Utility of Dysprosium as a Reductant in Coupling Reactions of Acyl

ARTICLE pubs.acs.org/Organometallics

Utility of Dysprosium as a Reductant in Coupling Reactions of Acyl Chlorides: The Synthesis of Amides and Diaryl-Substituted Acetylenes Weifeng Chen, Kebin Li, Ziqiang Hu, Liliang Wang, Guoqiao Lai,* and Zhifang Li* Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou, 310012, Zhejiang, People’s Republic of China

bS Supporting Information ABSTRACT: Reduction of acyl chlorides with dysprosium metal has been studied. The reducing ability of dysprosium metal is solvent-dependent. Dysprosium metal, which requires neither any additive nor pretreatment, can promote the crosscoupling of acyl chlorides in DMF or DEF to give amides in good yields. When the reaction was performed in N,N-dimethylacetamide, the reductive self-coupling reaction of aroyl chloride took place smoothly and afforded the diaryl-substituted acetylenes in moderate to good yield.

’ INTRODUCTION Samarium diiodide as a versatile reagent has been widely used in organic synthesis.1
3 However, a stronger single electron transfer reducing reagent than samarium diiodie is sometimes required. The addition of hexamethylphosphoramide (HMPA) to samarium diiodide is a solution because it can effectively enhance its reduction potential.4 Nevertheless, HMPA is highly carcinogenic; thus powerful and readily available alternatives are strongly desirable. Recently, some “new” soluble divalent lanthanide reagents, LnI2 (Ln = Nd,6 Dy,7 Tm8) whose reduction potential is much higher than that of SmI2 [oxidation reduction potential of Ln3þ/Ln2þ in aqueous solution, Nd (
2.6), Dy (
2.5), Tm (
2.3), and Sm (
1.55)]9 have been reported.10 For example, NdI2,6a DyI2,7b or TmI28b allows the Barbier-type reaction of ketone and alkyl chloride to take place smoothly even at low temperature, while the SmI2/HMPA system does not work under similar conditions. The zerovalent lanthaniod metals, such as Sm, Nd, Gd, Dy, and Yb, are stable in air, nontoxic, cheap, and also bear strong reduction potential. However, direct use of zerovalent lanthanoid metals themselves in organic synthesis is still rare. The advantages of the direct use of lanthaniod metals are the easy operation and electron economies compared with divalent lanthanoid species. However, the disadvantages are also distinct such as the heterogeneous nature in organic solvents and lower reactivity. In order to enhance the reduction ability of lanthanoid metals, usually activating agents are required. The additive commonly employed include iodine,11 Me3SiBr/HMPA,12 MenSiCl4
n,13 hydrochloric acid,14 alkyl halides,15 etc. Recently, Ogawa reported that photoirradiation could dramatically increase the reduction ability of some rare earth metals (e.g., Ce, Nd, Sm, and Eu) toward the reductive deiodation of iodoalkanes.16 By applying this technology, the reductive coupling of acid chlorides with styrenes using neodymium metal as a reductant has been realized.17 Metallic dysprosium is stable in air, and its reducing power (Dy3þ/Dy =
2.35) is comparable with that of magnesium (Mg3þ/Mg =
2.37). Therefore, the direct use of dysprosium r 2011 American Chemical Society

metal as a reducing agent in organic transformation will open a new fertile area. However, to the best of our knowledge, only few papers using dysprosium metal itself directly in organic synthesis without any activator or pretreatment have been reported.13b Herein, we describe that the synthesis of amides and diarylsubstituted acetylenes by applying a dysprosium powder-promoted coupling reaction of acyl chlorides and found that the reduction ability of dysprosium metal and the kind of coupling products were significantly solvent-dependent.

