of their superior reactivity and ready availability.¹⁵ They are less susceptible to homocoupling, thereby suppressing the competing diyne byproduct in the Sonogashira reaction.¹⁶

Fig. 1 Natural products and biologically active compounds containing 3-methyleneisoindolin-1-one.

Scheme 1 Various approaches for the synthesis of 3-methyleneisoindolin-1-one.

Results and discussion

Encouraged by the initial success, further optimisations were carried out to attain an improved yield of the product (1a). Several reports revealed that for substrates undergoing metal catalysed cross-coupling reactions involving a bromo substituent, assistance of a ligand often facilitates the process.¹⁷ Furthermore, an orthogonal selectivity has been demonstrated during a ligand assisted Cu-catalysed reaction.^17f,i A ligand functions by inhibiting the aggregation of the metal and improves the solubility of the catalyst/co-catalyst, thereby resulting in better catalytic activity of the metal. Taking cues from the aforementioned reports, when the coupling between 2-bromobenzamide (1) (1 equiv.) and phenylpropiolic acid (a) (1.2 equiv.) was performed in the presence of ligand 1,10-phenanthroline (10 mol%), CuI (10 mol%), Cs₂CO₃ (1.5 equiv.), the product (1a) was obtained in an improved yield of 59% (entry 2, Table 1). During the catalyst screening, CuI was found to be superior compared to all other Cu(I) and Cu(II) salts such as CuBr, CuCl, Cu(OAc)₂, CuCl₂, CuBr₂ tested (entries 3–7, Table 1). Replacement of the base Cs₂CO₃ with other inorganic bases such as K₂CO₃, K₃PO₄ resulted in lower yields (entries 8 and 9, Table 1). The use of organic bases like NEt₃ was found to be almost ineffective in bringing about the transformation (entry 10, Table 1). Other ligands such as diethylmalonate (DEM), L-proline, tetramethylethylenediamine (TMEDA), dimethylethylenediamine (DMEDA) tested were found to be inferior to 1,10-phenanthroline (entries 11–14, Table 1). Furthermore, an improvement in the yield (69%) was observed when the reaction was performed in DMSO instead of DMF (entry 15, Table 1). Other solvents such as 1,4-dioxane, 1,2-dichloroethane (DCE) and N-methyl-2-pyrrolidone (NMP) were found to be unsuitable for this transformation (entries 16–18, Table 1). Thus, CuI (10 mol%), 1,10-phen (10 mol%), Cs₂CO₃ (1.5 equiv.) in DMSO solvent at 120 °C were found to be the optimal conditions and rest of the reactions were performed under exactly identical conditions.

Table 1 Optimisation of reaction conditions^a

Entry	Catalyst (mol%)	Base (equiv.)	Ligand (mol%)	Solvent	Yield^b (%)
a Reactions were monitored by TLC.b Isolated yield.
1	CuI (10)	Cs2CO3 (1.5)	—	DMF	32
2	CuI (10)	Cs2CO3 (1.5)	1,10-Phen (10)	DMF	59
3	CuBr (10)	Cs2CO3 (1.5)	1,10-Phen (10)	DMF	51
4	CuCl (10)	Cs2CO3 (1.5)	1,10-Phen (10)	DMF	43
5	Cu(OAc)2 (10)	Cs2CO3 (1.5)	1,10-Phen (10)	DMF	38
6	CuCl2 (10)	Cs2CO3 (1.5)	1,10-Phen (10)	DMF	40
7	CuBr2 (10)	Cs2CO3 (1.5)	1,10-Phen (10)	DMF	49
8	CuI (10)	K2CO3 (1.5)	1,10-Phen (10)	DMF	53
9	CuI (10)	K3PO4 (1.5)	1,10-Phen (10)	DMF	41
10	CuI (10)	NEt3 (1.5)	1,10-Phen (10)	DMF	<5
11	CuI (10)	Cs2CO3 (1.5)	DEM (10)	DMF	35
12	CuI (10)	Cs2CO3 (1.5)	L-proline (10)	DMF	54
13	CuI (10)	Cs2CO3 (1.5)	TMEDA (10)	DMF	39
14	CuI (10)	Cs2CO3 (1.5)	DMEDA (10)	DMF	46
15	CuI (10)	Cs2CO3 (1.5)	1,10-Phen (10)	DMSO	69
16	CuI (10)	Cs2CO3 (1.5)	1,10-Phen (10)	1,4-Dioxane	42
17	CuI (10)	Cs2CO3 (1.5)	1,10-Phen (10)	NMP	49
18	CuI (10)	Cs2CO3 (1.5)	1,10-Phen (10)	DCE	53

