Energy efficient, clean and solvent free photochemical ...

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ScienceDirect Solar Energy 113 (2015) 332–339 www.elsevier.com/locate/solener

Energy efficient, clean and solvent free photochemical benzylic bromination using NBS in concentrated solar radiation (CSR) Saurabh Deshpande a, Balu Gadilohar a, Yogesh Shinde b, Dipak Pinjari b, Aniruddha Pandit b, Ganapati Shankarling a,⇑ a

Department of Dyestuff Technology, Institute of Chemical Technology, Mumbai, India Chemical Engineering Department, Institute of Chemical Technology, Mumbai, India

b

Received 15 October 2014; received in revised form 3 December 2014; accepted 7 January 2015

Communicated by: Associate Editor Dr. Gion Calzaferri

Abstract An environmentally benign, clean, solvent free approach for the benzylic bromination have been developed using concentrated solar radiation (CSR). CSR methodology was used as an energy source to the reaction media. The protocol was found to be superior to the conventional photochemical and thermal methods in terms of reaction time and total energy requirement. This method was adapted with concentrated solar radiations in solvent free conditions without the use of radical initiators and has proved to provide good yields. Incorporation of renewable energy source, minimisation of raw materials makes the process not only green but energy efficient also. Ó 2015 Elsevier Ltd. All rights reserved.

Keywords: Concentrated solar radiation; Benzylic bromination; Photochemical reaction

1. Introduction In order to develop environmental benign synthetic pathways, it is necessary that any process in addition to being high yielding and simple, it should have a reduced number of steps, minimum energy consumption and should involve a renewable source of energy (Clark, 1999). In an attempt to have a greener process, much of the work has been done on the solvent free reaction conditions (SFRC) (Tanaka and Toda, 2000), to carry out reactions using greener solvents like water, diethyl carbonate, etc. As a matter of fact, reusable media such as ionic liquids (Sheldon, 2005; Scott et al., 2000) deep eutectic mixtures (DEM) which has less impact ⇑ Corresponding author. Tel.: +91 2233612708; fax: +91 22 33611020.

E-mail addresses: [email protected], gs.shankarling@ictmumbai. edu.in (G. Shankarling). http://dx.doi.org/10.1016/j.solener.2015.01.008 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.

or is benign to the environment (Paiva et al., 2014) has also been employed previously. In order to reduce the overall energy consumption requirement, many initiatives have been taken by employing microwave and UV irradiation (Alimenla et al., 2006; Pingali et al., 2011) or to utilise sonochemical methods (Adhikari and Samant, 2002). However, these sources consume some sort of energy and cannot be relied upon for a long period; hence there is always a need to find a reliable, green, renewable source of energy. In this regards solar energy is a promising tool as the green energy source and proves to be very beneficial. The utilisation of solar energy for carrying out organic reactions has been reported previously for various photochemical reactions such as cycloadditions, Diels–Alder reactions and Paterno–Buchi (Pohlmann et al., 1997). The solar energy as mentioned earlier has been used to carry out synthesis of 2-aminothiophenes, Juglone etc. (Mekheimer et al.,

