Fluorination of fluorene, dibenzofuran and their open analogues with caesium fluoroxysulfate and related fluorinating reagents

Article abstract of DOI:10.1135/cccc20081671

Fluorination of fluorene (1) with caesium fluoroxysulfate (CFS), 2,6-dichloro-l-fluoropyridinium tetrafluoroborate (FP-B800) and 1-fluoro-4-hydroxy-1,4-diazoniabicyclo[2.2.2]-octane bis(tetrafluoroborate) (Accufluor NFTh) occurred only on the aromatic ring in the position ortho and para to the biphenyl central bond with the ratio 2-fluoro- (2a) vs 4-fluorofluorene (2b) 1.7-2.4:1. Regioselectivity of fluorination of both open-chain analogues - diphenylmethane (3a) and biphenyl (3b) was different and more ortho-fluorinated product was formed. Furthermore, the reaction of diphenylmethane (3a) with CFS occurred also on central carbon forming benzophenone (6) and fluorodiphenylmethane (7), while fluorination with FP-B800 and Accufluor NFTh occurred only at the aromatic ring. Similar effect of the structure of fluorinating reagent on the regioselectivity was also observed with dibenzofuran (8) and its open-chain analogues diphenyl ether (10) and biphenyl (3b), where the regioselectivity of fluorination with CFS (1- (9a):2- (9b):3- (9c) = 27:46:27) was similar to fluorination with Selectfluor. Product distribution of fluorination of fluorene (1) and dibenzofuran (8) with CFS is similar to nitration and is in accordance with the calculated HOMO electron density, which indicates the presence of the electron transfer pathway.

Full text of DOI:10.1135/cccc20081671

Fluorination of Fluorene, Dibenzofuran and Their Open Analogues
1671
FLUORINATION OF FLUORENE, DIBENZOFURAN AND THEIR OPEN
ANALOGUES WITH CAESIUM FLUOROXYSULFATE AND RELATED
FLUORINATING REAGENTS
Jernej ISKRAa1,*, Stojan STAVBERa2 and Marko ZUPANa,b
a
Laboratory of Organic and Bioorganic Chemistry, Department of Physical and Organic Chemistry,
“
Jožef Stefan” Institute, Jamova 39, 1000 Ljubljana, Slovenia; e-mail: 1 jernej.iskra@ijs.si,
stojan.stavber@ijs.si
2
b
Faculty of Chemistry and Chemical Technology, University of Ljubljana,
Aškerčeva 5, 1000 Ljubljana, Slovenia; e-mail: marko.zupan@fkkt.uni-lj.si
Received May 5, 2008
Accepted Septem ber 9, 2008
Publish ed on lin e Decem ber 5, 2008
Dedicated to Professor Oldřich Paleta on the occasion of his 70th birthday in recognition of his
outstanding contributions to the area of organofluorine chemistry.
Fluorin ation of fluoren e (1) with caesium fluoroxysulfate (CFS), 2,6-dich loro-1-fluoro-
pyridin ium tetrafluoroborate (FP-B800) an d 1-fluoro-4-h ydroxy-1,4-diazon iabicyclo[2.2.2]-
octan e bis(tetrafluoroborate) (Accufluor NFTh ) occurred on ly on th e arom atic rin g in th e po-
sition ortho an d para to th e biph en yl cen tral bon d with th e ratio 2-fluoro- (2a) vs 4-fluoro-
fluoren e (2b ) 1.7–2.4:1. Regioselectivity of fluorin ation of both open -ch ain an alogues –
diph en ylm eth an e (3a) an d biph en yl (3b) was differen t an d m ore ortho-fluorin ated product
was form ed. Furth erm ore, th e reaction of diph en ylm eth an e (3a) with CFS occurred also on
cen tral carbon form in g ben zoph en on e (6) an d fluorodiph en ylm eth an e (7), wh ile fluorin a-
tion with FP-B800 an d Accufluor NFTh occurred on ly at th e arom atic rin g. Sim ilar effect of
th e structure of fluorin atin g reagen t on th e regioselectivity was also observed with diben zo-
furan (8) an d its open -ch ain an alogues diph en yl eth er (10) an d biph en yl (3b), wh ere th e
regioselectivity of fluorin ation with CFS (1- (9a):2- (9b):3- (9c) = 27:46:27) was sim ilar to
fluorin ation with Selectfluor. Product distribution of fluorin ation of fluoren e (1) an d
diben zofuran (8) with CFS is sim ilar to n itration an d is in accordan ce with th e calculated
HOMO electron den sity, wh ich in dicates th e presen ce of th e electron tran sfer path way.
