8th International Electronic Conference on Synthetic Organic Chemistry. ECSOC-8. 1-30 November 2004. http://www.lugo.usc.es/~qoseijas/ECSOC-8/

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[A030]

Use of arylantimony(III,V), bismuth(III) compounds and peroxides in cross-coupling reactions with alkenes

Alexey V. Gushchin, Dmitry V. Moiseev, Vera A. Morugova, Julia B. Malysheva,

Victor A. Dodonov

Organic Chemistry Department, Nizhny Novgorod State University, 23 Gagarin Avenue,

Nizhny Novgorod, 603950, Russian Federation

Tel.: +7-831-233-7865; fax: +7-831-265-8592; e-mail: [email protected]

Keywords: tetraphenylantimony(V) carboxylates; triphenylantimony; triphenylbismuth; peroxides; arylation; palladium, homo-coupling, cross-coupling.

Tetraphenylantimony(V) carboxylates, triphenylantimony, triphenylbismuth have been used in the Pd-catalyzed C-phenylation reaction of methyl acrylate in the presence of peroxides such as (PhCO₂)₂, t-BuOOH, H₂O₂ under mild conditions (20-50 °С). The peroxides promote a cascade participation of organoantimony compound to provide transfer of two or three phenyl groups with the formation of methylcinnamate as a sole product. Organobismuth compound gives methylcinnamate, biphenyl and benzene simultaneously.

1. Introduction

Organoantimony compounds have been used in organic synthesis either as reagents or as catalysts for a number of years [1]. They also have been used in the Pd-catalyzed reactions [2]. We have recently reported that triarylantimony dicarboxylates were efficient reagents in the Pd-catalyzed C‑arylation reaction of unsaturated compounds with involvement of two aryl groups [3]. Triarylstibines Ar₃Sb can also be used as reagents in the C-arylation reaction via formation of Ar₃Sb(OAc)₂ in situ in the presence of peroxides [4]. Tetraphenylantimony derivatives Ph₄SbX have shown lower activity in comparison with Ar₃Sb(OAc)₂. Only one phenyl group was involved in the C-phenylation reaction [5].

Organobismuth compounds are of current interest [6]. There are several examples of palladium-mediated conversation of a C-Bi bond to a C-C bond. The cross-coupling have been observed in the reaction of Ar₃Bi with acyl chloride [7], aryl halides and triflates [8], allyl halides [9] and in the reaction of organobismuth dialkoxides with aryl or alkenyl triflates and aryl halides [10,11]. Pentavalent triarylbismuth compounds Ar₃BiX₂ (X = Hal, OAc) have been used in the palladium-catalysed cross-coupling reaction with hypervalent iodonium salts at room temperature [12]. In the works [7,13] the homo-coupling of organobismuth compounds have been studied. Triarylbismuth has been used in the carbonylation reaction as well [14]. In the work [15] arylation of vinyl epoxides, diol acetonides and diol carbonates has been done with both triarylbismuth(III) and triarylbismuth(V) dichloride or carbonate. Several types of organobismuth(III) compounds have been tested in the Heck-type reactions [16-18]. In the work [19] some bismuth ylides have been used to study a homo- and cross-coupling competition.

In the present work organic peroxides are used to increase an efficiency of organoantimony and organobismuth derivatives in the C-phenylation of alkenes.

2. Results and discussion

2.1. Ph₄SbX + Peroxide system in the Pd-catalyzed C-phenylation reaction

We investigated a model C-phenylation reaction of methyl acrylate 1 with tetraphenylantimony hydroxide Ph₄SbOH 2 (4:1) in AcOH in the presence of t-BuOOH 3 (1.2 equivalent per Sb compound) and 4 mol% Li₂PdCl₄. Ph₄SbOH was selected for the following reasons. It is one of the most accessible compounds of this type and can easy give tetraphenylantimony acetate Ph₄SbOAc 4 in AcOH solution (Scheme 1). Peroxide 3 is the best oxidant of Sb(III) derivatives [4]. Li₂PdCl₄ is the optimum precatalyst for the C-phenylation in the presence of peroxides [4]. Methyl acrylate 1 was taken in a molar excess to 2 (4:1), one molecule of 1 per one phenyl group of 2.

Scheme 1.