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After establishing the optimised conditions, the substrate scope for this decarboxylation–heteroannulation methodology was explored. A variety of substituted 3-arylmethylene-isoindolin-1-ones has been synthesised as shown in Scheme 2. At first, the effect of substituents present on the N-aryl ring of benzamide was examined by treating them with phenylpropiolic acid (a). The N-aryl ring of amide bearing electron-donating groups such as 3-Me (2), 3,4-diMe (3), and 4-OMe (4) yielded their respective products (2a), (3a) and (4a) in 62%, 78% and 66%, respectively (Scheme 2); while benzamides derived from aryl amines possessing moderately electron-withdrawing groups such as 4-Cl (5), 4-Br (6), 4-F (7) provided their corresponding isoindolin-1-one (5a), (6a) and (7a) in 72%, 73% and 76% yields, respectively (Scheme 2). A comparison between two sets of substrates one possessing electron-donating groups and the other electron-withdrawing groups shows that yields are marginally better with the latter set with the lone exception of the 3,4-diMe substrate (3). The structure of the product (5a) has been confirmed by X-ray crystallography (Fig. 2).

Scheme 2 Substrate scope of substituted 3-arylmethyleneisoindolin-1-ones. ^a Yields of isolated pure products are reported. ^b X = Br. ^c X = I.

Fig. 2 ORTEP view of compound (5a).

Instead of an aryl group, when a benzyl substituent was attached to the N-atom of the benzamide (8), the product (8a) was obtained in good yield (84%). However, with analogous picolinamide (9) there was a substantial drop in the yield (68%). The variation of substituents in aryl alkynyl acids were scrutinised by treating them with benzamide (1). Aryl propiolic acids possessing electron-donating groups such as 4-Me (b), 4-^tBu (c) and electron-withdrawing groups such as 4-Br (d), 4-Cl (e) underwent facile reactions with (1) affording their respective products (1b), (1c), (1d) and (1e) in moderate yields as shown in Scheme 2.

From the trends in products yield it is evident that no significant electronic effects of substituents present in aryl propiolic acids could be ascertained. An attempt was made to find out whether some combinations of substituents present in the N-aryl ring of the benzamides and the aryl ring of alkynyl acids (based on their electronic effects) could be the most suitable for this transformation. For this reason, two independent reactions were carried out with benzamide containing electron-donating 3,4-diMe group in the N-aryl ring (3) with aryl alkynyl acids bearing the electron-donating 4-OMe group (f) and having the electron-withdrawing 4-Br group (g). Another set of reactions were performed with benzamide possessing the electron withdrawing 4-Cl group (5) with aryl alkynyl acids bearing the electron-donating group 4-Me (b) and having the electron-withdrawing 3-F group (h). However, judging by the yield pattern of products (3f), (3g), (5b) and (5h) obtained from the aforementioned combinations, again no correlation between yields and the substituent effect could be obtained (Scheme 2). To check the efficacy of this transformation with aliphatic alkynyl acid, a reaction was performed between 2-bromobenzamide (1) and 2-butynoic acid. The reaction failed to give the desired isoindolin-1-one product, instead a hydroxylation in the ortho position to the amide (1a′) moiety was observed under the reaction conditions (Scheme 3). A similar hydroxylation of o-haloanilides and o-halo benzamides has been observed during the Cu-catalysed reaction in an aqueous medium.¹⁸ Due to lower reactivity of the bromo group towards oxidative addition with aliphatic alkyne species, the hydroxylation path is preferred and the possible hydroxyl source is the water present in the commercial grade DMSO.

Scheme 3 Competitive hydroxylation of 2-bromobenzamide.

