PROCESS FOR PREPARING MONOHALOGENATED BENZOCYCLOBUTENE

Abstract: Monobrominated cyclobutarenes are prepared by brominating a cyclobutarene in the presence of an organic complexing agent, an acid scavenger, or water. Faster reaction rates highly selective to monobrominated cyclobutarenes are obtained without conventional heavy metal or halogen catalysts.

Description: This invention relates to a process for preparing a benzocyclobutene monohalogenated on the benzene ring. Monohalogenated cyclobutarenes are intermediates for the preparation of high performance monomeric and polymeric compositions for the electronics and aerospace industries. U S-A-4,540,763 discloses that monohalogenated cyclobutarenes can be processed to prepare poly(cyclobutarene) polymeric compositions. These compositions possess thermal stability at high temperatures, as well as chemical resistance and low sensibility to water. K.P.C. Vollhardt et al. J.Am.Chem.Soc, 99 , (1977), p 4058-4069, disclose the preparation of 4-trimethylsilyl-5-bromobenzocyclobutene by reacting 4,5-bis(trimethylsilyl)benzocyclobutene with two equivalents of bromine in the presence of one equivalent of pyridine. This type of reaction explores the properties of a silyl substituent as excellent leaving group in an electrophilic substitution reaction on an aromatic ring. But the preparation of the silyl substituted starting material is complicated and involves the employment of an organo metallic catalyst like CpCo(CO)₂. Processes for preparing monohalogenated cyclobutarenes are difficult because multiple halogenation reactions occur and the strained cyclobutane ring of the cyclobutarene is easily susceptible to ring-opening side reactions (see J. B. F. Lloyd et al., Tetrahedron , 20 , pp. 2185-94 (1964)). U S-A-4,540,763 discloses a process for preparing monobrominated cyclobutarenes which involves diluting a cyclobutarene in acetic acid and then contacting the solution with pyridinium perbromide hydrobromide in the presence of a mercuric acetate catalyst. The reaction occurs over a four day period and uses approximately 300 percent excess brominating agent. J. B. F. Lloyd et al., Tetrahedron , 21 , pp. 245-54, (1965), disclose a process for preparing monobrominated benzocyclobutene which involves diluting benzocyclobutene in a 95 percent aqueous solution of acetic acid and then contacting the solution with molecular bromine in the presence of an iodine catalyst. The yield of monobrominated benzocyclobutene is 78 percent after 48 hours. Unfortunately, both of these processes require large quantities of brominating agent to complete a very slow bromination reaction. Also, both processes require either a heavy metal catalyst or a halogen catalyst. The residual catalyst that inevitably finds its way into the final product is detrimental for electronics and aerospace industry applications Furthermore, these catalysts create environmental problems related to their disposal. Therefore the object of the present invention is to provide a process for preparing benzocyclobutene monohalogenated on the benzene ring that does not require a halogen catalyst or a heavy metal catalyst in a faster halogenation reaction highly selective to monohalogenated benzocyclobutene without requiring excessive quantities of halogenating agent. This object has been attained by a method for preparing a benzocyclobutene monohalogenated on the benzene ring comprising reacting the benzocyclobutene with a halogenation agent selected from molecular bromine, bromine chloride or molecular chlorine, characterized by carrying out the halogenation in the absence of an iodine catalyst and in the presence of either methanol or water. Surprisingly, reaction rates faster than the rates disclosed in the prior art are achieved by the method of this invention without requiring a catalyst. In addition, the reaction is highly selective to monohalogenated benzocyclobutene and neither requires excessive quantities of halogenating agent nor creates an environmental problem related to the disposal of the catalysts. The monohalogenated benzocyclobutene of this invention is useful as intermediates for the preparation of high performance monomeric and polymeric compositions for the electronics industry. As the term is used herein, "cyclobutarene" refers to a compound containing at least one aromatic ring to which is fused one or more cyclobutane rings or one or more substituted cyclobutane rings. An aromatic ring contains (4N + 2)π electrons as described in Morrison and Boyd, Organic Chemistry , 3rd Edition, (1973). The cyclobutarene suitable for the present invention is benzocyclobutene. As disclosed in U S-A-4,570,011, benzocylobutene useful in this invention can be prepared by dissolving ortho methylchloromethylbenzene, in an inert solvent, and then pyrolyzing the solution under suitable reaction conditions. "Halogenating" refers to the introduction of halogen into an organic compound by treating the compound with a halogenating agent. Suitable halogenating agents