The C-phenylation reaction of 1 with the (2 + 3) system gives methyl cinnamate 5 (Scheme 1) in a 247% yield for 24 h (the yield being based on the initial organoantimony reagent, the yield of 100% corresponds to the transfer of only one phenyl group) (Table 1, entry 1). The yield of 5 is not changed significantly in the presence of an excess of 3 (Table 1, entries 2 and 3). In the absence of the peroxide the product yield is only 30% (Table 1, entry 5). An application of tetraphenylantimony propionate Ph₄SbO₂CEt 6 instead of 2 shows the same activity, the yield of 5 is 254% (Table 1, entry 4). The high yield of 5 corresponds to the transfer of three phenyl groups. However, in these conditions the complete transfer do not occur and the product yield do not exceed 260%.

Table 1.

Pd-catalyzed reaction of Ph₄SbOH with methyl acrylate 1 in the presence of a peroxide: influence of the nature and the amount of the peroxide ^a

Entry	Peroxide	Ph4SbOH : Peroxide	Yields of 5 (%) ^b
1	t-BuOOH	1 : 1.2	247
2	t-BuOOH	1 : 1.5	257
3	t-BuOOH	1 : 2	240
4 ^c	t-BuOOH	1 : 1.2	254
5	-	-	30
6 ^d	(PhCO2)2	1 : 1.5	240
7	(PhCO2)2	1 : 1.5	300
8 ^e	(PhCO2)2	1 : 1.5	300

^a The reactions were performed in AcOH at 50 °С for 24 h, under air with ratio between 1, [Sb] and Li₂PdCl₄ of 4:1:0.04.

^b Yields were determined by GLC, yield of 100% corresponds to involvement of one Ph-group.

^c Ph₄SbO₂CEt was used as a phenylating agent.

^d Reaction duration was 12 h.

^e Ph₄SbO₂CEt was used as a phenylating agent and CH₃CN was used as a solvent.

The substitution of 3 for benzoyl peroxide (PhCO₂)₂ 7 gives rise to the yield of 5. It is 240% and 300% in the case of 12 h and 24 h duration of the reaction, respectively (Table 1, entries 6 and 7). This yield corresponds the full involvement of three phenyl groups of 2. It is known that peroxide 7 in contrast with 3 can oxidize Sb(III) into active Sb(V) without involvement of carboxylic acid [4,8]. This allows to use the new system based on tetraphenylantimony derivatives in a neutral solvent, e.g. CH₃CN. However Ph₄SbOH is nоt effective in acetonitryle where it gives mainly a homo-coupling product, namely, biphenyl [5]. Therefore, propionate 6 was used as a phenylating agent. The reaction of 1 with the (6 + 7) system (1:1.5) gives the yield of 5, as expected, in 300% (CH₃CN, 50 °C, 24 h) (Table 1, entry 8). When 7 is heated in the absence of organoantimony compound under the same conditions the phenylated product 5 is not formed.

Thus, tetraphenylantimony derivatives can be effective cross-coupling agents. In the presence of (PhCO₂)₂ they phenylate methyl acrylate under mild conditions (50 °С) to provide a transfer of three phenyl groups. It was found that Ph₄SbOH was the preferable agent in AcOH, and Ph₄SbO₂CEt - in CH₃CN, respectively.

2.2. Mechanistic aspects of organoantimony(V) involvement in the Pd-catalyzed C-phenylation reaction

To understand the whole process of the tetraphenylantimony compounds participation in the investigated catalytic reaction we isolated three main stages, on each of which only one phenyl group is transferred. The catalytic cycle consists of consecutive redox reactions of Sb atom, which are caused by the presence of a peroxide and a palladium in the system.

2.2.1.Transfer of the first phenyl group

At the first stage of a catalytic cycle tetraphenylantimony carboxylate 4 reacts with the active Pd(0) species formed on the activation stage. As result, phenylpalladium intermediate 8 and Ph₃Sb 9 are formed (Eq. 1). The intermediate 8 interacts with 1 to give the phenylation product 5 (Eq. 2). On this stage the yield of 5 could reach a value of 100%. In reality, in the absence of peroxide it is only 30% (Table 1, entry 5) because of the inhibiting effect of Ph₃Sb [3].

	(1)
	(2)

2.2.2.Transfer of the second phenyl group

On the second stage the peroxide oxidizes 9 into an active pentavalent state [3,4]. The mechanism of the oxidation has been studied well [9,10]. Ph₃Sb forms either triphenylantimony diacetate Ph₃Sb(OAc)₂ 10 in AcOH solution in the presence of hydroperoxide 3 (Eq. 3) or triphenylantimony dibenzoate Ph₃Sb(O₂CPh)₂ 11 in CH₃CN in the presence of benzoyl peroxide 9 (Eq. 4). The triphenylantimony dicarboxylates oxidize Pd(0) into PhPdOAc 8 (Eq. 5), which phenylates methyl acrylate due to equation 2.

