Calcium mediated C–F bond substitution in fluoroarenes towards C–chalcogen bond formation

Abstract

A new calcium chloride mediated C–F bond cleavage in electron deficient fluoroarenes followed by functionalisation via C–Se/Te bond formation has been achieved in the absence of any additive/ligand/organometallic reagent. A series of functionalised organo-selenides and -tellurides were obtained in high yields by this procedure. The precise role of calcium and the possible mechanism have been proposed based on DFT analysis.

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Introduction

The C–F bond dissociation energy is the largest among all other C–X bonds (X = halogen).¹ Moreover, the reluctance of the C–F moiety to co-ordinate with the metal centres makes the C–F bond relatively inert² and thus cleavage of a C–F bond towards further functionalisation is a challenging task. In practice, the activation of a C–F moiety is initiated either by reductive cleavage of the C–F bond by a low valent transition metal³ or intermolecular hydrogen–fluorine exchange mediated by a trivalent organolanthanide complex.⁴ Several reports of C–F bond activation in fluoroarenes towards C–C bond formation catalyzed by Pd- and Ni-complexes are also available.³ Besides the transition metals, the use of group 2 alkaline earth metals such as calcium, strontium and barium have also been reported for specific organic transformations. These elements show excellent Lewis acid property because of their large radii and electropositive character. In addition to these, their low toxicity and cost efficiency have made them more attractive to be explored for new reactions. Recently calcium complexes have been reported to be employed for different types of organic reactions such as alkene hydroamination,⁵ hydrophosphonations,⁶ Michael reaction,⁷ aldol condensation,⁸ epoxidation,⁹ Tishchenko reactions,¹⁰ Sonogashira reaction,¹¹ Pictet Spengler reactions,¹² Friedel Crafts alkylation,¹³ cyclopropanation,¹⁴ dynamic multicomponent reaction,¹⁵ intermolecular carbohydroxylation of alkynes etc.¹⁶ Shu Kobayashi and others have applied chiral complexes of calcium in an enantioselective process too.¹⁷ Recently Hill and co-workers demonstrated the use of a calcium β-diketiminate complex for C–F bond cleavage via an intermediate heteroleptic calcium fluoride complex.¹⁸ To add a simpler procedure we report here a calcium chloride mediated C–F bond cleavage in electron deficient fluoroarenes for C–Se and C–Te functionalisation in the absence of any additive and ligand leading to the synthesis of a series of organoselenides and tellurides (Scheme 1a). Nevertheless, zinc dust has been used to cleave the Se–Se/Te–Te bond to make the PhSe/PhTe moiety available for the reaction and in no way has any role in C–F bond cleavage. To the best of our knowledge we are not aware of any direct Ca-mediated defluorination of fluoroarenes towards functionalisation in the absence of any additive/ligand/base. However, one report for Sonogashira-type cross-coupling of alkynes with fluoroarenes in the presence of excess sodium, NaOMe, Ca(OH)₂ and n-butylmagnesium chloride was found (Scheme 1b).¹¹

Scheme 1 Calcium mediated C–F bond activation.

Needless to say, the organoselenides are of much importance and of continued interest because of their presence as structural motifs in a variety of molecules of biological, pharmaceutical and material interest.¹⁹

They are used as catalysts for various reactions such as oxyselenation–elimination and valuable intermediates in organic synthesis.²⁰ Thus a number of methods have been developed for their synthesis.²¹ These include transition metal catalyzed C–Se bond formation through cross coupling of aryl halides (mainly I, Br, Cl), aryl boronic acid, and aryl triflates with selenol/PhSeNa/diphenyl diselenides.²² Recently some metal free procedures²³ (microwave irradiation, ball-milling, visible light photocatalysis etc.) have also been developed.