After the successful accomplishment of the present transformation with 2-bromo benzamides, next we turned our attention towards the more reactive 2-iodo benzamides. The reaction of 2-iodo benzamide (1′) with phenylpropiolic acid (a) under the previously optimised conditions provided 3-phenylmethyleneisoindolin-1-one (1a) in a 77% isolated yield. Interestingly, the same reaction, when performed in the absence of ligand 1,10-phen, proceeded smoothly virtually unaffecting the yield and reaction time. This illustrates the higher reactivity of the C–I bond compared to the C–Br bond towards the decarboxylative C–C bond formation. Further, ligand free conditions were then applied for the reaction of 2-iodo benzamides (2′) and (3′) with phenylpropiolic acid (a), which provided their corresponding isoindolin-1-one products (2′a) and (3′a) in 73% and 79% yields, respectively. The trend in the yields obtained from 2-iodo precursors without the involvement of ligand was found to be identical to that of 2-bromo benzamides with the assistance of a ligand.

Based on literature reports,^2a,e,14,19 a plausible mechanism for the formation of 3-phenylmethyleneisoindolin-1-one is outlined in Scheme 4. Initially a Cu(I) intermediate (A) is formed with phenylpropiolic acid (a). This Cu(I) species via a decarboxylative path gives a Cu-alkynylide species (B), which undergoes oxidative addition to 2-bromobenzamide (1) to produce the Cu(III) intermediate (C). Reductive elimination of Cu from (C) gives the ortho-alkynylated product (D). Deprotonation of the amide N–H of the intermediate (D) and subsequent hydroamination²⁰ of the C–C triple bond, promoted by co-ordination of Cu(I) with the alkynyl group, results in the formation of 3-phenylmethyleneisoindolin-1-one (1a) via the intermediacy of (E) along with the regeneration of Cu(I). The formation of intermediate species (A), (B), (C) and (D) has been detected by the ESI/MS analysis of reaction aliquots, which supports the proposed mechanism in Scheme 4 (see ESI†).

Scheme 4 Plausible mechanism for the formation of 3-methyleneisoindolin-1-one.

However, an alternative mechanism involving the initial oxidative addition of Cu(I) with 2-halobenzamide to give intermediate (A′) as proposed in Scheme 5 cannot be completely ruled out. Intermediate (A′) couples with phenylpropiolic acid (a) to give intermediate (B′). The loss of CO₂ from intermediate (B′) would give intermediate (C), which eventually leads to the formation of the desired product (1a), as shown in Scheme 5.

Scheme 5 Plausible mechanism for the formation of 3-methyleneisoindolin-1-one.

Conclusion

In conclusion, we have developed a Cu(I)-catalysed synthesis of substituted 3-methyleneisoindolin-1-ones involving the decarboxylative cross-coupling of 2-halobenzamides with aryl alkynyl acids, followed by 5-exo-dig heteroannulation. In this transformation, alkynyl acids have been utilised to generate an alkyne intermediate for the synthesis of this important heterocycle scaffold. While reactions of 2-iodo benzamides proceeded without a ligand, for 2-bromo substrates, the assistance of a ligand is essential.

General procedure for the synthesis of (Z)-3-benzylidene-2-phenylisoindolin-1-one (1a)

To a solution of 2-bromobenzamide (1) (138 mg, 0.5 mmol) in DMSO (2 mL), CuI (9.5 mg, 0.05 mmol), 1,10-phen (9 mg, 0.05 mmol), Cs₂CO₃ (245 mg, 0.75 mmol) and phenylpropiolic acid (a) (87.6 mg, 0.6 mmol) were added, and the resultant mixture was stirred in a preheated oil bath at 120 °C for 2 h. The reaction mixture was then cooled to room temperature, admixed with water (5 mL) and the product was extracted with ethyl acetate (2 × 20 mL). The organic phase was dried over anhydrous sodium sulphate and concentrated in vacuo. The crude product was purified over a column of silica gel and eluted with 19 : 1 hexane–ethyl acetate to give (Z)-3-benzylidene-2-phenylisoindolin-1-one (1a) (102.5 mg, 69% yield).

Acknowledgements

B. K. P acknowledges the support for this research by the Department of Science and Technology (DST) (SR/S1/OC-79/2009), New Delhi, and the Council of Scientific and Industrial Research (CSIR) (02(0096)/12/EMR-II). AG and GM thank UGC for the fellowships. Thanks are due to Central Instruments Facility (CIF) IIT Guwahati for NMR spectra, and DST-FIST for the XRD facility.

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR spectra. CCDC 1024354. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra12782j

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