Bismuth (III) salts in friedel-crafts acylation

B I S M U T H (III) SALTS IN F R I E D E L - C R A F T S A C Y L A T I O N

JEAN-ROGER DESMURS a), MIREILLE LABROUILLERE b), JACQUES DUBAC b) ANDRE LAPORTERIE b) HAFIDA GASPARD b~ AND FRAN(~OIS METZ a~ a) Rh6ne-Poulenc Industrialisation, Centre de Recherche, d'Ing6nierie et de Technologie des Carri6res, 85, Avenue des Fr6res Perret, BP 62, 69192 Saint-Fons Cedex, France. b) H6t6rochimie Fondamentale et Appliqu6e (URA CNRS 477), Universit6 Paul-Sabatier, 118 route de Narbonne, 31062 Toulouse Cedex, France.

INTRODUCTION The acylation reaction is one of the most important reactions in organic chemistry (ref. 1) (eqn. 1). The substituted atom Y is generally hydrogen, but can be an organometallic group of silyl type (refs. 2, 3). ZY

-~

z--C--R II 0

(1)

This reaction involves an acylating reagent (acyl halides, carboxylic acids or anhydrides) in the presence of an activator, usually a Lewis acid. However, as a result of the complexation of this Lewis acid with the formed ketone, more than one mole of catalyst is required per mole of reagent. It cannot be reused because the ketone is isolated after hydrolysis of the complex. Such is the dilemma of Friedel-Crafts acylation (refs. 4-6) in the presence of the traditional catalyst, aluminum chloride (eqn. 2). ArH + RCOX + A1C13

~

Ar~C--R + HX II O ... A1C13

H-,O -

CX- c~ o c (o), R_..)

~

Ar~CO--R

+

Alsalts

(2)

Consequently, a lot of research has been carried out in this area in order to find convenient catalysts, i.e. those able to activate the acylating reagent while giving labile complexes with ketones, in particular in hot conditions. Ferric chloride is the most common catalyst when the reaction is achieved in this manner (refs. 7, 8). With this same view, Friedel-Crafts acylation in the presence of small quantities of catalysts (for example FeC13), is strongly activated by microwave irradiation, in particular when the catalyst is on a graphite substrate (ref. 9). Other recent works concerning the catalytic acylation of aromatic ethers concern : - t h e use of Lewis acid-lithium or silver salt (AgC104 or AgSbF6) systems (ref. 10), boron (ref. 11) or metallic triflates (ref. 12), the latter being reusable catalysts, or also zinc chloride on a clay substrate ("Clayzic") (ref. 13) ; the study of the regiochemistry of the acylation of 2-methoxynaphtalene in the presence of metallic chlorides (ref. 14) ; and the preparation of 4-alkanoylaryl-benzylethers (ref. 15). Catalysis by Br6nsted acids requires very strong concentrations (refs. 4-6), and is restricted to the more stable reagents and substrates. In this respect, anisole is not acylated with a good yield in presence of 1 % of triflic acid (ref.tl6). -

-

R E C E N T RESULTS CONCERNING CATALYSIS BY BISMUTH (HI) SALTS Whilst numerous metallic salts are used in catalysis, some of them have been relatively little studied. Such is the case of bismuth (III) salts including the chloride. Bismuth, relatively inexpensive, with a metallic character marldedly more pronounced than that of As or Sb, and giving much less toxic derivatives (ref. 17), might play an increasing role in catalysis, in particular for the substitution of some industrial catalysts affected by stricter standards on wastes. Many works concern Bi (V) compounds in stoichiometric oxidations involving the BiV/Bi m redox pairing (ref. 18). The use of Bi (III) compounds in organic reactions is less developed, but the literature includes some references, especially for the catalytic oxidation of alkenes or arenes (Bi (III) complexes and molybdates) (refs. 19-23), acyloins (Bi203/AcOH) (ref. 24), and in oxidative cleavage of epoxides (Bi (III) mandelate / DMSO) (ref. 25), ~-glycols (Ph3Bi / NBS) (ref. 26), and ~-ketols (Bi (III) mandelate) (ref. 27).