Results and discussion

To optimise the reaction conditions, a series of experiments were performed under varying reaction parameters such as different metal salts, solvents, temperature and time for a representative reaction of 4-fluorobenzonitrile and diphenyl diselenide. Among a variety of metal salts and solvents studied, calcium chloride in DMSO is the most effective combination and the best result was observed in a reaction at 110 °C for 12 h in the presence of Zn dust (Table 1, entry 4). A reaction at lower temperature and reduced time decreased the yield (Table 1, entries 1–3). The reaction did not initiate in the presence of other metal salts such as MnCl₂, SnCl₂, ZnCl₂, SbCl₅, NiCl₂, AgF and AgOAc (Table 1, entries 5–11). Other calcium salts such as CaCO₃, Ca(OAc)₂ and Ca(NO₃)₂ have been investigated and none of these is considerably efficient although only Ca(OAc)₂ provides 25% yield (Table 1, entries 12–14). DMF and NMP are also effective for the reaction (Table 1, entries 15 and 16). The reaction did not proceed at all in other solvents such as H₂O, dimethyl carbonate (DMC) and xylene under similar conditions (Table 1, entries 17, 19 and 20). However a trace amount of the product was observed in CH₃CN (Table 1, entry 18). Instead of Zn, the use of KOH and indium as cleaving agents failed to produce the product (Table 1, entries 21 and 22). The reaction did not proceed in the absence of calcium chloride (Table 1, entry 24). A reduction of the amount of CaCl₂ decreased the yield (Table 1, entries 25 and 26). The reaction produced lower yields of the product on using group 1 metal salts such as LiCl, NaCl, KCl and CsF instead of CaCl₂ (Table 1, entries 27–30).

Table 1 Optimisation of reaction conditions^a

Entry	Metal salt	Time (h)	Temp. (°C)	Solvent	Yield^b (%)
a Reaction conditions: 4-fluorobenzonitrile (1.0 equiv.), diphenyl diselenide (0.6 equiv.), metal salt (3.0 equiv.), Zn dust (1.1 equiv.), argon atmosphere. b Isolated yield. c KOH (1.1 equiv.). d In (1.1 equiv.). e Without Zn dust. f Calcium salt (1.0 equiv.). g Calcium salt (2.0 equiv.).
1	CaCl2	8	100	DMSO	45
2	CaCl2	10	100	DMSO	57
3	CaCl2	12	100	DMSO	65
4	CaCl2	12	110	DMSO	92
5	MnCl2	12	110	DMSO	—
6	SnCl2	12	110	DMSO	—
7	ZnCl2	12	110	DMSO	—
8	SbCl5	12	110	DMSO	—
9	NiCl2	12	110	DMSO	—
10	AgF	12	110	DMSO	—
11	Ag(OAc)	12	110	DMSO	—
12	CaCO3	12	110	DMSO	—
13	Ca(OAc)2	12	110	DMSO	25
14	Ca(NO3)2	12	110	DMSO	—
15	CaCl2	12	110	DMF	85
16	CaCl2	12	110	NMP	81
17	CaCl2	12	110	H2O	—
18	CaCl2	12	110	CH3CN	Trace
19	CaCl2	12	110	DMC	—
20	CaCl2	12	110	Xylene	—
21^c	CaCl2	12	110	DMSO	11
22^d	CaCl2	12	110	DMSO	—
23^e	CaCl2	12	110	DMSO	Trace
24	—	12	110	DMSO	—
25^f	CaCl2	12	110	DMSO	65
26^g	CaCl2	12	110	DMSO	84
27	LiCl	12	110	DMSO	31
28	NaCl	12	110	DMSO	12
29	KCl	12	110	DMSO	15
30	CsF	12	110	DMSO	38

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Thus in a general experimental procedure a mixture of fluoroarene (1.0 mmol), diaryl diselenide (0.6 mmol), calcium chloride (3.0 mmol), and Zn dust (1.1 mmol) in DMSO (3 mL) was stirred under an argon atmosphere at 110 °C for 12 h. After evaporation of DMSO, the crude product was extracted with ethyl acetate. The pure product was isolated by column chromatography over silica gel. A wide range of diversely substituted diphenyl selenides have been obtained by this procedure. All the products are summarized in Table 2. Several (ten) of these compounds are reported for the first time.

Table 2 Calcium mediated selenylation of fluoroarenes via C–F bond cleavage^a,b

a Reaction conditions: fluoroarenes (1.0 equiv.), diaryl diselenide (0.6 equiv.), calcium chloride (3.0 equiv.), Zn dust (1.1 equiv.), argon atmosphere, 12 h, 110 °C. b Isolated yield.

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The fluoroarenes containing an electron withdrawing group (–NO₂, –CN, –CHO, –COCH₃, –COPh) underwent an efficient reaction with substituted diphenyl diselenides (–Me, –Cl, –OMe) to afford the corresponding selenides (3a–u). The unsubstituted and electron donating group substituted fluoroarenes are not effective for this reaction. Perfluoroarenes such as hexafluorobenzene and octafluorotoluene also did not produce the corresponding product under standardized conditions. It may be due to the less electrophilic nature of the arenes as well as less nucleophilic nature of PhSe under our experimental conditions. The other halogen (Cl, Br, I) substituted arenes also remained inert under these conditions. The Cl-substituted diphenyl diselenide reacted in the usual way keeping the Cl group unaffected during the reaction (3e and 3s) and it may be used for further functionalisation. In addition, the other functionalities (–NO₂, –CN, –CHO, –COCH₃, –COPh) present in the selenide products may be of much importance for further derivatisation. For example, 2-nitrophenyl-phenyl selenide obtained from 1-fluoro-2-nitrobenzene was successfully converted to 2-furylphenyl-phenyl selenide 4a by manipulation of nitro functionality (Scheme 2).^24a,b

Scheme 2 Synthesis of 2-furylphenyl-phenyl selenide from 2-fluoronitrobenzene.