useful in this invention are those compounds which are capable of reacting with the aromatic ring of the cyclobutarene to break the carbon-hydrogen bond and to form a carbon-halogen bond under the reaction conditions. Halogenating agents useful for halogenating cyclobutarenes are disclosed in H. P. Braendlin et al. Friedel-Crafts and Related Reactions , Vol. III, Chapter 46, pp. 1517-1593, John Wiley & Sons, New York (1964); Wagner et al. Synthetic Organic Chemistry , pp. 98-147, John Wiley & Sons, New York, (1965); and March, Advanced Organic Chemistry, 34d ed. pp. 476-479, John Wiley & Sons, New York (1985). Preferred halogenating agents are brominating and chlorinating agents. The most preferred halogenating agent is a brominating agent. The brominating agents that can be employed in this invention can include molecular bromine, bromine chloride, The preferred brominating agent is molecular bromine. The chlorinating agent is molecule chlorine. The monohalogenated benzocyclobutene useful in this invention are prepared by halogenating benzocyclobutene. The term "monohalogenated" refers to the replacement of one hydrogen atom on the aromatic ring with one halogen atom. The products produced from the halogenation of the benzocyclobutene include not only the monohalogenated benzocyclobutene but also small quantities of hydrogen halide, unreacted halogenating agent and undesirable side reaction products. The hydrogen halide can either dissolve in the reaction mixture or evolve from the reaction mixture as a gas. The organic complexing agents that improve the selectivity of the reaction to monohalogenated cyclobutarenes are organic compounds that will donate electrons to form donor-acceptor adducts with the unreacted halogenating agent and the hydrogen halide produced during the reaction. The adduct formed reduces the reactivity of the halogenating agent and hydrogen halide with the cyclobutane ring of the cyclobutarene and therefore reduces formation of undesirable side products. A. J. Downs et al., Comprehensive Inorganic Chemistry , Chapter 26, pp. 1196-1197 and pp. 1201-1209, New York, New York, (1973), discuss the crystalline structure of halogen adducts based on X-ray diffraction studies. They describe organic compounds which form halogen adducts and the factors influencing their stability. They also describe the relative capacities of organic compounds to donate electrons. Preferably, the organic complexing agent has an electron donor capacity equal to or slightly greater than the electron donor capacity of the cyclobutarene. In the present process methanol is used as complexing agent. When the benzocyclobutene is halogenated in the presence of water, the water acts in a manner similar to that of the organic complexing agent by forming donor-acceptor adducts with the unreacted halogenating agent and the hydrogen halide. In a preferred embodiment of this invention, the solubility of hydrogen halide produced during halogenation in the reaction mixture is reduced. The reduced solubility will increase the quantity of hydrogen halide that will evolve from the reaction mixture as a gas. Since more hydrogen halide will evolve from the reaction mixture as a gas, there will be less hydrogen halide in the reaction mixture that can react with the cyclobutane ring of the cyclobutarene to produce undesirable side products. Therefore, an increased selectivity of monohalogenated benzocyclobutene will result. One method of reducing the solubility of hydrogen halide in the reaction mixture is to dilute the benzocyclobutene in an appropriate nonreacting diluent before halogenation. Appropriate diluents are those in which the solubility of hydrogen halide is low. Ahmed et al., Journal of Applied Chemistry , 20 ., pp. 109-116, (April 1970), disclose the solubilities of hydrogen halides in various diluents. Suitable diluents that can be employed in this invention include methylene chloride, chloroform, carbon tetrachloride, ethylene dichloride, bromochloromethane, and hexane. The preferred diluents are methylene chloride, chloroform, and bromochloromethane. The most preferred diluent is methylene chloride. Certain organic complexing agents can also act as appropriate nonreacting diluents. Examples of such organic complexing agents include methanol, and water. The mole ratio of the benzocyclobutene to methanol or water employed in the practice of this invention can range from 0.05:1 to 6.0:1. If a diluent is employed to dilute the benzocyclobutene before halogenation, the weight ratio of the diluent to the benzocyclobutene can range from 0.1:1 to 100:1. A more preferable range is from 0.5:1 to 20:1. The mole ratio of the brominating agent to the benzocyclobutene can range from 0.90:1 to 1.10:1. The operating temperature and pressure of the reaction system are limited solely by practical considerations. The temperature can range from the freezing point to the boiling point of the reaction mixture. Preferably, the operating temperature ranges from 25°C to 60°C. Although the