International Scholarly Research Notices

International Scholarly Research Notices / 2014 / Article

Research Article | Open Access

Volume 2014 |Article ID 751298 | 11 pages | https://doi.org/10.1155/2014/751298

Substrate Directed Regioselective Monobromination of Aralkyl Ketones Using N-Bromosuccinimide Catalysed by Active Aluminium Oxide: α-Bromination versus Ring Bromination

Academic Editor: N. Zanatta
Received23 Nov 2013
Accepted24 Dec 2013
Published04 Mar 2014

Abstract

Bromination of aralkyl ketones using N-bromosuccinimide in presence of active Al2O3 provided either α-monobrominated products in methanol at reflux or mononuclear brominated products in acetonitrile at reflux temperature with excellent isolated yields depending on the nature of substrate employed. The α-bromination was an exclusive process when aralkyl ketones containing moderate activating/deactivating groups were subjected to bromination under acidic Al2O3 conditions in methanol at reflux while nuclear functionalization was predominant when aralkyl ketones containing high activating groups were utilized for bromination in presence of neutral Al2O3 conditions in acetonitrile at reflux temperature. In addition, easy isolation of products, use of inexpensive catalyst, short reaction time (10–20 min), and safe operational practice are the major benefits in the present protocol.

1. Introduction

Nowadays, researchers are focusing on the development of more acceptable bromination protocols to accomplish increasing demands for “organohalogen” chemistry and to achieve higher efficiency and selectivity of the bromination reactions which include α-bromination and nuclear bromination. The resulting α-brominated or nuclear brominated products acquired wide range of utility in organic synthesis [13]. Nuclear brominated ketones are found to be useful intermediates in C–C coupling reactions, as precursors to organometallic species and in nucleophilic substitutions.

It is well known that use of molecular bromine [4] as a basic electrophilic brominating reagent has several drawbacks. Alternative reagents were reported in the literature, for example, cupric bromide [5], dioxane dibromide [6], tetrabutyl ammonium tribromide [7], H2O2-HBr [8], bromodimethyl sulfoniumbromide [9], ethylene bis(N-methyl imidazolium) ditribromide [10], trihaloisocyanuric acids [11], pyridinium bromochromate [12], and NH4Br-oxone [13]. In addition, a popular and superior brominating agent such as N-bromosuccinimide [14] was utilized for α-bromination of carbonyl compounds using a radical initiator such as azobisisobutyronitrile (AIBN) or dibenzoyl peroxide (BPO) [15] and, later, it has been demonstrated that the reactivity of NBS could be modulated with ionic liquids [16], photochemical energy [17]; sonochemical energy [18], solvent free reaction conditions (SFRC) [19] and various catalysts such as Mg(ClO4)2 [20], NH4OAc [21], amberlyst-15 [22], silica supported NaHCO3 [23], sulfonic acid functionalized silica [24], FeCl3 [25], montmorillonite-K10 [26], and Silica gel [27]. NBS also is utilized for ring or nuclear bromination using various catalysts such as H2SO4-CF3CO2H [28], p-toluenesulfonic acid [29], dibromodimethylhydantoin in aqueous base [30], amberlyst [31], and HZSM-5 [32].

It is well recognized that solids play a significant role in the development of cleaner technologies through their abilities to act as catalysts, support reagents, entrain by-products, and influence product selectivity, and several books on the applications of solids in organic synthesis have appeared [3335]. However, the catalysts and any accompanying reagent used along with N-bromosuccinimide should be easily available to develop simple and efficient bromination procedure. Alumina (Al2O3) has been used as a catalyst [36] in a wide variety of industrial processes for many years [37]. But Alumina (Al2O3) has limited catalytic applications in synthetic organic chemistry [38]. However, on deep investigation it is found that aralkyl ketones with moderate activating/moderate or high deactivating groups undergo exclusively α-bromination in the presence of acidic alumina in methanol, while substrates with high activating groups undergo nuclear bromination predominantly in the presence of neutral alumina in acetonitrile at reflux temperature, as shown in Scheme 1

751298.sch.001

Plausible mechanism for bromination of aralkyl ketones containing moderate activating/deactivating groups may be involved in the exclusive formation of enolic form of ketone in presence of acidic Al2O3 aids predominant formation of α-brominated product (2). In addition, acidic Al2O3 may enhance the rate of release of bromonium ion from NBS and subsequent capture of bromonium ion by nucleophilic solvent such as methanol leads to rapid completion of the reaction (10–15 min) as shown in Scheme 2.

