Transannular additions of selectfluor and xenon difluoride: Regioselectivity and mechanism

Article abstract of DOI:10.1002/poc.1770

The addition of 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis-tetrafluoroborate (F-TEDA) to unsaturated systems was modelled computationally at the ab initio levels and studied experimentally. The reaction of olefins with F-TEDA is driven exclusively by charge transfer and displays the antibonding orbital picture in the transition structure for F-transfer, similarly to that for the reactions of olefins with F-radical. In contrast, the electrophilic and concerted fluorinations, respectively with H ₂O???F⁺ complex and with F₂, show strong bonding interactions between the fluorine and olefin moieties in the transition structures. The reaction with F-TEDA involves an initial formation of highly delocalized charge-transfer complexes in the first step with further low-barrier (ca 4 kcal) migration of fluorine and is best described as an inner-sphere electron transfer. This nonelectrophilic mechanism is operative for the transannular addition of F-TEDA to 3-methylene-7-ethylidenebicyclo[3.3.1] nonane studied experimentally. The addition mode is determined by the formation of a more stable complex via the ethylidene fragment and demonstrates selectivities that differ from conventional electrophilic additions. This mechanistic scenario may be extended to the fluorination with xenon difluoride where similar products are formed in high yields. Copyright

Full text of DOI:10.1002/poc.1770

Research Article
Received: 24 November 2009,
Revised: 5 April 2010,
Accepted: 16 June 2010,
Published online in Wiley Online Library: 24 August 2010
(wileyonlinelibrary.com) DOI 10.1002/poc.1770
Transannular additions of selectfluor and
xenon difluoride: regioselectivity and
mechanism
a
a
a
*
Yurii A. Serguchev , Maxim V. Ponomarenko , Lyudmila F. Lourie
and Andrey A. Fokin
b**
The addition of 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis-tetrafluoroborate (F-TEDA) to unsatu-
rated systems was modelled computationally at the ab initio levels and studied experimentally. The reaction of olefins
with F-TEDA is driven exclusively by charge transfer and displays the antibonding orbital picture in the transition
structure for F-transfer, similarly to that for the reactions of olefins with F-radical. In contrast, the electrophilic and
concerted fluorinations, respectively with H2OꢀꢀꢀFR complex and with F2, show strong bonding interactions between
the fluorine and olefin moieties in the transition structures. The reaction with F-TEDA involves an initial formation of
highly delocalized charge-transfer complexes in the first step with further low-barrier (ca 4 kcal) migration of fluorine
and is best described as an inner-sphere electron transfer. This nonelectrophilic mechanism is operative for the
transannular addition of F-TEDA to 3-methylene-7-ethylidenebicyclo[3.3.1]nonane studied experimentally. The
addition mode is determined by the formation of a more stable complex via the ethylidene fragment and
demonstrates selectivities that differ from conventional electrophilic additions. This mechanistic scenario may be
extended to the fluorination with xenon difluoride where similar products are formed in high yields. Copyright ß
2010 John Wiley & Sons, Ltd.
Keywords: ab initio; charge transfer fluorination; mechanism
ethylenes,[12] and later for electrophilic aromatic substitu-
INTRODUCTION
tions.[13,14] Authors suggested that the intermediate formation
of coloured charge transfer complexes in these reactions result
from a single-electron transfer from the substrates to the N–F
agents. The SET mechanism also was suggested for fluorination
of phenyl-substituted olefins with N-fluorobis[(trifluoromethyl)
sulfonyl]imide,[15] since this reaction is inhibited by p-dinitro-
benzene as an effective electron-transfer quencher. In general,
F-TEDA is a good oxidizer since it reacts with 2,2,6,6-tetramethyl-
piperidine N-oxide similarly to other oxidants (Cl2, Br2, XeF2, etc.)