The unique feature of this protocol is the use of fluoroarenes for the C–Se bond formation involving diphenyl diselenide which was not addressed earlier except in one reaction with a specific substrate^24c where the selenylation was performed using diphenyl diselenide and sodium borohydride. Notably, our method uses zinc dust and thus is compatible with reducible functionalities. Moreover, this has general applicability with all types of electron deficient fluoroarenes.

This strategy is also applied for the synthesis of unsymmetrical diaryl tellurides (Scheme 3). In general we obtained relatively low yield in tellurides. This may be due to the low reactivity of tellurium species probably dictated by the larger size of Te.

Scheme 3 Calcium mediated C_(sp²)–Te bond formation.

We then turned our attention to find the mechanism for this Ca-mediated selenylation reaction. To check the involvement of a radical process, the reaction of 4-fluoro benzonitrile and diphenyl diselenide was performed in the presence of TEMPO (radical quencher), THF (electron receptor), and nitroarene (electron acceptor) separately. Notably, the reactions remained unaffected. So the possibility of a radical pathway is unlikely for this reaction. In this reaction, zinc dust has been used to cleave the Se–Se/Te–Te bond to make the PhSe/PhTe moiety available. It has been observed that no product was formed in the absence of calcium chloride (Table 1, entry 24). However, if a free nucleophile (PhSe/PhTe⁻) is available in the medium the reaction is expected to produce some amount of product. Thus the reaction is not likely to follow the classical Nucleophilic Aromatic Substitution (SNAr) pathway. To have a better understanding on the role of Ca in C–F bond cleavage the mechanism of the process has been investigated using quantum mechanical (QM) calculations. For this purpose, we have prepared six different models. Herein, models A, B, C, D and E denote 4-fluorobenzonitrile and Ph–Se–Zn–Se–Ph structures with Ca²⁺, without any metal cation, with Zn²⁺, with Ni²⁺ and with one explicit DMSO molecule ligating with Ca²⁺, respectively. On the other side, model F is represented by 4-fluorobenzonitrile and the Ph–Se–Ca–Se–Ph structure with a Ca²⁺ cation. The computational details of QM modeling approaches are described in the ESI.† Cyclic six membered transition state (TS_A) structures for the formation of selenides are found more stable with lower activation free energy (ΔG^‡ = 32.5 kcal mol⁻¹) in the presence of Ca²⁺ than the four membered TS_B (ΔG^‡ = 41.1 kcal mol⁻¹) structure obtained in the absence of metal salts. The C_(sp²)–Se and C_(sp²)–F bond distances for six membered TS structures for the models A, C, and D in the presence of Zn and metal salts (such as Ca²⁺, Zn²⁺, and Ni²⁺) are in the range of 2.77–2.94 Å and 1.84–2.18 Å, respectively. For the four membered TS_B, C_(sp²)–Se and C_(sp²)–F bond distances are 2.37 and 1.70 Å, respectively, which are significantly shorter than the distances obtained from the six TS structures. On the other side, the predicted activation barriers for the formation of selenides in the presence of Zn²⁺ and Ni²⁺ are 37.4 and 35.1 kcal mol⁻¹, respectively, at the SMD_DMSO/B3LYP/6-311+G(d,p) level of theory,²⁵ which are significantly higher than the activation energy barrier (32.5 kcal mol⁻¹) obtained in the presence of calcium metal salts. Moreover, Ca²⁺ having a vacant d orbital is a hard Lewis acid and possesses a high charge to size ratio whereas Zn²⁺ and Ni²⁺ are border line Lewis acids. According to the hard soft acid base principle, the hard Lewis base F⁻ prefers to bind to the hard Lewis acid Ca²⁺ than other border line Lewis acids. Therefore, Ca²⁺ makes F⁻ a good leaving group compared to the other metal salts which have been used in our studies. Hence, calcium salt mediated C–F bond cleavage for the formation of diaryl selenides is favorable than other metal salts. Our experimental results also showed this trend. However, the possibility of transmetallation during the reaction is also investigated. In the model F, transmetallation leads to a Ph–Se–Ca–Se–Ph type intermediate from Ph–Se–Zn–Se–Ph. Moreover, in the TS_F structure, Se–Ca distances are significantly elongated (2.87 and 2.97 Å) compared to those in other TS structures (Fig. 1). The free energy of activation for the model F via TS_F for the formation of selenides is 26.6 kcal mol⁻¹ at the SMD_DMSO/B3LYP/6-311+G(d,p) level of theory which is lower than the value derived from the other models (Fig. 2). Moreover, the C_(sp²)–Se and C_(sp²)–F bond distances in the TS_E are 2.67 and 1.57 Å, respectively, which are slightly shorter than the aforementioned distances in the other TS structures. The computed reaction free energy along the model E is −28.6 kcal mol⁻¹ which indicates that the formation of selenides is energetically favorable. According to our study, transmetallation helps to decrease the free energy barrier from 32.5 (model A) to 26.6 kcal mol⁻¹ (model F). Therefore, our results show that formation of diaryl selenides proceeds via the Ph–Se–Ca–Se–Ph intermediate than Ph–Se–Zn–Se–Ph. Furthermore, we have also explored the role of DMSO in the mechanism using model E, where one DMSO molecule possesses substantial ligating interaction with the Ca²⁺ metal centre. In the TS_E, the DMSO molecule is interacting with the Ca²⁺ ion (Ca²⁺–O: 2.15 Å) although, the computed activation free energy for the model E is higher than the value obtained from the model F.

