Tetraalkynylstannanes in the Stille cross coupling reaction: a new effective approach to arylalkynes

Abstract

The Stille-type cross coupling reaction with tetraalkynylstannanes was studied in detail for the first time. The reaction provides a simple and effective route towards a variety of arylalkynes. The advantages and limitations of the proposed procedure are discussed.

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Introduction

For almost forty years after the pioneering research of John K. Stille, the cross coupling reaction named after him of organic electrophiles with organostannanes has been recognized as a powerful tool for the formation of carbon–carbon bonds (see reviews^1–5). The organotin compounds useful in the Stille reaction are mild reagents that tolerate a variety of functional groups and are the reagents of choice for delicate cross coupling syntheses of complex functionalized molecules.⁵ The Stille reaction has been thoroughly investigated and many advances have been made to expand both the scope and utility of this process.^5,6 In contrast to other cross coupling reactions, the Stille reaction has often been found to be effective and relatively undemanding, allowing for harsher conditions, as organostannanes are relatively insensitive to moisture and oxygen.⁵ On the other hand, the use of organostannanes such as Bu₃SnR raises problems with organotin contamination and waste. Both acute and long-term toxicities have been reported for many organotin reagents,^7,8 and methods designed to limit or avoid the presence of organotin by-products in reaction products have been developed.⁹ In general, the toxicity of alkylstannanes decreases as the size of the alkyl groups increases (Me₃SnX ∼ Et₃SnX ≫ Bu₃SnX ≫ Octyl₃SnX) and the number of alkyl groups decreases (R₃SnX > R₂SnX₂ > RSnX₃);⁹ R₄Sn may reveal enhanced delayed toxicity due to in vivo transformation into R₃SnX.¹⁰ However, the toxicity strongly depends on the nature of the organic group R.¹⁰ Due to the easy hydrolysis of the C(sp)–Sn bond, it is generally accepted that tetraalkynyltin compounds (RC≡C)₄Sn are far less toxic than other organotin species having C(sp²)–Sn or C(sp³)–Sn bonds.

Another feature of tetraalkynylstannanes is their high atom economy. A practical disadvantage of the use of tin mono-functional reagents such as R–Sn(alkyl)₃ (R is aryl, vinyl or alkynyl) is that a reactant of a high molecular weight is used to introduce a hydrocarbon group of a (relatively) low molecular weight, at the same time producing bulky and highly toxic triorganotin waste. Since each of the four alkynyl fragments in (RC≡C)₄Sn is reactive, tetraalkynyltin compounds may be compared with sodium acetylides with respect to low molecular weight and producing only inorganic Sn(IV) waste of low toxicity. It is noteworthy that, generally, the reactions involving organostannanes (e.g., classical Stille coupling or any other organotin-mediated process) are considered to be of a low atom economy, due to the loss of heavy and toxic tin-containing moieties.¹¹ In other words, the E-factor¹² (which is defined as the mass ratio of waste to desired product) of the Stille reaction with R–Sn(alkyl)₃ agents is much higher than that expected for coupling reactions with tetraalkynylstannanes, and the latter reactions could be considered as more environmentally benign. The advantages of the use of organotin compounds capable of transferring more than one organyl group are illustrated by the reactions of tetraallylstannane with electrophilic substrates.¹³ Tetraallylstannane is a gentle nucleophile for allylation reactions and easily reacts with imines,¹⁴ aldehydes,^15,16 phenacyl bromide,¹⁷ other ketones¹⁸ or carbon dioxide¹⁹ (Scheme 1). In contrast to allyltrialkylstannyl reagents which transfer only one organyl moiety out of four groups on the tin atom, from two to four allyl residues can be utilized in the case of Sn(CH₂CH=CH₂)₄.

Scheme 1 Typical reactions of tetraallylstannane.

The alkynyl fragment can be easily introduced onto a tin atom in different ways. Recently we have developed two convenient and effective methods of synthesis of tetraalkynylstannanes 1 (Scheme 2); the first is based on the direct reaction of terminal alkynes with SnCl₄ in the presence of anhydrous ZnCl₂ and diethylamine^20,21 and the next is based on the reaction of tin tetra(N,N-diethylcarbamate) with phenylacetylene.²² Tetraalkynylstannanes 1 are oily or solid compounds that can be easily purified and isolated in good yields after column chromatography on a silanized silica. Also they are stable enough to be stored in a freezer for months.

Scheme 2 Synthesis of tetraalkynylstannanes 1.

While much is known about the mono-, di- and trialkynyl tin compounds, less is known about the chemistry of tetraalkynyltin compounds. Only a limited number of reactions are reported encompassing (RC≡C)₄Sn as reagents (Scheme 3). Thus, the reactions with Grignard reagents were recognized as a convenient method of smooth transmetallation of tetraalkynylstannanes for the preparation of tri- and dialkynyltins.^23,24 The organoboration of (RC≡C)₄Sn with trialkylboranes led to the formation of 1,1′-spirobistannoles 2.^25–28 Tetra(phenylethynyl)-tin was reported to be an efficient catalyst of ring-opening polymerization of L-lactide to poly(L-lactide).²⁹ As expected for tetraalkynylstannanes 1, they may also react with acyl chlorides (4 eq.) to afford alkynyl ketones.³⁰

Scheme 3 The known reactions of tetraalkynylstannanes 1.

To the best of our knowledge, Stille-type cross coupling reactions with tetraalkynylstannanes have not been described in the literature prior to the present work. Recently, French researchers reported³¹ the Stille cross coupling reaction of di- or trialkynylstannanes with iodovinylic acids/esters, first introducing a half or a third equivalent of di- or tri-functional organotin compounds. This is the only previous report on Stille cross coupling with multi-functional C(sp)–Sn organotin compounds. In this paper, we wish to report the first example of a Stille-type cross coupling reaction of aryl halides 3 with tetraalkynyltin compounds 1.

Results and discussion

We found that tetraalkynylstannanes 1 easily react with a variety of aryl iodides and bromides under Stille conditions according to the following scheme (Scheme 4):

Scheme 4 The reaction of tetraalkynylstannanes 1 with aryl halides 3.

The tetraorganylstannanes 1 and aryl halides 3 used in the reaction are shown in Fig. 1. To prevent side reactions such as hydrolysis of stannanes 1 or oxidative couplings, the reactions should be conducted in an inert atmosphere (argon) wherein water and oxygen are excluded. Diaryl diacetylenes 5 are by-products probably derived from the Pd-mediated Glaser-type coupling reaction occurring in the presence of trace oxygen.

