Process for preparing monobrominated cyclobutarenes

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.

BACKGROUND OF THE INVENTION 
This invention relates to a process for preparing brominated organic 
compounds. More specifically, it relates to a process for preparing 
monobrominated cyclobutarenes. 
Monobrominated cyclobutarenes are intermediates for the preparation of high 
performance monomeric and polymeric compositions for the electronics and 
aerospace industries. U.S. Pat. No. 4,540,763 discloses that 
monobrominated 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. 
Processes for preparing monobrominated cyclobutarenes are difficult because 
multiple bromination 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. Pat. 
No. 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 benzocyolobutene 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, it would be desirable to have a process for preparing 
monobrominated cyclobutarenes that does not require a halogen catalyst or 
a heavy metal catalyst. It would also be desirable to have a process 
providing a faster bromination reaction highly selective to monobrominated 
cyclobutarenes without requiring excessive quantities of brominating 
agent. 
SUMMARY OF THE INVENTION 
This invention is a method of preparing monobrominated cyclobutarenes 
consisting essentially of brominating a cyclobutarene in the presence of 
an organic complexing agent, an acid scavenger, 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 monobrominated 
cyclobutarenes and neither requires excessive quantities of brominating 
agent nor creates an environmental problem related to the disposal of the 
catalysts. 
The monobrominated cyclobutarenes of this invention are useful as 
intermediates for the preparation of high performance monomeric and 
polymeric compositions for the electronics industry. 
DETAILED DESCRIPTION OF THE INVENTION 
As the term is used herein, "cyclobutarene" refers to a compound oontaining 
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)n electrons as described in Morrison and Boyd, Organic Chemistry. 
3rd Edition, (1973). Suitable compounds containing at least one aromatic 
ring include benzene, naphthalene, biphenyl, binaphthyl, phenanthrene, 
anthracene, and diphenylbenzene. The aromatic ring of the cyclobutarene 
can be substituted with groups stable to the bromination reaction, 
including but not limited to groups such as methyl, methoxy, and acetate. 
Heterocyclic compounds such as pyridine and picoline are also included. 
Preferred compounds are benzene, naphthalene, and biphenyl. The most 
preferred compound containing at least one aromatic ring is benzene. 
Therefore, the most preferred cyclobutarene is benzocyclobutene. 
As disclosed in U.S. Pat. No. 4,570,011, cyclobutarenes useful in this 
invention can be prepared by dissolving an ortho alkyl halomethyl aromatic 
hydrocarbon, such as ortho methylchloromethylbenzene, in an inert solvent, 
and then pyrolyzing the solution under suitable reaction conditions. 
"Brominating" refers to the introduction of bromine into an organic 
compound by treating the compound with a brominating agent. Suitable 
brominating agents useful in this invention are those compounds which are 
capable of reacting with the aromatic ring of the cyclobutarene to break 
the carbonhydrogen bond and to form a carbon-halogen bond under the 
reaction conditions. H. P. Braendlin et al. Friedel-Crafts and Related 
Reactions. Vol. III, Chapter 46, pp. 1517-1593, John Wiley & Sons, New 
York (1964), disclose brominating agents useful for brominating organic 
compounds. The brominating agents that can be employed in this invention 
can include molecular bromine, bromine chloride, pyridinium perbromide 
hydrobromide, dioxane dibromide, and N-bromosuccinimide. Preferred 
brominating agents include molecular bromine and bromine chloride. The 
most preferred brominating agent is molecular bromine. 
The monobrominated cyclobutarenes useful in this invention are prepared by 
brominating a cyclobutarene. The term "monobrominated" refers to the 
replacement of one hydrogen atom on the aromatic ring with one bromine 
atom. The products produced from the bromination of the cyclobutarene 
include not only the monobrominated cyclobutarenes but also small 
quantities of hydrogen bromide, unreacted brominating agent and 
undesirable side reaction products. The hydrogen bromide 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 monobrominated cyclobutarenes are organic compounds that will donate 
electrons to form donor-acceptor adducts with the unreacted brominating 
agent and the hydrogen bromide produced during the reaction. The adduct 
formed reduces the reactivity of the brominating agent and hydrogen 
bromide 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 cyolobutarene. 
