Thermal decomposition of alkenyliodonium tetrafluoroborates: A novel route to fluoroalkenes

Article abstract of DOI:10.1016/S0040-4039(00)00788-7

Alkenyl(phenyl)iodonium tetrafluoroborates dissolved in chloroform, or in the solid state, decompose thermally at 60°C to yield fluoroalkenes and iodobenzene as major products via an S(N)1- or S(N)2-type reaction within the ion pair of the substrates. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00788-7

Tetrahedron Letters 41 (2000) 5125±5129
Thermal decomposition of alkenyliodonium
tetra¯uoroborates: a novel route to ¯uoroalkenes
Tadashi Okuyama,a,* Morifumi Fujita,a Roel Gronheidb and Gerrit Lodderb
aFaculty of Science, Himeji Institute of Technology, Kamigori, Hyogo 678-1297, Japan
bLeiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, PO Box 9502, 2300 RA Leiden,
The Netherlands
Received 17 April 2000; accepted 1 May 2000
Abstract
Alkenyl(phenyl)iodonium tetra¯uoroborates dissolved in chloroform, or in the solid state, decompose
thermally at 60ꢀC to yield ¯uoroalkenes and iodobenzene as major products via an SN1- or SN2-type
reaction within the ion pair of the substrates. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: ¯uorine and compounds; pyrolysis; hypervalent elements; carbenium ions.
Alkenyliodonium salts undergo a variety of reactions owing to the high nucleofugality of the
iodonio group.1,2 The reactions include unimolecular as well as bimolecular nucleophilic sub-
stitutions and eliminations.1b The unimolecular reactions involve vinylic cations as intermediates
which yield substitution and/or elimination products via trapping by a nucleophile/base2 4 while
the bimolecular (SN2) reactions a€ord inverted nucleophilic substitution products.5 We have
found that the tetra¯uoroborate salts of alkenyl(phenyl)iodonium compounds decompose thermally
to give ¯uoroalkenes in the absence of any added nucleophile or base. Depending on the structure
of the substrates, ¯uoroalkenes of inverted, retained, or scrambled con®guration are produced.
The reaction very likely takes place through an SN1- or SN2-type mechanism with the tetra-
¯uoroborate anion acting as the ¯uoride donor. The reaction provides a novel route to ¯uoro-
alkenes from alkynes or other haloalkenes that can be converted into the iodonium salts via
alkenylsilane, stannane, or boronic acid derivatives.1a,6
The thermolysis of the b-alkyl substituted (E)-1-alkenyl(phenyl)iodonium tetra¯uoroborates
1a±c dissolved in chloroform (or CDCl3) at 60ꢀC for 34±40 h yields 1-¯uoro-1-alkenes (2), 1-alkynes
(3) and iodobenzene as products (Eq. (1)). The yields summarized in Eq. (1) are percentages,
1
determined by GC, and/or relative amounts (in parentheses), determined from H NMR spectra
of the reaction mixtures in CDCl3.
* Corresponding author. Tel: +81-791-58-0169; fax: +81-791-58-0132; e-mail: okuyama@sci.himeji-tech.ac.jp
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00788-7

5126
ꢁ1†
The stereochemistry of the ¯uoroalkenes 2a±c was determined to be Z in all cases on the basis
1
of the trans H-F, vicinal H-F and cis H-H couplings of about 45, 85, and 5 Hz in the H NMR
spectra,7 respectively. No E isomers were detected. Thus, the substitution occurs exclusively by
inversion of con®guration, probably via the in-plane SN2 pathway. For compounds 1a and b
elimination is a minor reaction, but for the t-butyl substrate 1c the major product is alkyne 3c. A
similar trend has been observed for the in-plane SN2 substitution of these iodonium salts by
chloride and bromide ions.5,8
In contrast, the b-phenyl substituted (E)-styryl(phenyl)iodonium tetra¯uoroborate 1d gave (E)-
1-¯uoro-2-phenylethene (2d)9 as well as (E)-1-chloro-2-phenylethene (Eq. (2)). The con-
®gurationally retained ¯uoroalkene 2d must result from an intermediate vinylenebenzenium ion,
similarly as in the reaction of 1d with nucleophilic solvents.3 The chloroalkene product must also
be derived from the benzenium intermediate that attacks a chlorine atom of the solvent chloro-
form. Abstraction of a solvent chlorine atom by unstable carbenium ions was observed during
dediazotization of arenediazonium ions10 and photosolvolysis of iodoalkenes.11 The formation of
the chloroalkene in the thermal decomposition of 1d may be considered as supporting evidence
for the formation of a cationic intermediate. It should be noted that no chloroalkenes were
detected during the thermolysis of 1-alkenyliodonium tetra¯uoroborates 1a±c and that these
compounds a€orded only the inverted ¯uoroalkenes.
ꢁ2†
The thermolysis of (E)- and (Z)-2-phenyl-1-propenyl(phenyl)iodonium tetra¯uoroborates E-1e
and Z-1e in chloroform gave extensively rearranged ¯uoroalkenes and elimination products
together with a small amount of a chlorine containing product (Eqs. (3) and (4)).12 The reaction
of the E isomer E-1e is much faster than that of Z-1e and the product distributions are strikingly
di€erent. Similarly di€erent product distributions have been observed in the solvolysis of E-1e
and Z-1e.14
ꢁ3†