Fig. 1 The scope of stannanes 1 and aryl halides 3 used.

A number of attempts have been made to optimize the coupling reaction conditions. We found that a variety of factors may affect the reaction outcome, such as the nature and quantity of amine additive used, the nature of the Pd catalysts and solvents, the temperature and the reaction time. As a model reaction, we examined the coupling of tetra(phenylethynyl)tin 1a with p-nitroiodobenzene 3d under different conditions (Scheme 5). First, we examined the effect of different solvents and amine additives on the yields of the target aryl acetylene 4ad, using Pd(PPh₃)₂Cl₂ as a catalyst. The selected results are summarized in Table 1; the complete set of data is given in ESI,† Table S1.

Scheme 5 The model reaction of tetra(phenylethynyl)tin 1a with p-nitroiodobenzene 3d.

Table 1 The effect of solvents and amines on the yields of aryl acetylene 4ad and the side product, diphenyl diacetylene 5a^a

Solvent	T (°C)	Et2NH			Et3N			DABCO
		Yield of 4ad^b (%)	Yield of 5a^b (%)	Time^c (h)	Yield of 4ad^b (%)	Yield of 5a^b (%)	Time^c (h)	Yield of 4ad^b (%)	Yield of 5a^b (%)	Time^c (h)
a The reaction conditions were as follows: (PhC≡C)4Sn 1a (20 mg, 0.038 mmol), 4-NO2C6H4I 3d (0.153 mmol), Pd(PPh3)2Cl2 (5.4 mg, 0.0077 mmol; 5 mol% vs.3d), amine (0.15 mmol), and solvent (2 mL). b Yields were determined by GC-MS. c Reaction time when the highest yield was achieved.
Et2O	35	0	0	5	0	3	5	71	6	5
THF	80	—	—	—	20	22	5	50	6	1
PhMe	100	55	10	7	27	14	5	87	8	0.5
MeCN	85	89	6	5	68	13	5	63	6.5	5
Dioxane	100	84	2	3	44	6	2	78	8	5
AcOEt	80	91	1	9	87	4	2	87	4	0.5
AcOBu	125	98	2	1	98	2	2	87	5	2
AcOBu	100	98	2	2	85	2	5	86	5	2
Et3N	80	—	—	—	96	4	2.5	—	—	—
DMF	100	—	—	—	89	9	2.5	—	—	—

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As can be seen, the best results were obtained with BuOAc, EtOAc, DMF and pure Et₃N as solvents in the temperature range of 80–125 °C, while the use of less polar (dioxane, PhMe) and low-boiling (Et₂O) solvents resulted in lower yields of the aryl acetylene 4ad with increased amounts of the diacetylene by-product 5a. Though only a few examples^32–34 have been reported for the amine-promoted Stille reaction, we found that the presence of an amine additive is strongly required for the reaction of tetraalkynylstannanes 1 with aryl halides 3. In the absence of an amine no reaction occurs, while even trace amounts give coupling products, albeit in low yields. The nature of the amine additive as well as its amount has a dramatic effect on the reaction course, as shown in Table 2. The best results were obtained with the strong bases such as Et₃N, Bu₃N, DABCO, and especially with Et₂NH and Pr₂NH. The application of benzylic amines, piperidine, morpholine and N-methylmorpholine lowered significantly the yield of 4ad, whereas pyridine and ethylene diamine were found to be completely inactive. The full set of data on the amines used is given in ESI,† Table S1. The amine concentration was also important and strongly influenced the reaction rate. Thus, the reaction proceeded more slowly in the presence of 1 eq. Et₂NH (the yield of 4ad reached only 66% after 5 h in EtOAc at 80 °C), and quite rapidly when the amount of Et₂NH was increased. The use of a large excess of Et₂NH under the same conditions produced tolane 4ad in almost quantitative yields after 2 h (Fig. 2).

Table 2 Effects of the different amines and solvents on the yields of 4ad

Amine	The yield of aryl acetylene 4ad^a^,^b (%)
	AcOBu	Time^c (h)	PhMe	Time^c (h)	Dioxane	Time^c (h)
a Yields were determined by GC-MS. b The reaction conditions were as follows: 1a (0.038 mmol), 3d (0.153 mmol), Pd(PPh3)2Cl2 (5 mol% vs.3d), amine (0.153 mmol) and solvent (2 mL) at 100 °C. c Reaction time when the highest yield was achieved.
Et2NH	98	2	55	7	84	3
Et3N	85	5	27	5	44	2
Bu3N	97	3	31	5	87	3
DABCO	68	2	87	0.5	78	0.5
Isophorone diamine	—	—	54	7	86	9
Morpholine	—	—	—	—	54	1
N-Methyl-morpholine	12.5	1	—	—	73	2
Pyridine	—	—	—	—	0	3
Piperidine	62	2	49	5	55	2
(CH2)2(NH2)2	0	3	—	—	—	—
PhCH2NH2	41	5	43	5	56	5
(PhCH2)2NH	—	—	11	3	26	5
Pr2NH	99.5	1	—	—	—	—

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Fig. 2 The kinetics of the reaction 1a + 3d → 4ad with different amounts of Et₂NH. The reaction conditions were as follows: (PhC≡C)₄Sn 1a (20 mg, 0.038 mmol), 4-NO₂C₆H₄I 3d (0.153 mmol), Pd(PPh₃)₂Cl₂ as the catalyst (5 mol% with respect to 3d), AcOEt (2.0 mL), 80 °C.

To optimize the conditions, we studied the effect of excessive amounts of different amines on the kinetics of the reaction. It was found that other amines were also as effective as Et₂NH when they were used in high excess. The results are summarized in Table 3.