through a single-electron transfer.[16,17] However, the reduction
potential of F-TEDA is difficult to measure or to compute.[18]
Some basic evidence for the SN2-mechanism using vinyl
esters as radical traps has also been found.[16,19,20] The absence of
rearranged products in the reactions of these substrates with F-TEDA
and other N–F reagents allowed to exclude the intermediacy of any
The substitution of hydrogen with fluorine dramatically alters
the physical, chemical and biological properties of organic
compounds.[1–4] Consequently, the development of mild and
selective synthetic methods for incorporation of fluorine into
organic molecules, as well as the elucidation of the fluorination
mechanisms are of great importance. Traditional electrophilic
fluorinations with highly toxic F2, XeF2, CF3OF, RCOOF, FClO3
proceed with low selectivities that reduce their preparative
interest and make mechanistic interpretations rather difficult.[5]
Currently, the mild electrophilic N–F reagents with general struc-
ture R2N–F or R3Nþ–F that are easy to handle, increasingly apply
to selective fluorination of organic compounds.[6–11] Among
many electrophilic N–F reagents, the commercially available 1-
chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis-tetra-
fluoroborate[6–11] (F-TEDA-BF4, hereafter F-TEDA or Selectfluor1)
is the most popular.
The mechanism of fluorinations with N–F reagents has been
a subject of debates since their discovery. Namely the single-
electron transfer (SET) and the nucleophilic SN2-substitution on
fluorine atom (Scheme 1) were suggested for the fluorination of
unsaturated compounds. These two mechanisms are difficult to
distinguish experimentally since both lead to the same fluori-
nated carbocationic intermediate (A), which gives the products
after its reaction with a nucleophile.
The key mechanistic studies on the addition reactions of the
N–F reagents attempted to identify the radical or radical ionic
intermediates in accord with a SET mechanism[6–15] first eviden-
ced for the reaction of N-fluoropyridinium salts with alkylated
*
Correspondence to: Y. A. Serguchev, Institute of Organic Chemistry, National
Academy of Sciences of Ukraine, 5 Murmanskaya Str., 02094 Kiev, Ukraine.
E-mail: serguch@gmail.com
** Correspondence to: A. A. Fokin, Department of Organic Chemistry, Kiev
Polytechnic Institute, pr. Pobedy 37, 03056 Kiev, Ukraine.
E-mail: aaf@xtf.ntu-kpi.kiev.ua
a
Y. A. Serguchev, M. V. Ponomarenko, L. F. Lourie
Institute of Organic Chemistry, National Academy of Sciences of Ukraine, 5
Murmanskaya Str., 02094 Kiev, Ukraine
b
A. A. Fokin
Department of Organic Chemistry, Kiev Polytechnic Institute, pr. Pobedy 37,
03056 Kiev, Ukraine
J. Phys. Org. Chem. 2011, 24 407–413
Copyright ß 2010 John Wiley & Sons, Ltd.

Y. A. SERGUCHEV ET AL.
is well documented.[23–28]. Herein we report the experimental
results on the transannular cyclization of bicyclo[3.3.1]nonane
dienes with F-TEDA and xenon difluoride which are associated
to ab initio computations on model reactions of olefins with
F-TEDA. We have recently described the reaction of dienes
with F-TEDA,[30–32] involving the formation of a number of
compounds with potential antiviral, and other types of biological
activities.[33–38]
Scheme 1. Mechanistic pathways for fluorination of olefins with N–F
reagents
RESULTS AND DISCUSSION
We first verified the single-electron transfer oxidative properties
of F-TEDA by computing the vertical and adiabatic reduction
potentials of the dication. Because many of our recent[39,40] and
other[41–44] data questioned the reliability of the DFT methods we
have chosen the MP2 method with correlation-consistent basis
set cc-pVDZ,[45] as well as the standard 6-31G(d) basis set for
larger systems. The MP2/cc-pVDZ optimized geometry of F-TEDA
dication (MIN1, Fig. 1) agrees well with the X-ray crystal structure
geometry.[46] The adiabatic SET reduction of the F-TEDA dication
leads to the radical cation (MIN2) and accompanied by subs-
tantial geometrical changes. Placing an electron on the LUMO,
which displays mostly N–F antibonding, elongates this bond
