Selective Fluorination in Organic and Bioorganic Chemistry

Chapter 4

New Oxidants Containing the O - F Moiety and Some of Their Uses in Organic Chemistry Shlomo Rozen

Downloaded by CORNELL UNIV on August 23, 2016 | http://pubs.acs.org Publication Date: February 22, 1991 | doi: 10.1021/bk-1991-0456.ch004

School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel

The synthesis and chemistry of compounds containing the O-F moiety not attached to perfluorinated alkyl group are described. In general such compounds are less stable than their perfluoroalkyl counterparts. With the exception of AcOF, they should possess at least one electron withdrawing group α to the O-F moiety. It was found that the bigger the alkyl chain of these derivatives the less stable is the compound itself. HOF is also a member of this group and when stabilized by acetonitrile it can perform efficient epoxidation, as well as tertiary hydroxylation on remote and deactivated sites. It also provides an easy way for incorporating various oxygen isotopes such as 18O into organic molecules. The chemistry of the fluoroxy compounds began with Cady's synthesis of CF3OF more than forty years agofij. His main idea, of reacting fluorine with fluoro-phosgene in the presence of a dry fluoride serving as a catalyst, was subsequently used by Prager and Thompson and many others who developed several useful variations.(2- 4) In the 50's Cady opened another area of fluoroxy chemistry by passing fluorine through a hot mixture of trifluoroacetic acid and water, obtaining trifluoroacetyl hypofluorite CF3COOF (figure 1).

Π F /F" R-C-F 2

RCF OF 2

CF3COOH + H 0 + F 2

2

*~ C F 3 C O O F

Figure 1. Formation of trifluoroacetyl hypofluorite 0097-6156/91/0456-0056$06.00/0 © 1991 American Chemical Society

Welch; Selective Fluorination in Organic and Bioorganic Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

4.

ROZEN

Oxidants Containing the Ο—F Moiety

57

Apart from the synthesis itself and some studies of physical properties, these compounds did not find much use in organic chemistry for almost 20 years. It was Barton in the late 60's who realized the synthetic potential of C F 3 O F (5,6) and once the direction was set, a flood of works appeared in the literature using this commercial electrophilic fluorinating reagent (7- 9). Not offered commercially, C F 3 C O O F did not gain as much popularity until the early 80 s when we modified its synthesis and showed that it can be an useful reagent in organic chemistry as for example in the preparation of fluorohydrins or a-fluoroketones (10,11) -(figure 2). f


CF3COOF

RCH=CHR'

•

RCH-CHR'

I

I

— R C H - C H R '

I

OCOCF3

Il

Ο

£

I

OH

Ο

OR" C F 3 C O O F

II

RCH —C-R'

•

2

RCH=c' R'

Π

RCHF-C-R'

X

Figure 2. Use of trifluoroacetyl hypofluorite as a reagent Later we developed higher homologs in the series of RfCOOF using the same synthetic pathway and used them as unique initiators for polymerization of perfluoro olefins in a way that no undesirable "end groups" were formed (12) (figure 3). F C F COOK 8

1 7

CF =CF

2

2

•

C F COOF 8

2

•

1 7

- C 0

F-(CF -CF ) -F 2

2

rt

2

Figure 3. Use of higher homologs as initiators Acyl Hypofluorites -

RHCOOF

Until recently the carbon skeletons to which the O-F moiety was attached contained no hydrogens and practically always consisted of a perfluoro alkyl group. It was assumed that a hydrogen would trigger an immediate decomposition through an easy HF elimination. A few years ago we showed that there are some exceptions to this "well known fact". We prepared, for the first time, acetyl hypofluorite - AcOF - by reacting sodium acetate with


58

SELECTIVE FLUORINATION IN ORGANIC AND BIOORGANIC CHEMISTRY

fluorine (13) and used it for synthetic purposes without any isolation or purification (figure 4). CH COONa 3

p

2

CH3COOF

o r

C H 3 C O O H + NaF


Figure 4. Use of acetyl hypofluorite for synthetic purposes Since its discovery, AcOF has been widely used for fluorination purposes(74-i7,) including positron emitting tomography (18-19) and general organic synthesis leading to fluorine free derivatives (20). It should be noted that at the beginning not everyone was fully convinced that the reagent consisted of a single molecule until we (21), but mainly Appelman (22), unequivocally proved its existence. For several years acetyl hypofluorite remained the only fluoroxy compound containing hydrogens in its alkyl group. Recently we decided to broaden the field by attempting to make additional acyl hypofluorites of this type which might be useful in organic chemistry (23). The first natural choice was to perform a reaction between a cold suspension of sodium propionate and F2 with or without water, HF and propionic acid. The reason for the addition of the propionic acid and water is that adding acetic acid to AcONa greatly enhances the efficiency of the AcOF production. The corresponding acid and the water have also an essential role in directing the reaction toward hypofluorite formation rather than fluorooxy compounds (see discussion in ref. 10,11). However in each case only traces of oxidizing material were obtained and the only isolable compound was NaF formed quantitatively. It seems that at the reaction temperature of -75 ° C the hypofluorite, if formed, is quite unstable and indeed H F elimination takes place quite easily. It was assumed at the beginning that there are many conformations where either the two oc or the three β hydrogens can be very close to the oxygen bound fluorine prompting the elimination. Replacing these hydrogens by methyl groups did not much change the outcome. Sodium pivalate or r-butyl acetate produced again only NaF and very volatile components when reacted with fluorine as did also sodium undecanoate. Trying to avoid too many hydrogens in sodium propionate we replaced die two in the oc position with chlorine atoms. This time the reaction with fluorine at -75 ° C resulted in a stable oxidizing solution. The low temperature 19p NMR spectrum revealed a sharp singlet at +134 ppm, a characteristic signal for the COOF fluorine nucleus. Also this oxidizer was reacted with the enol acetate of a-tetralone to give, in 85% yield, the


4. ROZEN

59

Oxidants Containing the O-F Moiety

expected a-fluorotetralone, a very characteristic product of electrophilic fluorination (figure 5). OAc

CO 3

®5f

CH CCl COOF

CH CCI COONa

3

2

2


Figure J. Formation of a-fluorotetralone by electrophilic fluorination The above result indicated that the (3 hydrogen atoms do not prevent the formation of the O-F bond, but the question remains whether such hypofluorites can be stable in the presence of a hydrogen atom at the α position. The reaction of sodium 2,3-dichloropropionate and sodium 2chloropropionate with F2, produced oxidizing solutions, stable at -75 ° C with physical and chemical properties very similar to those described above, (figure 6). It should be noted here that a single chlorine atom at the p position does not contribute much to the stability of the hypofluorite as is concluded from the case of sodium 3-chloro-propionate which did not produce any oxidizing material when treated with F2.

CH XCHClCOOF

CH XCHClCOONa

2

2

enol

X = CI, Η

acetate

Selective Fluorination in Organic and Bioorganic Chemistry

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