Using blue-light driven iron-catalysis to afford molecular diversity in phosphindole oxide derivatives

Abstract

We have developed original blue-light promoted iron-catalyzed reactions of activated alkynes with secondary phosphine oxides in the presence of tert-butylhydroperoxide under argon at room temperature. Depending on the nature of the iron precursors, benzo[b]phosphole oxides were obtained using Fe(NO₃)₃ in methanol and 9-phenyltribenzo[b,e,g]phosphindole 9-oxide derivatives utilizing Fe(OTf)₂ in acetic acid. Control experiments revealed that radical processes are involved, permitting the proposal of a reasonable mechanism.

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Introduction

Phosphorus-containing heterocycles have shown wide applications in organic synthesis, medicinal chemistry, and materials science.^1–4 Among these compounds, phosphole-based conjugated systems, and in particular benzo[b]phosphole derivatives have attracted significant interest from synthetic chemists in the last few years due to their intrinsic physical and optical properties, which have enabled the development of novel organic semiconductors for OLEDs (organic light-emitting diodes), OPVs (organic photovoltaics), cell imaging dyes or phototherapy^5–8 (Fig. 1).

Fig. 1 Representative examples of benzo[b]phosphole for optoelectronics or biophotonics.

Most of the conventional preparations of benzo[b]phosphole derivatives⁹ were performed starting from alkynes bearing ortho-phosphorus substituents involving one intramolecular cyclization step,^10–15 the main limitation of such pathways being the preparation of the starting materials (Scheme 1a and b). Transition metal catalysis has also shown its advantages for the preparation of benzo[b]phosphole oxides starting from o-diaryl-phosphinophenylalkynes using Pd(OAc)₂/CuI cocatalysts¹⁶ (Scheme 1c). Benzophosphole synthesis can be also done through electrophilic trapping with PhPCl₂ of a zirconaindene, obtained from Cp₂ZrPh₂ and an alkyne¹⁷ (Scheme 1d).

Scheme 1 Classical methodologies to prepare benzo[b]phosphole derivatives.

Starting from metallated arenes (M = ZnX, MgBr), benzo[b]-phosphole oxides can be obtained via the insertion of the alkyne catalyzed by nickel or cobalt species, then electrophilic trapping reaction with R³PCl₂, Friedel–Crafts type cyclisation and oxidation (Scheme 1e).^18,19 On one hand, during the past decade, transition metal-catalyzed C–H bond functionalization reactions have been extensively studied to afford a wide variety of heterocyclic compounds, notably for the synthesis of P-containing heterocyclic compounds. Indeed, catalyzed oxidative C–H/P–H functionalization reactions starting from internal alkynes and secondary phosphine oxides or H-phosphonates were reported using metal salts under oxidative conditions, such as copper,^20,21 manganese,²² and silver (Scheme 1f).²³ Additionally, using a well-defined cobaloxime based catalyst Co(dmgH₂)₂PyCl (15 mol%) in the presence of HCO₂Na at 60 °C for 20 h under blue LED irradiation, internal alkynes reacted with diarylphosphine oxides to lead to benzo[b]phosphole oxides in 43–90% yields.²⁴ Noticeably, metal-free promoted preparation can be conducted such as using Eosin Y as a photocatalyst and N-ethoxy-2-methylpyridinium tetrafluoroborate as the oxidant under green light.²⁵

On the other hand, highly condensed heteroaromatics based on phosphole motifs have received significant attention thanks to their physical and optoelectronic properties and were applied for the design and synthesis of organic semi-conductors or fluorescent bioprobes.^5,8 In terms of preparation, one efficient approach deals with the Mallory photo-promoted oxidative cyclization.^26–28 Indeed, as a representative example, the synthesis of highly condensed dibenzophosphapentaphenes bearing peripherally electron-donating dialkoxyphenyl or thienyl groups was reported by Bouit, Hissler, Réau, Nyulászi et al. (Scheme 2).²⁹ In 2023, Zhao described the oxidative cyclisation of 2,3-diarylbenzo-phospholes under aerobic conditions under UV light irradiation (365 nm) (Scheme 1).³⁰ Noticeably, when starting from 1-phenyl-2,3-di(p-substituted-aryl)phosphindole oxides, the corresponding tribenzophosphindole oxides were obtained in 72–91% yields, whereas conducting the cyclisation with 1-phenyl-2,3-di(m-substituted-aryl)phosphindole oxides, low to moderate yields were observed (1–36% yields). Recently, Hirano reported a Bi(OTf)₃-mediated oxidative cyclization of 2,3-diarylbenzophopholes under blue visible light irradiation (456 nm) in the presence of 1 equiv. of NaHCO₃ as the base, but without any additional oxidants.³¹ Alternative preparations can be done starting from o-arylalkynylanilines with R₂HP(=O) in the presence of 2 equiv. of DTBP and 2 equiv. of Mg(NO₃)₂·6H₂O in 1,4-dioxane under air at 100 °C.³²