S. Deshpande et al. / Solar Energy 113 (2015) 332–339

2008; Oelgemo¨ller et al., 2006; Protti and Fagnoni, 2009). Metal oxide nanoparticles prepared by such methods as a catalyst have been found to be more efficient than those prepared by conventional methods (Patil et al., 2012). Similarly many reactions have been studied to date on different reactions whereas in this paper we tried to explore one of the versatile application of solar energy on well-known Wohl– Ziegler reaction. This is a free radical benzylic bromination and is traditionally performed with N-bromosuccinimide (NBS) in refluxing carbon tetrachloride in the presence of a radical initiator (Smith and March, 2006). Vogtle and co-workers showed that NBS reactions are solvent dependent and has been reported to exhibit good selectivity in solvents which have low refractive index (Offermann and Vo¨gtle, 1980). Aromatic nuclear bromination occurs mostly when Lewis acids are employed in a stoichiometric amount or in polar solvents with high dielectric constants (Schmid and Karrer, 1946; Pravst et al., 2006). In an attempt to use greener solvents, Water was described as an excellent medium for free radical reaction (Chhattise et al., 2008), due to its remarkable non-reactivity towards radicals (OH bond resistance to homolytic breaking). Again, it has been reported that visible-light-promotes solvent-free N-bromosuccinimide (NBS) benzyl bromination (Jereb et al., 2009). However, these methods are borne with some limitations, as they require longer time, low to moderate yields and relatively harsh condition. During the last decade, various energy sources like light, microwaves and ultrasound have been employed with an attempt to avoid organic solvents by performing the bromination under solvent-free conditions, in ionic liquids or in water (Togo and Hirai, 2003; Podgorsˇek et al., 2006). So far, research has been focused on the utilisation of more benign solvents (e.g., ionic liquids, ethyl or methyl acetate, and biphasic media) (Bedel et al., 2002) and different modes of activation (e.g., light, microwaves, use of zeolites, and grinding) (Esakkidurai et al., 2004; Rahman et al., 2005; Heropoulos et al., 2007) but very few report the utilisation of such bromination using solar energy as a source of light and heat. Hence, there is a need to design and develop protocols to carry out solvent free reactions requiring shorter reaction times and utilisation of renewable energy sources. Solar energy in this regard not only provides conservation of energy but also helps for the process to occur without solvent. Considering the scope of renewable energy sources in organic synthesis herein, we report solvent free benzylic bromination with good to excellent yield and in shorter reaction time under concentrated solar radiation using Fresnel lens.

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layered chromatography (TLC) using 0.25 mm E-Merck silica gel 60 F254 precoated plates, which were used for detection and visualisation of organic compounds using UV light. All products were confirmed by GC–MS and GC analysis (Shimadzu GC–MS QP2010 and Thermo-scientific respectively). All compounds have been characterised by Mass spectrometry and is confirmed by physical constants. 2.2. Experimental set-up From a point on the Earth’s surface, the Sun appears to move in the sky in a tight helix whose axis is oriented in the North–South direction. The axis of the said helical path makes an angle with the Earth based observer’s horizontal axis and this angle is equal to the latitude of the point at which the observer is placed. Hence, at the equator, the Sun appear to move in a helix whose axis appears to be exactly aligned with the North–South direction. At a latitude of h degrees, the axis of the helix appears to make an angle of h degrees with the horizontal. If the observer is placed in the Northern Hemisphere, the axis of the helix will appear to be pointed into the ground southwards: so that the Sun appears to travel “below” the southern horizon in the winter and travel back “above” the southern horizon in the summer. The angle formed by the direction of the Sun (wrt the observer) during the winter solstice with that during the summer solstice is twice the tilt of the Earth’s axis i.e. 2 23.5° = 47° Hence the angular advancement of the helix is less than 0.13° in a day or less than 0.032° over a 6 h period.

2. Experimental 2.1. Materials and methods All reagents and solvents were used directly as procured by the SD Fine Chemical Ltd. (Mumbai, India) without further purification. The reactions were monitored by thin

Fig. 1. Experimental setup of benzylic bromination using Fresnel lens (schematic representation).