Fig. 1 Key geometrical distances (Å) of the TS structures from different models for the formation of selenides are obtained at the SMD_(DMSO)B3LYP/6-311+G(d,p) level of theory at 110 °C.

Fig. 2 Computed Gibbs free energy (in kcal mol⁻¹) profiles for the C_(sp²)–F bond activation in fluoroarenes via C_(sp²)–Se bond formation are obtained using different models at the SMD_DMSO/B3LYP/6-311+G(d,p) level of theory at 110 °C. Moreover, RS, TS and PS represented the reactant state, transition state and product state, respectively.

Based on the results of the computational study we have proposed a possible reaction pathway (Scheme 4). In the first step diphenyl diselenide reacts with Zn dust to produce (PhSe)₂Zn (A). Then the (PhSe)₂Zn (A) interacts with calcium to give an intermediate (B) through transmetallation. Subsequently, (PhSe)₂Ca reacts with electron deficient fluoroarene in the presence of calcium chloride to form the stable six membered transition state C which ultimately leads to the corresponding diphenyl selenide.

Scheme 4 Possible reaction pathway.

Conclusion

In conclusion, we have achieved direct calcium chloride mediated C–F bond activation in electron deficient fluoroarenes towards C–chalcogen bond formation in the absence of any additive/ligand leading to the synthesis of functionalised organo-selenides and tellurides. This protocol provides enormous scope for further manipulation to obtain new selenide derivatives. To the best of our knowledge this is the first report of C–F functionalisation of fluoroarenes using only Ca-salt in the absence of any strong base and organometallic reagent. The role of calcium in the cleavage of the C–F bond and the mechanism of the reaction has been established by using DFT analysis. This work thus opens a new vista for functionalisation of the apparently inert C–F bond by a simple operation using an inexpensive reagent, calcium chloride.

Experimental section

General

IR spectra were taken as thin films for liquid compounds and as KBr pellets for solids. NMR spectra were recorded at 300, 400 and 500 MHz for ¹H NMR spectra and at 75, 100 and 125 MHz for ¹³C NMR spectra in CDCl₃ solutions. HRMS analysis was performed in a Qtof mass analyzer using an ESI ionization method. Elemental analyses are done at our institute using an autoanalyser. The all substituted diaryl diselenides were synthesized using previously reported procedures.^26e

Representative experimental procedure for the synthesis of (2-nitrophenyl)(m-tolyl)selane (Table 2, 3a)