751298.sch.002

In contrast, aralkylketones containing high electron donating groups may be susceptible to rapid activation of the aromatic ring of substrate and rapid release of bromonium ion from NBS via surface interaction with neutral Al2O3 which leads to exclusive formation of nuclear brominated product (4) in acetonitrile based on substrate employed and the catalyst may assist in abstracting the proton during the course of bromination as shown in Scheme 3.

751298.sch.003

2. Results and Discussion

Initially, two types of substrates were selected for optimization of reaction conditions using N-bromosuccinimide in presence of either acidic or neutral Al2O3. Accordingly, acetophenone and 4′-hydroxy acetophenone were utilized for the evaluation of reaction conditions such as effect of solvent, temperature, catalyst, and catalyst load and the obtained results are discussed below.

The effect of catalyst on the course of bromination of acetophenone was studied and the obtained results were summarized in Table 1. It was observed that acidic Al2O3 (entry 3, Table 1) provided better yields of desired α-brominated product (2a) compared to neutral Al2O3 (entry 5) in methanol at reflux temperature. It might be attributed that acidic nature of Al2O3 may enhance the formation of enol form of substrate and subsequent bromination also. In addition, acidic nature of catalyst might boost the rate of release of bromonium ion from N-bromosuccinimide followed by the capture of bromonium ion by nucleophilic solvent such as methanol. Reuse of acidic Al2O3 provided 89%, 86%, 81%, and 72% yields of product (2a) for first, second, third, and fourth time, respectively.


751298.table.001

EntryCatalystTemp/°CTime/hProductYieldb/%Selectivity/%
2a : 3a : 

1
1

32 24
0

26524
3Acidic Al2O3321.32a6161 : 18 : 21
4Acidic Al2O3650.12a8989 : 08 : 03
5Neutral Al2O3322.02a4848 : 19 : 33
6Neutral Al2O3650.122a6666 : 12 : 22

Reaction conditions: acetophenone 1a (10 mmol), N-bromosuccinimide (12 mmol), 10% (w/w) catalyst, and methanol (20 vol) at reflux temperature.
Isolated yield of product 2a.
Unreacted acetophenone.

We also studied the effect of acidic Al2O3 catalyst load on the course of α-bromination. Towards this direction, 5%, 10%, and 15% of catalyst (w/w) was applied and the obtained yields of product 2a were 71%, 89%, and 78%, respectively. The study revealed that 10% (w/w) of acidic Al2O3 is optimum for better isolated yield of product 2a.

Later, we focused on the optimization of other reaction parameters such as effect of solvent in presence of 10% of acidic Al2O3. Consequently, the effect of solvents on the course of α-bromination was studied using different types of solvents and the obtained results were presented in Table 2. For example MeOH, EtOH, H2O, CH3CN, THF, CH2Cl2, are CHCl3 were provided 89%, 74%, 15%, 51%, 56%, 44%, 55%, and 48% yields of desired product 2a, respectively. Interestingly, a high yield (89%) of product 2a was obtained when methanol was used as solvent (entry 1, Table 1).


EntrySolventTime/minProductYieldb/%

1MeOH102a89
2EtOH402a74
3H2O24 hrs2a15
4CH3CN602a51
c5 dEther302a56
6 dTHF652a44
7CH2Cl21802a55
8CHCl31202a48

Reaction conditions: acetophenone 1a (10 mmol), N-bromosuccinimide (12 mmol), 10% (w/w) acidic Al2O3, and solvent (20 vol) at reflux temperature.
bIsolated yield.
cReaction was conducted at 32°C.
dFreshly distilled ether and THF were used (peroxide free).

As evident from the literature [26], portion wise addition of N-bromosuccinimide caused to control the release of bromonium ion and it provided improved yields of α-brominated product (2a). The same was implemented in the present investigation and N-bromosuccinimide was added portion wise (10 portions). As a consequence, the isolated yield of product 2a was improved up to 89% compared to one time NBS addition, which provided lower yield (74%) of product 2a.