Scheme 2 Methodologies to prepare 9-phenyltribenzo[b,e,g]-phosphindole 9-oxide derivatives.

Additionally, since the beginning of this millennium, iron has emerged as a powerful substitute to noble metals in homogeneous catalysis, more particularly in hydroelementation areas,³³ including in regioselective hydrophosphination. Following our continuous contribution in iron catalyzed transformations, notably involving organophosphorous derivatives such as α-alkylation reaction of methyldiarylphosphine oxides,³⁴ or oxophosphinylation of activated alkenes by secondary phosphine oxides leading to β-ketophosphine oxides,³⁵ we report hereafter a blue-light driven iron-controlled oxidative C–H/P–H functionalization method for benzo[b]phosphole oxide and tribenzophosphindole oxide synthesis using secondary phosphine oxides with alkynes, conducting the reaction at ambient conditions.

To the best of our knowledge, there are no reports addressing such transformations using only iron as a catalyst.³⁶

Results and discussion

First of all, in order to evaluate the ability of iron salts to lead to the annulation of secondary phosphine oxides 1 with internal alkynes 2 leading to benzo[b]phosphole oxide derivatives 3, the reaction of diphenylphosphine oxide 1a with diphenylacetylene 2a was selected as the benchmark reaction. Thus, in the presence of 10 mol% Fe(NO₃)₃, and 2 equiv. of tert-butylhydroperoxide (TBHP) as the oxidant, 1a reacted with 2 equiv. of 2a in acetonitrile upon blue light irradiation (using two 24-watt LED lamps, 450–460 nm) at room temperature (rt) for 12 h, and the corresponding benzo[b]phosphole oxide 3a was obtained in 40% isolated yield (Table 1, entry 1).

Table 1 Optimization of the light driven iron-catalyzed annulation reaction of diphenylacetylene 2a with diphenylphosphine oxide 1a ^a

Entry	[Fe] (10 mol%)	Light	Solvent	3a Yield (%)
a Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), [Fe] (10 mol%), TBHP (2 equiv.), solvent (1 mL), 2 blue-LEDs 24 W (450–460 nm), rt, 12 h under argon. Isolated yields after purification by flash chromatography on silica. b A by-product was observed in 40% yield and was identified as 9-phenyltribenzo[b,e,g]phosphindole 9-oxide 4a. c 4a was obtained in 22% yield. d >5% of 4a was observed as a by-product. e No TBHP was used.
1	Fe(NO3)3	Blue-LED	CH3CN	40
2	FeBr3	Blue-LED	CH3CN	—
3	FeF3	Blue-LED	CH3CN	6
4	Fe(OTf)2	Blue-LED	CH3CN	—^b
5	FeCl3	Blue-LED	CH3CN	—^c
6	FeSO4	Blue-LED	CH3CN	12^d
7	Fe(NO3)3	Blue-LED	Toluene	30
8	Fe(NO3)3	Blue-LED	1,4-Dioxane	54
9	Fe(NO3)3	Blue-LED	1-Pentanol	50
10	Fe(NO 3 ) 3	Blue-LED	MeOH	73
11	Fe(NO3)3	White light	MeOH	53
12	Fe(NO3)3	No light	MeOH	2
13^e	Fe(NO3)3	Blue-LED	MeOH	—
14	—	Blue-LED	MeOH	—
15^e	—	Blue-LED	MeOH	—

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We then evaluated the effect of the iron salt nature on the transformation. Among the tested salts (10 mol%), FeBr₃ and FeF₃ exhibited no or very low activity. Noticeably, when conducting the reaction with Fe(OTf)₃ or FeCl₃, no benzo[b]phosphole oxide 3a was detected, but a by-product identified as 9-phenyltribenzo-[b,e,g]phosphindole 9-oxide 4a was obtained in 40% and 22% yields, respectively (entries 4 and 5). With FeSO₄, the reaction led to a mixture of 12% of 3a and <5% of 4a (entry 6).