radical species. As a possible explanation it was suggested[21] that
if the SET mechanism with F-TEDA is operative, the intermediate
radical cation would immediately react with fluorine radical and,
hence, the use of radical traps led to inconclusive results. Very
clear evidence for the SET mechanism was obtained for the
fluorination of triphenylethylene and tetraphenylethylene with
F-TEDA from the ESI-MS and ESI-MS/MS spectra data, in which
radical cationic intermediates were observed.[22] It was assu-
med[21,22] that for many reactions the difference between the two
mechanisms is probably smaller than it appears, and the limits of
current methods of studying extremely fast reactions inhibit the
definitive elucidation of the reaction mechanism. The dienes of
bicyclo[3.3.1]nonane series are a good choice for mechanistic
studies since their radical, electrophilic and oxidative chemistry
˚
dramatically (1.390 vs. 1.968 A). Such substantial geometrical
changes are responsible for an unusually high divergence
between the adiabatic (DE ¼ ꢁ226.2 kcal/mol) and vertical
˚
Figure 1. The MP2/cc-pVDZ structures (bond distances in A, X-ray experimental values in parentheses) of the F-TEDA dication (MIN1), radical cation
(MIN2), and the frontier orbitals of F-TEDA dication (energies in au)
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J. Phys. Org. Chem. 2011, 24 407–413

TRANSANNULAR ADDITIONS OF SELECTFLUOR AND XENON DIFLUORIDE
(DE ¼ ꢁ152.0 kcal/mol) reduction potentials of F-TEDA. Notably,
the F-TEDA dication is only a moderate SET oxidant but
theoretically is able to oxidize unsaturated systems even via
an outer-transfer mechanism.
In order to gain information about the mechanism of F-TEDA
interaction with olefins we modelled its reaction with isobutene,
which is characterized by the moderate ionization potential of
212kcal/mol.[47] The reaction starts with an exergonic (ꢁ21.7 kcal/
mol, MP2/cc-pVDZ) formation of the complex (MIN3) with subs-
tantial charge transfer (ca 0.6 e) from the olefin to F-TEDA (Fig. 2).
As a result the population of the antibonding LUMO of F-TEDA
TS3 (Fig. 2). Thus, the fluorinations of olefins with F-TEDA closely
resemble the reactions with the fluorine radical. We recomputed
TS3 with a spin-broken wavefunction and indeed found a subs-
tantial contribution from higher spin states (hS2i ¼ 0.65 before
and 0.1 after the spin projection). At the same time, a relatively
low barrier for the F-transfer through TS3 indicates that the
overall reaction is indeed controlled by the exergonic formation
of the charge transfer complex MIN3 and thus can be classified as
‘inner-sphere’ ET.[52] The low barrier for the fluorine atom transfer
is in accord with only little geometrical changes in TS3 relative to
MIN3. We imply that the slow formation of the charge transfer
complex is accompanied by the fast F-atom transfer (Scheme 2).
The formation of charge transfer complexes with N–F reagents
has been previously suggested based on the kinetic isotope
effects of fluorinations of aromatic compounds.[53,54] However
the authors were not able to distinguish between the SET
and SEAr mechanisms. From other data it is not clear yet[53–57]
whether formation of or recombination of Arꢀþ and Fꢀ radicals
represent the more energetically favourable pathway for the for-
mation of the Wheland-like intermediates. Although some other
data support[58–60] or suggest[12,61,62] the formation of such inter-
mediates, there is still no experimental evidence for their
formation in the reaction of N–F reagents with olefins.
˚
increases and the N–F distance elongates substantially (1.804 A in
˚
MIN3 vs. 1.390 A in the F-TEDA dication). Only a small barrier of
4.2 kcal/mol is required for fluorine transfer to the olefin through
TS3. The nature of MIN3 and TS3, gives a key mechanistic infor-
mation about the reaction mechanism. Remarkably, there are
only antibonding interactions between the reactants in this
transition structure for fluorine transfer (Fig. 2) and formation of
TS3 is driven exclusively by the charge transfer from olefin to
F-TEDA. The charge transfer value is only slightly higher in TS3
than that in MIN3.