With the help of optimized reaction conditions, further scope and generality of the α-bromination was tested by utilizing a series of aralkyl ketones containing moderate activating/deactivating groups and highly deactivating groups in presence of acidic Al2O3 in methanol and the obtained results were presented in Table 3. It was found that presence of moderate activating and deactivating groups on phenyl ring of aralkylketones favors the α-bromination (entries 1–10 and 11, Table 3 and entries 1 and 2, Table 4) with excellent isolated yields of desired product(s), whilst presence of highly deactivating groups provided lower yields of product (entries 12 and 13, Table 3). Fascinatingly, α-brominated product (2k) was formed exclusively even though 4′-methoxyacetophenone contains high activating group (-OMe) (entry 11, Table 3).


751298.table.003

EntrySubstrate (1)Product (2) Time/min Yieldb/% Selectivity/%
2 : 3 : 
Lit. Ref.
R′R

1H  
1a
H 2a108989 : 09 : 02[26]
2H  Me
1b
2b107474 : 22 : 04[26]
32′-Me  
1c
H2c128686 : 08 : 06[26]
43′-Me  
1d
H2d118888 : 05 : 07[26]
54′-Me  
1e
H2e109090 : 06 : 04[26]
64′-Et  
1f
H2f128585 : 09 : 06[26]
74′-CN  
1g
H2g97373 : 11 : 16[39]
83′-Cl  
1h
H2h77676 : 10 : 14[26]
94′-Cl  
1i
H2i99292 : 05 : 03[26]
104′-Br  
1j
H2j98787 : 08 : 05[26]
114′-MeO
1k
H2k67272 : 19 : 09[26]
d123′-NO2
1l
H2l114141 : 19 : 40[26]
d134′-NO2
1m
H2m144949 : 16 : 35[26]

Reaction conditions: 1 (10 mmol), N-bromosuccinimide (12 mmol), and 10% (w/w) acidic Al2O3 in methanol (20 mL) at reflux temperature.
Isolated yield.
Unreacted substrate.
5% acidic Al2O3 was applied.

751298.table.004a

EntrySubstrate (1)Product (2)Time (min)Yieldb/%Selectivity/%
2 : 3 : 
Lit. Ref.

1751298.table.004b751298.table.004c148484 : 11 : 05[26]
2751298.table.004d751298.table.004e119191 : 06 : 03[26]

Reaction conditions: acenaphthone 1 (10 mmol), N-bromosuccinimide (12 mmol), and 10% acidic Al2O3 in methanol (20 mL) at reflux temperature.
Isolated yield of desired product(s).
Unreacted substrate.

Acenaphthones(s) were subjected to bromination using N-bromosuccinimide in presence of acidic Al2O3 in methanol at reflux temperature and they provided respective α-brominated products (2n and 2o, entries 1 and 2, Table 4) exclusively. The obtained results were presented in Table 4.

In continuation of our agenda to explore the effect of Alumina catalyst on different types of substrates, we focused on the effect of neutral alumina on the course of bromination. As a result, we selected 4′-hydroxy acetophenone (1p) as model substrate and the reaction conditions were optimized with respect to it. The obtained results were presented in Table 5.


751298.table.005

EntryCatalystTemp/°CTime/hProductYieldb/%Selectivity
4a : 5a :

1
1

3224
0

26524
3Acidic Al2O33224a4343 : 16 : 41
4Acidic Al2O3650.34a5858 : 22 : 20
5Neutral Al2O3321.54a6262 : 21 : 17
6Neutral Al2O3650.124a8686 : 12 : 02

Reaction conditions: 4′-hydroxy acetophenone 1p (10 mmol), N-bromosuccinimide (12 mmol), 10% (w/w) catalyst, and methanol (20 mL) at reflux temperature.
Isolated yield.
Unreacted 4′-hydroxy acetophenone (1p).

When we applied neutral Al2O3 (entry 6, Table 5) for bromination reaction of 4′-hydroxy acetophenone (1p), good yield of nuclear brominated product (4a) was obtained compared to acidic Al2O3 (entry 4, Table 5) in methanol at reflux temperature. It may be attributed that acidic Al2O3 might deactivate the electron donating efficiency of high activating groups. In contrast, neutral Al2O3 might facilitate the activation of the aromatic ring of aralkyl ketone as well as the rate of formation of bromonium ion from NBS via surface interaction with reactants.