Table 3 Effects of the amine additive and its amount on the reaction outcome^a

Amine	Volume (μL)	Aryl iodide 3a : amine ratio	The yields^b of		Time^d (h)
			4ad (%)	5a (%)
a Unless otherwise stated, the conditions were as follows: 1a (0.038 mmol), 3d (0.153 mmol), Pd(PPh3)2Cl2 (5 mol% with respect to 3d), BuOAc, 100 °C, total volume solvent + amine = 1000 μL. b Yields were determined by GC-MS. c TMEDA = Me2NCH2CH2NMe2. d Reaction time when the maximum yield was achieved.
Et2NH	900	1 : 57	96.4	1.5	6
Pr2NH	21	1 : 1	99.5	0.5	1
Pr2NH	105	1 : 5	99.2	0.8	1
Pr2NH	210	1 : 10	98.8	1.2	1
Pr2NH	840	1 : 40	98.9	1.1	1
Pr2NH	900	1 : 43	99.0	1.0	1
Pr2NH	1000	1 : 47	98.3	1.7	1
Bu3N	36	1 : 1	75.7	2.0	4
Bu3N	182	1 : 5	99.5	0.5	1
Bu3N	640	1 : 10	99.5	0.5	1
Bu3N	900	1 : 25	83.7	9.2	4
TMEDA^c	23	1 : 1	95.0	1.5	4
TMEDA	115	1 : 5	97.7	2.3	1
TMEDA	229	1 : 10	97.2	2.8	1
TMEDA	920	1 : 40	97.3	2.7	1
TMEDA	1000	1 : 43	94.0	6.0	1

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We have to admit that the mechanistic picture of the Stille reaction is rather complex³⁵ and details cannot be specified with confidence, so the role of an amine and its amount still remains unclear and requires further investigation. We suggest that the reaction proceeds by way of formation of alkynyl-palladium complexes 6 and 7 according to Scheme 6.

Scheme 6 A possible mechanism for the reaction of tetraalkynylstannanes 1 with aryl halides 3.

The effect of different catalysts was studied to determine the best catalytic system. No reaction occurs without a catalyst: thus, when (PhC≡C)₄Sn 1a was added to 4-NO₂C₆H₄I 3d in pure Et₃N (80 °C, 2 h), no conversion was observed.

However, when a catalytic amount of CuI was added under the same conditions, a trace amount of the coupling product was detected by GC-MS. It is noteworthy that when (PhC≡C)₄Sn 1a was treated with a 4-fold excess of CuBr₂ (THF, 0.5 h, 25 °C), the oxidative Glaser-type coupling product (Ph–C≡C–)₂5a was formed in a good yield. The first success came with the use of Pd catalysts, especially Pd(PPh₃)₂Cl₂. To our surprise, the reaction was completely suppressed by the addition of an excess of phosphine ligand. Thus, no reaction between stannane 1a and 1-iodo-4-nitrobenzene 3d occurred in the presence of Pd(PPh₃)₂Cl₂ and PPh₃ (5 mol% and 20 mol% with respect to 3d, respectively), while Pd(PPh₃)₂Cl₂ with no PPh₃ additive gave the best yields. The results using different Pd catalysts are summarized in Table 4.

Table 4 Effects of the different catalysts on the yields of 4ad^a

Amine	The yield of 4-NO2C6H4C≡CPh 4ad^b (%)
	PdCl2	Time^c (h)	Pd(PhCN)2Cl2	Time^c (h)	Pd(PPh3)2Cl2	Time^c (h)
a The reaction conditions were as follows: 1a (0.038 mmol), 3d (0.153 mmol), Pd catalyst (5 mol% with respect to 3d), amine (0.153 mmol), BuOAc, 100 °C. b Yields were determined by GC-MS. c Reaction time when the maximum yield was achieved.
Et2NH	13	7	7	5	98	2
Et3N	72	3	71	2	89	7
Bu3N	60	7	50	5	97	3
DABCO	57	2	56	1	68	2

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Next, using an optimized protocol (BuOAc or DMF, 100 °C, 5 mol% Pd(PPh₃)₂Cl₂, and an excess (4-fold or more with respect to stannane 1) of an amine additive – Et₂NH, Pr₂NH, DABCO or Bu₃N), we studied the reactivity of different tetraalkynylstannanes 1 and aryl halides 3. As expected, aryl halides bearing electron-withdrawing groups showed the best reactivity and gave the highest yields of acetylenes 4. Selected results are summarized in Table 5; the complete set of data is given in ESI,† Table S1.

Table 5 The reactivity of different aryl halides 3 towards (PhC≡C)₄Sn 1a under the optimized conditions^a

Aryl Halides 3	Solvent	Time^b (h)	Amine	Yield of 4
a Yields were determined by GC-MS. Unless otherwise stated, the reaction conditions were as follows: 1a (0.038 mmol), 3d (0.153 mmol), Pd(PPh3)2Cl2 (5 mol% with respect to 3d), amine (0.153 mmol) and solvent (DMF or BuOAc, 2 mL) at 100 °C. b Reaction time when the maximum yield was achieved.
Ph–I 3a	DMF	2.5	Et3N	87
4-MeC6H4I 3b	DMF	5	Et3N	95
4-MeC6H4I 3b	BuOAc	9	Et2NH	54
4-NO2C6H4I 3d	DMF	2.5	Et3N	89
4-NO2C6H4I 3d	BuOAc	6	Et2NH	98
4-NO2C6H4I 3d	BuOAc	1	Pr2NH	99.5
4-NO2C6H4I 3d	BuOAc	3	Bu3N	97
4-MeC6H4Br 3h	BuOAc	9	Et2NH	11
4-MeOC6H4Br 3i	BuOAc	5	Et2NH	7
4-MeOC6H4Br 3i	BuOAc	3	DABCO	15
2-NO2C6H4I 3k	BuOAc	10	Et2NH	83
2-IC6H4CO2H 3l	BuOAc	5	Et2NH	0
2-IC6H4CO2H 3l	Et3N	4	Et3N	63

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However, when the reaction was carried out using DMF as a solvent instead of BuOAc, the coupling products with aryl halides 3 bearing electron-donating substituents were obtained in good yields. The analogs of (PhC≡C)₄Sn 1a, i.e., tetraalkynylstannanes 1b–g, reacted with 4-NO₂C₆H₄I 3d to form the desired acetylenes 4bd–gd, but the yields were generally lower than those obtained with 1a. The selected results are presented in Table 6. Finally, the Stille coupling products 4 were obtained in 40–93% yields in preparative-scale experiments using conditions similar to those of the kinetic runs and the optimization protocols. The results are given in Table 7.