Suitable organic complexing agents include aliphatic alcohols and diols 
having less than 10 carbon atoms, such as methanol, isobutyl alcohol, and 
ethylene glycol: aliphatic polymeric diols having an average molecular 
weight ranging from about 100 to about 15,000, such as the commercial 
grades of polyethylene glycol and polypropylene glycol; saturated 
aliphatic ethers having less than 10 carbon atoms, such as ethylene glycol 
ethyl ether and tripropylene glycol methyl ether: saturated cyclic ethers 
such as dioxane and 12-crown-4 ether; saturated aliphatic carboxylic acids 
and their anhydrides having less than 10 carbon atoms, such as acetic acid 
and acetic anhydride; other complexing agents such as dimethyl formamide 
and dimethyl sulfoxide; and mixtures of these organic complexing agents. 
Preferred organic complexing agents are methanol and ethylene glycol ethyl 
ether. The most preferred organic complexing agent is methanol. 
Other organic complexing agents that improve the selectivity of the 
reaction to monobrominated cyclobutarenes include saturated quaternary 
ammonium salts, such as tetraalkylammonium salts and trialkylamine salts. 
Although these compounds do not donate electrons to form donor-acceptor 
adducts, their effectiveness as complexing agents has been demonstrated. 
The Dictionary of Scientific and Technical Terms, McGraw-Hill, Second 
Edition (1978) defines a scavenger as "a substance added to a mixture or 
other system to remove or inactivate impurities". Acid scavengers useful 
in this invention remove or inactivate hydrogen bromide produced during 
the bromination by reacting with the hydrogen bromide to form a side 
product. The scavenger does not react with the cyclobutarene. Preferably, 
the scavenger reacts readily with hydrogen bromide but does not react 
readily with the brominating agent to prevent the bromination of the 
cyclobutarene. The acid scavenger can be organic or inorganic. 
Suitable organic acid scavengers include epoxides having less than 10 
carbon atoms, such as ethylene oxide, propylene oxide, epichlorohydrin, 
and epibromohydrin; aliphatic tertiary alcohols having less than 10 carbon 
atoms, such as tertiary butyl alcohol; aliphatic primary, secondary and 
tertiary amines, such as ethylamine, diethylamine, and triethylamine; 
heterocyclic compounds such as pyridine and picoline, and 
triarylphosphines such as triphenylphosphine. The preferred scavengers are 
the epoxides having less than 10 carbon atoms and the tertiary amines. The 
most preferred epoxide is epichlorohydrin and the most preferred tertiary 
amine is triethylamine. 
Suitable inorganic acid scavengers include alkali metal and alkali earth 
metal salts of alcohols and carboxylic acids, such as sodium methylate, 
sodium ethylate, and sodium acetate; alkali metal and alkali earth metal 
bases, such as sodium hydroxide and calcium hydroxide; and carbonates and 
bicarbonates of alkali metal and alkali earth metals, such as sodium 
bicarbonate and potassium carbonate. 
When the cyclobutarene is brominated 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 brominating agent and 
the hydrogen bromide. 
In a preferred embodiment of this invention, the solubility of hydrogen 
bromide produced during bromination in the reaction mixture is reduced. 
The reduced solubility will increase the quantity of hydrogen bromide that 
will evolve from the reaction mixture as a gas. Since more hydrogen 
bromide will evolve from the reaction mixture as a gas, there will be less 
hydrogen bromide in the reaction mixture that can react with the 
cyclobutane ring of the cyclobutarene to produce undesirable side 
products. Therefore, an increased selectivity of monobrominated 
cyclobutarene will result. 
One method of reducing the solubility of hydrogen bromide in the reaction 
mixture is to dilute the cyclobutarene in an appropriate nonreacting 
diluent before bromination. Appropriate diluents are those in which the 
solubility of hydrogen bromide 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 oomplexing agents include acetic acid, 
methanol, and water. 
The mole ratio of the cyclobutarene to the complexing agent or water 
employed in the practice of this invention can range from about 0.001:1 to 
about 100:1. A more preferable range is from about 0.005:1 to about 70:1. 
The most preferable range is from about 0.05:1 to about 6.0:1. The mole 
ratio of the cyclobutarene to the scavenger employed in the practice of 
this invention can range from about 0.1:1 to about 100:1. A more 
preferable range is from about 0.3:1 to about 20:1. The most preferable 
range is from about 0.5:1 to about 2.0:1. 