A mixture of 2-fluoronitrobenzene (141 mg, 1.0 mmol), 3-methyl diphenyl diselenide (204 mg, 0.6 mmol), calcium chloride (333 mg, 3.0 mmol), and Zn dust (72 mg, 1.1 mmol) in DMSO (3 mL) was heated at 110 °C (bath temperature) for 12 h (TLC) in an oil bath under an argon atmosphere. The reaction mixture was then allowed to cool and DMSO was evaporated under vacuum. The residue was extracted with EtOAc (3 × 20 mL). The extract was washed with water (10 mL) and brine (10 mL). The organic phase was dried (Na₂SO₄) and evaporated to afford the crude product, which was purified by column chromatography over silica gel (hexane) to provide the pure (2-nitrophenyl)(m-tolyl)selane as a pale yellow solid (245 mg, 84%), mp 79–82 °C, ¹H NMR (CDCl₃, 300 MHz) δ 2.40 (s, 3H), 7.01–7.04 (m, 1H), 7.28–7.38 (m, 4H), 7.49–7.53 (m, 2H), 8.28–8.32 (m, 1H); ¹³C NMR (CDCl₃, 75 MHz) δ 21.4, 125.8, 126.1, 127.9, 130.0, 130.4, 130.8, 133.7, 134.5, 136.2, 138.0, 140.1, 145.7; IR (KBr): 2984, 2883, 1587, 1512, 1330 cm⁻¹; anal. calcd for C₁₃H₁₁NO₂Se: C 53.44, H 3.79, N 4.79; found: C 53.42, H 3.82, N 4.75%.

This procedure was followed for all the reactions listed in Table 2, Schemes 2 and 3. Many of these compounds have not been reported earlier and these unknown compounds were characterized suitably by their spectroscopic and spectrometric data (IR, ¹H NMR, ¹³C NMR, HRMS and elemental analysis). The known compounds were identified by comparison of their spectroscopic and spectrometric data with those reported earlier. All these data are provided here.

2-((4-Methoxyphenyl)selanyl)benzonitrile (Table 2, 3b)^26a. (256 mg, 89%), ¹H NMR (CDCl₃, 300 MHz) δ 3.83 (s, 3H), 6.90–6.93 (m, 2H), 7.10–7.13 (m, 1H), 7.19–7.34 (m, 2H), 7.56–7.59 (m, 3H); ¹³C NMR (CDCl₃, 75 MHz) δ 55.5, 113.5, 115.7 (2C), 117.5, 117.6, 126.4, 130.8, 133.0, 133.6, 138.1 (2C), 139.4, 160.8.

2-(p-Tolylselanyl)benzaldehyde (Table 2, 3c). Light red liquid (184 mg, 67%), ¹H NMR (CDCl₃, 300 MHz) δ 2.42 (s, 3H), 7.01–7.04 (m, 1H), 7.22–7.32 (m, 4H), 7.54–7.56 (m, 2H), 7.81–7.84 (m, 1H), 10.19 (s, 1H); ¹³C NMR (CDCl₃, 75 MHz) δ 21.5, 124.5, 125.4, 129.9, 130.8 (2C), 133.9, 135.2, 137.0 (2C), 139.5, 140.3, 192.6; IR (neat): 3016, 2921, 1691, 1672, 1583, 1452, 1218 cm⁻¹; anal. calcd for C₁₄H₁₂OSe: C 61.10, H 4.40; found: C 61.14, H 4.35%.

1-((4-Phenylselanyl)phenyl)ethanone (Table 2, 3d)^23a. (247.5 mg, 90%), ¹H NMR (CDCl₃, 300 MHz) δ 2.54 (s, 3H), 7.34–7.39 (m, 5H), 7.57–7.60 (m, 2H), 7.77–7.79 (m, 2H); ¹³C NMR (CDCl₃, 75 MHz) δ 26.5, 128.7, 128.9 (2C), 129.8 (2C), 130.4 (2C), 133.6, 135.2 (2C), 135.3, 140.3, 197.3.

4-((4-Chlorophenyl)selanyl)benzonitrile (Table 2, 3e)^26d. (255 mg, 87%), ¹H NMR (CDCl₃, 500 MHz) δ 7.34–7.37 (m, 4H), 7.48 (d, J = 8.5 Hz, 2H), 7.53 (d, J = 8.5 Hz, 2H); ¹³C NMR (CDCl₃, 125 MHz) δ 110.2, 118.7, 126.0, 130.3 (2C), 130.6 (2C), 132.6 (2C), 135.7, 136.9 (2C), 140.3.

4-(p-Tolylselanyl)benzonitrile (Table 2, 3f)^26d. (247.5 mg, 91%), ¹H NMR (CDCl₃, 300 MHz) δ 2.39 (s, 3H), 7.20 (d, J = 7.8 Hz, 2H), 7.27–7.31 (m, 2H), 7.40–7.44 (m, 2H), 7.49–7.52 (m, 2H); ¹³C NMR (CDCl₃, 75 MHz) δ 21.4, 109.4, 118.9, 123.6, 129.8 (2C), 130.9 (2C), 132.4 (2C), 136.1 (2C), 139.7, 141.8.