’ RESULTS AND DISCUSSION The reduction of acyl chlorides has been investigated extensively, and the products are highly dependent on the reducing agents used. For example, symmetrical ketones were formed as a nickel or enneacarbonyl diiron lead was used as the reductant.18 Tetraphenyl furan was isolated as the major product when low-valent titanium species were used as the reducing agents.19 The reduction of aroyl chlorides with samarium diiodide yielded R-diketones, R-hydroxy ketones, and (Z)-R,R0 -stilbenediol dibenzoates based on the reaction conditions employed.20 Recently, Zhang reported the reduction of aroyl chlorides with samarium metal in DMF and afforded o-aroylbenzoins, 1,2-diarylethanones, and (Z)-R,R0 -stilbenediol dibenzoates in moderate to good yields.21 Herein, we wish to further report the lanthanoid metals (Nd, Gd, and Dy)-promoted coupling reactions of acyl chlorides. Two novel kinds of products resulting from the reductive coupling were afforded, and still different pathways other than those previously reported were proposed. Our studies found that the reducing ability of lanthanoid metals (Nd, Gd, and Dy) and the kind of products in this coupling reaction are highly solvent-dependent. The effects of the lanthanoid metals, solvent, time, and temperature on the reductive coupling of acyl chlorides are summarized in Table 1. Received: January 27, 2011 Published: March 09, 2011 2026

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Organometallics

ARTICLE

Table 1. Ln-Induced Reductive Coupling of Benzoyl Chloride in a Series of Solventsa

Table 2. Dysprosium-Induced Cross-Coupling of Acyl Chlorides in N,N-Dimethylformamide (DMF) or N, N-Diethylformamide (DEF)a

yield (%)b entry

Ln

solvent

temperature (°C)

time (min)

1 2

Dy Nd

DMF DMF

25 25

20 20

95 88

3 4

Gd

DMF

25

20

89

Dy

DMAC

25

20

7

5

Nd

DMAC

25

6

Gd

DMAC

7

Sm

8

2a

3a

entry

acyl chloride (1a
i)

solvent

product

yield (%)b

1

benzoyl chloride (1a)

DMF

2a

91

2

benzoyl chloride (1a)

DEF

4a

81

trace

3

4-methylbenzoyl chloride (1b)

DMF

2b

89

43

4

2-chlorobenzoyl chloride (1c)

DMF

2c

93

30

5

3-chlorobenzoyl chloride (1d)

DMF

2d

91

25

30

6

3-chlorobenzoyl chloride (1d)

DEF

4b

92

DMAC

25

30

Dy

DMAC

25

30

7 8

4-bromobenzoyl chloride (1e) 1-naphthoyl chloride (1f)

DMF DMF

2e 2f

93 94

9 10

Dy Dy

DMAC THF

80 65

180 120

9

2-naphthoyl chloride (1g)

DMF

2g

89

2-phenylacetyl chloride (1h)

DMF

2h

82

11

Dy

CH3CN

81

120

d

10

heptanoyl chloride (1i)

DMF

2i

66c

12

Dy

DME

70

120

d

11

8

65

5

86c

d

13

Dy

toluene

100

120

d

14

Dy

pyridine

80

60

e

15

Dy

DME

25

120

f

a

Unless otherwise noted, aroyl chloride (5 mmol) and dysprosium (2 mmol) were allowed to react in DMF (10 mL) or DEF (10 mL) at rt whin 20 min. b Isolated based on acyl chloride used. c 2 h.

a

Reaction conditions: Benzoyl chloride (5 mmol), Ln (2 mmol), solvent (10 mL). b Isolated yield based on benzoyl chloride used. c Excess amount of dysprosium (4 mmol) was used. d No reaction. e Complex mixture was formed. f Catalytic amount of iodine was used; 1, 2-diphenylethanone (40%) and methyl benzoate (35%) were isolated.