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The tightness of this helix can be exploited to devise tracking systems that need to be rotated along a single axis during the course of a day. This “polar axis tracking” mechanism is well known to astronomers who have long used it to circumvent the inconvenience of fiddling with two dials to keep track of a star. The device (Fig. 1) can be used to hold the focus of a Fresnel lens at a fixed point while rotating the lens about a single axis to track the sun. The device is composed of a frame for the Fresnel lens that that is capable of rotating about two perpendicular axes, both of which pass through its focal point. The whole assembly is arranged so that one of the axes is directed parallel to the axis of the Solar Helix (mentioned above). We will call this the helical axis. This is the axis about which the rotation occurs. The angular position of the lens about the other axis (which we will call the travel axis) is fixed at the beginning of each day and not changed thereafter. The Sun can now be tracked with 0.04° accuracy by adjusting the angular position about the helical axis. The device is fabricated in the following manner: The Fresnel lens is mounted on a frame which is held along the sides rigidly by two arms which are themselves fixed to pivots on another frame. The length of the arms is such that the perpendicular distance between the lens and the pivots is equal the focal length of the lens: the line joining these pivots is the travel axis. The second frame is pivoted about an axis perpendicular to the travel axis and this is the helical axis. The point of intersection of both axes is the stationary focus of the system. To align the helical axis along the Solar Helix, the whole assembly is mounted on a base which is inclined at the latitude angle h wrt the horizontal and positioned so that the face of the Fresnel lens is southwards (for Northern Hemisphere mounting). At the beginning of each day, the lens is adjusted about the travel axis and then the sun is tracked by changing its angular displacement about the helical axis. The point of intersection of the two axes is stationary and if properly tracked, the solar focal spot will coincide with it. This position can then be used for photochemistry or any other applications. This assembly was used at Matunga, Mumbai, India (19°010 1800 N 72°510 5300 E/19.021632°N), the peak value of the direct normal irradiation is 950 W/m2 as recorded by Pyranometer (dynalab Tech. Ltd.). This lens concentrates solar energy by refraction that is achieved within a volume of 30 cm (x-axis) 30 cm (y-axis). The experiments were carried out under solar illumination on sunny days of May 2013, between 10 AM and 1 PM. 2.3. Experimental procedure All the experiments were conducted at Matunga, Mumbai, India (19°010 1800 N 72°510 5300 E/19.021632°N). In a typical experimental procedure, toluene (1.0 mmol) and NBS (1.1 mmol) were charged into round bottom flask and was placed on the stirrer with such an orientation that focal

point of the lens directly gets focused over round bottom flask. The reaction mixture was monitored by TLC. The mixture was added with water which made ensured that N-succinimide gets dissolved and the powder obtained could be filtered dried, and recrystallized (in case of solids), was extracted with ethyl acetate, dried over Na2SO4 which was then evaporated in vacuum. The method showed good to excellent yields ranging from 75% to 90%. The obtained crude product was purified through recrystallization or column chromatography with Hexane: ethyl acetate as eluting solvent. 3. Results and discussions The solar radiations contains a wide spectrum ranging from ultraviolet (UV – 280 nm) to near Infra-red (4000 nm) which means that it can provide both photochemical energy (from UV) and thermal (by infra-red) energy. As a matter of fact, we decided to carry out benzylic bromination of toluene (Scheme 1) and observed that reactions would proceed without the use of solvents minimising the tedious work up procedure. The tropical regions receive sunrays of maximum intensity during the month of May; hence these experiments were carried out during the month of May for maximum utilisation of solar energy. The global solar intensity during any typical day in the month of May is shown in Fig. S1 (see Suppo. Info) which indicates that maximum intensity is observed during 11:00–13:00 ranging from 900 to 939 W/m2. It was thus decided to carry out all the experiments during 11:00–13:00. It is quite apparent from the Fig. 2 that the solar intensity during the daytime course is highest between 11:30 h and 13:00 as the values are above the mean values, considering 907 as the mean value with the standard deviation of ±50 W/m2. Previous literature, which is tabulated in Table 1, involves solvents and the environmentally harmful radiation source which in addition, also requires longer time to complete the reaction (Table 1, entry 1–4). On the other hand CSR method involves green and eco-friendly radiation source, no solvent and also requires less time with high yields (Table 1, entry 5). In the first phase, the reaction conditions were studied using 4-nitrotoluenes in different solvents and the reaction yields and time was being monitored (Table 2, Fig. 2). The error bars related to the percentage deviation of the yields of the reaction has been included in the Fig. 2 with ±5% deviation from the normal yields. It was observed during the optimisation of conditions, that reaction proceeds faster with good yields when neat conditions were employed (Table 2, entry 7). Also reaction was carried out in CCl4 as solvent under CSR, this reaction showed 70% yield in 5 min (Table 2, entry 1). Entry 2 shows the use of EDC as solvent, these reaction exhibited efficient conversion but provides only 81% yields in 6 min. As we move on to polar solvents it shows increase in yield (Table 2, entry 2–6).