Concerning BiC13, this weak Lewis acid proved an unexpected catalyst in the Mukaiyama-cross aldol and -Michael reactions from enoxysilanes (ref. 28), because other metallic chlorides (TIC14, A1C13, SnC14...) and stronger Lewis acids, are required in stoichiometric proportion for these reactions (ref. 29). On the other hand, more recently, i has been shown that : the BiC13 catalytic activity in these reactions could be considerably enhanced by addition of some metallic iodides (ref. 30) ; the coupling of aldolisation and halogenation reactions, giving 13-haloketones or -esters, is possible owing to these catalytic systems (ref. 31) ; these Bi (III) halide systems allow strong Lewis acid sensitive compounds to be used (furane cpds) (ref. 32). Moreover, BiC13 on its own acts as catalyst for the Knoevenagel reaction (ref. 33) and is a strong activator of the Si-C1 bond (ref. 34). Associated with some metals (A1, Fe, Zn), BiC13 gives Bi(0) which is a catalyst for the allylation of aldehydes and amines (ref. 35), and for the reduction of aromatic nitro compounds to azoxy compounds (ref. 36). When associated with sodium borohydride, BiC13 gives an efficiem system for the selective reduction of nitroarenes and azomethines (ref. 37). As far as catalysis of acylation is concerned, BiC13 has been little studied. Two references report the use of this salt for the acetylation of toluene (ref. 38) and for the benzoylation of anisole (ref. 39), with average results for the latter, but poor for toluene. More recently, Le Roux and al. showed that BiC13metallic iodide systems efficiently catalyze the acylation reaction of enoxysilanes and allylsilanes for which they represent the first known catalysts (refs. 30b, 40). Although involving organosilanes, these last results encouraged us on to investigate the catalytic approach of Friedel-Crafts acylation using bismuth (III) salts, on their own, associated with co-catalysts, or on a substrate. We present here our initial results (ref. 41). -

-

-

BISMUTH (III) SALTS IN THE FRIEDEL-CRAFTS ACYLATION OF AROMATIC ETHERS Anisole is a reactive aromatic substrate for acylating reagents. For large extent, the recent works of Scheele (ref. 42) focus on this compound. Therefore, in order to give good comparison, we have chosen to carry out our first tests on Friedel- Crafts acylation using anisole 1 (eqn. 3).

MeO-~

RCOX_ HX -~ M e O ~ C R

+ MeO-~ o

(3)

/

RC [I O

2

3_

X = CL OC (O) R R = Me (a), Me2CH (b), Me3C (e), Me (CH2)4(d), Ph (e)

Acylation of anisole by acyl chlorides The reaction between anisole and acyl chlorides (eqn. 3, Z = C1) was carried out with an excess of aromatic substrate, without solvent (Table 1).

Table 1. Acylation of anisole by acyl chlorides (eq. 1, E = C1)a Entry

Catalyst (% mol)

R

Conditions b

Yield (%)~

1

BiC13 (10)

Me

50~

2h

60

;

50 d

2

BiC13 (10)

Me2CH

85~

6h

100

;

87 d

3

BiC13 (1)

Me2CH

85 ~

6h

40

4

BiC13 (10)

Me3C

85~

6h

90

;

80 a

5

BiC13 (10)

Ph

85~

6h

100

;

90 d

6

Claybis c (5) f

Me

50~

lh

50

7

Claybis: (5) f

Me2CH

70~

2h

92

;

85 d

8

Claybis c (5) f

MeaC

70~

2h

90

;

80 d

9

Bi203 (5)

Me2CH

85~

4h

90

;

80 d

l0

Bi203 (0.5)

Me2CH

85 ~

4h

35

a. b. c. d. e. f.