Benzoyl chloride was chosen as the model substrate to investigate the coupling reaction with lanthannoid metals (Nd, Gd, and Dy). When benzoyl chloride (5 mmol) was treated with lanthanoid metals (Dy, Nd, Gd) (2 mmol) in DMF at room temperature, the color of the reaction mixture changed rapidly from colorless to brown, and an exothermic reaction was observed (Table 1, entries 1
3). N,N-Dimethylbenzamide (2a) was unexpectedly obtained in high yields after simple workup (Table 1, entries 1
3). The reactions were generally clean, and only N, N-dimethylbenzamide (2a) was isolated in good yield. The formation of 2a should be ascribed to the cross-coupling of benzoyl chloride and DMF. It should be noted that no selfcoupling products of the benzoyl chloride could be detected in the GC-MS analysis of the crude products. Interestingly, when benzoyl chloride (5 mmol) was treated with dysprosium (2 mmol) in N,N-dimethylacetamide (DMAC) at room temperature, 1,2-diphenylethyne (3a) was obtained in 43% isolated yield (entry 4). The yield of 3a was increased to 65% by prolonging the reaction time (30 min) (entry 8). It should be noted that when an excess amount of dysprosium (4 mmol) was used, the yield of 3a was greatly improved (entry 9). Neodymium, gadolinium, or samarium did not promote the coupling reaction, and the starting benzoyl chloride was recovered quantitatively (entries 5
7). When the coupling reaction was carried out in THF, CH3CN, DME, toluene, etc., no reaction occurred at all (entries 10
13). With pyridine as the solvent, a complex mixture was obtained. When a catalytic amount of iodine was used in DME, 1,2-diphenylethanone (40%) and methyl benzoate

(35%) rather than 3a were isolated (entry 15). Thus the selfcoupling of benzoyl chloride with dysprosium powder at 80 °C in N,N-dimethylacetamide to give 1,2-diphenylethyne (3a) was developed as the standard reaction conditions. On the basis of the reductive cross-coupling of benzoyl chloride with dysprosium metal, next, we subsequently studied the scope of the dysprosium/DMF system induced coupling reaction of acyl chlorides. As shown in Table 2, a variety of amides were obtained in good to excellent yields. Notably, in this Dy-promoted reaction, aliphatic acyl chlorides, such as 2-phenylacetyl chloride (1h) and heptanoyl chloride (1i), also worked well to give the corresponding N,N-dimethyl-2-phenylacetamide (2h) and N,N-dimethylheptanamide (2i) in 82% and 66% isolated yields, respectively (entries 10 and 11). When benzoyl chloride and 3-chlorobenzoyl chloride were treated with metallic dysprosium in N,N-diethylformamide (DEF), N,N-diethylbenzamide (4a) and 3-chloro-N,N-diethylbenzamide (4b) were produced in good yields (entries 2 and 6). From the experimental results, it is reasonable to propose that the reaction proceeded through two pathways. Path 1 involves a radical mechanism. Thus DMF as a solvent with strong polarity may play an important role in stabilizing the intermediate and dissolving the dysprosium salts formed in the reaction since the reaction does not work in THF, toluene, and DME. In path 2, first, DMF decomposed into dimethylamine and carbon monoxide induced by dysprosium; then the released dimethylamine is captured by benzoyl chloride to give N, N-dimethylbenzoylamide (2a). However, when DMF or Nmethyl-N-phenylformamide in neat was treated with dysprosium in 3 h, neither dimethylamine or N-methylbenzenamine could be detected. It should be noted that when benzoyl chloride was treated with neat N-methyl-N-phenylformamide in the presence of dysprosium, the cross-coupling product 2027

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Scheme 1. Possible Mechanism of Dysprosium-Promoted Cross-Coupling of Benzoyl Chloride in DMF

Table 3. Dysprosium-Induced Self-Coupling of Aroyl Chloride in N,N-Dimethylacetamide (DMAC)a

entry

aroyl chloride

time (h)

product

Scheme 2. Possible Mechanism of Dysprosium-Induced Coupling of Benzoyl Chloride in DMAC

yield (%)b

1

benzoyl chloride (1a)

1

3a

46

2

benzoyl chloride (1a)

3

3a

86

3 4

2-methylbenzoyl chloride (1b) 3-methylbenzoyl chloride (1c)

3 3

3b 3c

85 81

5

4-methylbenzoyl chloride (1d)

3

3d

80

6

4-ethylbenzoyl chloride (1e)

3

3e

72

7

4-butylbenzoyl chloride (1f)

3

3f

70

8

4-methoxybenzoyl chloride (1g)

5

3g

60

9

1-naphthoyl chloride (1h)

5

3h

35c

10

4-chlorobenzoyl chloride (1i)