2-(p-Tolylselanyl)benzonitrile (Table 2, 3g)^23a. (220 mg, 81%), ¹H NMR (CDCl₃, 300 MHz) δ 2.38 (s, 3H), 7.17–7.34 (m, 4H), 7.50–7.52 (m, 2H), 7.58–7.64 (m, 2H); ¹³C NMR (CDCl₃, 75 MHz) δ 21.4, 114.3, 116.7, 117.7, 124.2, 126.7 (2C), 130.8 (2C), 131.7, 133.0, 133.7, 135.9, 139.5.

(2-Nitrophenyl)(phenyl)selane (Table 2, 3h)^23d. (245 mg, 88%), ¹H NMR (CDCl₃, 500 MHz) δ 7.00 (d, J = 7.5 Hz, 1H), 7.25–7.32 (m, 2H), 7.45–7.53 (m, 3H), 7.70 (d, J = 7.0 Hz, 2H), 8.30–8.32 (m, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 125.9, 126.2, 128.3, 130.0, 130.2 (2C), 130.4, 133.7, 136.0, 137.5 (2C), 145.7.

(4-Nitrophenyl)(phenyl)selane (Table 2, 3i)^23d. (256 mg, 92%), ¹H NMR (CDCl₃, 500 MHz) δ 7.33–7.38 (m, 2H), 7.50 (d, J = 8.5 Hz, 2H), 7.58–7.60 (m, 2H), 7.79 (d, J = 8.0 Hz, 2H); ¹³C NMR (CDCl₃, 100 MHz) δ 123.7 (2C), 128.3, 129.7, 129.8 (2C), 132.1 (2C), 134.4 (2C), 136.7, 151.5.

1-(2-(Phenylselanyl)phenyl)ethanone (Table 2, 3j)^26b. (206 mg, 75%), ¹H NMR (CDCl₃, 500 MHz) δ 2.69 (s, 3H), 6.97–6.99 (m, 1H), 7.19–7.23 (m, 2H), 7.40–7.47 (m, 3H), 7.69–7.70 (m, 2H), 7.94–7.96 (m, 1H); ¹³C NMR (CDCl₃, 100 MHz) δ 27.3, 124.8 (2C), 129.2, 129.7, 129.8 (2C), 131.7, 132.6, 134.2, 137.5, 140.6, 198.8.

4-(Phenylselanyl)benzonitrile (Table 2, 3k)^23a. (237 mg, 92%), ¹H NMR (CDCl₃, 500 MHz) δ 7.23 (d, J = 7.5 Hz, 2H), 7.28–7.36 (m, 5H), 7.51 (d, J = 7.0 Hz, 2H); ¹³C NMR (CDCl₃, 125 MHz) δ 109.7, 118.8, 127.6, 129.2, 130.0 (2C), 130.3 (2C), 132.4 (2C), 135.7 (2C), 141.0.

4-(m-Tolylselanyl)benzonitrile (Table 2, 3l). Pale yellow solid (245 mg, 90%), mp 45–47 °C, ¹H NMR (CDCl₃, 300 MHz) δ 2.34 (s, 3H), 7.20–7.25 (m, 2H), 7.29–7.33 (m, 2H), 7.37–7.43 (m, 4H); ¹³C NMR (CDCl₃, 75 MHz) δ 21.3, 109.6, 118.9, 127.2, 129.8, 130.1 (2C), 130.2, 132.4 (2C), 132.8, 136.3, 140.0, 141.3; IR (KBr): 3072, 2921, 2852, 2225, 1585, 1477 cm⁻¹; anal. calcd for C₁₄H₁₁NSe: C 61.77, H 4.07, N 5.15; found: C 61.73, H 4.10, N 5.13%.

1-(4-(p-Tolylselanyl)phenyl)ethanone (Table 2, 3m). White solid (246 mg, 85%), mp 103–106 °C, ¹H NMR (CDCl₃, 500 MHz) δ 2.38 (s, 3H), 2.54 (s, 3H), 7.18 (d, J = 8.0 Hz, 2H), 7.32 (d, J = 8.5 Hz, 2H), 7.50 (d, J = 7.5 Hz, 2H), 7.76 (d, J = 8.5 Hz, 2H); ¹³C NMR (CDCl₃, 125 MHz) δ 21.4, 26.6, 124.5, 129.0 (2C), 129.8 (2C), 130.7 (2C), 135.0, 135.8 (2C), 139.1, 141.3, 197.4; IR (KBr): 3444, 2922, 2852, 1674, 1581, 1271 cm⁻¹; anal. calcd for C₁₅H₁₄OSe: C 62.29, H 4.88; found: C 62.32, H 4.83%.