Without solvent; anisole / RCOC1 = 4/1; heating in an oil bath. Temperature of the oil bath. Conversion toward RCOCI ; 2_a,b,c,e / 3 a,b,c,e > 90/10 Yield in isolated product after aqueous workup. BiC13 / K 10 Montmorillonite prepared as "Clayzic" (ref. 13). 5% mol. in BiC13.

The Bi (III) salts, used bath chloride and oxide, gave comparable or often better results to those previously described in the literature. For instance, in the same experimental conditions Scheele observed 40 to 50% conversion of acetyl chloride in presence of 25 % mol. of catalyst (FeC13, TiCI4, SnCI4) (ref. 42). We have obtained 60 % conversion of acetyl chloride with only 10 % mol. of BiC13 (Table 1, entry 1). With a less volatile reagent, isobutyryl chloride, the reaction temperature can be increased, and the conversion was quantitative after 6h of heating at 85~ (Table 1, emry 2). In this case, 1% mol. of BiC13 led to 40 % conversion (entry 3). Isovaleryl chloride and benzoyl chloride also gave high yields (entries 4, 5). With the oxide Bi203, the yields are comparable to those obtained with the chloride (entries 9, 10). Deposited on K 10 Montmorillonite ("Claybis"), BiC13 becames more active than ZnC12 (ref. 13), the acylation reaction takes place at lower temperature and with a shorter reaction time (entries 7, 8), with only 5 % mol. of BiC13equivalent. Furthermore, some experiments with various Bi (III) salts and organometallic derivatives (for example : bismuth oxychloride, -acetate, salicylate, -carbonate oxide, -zirconate, -germanium oxide, triphenylbismuth) have shown that these derivatives are also catalysts for the acylation of anisole by acyl chlorides (ref. 41). Comparative tests were carried out with some catalysts under the same experimental conditions (Table 2). BiC13 is the most efficient of the 4 metallic chlorides used (Table 2, entries 1-6), but the difference is more pronounced with the oxides (entries 7-10), a point that will be turned to account and discussed further.

Table 2. Acylation of anisole by catalystsa Entry

Catalyst (% mol)

hexanoyl chloride. Comparative tests with various

Reaction time

Y/d (%)b 65

1

BiCI3 (5)

6h

2

BiC13 (10)

6h

87

3

SbC13 (10)

6h

27

4

FeC13 (5)

6h

58

5

FeCI3 (10)

6h

44

6

ZnC12 (5)

6h

46

7

Bi203 (5)

4h

80

8

Sb203 (5)

4h

20

9

Fe203 (5)

6,5h

< 5

10

ZnO (5)

6,5h

60

a. Without solvent 9 anisole / RCOCI = 4/1 9 heating at 80~ in an oil bath. b. In isolated product after aqueous workup; 2 d / 3 d > 90/10.

A study of solvents (Table 3) shows that dichloromethane associated with ether (10/1) gives acceptable yields, but benzene, not acylated in these conditions, and nitromethane give better yields. BiC13 is soluble in these last two solvents, at room temperature in MeNO2, and the reaction proceeds in homogeneous conditions.

Acylation of anisole by acid anhydrides Table 4 reports the results using 10 % tool. of BiC13 with acetic and isobutyric anhydrides as reagents (eq. 3, Z = OC(O)R). The acetylation of anisole by (MeCO)20 is outstanding (Table 4, entries 1,2), particularly by refluxing, towards acylation by MeCOC1 (Table 1, entries 1,6). On the other hand, acylation with (MezCHCO)20 is more difficult than with MezCHCOC1. The oxide Bi203 is not an acylation catalyst with an acid anhydride as reagent. However, the addition of chlorine-mobile agent to Bi203 gives an efficient catalytic system, for example Bi203,6 MeCOCI (5 % mol) or BieO 3, 6 Me3SiC1 (5 % mol). In the acylation of anisole by acetic anhydride, bismuth trichloride appears as a better catalyst than iron and zinc chlorides (Table 4, entries 2,5,6).