5

3i

30d

11 12

4-chlorobenzoyl chloride (1i) heptanoyl chloride (1j)

12 5

3j

12e NR

a

Unless otherwise specified, aroyl chloride (5 mmol) and dysprosium (4 mmol) were allowed to react in DMAC at 80 °C. b Isolated yields based on aroyl chloride used. c 1,2-Di(naphthalen-1-yl)ethane-1,2-dione was obtained in 40% yield. d 1,2-Bis(4-chlorophenyl)ethane-1,2-dione was obtained in 45% yield. e 1,2-Bis(4-chlorophenyl)ethane was isolated in 12% yield.

N-methyl-N-phenylbenzamide was obtained in 85% isolated yield (eq 1). Thus, for the mechanism of the cross-coupling reaction of benzoyl chloride in DMF or DEF, path 1 is preferred.

The reductive conversion of aroyl chlorides into diarylacetylenes with lithium amalgam was first reported by Horner.22 The electrochemical reduction of aroyl chlorides on a Hg cathode in DMF/LiClO4 yielded 1,2- diphenylacetylenes using a two-step procedure.23 We found that when aroyl chloride was treated with

metallic dysprosium in DMAC, diaryl-substituted acetylenes were obtained. As shown in Table 3, aroyl chlorides with electron-donating groups underwent the self-coupling reaction smoothly to give the corresponding diaryl-substituted acetylenes in good yields (entries 1
7). With aroyl chlorides with electronwithdrawing groups, such as 1-naphthoyl chloride and 4-chlorobenzoyl chloride, the corresponding diaryl-substituted acetylenes were isolated in low yields. In these reactions, R-diketones were isolated as the major products (entries 9 and 10). In the case of 1-naphthoyl chloride, the low yield may be due to the steric effect, not the inductive effect (Table 3, entry 9). When the reaction time was prolonged, the overreduction product, 1,2-bis(4-chlorophenyl)ethane, was detected in 12% yield by GC-MS analysis of the crude products (entry 11).7b However, when heptanoyl chloride was treated under similar reaction conditions, no reaction occurred (entry 12). A possible mechanism for this self-coupling reaction of aroyl chlorides with metallic dysprosium is shown in Scheme 2. It is likely that the reaction begins with an electron transfer from dysprosium to benzoyl chloride (1a); thus benzoyl radical is formed (A). The benzoyl radical (A) may dimerize into benzyl (B), which immediately accepts one equivalent of electrons from dysprosium metal to give C. (Z)-Enediolate (E) is formed when C accepts two more equivalents of electrons.21 (Z)-Enediolate (E) may undergo reductive deoxygenation in the presence of low-valent dysprosium species, giving the corresponding 1, 2-diphenylethyne (3a).22,23 To provide support for the above mechanism, benzil (B) was treated with dysprosium metal in DMAC. Gratifyingly, the 2028

dx.doi.org/10.1021/om200080f |Organometallics 2011, 30, 2026–2030

Organometallics reaction did afford 1,2-diphenylethyne (3a) in 56% isolated yield.24

In conclusion, we have found that dysprosium metal without any activation and pretreatment could promote the cross- or selfcoupling reaction of acyl chlorides effectively in DMF, DEF, or DMAC, which afford the amides or diaryl-substituted acetylenes in good yields. Especially, it should provide an attractive alternative route to diaryl-substituted acetylenes due to the readily available and less expensive starting materials, operational simplicity, high yields of the products, and high potential for largescale synthesis. Furthermore, the investigation may demonstrate that dysprosium metal as a very powerful reductant would find its unique usages in organic transformation since it proved to have somewhat different reactivity and oxygen-philicity compared with the samarium and traditional divalent lanthanide complexes.

