Phosphorus, Sulfur, and Silicon and the Related ...

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AlCl3 Catalyzed Oxo-Diels-Alder and Wittig-Type Reaction of 2,3-Dimethyl-1,3-Butadiene With Acyl Phosphonates S. Nadeem a

ab

& Ayhan S. Demir

a

Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey

b

International Center for Chemical and Biological Sciences, HEJ Research Institute of Chemistry, University of Karachi, Karachi 75270, Pakistan Accepted author version posted online: 30 Apr 2014.

To cite this article: S. Nadeem & Ayhan S. Demir (2014): AlCl3 Catalyzed Oxo-Diels-Alder and Wittig-Type Reaction of 2,3-Dimethyl-1,3-Butadiene With Acyl Phosphonates, Phosphorus, Sulfur, and Silicon and the Related Elements, DOI: 10.1080/10426507.2014.884089 To link to this article: http://dx.doi.org/10.1080/10426507.2014.884089

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ACCEPTED MANUSCRIPT AlCl3 Catalyzed Oxo-Diels-Alder and Wittig-Type Reaction of 2,3-Dimethyl-1,3-Butadiene With Acyl Phosphonates S. Nadeem,a,b,* Ayhan S. Demir,a a

Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey

b

International Center for Chemical and Biological Sciences, HEJ Research Institute of

Downloaded by [Mugla University] at 03:56 18 September 2014

Chemistry, University of Karachi, Karachi-75270, Pakistan *corresponding author: [email protected] Phone: +90-5335499880 Abstract: Different aliphatic and aromatic acyl phosphonates were synthesized using Arbuzov reaction and subjected to Oxo-Diels-Alder (ODA) reaction with 2,3-dimethyl-1,3-butadiene at -78 °C. AlCl3 was found the best catalyst to obtain the desired ODA products from acyl phosphonates in only 30 minutes at low temperature, as compared to 100 °C - previously reported. The same reaction was repeated at room temperature but, instead of ODA, Wittig-type product was obtained. All the products were characterized by NMR spectroscopy and mass spectrometry. 1H,31P and 13C,31P couplings were observed in the NMR spectra. O

O P O O

O P O

O

DCM, AlCl 3

O -78 oC, 1h

+

DCM, AlCl3 rt, 1h

O P O O

Keywords: Oxo-Diels-Alder; Arbuzov reaction; Wittig reaction; acyl phosphonates

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INTRODUCTION

Acyl phosphonates (ACPS) are potent acyl anion precursors. They generate acyl anion equivalents in the presence of cyanide anion via phosphonate-phosphate rearrangement. These


anions readily attack activated ketones, which results in the formation of acyloin-type coupling products.1 ACPS are useful reagents in cross-benzoin reactions with certain advantages: they are easy to prepare from carboxylic acids or aldehydes and are stable and reactive under a variety of conditions.2-5 The formation of acyl anions from ACPS is very similar to that from acyl silanes.6 ACPS are normally synthesized through Arbuzov reaction.7,8 In a typical reaction, acyl chlorides are reacted with trialkyl phosphites which produce ACPS as the main product, while alkyl chlorides are formed as the side product. ACPS are mostly greenish liquids which are distilled at reduced pressure with the help of high vacuum pumps connected to a glass assembly. Our group is already involved in ACPS chemistry11,12 and successfully employed acyl phosphonates in Hetero-Diels-Alder reactions as dienophile partner. Our group already reported the thermally activated Hetero-Diels-Alder (HDA) reaction between benzoylphosphonate and 1-methoxy-3-trimethylsilyloxy-1,3-butadiene (Danishefsky’s diene).7 The reactants were mixed and heated up to 60 °C, but no HDA product was observed. The desired glycosyl-type phosphonates were obtained in 89 % yield when the temperature was increased to 100 °C. Herein we report the Oxo-Diels-Alder (ODA) reaction of different ACPS with 2,3-dimethyl-1,3-butadiene at low temperatures in the presence of AlCl3 as catalyst. Thus we were able to perform ODA reaction of ACPS as dienophile and 2,3-dimethyl-1,3-butadiene

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ACCEPTED MANUSCRIPT as diene at mild conditions, i.e. at low temperature. It is well established that at low temperature, chemical reactions show high selectivity. ACPS were also reacted with 2,3-dimethyl-1,3butadiene in the presence of AlCl3 at room temperature in dichloromethane. Interestingly, Wittig-type products were obtained instead of ODA type products. The oxygen atom of the carbonyl group was replaced by a =CH2 moiety. We propose that the =CH2 moiety originates


from dichloromethane (solvent), but to investigate our assumption, a separated project has been started.

