(9H-fluoren-9-ylidene)fluoromethyllithium, a stabilized fluoroalkenyl carbanion. Preparation, reactions, 13C and 19F NMR spectra

Article abstract of DOI:10.1135/cccc20011508

(9H-Fluoren-9-ylidene)fluoromethyllithium (1) was prepared by a low-temperature transmetallation of bromo(9H-fluoren-9-ylidene)fluoromethane (2). Whereas the synthesis of unlabeled bromofluoroalkene 2a was based on Wittig-Horner reaction of fluorenone (3)

Full text of DOI:10.1135/cccc20011508

1508
Kvíčala, Pelter:
(9H-FLUOREN-9-YLIDENE)FLUOROMETHYLLITHIUM, A STABILIZED
FLUOROALKENYL CARBANION. PREPARATION, REACTIONS,
13C AND 19F NMR SPECTRA+
a,
b
Jaroslav KVÍČALA * and Andrew PELTER
a
Department of Organic Chemistry, Institute of Chemical Technology, Prague, Technická 5,
166 28Prague 6, Czech Republic; e-mail: kvicala@vscht.cz
Department of Chemistry, University of Wales Swansea, Singleton Park, Swansea SA2 8PP,
U. K.; e-mail: a.pelter@swansea.ac.uk
b
Received Septem ber 5, 2001
Accepted October 19, 2001
(9H-Fluoren -9-yliden e)fluorom eth yllith ium (1) was prepared by
a
low-tem perature
tran sm etallation of brom o(9H-fluoren -9-yliden e)fluorom eth an e (2). Wh ereas th e syn th esis of
un labeled brom ofluoroalken e 2a was based on Wittig–Horn er reaction of fluoren on e (3)
with eth yl (dieth oxyph osph oryl)fluoroacetate (4), (1-13C)-labeled com poun d 2b was ob-
tain ed via an addition of labeled lith ium 1-eth oxy-2-fluoro(2-13C)eth en -1-olate (5) to keton e
3. Fluoroeth en yllith ium 1 was foun d by a low-tem perature 19F NMR spectroscopy to be sta-
ble up to –40 °C; it was reacted with th e series of electroph iles, e.g. ben zaldeh yde (6),
m eth yl iodide (7) or ch loro(trim eth yl)silan e (8). 13C NMR experim en ts with (1-13C)-labeled
1a proved th at fluorocarben oid 1 is probably m on om eric in THF solution in an alogy to
oth er h alocarben oids.
Keyw ord s: 1-Fluoro-1-lith ioalken e; Carben oids; Carban ion s; Fluorocarben oids; Fluoro-
carban ion s; Fluoroalken es; Fluoroorgan om etallics; NMR spectroscopy.
Carban ion s wh ich con tain both electropositive m etal an d electron egative
h alogen atom s at th e cen tral atom are called carben oids as th ey often be-
h ave sim ilarly to carben es. Ch em istry of carben oids is govern ed by a stron g
in teraction between th e m etal an d h alogen atom s. Th is in teraction lowers
ch aracteristically th e carben oid stability1 an d h en ce lim its th eir syn th etic
use. Alth ough brom o- an d ch lorocarben oids h ave been in ten sively studied1,
little atten tion h ave been paid to fluorocarben oids2. Organ olith ium com -
poun ds are widely used in syn th etic organ ic ch em istry3. However, in con -
+ Part III in the Series Fluorinated Carbanions. Part II. Kvíčala J., Štam baský J., Böhm S., Paleta O.:
J. Fluorine Chem., in press.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)
doi:10.1135/cccc20011508

Stabilized Fluoroalkenyl Carbanion
1509
trast to oth er m etals such as zin c or cadm ium , h igh ly electropositive
lith ium brin gs addition al in stability to th e fluorocarben oid m olecule an d
th e ways of decom position of both alkan e-4 an d alken e-based5 fluoro-
carben oids h ave been studied. Un til n ow, on ly on e exam ple of th e syn -
th etic use of th e n on -stabilized alkan e-based fluorocarben oid, dibrom o-
fluorom eth yllith ium , at low tem perature is kn own 6, an d few oth ers rely on
adjacen t h eteroatom , like sulfur7, ph osph orus8 or silicon 9, stabilization .