’ EXPERIMENTAL SECTION 1 H and 13C NMR spectra were recorded on a Bruker AV-400 MHZ instrument as DMSO solutions using TMS as an internal standard. Chemical shifts (δ) are reported in ppm, and coupling constants J are given in Hz. GC-MS were measured with a Trance 2000 DSQ mass spectrometer. IR spectra were taken as thin films with a Nicolet 5700 infrared spectrometer. Elemental analysis was performed on a Vario ELIII instrument. Melting points are uncorrected. Column chromatography was performed with silica gel (Wako Pure Chemical Industries, Ltd., Wakogel C-300). Thin-layer chromatography was performed on 0.25 mm E. Merck silica gel plates (GF-254). All the reactions in this paper were performed under a nitrogen atmosphere.

General Procedure for Dysprosium-Induced Self-Coupling of Aroyl Chlorides. A mixture of aroyl chloride (5 mmol) and dysprosium powder (4 mmol) in DMAC (10 mL) was heated at 80 °C under a N2 atmosphere for the time given in Table 3. The reaction was monitored by thin-layer chromatography. The volatile materials were removed under reduced pressure, and the residue was purified by flash column chromatograph (SiO2, petroleum ether/ethyl acetate, 10:1) to afford the pure diaryl-substituted acetylenes. 1,2-Diphenylethyne (3a) (ref 22): white solid; mp 59
60.5 °C (lit.22 mp 60 °C); 1H NMR (DMSO, 400 MHz) δ 7.55
7.53 (m, 4H), 7.42
7.40 (m, 6H); 13C NMR (DMSO, 100 MHz) δ 129.3, 129.2, 122.8, 89.8; MS m/z (%) 178 (Mþ, 100), 152 (12), 126 (7), 89 (13),76 (17), 51 (3). 1,2-Di-o-tolylethyne (3b) (ref 25): white solid; mp 58
60 °C (lit.25 mp 57
59 °C); 1H NMR ((CD3)2CO, 400 MHz) δ 7.53
7.51 (d, 2H, J = 8 Hz), 7.32
7.20 (m, 6H), 2.51 (s, 6H); 13C NMR ((CD3)2CO, 100 MHz) δ 139.7, 131.7, 129.6, 128.5, 125.8, 123.1, 92.1, 20.1; MS m/z (%) 206 (Mþ, 100), 191 (31), 178 (16), 165 (13), 128 (8), 115 (13), 91 (18), 89 (24), 63 (7). 1,2-Di-m-tolylethyne (3c) (ref 23): white solid; mp 75
76 °C (lit.23 mp 74 °C); 1H NMR ((CD3)2CO, 400 MHz) δ 7.35
7.21 (m, 8H), 2.34 (s, 6H); 13C NMR ((CD3)2CO, 100 MHz) δ 138.2, 131.9, 129.2, 128.5, 123.1, 88.9, 20.3; MS m/z (%) 206 (Mþ, 100), 189 (18), 165 (7), 115 (5), 102 (10), 76 (5). 1,2-Di-p-tolylethyne (3d) (ref 22): white solid; mp 134 °C (lit.22 mp 131
133 °C); 1H NMR ((CD3)2CO, 400 MHz) δ 7.42
7.40 (d, 4H, J = 8 Hz), 7.23
7.21 (d, 4H, J = 8 Hz), 2.35 (s, 6H); 13C NMR ((CD3)2CO, 100 MHz) δ 138.4, 131.3, 129.2, 120.3, 88.6, 20.5; MS m/z (%)