To improve the yields of product 4a further, we have studied the effect of solvent on the course of bromination and the obtained results were depicted in Table 6. Solvents such as MeOH, EtOH, H2O, CH3CN, THF, CH2Cl2, and CHCl3 were provided 86%, 61%, 22%, 94%, 48%, 40%, 30%, and 34% yields of desired product 4a, respectively. The solvent study disclosed that acetonitrile is the best to obtain maximum yield (94%) of the product 4a (entry 4, Table 6) compared to methanol (86%) (entry 1, Table 6). Reuse of catalyst provided 94%, 89%, 80%, and 74% yields of product (4a) for first, second, third, and fourth time, respectively.


EntrySolventTime/minProductYieldb/%

1MeOH124a86
2EtOH604a61
3H2O24 hrs4a22
4CH3CN144a94
c5 dEther454a48
6 dTHF754a40
7CH2Cl21504a30
8CHCl3904a34

Reaction conditions: 4′-hydroxy acetophenone 1p (10 mmol), N-bromosuccinimide (12 mmol), 10% (w/w) neutral Al2O3, and solvent (20 vol) at reflux temperature.
Isolated yield.
Reaction was conducted at 32°C.
Freshly distilled ether and THF were used (peroxide free).

It was observed that portion wise (10 portions) addition of N-bromosuccinimide provided improved yield (94%) of product 4a compared to one-time addition of N-bromosuccinimide which provided lower yield (65%) of product 4a. The study revealed that the portion wise addition of NBS resulted in improved yields of product 4a.

With the help of optimized reaction conditions further generality of the nuclear bromination was tested with a variety of substrates (1pr) containing high activating groups in presence of neutral Al2O3 in acetonitrile at reflux temperature and the obtained results were summarized in Table 7. It was found that neutral Al2O3 provided good yields of nuclear monobrominated products (entries 1–5, Table 7).


751298.table.007

EntrySubstrate Product Time/min Yieldb/%
RR′′

1HOH  
1p
4a1294
2OH  
1q
H4b1168
3NH2
1r
H4c1872
4HNH2
1s
4d1394
5HOMe  
1k
4e1591
c6HOH  
1p
5a2099
c7NH2
1r
H 5b1998

Reaction conditions: aralkyl ketone(s) 1p–r (10 mmol), N-bromosuccinimide (12 mmol), and 10% (w/w) neutral Al2O3 in acetonitrile (20 mL) at reflux temperature.
bIsolated yield.
c22 mmol of NBS was utilized.

Excess use of N-bromosuccinimide (24 mmol) provided excellent yields of nuclear dibrominated products 5a and 5b (entries 6 and 7, Table 7). The 4′-methoxy acetophenone (1k) in acetonitrile solvent in presence of neutral Al2O3 provided exclusively nuclear brominated product 5e instead of 2k.

Encouraged by the above fruitful results, we attempted to prepare 1-(3,5-bis(benzyloxy) phenyl)-2-bromoethanone (2p), a key α-brominated intermediate product in the synthesis of terbutaline sulphate drug starting from the substrate, 3′,5′-dibenzyloxy acetophenone (1s).

Interestingly, we obtained 95–98% yields of mononuclear brominated product of 1-(3,5-bis(benzyloxy)-4-bromophenyl)ethanone (4f) exclusively both in presence of acidic Al2O3 in methanol and neutral Al2O3 in acetonitrile as depicted in Scheme 4.

751298.sch.004

3. Conclusion

In summation, we demonstrated that using Al2O3 as catalyst, monobromination of various aralkyl ketones was achieved in high yield with high substrate directed regioselectivity (α-bromination versus nuclear bromination) using NBS. The α-bromination was the exclusive process when aralkyl ketones containing moderate activating/deactivating groups were subjected to bromination under acidic Al2O3 conditions in methanol at reflux while nuclear functionalization was predominant when aralkyl ketones containing high activating groups were utilized for bromination in presence of neutral Al2O3 conditions in acetonitrile at reflux temperature. Thus, our new protocol offers safe operational procedure, short reaction time (10–20 min), and use of inexpensive and recyclable catalyst for 3 times without loss of activity.

4. Experimental

4.1. General

All chemicals used were reagent grade and were used as received without further purification. Ketones were purchased from Acros, Merck, and SD Fine Chemicals Ltd., Mumbai, India and Avra Laboratories Ltd., Hyderabad, India. N-bromosuccinimide was purchased from Merck. Methanol (99.0%) was purchased from SD Fine Chemicals Ltd. EtOAc was purchased from Merck. The double distilled millipore deionized water was used for work up. Melting points were determined in open capillaries on REMI melting point apparatus and were uncorrected. 1H NMR spectra were recorded on a Varian 400 MHz. Chemical shifts were expressed in parts per million (ppm). Splitting patterns describe apparent multiplicities and are designated as s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), or br (broad). Mass spectra (MS) are acquired on agilent, model-6410, Triple Quad LC MS. Thin-layer chromatography was performed on 0.25 mm Merck silica gel plates (60F-254) and visualized with UV light. Column chromatography was performed on silica gel (finer than 200 mesh, Merck).