RSULTS AND DISCUSSION Different acyl phosphonates were synthesized according to literature procedures utilizing Arbuzov reaction. In this reaction the phosphorus atom of the phosphite molecule attacks the acyl carbon atom with chloride acting as a leaving group. The chloride anion subsequently attacks the methyl carbon atom which results in the formation of methyl chloride. Due to the low boiling point of methyl chloride (-23.8 °C), it is a gas at room temperature and develops pressure in the vessel during the reaction. To avoid explosion in this reaction, continues argon supply and an exhaust line is necessary. The formation of ACPS 3 can be observed from its color change from colorless to greenish.

Oxo-Diels-Alder Reactions The acyl phosphonates 1 were subjected to ODA reactions with 2,3-dimethyl-1,3-butadiene (2). In a typical reaction, the phosphonate 1 was transferred into a dry flask filled with argon and sealed with a rubber septum. Dichloromethane was added with a syringe and the temperature of

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ACCEPTED MANUSCRIPT the solution was lowered to -78 °C using dry ice in acetone in a thermostat. Dry aluminum chloride was added to the flask while flushing argon. Butadiene 2 was added at the same temperature drop-wise with continuous stirring. After the addition was completed, the reaction mixture was stirred for half an hour and allowed to warm to room temperature. The reaction mixture was poured into distilled water and stirred. The crude product 3 was extracted with


diethyl ether. The ODA products showed special behavior on the TLC. After developing, TLC was dipped into 4-methoxybenzaldehyde and heated with a heat gun - dark blue spot appeared on TLC plate. AlCl3 was used to increase the polarity of the carbonyl double bond. The benzoyl phosphonates 1 were allowed to react with 2 in dichloromethane as shown in Scheme 1. The crude product was distilled in high vacuum and the pure product was investigated by 1H NMR spectroscopy to confirm the formation of the ODA product.

Wittig-Type Reaction The same reactants were used and same reaction conditions were applied in this reaction except that the reaction was performed at room temperature. In a typical reaction, phosphonate 1 (R = Ph) and 2,3-dimethyl-1,3-butadiene 2 were added to the reaction tube. AlCl3 was added to the reaction mixture under argon (Scheme 2). After one hour stirring, the reaction mixture was poured into dilute HCl and stirred. The organic phase was extracted with diethyl ether and purified chromatographically. The main product obtained was subjected to NMR analysis. OxoDiels-Alder product was expected but no NMR signals for the methyl and –CH2– groups were observed. Instead, 1H NMR signals for two protons were observed in the form of doublet of

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ACCEPTED MANUSCRIPT doublets with high P,H coupling constants of 22.1 Hz and 45.7 Hz. The doublet of each proton was further split into a doublet with coupling constant of 1.3 Hz. So obviously a Wittig-type reaction occurred and the oxygen atom of the carbonyl group has been exchanged with by =CH 2 moiety as shown in Scheme 2. Most probably the =CH2 group originates from dichloromethane used in the reaction as solvent.


A separate project has been started to put light on the mechanism of the reaction. If we would be able to prove that, it will be a cheap and easy reagent to replace Wittig reagent.

CONCLUSION Oxo-Diels-Alder reaction was performed on different acyl phosphonates with 2,3-dimethyl-1,3butadiene at -78 °C. This temperature used is much lower as compared to that of 100 °C reported previously. AlCl3 was used as a catalyst in Oxo-Diels-Alder reaction. When the same reaction was performed at room temperature, instead of ODA product a Wittig-type product was obtained, where the oxygen atom of the carbonyl group is replaced by =CH2 moiety. We propose that the =CH2 moiety originates from the solvent dichloromethane – similar to Friedel-Craft reaction. Studies are underway to prove our proposed mechanism. This will open new ways to perform Wittig reaction at mild conditions, using less expense reagents and a cheape catalyst instead of Wittig reagent.

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EXPERIMENTAL All the reactions were performed according to standard methods with clean glassware and dry solvents. Trimethyl phosphite, AlCl3, and all solvents used were purchased from Merck. Compounds were purified by column chromatography using flash silica. All acyl phosphonates


were synthesized using Arbuzov reaction according to the literature procedure. 8 Freshly distilled solvents were used in all reactions. Dry AlCl3 was handled in a dry box. Reactions were performed under argon. Characterization of the pure products was done by different techniques including EIMS (JEOL JMS600), NMR (Bruker Avance 400, Avance AV 300), and TLC.