For comparison we computed the transition structures for
the electrophilic, radical[48,49] and concerted[50,51] fluorinations
with respect to the reactions of isobutene with the complex of
‘fluorine cation’ with water (TS1), with F2 (TS2), as well as with the
fluorine radical (TS4). The HOMOs for these transition structures
are shown in Fig. 3, clearly representing the bonding between
the fluorine and olefin moieties in the TS1 and TS2, in contrast to
the antibonding interactions for the fluorine radical addition
(TS4). The latter comprises orbital distributions similar to that in
Our computations indicate that the charge transfer complexes
with F-TEDA are very tight, characterized by strong charge
transfer and thus, can be classified as s-complexes. Their
formation must be slow relative to further fast and almost
barrierless fluorine atom transfer. Accordingly, the low concen-
tration of such complexes makes their experimental identification
rather difficult. As a result, substantial charge transfer in the
Figure 2. The key stationary points for the reaction of F-TEDA dication with isobutylene and the frontier orbitals in the transition structure (TS3) for
˚
fluorine transfer (MP2/cc-pVDZ, selected bond distances in A)
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Y. A. SERGUCHEV ET AL.
Figure 3. The highest occupied orbitals in the transition structures for the electrophilic (TS1), concerted (TS2) and radical (TS4) additions of model
fluorine-containing reagents to isobutene (MP2/cc-pVDZ)
limiting step followed by fast F-transfer in the reactions of olefins
with N–F reagents probably hampers the experimental obser-
vation of radical cationic or radical intermediates.
isms. The regioselectivities of these reactions are clearly different
for that observed for F-TEDA. The formation of ‘anomalous’
fluorinated adamantanes (3a,b and 4a,b) excludes the elect-
rophilic schemes and supports strongly the non-electrophilic
mechanism of fluorocyclization of bicyclo[3.3.1]nonane dienes
with F-TEDA. We modelled two different modes of attack of
the reagent on 3-methylene-7-ethylidenebicyclo[3.3.1]nonane
(1a). Diene 1a exothermally (ca 50 kcal/mol) forms two isomeric
complexes with F-TEDA (MIN5 and MIN6) (Fig. 4). The complex
via the more substituted ethylidene (MIN6) fragment is 2.3 kcal/
mol more stable than MIN5. The transition structures for the
fluorine atom transfer from F-TEDA dication (TS5 and TS6) are of
similar nature as TS3 and are also characterized by a substantial
charge transfer value (0.7 e). Thus, the higher donor ability of
ethylidene moiety determines the mode of addition of F-TEDA
that agrees with the experiment where products 3a or 4a domi-
nate (Scheme 4).
Further F-atom transfers in the charge transfer complexes
through TS5 and TS6 are characterized by low barriers (2–4 kcal/
mol) as in the case of isobutene. The main difference is that these
TSs describe not only the migration of fluorine atom to the olefin
moiety, but the transannular cyclization with formation of the
adamantane cage as well. Structurally these TSs are close to the
transition structures for the transannular cyclizations of 3,7-di-
methylenebicyclo[3.3.1]nonane with radical reagents suggested
previously.[64]
This is in marked contrast to the SET[12–15,22,53–62] and
[16,19–21]
SN
mechanistic scenarios that require either a complete
electron transfer in the entrance complex (which is much less in
our case) or slow fluorine transfer (which we found to have a low
barrier, TS3).