ARTICLE

206 (Mþ, 100), 191 (9), 165 (6), 139 (4), 102 (11), 89 (12), 76 (7), 63 (4). 1,2-Bis(4-ethylphenyl)ethyne (3e) (ref 26): white solid; mp 110
112 °C; 1H NMR ((CD3)2CO, 400 MHz) δ 7.45
7.43 (d, 4H, J = 8 Hz), 7.26
7.24 (d, 4H, J = 8 Hz), 2.67
2.63 (q, 4H, J = 4 Hz), 1.23
1.19 (t, 6H, J = 4 Hz); 13C NMR ((CD3)2CO, 100 MHz) δ 144.8, 131.4, 128.0, 120.6, 88.7, 28.4, 14.9; MS m/z (%) 234 (Mþ, 93.0), 219 (100), 204 (32), 189 (9), 139 (4), 117 (4), 102 (25), 89 (8), 76 (3). 1,2-Bis(4-butylphenyl)ethyne (3f): white solid; mp 41
42 °C (lit. mp 41 °C); 1H NMR ((CD3)2CO, 400 MHz) δ 7.47
7.42 (d, 4H, J = 8 Hz), 7.24
7.22 (d, 4H, J = 8 Hz), 2.65
2.59 (m, 4H), 1.63
1.56 (m, 4H), 1.39
1.28 (m, 4H), 0.94
0.90 (m, 6H); 13C NMR ((CD3)2CO, 100 MHz) δ 143.4, 131.3, 128.7, 128.6, 120.6, 88.8, 35.2, 33.4, 22.1, 13.3; MS m/z (%) 290 (Mþ, 73), 247(100), 204 (51), 189 (8), 161 (7), 115 (3), 102 (2). 1,2-Bis(4-methoxyphenyl)ethyne (3h) (ref 22): white solid; mp 140
144 °C (lit.22 mp 142
144 °C); 1H NMR ((CD3)2CO, 400 MHz) δ 7.45
7.43 (d, 4H, J = 8.6 Hz), 6.96
6.94 (d, 4H, J = 8.6 Hz), 3.82 (s, 6H); 13C NMR ((CD3)2CO, 100 MHz) δ 159.7, 132.7, 115.5, 114.1, 88.7, 54.8; MS m/z (%) 238 (Mþ, 100), 223 (77), 195 (22), 180 (12), 152 (29), 119 (18), 98 (3), 75 (2). 1,2-Di(naphthalen-1-yl)ethyne (3g) (ref 26): white solid; mp 124
126 °C (lit.26 mp 125
126 °C); 1H NMR ((CD3)2CO, 400 MHz) δ 8.60
8.58 (d, 2H, J = 8 Hz), 8.10
8.00 (m, 6H), 7.73
7.59 (m, 6H); 13C NMR ((CD3)2CO, 100 MHz) δ 133.5, 133.1, 130.7, 129.2, 128.6, 127.2, 126.7, 125.9, 125.6, 120.7, 92.2; MS m/z (%) 278 (Mþ, 100), 138 (30), 125 (10), 112 (3), 87 (1), 63 (1). 1,2-Bis(4-chlorophenyl)ethyne(3i) (ref 22): white solid; mp 175
177 °C (lit.22 mp 174 °C); 1H NMR ((CD3)2CO, 400 MHz) δ 7.53
7.51 (d, 4H, J = 8.8 Hz), 7.45
7.44 (d, 4H, J = 8.8 Hz); 13C NMR ((CD3)2CO, 100 MHz) δ 133.9, 129.8, 128.0, 121.7, 89.2; MS m/z (%) 246 (Mþ, 100), 176 (50),98(4), 75 (2).

’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed experimental procedure for the synthesis of amides (2a
i and 4a,b). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (20802014) and Zhejiang Provincial High Technology Research Program (2009R50016). ’ REFERENCES (1) Namy, J. L.; Girard, P.; Kagan, H. B. New J. Chem. 1977, 1, 5–7. (2) Girard, P.; Namy, J. L.; Kagan, H. B . J. Am. Chem. Soc. 1980, 102, 2693–2698. (b) Namy, J. L.; Girard, P.; Kagan, H. B.; Caro, P. E. New J. Chem. 1981, 5, 479–484.(c) Kagan, H. B.; Namy, J. L. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Jr., Eyring, L., Eds.; Elsevier: Amsterdam, 1984; Vol. 6, Chapter 50. (3) For reviews, see: (a) Kagan, H. B.; Namy, J. L. Tetrahedron 1986, 42, 6573–6614. (b) Soderquist, J. A. Aldrichim. Acta 1991, 24, 15–23. (c) Molander, G. A. Chem. Rev. 1992, 92, 29–68. (d) Krief, A.; Laval, A. M. Chem. Rev. 1999, 99, 745–777.(e) Molander, G. A.; Harris, C. R. In Encyclopedia of Reagents for Organic Synthesis; Paquette, L. A., Ed.; Wiley: New York, 1995; Vol. 6, pp 4428
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