Table 6 The reactivity of different tetraalkynylstannanes 1 towards 4-NO₂C₆H₄I 3d under the optimized conditions^a

Tetraalkynylstannanes 1	Time^b (h)	Amine	Yields of 4 (%)
a Yields were determined by GC-MS. The reaction conditions were as follows: 1 (0.038 mmol), 3d (0.153 mmol), Pd(PPh3)2Cl2 (5 mol% with respect to 3d), amine (0.153 mmol), BuOAc (2 mL) at 100 °C. b Reaction time when the maximum yield was achieved.
(PhC≡C)4Sn 1a	2	Et2NH	98
(t-BuOCH2C≡C)4Sn 1e	1	DABCO	74
(t-BuOCH2C≡C)4Sn 1e	2	Et2NH	33
(n-C8H18CH2C≡C)4Sn 1g	7	Et2NH	50
(n-C8H18CH2C≡C)4Sn 1g	5	DABCO	52

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Table 7 The preparative-scale synthesis of acetylenes 4

Tetraalkynyl stannane 1	Aryl halide 3	Product	Yield^a (%)
a Isolated yields are given. b When 2-Br-4-NO2C6H3I 3f and (PhC≡C)4Sn 1a were added in a 2 : 1 ratio, acetylene 4af was obtained in 81% yield.
(PhC≡C)4Sn 1a	Ph–I 3a	PhC≡CPh 4aa	78
(PhC≡C)4Sn 1a	4-MeC6H4I 3b	4-MeC6H4C≡CPh 4ab	81
(PhC≡C)4Sn 1a	4-BrC6H4I 3c	4-BrC6H4C≡CPh 4ac	80
(PhC≡C)4Sn 1a	4-NO2C6H4I 3d	4-NO2C6H4C≡CPh 4ad	93
(PhC≡C)4Sn 1a	2-Br-4-NO2C6H3I 3f	2-Br-4-NO2C6H3C≡CPh 4af	40^b
(PhC≡C)4Sn 1a	2-NO2C6H4I 3k	2-NO2C6H4C≡CPh 4ak	88
(PhC≡C)4Sn 1a	2-EtO2CC6H4I 3m	2-EtO2CC6H4C≡CPh 4am	81
(PhC≡C)4Sn 1a	4-BrC6H4CHO 3n	PhC≡CC6H4CHO 4an	82
(PhC≡C)4Sn 1a	4-BrC6H4C(O)Me 3o	PhC≡CC6H4C(O)Me 4ao	65
(4-MeC6H4C≡C)4Sn 1b	4-NO2C6H4I 3d	4-MeC6H4C≡CC6H4NO24bd	91
(4-ClC6H4C≡C)4Sn 1c	4-NO2C6H4I 3d	4-ClC6H4C≡CC6H4NO24cd	82
(t-BuOCH2C≡C)4Sn 1e	4-NO2C6H4I 3d	t-BuOCH2C≡CC6H4NO24ed	49
(n-BuC≡C)4Sn 1f	4-NO2C6H4I 3d	n-BuC≡CC6H4NO24fd	47

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Conclusions

In conclusion, we have developed an effective synthetic protocol based on the Stille cross coupling reaction of easily available tetraalkynylstannanes with aryl halides. The reported method provides atom-economical access to aryl acetylenes and diaryl acetylenes (tolanes) which are valuable reagents for further transformations. The scope and limitations of the reaction were studied and the conditions were optimized.

Experimental

Materials and methods

Solvents and starting reagents were thoroughly dried and purified according to common procedures.³⁶ All reactions were carried out and the target compounds were isolated in an argon (99.993%) atmosphere. ¹H, ¹³C, and ¹¹⁹Sn NMR spectra were recorded on a JEOL ECA 400 instrument at operating frequencies of 399.78, 100.52 and 149.08 MHz respectively, in CDCl₃ (Aldrich) with reference to TMS or to the residual signals of a solvent (with SnMe₄ as a standard for ¹¹⁹Sn NMR). Chemical shifts are given in ppm; coupling constants are given in Hz. IR spectra were recorded on an InfraLUM FT-02 instrument in the range of 400–4200 cm⁻¹ (KBr or HCCl₃ solution) and on a Bruker Vertex 70 instrument in ATR (attenuated total reflection) mode. Mass spectra (EI, 70 eV) were obtained on a Shimadzu GCMS-QP 2010 spectrometer. The purity of the compounds was checked by TLC (Sorbfil A plates) with Et₂O : hexane (10 : 1), MeOH : HCCl₃ (1 : 10) or HCCl₃ : Me₂CO (10 : 1) mixtures as eluents. The spots were visualized with iodine vapors, KMnO₄–H₂SO₄ solution or UV-light. The starting tetraalkynylstannanes 1a–g were obtained according to the reported methods;^20,21 the detailed procedures are given in the ESI.†

General procedure for the synthesis of 4-nitrotolane (1-nitro-4-(phenylethynyl)benzene) (4ad) (the model reaction, Scheme 5, Tables 1, 2 and 4)

A 5 mL sealable Wheaton vial was charged with 0.00765 mmol of Pd catalyst (PdCl₂, Pd(PPh₃)₂Cl₂, or Pd(PhCN)₂Cl₂) and 0.153 mmol of the amine additive (Et₂NH, Et₃N, Bu₃N, DABCO, morpholine, etc.). Then the vial was flushed with a stream of dry argon, and subsequently a solution of 0.0382 mmol of (PhC≡C)₄Sn 1a in a dry solvent (1 mL) and a solution of 0.153 mmol of 4-NO₂C₆H₄I in a dry solvent (1 mL) were added through a syringe. The mixture was stirred for the indicated time, and the yield was determined by GC-MS.