Due to th ese lim itation s, m uch m ore atten tion h as been paid to m ore sta-
ble alken yl fluorocarben oids.
Alth ough 1-fluoroeth en yllith ium was proved to be un stable10,11, stabiliz-
in g β-substituen ts such as fluorin e, ph en yl or trifluorom eth yl groups en -
h an ced th e stability of correspon din g 1-fluoro-1-lith ioeth en es, wh ich h ave
been em ployed in organ ic syn th eses12. Th eir stability could be estim ated by
a low-tem perature 19F NMR spectroscopy11 an d in m ost cases sign ifican t
differen ces in stabilities of both geom etric isom ers h ave been observed11,12
Recen tly, surprisin gly stabilizin g role of oxygen substituen t in a greater dis-
tan ce from th e carben oid cen tre h as been reported13
.
.
Most organ olith ium com poun ds form oligom ers in solution . Th us, with
th e aid of a low-tem perature 13C NMR spectroscopy, eth en yllith ium proved
to form tetram ers in THF (ref.14). On th e oth er h an d, brom o- an d ch loro-
carben oids are on ly m on om eric in solution 15 an d th is h as been con firm ed
by X-ray spectroscopy of the 2,2-diaryl-1-chloroethenyllithium –THF–TMEDA
com plex16. In con trast to th is experim en tal observation , calculation s of
fluorom eth yllith ium in sim ulated solven t suggested th at dim ers con tain in g
two C–F–Li–C lin kages could also be form ed17
.
Aryl substituen ts in th e β-position stabilize h alocarben oids, but are pron e
to th e Fritsch –Butten berg–Wiech ell rearran gem en t1. Th is can be efficien tly
suppressed by con n ectin g arom atic rin gs1. We publish ed prelim in ary n otes
about syn th esis an d low-tem perature NMR spectra of (9H-fluoren -
9-yliden e)fluorom eth yllith ium 18,19 (1). Here we wish to report full experi-
m en tal details of th is work, as well as som e reaction s of carben oid 1 wh ich
sh ow th e scope an d lim itation s of its use.
EXPERIMENTAL
Gen eral Com m en ts
Tem perature data are un corrected. Low-tem perature NMR experim en ts h ave been perform ed
in a Wilm ad n arrow m outh NMR tubes equipped with a Wilm ad Om n ivalve adapter. 1H NMR
1D an d 2D spectra were recorded with a Varian Gem in i 300 HC spectrom eter at 400.1 MHz
usin g TMS as in tern al stan dard, oth er NMR spectra were recorded with a Bruker AM 400
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

1510
Kvíčala, Pelter:
spectrom eter, viz. 1H NMR spectra at 400.1 MHz in CDCl3 an d 13C NMR spectra at 100.6 MHz
in CDCl3 usin g TMS as in tern al stan dard, 19F NMR at 376.5 MHz usin g CFCl3 as in tern al
stan dard with upfield values design ed n egative. In NMR spectra, ch em ical sh ifts are given in
ppm (δ-scale), couplin g con stan ts (J) in Hz. FTIR spectra were recorded with a Nicolet 740
in strum en t in CHCl3.
All m an ipulation s an d reaction s in cludin g organ om etallic reagen ts were perform ed with
an exclusion of m oisture an d atm osph eric oxygen in oven -dried apparatuses. Eth yl
brom o(2-13C)acetate (12) was purch ased from Aldrich . KF (Aldrich ) was dried by h eatin g to
150 °C/100 Pa prior to use. THF was dried over sodium ben zoph en on e ketyl an d distilled
prior to use. Eth yl (dieth oxyph osph oryl)fluoroacetate (4) was prepared accordin g to ref.20
Con cen tration s of BuLi an d MeLi solution s were estim ated by titration prior to use accord-
in g to ref.21
.
.