4.2. Active Al2O3 Catalyst Specification(s)
4.2.1. Acidic Al2O3

Acidic Al2O3 is commercially available (Merck made) and has the following characteristics (i) White crystalline solid, (ii) pH value (10% aqueous suspension) is 4-5; (iii) active according to Brockmann for column chromatography.

4.2.2. Neutral Al2O3

Neutral Al2O3 is commercially available (Fisher made) and has the following characteristics (i) White crystalline solid, (ii) pH value (10% aqueous suspension) is 6.8–7.8; (iii) active according to Brockmann for column chromatography.

4.3. General Experimental Procedure for α-Bromination

In a 100 mL RB flask fitted with condenser, the aralkylketone 1a (10 mmol), 10% (w/w) active Al2O3 catalyst, and methanol (20 vol) were added. The temperature of the reaction mass was raised to reflux. Then, N-bromosuccinimide (12 mmol) was added portion wise (10 portions). After completion of the reaction, as monitored by TLC (mobile phase is a mixture of 5 mL n-hexane and 3 drops of EtOAc; Caution: there is need to run TLC for 3–5 times for clear distinction between mono versus dibrominated products), the reaction mixture was filtered to collect the catalyst and the solvent was removed under reduced pressure. Water (100 mL) was added to the reaction mass and the formed α-brominated product was extracted thrice with EtOAc (3 × 50 mL). Layers were separated and the organic layer was collected and then washed thrice with water (3 × 50 mL). The organic layer was collected and dried over anhydrous Na2SO4 and the solvent was removed using rotary evaporator. Pure α-brominated product (2a) was obtained from crude residue after purification by column chromatography over silica gel using a mixture of n-hexane and EtOAc (99 : 1 ratio).

4.4. General Experimental Procedure for Nuclear Bromination

In a 100 mL RB flask fitted with condenser, the aralkylketone 1p (10 mmol), 10% (w/w) neutral Al2O3 catalyst, and acetonitrile (20 vol) were added. The temperature of the reaction mass was raised to reflux. Then, N-bromosuccinimide (12 mmol) was added portion wise (10 portions). After completion of the reaction, as monitored by TLC (mobile phase is a mixture of 5 mL n-hexane and 3 drops of EtOAc; caution: there is need to run TLC for 3–5 times for clear distinction between mono- versus dibrominated products), the reaction mixture was filtered to collect the catalyst and the solvent was removed under vacuum. Water (100 mL) was added to the reaction mass and the formed nuclear brominated product was extracted thrice with EtOAc (3 × 50 mL). Layers were separated and the organic layer was collected and it was washed thrice with water (3 × 50 mL). The organic layer was collected and dried over anhydrous Na2SO4 and the solvent was removed using rotary evaporator. Pure nuclear brominated product (4a) was obtained from crude residue after purification by column chromatography over silica gel using a mixture of n-hexane and EtOAc (99 : 1 ratio).

4.5. Physical and Spectral Characterization Data
4.5.1. 2-Bromo-1-phenyl Ethanone (2a)

Off-white solid, yield: 89%; m.p. 48–50°C; FT-IR (KBr, cm−1): 3085.6, 2999.6, 1694.3, 1589.7, 1485.8, 1283.6, 1198.8, 811.8, 665.7, 548.1. 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.00–8.10 (2H, m, arom H), 7.43–7.78 (3H, m, arom H), 4.50 (2H, s, –CH2). MS (ESI) m/z 199.05 [+H, 79Br], 201.1 [+H+2, 81Br].

4.5.2. 2-Bromo-1-phenylpropan-1-one (2b)

Colorless liquid, yield: 74%; b.p. 247–251°C; 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.10 (2H, d, J = 7.2 Hz, arom H), 7.72–7.64 (1H, m, arom H), 7.50−7.60 (2H, m, arom H), 5.30 (1H, q, J = 6.8 Hz, alkyl H), 2.10 (3H, d, J = 7.2 Hz, –CH3); MS (ESI) m/z 213.03 [+H, 79Br], 215.2 [+H+2, 81Br].

