TFPX synthesis

Details TFPX synthesis TFPX synthesis

1996-12-06

2001-09-04

Siegel, Alan (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Halogen containing

Reexamination Certificate

active

06284933

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to methods of synthesizing TFPX, and more specifically to such methods which utilize S
N
2 nucleophilic substitution reactions.
2. Discussion of the Related Art
Dielectric films are widely used throughout both the electronics and coatings industries. Due to their relatively high dielectric constants and melting points, there is an increasing interest in forming dielectric layers from parylene polymers having the molecular structure:
wherein X is typically a hydrogen atom or a fluorine atom.
Parylene polymers are usually formed by chemical vapor deposition processes. One such process is the Gorham process in which a parylene dimer having the molecular structure:
is vaporized and the dimer bonds are then cleaved to yield parylene monomers. The parylene monomers are deposited onto a surface and subsequently polymerized. Because the dielectric constant and melting temperature of parylene polymers usually increases as the number of fluorine atoms within the polymer increases, it is desirable to use octafluoro-[2,2]paracyclophane (hereinafter “AF4”) having the molecular structure:
as the parylene dimer. &agr;,&agr;,&agr;′,&agr;′-tetrafluoro-p-xylene (hereinafter “TFPX”) having the molecular structure:
is often used as the starting material in AF4 synthesis. The conventional procedure for synthesizing TFPX involves the fluorination of terephthaldehyde, which has the molecular structure:
SF
4
and MoF
6
are the most commonly used reagents for terephthaldehyde fluorination. However, SF
4
and MoF
6
are expensive, reducing the industrial utility of this synthetic scheme. In addition, SF
4
and MoF
6
are toxic materials, so a large amount of hazardous waste is produced using these reagents.
Russian Patent No. 2,032,654 discloses an alternate method of synthesizing TFPX in which &agr;,&agr;,&agr;′,&agr;′-tetrabromo-p-xylene (hereinafter “TBPX”) having the molecular structure:
is reacted with SbF
3
to produce TFPX. This method employs the well established electrophilic catalyzed S
N
1 reaction mechanism for replacement of benzylic halogen atoms of the TBPX with fluorine atoms. According to this method, the antimony atom in SbF
3
acts as an electrophile which removes bromine from TBPX to form a carbocation. The carbocation subsequently reacts with fluorine to form TFPX. While this reaction is reported to provide a good yield when carried out under comparatively mild reaction conditions, antimony containing compounds are highly toxic and expensive. Furthermore, the SbF
3
is used in a stoichiometric amount rather than a catalytic amount, resulting in large quantities of hazardous waste materials. Therefore, this method of synthesizing TFPX has limited use for industrial applications.
Therefore, it remains a challenge in the art to synthesize TFPX using a method that is comparatively inexpensive and results in a reduced amount of hazardous waste.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method of synthesizing TFPX that results in reduced amounts of hazardous waste.
It is another object of the present invention to provide such a method that utilizes reagents that are comparatively nontoxic.
In one illustrative embodiment, the present invention provides a method of synthesizing TFPX. This method includes reacting a nonfluorinated tetrahalo-p-xylene compound with a nucleophilic fluorine molecule.
In another illustrative embodiment of the present invention, the method of synthesizing TFPX includes reacting &agr;,&agr;,&agr;′,&agr;′-tetrachloro-p-xylene with CsF.
In a further illustrative embodiment of the present invention, the method of synthesizing TFPX includes reacting &agr;,&agr;,&agr;′,&agr;′-tetrachloro-p-xylene with KF.
DETAILED DESCRIPTION
The present invention relates to the synthesis of TFPX by the reaction of nucleophilic fluorine molecules with nonfluorinated tetrahalo-p-xylenes via S
N
2 type nucleophilic displacement reactions. According to this synthetic scheme, the benzylic halogen atoms of a nonfluorinated tetrahalo-p-xylene are replaced by the fluorine atoms in a nucleophilic fluorine molecule without formation of a carbocation intermediate. This result is unexpected because the conventional approach to the displacement of benzylic halogen atoms involves electrophilically catalyzed S
N
1 reaction mechanisms (i.e., the formation of carbocation intermediates).
A “nucleophilic fluorine molecule” as used herein refers to a molecule that donates a pair of electrons to an atomic nucleus by the addition of a fluorine atom to the atomic nucleus. According to the present invention, nucleophilic fluorine molecules should be chemically stable under TFPX synthesis reaction conditions as described below. Such nucleophilic fluorine molecules include, for example, CsF, NaF, KF, LiF and CaF
2
. Preferably, the nucleophilic fluorine molecule is CsF or KF. While certain nucleophilic fluorine molecules have been disclosed herein, other nucleophilic fluorine molecules appropriate for use in the present invention will be apparent to those skilled in the art.
As used herein, the term “nonfluorinated tetrahalo-p-xylene” denotes a molecule having the structure:
wherein X, X′, X″ and X′″ are benzylic halogen atoms that may each be a chlorine atom, a bromine atom or an iodine atom. An exemplary and nonlimiting list of nonfluorinated tetrahalo-p-xylenes includes &agr;,&agr;,&agr;′,&agr;′-tetraiodo-p-xylene (hereinafter “TIPX”) having the molecular structure:
&agr;,&agr;,&agr;′,&agr;′-tetrachloro-p-xylene (hereinafter “TCPX”) having the molecular structure:
and TFBX. In a preferred embodiment, the nonfluorinated tetrahalo-p-xylene is TCPX.
By “benzylic halogen” it is herein meant to refer to a halogen atom that is bonded to a carbon atom that is directly bonded to a benzene ring (i.e., alpha to a benzene ring). For example, in the molecular structure:
X, X′ and X″ are each benzylic halogen atoms.
In certain embodiments, the S
N
2 type nucleophilic displacement reactions of the present invention may be performed in a solvent system. These solvents are generally aprotic polar solvents, such as dimethyl formamide (DMF) or dimethyl sulfoxide (DMSO). Other solvents appropriate for use in the present invention will be apparent to those skilled in the art and are intended to be within the scope of the present invention.
In alternate embodiments, TFPX synthesis may be carried out under conditions that are substantially free from any solvents (i.e., conditions under which the reaction mixture includes less than 0.5% solvent by weight). By making TFPX in a substantially solvent free environment, the increased costs associated with the use of solvents (e.g., for separation of solvents from product and solvent disposal) can be avoided.
Typically, substantially no volatile products are formed during TFPX synthesis, so the pressure of the reaction vessel does not normally increase during the reaction. As a result, TFPX may be synthesized using open systems or closed systems. For example, in some embodiments, TFPX synthesis may be performed in standard laboratory containers such as, for example, three-necked flasks. In other embodiments, TFPX may be made using a closed container, such as an autoclave or a low pressure bottle system (e.g., a parr) which is held at a few atmospheres pressure. Furthermore, to provide compatibility with certain industrial settings, the reaction may be scaled up and run at pressures of about 1.5 atmospheres.
The temperature at which TFPX is synthesized depends upon the reactants and the reaction conditions. For example, by varying the nucleophilic fluorine molecule and/or the nonfluorinated tetrahalo-p-xylene molecule, the reaction temperature may be increased or decreased. Furthermore, the temperature is generally lower when the reaction is carried out in a closed system compared to an open system. In embodiments in

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