Keyw ord s: Fluorin ation ; Mech an ism ; Electroph ilic arom atic substitution ; Electron tran sfer;
Caesium fluoroxysulfate.
In troduction of a fluorin e atom in to an organ ic m olecule could brin g sig-
n ifican t ch an ges in its ch em ical an d biological properties th rough special
ph ysicoch em ical properties of th e fluorin e atom an d its bioisosterism with
1
h ydrogen an d h ydroxy group . Th erefore, selective in troduction of fluorin e
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©
2008 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc20081671

1
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Iskra, Stavber, Zupan:
atom is an im portan t an d ch allen gin g task, especially wh en h igh reactivity
of m olecular fluorin e is taken in to con sideration . On e of th e m ost im por-
tan t processes for selective in troduction of fluorin e atom in to organ ic m ole-
cules is “electroph ilic” fluorin ation wh ere h ydrogen atom is replaced by
fluorin e. However, selective fluorin ation with m olecular fluorin e as a basic
fluorin atin g reagen t is difficult to ach ieve due to a large differen ce in C–H
an d C–F bon d en ergy leadin g to exoth erm ic reaction s. Furth erm ore, it is a
dan gerous an d corrosive ch em ical. A very im portan t breakth rough in th is
field was ach ieved by substitution of m olecular fluorin e for less reactive
an d easier-to-h an dle electroph ilic fluorin atin g reagen ts, grouped in to th ree
1
e,2,3
m ajor classes: xen on fluorides, fluoroxy reagen ts an d N–F reagen ts
. Th e
fluoroxy reagen ts – perch loryl fluoride (FClO ), acyl an d alkyl h ypofluorites
3
(
AcOF, MeOF) an d m etal fluoroxysulfate (CsSO F) – are stron g fluorin atin g
4
4
,5
reagen ts an d oxidan ts . Caesium fluoroxysulfate was first prepared by th e
group of Appelm an . Alth ough it was sh own to be a powerful reagen t for
6
fluorin ation of arom atic m olecules, little in form ation is available on its
7
m ech an ism of action .
Valuable in form ation about th e role of th e structure of fluorin atin g re-
agen t on th e fluorin ation process could be obtain ed by studyin g m odel
m olecules un der com parable reaction con dition s. In terestin g m odel com -
poun ds are diben zofuran , fluoren e an d th eir open -ch ain an alogues –
biph en yl, diph en ylm eth an e an d diph en yl eth er th at were used m an y tim es
in studies of th e effect of geom etry, electron egativity, con jugation an d
8
strain on th e electroph ilic arom atic substitution . Furth erm ore, com parison
of th e results of fluorin ation of th ese substrates with th ose of oth er
electroph ilic substitution reaction s would brin g furth er in sigh t in to th e
m ech an ism of fluorin ation of arom atic m olecules with F–L reagen ts (wh ere
L represen ts ligan d part of reagen t). Studies on th e fluorin ation of fluoren e,
diben zofuran an d th eir open an alogues with Selectfluor (F-TEDA, 1-ch loro-
m eth yl-4-fluoro-1,4-diazon iabicyclo[2.2.2]octan e bis(tetrafluoroborate))
an d xen on difluoride h ave already provided an in terestin g in sigh t in to th e
m ech an ism of fluorin ation of arom atics in th e ligh t of th e dich otom y be-
9
tween th e polar an d electron tran sfer m ech an ism s .