4.5.3. 2-Bromo-1-o-tolylethanone (2c)

Colorless liquid, yield: 86%; b.p. 81–83°C; 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.10 (1H, d, arom H, J = 8.0 Hz), 7.70 (1H, t, J = 7.6 Hz, arom H), 7.40 (1H, t, J = 7.2 Hz, arom H), 7.10 (1H, d, J = 8.0 Hz, arom H), 4.50 (2H, s, –CH2), 2.50 (3H, s, –CH3); MS (ESI): m/z 212.96 [+H, 79Br], 214.97 [+H+2, 81Br].

4.5.4. 2-Bromo-1-m-tolylethanone (2d)

Colorless liquid, yield: 88%; b.p. 234°C; 1H-NMR (400 MHz, CDCl3, δ/ppm): 7.83 (1H, d, J = 8.0 Hz, arom H), 7.58 (1H, s, arom H), 7.35 (1H, d, J = 7.6 Hz, arom H), 7.10 (1H, t, J = 7.2 Hz, arom H), 4.60 (2H, s, –CH2), 2.48 (3H, s, –CH3); MS (ESI): m/z 213.15 [+H, 79Br], 215.1 [+H+2, 81Br].

4.5.5. 2-Bromo-1-p-tolylethanone (2e)

Off-white solid, yield: 90%, m.p. 51–53°C. 1H-NMR (400 MHz, CDCl3, δ/ppm): 7.65 (2H, d, J = 7.6 Hz, arom H), 7.00 (2H, d, J = 7.2 Hz, arom H), 4.70 (2H, s, –CH2), 2.54 (3H, s, –CH3); MS (ESI): m/z 212.89 [+H, 79Br], 214.81 [+H+2, 81Br].

4.5.6. 2-Bromo-1-(4-ethylphenyl)ethanone (2f)

Colorless liquid, yield: 85%; b.p. 288.5°C. 1H-NMR (400 MHz, CDCl3, δ/ppm): 7.45 (2H, d, J = 8.0 Hz, arom H), 7.20 (2H, d, J = 7.6 Hz, arom H), 4.60 (2H, s, –CH2), 2.64 (2H, q, J = 7.6 Hz, –CH2), 1.54 (3H, t, J = 6.8 Hz, –CH3); MS (ESI): m/z 226.95 [+H, 79Br], 229.08 [+H+2, 81Br].

4.5.7. 4-(2-Bromoacetyl)benzonitrile (2g)

Off-white solid, yield: 73%, m.p. 90–92°C. 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.24 (2H, d, J = 7.2 Hz, arom H), 7.91 (2H, d, J = 7.6 Hz, arom H), 4.32 (2H, s, –CH2); MS (ESI): m/z 224.15 [+H, 79Br], 226.01 [+H+2, 81Br].

4.5.8. 2-Bromo-1-(3-chlorophenyl)ethanone (2h)

Off-white solid, yield: 76%; m.p. 39–42°C. 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.35 (1H, s, arom H), 7.78–7.88 (2H, m, arom H), 7.40 (1H, t, J = 8.0 Hz, arom H), 4.68 (2H, s, –CH2); MS (ESI): m/z 233.12 [+H, 79Br 35Cl], 235.1 [+H+2, 79Br 37Cl or 81Br 35Cl], 237.20 [+H+4, 81Br, 37Cl].

4.5.9. 2-Bromo-1-(4-chlorophenyl)ethanone (2i)

Off-white solid, yield: 92%, m.p. 94–96°C. FT-IR (KBr, cm−1) 3000.9, 2947.9, 1689.8, 1625.5, 1384.8, 1267.5, 853.3, 679.8, 564.8. 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.12 (2H, d, J = 8.0 Hz, arom H), 7.72 (2H, d, J = 8.0 Hz, arom H), 4.52 (2H, s, –CH2); MS (ESI): m/z 233.01 [+H, 79Br 35Cl], 234.90 [+H+2, 79Br 37Cl or 81Br 35Cl], 237.00 [+H+4, 81Br, 37Cl].