20

Table 3. Acylation of anisole by acyl chlorides RCOC1 in the presence of a solvent a Entry

Catalyst (% tool)

R

Solvent Conditions

Conversion ( ~o)b

1

BiC13 (5)

Me

28

2 3 4 5

BiC13 (10) Bi203 (5) BiC13 (10) Bi203 (5)

Me Me Me Me2CH

6

BiC13 (10)

Me2CH

CH2C12, Et20 (10/1) reflux, 3h MeNO2, 80~ 2h CH2C12, Et20 (10/1) reflux 4h C6H6, reflux 4h

7

Bi203 (5)

Me2CH

C6H 6,

8

BiC13 (10)

Me2CH

MeNO2, reflux 3,5h

a. b. c.

40 55 80 70 70 85 9 68c

reflux 4h

90" 80c

Anisole / RCOC1 = 1/1.2. Conversion toward anisole" 2 a,b / ~ a,b > 90/10. Yield in isolated product after aqueous workup.

Acylation of veratrole The acylation of 1,2-dimethoxybenzene or veratrole was carried out in the same conditions

as those of anisole,

either using acyl chlorides

or acid

anhydrides (eqn. 4) (Table 5). MeO MeO

MeO /N~

RCOX

MeO~N~~_C[

R

o

_4

5 x = EL OC(O)R R = Me (a), Me2CH (b)

21

(4)

Table 4. Acylation of anisole by acid anhydrides (eq. 1, Z = OC(O)R) a.

Entry

Catalyst (% mol)

R

Conditions

Conversion ( %)b

1 2 3 4 5 6

BiC13 (10) BiC13 (10) BiC13 (10) BiC13 (10) FeC13 (10) ZnC12 (10)

Me Me Me2CH Me2CH Me Me

85~ c, 6h reflux, 3h 85~ r 6h reflux, 3h reflux, 6h reflux, 6h

87 100 67 75 70 66

a. b. c.

Without solvent 9 anisole / (RCO)20 = 4/1. Conversion toward the acid anhydride; 2 a,b / 3 a,b > 90/10. Temperature of the oil bath.

Bismuth chloride is a good catalyst for veratrole acylation by RCOC1 (entries 1,2), but the crude product may contain about 5 % of an impurity idemified (GC-MS) as compound 6, which would arise from cleavage of an ether group by HC1. The reaction is more difficult with acid anhydrides (entries 4,5) relative to anisole (Table 4, entries 2,4). MeO

M e O ~ c o

R

22

Table 5. Acylationof veratrolea Entry

Catalyst (% mol)

Acylating reagent

Conditions b

Conversion ( %)c

1

BiCI3 (10)

MeCOCI

50~

lh

2

BiC13 (10)

Me2CHCOC1

85~

6h

80 100 " 88 d

3

Bi203 (5)

Me2CHCOC1

85~

4h

70

4

BiC13 (10)

(MeCO)20

140~

7h

65

5

BiC13 (10)

(Me2CHCO)20

140~

7h

24

a. b. c. d.

9 58 d

Without solvent; veratrole / acylating agent = 4/1 ; heating in an oil bath. Temperature of the oil bath. Conversion of 5 toward acylating agent. Yield in isolated product after aqueous workup.