halogenation reaction will proceed at both high and low operating pressures, it is preferable to run as close to atmospheric pressure as possible because higher pressures will increase the solubility of the hydrogen halide in the reaction system and therefore generate more side reactions. Also, high operating pressures necessitate the use of more expensive pressure rated equipment. In a preferred embodiment of this invention, the halogenating agent is added continuously or periodically to the reaction mixture to control the evolution of gaseous hydrogen halide. By controlling the evolution of the gaseous hydrogen halide, the operating pressure of the system can be maintained as close to atmospheric pressure as possible. The halogenation reaction proceeds almost instantaneously when the halogenating agent contacts the benzocyclobutene. In most instances, the required reaction time depends on the rate of addition of the halogenating agent to the reaction system. The rate of addition of the halogenating agent depends on the ability of the system to remove the gaseous hydrogen halide and the design pressure of the reactor. The selectivity of the reaction to monohalogenated benzocyclobutene decreases with conversion because the monohalogenated benzocyclobutene prepared from the halogenation can react further with the reaction mixture to form undesirable side products. Advantageously, the monohalogenated benzocyclobutene are separated quickly from the reaction mixture. In preferred embodiments of this invention, the selectivity will range from 75 mole percent to 95 mole percent. Selectivity is defined as the mole percentage of the reacted benzocyclobutene that forms monohalogenated benzocyclobutene. After the halogenation reaction, the monohalogenated benzocyclobutene can easily be separated from the side products produced by the reaction. One method of separation is to fractionally distill all of the impurities from the reaction system. Another method of separation involves adding an aqueous solution of a reducing agent, such as sodium metabisulfite, to neutralize the residual halogenating agent and to extract the hydrogen halide from the organic phase of the reaction mixture to the aqueous phase. The aqueous phase can then be physically separated from the organic phase and then the organic phase can be fractionally distilled to recover the monohalogenated benzocyclobutene. Preferably, the recovered monohalogenated benzocyclobutene have a purity of at least 97 percent by weight. The recovered monohalogenated benzocyclobutene are useful intermediates which can be processed to prepare poly(benzocyclobutene) monomeric and polymeric compositions. U S-A-4,540,763 discloses methods of preparing these compositions from monohalogenated benzocyclobutene. The polymeric compositions have excellent thermal stability at high temperatures, good chemical resistance to most industrial solvents, and a low sensitivity to water. These properties are highly desirable for applications in the electronics and aerospace industries. The following examples are illustrative of this invention. All percentages are mole percent unless otherwise indicated. Example 1 2005 grams (g) Benzocyclobutene (19.25 moles), 2000 g methylene chloride (23.55 moles) and 200 g methanol (6.24 moles) were charged to a jacketed, 8 liter cylindrical 3-neck round bottom reactor equipped with a mechanical stirrer, a digital thermocouple, and a reflux condenser connected to a caustic scrubber. The mixture was heated to 40°C by recirculating an aqueous solution of ethylene glycol from a constant temperature bath through the jacket. 3275 g Bromine (20.49 moles) was fed to the reactor at a constant flow rate of 728 g/hr. During the addition, the temperature increased to a range between 48°C and 57.5°C and reflux was observed. A Sample of the reaction mixture was taken each hour for 4 hours. Another sample was taken after 4 hours and 30 minutes when all of the bromine has been fed to the reactor. The residual bromine of each sample was neutralized with the requisite amount of an aqueous solution of sodium metabisulfite. Each organic layer was separated and analyzed using a capillary gas chromatograph to determine its composition. A final sample of the reaction mixture was taken after 5 hours and 30 minutes. It was washed with aqueous sodium metabisulfite and the organic layer was separated and analyzed in a similar manner. The analysis of each sample is shown in Table I. Table I indicates that a significantly improved selectivity of the reaction to monobrominated benzocyclobutenes is obtained by the method of this invention without the use of the catalysts of the prior art. Table I also indicates high selectivities are achieved at much faster reaction rates than the rates achieved by the prior art. Example 2 100.95 g Benzocyclobutene (0.969 moles), 115.52 g methylene chloride (1.36 moles) and 6.00 g methanol (0.187 moles) were charged to the same reactor as that of Example 1 equipped with a 500 ml dropping funnel. The