Fluoro(9H-fluoren -9-yliden e)acetic Acid (9a)
A flask was ch arged with eth yl (dieth oxyph osph oryl)fluoroacetate (4; 12.11 g, 49.9 m m ol)
an d dry THF (100 m l). Th e m ixture was cooled to –80 °C an d BuLi (23.3 m l, 2.15 M solution
in h exan es, 50.1 m m ol) was added. Th e m ixture was allowed to warm to –30 °C an d a solu-
tion of fluoren -9-on e (3; 8.55 g, 47.5 m m ol) in THF (20 m l) was dropwise added to it. Th e
m ixture was stirred at –30 °C for 30 m in an d at 0 °C for 2 h . Solven ts were rem oved on a
vacuum rotary evaporator an d th e residue was partition ed between CH2Cl2 (100 m l) an d wa-
ter (100 m l). Th e water layer was extracted twice with CH2Cl2 (50 m l), organ ic layers were
com bin ed, wash ed with water (50 m l) an d dried with an h ydrous MgSO4. Evaporation of sol-
ven t afforded crude eth yl fluoro(9H-fluoren -9-yliden e)acetate (10a; 13.26 g), wh ich was
added to a m ixture of dioxan e (90 m l) an d aqueous (90 m l) NaOH (7.60 g, 190 m m ol) solu-
tion . Th is m ixture was stirred at room tem perature for 24 h , acidified by 5% HCl (150 m l)
an d extracted with CH2Cl2 (70 + 3 × 40 m l). Com bin ed organ ic layers were dried with an h y-
drous MgSO4 an d solven t rem oved on a vacuum rotary evaporator to afford crude acid 9a
(11.52 g), wh ich was wash ed four tim es with h ot petroleum eth er an d recrystallized from
MeCN to afford pure fluoro acid 9a (8.61 g, 75.5%, m .p. 228–230 °C). 1H NMR: 7.34 dt, 1 H,
4
3
3
3
3JHH = 7.9, JHH = 1.2; 7.37 t, 1 H, JHH = 7.7; 7.44 t, 1 H, JHH = 7.5; 7.47 dt, 1 H, JHH = 7.6,
4JHH = 0.8; 7.84 d, 1 H, JHH = 6.1; 7.85 d, 1 H, JHH = 6.9; 7.98 d, 1 H, JHH = 7.7; 8.64 d, 1 H,
3JHH = 7.9; 14.3 bs, 1 H. 13C NMR: 120.1 s; 126.3 d, JCF = 15; 127.3 s; 127.4 s; 127.5 s; 127.7 s;
128.2 s; 130.0 s; 130.2 s, 7 C; 133.0 d, JCF = 8; 135.6 s; 139.5 s; 140.9 d, JCF = 3; 147.5 d,
1JCF = 276; 162.2 d, JCF = 35. 19F NMR: –105.4 s. IR: 923 s, 946 s, 1 146 s, 1 203 s, 1 285 s,
3
3
3
2
3
2
1 304 s, 1 423 s, 1 438 s, 1 496 s, 1 623 s, 1 685 s, 1 708 s, 2 520 m , 2 970 m , 3 000 m . For
C
15H9FO2 (240.2) calculated: 75.0% C, 3.8% H; foun d: 75.1% C, 3.7% H.
Brom o(9H-fluoren -9-yliden e)fluorom eth an e (2a)
A flask was ch arged with fluoro acid 9a (3.60 g, 15.0 m m ol) an d CHCl3 (50 m l). Brom in e
(2.52 g, 15.8 m m ol) was dropwise added to th e stirred dispersion form ed, th e flask was
stoppered an d th e reaction m ixture was stirred at room tem perature wh ile irradiated by an
in frared lam p for 6 h . Evaporation of solven t on a vacuum rotary evaporator afforded crude
brom o(9-brom o-9H-fluoren -9-yl)fluoroacetic acid (11a; 6.55 g), wh ich was added to th e
suspension of NaHCO3 (2.52 g, 30.0 mmol) in acetone (50 ml). The mixture was refluxed for 16 h.
Solven t was th en rem oved on a vacuum rotary evaporator to afford crude product (4.04 g),
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Stabilized Fluoroalkenyl Carbanion
1511
wh ich was purified by sh ort colum n ch rom atograph y (SiO2; colum n len gth 4 cm , eluen t
petroleum eth er) to afford fluoroeth en e 2a (3.94 g, 95.6%, m .p. 80–84 °C; ref.22 87–88 °C).