(4-Nitrophenyl)(m-tolyl)selane (Table 2, 3n). Yellow solid (257 mg, 88%), mp 49–51 °C, ¹H NMR (CDCl₃, 300 MHz) δ 2.36 (s, 3H), 7.23–7.28 (m, 2H), 7.31–7.36 (m, 2H), 7.40–7.45 (m, 2H), 7.99–8.03 (m, 2H); ¹³C NMR (CDCl₃, 75 MHz) δ 21.4, 124.1 (2C), 127.0, 129.7 (2C), 129.9, 130.4, 136.6, 140.2, 144.3, 146.3; IR (KBr): 3053, 2921, 1573, 1508, 1340 cm⁻¹; anal. calcd for C₁₃H₁₁NO₂Se: C 53.44, H 3.79, N 4.79; found: C 53.47, H 3.75, N 4.81%.

2-(Phenylselanyl)benzonitrile (Table 2, 3o)^23a. (214 mg, 83%), ¹H NMR (CDCl₃, 300 MHz) δ 7.25–7.35 (m, 2H), 7.36–7.39 (m, 4H), 7.56–7.61 (m, 3H); ¹³C NMR (CDCl₃, 75 MHz) δ 114.8, 117.5, 127.1, 128.0, 128.9, 129.8 (2C), 132.3, 133.1, 133.7, 135.2 (2C), 137.5.

4-((4-Methoxyphenyl)selanyl)benzonitrile (Table 2, 3p)^23d. (262 mg, 91%), ¹H NMR (CDCl₃, 300 MHz) δ 3.84 (s, 3H), 6.91–6.94 (m, 2H), 7.23–7.26 (m, 2H), 7.39–7.42 (m, 2H), 7.54–7.57 (m, 2H); ¹³C NMR (CDCl₃, 75 MHz) δ 55.5, 109.2, 115.8 (2C), 117.1, 119.0, 129.2 (2C), 32.3 (2C), 138.2 (2C), 142.5, 160.8.

Phenyl(4-(m-tolylselanyl)phenyl)methanone (Table 2, 3q). Pale yellow gummy mass (295 mg, 84%), ¹H NMR (CDCl₃, 300 MHz) δ 2.34 (s, 3H), 7.16–7.24 (m, 2H), 7.36–7.47 (m, 6H), 7.52–7.58 (m, 1H), 7.62–7.66 (m, 2H), 7.73–7.74 (m, 2H); ¹³C NMR (CDCl₃, 75 MHz) δ 21.4, 128.3 (2C), 128.4, 129.6 (2C), 130.0 (2C), 130.1, 130.9 (2C), 132.4, 132.5, 135.6, 135.9, 137.7, 139.8, 139.9, 196.1; IR (neat): 3055, 2921, 2852, 1655, 1581, 1278 cm⁻¹; HRMS: m/z calcd for C₂₀H₁₇OSe: 353.0439 [M + H]⁺; found: 353.0469.

Phenyl(4-(phenylselanyl)phenyl)methanone (Table 2, 3r)^26c. (290 mg, 86%), ¹H NMR (CDCl₃, 500 MHz) δ 7.35–7.42 (m, 5H), 7.45–7.51 (m, 2H), 7.56–7.67 (m, 5H), 7.71–7.78 (m, 2H); ¹³C NMR (CDCl₃, 125 MHz) δ 128.4 (2C), 128.7 (2C), 129.8 (2C), 130.0, 130.3 (2C), 130.9 (2C), 132.5, 135.2 (2C), 135.7, 137.7, 139.6, 196.0.

(4-((4-Chlorophenyl)selanyl)phenyl)(phenyl)methanone (Table 2, 3s). Pale yellow solid (290 mg, 78%), mp 114–116 °C, ¹H NMR (CDCl₃, 300 MHz) δ 7.30–7.39 (m, 2H), 7.40–7.61 (m, 7H), 7.66–7.72 (m, 2H), 7.75–7.78 (m, 2H); ¹³C NMR (CDCl₃, 75 MHz) δ 127.1, 128.4 (2C), 130.0 (2C), 130.5 (2C), 131.0 (2C), 132.6, 135.1, 136.0, 136.3 (2C), 137.6, 138.7, 196.0; IR (KBr): 3064, 2923, 2852, 1649, 1583, 1471, 1276 cm⁻¹; HRMS: m/z calcd for C₁₉H₁₄ClOSe: 372.9893 [M + H]⁺; found: 372.9890.