In th is article, we presen t a study of th e effect of th e fluorin atin g reagen t
structure on th e fluorin ation of fluoren e, diben zofuran an d th eir open -
ch ain an alogues diph en ylm eth an e, biph en yl an d diph en yl eth er with th e
em ph asis on th e fluorin ation with caesium fluoroxysulfate (CFS, CsSO F),
4
N-fluoropyridin ium salt (FP-B800, 2,6-dich loro-1-fluoropyridin ium tetra-
fluoroborate) an d Accufluor NFTh (1-fluoro-4-h ydroxy-1,4-diazon iabicyclo-
[
2.2.2]octan e bis(tetrafluoroborate)), wh ile com parison with literature data
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 12, pp. 1671–1680

Fluorination of Fluorene, Dibenzofuran and Their Open Analogues
1673
about fluorin ation with Selectfluor an d xen on difluoride an d oth er
electroph ilic processes (n itration , brom in ation etc.) will also be presen ted
(
Sch em e 1).
2
–
4
–
2,
–
4
4
2
2
SCHEME 1
RESULTS
Fluorin ation of fluoren e (1) with caesium fluoroxysulfate (CsSO F, CFS) at
4
3
0 °C in aceton itrile yielded 15% of 2-fluoro- (2a) an d 4-fluorofluoren e (2b)
in a 71:29 ratio (Table I, en try 1). No fun ction alization of th e m eth ylen e
group of 1 was observed, alth ough CFS is kn own to react with alkyl-
substituted ben zen es in th e side ch ain (i.e. th e reaction of 1,2,4,5-tetra-
1
0
alkylben zen e occurs exclusively in th e side ch ain ) . Yield of fluorin ated
products is n ot optim ized, h owever, we were in terested in th e ratio of th e
products form ed in order to get an in sigh t in to th e course of fluorin ation .
Hen ce, on ly on e equivalen t of fluorin atin g reagen t was used to m in im ize
side-product form ation . Decom position of fluorin atin g reagen t un der th e
1
1
reaction con dition s also con tributes to th e lower yield of fluorin ation . Re-
agen ts of th e N–F group (FP-B800 an d Accufluor NFTh ) reacted in a sim ilar
m an n er givin g on ly products fluorin ated in th e arom atic rin g (en tries 2, 3).
Th e regioselectivity was sim ilar to fluorin ation with Selectfluor an d XeF ,
2
1
2
bein g in accordan ce with th e calculated electron den sity of fluoren e . A
com parison of regioselectivity of oth er electroph ilic reagen ts reveals th at
1
3
fluorin ation an d n itration (en try 6) are very sim ilar , wh ile brom in ation
1
4
an d ch lorin ation occurs m ain ly in th e position 2 (en try 7) .
A sim ilar study on th e effect of reagen t structure on th e course of fluori-
n ation was perform ed with both open an alogues of fluoren e (1), n am ely
diph en ylm eth an e (3a) an d biph en yl (3b). Th e m ajor reaction site in th e re-
action of 3a with CFS is th e CH group, wh ere th e form ation of two prod-
2
ucts was observed; ben zoph en on e (6) as th e m ain product in 71% yield an d
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1
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Iskra, Stavber, Zupan:
fluorodiph en ylm eth an e (7) in a yield of 11%. Rin g fluorin ation was a side
reaction an d occurred on ly in th e ortho position givin g 4a in 18% yield
(
Table II, en try 1). Th e yield of fluorin ation can be en h an ced by takin g
h igh er am oun t of CFS an d perform in g reaction in an in ert atm osph ere,
n everth eless, th e ratio between CH fun ction alization an d rin g fluorin ation
2
1
5
rem ain s alm ost th e sam e (82% (6 an d 7):18% (4a) vs 85%:15% , respec-
tively). A differen t course of fluorin ation was observed with N-fluoropyridi-
n ium salt FP-B800 wh ere 3a was fluorin ated on ly in th e arom atic rin g an d
form ation of 2-(fluoroph en yl)ph en ylm eth an e (4a) prevailed over th e para
derivative 5a (en try 2). Sim ilar reactivity was also observed with Accufluor
NFTh wh ere ortho- an d para-fluorodiph en ylm eth an es 4a an d 5a were
form ed in th e ratio 71:29 (en try 3). Alth ough sim ilar in structure to
Selectfluor, Accufluor NFTh did n ot sh ow an y reactivity on th e CH group,
2
wh ile XeF was th e on ly reagen t th at form ed a h igh er am oun t of th e para-
2
substituted product 5a th an of th e ortho 4a (en try 5).