Preparative procedure for the synthesis of tolane (diphenyl acetylene) (4aa) from (PhC≡C)₄Sn (1a). A dry 25 mL, two-necked, round-bottomed flask equipped with an argon gas inlet tube and a magnetic stirrer was flushed with argon and charged with iodobenzene 3a (212.7 mg, 1.043 mmol), Pd(PPh₃)₂Cl₂ (36.6 mg, 0.052 mmol; 5 mol% vs.3a) and (PhC≡C)₄Sn 1a (150 mg, 0.287 mmol). Then Pr₂NH (1.43 mL, 10.43 mmol) and dry BuOAc (6 mL) were added, the solution was degassed by freezing in liquid nitrogen and pumping under vacuum several times, and then it was flushed with argon. The reaction mixture was stirred at 100 °C for 5 h, then allowed to cool and quenched with EtOH (10 mL). The mixture was treated with 0.5 g of silica modified with 3-aminopropyltriethoxysilane, the solvent was removed on a rotary evaporator and the traces of BuOAc were removed in vacuo. The resulting mixture was purified by column chromatography over silica gel (2 g) with pure dry PhMe (50 mL). The eluent was evaporated, the residue was dissolved in n-hexane and purified by column chromatography on a mixture of silica gel (6 g) and silica gel modified with 3-aminopropyltriethoxysilane (1 g, 1.14 mmol g⁻¹ of NH₂ groups; the use of the modified silica gel allowed us to remove easily the by-product SnI₄, which appears to be hard to separate when a non-modified silica gel is used as sorbent). Hexane was used as the eluent. Column fractions were analyzed by GC-MS. The eluent was evaporated to give 152.3 mg of tolane 4aa as a colorless crystalline solid (78%, purity by GCMS – 95.3%).

¹H NMR (400 MHz, CDCl₃) δ 7.33–7.73 (m, 6H, Ph), 7.52–7.54 (m, 4H, Ph); ¹³C NMR (100 MHz, CDCl₃) δ 89.4, 123.3, 128.2, 128.3, 131.6. IR (KBr, cm⁻¹) ν_max 3063, 2928 (C–H, C–C), 1599 (C=C). MS (m/z, EI, 70 eV) 178 ([M⁺], 100), 152 ([M − C₂H₂]⁺, 17.6), 77 ([Ph]⁺, 3.3).

4-Methyltolane (1-methyl-4-(phenylethynyl)benzene) (4ab). A dry 25 mL, two-necked, round-bottomed flask equipped with an argon gas inlet tube and a magnetic stirrer was flushed with argon and charged with 4-iodotoluene (3b) (227.4 mg, 1.043 mmol), Pd(PPh₃)₂Cl₂ (36.6 mg, 0.052 mmol; 5 mol% vs.3b) and (PhC≡C)₄Sn 1a (150 mg, 0.287 mmol). Then Pr₂NH (1.43 mL, 10.43 mmol) and dry BuOAc (6 mL) were added, the solution was degassed by freezing in liquid nitrogen and pumping under vacuum several times, and then it was flushed with argon. The reaction mixture was stirred at 100 °C for 5 h, then allowed to cool and quenched with EtOH (10 mL). The mixture was treated with 0.5 g of silica modified with 3-aminopropyltriethoxysilane, the solvent was removed on a rotary evaporator and the traces of BuOAc were removed in vacuo. The resulting mixture was purified by column chromatography on a mixture of silica gel (6 g) and silica gel modified with 3-aminopropyltriethoxysilane (1 g, 1.14 mmol g⁻¹ of NH₂ groups). Hexane was used as the eluent. The yield of 4ab was 81% (161.8 mg), white crystalline solid. ¹H NMR (400 MHz, CDCl₃) δ 2.35 (s, 3H, Me), 7.114 (d, ³J = 8.0 Hz, 2H, Ar), 7.29–7.35 (m, 3H, Ph), 7.42 (d, ³J = 8.0 Hz, 2H, Ar), 7.50–7.53 (m, 2H, Ph); ¹³C NMR (100 MHz, CDCl₃) δ 21.5, 88.7, 89.6, 120.2, 123.5, 128.1, 128.3, 129.1, 131.6, 132.5, 138.4. IR (KBr, cm⁻¹) ν_max 2920.6, 2853.1 (C–H, C–C), 2214.6 (C≡C), 1595.3 (C=C). MS (m/z, EI, 70 eV) 192 ([M⁺], 100), 115 ([M − Ph]⁺, 8.3), 77 ([Ph]⁺, 2.6).

4-Bromotolane (1-bromo-4-(phenylethynyl)benzene) (4ac). 4-Bromotolane (4ac) was prepared using a similar procedure to that for 4ab, using 1-bromo-4-iodobenzene (3c) (295 mg, 1.043 mmol) instead of 4-iodotoluene (3b). The yield was 80% (214.4 mg), white crystalline solid. ¹H NMR (400 MHz, CDCl₃) δ 7.32–7.35 (m, 3H, Ar), 7.38 (d, ³J = 8.7 Hz, 2H, Ar), 7.47 (d, ³J = 8.7 Hz, 2H, Ar), 7.50–7.53 (m, 2H, Ar); ¹³C NMR (100 MHz, CDCl₃) δ 88.3, 90.5, 122.3, 122.5, 122.9, 128.4, 128.5, 131.60, 131.62, 133.0. IR (KBr, cm⁻¹) ν_max 3049.8, 2924.4, 2855 (C–H, C–C), 2214.6 (C≡C), 1599.2 (C=C). MS (m/z, EI, 70 eV) 258 ([M^{+ 81}Br], 97.2), 256 ([M^{+ 79}Br], 100), 177 ([M − Br]⁺, 13.6), 77 ([Ph]⁺, 6.8).

4-Nitrotolane (1-nitro-4-(phenylethynyl)benzene) (4ad). 4-Nitrotolane was prepared using a similar procedure to that for 4ab, using 1-iodo-4-nitrobenzene (3d) (259.7 mg, 1.043 mmol) instead of 4-iodotoluene (3b). The reaction time was 1 h 40 min. The yield was 93% (216.5 mg), light yellow crystals. ¹H NMR (400 MHz, CDCl₃) δ 7.38–7.39 (m, 3H, Ph), 7.54–7.56 (m, 2H, Ph), 7.65 (d, ³J = 8.7 Hz, 2H, Ar), 8.20 (d, ³J = 8.7 Hz, 2H, Ar); ¹³C NMR (100 MHz, CDCl₃) δ 87.6, 94.7, 122.1, 123.6, 128.6, 129.3, 130.3, 131.9, 132.3, 147.0. IR (KBr, cm⁻¹) ν_max 2922.5, 2851.1 (C–H, C–C), 2216.5 (C≡C), 1591.5 (C=C), 1346.5 (symm NO₂). MS (m/z, EI, 70 eV) 223 ([M⁺], 100), 177 ([M − NO₂]⁺, 21.1), 77 ([Ph]⁺, 8.8).