The nature of the solvent was then evaluated, and among the solvents tested (acetonitrile, 1,4-dioxane, toluene, 1-pentanol), methanol exhibited the best efficiency with 73% of 3a obtained (entries 1 and 7–9 vs. 10).

The nature of the light had also a great influence on the efficiency of the transformation. Indeed, using white light (two 24-watt compact fluorescent lamps) reduced the yield but did not inhibit the reactivity by contrast with the reaction conducted in dark (53% vs. 73%, entries 11 vs. 10 and 12). Control experiments showed that in the absence of catalyst, and of TBHP oxidant, no reaction took place (entries 13–15). Additionally, when the reaction was performed under air instead of under argon, only trace amount of product 3a was detected.

As mentioned, when using Fe(OTf)₂, FeCl₃ and Fe(SO₄), a by-product, named 9-phenyltribenzo[b,e,g]phosphindole 9-oxide 4a was isolated in >5–40% yields and fully characterized, including by X-ray analysis (see Fig. S1 for the molecular structure of 4a). This specific reaction is highly attractive as it allows obtaining tribenzo[b,e,g]phosphindole 9-oxide in a single step at rt from commercially available precursors, which appear as an interesting alternative to Hirano's 2-steps P-cyclization/Mallory reaction.³¹

Based on these promising results, the parameters of the reaction were evaluated to determine their effect on switching the chemoselectivity toward the formation of 9-phenyltribenzo[b,e,g]phosphindole 9-oxide 4a (Table 2). As already shown in Table 1, among the 3 tested salts, Fe(OTf)₂, FeCl₃ and FeSO₄ (10 mol%), for the reaction of 1a with 2 equiv. of 2a in the presence of 2 equiv. of TBHP in acetonitrile upon blue light irradiation at rt for 12 h, Fe(OTf)₂ exhibited the highest performance, 4a being obtained in 40% yield (Table 2, entries 1–3).

Table 2 Optimization of the light driven iron-catalyzed reaction of diphenylacetylene 2a with diphenylphosphine oxide 1a leading to 9-phenyltribenzo[b,e,g]phosphindole 9-oxide 4a ^a

Entry	[Fe] (10 mol%)	Light	Solvent	4a Yield (%)
a Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), [Fe] (10 mol%), TBHP (2 equiv.), solvent (1 mL), 2 blue-LEDs 24 W (450–460 nm), rt, 12 h under argon. Isolated yields after purification by flash chromatography on silica. b 3a was obtained in 22% yield. c Dicumylperoxide was used instead of TBHP. d Cumylhydroperoxide was used instead of TBHP. e DTBP was used instead of TBHP.
1	Fe(OTf)2	Blue-LED	CH3CN	40
2	FeCl3	Blue-LED	CH3CN	22
3	FeSO4	Blue-LED	CH3CN	>5^b
4	Fe(OTf)2	Blue-LED	CH2Cl2	26
5	Fe(OTf)2	Blue-LED	1,2-DCE	—
6	Fe(OTf)2	Blue-LED	Dioxane	54
7	Fe(OTf)2	Blue-LED	TFE	61
8	Fe(OTf) 2	Blue-LED	AcOH	70
9^c	Fe(OTf)2	Blue-LED	AcOH	Trace
10^d	Fe(OTf)2	Blue-LED	AcOH	27
11^e	Fe(OTf)2	Blue-LED	AcOH	20
12	Fe(OTf)2	White light	AcOH	Trace
13	Fe(OTf)2	No light	AcOH	—

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The nature of the solvents on the activity was then evaluated. In non-protonic chlorinated solvents such as dichloromethane and 1,2-dichloroethane (1,2-DCE), the formation of 4a was not favored with a maximum of 26% yield in CH₂Cl₂ (entries 4 and 5). Conducting the reaction in 1,4-dioxane led to a better efficiency and 54% of 4a (entry 6). However, protic solvents such as 2,2,2-trifluoromethanol (TFE) and acetic acid were found to favor the reaction (entries 7 and 8). Especially when acetic acid was used as the solvent, the product 4a can be isolated in 70% yield. Noticeably, replacing tert-butyl hydroperoxide (TBHP) to other peroxides such as dicumyl peroxide, cumyl hydroperoxide and di-tert-butyl peroxide (DTBP) didn't favor the efficiency of the reaction (up to 27% yield, entries 9–11).