We then employed the transannulated diolefins in the bicy-
clo[3.3.1]nonane series to distinct between the electrophilic and
nonelectrophilic addition modes in the reaction with F-TEDA. It
has been shown previously that bicyclo[3.3.1]nonane dienes 1a
and 1b substituted in one of exocyclic methylene groups, react
regioselectively with electrophilic reagents such as iodine,[23,24]
bromine,[25,26] acids[27] and halosuccinimides[30,63] with the for-
mation of adamantane derivatives with a phenyl or methyl group
in the bridge of the adamantane framework (Scheme 3). The
reactions in oxidizing media proceed similarly.[28,29]
In contrast to the electrophilic reactions, the fluorocyclization
of dienes 1a,b with F-TEDA occurs with formation of fluorinated
adamantanes with methyl- (3a, 4a) and phenyl- (3b, 4b) subs-
tituents in a side chain[30–32] (Scheme 4).
This dramatic distinction in regioselectivity of the transannular
cyclization of bicyclo[3.3.1]nonane dienes provides evidence for
different mechanisms of these reactions. The fluorination of die-
nes 1a,b with F-TEDA proceeds via an intermediate carbocation B
as it was suggested earlier.[30] Products 5a,b were determined
only as admixtures from the NMR data.
In accordance with previous mechanistic[23–25] studies the
transannular cyclizations of bicyclo[3.3.1]nonane dienes with
electron-deficient reagents proceed via AdE1 or AdE3 mechan-
Thus, we can conclude that the reactions of olefins or dienes
occur via the initial formation of highly delocalized charge trans-
fer complexes, where more than 50% of the electron charge is
transferred from the olefin to F-TEDA. Although such complexa-
tion is highly exothermic (ca 50 kcal/mol) it is likely that it deter-
Scheme 2. Mechanism of the fluorination with F-TEDA via the inner-
sphere electron transfer
Scheme 3. Reactions dienes 1a,b with electrophilic reagents
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TRANSANNULAR ADDITIONS OF SELECTFLUOR AND XENON DIFLUORIDE
Scheme 4. Reactions dienes 1a,b with F-TEDA
*
Figure 4. The MP2/6-31G optimized geometries of the charge-transfer complexes (MIN5 and MIN6) and transition structures (TS5 and TS6) for the
˚
transannular cyclization of 3-methylene-7-ethylidenebicyclo[3.3.1]nonane with F-TEDA dication (selected distances in A, relative energies in kcal/mol)
J. Phys. Org. Chem. 2011, 24 407–413
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Y. A. SERGUCHEV ET AL.
Scheme 5. Reactions dienes 1a,b,c with XeF2
mines the reaction rates. Further migration of fluorine to the
olefin is a low-barrier process and is best viewed as a radical
addition of a fluorine atom to the double bond.
bicyclo[3.3.1]nonane dienes with F-TEDA, and give additional
evidence for the two-fold one-electron process. The com-
putations show that the reaction of 3-methylene-7-ethylidene-
bicyclo[3.3.1]nonane with F-TEDA is determined by the formation
of a more stable s-complex through the ethylidene fragment and
agree well with the regioselectivities observed experimentally.
We also suggest that this mechanistic scenario may be extended
to the fluorination with xenon difluoride, where a similar addition
mode was indicated experimentally.
This mechanistic scenario may likely be extended to the fluori-
nations with XeF2. The SET and polar electrophilic mechanisms
have been postulated for reactions of XeF2 with unsaturated
compounds (trimethylsilyl enol ethers,[65] phenylsubstituted
ethylenes[66,67] and norbornene[68]), but both mechanisms
lead to the same products and no distinction could be made.
Because the reactions with XeF2 is difficult to compute, we tested
the experimental regioselectivities of transannular cyclization of
bicyclo[3.3.1]nonane dienes with this reagent. The reactions were
carried out in a quartz vessel in dry CH2Cl2, because the glass
surface and presence of moisture causes rapid decomposition of
EXPERIMENTAL AND COMPUTATIONAL
DETAILS
The 1H (200.13 MHz), and 19F (188.31 MHz) NMR spectra were
recorded on a Bruker DPX-200 spectrometer using CDCl3 as
solvent and TMS or CCl3F as internal standards. All computations
were performed with the GAUSSIAN03 program suite[69] utilizing
analytical first and second energy derivatives. Harmonic vibrat-
ional frequencies were computed to ascertain the nature of all
stationary points (NIMAG ¼ 0 for minima and 1 for the transition
structures). Reaction pathways along both directions from the
transition structures were followed by the IRC method.