If a diluent is employed to dilute the cyclobutarene before bromination, 
the weight ratio of the diluent to the cyclobutarene can range from about 
0.1:1 to about 100:1. A more preferable range is from about 0.5:1 to about 
20:1. The mole ratio of the brominating agent to the cyclobutarene can 
range from about 0.1:1 to about 2.0:1. A more preferable range is from 
about 0.90:1 to about 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 about 25.degree. C. to about 
60.degree. C. Although the bromination 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 bromide 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 brominating agent is added 
continuously or periodically to the reaction mixture to control the 
evolution of gaseous hydrogen bromide. By controlling the evolution of the 
gaseous hydrogen bromide, the operating pressure of the system can be 
maintained as close to atmospheric pressure as possible. 
The bromination reaction proceeds almost instantaneously when the 
brominating agent contacts the cyclobutarene. In most instances, the 
required reaction time depends on the rate of addition of the brominating 
agent to the reaction system. The rate of addition of the brominating 
agent depends on the ability of the system to remove the gaseous hydrogen 
bromide and the design pressure of the reactor. 
The selectivity of the reaction to monobrominated cyclobutarenes decreases 
with conversion because the monobrominated cyclobutarenes prepared from 
the bromination can react further with the reaction mixture to form 
undesirable side products. Advantageously, the monobrominated 
cyclobutarenes are separated quickly from the reaction mixture. In 
preferred embodiments of this invention, the selectivity will range from 
about 75 mole percent to about 95 mole percent. Selectivity is defined as 
the mole percentage of the reacted cyclobutarene that forms monobrominated 
cyclobutarenes. 
After the bromination reaction, the monobrominated cyclobutarenes 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 brominating agent and to extract the hydrogen 
bromide 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 monobrominated cyclobutarenes. Preferably, the recovered 
monobrominated cyclobutarenes have a purity of at least 97 percent by 
weight. 
The recovered monobrominated cyolobutarenes are useful intermediates which 
can be processed to prepare poly(cyclobutarene) monomeric and polymeric 
compositions. U.S. Pat. No. 4,540,763 discloses methods of preparing these 
compositions from monobrominated cyclobutarenes. 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 and are not intended to limit the 
scope of this invention. All percentages are mole percent unless otherwise 
indicated. 
EXAMPLES 
EXAMPLE 1 
2005 grams (g) Benzocyclobutene (19.25 moles), 2000 g methylene chloride 
(23.55 moles) and 200 g methanol (6.24 moles) are 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 is heated to 40.degree. C. by recirculating 
an aqueous solution of ethylene glycol from a constant temperature bath 
through the jacket. 3275 g Bromine (20.49 moles) are fed to the reactor at 
a constant flow rate of 728 g/hr. During the addition, the temperature 
increases to a range between 48.degree. C. and 57.5.degree. C. and reflux 
is observed. A sample of the reaction mixture is taken each hour for 4 
hours. Another sample is taken after 4 hours and 30 minutes when all of 
the bromine has been fed to the reactor. The residual bromine of each 
sample is neutralized with the requisite amount of an aqueous solution of 
sodium metabisulfite. Each organic layer is separated and analyzed using a 
capillary gas chromatograph to determine its composition. A final sample 
of the reaction mixture is taken after 5 hours and 30 minutes. It is 
washed with aqueous sodium metabisulfite and the organic layer is 
separated and analyzed in a similar manner. The analysis of each sample is 
shown in Table I. 
TABLE I 
__________________________________________________________________________ 
Mono- Multi- 
Unreacted 
brominated 
2-Bromo- 
brominated 
Reaction 
Benzo- Benzo- phenethyl 
Benzo- Phenethyl 
Time cyclobutene 
cyclobutenes 
Bromide 
cyclobutene 
Bromide 
Selectivity 
(Hours) 
(Percent) 
(Percent) 
(Percent) 
(Percent) 
(Percent) 
(Percent) 
__________________________________________________________________________ 
1.0 84.9 13.7 1.5 0 0 91 
2.0 58.4 36.9 4.6 0 0 89 
3.0 38.1 54.7 6.7 0.2 0.2 88 
4.0 19.2 70.1 8.9 1.1 0.6 87 
4.5* 
10.0 77.0 10.2 1.9 0.9 86 
5.5 4.4 81.0 10.9 2.7 1.1 85 
__________________________________________________________________________ 
*Bromine addition complete. 