TABLE I
Th e effect of reagen t structure on th e regioselectivity of fluorin ation of fluoren e (1)
Regioselectivitya
Reaction
con dition s
Yield
%
En try Reagen t
Ref.
a
2
- (2a)
4- (2b)
1
2
3
4
5
6
7
8
CFS
MeCN, 30 °C, 3 h
MeCN, 80 °C, 4 h
MeCN, 80 °C, 4.5 h
MeCN, 80 °C, 4.5 h
DCM, r.t., 3 h
15
25
19
27
23
71
68
63
67
65
67
29
32
37
33
35
33
FP-B800
Accuflor NFTh
Selectfluor
XeF2b
9c
9b
HNO3
Ac O
13
2
Br2
AcOH
AcOH
>97
89
14a
14b
Cl2
a
Yield an d regioselectivity were determ in ed by 19F NMR spectroscopy with octafluoro-
b
n aph th alen e as in tern al stan dard. On e drop of BF ·OEt was used as a catalyst.
3
2
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Fluorination of Fluorene, Dibenzofuran and Their Open Analogues
1675
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Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 12, pp. 1671–1680

Fluorination of Fluorene, Dibenzofuran and Their Open Analogues
1677
Regioselectivity of fluorin ation of biph en yl (3b) was sim ilar to th at of 3a.
CFS fluorin ated 3b preferen tially in th e ortho position givin g 2-fluoro-
biph en yl (4b) an d 4-fluorobiph en yl (5b) in th e ratio 7:3 (Table II, en try 6).
Ortho regioselectivity was less pron oun ced th an with Accufluor NFTh an d
Selectfluor but m ore th an with FP-B800 an d XeF2.
Diben zofuran (8) was taken as a testin g substrate for in vestigation on th e
course of fluorin ation with CFS, as it is ch aracteristic by affordin g differen t
products depen din g on th e reaction m ech an ism (Table III). Th us, n itration
proceeds th rough th e electron tran sfer m ech an ism (HNO in TFA) an d th e
3
reaction with 8 occurs predom in an tly in position 3, an ion ic reaction
(
Friedel–Crafts acylation an d brom in ation ) in position 2, wh ile a radical
process does n ot sh ow an y regioselectivity (Table III, en tries 6–10). In fluo-
rin ation of diben zofuran (8) with CFS n o 4-fluorodiben zofuran was form ed,
in dicatin g th e absen ce of a radical m ech an ism in th e fluorin ation with CFS
(
(
en try 1). However, product distribution is very sim ilar to th at in n itration
1
6
en try 7) for wh ich a pron oun ced electron -tran sfer ch aracter was sug-
9
a
gested, an d also to th e fluorin ation with Selectfluor . A com parison of th e
regioselectivity in fluorin ation of 8 with various fluorin atin g reagen ts
sh ows a sim ilarity between CFS, Selectfluor an d Accufluor NFTh , wh ile
N-fluoropyridin ium salt (FP-B800) gave th e h igh est ratio of 2-fluorodi-
ben zofuran (9b) am on g th e studied fluorin atin g reagen ts . In con trast,
XeF sh owed a sligh tly differen t regioselectivity an d 3-fluorodiben zo-
9
a
9
b
2
furan (9c) was th e product th at was form ed in th e h igh est yield.