mixture was heated to 40°C. 163.4 g Bromine (1.022 moles) was added dropwise to the reaction mixture through the dropping funnel. During the addition, the temperature increased to 44.2°C and reflux was observed. After 78 minutes the addition of bromine was completed. After 16 hours, the residual bromine of the reaction mixture was neutralized with 200 ml of an aqueous solution containing 10 g of sodium metabisulfite. The organic layer was separated and analyzed using a capillary gas chromatograph. The analysis indicates that the product contains 3.7 percent unreacted benzocyclobutene, 81.2 percent monobrominated benzocyclobutenes, 6.5 percent 2-bromophenethylbromide, 8.4 percent multibrominated benzocyclobutenes and less than 0.3 percent phenethyl bromide. Example 3 1.6 g Bromine (104 percent theoretical) was added to a solution containing 1 g benzocyclobutene and 0.1 g methanol at room temperature. After 12 hours a sample of the reaction mixture was washed with aqueous sodium metabisulfite. The organic layer was separated and analyzed using a capillary gas chromatograph. The analysis indicates that the product contains 24.8 percent benzocyclobutene, 56.5 percent monobrominated benzocyclobutenes, 9.1 percent 2-bromophenethyl bromide, 9.3 percent multibrominated benzocyclobutenes, and 0.3 percent phenethyl bromide. Example 4 In each of a series of runs, 1.6 g bromine was added to a solution containing 4 g methylene chloride, 1 g benzocyclobutene and 0.1 g of methanol or water at room temperature. After 12 hours a sample of the reaction mixture was washed with aqueous sodium metabisulfite. The organic layer was separated and analyzed using a capillary gas chromatograph to determine the percent conversion and the percent selectivity. The conversion and selectivity were compared to a first run in which neither the complexing agent (or water) nor methylene chloride were added and a second run in which the complexing agent (or water) was not added. Percent conversion is defined as the mole percentage of benzocyclobutene that reacted. The results are shown in Table II. Complexing Agent(or Water) Diluent Conversion (Percent) Selectivity (Percent) None None 92.3 71 None Methylene chloride 83.1 76 Methanol Methylene chloride 96.0 86 Water Methylene chloride 90.3 81 Table II indicates that a high selectivity of the reaction to monobrominated benzocyclobutenes is obtained by the method of the present invention using methanol or water. The selectivities of the two runs obtained without the complexing agent (or water) are poor relative to the selectivities obtained according to the present invention. Example 5 1.6 g Bromine was added to a solution containing 1 g benzocyclobutene and 4 g of methanol at room temperature. After 12 hours, a sample of the reaction mixture was washed with aqueous sodium metabisulfite. The organic layer was separated and analyzed using a capillary gas chromatograph to determine the percent conversion and the percent selectivity. The experiment was repeated replacing the 4 g of methanol with 4 g of water. The results are shown in Table III. Complexing Agent(or Water) Diluent Conversion (Percent) Selectivity (Percent) Methanol None 50.5 85 Water None 92.0 81 Table III indicates that a high selectivity of the reaction to monobrominated benzocyclobutenes is obtained without the use of a diluent. Example 6 The procedure of Example 4 was followed, except that the methylene chloride diluent was replaced with various diluents listed in Table IV and the complexing agent employed was methanol. The results are shown in Table IV. Complexing Agent Diluent Conversion (Percent) Selectivity (Percent) Methanol Chloroform 88.2 86 Methanol Carbon Tetrachloride 82.5 80 Methanol Ethylene Dichloride 94.9 81 Methanol Bromochloromethane 87.7 84 Methanol Hexane 80.9 81 Methanol Water 83.0 77 Table IV indicates that a high selectivity of the reaction to monobrominated benzocyclobutenes is still obtained using various diluents other than methylene chloride.

Claims: A method for preparing a benzocyclobutene monohalogenated on the benzene ring comprising reacting the benzocyclobutene with a halogenation agent selected from molecular bromine, bromine chloride or molecular chlorine, characterized by carrying out the halogenation in the absence of an iodine catalyst and in the presence of either methanol or water. The method of Claim 1 wherein the benzocyclobutene is reacted with molecular bromine. The method of Claims 1 or 2 wherein the benzocyclobutene is diluted in a nonreacting diluent in which the solubility of hydrogen halides is low. The method of Claim 3 wherein the diluent is methylene chloride. The method of Claim 1 wherein the mole ratio of benzocyclobutene to methanol or water ranges from 0.05:1 to 6.0:1. The method of Claims 1-5 wherein the mole ratio of bromine to benzocyclobutene ranges from 0.9:1 to 1.1:1.

Inventors: LIU, MING-BIANN
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1988
Main classification: C07C25
IPC codes: C07C25, C07C17, C07C67