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Scheme 1. Benzylic bromination using NBS in CSR.

100

Yield (%)

80

60

40

20

0

Conversion

Solvent

Yield

Fig. 2. Solvent optimisation study for the bromination of 4-Nitro toluene using NBS (error bars represent the percent deviation (±5%) of the yield of reaction).

Literature shows that in a microwave assisted bromination of toluene, reaction proceeds selectively to give side chain brominated product when carried out neatly in shorter reaction time. Fig. 2 shows error bars for the percent deviation of the yield of reaction for solvent optimisation study for the bromination of 4-Nitro toluene using NBS. Furthermore, neat reactions with NBS favours benzylic bromination than ring substitution. This reports prompted us to go for neat conditions. In benzylic bromination reaction, concentrated solar radiations melt the substrate so that solvent is not required, which is a green method. Water has been described as an excellent medium for free radical reaction (Shaw et al., 1997), due to its remarkable non-reactivity towards radicals (OH bond resistance to homolytic breaking). As per the reports, the results obtained in our case were in accordance and was found to give good yield of benzylic bromination. However, when the reaction mass was irradiated for a longer time, it was observed that it resulted into side products majorly being benzyl alcohol and the yield obtained was not very good

Table 1 A comparative overview of previous work for the benzylic bromination using NBS with current work. Entry No.

Source

Solvent

Time (min)

Yield (%)

Refs.

1 2 3 4 5

Visible light (hv) UV light (hv) UV light (hv) Microwave (lv) Concentrated solar radiation (hv)

H2O EtOAc ACN Neat Neat

1320 960 30 20 5

89 52 95 62 90

Podgorsˇek et al. (2006b) Amijs et al. (2003) Chhattise et al. (2008) Heropoulos et al. (2007) This work

Table 2 Solvent optimisation study for the bromination of 4-Nitro toluene using NBS. Sr. No

Solvent

Time (mins)

Conversion (% by GC)

Yield (%)a

Error bars

1. 2. 3. 4. 5. 6. 7.

CCl4 EDC EtOAc ACN:H2O (1:1) Acetonitrile H2O Neat

5 6 20 8 10 9 5

92 94 90 98 96 100 100

82 81 80 82 86 89 90

– – – – – – ±5%

Reaction conditions: 1.0 mmol toluene and 1.1 mmol NBS and 2 volumes of solvent. a Isolated yields.

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Table 3 Study of concentrated sunlight beam on different substrates with NBS. Time (mins)

Yield (%)a

M.P/B.P (°C)

1

5

90

98

2

10

89

44

3

12

87

54

4

10

87

201

5

20

81

45

6

5

56b, 38c, 6.29d

41b, 64c

7

20

88

155

8

19

85

36

9

10

63e, 31f

21e, 90f

Sr. No.

Reactants

Products

b

c

,

d

e

,

f

(continued on next page)

Line missing


337

Table 3 (continued) Time (mins)

Yield (%)a

M.P/B.P (°C)

10

7

75

80

11

30

NR

–

Sr. No.

Reactants

Products

Reaction conditions: reactant (1.0 eq), NBS (1.1 eq) and irradiated with CSR. a Isolated yields. b,e Mono brominated product. c,f Dibrominated product. d Ring brominated product as per the GC–MS, NR-No reaction.

Table 4 Energy calculations study for different methods. Entry No.

Energy source

Time (mins)

Yield (%)

Energy utilised (kJ)

1. 2.