2-Bromo-4-nitro-1-(phenylethynyl)benzene (4af). A vial was charged with 2-bromo-1-iodo-4-nitrobenzene (3f) (102.5 mg, 0.313 mmol), Pd(PPh₃)₂Cl₂ (11.0 mg, 0.016 mmol; 5 mol% vs.3f) and (PhC≡C)₄Sn 1a (90 mg, 0.172 mmol). Then Pr₂NH (0.43 mL, 3.13 mmol) and dry BuOAc (1.8 mL) were added, the solution was degassed by freezing in liquid nitrogen and pumping under vacuum several times, and then the vial was flushed with argon. The reaction mixture was stirred at 100 °C for 4.5 h, then allowed to cool and quenched with EtOH (2 mL). The mixture was treated with 0.2 g of silica gel modified with 3-aminopropyltriethoxysilane, the solvent was removed on a rotary evaporator and the traces of BuOAc were removed in vacuo. The resulting mixture was purified by column chromatography on a mixture of silica gel (6 g) and silica gel modified with 3-aminopropyltriethoxysilane (1 g, 1.14 mmol g⁻¹ of NH₂ groups), eluted with hexane, then hexane : PhMe 4 : 1. Column fractions were analyzed by GCMS. The eluent was evaporated to give 76 mg (81%) of acetylene 4af as a yellow crystalline solid. In addition, the sample could be recrystallized from n-heptane. ¹H NMR (400 MHz, CDCl₃) δ 7.38–7.42 (m, 3H, Ph), 7.59–7.61 (m, 2H, Ph), 7.68 (d, ³J = 8.2 Hz, 1H, H-6 Ar), 8.20 (dd, ³J = 8.2 Hz, ⁴J = 2.3 Hz, 1H, H-5 Ar), 8.48 (d, ⁴J = 2.3 Hz, 1H, H-3 Ar); ¹³C NMR (100 MHz, CDCl₃) δ 86.8, 99.5, 121.8, 122.1, 125.9, 127.6, 128.6, 129.7, 132.0, 132.1, 133.3, 146.9. IR (KBr, cm⁻¹) ν_max 3094.2, 3074.9 (C–H, C–C), 2218.4 (C≡C), 1583.8 (C=C), 1340.7 (symm NO₂). MS (m/z, EI, 70 eV) 303 ([M^{+ 81}Br], 59.4), 301 ([M^{+ 79}Br], 60.2), 257 ([(M − NO₂)⁺, ⁸¹Br], 0.8), 255 ([(M − NO₂)⁺, ⁷⁹Br], 0.9), 176 (([M − NO₂ − Br])⁺, 100), 77 ([Ph]⁺, 1.6).

2-Nitrotolane (2-nitro-4-(phenylethynyl)benzene) (4ak). 2-Nitrotolane was prepared using a similar procedure to that for 4ab, using 1-iodo-2-nitrobenzene (3k) (259.7 mg, 1.043 mmol) instead of 4-iodotoluene (3b). The reaction time was 3.5 h. The yield was 88% (205 mg), red oil. ¹H NMR (400 MHz, CDCl₃) δ 7.35–7.38 (m, 3H, Ph), 7.42–7.46 (m, 1H, Ar), 7.56–7.60 (m, 3H, Ar), 7.70 (d, ³J = 7.8 Hz, 1H, Ar), 8.06 (d, ³J = 8.3 Hz, 1H, Ar); ¹³C NMR (100 MHz, CDCl₃) δ 84.8, 97.1, 118.7, 122.4, 124.7, 128.45, 128.54, 129.2, 132.0, 132.8, 134.6, 149.6. IR (neat, cm⁻¹) ν_max 3059.9 (C–C, C–H), 2219.0 (C≡C), 1339.5 (symm NO₂). MS (m/z, EI, 70 eV) 223 ([M⁺], 5.2), 177 ([M − NO₂]⁺, 3.8), 77 ([Ph]⁺, 89.0).

Ethyl 2-(phenylethynyl)benzoate (4am). A dry 25 mL, two-necked, round-bottomed flask equipped with an argon gas inlet tube and a magnetic stirrer was flushed with argon and charged with 2-IC₆H₄C(O)OEt (3m) (287.9 mg, 1.043 mmol), Pd(PPh₃)₂Cl₂ (36.6 mg, 0.052 mmol; 5 mol% vs.3m) and (PhC≡C)₄Sn 1a (150 mg, 0.287 mmol). Then TMEDA (1.56 mL, 10.43 mmol) and dry BuOAc (6 mL) were added, the solution was degassed by freezing in liquid nitrogen and pumping under vacuum several times, and then it was flushed with argon. The reaction mixture was stirred at 100 °C for 5 h, then allowed to cool and quenched with EtOH (10 mL). The mixture was treated with 0.5 g of silica gel modified with 3-aminopropyltriethoxysilane, the solvent was removed on a rotary evaporator and the traces of BuOAc were removed in vacuo. The resulting mixture was purified by column chromatography on a mixture of silica gel (6 g) and silica gel modified with 3-aminopropyltriethoxysilane (1 g, 1.14 mmol g⁻¹ of NH₂ groups), eluted with hexane, then with hexane : PhMe 4 : 1 (after diphenyldiacetylene 5a was eluted). Column fractions were analyzed by GCMS. The eluent was evaporated to give 243 mg (purity by GCMS – 87.3%) of benzoate 4am as a yellow solid. The product was further purified with flash chromatography (2 g of silica gel, hexane). Yield 81%. ¹H NMR (400 MHz, CDCl₃) δ 1.40 (t, ³J = 7.3 Hz, 3H, OCH₂CH̲₃), 4.42 (q, ³J = 7.3 Hz, 2H, OCH̲₂CH₃), 7.34–7.39 (m, 4H, Ar), 7.46–7.50 (m, 1H, Ar), 7.56–7.58 (m, 2H, Ar), 7.64 (d, ³J = 8.2 Hz, 1H, Ar), 7.97 (dd, ³J = 7.8 Hz, ⁴J = 0.9 Hz, 1H, Ar); ¹³C NMR (100 MHz, CDCl₃) δ 14.4, 61.2, 88.3, 94.2, 123.4, 123.6, 127.9, 128.4, 128.5, 130.4, 131.5, 131.7, 132.3, 134.0, 166.4. IR (KBr, cm⁻¹) ν_max 3061.4, 2982.3 (C–H, C–C), 2218.4 (C≡C), 1726.5 (C=O). MS (m/z, EI, 70 eV) 250 ([M⁺], 94.4), 235 ([M − Me]⁺, 3.0), 222 ([M − CO]⁺, 100), 221 ([M − Et]⁺, 29.7), 205 ([M − EtO]⁺, 36.0), 177 ([M − COOEt]⁺, 22.1), 77 ([Ph]⁺, 13.9).