The nature of the light again played a crucial role. When activating the reaction with white light (24 W bulk lamp) in acetic acid, only trace amount of 4a was detected. Noticeably, in dark, no reaction took place (entries 12 and 13).

With these optimized reaction conditions in hands, the substrate scope for the preparation of benzo[b]phosphole oxide 3 derivatives was evaluated [Fe(NO₃)₃ (10 mol%), 2 equiv. of alkyne, 1 equiv. of secondary phosphine oxide and 2 equiv. of TBHP in methanol for 12 h at rt under blue light activation (2 × 24 W-blue LED) under argon] (Scheme 3). The reaction of diphenylphosphine oxide 1a was successfully performed with substituted diarylacetylenes bearing para-electron donating group such as n-butyl, leading to the corresponding benzo[b]phosphole oxide 3b in 58% isolated yield. By contrast, when starting from bis(4-nitrophenyl)acetylene or bis(4-dimethylaminophenyl)-acetylene, no reaction took place. Diarylacetylene with a p-substituted fluoro can be also used and was selectively converted in the benzo[b]phosphole oxide 3c in 41% yield. When conducting the reaction with di(p-tolyl)phosphine oxide and diphenylacetylene, benzo[b]phosphole oxide 3e was obtained in 37% yield with the methyl located at the para and meta position of the phosphorus of the benzophosphole moiety. Additionally, the reaction can be conducted successfully with ethyl 3-phenylprop-2-ynoate and diethyl but-2-yndioate yielding the corresponding benzo[b]phosphole oxide 3f and 3g in 68% and 53%, respectively.

Scheme 3 Scope of iron-catalyzed annulation reaction of internal alkynes 2 with diarylphosphine oxide 1 leading to benzo[b]phosphole oxides 3 and 9-phenyltribenzo-[b,e,g]phosphindole 9-oxide derivatives 4. ^a For the preparation of benzo[b]phosphole oxides 3: Fe(NO₃)₃ (10 mol%), alkyne (2 equiv.), secondary phosphine oxide (1 equiv.), TBHP (2 equiv.) in methanol, 12 h at rt under blue light activation (2 × 24 W-blue LED) and argon. Isolated yields. ^b ∼1 : 1 ratio based on ³¹P-NMR. For the preparation of 9-phenyltribenzo-[b,e,g]phosphindole 9-oxide derivatives 4: Fe(OTf)₂ (10 mol%), alkyne (2 equiv.), secondary phosphine oxide (1 equiv.), TBHP (2 equiv.) in acetic acid, 12 h at rt under blue light activation (2 × 24 W-blue LED) and argon. Isolated yields. ^c ∼1 : 1 ratio based on ³¹P-NMR; purity ∼ 80%. ^d 85% of p-substituted F, 15% of m-substituted F on the benzo[b]phosphindole motif, based on X-ray analysis. ^e Undefined ratio p-substituted CF₃ and m-substituted CF₃ on the benzo[b]phosphindole motif.

Then, the substrate scope and limitation of the formation of 9-phenyltribenzo[b,e,g]phosphindole 9-oxide derivatives 4 were evaluated using the optimized conditions [Fe(OTf)₂ (10 mol%), 2 equiv. of alkyne, 1 equiv. of secondary phosphine oxide and 2 equiv. of TBHP in acetic acid for 12 h at rt under blue light activation (2 × 24 W-blue LED) under argon] (Scheme 3). The reaction of diphenylphosphine oxide 1a can be conducted with bis(4-fluorophenyl)acetylene or bis(4-methylphenyl)-acetylene giving the corresponding compounds 4b and 4c in 42% and 68% yields, respectively. The nature of the diarylphosphine oxides can be also modulated and the reaction of di(p-tolyl)phosphine oxide, di(4-fluorophenyl)phosphine oxide and (4-trifluoromethylphenyl)-phosphine oxide with diphenyl-acetylene and di-p-tolylacetylene led to the derivatives 4d–f in 41–69% yields. In our hands, with diisopropylphosphite and diphenylphosphite, no reaction occurred. Noticeably, crystals suitable for X-ray analysis were grown by diffusion of cyclohexane into a solution of 4g in EtOAc. The molecular structure of 4g is depicted in Fig. 2. First of all, it demonstrated the formation of benzo[b]phosphindole motif, with the formation of the additional C–C bond (herein C19–C20, 1.455(3) Å). Importantly, the meta relationship between the trifluoromethyl substituent on the aromatic ring of the benzo[b]phosphindole motif and the phosphorus atom was unambiguously confirmed. In the phosphole ring, the bond lengths and angles are classical for such P-cycle.⁸

Fig. 2 Molecular structure of 4g.