	(3)
	(4)
	(5)

To prove the existing of reaction 5 we made a special investigation of the C‑arylation reaction of 1 with tris(para-tolyl)antimony diacetate p‑Tol₃Sb(OAc)₂ 13 by ¹H NMR. This system was selected for the following reasons. The initial compounds as well as the arylation products methyl 3-(para-tolyl)propionate 14 and bis(para-tolyl)antimony acetate 15 had clear and easy identifiable NMR signals (Scheme 2). D₃CCOOD was used as a solvent. The reaction was carried out under argon atmosphere.

Scheme 2.

The initial ¹H NMR spectrum of the reaction mixture contained the resolved signals of 13 at 7.88 (d, ortho-protons), 7.36 (d, metha-protons), 2.39 (s, CH₃), 2.06 (s, OAc) ppm and 1 at 6.41, 6.16, 5.86 (d, protons at the double bond), 3.74 (s, OMe) ppm (Fig. 1a). In the second spectrum, recorded after 3 h, two new sets of signals appeared (Fig. 1b). They corresponded to the expected products 14 at 7.71 (d), 7.48 (d, ortho-protons), 7.20 (d, metha-protons), 6.48 (d), 3.79 (s, OMe), 2.35 (s, CH₃) ppm and 15 at 7.55 (d, ortho-protons), 7.24 (d, metha-protons), 2.33 (s, CH₃), 2.06 (s, OAc) ppm. The 14:15 ratio was 1:1. An amount of 14 corresponded to an amount of consumed organoantimony compound 13. Spectra, recorded for 12 and 24 h of the reaction duration (Fig. 1c), confirmed that the product formation and the consumption of 13 occurred in equivalent amounts. However, new signals appeared at 7.66 (d), 7.29 (d), 2.33 (s), 2.06 (s) ppm were associated with mono(para-tolyl)antimony diacetate p-TolSb(OAc)₂ 16 and signals at 7.31 (d), 7.13 (d), 2.31 (s) ppm were associated with tris(para-tolyl)antimony p-Tol₃Sb 17. These compounds are the products of the ligand exchange reaction of 15 due to equation 6. Thus, we confirmed that triarylantimony dicarboxylates were involved in the arylation according to the equation 5. The total catalytic process (Eq. 1-3,5) can give 200% of phenylation product 5 on the initial Ph₄SbOAc.

(6)

a)

b)

c)

Fig. 1. ¹H-NMR spectra of the reaction mixture: p-Tol₃Sb(OAc)₂ + 1 (1:2), PdCl₂ (4 mol%), D₃CCOOD, argon, 50 °C: (a) at the start of the reaction; (b) after 3 h; (c) after 24 h.

2.2.3. Transfer of third phenyl group

Organoantimony(III) derivatives Ph₂SbX (X = OAc, O₂CPh) are involved in the third stage of the phenylation reaction. Like in the case of Ph₃Sb, a peroxide oxidizes Ph₂SbX to give an active Sb(V) derivative, diphenylantimony triacetate Ph₂Sb(OAc)₃ 18 (Eq. 7) or diphenylantimony tribenzoate Ph₂Sb(O₂CPh)₃ 19 (Eq. 8). These compounds react with Pd(0) to give the phenylpalladium intermediate 8 (Eq. 9), which phenylates the third molecule of 1 (Eq. 2). Sb(V) is reduced to phenylantimony(III) dicarboxylate. Thus, peroxide is consumed in oxidation of both triphenyl- and diphenylantimony(III) derivatives.

	(7)
	(8)
X = OAc, O2CPh	(9)

Another route of Ph₂SbX involvement can occur in the presence of oxygen. Matoba et al. [2e] suggested a radical mechanism of this process. In previous work [3] we suggested the speculative mechanism of the oxygen involvement via formation of a palladium hydroperoxide XPdOOH, which oxidized Sb(III) into Sb(V).