4-(Phenylethynyl)benzaldehyde (4an). Aldehyde 4an was prepared using a similar procedure to that for benzoate 4am, using 4-BrC₆H₄CHO (3n) (193 mg, 1.043 mmol), Pd(PPh₃)₂Cl₂ (36.6 mg, 0.052 mmol; 5 mol% vs.3n), (PhC≡C)₄Sn 1a (150 mg, 0.287 mmol), TMEDA (1.56 mL, 10.43 mmol) and dry BuOAc (6 mL). The reaction time was 3 h. The yield of crude aldehyde 4an was 176 mg. For further purification, the sample was recrystallized from n-heptane. Yield 82%, beige crystalline solid. ¹H NMR (400 MHz, CDCl₃) δ 7.36–7.38 (m, 3H, Ph), 7.54–7.57 (m, 2H, Ph), 7.67 (d, ³J = 8.2 Hz, 2H, Ar), 7.85 (d, ³J = 8.2 Hz, 2H, Ar), 10.00 (s, 1H, CHO); ¹³C NMR (100 MHz, CDCl₃) δ 88.5, 93.5, 122.5, 128.5, 129.0, 129.58, 129.60, 131.8, 132.1, 135.4, 191.4. IR (KBr, cm⁻¹) ν_max 3049.3, 2845.8 (C–C, C–H), 2216.1 (C≡C), 1697.3 (C=O), 1599.9 (C=C); MS (m/z, EI, 70 eV) 206 ([M⁺], 100), 205 ([M − H]⁺, 71.4), 178 ([M − CO]⁺, 13.3), 77 ([Ph]⁺, 5.9).

1-[4-(Phenylethynyl)phenyl]ethanone (4ao). Ketone 4ao was prepared using a similar procedure to that for benzoate 4am, using 4-BrC₆H₄C(O)CH₃ (3o) (151.4 mg, 0.761 mmol), Pd(PPh₃)₂Cl₂ (26.7 mg, 0.038 mmol; 5 mol% vs.3o), (PhC≡C)₄Sn 1a (109.4 mg, 0.209 mmol), TMEDA (0.57 mL, 3.8 mmol) and dry BuOAc (4.5 mL). The product was purified by column chromatography on a silica gel (7 g), eluted with hexane, then hexane : PhMe 4 : 1 (after diphenyldiacetylene 5a was eluted). The yield of crude ketone 4ao was 129.3 mg (purity by GCMS – 84%). For further purification, the sample was recrystallized from n-heptane. Yield 65%, white crystalline solid. ¹H NMR (400 MHz, CDCl₃) δ 2.61 (s, 3H, COCH₃), 7.36–7.37 (m, 3H, Ph), 7.53–7.56 (m, 2H, Ph), 7.61 (d, ³J = 8.5 Hz, 2H, Ar), 7.93 (d, ³J = 8.5 Hz, 2H, Ar); ¹³C NMR (100 MHz, CDCl₃) δ 26.6, 88.6, 92.7, 122.7, 128.2, 128.3, 128.5, 128.8, 131.7, 131.8, 136.2, 197.3. IR (KBr, cm⁻¹) ν_max 3061.4, 2997.7 (C–H, C–C), 2218.4 (C≡C), 1680.2 (C=O), 1603.0 (C=C). MS (m/z, EI, 70 eV) 220 ([M⁺], 72.5), 205 ([M − CH₃]⁺, 100), 177 ([M − COCH₃]⁺, 27.8), 77 ([Ph]⁺, 6.4).

1-Methyl-4-[(4-nitrophenyl)ethynyl]benzene (4bd). Acetylene 4bd was prepared using a similar procedure to that for 4ab, using 1-iodo-4-nitrobenzene (3d) (178.3 mg, 0.716 mmol), Pd(PPh₃)₂Cl₂ (25.1 mg, 0.036 mmol; 5 mol% vs.3d), (4-MeC₆H₄C≡C)₄Sn 1b (114.1 mg, 0.197 mmol), Pr₂NH (0.98 mL, 7.16 mmol) and BuOAc (4.5 mL). The reaction time was 3.5 h. The resulting crude product was purified by column chromatography on a mixture of silica gel (6 g) and silica gel modified with 3-aminopropyltriethoxysilane (1 g, 1.14 mmol g⁻¹ of NH₂ groups), and subsequently eluted with hexane, hexane : EtOAc 9 : 1, hexane : EtOAc 4 : 1 and pure toluene. Recrystallization from toluene yielded 108.6 mg (63.9%) of acetylene 4bd as a white crystalline solid. Another crop of product (49.1 mg, purity by GC-MS – 95%) was obtained from the mother liquor. The total yield of tolane 4bd was 155.2 mg (91%). ¹H NMR (400 MHz, CDCl₃) δ 2.38 (s, 3H, CH₃), 7.18 (d, ³J = 7.8 Hz, 2H, 4-MeC₆H₄), 7.44 (d, ³J = 7.8 Hz, 2H, 4-MeC₆H₄), 7.63 (d, ³J = 9.2 Hz, 2H, 4-NO₂C₆H₄), 8.19 (d, ³J = 9.2 Hz, 2H, 4-NO₂C₆H₄); ¹³C NMR (100 MHz, CDCl₃) δ 21.6, 87.1, 95.1, 119.1, 123.6, 129.3, 130.5, 131.8, 132.2, 139.7, 146.9. IR (KBr, cm⁻¹) ν_max 2922.5, 2847.3 (C–H, C–C), 2212.6 (C≡C), 1589.5 (C=C), 1342.6 (symm NO₂). MS (m/z, EI, 70 eV) 237 ([M⁺], 100), 191 ([M − NO₂]⁺, 13.6), 176 ([M − Me − NO₂]⁺, 17.1).