Table I indicates that a significantly improved selectivity of the reaction 
to monobrominated benzocyolobutenes 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) are charged to the same reactor as 
that of Example 1 equipped with a 500 ml dropping funnel. The mixture is 
heated to 40.degree. C. 163.4 g Bromine (1.022 moles) are added dropwise 
to the reaction mixture through the dropping funnel. During the addition, 
the temperature increases to 44.2.degree. C. and reflux is observed. After 
78 minutes the addition of bromine is completed. After 16 hours, the 
residual bromine of the reaction mixture is neutralized with 200 ml of an 
aqueous solution containing 10 g of sodium metabisulfite. The organic 
layer is 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) are 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 is washed with aqueous sodium 
metabisulfite. The organic layer is separated and analyzed using a 
capillary gas chromatograph. The analysis indicates that the product 
contains 24.8 percent benzocyolobutene, 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 are added to a solution 
containing 4 g methylene chloride, 1 g benzocyclobutene and 0.1 g of one 
of several selected complexing agents (or water) at room temperature. 
After 12 hours a sample of the reaction mixture is washed with aqueous 
sodium metabisulfite. The organic layer is separated and analyzed using a 
capillary gas chromatograph to determine the percent conversion and the 
percent selectivity. The conversion and selectivity are compared to a 
first run in which neither the complexing agent (or water) nor methylene 
chloride are added and a second run in which the complexing agent (or 
water) is not added. Percent conversion is defined as the mole percentage 
of benzocyclobutene that reacts. The results are shown in Table II. 
TABLE II 
______________________________________ 
Complexing Conversion Selectivity 
Agent (or Water) 
Diluent (Percent) (Percent) 
______________________________________ 
None* None 92.3 71 
None* Methylene 83.1 76 
chloride 
Methanol Methylene 96.0 86 
chloride 
Water Methylene 90.3 81 
chloride 
Ethyl Glycol 
Methylene 87.7 87 
Ethyl Ether chloride 
Glacial Acetic 
Methylene 94.5 81 
Acid chloride 
Tetra(n-butyl) 
Methylene 92.8 83 
Ammonium Chloride 
Hydrogen Sulfate 
______________________________________ 
*Not an embodiment of this invention. 
Table II indicates that high selectivity of the reaction to monobrominated 
benzocyclobutenes is obtained by the method of the present invention using 
various complexing agents 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. 
Examples 5 
1.6 g Bromine are 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 is washed with aqueous sodium metabisulfite. The organic 
layer is separated and analyzed using a capillary gas chromatograph to 
determine the percent conversion and the percent selectivity. The 
experiment is repeated replacing the 4 g of methanol with 4 g of water. 
The results are shown in Table III. 
TABLE III 
______________________________________ 
Complexing Conversion 
Selectivity 
Agent (or Water) 
Diluent (Percent) (Perent) 
______________________________________ 
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 is followed, except that the methylene chloride 
diluent is replaced with various diluents listed in Table IV and the 
complexing agent employed is methanol. The results are shown in Table IV. 
TABLE IV 
______________________________________ 
Complexing Conversion 
Selectivity 
Agent Diluent (Percent) (Percent) 
______________________________________ 
Methanol 95 percent 73.0 82 
Acetic Acid 
Methanol Chloroform 88.2 86 
Methanol Carbon 82.5 80 
Tetrachloride 
Methanol Ethylene 94.9 81 
Dichloride 
Methanol Bromochloro- 
87.7 84 
methane 
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. 
EXAMPLE 7 
The procedure of Example 4 is followed, except that the complexing agents 
(or water) are replaced with various scavengers listed in Table V. The 
results are shown in Table V. 
TABLE V 
______________________________________ 
Molar ratio 
of Scavenger 
to Benzo- Conversion 
Selectivity 
Scavenger 
cyclobutene 
Diluent (Percent) 
(Percent) 
______________________________________ 
T-Butyl 0.14 Methylene 75.6 78 
Alcohol chloride 
Epichloro- 
1.0 Methylene 85.1 80 
hydrin Chloride 
Triethyl- 
0.10 Methylene 78.8 85 
amine Chloride 
Sodium 1.0 Methylene 44.8 80 
Methylate Chloride 
______________________________________ 
Table V indicates that a high selectivity of the reaction to monobriminated 
benzocyclobutenes is obtained by the method of the present invention using 
various scavengers instead of complexing agents or water.