Synthesis of Phosphonates (1): General Procedure In a typical reaction, 1.24 g trimethyl phosphite (10 mmol) was added dropwise to 1.4 g benzoyl chloride (10 mmol) at 0 °C — under argon, no solvent and stirred constantly for 30 min. The flask was sealed with a rubber septum. As a byproduct, methyl chloride (gas) is produced during the reaction that develops pressure. An injection needle was put throughout the septum to release that pressure. Color of reaction mixture changed to green which is the indication of phosphonate formation. 30 min later, the reaction mixture was allowed to reach room temperature. The crude liquid was subjected to high vacuum distillation. Until the green product condensed, pre-distillate — white in color and contained mostly remaining benzoyl chloride — were collected and discarded. The green product was collected till the temperature was constant at 100 oC. the green compound was subjected NMR which showed clear spectrum without any impurity peaks. Different aromatic and aliphatic acyl phosphonates were synthesized by this way.

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Oxo-Diels-Alder Reaction: General Procedure A model reaction procedure for ODA is given below:

Dimethyl (4,5-Dimethyl-2-phenyl-3,6-dihydro-2H-pyran-2-yl)phosphonate (3a)


To an oven dried Schlenk tube equipped with a dried magnet stick, filled with argon and sealed with a rubber septum 1 mL dried DCM was added by a syringe followed by the addition of 150 mg (0.7 mmol) of benzoyl phosphonate. 3.52 mg (1.1 equiv.) of diene 2 was added by a syringe and the reaction mixture was stirred for 5 min at -78 °C. 111 mg (1.2 equiv.) of AlCl3 was added while passing argon at the same temperature. The tube was sealed again under argon and after stirring for 10 min the reaction mixture was warmed to 0 °C (ice bath) and stirred. After 1 h the reaction mixture was poured into 20 mL of distilled water and vigorously stirred. The crude product 5 was extracted with 3 × 10 mL of diethyl ether. The organic layer was dried with Na2SO4 and the solvent was evaporated. On TLC the product showed bluish color when dipped in 4-methoxybenzaldehyde and heated. The product was purified by column chromatography using 30 % ethyl acetate in hexane as eluent. The solvent was evaporated and the compound was dried in high vacuum. 145 mg of the product was obtained as slight yellow viscous liquid corresponding to 70 % yield. EI-MS: m/z = 296.3. 1H NMR (400 MHz, CDCl3): δ = 7.52 (m, 2 H), 7.37 (m, 2H), 7.3 (m, 1H), 4.0 (d, J = 16.0 Hz, 1H), 3.87 (d, J = 16.0 Hz, 1H), 3.69 (d, J = 8.0 Hz, 3H, OCH3), 3.61 (d, J = 8.0 Hz, 3H, OCH3), 2.98 (t, J = 16.0 Hz, 1H), 2.70 (d, J = 16.0 Hz, 1H), 1.70 (s, 3H, CH3), 1.35 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3): δ = 135.9, 129.9, 128.2 (d, JPC = 3 Hz), 127.9 (d, JPC = 2.9 Hz), 127.8 (d, JPC = 4.4 Hz), 123.6 (d, JPC = 1.5 Hz),

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ACCEPTED MANUSCRIPT 121.0 (d, JPC = 10.9 Hz), 64.6 (d, JPC = 10.9 Hz), 54.1 (d, JPC = 6.5 Hz), 53.9 (d, JPC = 7.3 Hz), 32.7, 18.7, 13.8.

Dimethyl (2-(4-Chlorophenyl)-4,5-dimethyl-3,6-dihydro-2H-pyran-2-yl)phosphonate (3b)


139 mg (70 %) of 3b was obtained as slight yellow viscous liquid. EI-MS: m/z = 330.08. 1H NMR (400 MHz, CDCl3): δ = 7.37 (dd, 3J = 12.0 Hz, 4J = 4.0 Hz, 2H), 7.27 (d, 3J = 8.0 Hz, 2H), 3.93 (d, J = 16.0 Hz, 1H), 3.74 (d, J = 16.0 Hz, 1H), 3.64 (dd, 3JPH = 10.6 Hz, 3H, OCH3), 3.60 (d, 3JPH = 10.3 Hz, 3H, OCH3), 2.89 (t, JPH = 16.1 Hz, 1H), 2.55 (d, J = 16.0 Hz, 1H), 1.62 (s, 3H, CH3), 1.28 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3): δ = 134.7, 133.9 (d, JPC = 3.6 Hz), 131.3, 129.3 (d, JPC = 5.1 Hz), 128.4 (d, JPC = 2.9 Hz), 123.7 (d, JPC = 1.5 Hz), 120.9 (d, JPC = 10.9 Hz), 64.6 (d, JPC = 10.9 Hz), 54.2 (d, JPC = 6.6 Hz), 54.0 (d, JPC = 7.3 Hz), 32.7, 18.7, 13.7.

