The following is a report written by Dr. Hajimu Kawa, Dr. Thomas R. Bierschenk, Dr. Timothy J. Juhlke, and Dr. Richard J. Lagow.  The report was presented by Dr. Kawa  at the Fluorine in Coatings III meeting in Orlando, Florida, January 25-27, 1999.

Synthesis of Unique Fluorinated Diols

Introduction

Most fluoropolymers currently available are made from fluoroolefins. Many research chemists, who are involved in the development of new fluoropolymers, are aware that commercially available fluorinated monomers other than fluoroolefins are difficult to find. Fluorinated versions of common polymers, such as polyurethanes, polyesters, polyamides and acrylic polymers, are not commercially available simply because of the lack of fluorinated monomers.

Exfluor Research Corporation has been engaged in the development of highly fluorinated functional monomers since more than a decade ago. Our technology is based on direct fluorination in which elemental fluorine is used to replace hydrogen atoms in organic compounds with fluorine atoms. For example, n-nonane is converted to perfluoro-n-nonane by direct fluorination.

Since there are unlimited source of hydrocarbons, direct fluorination enables one to synthesize many new fluorinated compounds. One of our most significant achievements was the development of a process that made it possible to synthesize a wide variety of perfluoroesters.  For example, pentyl pentanoate can be converted to the corresponding perfluoroester ( I ).

One mole of the perfluoroester ( I ) can be hydrolyzed to give two moles of perfluoropentanoic acid.

Similarly, one mole of ( I ) can be reacted with methanol to give two moles of methyl perfluoro-pentanoate.

Perfluoropentanoic acid can be treated with F₂/Br₂ to give Perfluoropentyl bromide¹⁾.

Methyl perfluoropentanoate can be reduced to give perfluoropentyl methanol.

Table-1 shows our products that are currently available in commercial quantities.

Synthesis of new diols

We have successfully synthesized various diols as shown above. Those diols have linear structures in that difluoromethylene groups are sandwiched by two hydroxymethyl groups. Inserting more or less difluoromethylene groups can control the total fluorine content.

In this paper, we wish to report our recent development of the synthesis of new fluorinated diols having branching perfluoroalkyl groups. Branching perfluoroalkyl groups extending out from polymer backbone would work as a protective layer to keep the polymer from severe environments. Using larger or smaller perfluoroalkyl groups can control the total fluorine content.

 Method

Branched perfluoroalkyl groups, such as secondary and tertiary perfluoroalkyl moieties, sometimes act as pseudo halogens and therefore are good leaving groups. Because of this, it is very difficult to synthesize perfluoroalkyl methanols, R_f-CH₂OH, that have branching at the carbon atom next to the CH₂OH group (this carbon atom can be referred to as the a -carbon atom). For example, it was observed that methyl perfluoro-2-hexyl decanoate which has branching at the a -carbon atom did not yield the corresponding perfluoroalkyl methanol when the ester was reduced under standard conditions for the reduction of linear perfluoroesters, such as by treatment of the ester with lithium aluminum hydride or sodium borohydride. The products of attempted conventional reduction reaction were quite complicated. It is believed that when there is a branch site at the carbon atom next to the carbonyl group, the secondary perfluoroalkyl group becomes a better leaving group than the alkoxy group because of the two strong electron withdrawing perfluoroalkyl groups. Thus, the secondary perfluoroalkyl group becomes a leaving group upon attack by hydride ion on the carboxylic ester functionality, producing a perfluoroalkyl anion and a formate ester, as shown in Scheme 1.

Scheme 1

The perfluoroalkyl anion can rapidly decompose into an olefin (Scheme 2), which can react with the reducing reagent or with solvent to give a complicated product mixture.

Scheme 2

On the other hand, when perfluoro (2-hexyldecyl acetate) was reduced with sodium borohydride, the corresponding branched perfluoroalkyl methanol, perfluoro-1H,1H-2-hexyldecanol, was obtained in an excellent yield.

A suggested reaction mechanism is shown in Scheme 3. After the initial attack of hydride ion, a perfluoroalkoxide anion forms. The anion rearranges into a perfluoroacyl fluoride and fluoride ion. When hydride anion attacks the perfluoroacyl fluoride, fluoride ion, instead of the secondary perfluoroalkyl group, leaves the molecule because fluoride ion is a better leaving group than the secondary perfluoroalkyl group. Thus, an a -branched perfluoroalkyl methanol was produced successfully by reduction of a perfluoroester

It was found that the reduction of perfluoroesters generally gave no complicated products but the expected perfluoroalkyl methanols in good to excellent yield under simple conditions. Since perfluoroalkoxy groups are generally better leaving groups than perfluoroalkyl groups, the reduction reaction proceeded smoothly to yield perfluoroalkyl methanols or perfluoroalkylene dimethanols even when there was a branch site at the carbon atom next to the carbonyl group.

An exemplary reaction scheme for the reduction of a perfluoroalkyl ester of a perfluoroalkyl carboxylic acid is shown in scheme 4.

Without wishing to be bound by theory, it is believed that initial hydride attack at the carbonyl group of (i) (in which R_f and R_f’ are both perfluoroalkyl moieties, which can be the same or different) initially yields a perfluoroalkanal (aldehyde) (ii) and a perfluoroalkoxide ion (iii). The perfluoroalkanal (ii) is further reduced to the perfluoroalkyl methanol (iv). Perfluoroalkoxide ion (iii), on the other hand, immediately decomposes into perfluorocarboxylic acid fluoride (v) and fluoride ion. Since the fluoride group is a very good leaving group, as mentioned earlier, the perfluorocarboxylic acid fluoride (v) can be reduced {via perfluoroaldehyde (vi)} to another perfluoroalkyl methanol (vii) even if the group R_f’ is a secondary or tertiary perfluoroalkyl group.

Synthesis of 1,3-propanediols

1,3-Propanediols having branching perfluoroalkyl groups were synthesized from alkyl-substituted diethyl malonates. An alkyl-substituted diethyl malonate was fluorinated to give the corresponding perfluoroester ( II ). The perfluoroester ( II ) was then directly reduced with sodium borohydride to give 2-fluoro-2-perfluoroalkyl-1,3-propanediol.

1,3-Propanediols having short chain, long chain or branched chain perfluoroalkyl groups were synthesized.

Synthesis of 1,4-butanediols

Perfluoroalkyl-substituted 1,4-butanediols were synthesized from commercially available gamma lactones. For example, undecanoic g -lactone was fluorinated to give the corresponding perfluorolactone ( III ). Reduction of ( III ) gave perfluoro-1H,1H,4H-undecane-1,4-diol.

Perfluoroalkyl-substituted 1,5-pentanediols were synthesized most conveniently from commercially available delta lactones. For example, dodecanoic d -lactone was fluorinated to give the corresponding perfluorolactone ( IV ). Reduction of ( IV ) with sodium borohydride gave perfluoro-1H,1H,5H-dodecane-1,5-diol.

Summary

A wide variety of new perfluorodiesters having branching perfluoroalkyl groups were synthesized by direct fluorination. It was found that the reduction of those perfluorodiesters proceeded smoothly to give the corresponding diols in good yields even when there was a branch site at the carbon next to the carbonyl group. The unique branching structure is expected to provide a protective layer to keep polymer backbone from severe environments.

Acknowledgement

This work was partially funded by the U.S. Air Force.

References

1) "Method of Producing Perfluorocarbon Halides" U.S. Patent No. 5,455,373.