XeF2 into more electrophilic species FXeþFꢁ or FXedþ. . .FHFdꢁ[65]
.
As a result the products 4a,b are formed in high yields with only
traces of electrophilic addition products 5a,b were detected
(Scheme 5).
The fact that the products were formed under the kinetic
control was confirmed by the prolonged exposure of the reaction
mixtures of 4a–5a that does not change the ratio of the products.
No change was observed also when individual products 4a–5a
and 4b-5b were exposed with XeF2 up to 80 8C. Taking into
account previous mechanistic considerations[65] we could sup-
pose that compounds 4a–c also form via the inner-sphere ET
mechanism with non-ionized XeF2.
General procedure for fluorination of dienes 1a,b,c with XeF2
To a solution of XeF2 (1.12 mmol) in CH2Cl2 (10 mL) in quartz flask
at ꢁ20 8C kept under inert atmosphere (argon, nitrogen) was
added dropwise a solution of 1a or 1b (0.93 mmol) in CH2Cl2
(5 mL). The mixture was stirred for 6 h at 0 8C, and then washed
with a 10% aqueous solution of NaHCO3 (4 ꢂ 5 mL). The organic
layer was dried over Na2SO4 and the solvent was removed under
reduced pressure. Column chromatography on silica gel (hexane)
gave 4 {a 0.143 g (77%), b 0.117 g (48%), c 0.126 g (73%)}.
Compounds 3a,b and 4a,b,c have been described by us
previously.[32] Compounds 5a,b were indicated in the corres-
ponding reactions mixtures by 1H and 19F NMR based upon the
characteristic signals. For 5a: 1H NMR (CDCl3): d ¼ 4.02 (dd, 1H, AB,
J ¼ 48.0, 9.0 Hz), 4.13 (dd, 1H, AB, J ¼ 48.0, 9.0 Hz) ppm; 19F NMR
(CDCl3): d ¼ –233.7 (t, J ¼ 48.0 Hz), ꢁ141.9 (s) ppm. For 5b: 1H NMR
(CDCl3): d ¼ 3.81 (dd, 1H, AB, J ¼ 48.0, 9.3 Hz), 3.85 (dd, 1H, AB,
J ¼ 48.0, 9.3 Hz) ppm; 19F NMR (CDCl3): d ¼ ꢁ233.3 (t, J ¼ 48.0 Hz),
ꢁ139.3 (s) ppm.
CONCLUSIONS
We have indicated the two-step mechanism in the reaction of
F-TEDA with olefins, where the rate-determining step involves
exothermic formation of highly delocalized charge transfer comp-
lexes, in which ca 50% of the electron charge is transferred to
the reagent. Further migration of fluorine to olefin is a low-barrier
process and is best viewed as a radical addition of a fluorine
atom to the double bond. The overall reaction mechanism can be
classified as an inner-sphere electron transfer. This explains why
for most of reactions of F-TEDA and other N–F reagents with
olefins the experimental observation of radical-cationic or radical
intermediates was hampered. This inner-sphere ET mechanism
eliminates the conceptual contradictions between the SET and
SN2 mechanisms, previously noticed for the reactions of N–F
reagents with unsaturated systems.
The dramatic distinction in the regioselectivities of transan-
nular cyclizations of unsymmetric bicyclo[3.3.1]nonane dienes
with F-TEDA and with conventional electrophilic reagents shows
evidence for different mechanisms of these reactions. These
results exclude the two-electron nucleophilic SN2-substitution of
fluorine or polar AdE-mechanisms for the fluorocyclization of
Acknowledgements
This work was supported in part by the Ukrainian State Fund
for Fundamental Research. AAF is grateful to the University of
Giessen (Germany) and the Institute of Applied System Analysis
(Kiev, Ukraine) for computing facilities.
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TRANSANNULAR ADDITIONS OF SELECTFLUOR AND XENON DIFLUORIDE
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