4.5.10. 2-Bromo-1-(4-bromophenyl)ethanone (2j)

Off-white solid,yield: 87%; m.p. 108–110°C; 1H-NMR (400 MHz, CDCl3, δ/ppm): 7.90 (2H, d, J = 7.6 Hz, arom H), 7.64 (2H, d, J = 7.2 Hz, arom H), 4.48 (2H, s, –CH2); MS (ESI): m/z 278.1 [+H, 2 79Br], 279.88 [+H+2, 79Br 81Br], 282.02 [+H+4, 2 81Br].

4.5.11. 2-Bromo-1-(4-methoxyphenyl)ethanone (2k)

Off-white solid, yield: 72%; m.p. 70–72%; FT-IR (KBr, cm−1): 3098, 2938.2, 1688.8, 1600, 1509.1, 1326.2, 1263.7, 1021.4, 817.8, 687.3, 557.8; 1H-NMR (400 MHz, CDCl3, δ/ppm): 7.95 (2H, d, J = 7.4 Hz, arom H), 6.95 (2H, d, J = 7.4 Hz, arom H), 4.40 (2H, s, –CH2), 3.90 (3H, s, –CH3). MS (ESI) m/z 229.0 [+H, 79Br], 231.01 [+H+2, 81Br].

4.5.12. 2-Bromo-1-(3-nitrophenyl)ethanone (2l)

Yellowish solid, yield: 41%; m.p. 93–96°C; FT-IR (KBr, cm−1): 3049, 2947, 1669, 1605.5, 1489.4, 1159.5, 929.6, 853.1, 679.3, 564.5; 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.70 (1H, s, arom H), 8.35–8.55 (2H, m, arom H), 7.80–7.90 (1H, m, arom H), 4.40 (2H, s, –CH2); MS (ESI): m/z 244.05 [+H, 79Br], 246.12 [+H+2, 81Br].

4.5.13. 2-Bromo-1-(4-nitrophenyl)ethanone (2m)

Yellowish solid, yield: 49%; m.p. 99–102°C; FT-IR (KBr, cm−1) 3088, 2992, 1690, 1523, 1347, 1195, 813.2, 729, 618, 490; 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.21 (2H, d, J = 7.2 Hz, arom H), 8.61 (2H, d, J = 7.2 Hz, arom H), 4.41 (2H, s, –CH2); MS (ESI): m/z 244.1 [+H, 79Br], 245.97 [+H+2, 81Br].

4.5.14. 2-Bromo-1-(naphthalen-1-yl)ethanone (2n)

Brownish liquid, yield: 84%; b.p. 348.5°C; FT-IR (KBr, cm−1): 3049.9, 2948, 1689.8, 1625.4, 1469.3, 1174.6, 1126.9, 1029.5, 853.3, 811.6, 564.7; 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.75 (1H, d, J = 7.2 Hz, arom H), 7.95–8.19 (4H, m, arom H), 7.65–7.75 (2H, m, arom H), 4.70 (2H, s, –CH2); MS (ESI): m/z 249.0 [+H, 79Br], 251.1 [+H+2, 81Br].

4.5.15. 2-Bromo-1-(naphthalen-2-yl)ethanone (2o)

Pale yellowish solid, yield: 91%; m.p. 81–83°C; FT-IR (KBr, cm−1): 3088.2, 2997.4, 1702.4, 1610.2, 1434.7, 1199.8, 1082.9, 873.4, 818.2, 739.3, 618.8; 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.80 (1H, d, J = 7.6 Hz, arom H), 7.95–8.19 (4H, m, arom H), 7.60–7.78 (2H, m, arom H), 4.30 (2H, s, –CH2); MS (ESI): m/z 249.1 [+H, 79Br], 251.05 [+H+2, 81Br].

4.5.16.,2-Dibromo-1-phenylethanone (3a)

Colorless liquid, yield: 8–10%; 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.1–8.15 (2H, m, arom H), 7.51–7.81 (3H, m, arom H), 6.60 (1H, s, –CH); MS (ESI): m/z 277.03 [+H, 2 79Br], 278.920 [+H+2, 79Br, 81Br], 281.01 [+H+4, 81Br].

4.5.17. 1-(3-Bromo-4-hydroxyphenyl)ethanone (4a)

Off-white solid, yield: 94%; 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.18 (1H, s, arom H), 7.95 (1H, d, J = 7.2 Hz, arom H), 7.22 (1H, d, J = 7.2 Hz, arom H), 5.42 (1H, s, OH), 2.58 (3H, s, –CH3); MS (ESI): m/z 215.1 [+H, 79Br], 217.08 [+H+2, 81Br].