Fluorin ation of an open -ch ain an alogue – diph en yl eth er (10) with CFS
was also studied. Reaction of 10 with CFS at 30 °C yielded 32% of ortho-
(
11) an d para- (12) fluorin ated products in th e ratio 56:44 (Sch em e 2). Both
open an alogues of diben zofuran 3b an d 10 sh owed a preferen ce in th e fluo-
rin ation to position ortho, wh ile th is is n ot reflected in th e fluorin ation of
diben zofuran (8) wh ere reaction proceeded m ain ly in th e position para to
eth er an d biph en yl bon d. Of th e F–L reagen ts studied, CFS was th e on ly
SCHEME 2
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 12, pp. 1671–1680

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Iskra, Stavber, Zupan:
on e th at reacted with diph en yl eth er (10) preferen tially in position ortho,
9
b
wh ile XeF h ad th e h igh est ratio of para substitution (ortho:para 38:62) .
2
A sim ilar effect was observed also in th e case of biph en yl (3b). Fluorin ation
occurred predom in an tly in th e ortho position (ortho:para 70–78:30–22)
wh ile FP-B800 an d XeF were less regioselective (ortho:para 60:40 an d 53:47,
2
respectively).
CONCLUSION
Regioselectivity of reaction of fluoren e (1), diben zofuran (8) an d th eir open
analogues diphenylmethane (3a), biphenyl (3b) and diphenyl ether (10) shows
in terestin g in sigh ts in to th e m ech an ism of fluorin ation with caesium
fluoroxysulfate an d oth er fluorin atin g reagen ts. Purely radical fluorin ation
of arom atic rin g could be ruled out due to th e regioselectivity of fluorin a-
tion . However, in diph en ylm eth an e (3a), wh ose low reactivity of ben zen e
rin gs preven ts th e fluorin ation , th e fun ction alization of CH group becam e
2
th e m ain reaction ch an n el in reaction with CFS. Th is reactivity is very sim i-
lar to th e Selectfluor reagen t. On th e oth er h an d, N-fluoropyridin ium salt
(
FP-B800) an d Accufluor NFTh reacted on ly at th e arom atic rin g. Regio-
selectivity of fluorin ation of fluoren e (1) an d diben zofuran (8) with all th e
studied fluorin atin g reagen ts sh ows a very good agreem en t with th at of
n itration , in con trast to reagen ts with a less pron oun ced oxidative power
(
brom in ation , Friedel–Crafts acylation ). Product distribution is sim ilar also
to th e HOMO electron den sity of 1 an d 8 an d th is could poin t to th e in -
volvem en t of th e electron tran sfer m ech an ism in th e fluorin ation with
studied reagen ts.
EXPERIMENTAL
Materials
Caesium fluoroxysulfate was syn th esized by a kn own procedure19. 2,6-Dich loro-1-fluoro-
pyridin ium tetrafluoroborate (FP-B800, Ch ich ibu On oda Cem en t Corp.), 1-fluoro-4-h ydroxy-
1
,4-diazon iabicyclo[2.2.2]octan e tetrafluoroborate (Accufluor NFTh , Allied Sign al Ch em icals),
fluoren e (Jan ssen Ch im ica), diben zofuran (Math eson Colem an ), diph en ylm eth an e (Fluka),
biph en yl (Fluka), diph en yl eth er (Fluka) an d BF -OEt (Merck) were obtain ed from com m er-
3
2
1
19
cial sources. H an d
F NMR spectra (δ, ppm ; J, Hz) were recorded by a Varian EM360L
spectrom eter at 60 or 56.45 MHz with Me Si or CCl F as in tern al stan dards. Gas ch rom atog-
4
3
raph y was carried out on Varian Models 3799 an d 3300, an d TLC on a Merck PSC-Fertigplatten
silica gel F-254.