Mechanistic aspects The mechanistic aspects of Friedel-Crafts acylation have been widely developed, in particular concerning identification of the intermediate complexes between the acylating reagent and the catalyst (refs. 1, 5, 6, 43). The general opinion is that these species exist in solution as an equilibrium mixture of ionic (oxocarbenium salts) and molecular (donor-acceptor complexes) forms whose relative concentrations depend on solvent and temperature. As a result of the insolubility of bismuth chloride in chloromethanes, a spectrometric study of RCOC1-BiC13 mixtures has been carried out in nitromethane. An equimolecular mixture of MeCOC1-BiC13 in MeNO2 (0.2 mol. 1-1) analyzed by infrared spectrometry showed the lack of characteristic absorptions of oxocarbenium ions around 2200-2300 cm -1 (refs. 43, 44), but a marked modification of the carbonyl stretching absorption. Two bands at 1715 and 1756 cm -1 took the place of the strong band at 1810 cm -1 of the free acetyl chloride, indicative of the perturbed carbonyl frequency of a dative structure C =O-+BiC13 (ref. 43). Multinuclear NMR has been very useful for the analysis of these complexes (ref. 43), in particular for the acetyl chloride-aluminum chloride system (ref. 45). A 1H and 13C-NMR study has been achieved with nitromethane solutions of acetyl chloride-BiC13 system (Table 6). Experiments using carbon 13 labelled acetyl-13C2 chloride in CD3NO 2 prove the existence of only one coordination

species. Indeed, the 13C-NMR spectra show one carbonyl signal (doublet) at low field (A6 = 6 ppm) of the carbonyl signal of free acetyl chloride, and one methyl signal (doublet) at high field (A6 = -13 ppm) of the methyl signal of free acetyl chloride. With two equivalents of BiC13, the acetyl chloride appears completely complexed. The chemical shifting of the protons of the methyl group is not influenced by the complexation. With 4-methoxyacetophenone 2, a similar experiment in nitromethane shows any modification of the chemical shift of the 13C-carbonyl signal after introduction of BiC13. Addition of MeCOC1 to this mixture causes the appearance of the characteristic signals of MeCOC1-BiC13 interaction. The actual stoichiometry of the RCOC1-BiC13 complex will be possible to define after further studies and its eventual isolation. Owing to the low basicity of bismuth, its interaction with the chlorine atom of RCOC1 must be envisaged. This would also explain the preferential complexation of BiC13 with acyl chlorides relative to ketones, that is the key of a catalytic system for FriedelCrafts acylation. We must point out the possibility of rt complexes between BiC13 and aromatic compounds (ref. 46). Considering the lability of these complexes, these interactions do not play a prominent role in the mechanism of FriedelCrafts acylation, but they can improve the solubility of bismuth salts.

Table 6. N M R data for acetyl-13C2 chloride-BiC13 a mixtures.

Mixtures

51H

513C (Me) b

513C(O)b

MeCOC1

2.66

33.9

172.5

33.9 20.6

172.5 176.5

20.9

178.8

MeCOC1, BiCI3

MeCOC1, 2 BiC13

a.

b.

2.67

Chemical shifts relative to TMS (ppm) 9 solvent CD3NO2 " t e m p e r a t u r e doublet, Ij(13C/13C) = 56 Hz.

24

300K.

A remarkable and somewhat surprising result is the catalytic activity of numerous bismuth (III) derivatives for the acylation of anisole by acyl chlorides. This result is indicative of likely oxygen-chlorine exchange between the Bi-O bond containing compound and acyl chloride giving BiC13, the true active species.This exchange is also involved in the Bi203-MeCOC1 and Bi203Me3SiC1 systems, active catalyst with acid anhydrides.

CONCLUSION Whilst bismuth (III) chloride is an efficient catalyst for the aromatic ether acylation by acid chlorides or anhydrides, it is not strong enough to carry out the acylation of non activated aromatics. However, the potential of using a wide range of Bi (III) salts as catalysts (ref. 41), in particular the oxide, the oxychloride and the carboxylates, all non hygroscopic compounds, offers advantages, and is indicative of the great versatility of Bi (III) derivatives. Moreover, the Bi salts obtained after hydrolytic workup are directly reusable. The para-selectivity of the described acylations is very high. In the case of the bismuth (III) salt, the ortho effect (refs. 42, 47), with its disadvantages, does not appear. Since the molecular chemistry is well developed, it seems to us possible to undertake the synthesis and study of the catalytic power of new bismuth (III) derivatives containing suitable ligands to activate the Lewis acidity of this element.

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25

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b)

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26

26. 27. 28. a)

b) 29. 30. a)

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