We investigated the arylation reaction of 1 with p‑Tol₃Sb(OAc)₂ 13 (Scheme 2) in the presence of oxygen by ¹H NMR. The same products 14-17 like under argon were registered. However, in the presence of oxygen the formation of the arylation product 14 was observed in an amount more than an amount of the 13 consumption. The organoantimony(III) compound p‑TolSb(OAc)₂ 16 was the main product. It was found that oxygen is absorbed 0.5 mol per 1 mol of 5. Thus, the following scheme of oxygen involvement in the transfer of the third phenyl group is realized (Scheme 3). The scheme could be explained by concerted oxidation of two molecules of 12 with an oxygen molecule in the coordination sphere of Pd like in the homogeneous oxidation of phosphines with oxygen on Rh, Co, Pt catalysts [11-13] (Scheme 4).

Scheme 3.

Scheme 4.

Compound 20 in AcOH forms 18 (Eq. 10), which acts with Pd(0) (Eq. 9). Thus, diphenylantimony(V) tricarboxylates are preliminarily formed in the both routes of transfer of the third phenyl group at the third stage of the catalytic cycle. To prove the equation 9 we used diphenylstibine acid Ph₂Sb(O)OH as a phenylating agent. In this reaction the yield of 5 was 92% (50 ºC, 12 h, AcOH) (Scheme 5). This fact confirms the mechanism via diphenylantimony tricarboxylates. This stage is a final one and the yield of 5 can reach a value of 300%.

(10)

Scheme 5.

The total catalytic cycle is shown on Scheme 6. The active Pd(0) species interact with the initial tetraphenylantimony compound to give phenylpalladium intermediate, which phenylates 1. Triphenylstibine is oxidized by a peroxide to give antimony(V), which reacts with Pd(0) giving PhPdX. As a result, the second molecule of 5 and antimony(III) derivative are formed. Oxygen (or peroxide in excess) oxidizes the latter into antimony(V), which transfers the third phenyl group to methyl acrylate. PhSbX₂ (X = OAc, O₂CPh)_, the final product of transformation, is not involved in the C-phenylation reaction in these conditions. The total yield of 5 is 300% in respect to the initial tetraphenylantimony derivative.

Thus, an addition of a peroxide increases an activity of tetraphenylantimony derivatives in the C‑phenylation reaction. The presence of the peroxide provides involvement of three phenyl groups from the initial organoantimony compound via consecutive redox stages. We found that (PhCO₂)₂ is the optimum peroxide for this system. The reactions can be performed both in AcOH and in CH₃CN. It was established that the initial Ph₄SbX derivative transforms in this cascade reaction in the following range: Ph₄SbX – Ph₃Sb – Ph₃SbX₂ – Ph₂SbX – Ph₂SbX₃ – PhSbX₂. The final PhSbX₂ derivative is inactive in the C-phenylation under these conditions. The key role of peroxides is oxidation of inactive Ph₃Sb or Ph₂SbX compounds into an active pentavalent state.

X = O₂CEt, OAc or O₂CPh

Scheme 6.

2.3. Ar₃Sb + Peroxide system in the Pd-catalyzed C-phenylation reaction

We have found for a model reaction, the C-arylation of methyl acrylate, that in the presence of equimolar amounts of peroxide and catalytic amounts of a palladium compound (4 mol %), triarylstibines can act as mild efficient arylating agents (Scheme 7):

Scheme 7

For the reaction of 1 with 21a (3:1) we tested a number of peroxides H₂O₂, t-BuOOH, (PhCO₂)₂, (cyclo-C₆H₁₁OCO₂)₂ in order to choose the best. All reactions proceeded under standard conditions (AcOH, 50 °C, 12 h).  The results are summarized in Table 2. In all cases the product yield 100% corresponds to the transfer of only one aryl group from an initial organoantimony compound.

 All the peroxides exhibited high activity. Peroxides t-BuOOH and H₂O₂ turned out to be the most effective, giving the yields of the reaction product – methyl cinnamate 171% and 174%, respectively. Peroxide (PhCO₂)₂ was less efficient (164%). In the case of (cyclo-C₆H₁₁OCO₂)₂ the yield was minimal (127%). A two-fold excess of hydroperoxide t-BuOOH ensured an increase in the product yield but only to a small extent (Table 2, entry 4). The yield of methyl cinnamate did not exceed the stoichiometric yield based on palladium in the absence of the oxidant (Table 2, entry 1). This fact shows that a catalytic reaction does not occur.

The principal role of the peroxides in this given system is the oxidation of Sb(III) to Sb(V) in the form of triarylantimony diacetates. The mechanism of this interaction has been well studied. Triarylantimony diacetates  are, as known, effective arylating agents.