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Fluorination of Fluorene, Dibenzofuran and Their Open Analogues
1679
Fluorin ation with CsSO F. Gen eral Procedure
4
To a solution of 0.5 m m ol of fluoren e (1) (83 m g) in 5 m l of aceton itrile, 0.5 m m ol of
CsSO F (124 m g) was added with in 15 m in an d th e m ixture was stirred at 30 or 40 °C for
4
addition al 3 h . Th e reaction m ixture was poured in to water an d organ ic products were ex-
tracted th ree tim es with 10 m l of CH Cl . Th e com bin ed organ ic extracts were wash ed with
2
2
1
0 m l of water, 10 m l of a saturated solution of NaHCO , dried with an h ydrous Na SO an d
3 2 4
th e solven t evaporated un der reduced pressure. Th e crude reaction m ixture was an alyzed by
1
19
H an d
F NMR spectroscopy. Th e yields of fluorin ated products were determ in ed usin g
octafluoron aph th alen e as in tern al stan dard.
Fluorin ation of Fluoren e (1) an d Diph en ylm eth an e (3a) with
Accufluor NFTh an d FP-B800
A solution of 1.0 m m ol of substrate (1, 3a) an d 1.0 m m ol of reagen t (Accufluor NFTh or
FP-B800) in 10 m l of MeCN was stirred at 80 °C for 4–24 h (con sum ption of reagen t was fol-
lowed by KI-starch strips). Th e solven t was partially rem oved un der reduced pressure, th e
residue was diluted with 30 m l of CH Cl , wash ed with water (10 m l) an d a saturated solu-
2
2
tion of NaHCO3 (10 m l) (in reaction s with FP-B800, th e organ ic ph ase was wash ed with
5 m l of 0.5 M HCl) an d dried over an h ydrous Na SO . After evaporation of th e solven t,
1
2
4
1
19
th e crude reaction m ixture was an alyzed by H an d F NMR spectroscopy an d th e yields
of fluorin ated products were determ in ed usin g octafluoron aph th alen e as in tern al stan dard.
Pure products were isolated by preparative GC (FFAP 30% on Ch rom osorb W A/W 80/100)
an d th e structures were determ in ed on th e basis of th eir spectroscopic data com pared with
1
9
th e kn own data or with th ose of in depen den tly prepared sam ples. F NMR (56.45 MHz,
2
0
(2a ) –116.2, ddd; 4-fluorofluoren e20 (2b ) –120.8, m ;
CDCl3) of: 2-fluorofluoren e
(
2
1
22
2-fluoroph en yl)ph en ylm eth an e
117.7, dd; 2-fluorobiph en yl
(9a ) –118.8, ddd; 2-fluorodiben zofuran
(9b ) –113.7, ddd; 2-fluorodiph en yl eth er25 (11) –130.5, m ;
-fluorodiph en yl eth er25 (12) –120.0, m .
(4a) –118.5, m ; (4-fluoroph en yl)ph en ylm eth an e
(5a)
2
3
23
–
1
3
4
(4b ) –116.0, m ; 4-fluorobiph en yl
(5b ) –118.2, m ;
9
a
24
-fluorodiben zofuran
-fluorodiben zofuran
(9b ) –121.0, ddd;
2
4
In fluorin ation of diph en ylm eth an e (3a) with CFS, fluorodiph en ylm eth an e (7) was iso-
2
6
1
lated by preparative TLC (SiO , petroleum eth er), 6 m g (3%) , oil. H NMR (60 MHz,
2
1
9
CDCl ): 6.4 (d, J = 47, 1 H); 7.2–7.4 (m , 10 H). F NMR (56.45 MHz, CDCl ): –168 (d, J = 47).
3
3
The financial support of the Ministry of Science and Technology of the Republic of Slovenia
is acknowledged. W e are indebted to Dr. G. A. Shia (Allied Signal Chemicals) for samples of
Accufluor NFTh and Dr. K. Nukui from New Field Research Laboratory, Chichibu Onoda Cement
Corp. for samples of FP-B800, to Dr. B. Kralj for MS and HRMS spectral measurements and to
Ms M. Kastelic and Prof. Dr. B. Stanovnik for elemental analyses.
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