3
4
3
4
1H NMR: 7.29 dt, 1 H, JHH = 7.5, JHH = 1.2; 7.30 dt, 1 H, JHH = 7.6, JH3H = 1.3; 7.35 dt,
3
4
3
4
4
1 H, JHH = 7.5, JH3H = 1.2; 7.37 dt, 1 H, JHH = 7.5, JHH = 1.1; 7.66 dt, 1 H, JHH = 7.5, JHH
=
4
3
4
1.1; 7.68 dd, 1 H, JHH = 7.0, JHH = 0.7; 7.85 dd, 1 H, JHH2 = 7.5, JHH = 0.8; 8.15 dd, 1 H,
3JHH = 7.7, JHH = 0.8. 13C NMR: 119.6 s; 120.0 s; 121.3 d, JCF = 14; 123.4 s; 125.3 d, JCF
=
4
4
1
15; 127.0 s; 127.8 s; 128.6 s; 128.7 s; 134.4 d, JCF = 288; 134.2 s; 136.2 s; 138.3 s; 140.5 s.
19F NMR: –54.6 s. IR: 938 s, 1 200 s, 1 212 s, 1 442 s, 1 665 s, 3 080 m . For C14H8BrF (275.1)
calculated: 61.1% C, 2.9% H; foun d: 61.0% C, 2.7% H.
Eth yl Fluoro(2-13C)acetate (13)
A flask topped by a Hickm an n still was ch arged with eth yl brom o(2-13C)acetate (12; 1.00 g,
5.95 m m ol), an h ydrous KF (689 m g, 11.9 m m ol) an d 18-crown -6 eth er (162 m g, 0.61 m m ol).
Th e m ixture was h eated to 140 °C for 2 h , durin g wh ich fluoro ester 13 (610 m g, 95.7%)
was collected in th e adapter.
Eth yl Fluoro(9-h ydroxy-9H-fluoren -9-yl)(2-13C)acetate (14)
A flask was ch arged with h exam eth yldisilazan e (0.33 g, 2.4 m m ol) an d THF (15 m l), an d
MeLi (1.45 m l, 1.5 m ol l–1, 2.18 m m ol, m ixture with LiBr) was added. After th e gas evolu-
tion ceased (15 m in ), th e m ixture was cooled to –110 °C an d HMPA (0.91 g, 5.0 m m ol) was
added followed by labeled fluoro ester 13 (180 m g, 1.68 m m ol). Th e m ixture was kept at
–110 °C wh ile stirred for 30 m in an d fluoren -9-on e (3; 357 m g, 1.98 m m ol) was added. Th e
m ixture was th en stirred at –110 °C for 2 h , at –90 °C for 15 m in an d quen ch ed at –90 °C
with saturated aqueous NH4Cl solution (1 m l). Th e m ixture was th en allowed to h eat to
room tem perature an d 30 m l of petroleum eth er was added. Th is solution was extracted
with water (5 × 25 m l) an d dried with an h ydrous MgSO4. Solven ts were rem oved on a
vacuum rotary evaporator an d pure fluoro ester 14 (225 m g, 46.6%) was obtain ed by col-
um n ch rom atograph y (SiO2, colum n len gth 8 cm , diam eter 1 cm , eluen t CH2Cl2). 1H NMR:
3
3
0.90 t, 3 H, JHH = 7.1; 3.34 dd, 1 H, J = 3.5, J = 1.1; 3.94 q, 2 H, JHH = 7.1; 5.29 dd, 1 H,
1JHC = 159.6, JHF = 48.4; 7.32 dt, 1 H, JHH = 7.5, JHH = 1.1; 7.33 dt, 1 H, JHH = 7.5, JHH
1.1; 7.41 dt, 1 H, JHH = 7.5, JHH = 1.1; 7.57 d, 1 H, JHH = 7.5; 7.62 m , 3 H. 13C NMR: 13.5 s;
61.5 s; 81.9 dd, JCC = 45, JCF = 20; 92.0 d (stron g sign al due to labelin g), JCF = 195; 120.1 s;
120.1 s; 124.8 s; 125.1 s; 140.1 s; 140.3 s; 143.4 s; 143.6 s; 167.0 dd, JCC = 60, JCF = 20. 19F NMR:
=
2
3
4
3
4
3
4
3
1
2
1
1
2
1
2
–195.6 dd, JCF = 195, JHF = 48.