Dimethyl (2-(4-Methoxyphenyl)-4,5-dimethyl-3,6-dihydro-2H-pyran-2-yl)phosphonate (3c) 116 mg (58 %) of 3c was obtained as slight yellow viscous liquid. EI-MS: m/z = 326.13. 1H NMR (400 MHz, CDCl3): δ = 7.35 (dd, 3J = 8.8 Hz, 4J = 2.3 Hz, 2H), 6.84 (d, 3J = 9.0 Hz, 2H), 3.89 (d, J = 15.6 Hz, 1H), 3.74 (s, 3H, OCH3), 3.73 (d, J = 16.0 Hz, 1H), 3.61 (d, 3JPH = 10.3 Hz, 3H, OCH3), 3.57 (d, 3JPH = 10.3 Hz, 3H, OCH3), 2.88 (t, JPH = 15.0 Hz, 1H), 2.57 (d, J = 16.8 Hz, 1H), 1.63 (s, 3H, CH3), 1.29 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3): δ = 159.3, 149.6, 134.5, 129.5, 121.2 (d, JPC = 10.9 Hz), 123.7, 120.9 (d, JPC = 10.9 Hz), 113.7 (d, JPC = 2.9 Hz), 64.4 (d, JPC = 10.9 Hz), 55.2, 54.1 (d, JPC = 7.3 Hz), 53.8 (d, JPC = 7.3 Hz), 32.6, 18.6, 13.8.

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ACCEPTED MANUSCRIPT Dimethyl (2-(4-Fluorophenyl)-4,5-dimethyl-3,6-dihydro-2H-pyran-2-yl)phosphonate (3d) 124 mg (61 %) of 3d was obtained as slight yellow viscous liquid. EI-MS: m/z = 314.11; 1H NMR (400 MHz, CDCl3): δ = 7.4 (m, 2H), 6.98 (m, 2H), 3.92 (d, J = 15.6 Hz, 1H), 3.73 (d, J = 16.0 Hz, 1H), 3.63 (d, 3JPH = 10.2 Hz, 3H, OCH3), 3.59 (d, 3JPH = 10.2 Hz, 3H, OCH3), 2.90 (t, JPH = 15.6 Hz, 1H), 2.57 (d, J = 16.0 Hz, 1H), 1.63 (s, 3H CH3), 1.29 (s, 3H CH3).

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C NMR


(100 MHz, CDCl3): δ = 163.7 (d, JPC = 3.7 Hz), 161.2 (d, JPC = 3.7 Hz), 132.4 (d, JPC = 5.0 Hz), 129.7 (d, JCF = 4.4 Hz, 13C, 19F), 123.7 (d, JPC = 2.2 Hz), 120.9 (d, JPC = 10.9 Hz), 113.7 (d, JFC = 2.9 Hz), 64.6 (d, JPC = 13.4 Hz), 54.1 (d, JPC = 7.3 Hz), 53.9 (d, JPC = 7.3 Hz), 32.8, 18.6, 13.7. Dimethyl (4,5-Dimethyl-2-(4-(trifluoromethyl)phenyl)-3,6-dihydro-2H-pyran-2-yl)phosphonate (3e) 163 mg (84 %) of 3e was obtained as slight yellow viscous liquid. EI-MS: m/z = 364.11; 1H NMR (400 MHz, CDCl3): δ = 8.21 (d, J = 8.0 Hz, 2H), 7.73 (d, J = 8.3 Hz, 2H), 4.05 (d, J = 15.8 Hz, 1H), 3.84 (d, J = 16.0 Hz, 1H), 3.75 (d, 3JPH = 10.3 Hz, 3H, OCH3), 3.69 (d, JPH = 10.3 Hz, 3H, OCH3), 3.01 (t, JPH = 13.8 Hz, 1H), 2.69 (d, J = 17.0 Hz, 1H), 1.71 (s, 3H, CH3), 1.37 (s, 3H, CH3). 13C NMR (100 MHz, CDCl3): δ = 142.0, 132.8, 132.7 (d, JPC = 9.5 Hz), 129.7 (d, J = 4.4 Hz), 123.7 (d, JPC = .2 Hz), 120.9 (d, JPC = 10.9 Hz), 115.7 (d, JPC = 22.0 Hz), 115.5, 68.6, 63.6 (d, JPC = 13.0 Hz), 53.3 (d, JPC = 7.6 Hz), 53.0 (d, JPC = 7.5 Hz), 31.7, 19.3, 13.9,