Similarly, meta relationships were also observed with fluoro substituants when starting with di(4-fluorophenyl)phosphine oxide and diphenylacetylene leading to the benzo[b]phosphole oxide derivative 3d and the 9-aryltribenzo[b,e,g]phosphindole 9-oxide 4e. Noticeably, in the X-ray structure of 4e, it was shown two structures in which the fluorine was located at the para position (4e-1) and at the meta position (4e-2) (see Fig. S3). Noticeably, no expected 9-phenyltribenzo[b,e,g]phosphindole 9-oxide product was obtained by reaction of 1a with di(2-naphthyl)acetylene.

In order to gain insights into those two transformations, we then conducted preliminary mechanistic studies. First, in order to check if a radical process was involved, 3 equiv. of TEMPO (2,2,6,6-tetramethyl-1-piperi-dinyloxy) was added to the reaction mixture (1 equiv. of diphenylphosphine oxide 1a, 2 equiv. of diphenylacetylene 2a, 2 equiv. of TBHP in the presence of 10 mol% of Fe(NO₃)₃ at rt under blue light activation for 14 h). Under such conditions, no product 3a was detected. Noticeably, TEMPO-B adduct was detected by LR-MS at m/z = 357³⁴ (Scheme 4a). Similarly, when conducted the reaction with 3 equiv. of TEMPO and 10 mol% of Fe(OTf)₂ catalyst, no product 4a was observed (Scheme 4b). These experimental evidences support a radical pathway.

Scheme 4 Control experiments.

Based on our previous work^35b demonstrating that the reaction of diphenylphosphine oxide 1a with 1 equiv. of TEMPO under blue light activation and oxidative conditions allowed for the detection of the TEMPO-1a adduct in HR-MS, thus highlighting the formation of a P-centered phosphinoyl radical B, and supported by the literature, a plausible mechanism is described in Scheme 5. In an initial step, the secondary phosphine oxide 1 is oxidized to phosphorus radical Bvia a blue light promoted oxidative process catalyzed by iron^28,35b,37 species.³⁸ This P-centered radical B then underwent an intermolecular addition to the alkyne to afford the corresponding alkenyl radical C ^23b,25,39 which thus reacted with the phenyl ring of the phosphine oxide via a 5-exo-trig cyclisation leading the cyclohexadienyl radical D. Two pathways can then be considered: (i) the oxidation of D in the arenium cation E either by Fe(III)⁴⁰ and/or TBHP²⁰ through a single electron transfer process followed by the elimination of a proton to generate the expected benzo[b]phosphole oxide; (ii) the production of 3 by the reaction of D with the tert-butoxyl radical generated during the formation of the phosphinoyl radical B, via the abstraction of one radical H˙. At this point, the para/meta relationship of fluoro and trifluoromethyl groups on the benzo[b]phosphindole motif can be rationalized starting from the intermediate C. Indeed, an alternative pathway can be conducted via a 4-exo-trig cyclisation at the ipso-carbon leading the spiro-cyclohexadienyl radical species F. Thus, via a ring expansion, the intermediate H was obtained and similarly to D finally gave the isomer 3′.^23a,b The variable regioselectivity of the transformation may be due to the equilibrium between the alkenyl radical C and the spiro-cyclohexadienyl radical F and their relative stability and reactivity.

Scheme 5 Proposed reaction mechanism for the benzo[b]phosphole oxide 3 and 9-phenyltribenzo-[b,e,g]phosphindole 9-oxide 4 preparation.