5127
ꢁ4†
The results conform to reaction pathways involving phenyl and methyl participation for E-1e and
Z-1e, respectively, and ensuing 1,2-hydride shift in a vinylic cationic intermediate as illustrated in
Scheme 1. The major product E-4 formed from E-1e is the result of trapping of the unsymmetrical
vinylenebenzenium ion intermediate I1. The unbridged a-methylvinyl cation I2 is also a possible
intermediate which leads to E-4 and Z-4, as well as to the elimination products, 7 and 8, by
deprotonation. Cation I2 further rearranges by a 1,2-hydride shift to the a-phenylvinyl cation I3
which a€ords 5 and phenylpropyne 7. The heterolytic C-I bond cleavage of Z-1e occurs with
methyl participation and also results in the phenyl-stabilized cation I3. Further rearrangement of
I3 is not possible because it is the most stable ion among isomeric vinylic cations.15 The unrearranged
¯uoroalkene 6 must be formed directly from Z-1e via an SN2 reaction. If the primary vinylic
cation I4 were involved, more rearranged products would have been formed.
Scheme 1.
4-t-Butyl-1-cyclohexenyl(phenyl)iodonium tetra¯uoroborate (1f) gave the corresponding ¯uoro-
cyclohexene 9 together with considerable amounts of chlorocyclohexene 10 and o-(cyclohexenyl)-
iodobenzene 11 (Eq. (5)). The products can be accounted for by the reaction of an intermediate
cyclohexenyl cation: its trapping with tetra¯uoroborate a€ords the ¯uorocyclohexene 9, its attack
on a chlorine atom of the solvent leads to the chlorocyclohexene 10, and internal return within its
ion-molecule pair with iodobenzene gives the Friedel±Crafts product 11 of ortho orientation.
Product 11 was also obtained in the solvolysis of 1f.2

5128
ꢁ5†
The thermal reactions of 1b, d and f were also examined in the solid state or in a suspension in
n-hexane. The results are largely the same as those obtained in chloroform except for the formation
of chloroalkenes. On the other hand, the reaction in acetonitrile a€orded essentially no ¯uoro-
alkenes (only 0.5% 2d from 1d and no 2b from 1b). E€ects of added salts are also dramatic:
Addition of 0.1 mol dm^3 of tetrabutylammonium perchlorate completely inhibited formation of
¯uoroalkene in the chloroform reaction, while added tetrabutylammonium tetra¯uoroborate
barely a€ected the yield of ¯uoroalkene. These results strongly suggest that the formation of
¯uoroalkene occurs within the undissociated ion pair of the substrate iodonium salts, and the
dissociated iodonium ion does not undergo the ¯uorination reaction.
The ¯uoride donor of this reaction is the paired tetra¯uoroborate ion and not a ¯uoride ion
generated therefrom, since ¯uoride ion induces exclusively a-elimination of 1-alkenyliodonium
salts owing to its basicity (pKa of HF=3.17).5 The present reaction resembles the Schiemann
reaction of arenediazonium tetra¯uoroborates yielding ¯uoroarenes16 where also the counter
anion has been concluded to be the direct ¯uoride donor.10
Acknowledgements
We are grateful to Professor Masahito Ochiai for his gift of some of the iodonium salts.
References
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5129
12. 1-Phenylpropyne (7)13a and phenylallene (8)13b were compared with authentic samples. The major product from
1
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