Wittig-Type Reaction Dimethyl (1-Phenylvinyl)phosphonate (4) To an oven dried Schlenk tube equipped with a dried magnet stick, filled with argon and sealed with a rubber septum, 1 mL of dried dichloromethane was added with a syringe. 150 mg (0.7

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ACCEPTED MANUSCRIPT mmol) of phosphonate 3 and 87 L (1.1 equiv.) of butadiene 2 was added to the reaction tube and the mixture was stirred for 5 minutes at room temperature. 111 mg (1.2 equiv.) of AlCl3 was added while passing argon. The reaction tube was sealed again with a rubber septum while flushing argon. The reaction mixture was stirred for one hour and then was poured in 20 mL of 0.5 M HCl solution and stirred vigorously. The crude product was extracted with 3 × 10 mL of


diethyl ether. The organic layer was washed with 0.1 M NaOH solution. The organic layer was dried with Na2SO4 and the solvent was evaporated by rotary evaporator. The product was purified by column chromatography using 5 % ethyl acetate in hexane as eluent. 97 mg (65 %) of 4 was obtained as colorless liquid. EI-MS: m/z = 212.13. 1H NMR (400 MHz, CDCl3): δ = 7.47 (dd, 3J = 8.5 Hz, 4J = 1.5 Hz, 2H) 7.34 (dd, 3J = 8.5 Hz, 4J = 1.5 Hz, 2H), 6.34 (dd, 3J = 22.1 Hz, J = 1.26 Hz, 1H, vinyl-H anti to P), 6.17 (dd, 3J = 45.7 Hz, J = 1.26 Hz, 1H, vinyl-H syn to P), 3.76 (d, 3JPH = 11.0 Hz, 6H, OCH3), 3.74 (s, 3H, OCH3).

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C NMR (400 MHz, CDCl3): δ =

138.6, 136.8, 135.0 (d, JPC = 46.7 Hz), 134.5 (d, JPC = 5.8 Hz), 132.5 (d, JPC = 32.1 Hz), 128.8, 128.7, 52.8 (d, JPC = 23.4 Hz, P-OCH3). ACKNOWLEDGEMENT We are thankful to the TUBITAK (The Scientific and Technological Research Council of Turkey) for financial support (Program# 2216) and Middle East Technical University, Ankara Turkey.

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2.

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10584-10587. 3.

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4.

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Demir, A. S.; Reis, O.; Esiringu, I.; Reis, B.; Baris, S. Tetrahedron 2007, 63, 160-165.

6.

Xin, L. H.; Johnson, J. S. Angew. Chem. Int. Ed. 2003, 42, 2534-2536.

7.

Polat-Cakir, S.; Demir, A. S. Tetrahedron 2011, 67, 2396-2401.

8.

Bhattacharya, A. K.; Thyagarajan, G. Chem. Rev. 1981, 81, 415-430.

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13.

Ning, Y.-C. In Interpretation of Organic Spectra; John Wiley & Sons (Asia) Pte Ltd, 2011; pp. 39-51.

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O R

O

P O O

O P

-78 oC to 0 oC

O

O O

2

1 Downloaded by [Mugla University] at 03:56 18 September 2014

R

DCM, AlCl3

+

3

Scheme 1 ODA reaction of aryl phosphonates 1 with 2,3-dimethyl-1,3-butadiene (2)

O O P O O 1

O

DCM, AlCl3

+

P O

rt, 1h

O O

2 3 H H

O

H H

P O O

O P

H O H

H 4

H H

O H

H H

H

Scheme 2 Wittig-type reaction of benzoyl phosphonate with 2,3-dimethyl-1,3-budadiene

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Table 1 Substituents R used in the reaction with diene 1 and yields of the products obtained R

Product (3)

3a

O P O O O

70

Cl

Cl Downloaded by [Mugla University] at 03:56 18 September 2014

Yield (%)

3b

O P O O O

73

O

O 3c

O P O O O

58

F

F

3d

O P O O O

61

CF3

CF3 3e

O P O O O

13

84

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