Phenyl(4-(p-tolylselanyl)phenyl)methanone (Table 2, 3t). White solid (281 mg, 80%), mp 87–89 °C, ¹H NMR (CDCl₃, 300 MHz) δ 2.38 (s, 3H), 7.18 (d, J = 7.8 Hz, 2H), 7.35–7.38 (m, 2H), 7.43–7.57 (m, 5H), 7.63–7.67 (m, 2H), 7.74–7.78 (m, 2H); ¹³C NMR (CDCl₃, 75 MHz) δ 21.4, 124.6, 128.4 (2C), 129.6 (2C), 130.0 (2C), 130.7 (2C), 130.8 (2C), 132.4, 135.4 (2C), 135.8, 137.8, 139.1, 140.5, 196.1; HRMS: m/z calcd for C₂₀H₁₇OSe: 353.0439 [M + H]⁺; found: 353.0442.

(4-((4-Methoxyphenyl)selanyl)phenyl)(phenyl)methanone (Table 2, 3u). Pale yellow solid (319 mg, 87%), mp 97–100 °C, ¹H NMR (CDCl₃, 300 MHz) δ 3.83 (s, 3H), 6.90–6.93 (m, 2H), 7.28–7.32 (m, 2H), 7.42–7.47 (m, 2H), 7.53–7.65 (m, 5H), 7.72–7.74 (m, 2H); ¹³C NMR (CDCl₃, 75 MHz) δ 55.4, 115.6 (2C), 117.9, 128.3 (2C), 128.8 (2C), 129.9 (2C), 130.8 (2C), 132.3, 135.1, 137.8, 138.0 (2C), 141.4, 160.6, 196.0; IR (KBr): 3471, 3055, 2923, 2846, 1647, 1583, 1489, 1280, 1246 cm⁻¹; HRMS: m/z calcd for C₂₀H₁₇O₂Se: 369.0388 [M + H]⁺; found: 369.0387.

2-(2-(Phenylselanyl)phenyl)furan (Scheme 2, 4a). Brown gummy mass (225 mg, 75%), ¹H NMR (CDCl₃, 300 MHz) δ 6.52–6.54 (m, 1H), 6.84 (d, J = 3.3 Hz, 1H), 7.06–7.11 (m, 1H), 7.16–7.28 (m, 2H), 7.32–7.36 (m, 3H), 7.55–7.59 (m, 3H), 7.66–7.69 (m, 1H); ¹³C NMR (CDCl₃, 75 MHz) δ 109.5, 111.6, 126.6, 128.3, 128.5, 129.7 (2C), 129.9, 131.1, 131.7, 132.1, 135.3 (2C), 142.3, 152.9; IR (neat): 3411, 3055, 1664, 1579, 1461, 1435 cm⁻¹; anal. calcd for C₁₆H₁₂OSe: C 64.22, H 4.04; found: C 64.30, H 4.01%.

1-(4-(Phenyltellanyl)phenyl)ethanone (Scheme 3, 5a)^23a. (165 mg, 51%), ¹H NMR (CDCl₃, 300 MHz) δ 2.55 (s, 3H), 7.26–7.38 (m, 3H), 7.60–7.63 (m, 2H), 7.71–7.79 (m, 2H), 7.80–7.83 (m, 2H); ¹³C NMR (CDCl₃, 75 MHz) δ 26.5, 113.4, 124.2, 128.8 (2C), 128.9, 129.9 (2C), 135.9, 136.0 (2C), 139.7 (2C), 197.7.

(4-Nitrophenyl)(phenyl)tellane (Scheme 3, 5b)^23a. (176 mg, 56%), ¹H NMR (CDCl₃, 300 MHz) δ 7.31–7.36 (m, 2H), 7.42–7.47 (m, 1H), 7.59 (d, J = 9.0 Hz, 2H), 7.85–7.88 (m, 2H), 7.97 (d, J = 8.7 Hz, 2H); ¹³C NMR (CDCl₃, 75 MHz) δ 112.7, 123.9 (2C), 127.9, 129.5, 130.3 (2C), 135.5 (2C), 140.4 (2C), 147.3

Acknowledgements

We are pleased to acknowledge the financial support from DST, New Delhi through an award of JCB Fellowship to B. C. R. (Grant no. SR/S2/JCB-11/2008). BCR also thanks Indian National Science Academy, New Delhi for the offer of INSA Senior Scientist to him. P. M. thanks CSIR for his fellowship. We thank the supercomputing facility at IACS for computational support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR, ¹³C NMR spectra of all the synthesized compounds and DFT calculation data. See DOI: 10.1039/c6qo00515b

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