Conventional methoda Concentrated solar beamb

300 5

89 90

153.18 25.65

a b

AIBN and CCl4 added. Neat, without AIBN (all the energy calculations has been carried out neglecting the energy losses during the course.).

i.e. around 48%. The results for the other types of substrates have been reported in the following Table 3. As depicted in Table 3, the yields of the compounds are good. When 4 was used as a substrate, the yield of benzyl bromide was found to be 81%. With 2 and 3, the reaction proceeds rapidly and with good yields of 89% and 87% respectively which can be understood due to the presence of the strong electron withdrawing group on the ring. When solid such as 7 were considered for the reaction, it gave side chain brominated product with the yield of 88%. It was observed that with 5 side chain bromination results to provide the benzyl derivative with 81% yield. Now, 6 has three active methyl groups which can undergo benzylic bromination, but as reports suggests that the mono bromo product is obtained in high yields (Heropoulos et al., 2007), 56% of monobromo was obtained as a major product apart from the 38% of dibromo product. Again, it has been reported that heating 6 with NBS in ultrasound or microwave irradiation leads to a ring brominated product which in our case was observed to be 6.29%. The lower yield of ring brominated product may be due to the absence of solvent media in the reaction (Heropoulos et al., 2007). Similarly, it was observed in the case of 9, mono brominated product was observed to be 63% while di bromo product was found to be 31%. Again, it was observed that substrate 10, gave mono brominated compound as a major product. It was really interesting to notice that when substrate 8 was used, only benzyl bromination occurs with no a-brominated

product with yield being of about 85%. In order to clear the ambiguity we carried out the reaction using 11 as the substrate, but reaction did not proceed at all. This indicated that no a-bromination occurs under Concentrated Solar (CS) conditions. The mechanism or the main reason behind benzylic bromination can be predicted as radical formation because of UV or visible light radiations (Dinda et al., 2013) and the heating of the medium can be due to as a result of infrared radiations. Though microwave radiations are known to cause a reaction neatly in the similar manner without the use of radical initiator (Heropoulos et al., 2007), solar spectrum do not consists of a microwave as such, and hence the mechanism must be due to the combined effect of UV, Visible and Infrared radiations.

3.1. Efficacy of energy utilisation Appendix A shows the comparison of energy based on the two types of synthesis method; conventional and CSR methods used for bromination. The energy utilised for the reaction to complete has been calculated and it can be clearly seen (Table 4) that large amount of energy (153.18 kJ) is required for conventional method whereas same reaction can be carried out with minimum energy consumption (25.65 kJ) with the help of solar radiations. Again, solar energy being the green energy source not only

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serves the purpose by saving this much energy but also causes the reaction to occur completely within short reaction time.

Net energy delivered during conventional method = Power input in magnetic stirrer Time required for completion of reaction ¼ 8:51ðJ=sÞ 18000 s

4. Conclusions A new energy efficient method of benzylic bromination was developed using concentrated solar radiation (CSR) beam for the bromination without the use of radical initiator. Furthermore, a green approach was also employed in the process as solvent usage was minimised and renewable energy source was brought into the use. A further improvement in the process would open up the new pathway for incorporation of solar energy in the organic reactions thereby decreasing the energy load. Finally, CSR methodology is simple, viable, eco-friendly, environmentally benign and promises to scale it up at larger scale very soon. Acknowledgement Authors are thankful to UGC, New Delhi for providing the financial assistance for the research work. Appendix A A.1. Energy calculations 1. Energy calculated by concentrated solar radiation method Energy delivered during CSR method = Energy required to synthesize product. Solar energy during irradiation will be = Cross sectional area of the lens Solar Intensity. ¼ 9 102 m2 950 W=m2 ¼ 85:5 W Maximum time required for the reaction is 5 mins. Hence the probable energy consumed would be equal to = 85.5 5 60 ¼ 25:65ðkJÞ 2. Energy delivered during conventional method. Voltage input in magnetic stirrer (Model RQ1210, Deepali United Mfg. Ltd, India) = 230 V. Current measured using digital multimeter (KUSAMMECO Model 2718, Kusam Electrical Industries Ltd., Mumbai, India) = 37 mA = 37 103 A. Power input in magnetic stirrer = Voltage input Current measured ¼ 230ðVÞ 37 103 ðAÞ ¼ 8:51 WðJ=sÞ Time required for completion of reaction = 5 h (18,000 s)

¼ 153:18 kJ

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