Table 2.  C-phenylation reaction of methyl acrylate 1 with  Ph₃Sb/Peroxide/Li₂PdCl₄ systems ^a

Entry	Peroxide	Yields of methyl cinnamate ^b, %
1	-	2
2	H2O2	171
3	t-BuOOH	174
4 ^c	t-BuOOH	189
5	(PhCO2)2	164
6 ^d	(PhCO2)2	121
7	(cyclo-C6H11OCO2)2	127
8	PhI(OAc)2	26
9 ^e	-	186

^aThe reactions were carried out with 0.5 mmol Ph₃Sb, 1.5 mmol methyl acrylate, 0.5 mmol peroxide, 0.02 mmol Li₂PdCl₄ in AcOH (4 ml) for 12 h at 50°C under air.

^bThe yields of methyl cinnamate in all cases were determined by GLC.

^cThe ratio 21a:t-BuOOH = 1:2.

^dAcetonitrile was used as solvent.

^ePh₃Sb(OAc)₂ was utilized instead of 21a.

With benzoyl peroxide the availability of an acid is not obligatory since Ar₃Sb transforms to triarylantimony dibenzoate without participation of an acid. Therefore, reactions with benzoyl peroxide can be carried out in neutral solvents, e.g. acetonitrile, but the rate becomes lower (Table 2, entry 6).

While comparing the new systems Ph₃Sb/peroxide with the known system based on Ph₃Sb(OAc)₂, it is seen that they display similar efficiencies (Table 2, entries 3 and 9). Nevertheless, the former differs advantageously from the latter in that the synthesis, isolation and purification necessary for triphenylantimony diacetate are not needed.

To compare with the peroxides, we used another known oxidant PhI(OAc)₂ which gives Ph₃Sb(OAc)₂ on interacting with 21a as well.

However, as seen from our results, the yield of the target product proved to be low (Table 2, entry 8). Moreover, as it turned out, the addition of an equimolar amount of PhI  inexplicably  inhibits the phenylation reaction under these conditions.

In this C-arylation reaction, other triaryl derivatives of antimony(III) were examined. All reactions were carried out under the standard conditions (AcOH, 50 °C, 12 h) using tert-butyl hydroperoxide.  The results are given in Table 3.

As seen, for 21b and 21c the yields of the C-arylated products remained high (180% and 192%, respectively). In the case of the ortho-substituted derivative of antimony 21d the product yield decreased sharply to 125%. With the most sterically hindered 21e, no reaction was observed at all. Thus, the C-arylation reaction depends on steric hindrance in the aromatic ring, especially, in the ortho-position.

As should be expected, better donor substituents reduced the rate of the arylation reaction. So, in the case of 21, the yield of the product was smaller as compared with 21b (Table 3, entries 1 and 5).

Hence, we offer a new catalytic system for C-arylation of unsaturated compounds based on triarylstibines in the presence of equimolar amounts of a peroxide and a catalytic amount of Li₂PdCl₄ (4 mol %). The role of the peroxides is in the in situ preparation of triarylantimony diacetates without isolation and purification of the latter. H₂O₂ and t-BuOOH can be considered as the best peroxides. This system enables the reaction to be conducted at low temperatures with transfer of two of the three possible aryl groups. The new system compares favorably with the earlier proposed system based on triarylantimony dicarboxylates.

Table 3.  C-arylation reactions of methyl acrylate with Ar₃Sb/t-BuOOH/Li₂PdCl₄ systems ^a

Entry	Triarylstibine	Product	Yields of phenylation product ^b, %
1			180
2			192
3			125
4			-
5			158

^aThe reactions were carried out with 0.5 mmol Ar₃Sb, 1.5 mmol methyl acrylate, 0.5 mmol peroxide, 0.02 mmol Li₂PdCl₄ in AcOH (4 ml) for 12 h at 50 °C under air.

^bAll C-arylated products were isolated in pure state by column chromatography on silica gel 60 mm and characterized by ¹H NMR.

2.4. Ph₃Bi + Peroxide system in the Pd-catalyzed C-phenylation reaction

We have found for a model reaction with methyl acrylate, that in the presence of equimolar amounts of benzoyl peroxide and catalytic amounts of a palladium compound (4 mol %), triphenylbismuth can give methyl cinnamate? biphenyl and benzene as coupling products. The results for the three Pd compounds are given in the table 4. Li₂PdCl₄ is the most active catalyst. BF₃∙OEt₂ was added to promote peroxide oxidation of triphenylbismuth.