(9H-Fluoren -9-yliden e)fluoro(2-13C)acetic Acid (9b)
A flask was ch arged with toluen e (25 m l), 4-m eth ylben zen e-1-sulfon ic acid (14 m g, 74 µm ol)
an d labeled flu oro ester 14 (215 m g, 0.75 m m ol). Th e m ixtu re was h eated to reflu x for
48 h and toluene was rem oved on a vacuum rotary evaporator. Ethyl (9H-fluoren-9-ylidene)-
fluoro(2-13C)acetate (10b; 164 m g) was obtain ed by colum n ch rom atograph y (SiO2, colum n
len gth 8 cm , diam eter 1 cm , eluen t petroleum eth er–CH2Cl2 5 : 1) of th e residue. Ester 10b
(164 m g, 0.61 m m ol) was added to th e m ixture of THF (8 m l) an d aqueous solution (8 m l)
of NaOH (114 m g, 2.85 m m ol), followed by stirrin g at room tem perature for 12 h . THF was
rem oved on a vacuum rotary evaporator. Th e obtain ed aqueous suspen sion of crude acid 9b
was extracted with Et2O (4 × 20 m l), organ ic layers were com bin ed an d dried with an h y-
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

1512
Kvíčala, Pelter:
drous MgSO4. Solven t was rem oved on a vacuum rotary evaporator to afford labeled fluoro
acid 9b (145 m g, 80.3%), wh ich h ad iden tical properties an d NMR spectra as un labeled
fluoro acid 9a with th e exception of two sign als in 13C NMR spectra: 147.9 d (stron g sign al
1
1
2
due to labelin g), JCF = 276; 162.4 dd, JCC = 85, JCF = 45, an d th e 19F NMR sign al: 107.4 d,
1JCF = 276.
Brom o(9H-fluoren -9-yliden e)fluoro(1-13C)m eth an e (2b)
A flask was ch arged with labeled fluoro acid 9b (135 m g, 0.52 m m ol), CHCl3 (50 m l) an d
brom in e (100 m g, 0.63 m m ol), stoppered an d th e reaction m ixture was stirred at room tem -
perature for 8 h wh ile irradiated with a in frared lam p. Evaporation of solven ts by vacuum
rotary evaporator afforded crude brom o(9-brom o-9H-fluoren -9-yl)fluoro(2-13C)acetic acid
(11b; 247 m g), wh ich was added to th e suspen sion of NaHCO3 (82 m g, 0.98 m m ol) in ace-
ton e (15 m l). Th e m ixture was refluxed for 12 h , cooled to room tem perature an d solven t
was rem oved on a vacuum rotary evaporator. Pure labeled brom ofluoroeth en e (2b; 92 m g,
59.5%) was obtain ed by colum n ch rom atograph y (SiO2, colum n len gth 8 cm , diam eter 1 cm ,
eluen t petroleum eth er) of th e residue. Th e product h ad iden tical ph ysical properties an d
NMR spectra with th e exception of on e sign al in th e 13C NMR spectra: 134.6 d (stron g sig-
1
1
n al due to labelin g), JCF = 331, an d th e 19F NMR sign al: –53.2 d, JCF = 332.
Form ation of (9H-Fluoren -9-yliden e)fluorom eth yllith ium (1a) an d Its Reaction with
Electroph iles. Gen eral Procedure
A flask was ch arged with brom ofluoroalken e 2a (275 m g, 1.00 m m ol) an d THF (10 m l),
cooled to –110 °C an d BuLi solution in h exan es (1.10 m m ol) was slowly (15 m in ) dropwise
added. Th e m ixture was stirred at –110 °C for 15 m in form in g (9H-fluoren -9-yliden e)-
fluorom eth yllith ium (1a), followed by addition of an electroph ile. Th e m ixture was th en
stirred at –110 °C for 10 m in , at 90 °C for 30 m in an d allowed slowly (2 h ) to warm to room
tem perature Th e product was obtain ed after rem oval of a solven t on a vacuum rotary evapo-
rator by colum n ch rom atograph y.