1-Chloro-4-[(4-nitrophenyl)ethynyl]benzene (4cd). Acetylene 4cd was prepared using a similar procedure to that for 4ab, using 1-iodo-4-nitrobenzene (3d) (249 mg, 1.00 mmol), Pd(PPh₃)₂Cl₂ (35.1 mg, 0.05 mmol; 5 mol% vs.3d), (4-ClC₆H₄C≡C)₄Sn 1c (181.7 mg, 0.275 mmol), Pr₂NH (1.37 mL, 10.0 mmol) and BuOAc (6 mL). The reaction time was 2.5 h. The resulting crude product was purified by column chromatography on a mixture of silica gel (6 g) and silica gel modified with 3-aminopropyltriethoxysilane (1 g, 1.14 mmol g⁻¹ of NH₂ groups), and subsequently eluted with hexane, hexane : HCCl₃ 9 : 1, hexane : HCCl₃ 3 : 2 and hexane : HCCl₃ 1 : 1. Recrystallization from toluene yielded 160.9 mg (62%) of acetylene 4cd as a white crystalline solid. Another crop of product (49.8 mg) was isolated from the mother liquor. The total yield was 210.7 mg (82%). ¹H NMR (400 MHz, CDCl₃) δ 7.36 (d, ³J = 8.2 Hz, 2H, Ar), 7.49 (d, ³J = 8.2 Hz, 2H, Ar), 7.65 (d, ³J = 8.7 Hz, 2H, Ar), 8.22 (d, ³J = 8.7 Hz, 2H, Ar); ¹³C NMR (100 MHz, CDCl₃) δ 88.4, 93.4, 120.6, 123.7, 129.0, 129.9, 132.3, 133.1, 135.5, 147.2. IR (KBr, cm⁻¹) ν_max 3092.3, 2926.4, 2851.1 (C–H, C–C), 2210.7 (C≡C), 1587.6 (C=C), 1346.5 (symm NO₂). MS (m/z, EI, 70 eV) 259 ([M⁺] ³⁷Cl, 32.7), 257 ([M⁺] ³⁵Cl, 100), 213 ([M − NO₂]^{+ 37}Cl, 2.8), 211 ([M − NO₂]^{+ 35}Cl, 9.5), 176 ([M − Cl − NO₂]⁺, 82.7).

tert-Butyl 3-(4-nitrophenyl)prop-2-ynyl ether (4ed). Acetylene 4ed was prepared using a similar procedure to that for 4ab, using 1-iodo-4-nitrobenzene (3d) (289.8 mg, 1.164 mmol), Pd(PPh₃)₂Cl₂ (40.8 mg, 0.058 mmol; 5 mol% vs.3d), (t-BuOCH₂C≡C)₄Sn 1e (180 mg, 0.32 mmol), Pr₂NH (1.6 mL, 11.64 mmol) and BuOAc (6 mL). The reaction time was 5.5 h. The resulting crude product was purified by column chromatography on a mixture of silica gel (6 g) and silica gel modified with 3-aminopropyltriethoxysilane (1 g, 1.14 mmol g⁻¹ of NH₂ groups), and subsequently eluted with hexane and hexane : HCCl₃ 9 : 1. The yield of acetylene 4ed was 132 mg (49%), yellow crystalline solid. ¹H NMR (400 MHz, CDCl₃) δ 1.30 (s, 9H, tBu), 4.34 (s, 2H, OCH₂), 7.57 (d, ³J = 9.2 Hz, 2H, Ar), 8.16 (d, ³J = 9.2 Hz, 2H, Ar); ¹³C NMR (100 MHz, CDCl₃) δ 27.5, 51.0, 74.8, 83.0, 92.7, 123.5, 130.0, 132.4, 147.1. IR (KBr, cm⁻¹) ν_max 3105.8, 2976.5, 2862.7 (C–H, C–C), 2220.3 (C≡C), 1593.4 (C=C), 1342.6 (symm NO₂). MS (m/z, EI, 70 eV) 233 ([M⁺], 0.3), 218 ([M − CH₃]⁺, 10.4), 160 ([M − tBuO]⁺, 100), 57 ([tBu]⁺, 60.5).

1-Hex-1-ynyl-4-nitrobenzene (4fd). Acetylene 4fd was prepared using a similar procedure to that for 4ab, using 1-iodo-4-nitrobenzene (3d) (249.0 mg, 1.00 mmol), Pd(PPh₃)₂Cl₂ (35.1 mg, 0.05 mmol; 5 mol% vs.3d), (CH₃CH₂CH₂CH₂C≡C)₄Sn 1f (121.9 mg, 0.275 mmol), Pr₂NH (1.37 mL, 10.0 mmol) and BuOAc (6 mL). The reaction time was 15 h. The resulting product was purified by column chromatography over a mixture of silica gel (6 g) and silica gel modified with 3-aminopropyltriethoxysilane (1 g, 1.14 mmol g⁻¹ of NH₂ groups). Hexane was used as the eluent. The column fractions were concentrated, the unreacted 1-iodo-4-nitrobenzene (3d) was filtered off, and the crude product was again purified under the same column chromatography conditions as previously to give 96.1 mg (47%) of acetylene 4fd as a light yellow oil. ¹H NMR (400 MHz, CDCl₃) δ 0.96 (t, ³J = 7.3 Hz, 3H, Me), 1.44–1.53 (m, 2H, CH₂), 1.58–1.65 (m, 2H, CH₂), 2.45 (t, ³J = 6.9 Hz, 2H, CH₂C≡), 7.51 (d, ³J = 8.7 Hz, 2H, Ar), 8.15 (d, ³J = 8.7 Hz, 2H, Ar); ¹³C NMR (100 MHz, CDCl₃) δ 13.6, 19.3, 22.0, 30.5, 79.3, 96.8, 123.5, 131.3, 132.3, 146.6. IR (KBr, cm⁻¹) ν_max 3107.7, 3080.7, 2957.2, 2932.2, 2872.4 (C–H, C–C), 2230.0 (C≡C), 1592.3 (C=C), 1342.6 (symm NO₂). MS (m/z, EI, 70 eV) 203 ([M⁺], 39.9), 188 ([M − CH₃]⁺, 58.3), 174 ([M − CH₂CH₃]⁺, 6.6), 157 ([M − NO₂]⁺, 17.9).

Further details on the experimental procedures and spectra are given in the ESI.†

Acknowledgements

VVD is grateful for financial support from the Russian Ministry of Education and Science (State Assignment to Higher Education Institutions, project no. 4.5547.2017/).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Spectral data. See DOI: 10.1039/c6nj03905g

‡ Deceased.

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