The formation of compound 4a from 3a was then studied. It was shown that all the reactant (AcOH, TBHP, Fe(OTf)₂ catalyst) and blue light are crucial to be able to observe the transformation of 3a to 4a (see Table S7). Noticeably, when the reaction was performed with 3a in the reaction conditions [Fe(OTf)₂ (10 mol%), 2 equiv. of TBHP in acetic acid at rt under blue light activation (2 × 24 W-blue LED) under argon], 20% of 4a was detected after 7 h. Thus, the formation of the 9-phenyltribenzo[b,e,g]-phosphindole 9-oxide derivatives 4 from benzo[b]phosphole oxides 3 can be described a Mallory type reaction promoted by blue light with assistance of iron catalyst, even if its nature is not fully rationalized.^29b,41

Spectroscopic analyses of obtained tribenzophosphindole oxides showed, as expected, absorption bands in the visible range associated with intense blue luminescence (Fig. S73–S75 and Table S8).^30,31 In addition, they also display easily accessible reduction (Fig. S67) which is an advantage in the context of organic batteries.^42,43

Conclusions

In summary, we have developed a straightforward protocol for the synthesis of benzo[b]phosphole oxides 3 and 9-phenyltribenzo[b,e,g]phosphindole 9-oxides 4 starting from secondary diarylphosphine oxides and internal alkynes catalyzed by Fe(NO₃)₃ and Fe(OTf)₂, respectively, in the presence of TBHP as the oxidant and promoted by blue light activation at ambient temperature. Control experiments revealed that radical processes are involved which permitted to propose a reasonable mechanism. This innovative synthetic approach will open new perspectives for the synthesis of phosphindoles which are strategic compounds in the fields of optoelectronics, biophotonics or energy.

Experimental section

General procedure for blue-light driven iron-catalyzed synthesis of benzo[b]phosphole oxide 3

In a Schlenk tube, under argon atmosphere, secondary phosphine oxide 1 (0.2 mmol), alkyne 2 (0.4 mmol), Fe(NO₃)₃ (10 mol%), TBHP (2.0 equiv.), were dissolved in 1.0 mL of MeOH and the resulting mixture was stirred at rt under blue light irradiation (2 × 24 W LED; λ = 450–460 nm) for 12 h. Thus, the crude reaction mixture was diluted with EtOAc (5 mL), filtered through a Celite pad which was then washed with 10 mL of CH₂Cl₂. The volatiles were then removed under reduced pressure, and the residue was subjected to silica gel column chromatography [eluting with petroleum ether (PE)/ethyl acetate (EA)] to afford the corresponding product 3.

General procedure for the one pot synthesis of 9-phenyltribenzo[b,e,g]-phosphindole 9-oxide 4

In a Schlenk tube, under argon atmosphere, secondary phosphine oxide 1 (0.2 mmol), alkyne 2 (0.4 mmol), Fe(OTf)₂ (10 mol%), TBHP (2.0 equiv.), were dissolved in 1.0 mL of AcOH and the resulting mixture was stirred at rt under blue light irradiation (2 × 24 W LED; λ = 450–460 nm) for 12 h. Thus, the crude reaction mixture was diluted with EtOAc (5 mL), filtered through a Celite pad which was then washed with 10 mL of CH₂Cl₂. The volatiles were then removed under reduced pressure, and the residue was subjected to silica gel column chromatography [eluting with petroleum ether (PE)/ethyl acetate (EA)] to afford the corresponding product 4.

Conflicts of interest

There are no conflicts to declare.

Data availability

A data supporting this article have been included as part of the SI.

Experimental details and analytical data, NMR data and X-ray crystallographic data. See DOI: https://doi.org/10.1039/d5dt01551k.

CCDC 2448328 and 2448333 contains the supplementary crystallographic data for this paper.^44a,b

Acknowledgements

This work is supported by the Université de Rennes, the Centre National de la Recherche Scientifique (CNRS) and the FR Increase. Y. Y. thanks the China Scolarship Council for a PhD fellowship and L. F. thanks the Foundation Université de Rennes for a Master fellowship.

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43. Due to tedious purification, some derivatives were obtained in mediocre purities. For the reaction of 1a with di(p-tolyl)acetylene in the presence of Fe(NO₃)₃ in MeOH under optimized conditions, the expected benzo[b]phosphindole was obtained as the major compound (δ = 39.1 ppm) in mixture with non-identified derivatives (in ³¹P-NMR at δ = 39.4, 39.0, and 25.1 ppm) in ratio 1/0.27/0.20/0.13 (purity ∼ 66%). Similarly, starting from bis(4-fluorophenyl)phosphine oxide and diethyl but-2-yndioate, the expected benzo[b]phosphindole was the major compound (δ = 32.5 ppm) associated to 5 by products (δ = 47.2, 44.1, 43.9, 30.0, and 25.1 ppm), in ratio 1/0.08/0.03/0.04/0.08/0.05 (purity ∼ 75%).
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Footnote

† These authors contributed equally.

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