Field of the invention
The present invention provides an improved process for preparation of fluoro substituted benzyl bromides.

Background of the invention
The fluoro substituted benzyl bromides are very useful intermediates in preparation of pharmaceuticals and agro chemicals.
Various methods are known for preparation of these compounds e.g., the Solar Energy, Volume: 113, Pages: 332-339 discloses a process for preparation of 2-fluoro-benzyl bromide by brominating 2-fluoro toluene in presence of concentrated solar radiations and in absence of a radical initiator.
However, the use of concentrated solar radiations for bromination is found to be industrially uneconomic.
The Indian Patent Application Number 805/MUM/2013 also discloses a process for preparation of 2-fluoro-benzyl bromide by brominating 2-fluoro toluene in presence of N-bromosuccinimide and azobisisobutyronitrile using cyclohexane as a solvent at 78 to 80°C.
However the use of solvent results in low purity i.e., 80- 90% only.
Thus there is an urgent need to develop a cost effective, industrially applicable process for preparation of fluoro substituted aromatic benzyl bromides.
Surprisingly the inventors of the present invention found that the bromination of said compounds can be carried out in solvent free conditions which gives very good purity of the title compounds.

Object of the invention
The main object of the present invention is to provide a simple, safe and economic process for preparation of fluoro substituted benzyl bromides.
Summary of the invention
The present invention provides an improved process for preparation of a compound of formula I,

comprising:
a) brominating a compound of formula II using N-bromosuccinimide and a radical initiator in absence of a solvent to obtain the compound of formula I, and

b) isolating the compound of formula I obtained from step a).

Detailed description of the invention
The present invention provides an improved process for preparation of a compound of formula I,

comprising:
a) brominating a compound of formula II using N-bromosuccinimide and a radical initiator in absence of a solvent to obtain the compound of formula I, and

b) isolating the compound of formula I obtained from step a).

In particular, the present invention provides an improved process for preparation of a compound of formula Ia,

comprising:
a) brominating a compound of formula IIa using N-bromosuccinimide and a radical initiator in absence of a solvent to obtain the compound of formula Ia, and

b) isolating the compound of formula Ia obtained from step a).

The compound of formula II which is used as a starting material in step a) can be prepared by any method known in the art or may be obtained commercially.
The step a) is particularly carried out using N-bromosuccinimide as a brominating agent in presence of radical initiator, wherein the radical initiator can be selected from the group consisting of benzoyl peroxide, azobisisobutyronitrile, benzyl peroxide, tert-butyl peroxide and the alike.
The bromination using N-bromosuccinimide involves the formation of a stable benzylic radical intermediate which aids the formation of benzylic bromides. In particular, the reaction is carried out in solvent free conditions which increase the reaction selectivity and reduces the formation of undesired impurities.
The formation of radical intermediates is aided by the use of radical initiators as stated above.
The N-bromosuccinimide which is used as a brominating agent is easily available in commercial grades.
The bromination reaction of step a) can be carried out at a temperature selected from 60 to 85°C, preferably at 70 to 80°C. The reaction time is selected in the range of 12 to 16 hours.
The addition of N-bromosuccinimide is done in a lot-wise manner and 3 to 6 moles of N-bromosuccinimide are used. Similarly 0.00001 to 0.00006 moles of radical initiators are used.
In a preferred embodiment, 2-fluorotoluene was charged in a 2 litre round bottom flask fitted with a mechanical stirrer and heated to 70°C. Azobisisobutyronitrile was added and N-bromosuccinimide was then added in a lot-wise manner. After complete addition, reaction mass was heated and maintained at 75 to 80°C for about 12 to 16 hours. After completion of the reaction, the reaction mass was cooled to 20-25oC and was diluted with n-heptanes or n-hexanes. The reaction mass was filtered and the solvent was boiled-off completely from filtrate and then taken for distillation at 66-70°C under 8-11 mmHg vacuum, yielding the titled compound.
The compound of formula-I is isolated by using techniques known in the art for example distillation, evaporation, column chromatography and layer separation or combination thereof.
While the present invention has been described in terms of its specific embodiments, certain modifications and equivalents will be apparent to those skilled in the art and are intended to be included within the scope of the present invention.
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.

EXAMPLE 1: Preparation of 2-Fluorobenzylbromide.
600g of 2-fluorotoluene was charged in a 2 litre round bottom flask fitted with a mechanical stirrer and heated to 70°C. 0.035 g of azobisisobutyronitrile was added and 534 g of N-bromosuccinimide was then added in a lot-wise manner. After complete addition, reaction mass was heated and maintained at 75 to 80°C for about 12 to 16 hours. After completion of the reaction, the reaction mass was cooled to 20-25oC and was diluted with n-heptane (also we can use n-hexane or hexanes but preferably n-heptane). The reaction mass was filtered and the solvent was boiled-off completely from filtrate and then taken for distillation at 66-70°C under 8-11 mmHg vacuum, yielding the titled compound.
Yield: 80%
Purity: 99.4%

EXAMPLE 2: Preparation of 3-Fluorobenzylbromide.
600g of 3-fluorotoluene was charged in a 2 litre round bottom flask fitted with a mechanical stirrer and heated to 70°C. 0.035 g of azobisisobutyronitrile was added and 534 g of N-bromosuccinimide was then added in a lot-wise manner. After complete addition, reaction mass was heated and maintained at 75 to 80°C for about 12 to 16 hours. After completion of the reaction, the reaction mass was cooled to 20-25oC and was diluted with n-heptane (also we can use n-hexane or hexanes but preferably n-heptane). The reaction mass was filtered and the solvent was boiled-off completely from filtrate and then taken for distillation at 88°C under 20 mmHg vacuum, yielding the titled compound.
Yield: 85%
Purity: 98.4%

EXAMPLE 3: Preparation of 4-Fluorobenzylbromide.
600g of 4-fluorotoluene was charged in a 2 litre round bottom flask fitted with a mechanical stirrer and heated to 70°C. 0.035 g of azobisisobutyronitrile was added and 534 g of N-bromosuccinimide was then added in a lot-wise manner. After complete addition, reaction mass was heated and maintained at 75 to 80°C for about 12 to 16 hours. After completion of the reaction, the reaction mass was cooled to 20-25oC and was diluted with n-heptane (also we can use n-hexane or hexanes but preferably n-heptane). The reaction mass was filtered and the solvent was boiled-off completely from filtrate and then taken for distillation at 85°C under 15 mmHg vacuum, yielding the titled compound.
Yield: 87%
Purity: 96.4%

We Claims:
1. A process for preparation of a compound of formula I,

comprising:
a) brominating a compound of formula II using N-bromosuccinimide and a radical initiator in absence of a solvent to obtain the compound of formula I, and

b) isolating the compound of formula I obtained from step a).

2. The process as claimed in claim 1, wherein radical initiator is selected from a group consisting of benzoyl peroxide, azobisisobutyronitrile, benzyl peroxide and tert-butyl peroxide or a mixture thereof.

3. The process as claimed in claim 1, wherein step a) is carried out at a temperature selected in the range of 60 to 85°C.

4. The process as claimed in claim 1, wherein step a) is carried out using 3 to 6 moles of N-bromo succinimide.

5. The process as claimed in claim 1, wherein step a) is carried out using 0.00001 to 0.00006 moles of radical initiator.