4.5.18. 1-(5-Bromo-2-hydroxyphenyl)ethanone (4b)

Off-white solid, yield: 68%; m.p. 43–45°C; 1H-NMR (400 MHz, CDCl3, δ/ppm): 12.10 (1H, s, –OH), 8.2 (1H, s, arom H), 8.02 (1H, d, J = 7.2 Hz, arom H), 7.22 (1H, d, J = 7.6 Hz, arom H), 2.62 (3H, s, –CH3); MS (ESI): m/z 215.21 [+H, 79Br], 216.94 [+H+2, 81Br]

4.5.19. 1-(2-Amino-5-bromophenyl)ethanone (4c)

Off-white solid, yield: 72%; m.p. 83–85°C; 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.03 (1H, s, arom H), 7.58 (1H, d, J = 6.8 Hz, arom H), 7.13 (1H, d, J = 7.2 Hz, arom H), 6.42 (2H, s, –NH2), 2.7 (3H, s, –CH3); MS (ESI): m/z 214.09 [+H, 79Br], 215.89 [+H+2, 81Br].

4.5.20. 1-(4-Amino-5-bromophenyl)ethanone (4d)

Off-white solid, yield: 94%; m.p. 155–157°C; 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.15 (1H, s, arom H); 7.82 (1H, d, J = 7.2 Hz, arom H), 6.98 (1H, d, J = 7.2 Hz, arom H), 6.23 (2H, bs, –NH2), 2.7 (3H, s, –CH3); MS (ESI): m/z 214.1 [+H, 79Br], 215.89 [+H+2, 81Br].

4.5.21. 1-(3-Bromo-4-methoxyphenyl)ethanone (4e)

Off-white solid, yield: 91%; m.p. 70–72°C; FT-IR (KBr, cm−1): 3098, 2938.2, 2840, 1688.8, 1600, 1509.1, 1326.2, 1263.7, 1207.5, 1168.3, 1021.4, 940.8, 817.8, 687.3, 580.1, 557.8; 1H-NMR (400 MHz, CDCl3, δ/ppm): 8.1 (1H, s, arom H), 7.83 (1H, d, J = 7.6 Hz, arom H), 6.95 (1H, d, J = 7.2 Hz, arom H), 4.01 (3H, s, –CH3), 2.71 (3H, s, –CH3); MS (ESI) m/z: 229.0 [+H, 79Br], 231.06 [+H+2, 81Br].

4.5.22. 1-(3,5-Bis(benzyloxy)-2-bromophenyl)ethanone (4f)

Off-white solid, yield: 98%; m.p. 83–85°C; 1H-NMR (400 MHz, CDCl3, δ/ppm): 7.45–7.25 (10H, m, arom H), 7.59 (1H, s, arom H), 7.68 (1H, s, arom H), 5.02 (2H, s, –CH2), 5.14 (2H, s, –CH2), 2.6 (3H, s, –CH3); MS (ESI): m/z 411.00 [+H, 79Br], 412.93 [+H+2, 81Br].

4.5.23. 1-(3,5-Dibromo-4-hydroxyphenyl)ethanone (5a)

Off-white solid, yield: 99%; m.p. 184–188°C; 1H-NMR (400 MHz, CDCl3, δ/ppm): 9.86 (1H, s, –OH), 8.10 (2H, s, arom H), 2.60 (3H, s, –CH3); MS (ESI): m/z 293.01 [+H, 2 79Br], 294.89 [+H+2, 79Br 81Br], 297.09 [+H +4, 2 81Br].

4.5.24. 1-(2-Amino-3,5-dibromophenyl)ethanone (5b)

Off-white solid, yield: 98%; m.p. 121–123°C; 1H-NMR (400 MHz, CDCl3, δ/ppm): 7.81 (1H, s, arom H), 7.69 (1H, s, arom H), 6.82 (2H, bs, –NH2), 2.59 (3H, s, –CH3); MS (ESI): m/z 293.01 [+H, 2 79Br], 294.89 [+H+2, 79Br 81Br], 297.1 [+H+4, 2 81Br].

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

This work has been supported financially by the Council of Scientific and Industrial Research (CSIR), New Delhi, Government of India through a major research Project (no. 01 (2391)/10/EMR-II).

Supplementary Materials

Supplementary information (1H NMR, and mass spectral data of compounds) associated with this article can be found in the online version.

  1. Supplementary Material

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