Table 4.  C-arylation reactions of methyl acrylate with Ph₃Bi/(PhCO₂)₂/BF₃∙OEt₂/Li₂PdCl₄ systems ^a

Entry	Catalyst	Yields, %
		PhCH=CHCO2Me	Ph2	PhH
1^b	Li2PdCl4	22	80	28
2	Li2PdCl4	45	70	33
3^c	Li2PdCl4	37	52	0
4	PdCl2	25	57	27
5	Pd(OAc)2	30	61	28

^aThe reactions were carried out with 0.5 mmol Ph₃Bi, 1.5 mmol methyl acrylate, 1 mmol peroxide, 0.02 mmol Li₂PdCl₄ in MeCN (4 ml) for 4 h at 50 °C under air.

^b The reaction free of BF₃∙OEt₂.

^c 30 h at 20ºС.

Benzoyl peroxide is more efficient oxidant than (cyclo-C₆H₁₁OCO₂)₂, H₂O₂ and especially t-BuOOH.

Table 5.  C-arylation reactions of methyl acrylate with Ph₃Bi/Peroxide/BF₃∙OEt₂/Li₂PdCl₄ systems ^a

Entry	Peroxide	Yields, %
		PhCH=CHCO2Me	Ph2	PhH
1^b	t-BuOOH	<1	20	23
2^c	t-BuOOH	<1	20	117
3^d	(PhCO2)2	22	80	28
4	(PhCO2)2	45	70	33
5	(cyclo-C6H11OCO2)2	10	22	0
6	H2O2	07	14	0

^aThe reactions were carried out with 0.5 mmol Ph₃Bi, 1.5 mmol methyl acrylate, 1 mmol peroxide, 0.02 mmol Li₂PdCl₄ in MeCN (4 ml) for 4 h at 50 °C under air.

^b,c,d The reaction free of BF₃∙OEt₂.

^b 24 h at 20ºС.

^b,c The reaction in the presence of AcOH 1.5 mmol.

3. Experimental

3.1. General methods

Gas chromatographic analyses were performed with a LKhM-80 chromatograph using helium as the carrier gas, column 100 cm length, 15%-Apiezon-L on the Chromaton N-AW at 220 °C. ¹H NMR spectra were measured on a Bruker Avance DPX-200 spectrometer for solutions in D₃CCOOD. Column chromatographies were performed with silica gel 60 Merck.

3.2. Typical procedure for the C-phenylation reaction [20]

Ph₄SbOH (0.224 g, 0.5 mmol) was dissolved in AcOH (1 ml) in a 50 ml tube. A mixture of t-BuOOH (0.075 ml, 0.75 mmol), Li₂PdCl₄ (5.2 mg, 0.02 mmol) and methyl acrylate (0.18 ml, 2 mmol) in acetic acid (3 ml) was added to Ph₄SbOH solution. The tube was sealed and the reaction mixture was kept at 50 °C for 24 h. The solvent was then evaporated under reduced pressure. The solid residue was purified from inorganic products by elution through a short column on silica gel using a mixture of hexane-diethyl ether (v/v 4:1) as the eluant. The filtrate was analyzed by GLC. Methyl cinnamate (0.208 g) was found.

3.3. Procedure for measurement of oxygen consumption in C-phenylation of methyl acrylate with Ph₃Sb(OAc)₂

A mixture of Ph₃Sb(OAc)₂ (0.236 g, 0.5 mmol), PdCl₂ (3.6 mg, 0.02 mmol), methyl acrylate (0.135 ml, 1.5 mmol) in acetic acid (4 ml) under air was placed in the 10 ml tube being the reflux condenser. The tube was connected with a gas burette filled by the brine and was placed into the thermostat at 50 ºC. After equating the pressure in the tube and the burette with atmosphere pressure, the tube was locked on the gas burette by the three-way cock. The reaction mixture was kept at 50 ºC. After 12 h, the 0.21 mmol of oxygen was consumed. The yield of methyl cinnamate was 0.88 mmol. The same reaction under argon yielded 0.44 mmol of methyl cinnamate.

3.4. Procedure for ¹H-NMR spectroscopic study

PdCl₂ (0.9 mg, 0.005 mmol) and p-Tol₃Sb(OAc)₂ (64.1 mg, 0.125 mmol) were placed into NMR tube under argon. Air-free D₃CCOOD (1 ml) and methyl acrylate (0.034 ml, 0.375 mmol) were placed into the tube under argon. The tube was sealed and ¹H-NMR analysis was carried out. The reaction mixture was heated at 50 °C. The ¹H-NMR spectra were recorded in 3, 12, and 24 h.

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