Reaction of Fluoroeth en yllith ium 1a with Trifluoroacetic Acid
(9H-Fluoren -9-yliden e)fluorom eth yllith ium (1a), form ed by reaction of 2a with 2.28 M BuLi
solution (0.44 m l), was quen ch ed with a solution of trifluoroacetic acid (228 m g, 2.00 m m ol)
in THF (1 m l) affordin g after colum n ch rom atograph y (SiO2, colum n len gth 8 cm , diam eter
1.5 cm , eluen t petroleum eth er) 9-(fluoromethylidene)-9H-fluorene (15; 148 m g, 75.5%, m .p.
58–62 °C; in ref.24 n ot given ).
Reaction of Fluoroeth en yllith ium 1a with Deuteriotrifluoroacetic Acid
(9H-Fluoren -9-yliden e)fluorom eth yllith ium (1a), form ed by reaction of 2a with 2.28 M BuLi
solution (0.44 m l), was quen ch ed with a solution of deuteriotrifluoroacetic acid (230 m g,
2.00 m m ol) in THF (1 m l) affordin g after colum n ch rom atograph y (SiO2, colum n len gth 8 cm ,
diam eter 1.5 cm , eluen t petroleum eth er) 9-[fluoro(2H-methylidene)]-9H-fluorene (16; 167 m g,
3
4
3
84.7%, m .p. 63–65 °C). 1H NMR: 7.23 dt, 1 H, JHH = 7.5, JHH = 1.2; 7.32 dt, 1 H, JHH = 7.4,
4JHH = 1.8; 7.32 tm , 1 H, JHH = 6.8; 7.36 dt, 1 H, JHH = 7.4, JHH = 1.3; 7.49 dd, 1 H, JHH
7.6, JHH = 1.6; 7.68 dd, 1 H, JHH = 7.6, JHH = 1.8; 7.69 dd, 1 H, JHH = 7.2, JHH = 1.3;
=
3
3
4
3
4
3
4
3
4
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Stabilized Fluoroalkenyl Carbanion
1513
3
2
7.96 dm , 1 H, JHH = 6.3. 13C NMR: 119.8 s; 119.8 s; 120.2 s; 121.6 d, JCF = 8; 125.9 d, JCF
=
3
11; 127.0 s; 128.0 s; 128.5 s; 134.7 s; 136.0 d, JCF = 12; 139.4 s; 139.5 d, JCF = 5; 147.6 dt,
1JCF = 277, JCD = 26. 19F NMR: –126.3 t, JDF = 12. IR: 977 s, 1 065 s, 1 119 s, 1 177 s, 1 196 s,
1 231 s, 1 442 s, 1 473 s, 1 642 s, 1 662 s, 3 030 m , 3 080 s. For C14H8DF (197.2) calculated:
85.3% C, 5.1% H + D; foun d: 85.0% C, 4.8% H + D.
1
2
Reaction of Fluoroeth en yllith ium 1a with Meth yl Iodide (7)
A flask was ch arged with brom ofluoroeth en e 2a (275 m g, 1.00 m m ol), THF (10 m l), cooled
to –110 °C wh ile stirred an d 2.28 M BuLi solution (0.48 m l) was dropwise added. After stir-
rin g at –110 °C for 45 m in , m eth yl iodide (7; 284 m g, 2.00 m m ol) dissolved in THF (1 m l)
was dropwise added. Th e m ixture was th en stirred at –90 °C for 2 h an d m ore m eth yl iodide
(7; 150 m g, 1.06 m m ol) was added, followed by 1 h stirrin g. More m eth yl iodide (7; 142 m g,
1.00 m m ol) was added an d th e m ixture stirred at –90 °C for 30 m in , at –70 °C for 30 m in
an d at –40 °C for 30 m in . Th en it was cooled to –90 °C an d quen ch ed by a solution of
trifluoroacetic acid (250 m g, 2.17 m m ol). After allowin g th e m ixture to warm to room tem -
perature, solven ts were rem oved on
a vacuum rotary evaporator an d 9-(1-fluoro-
ethylidene)-9H-fluorene (17; 100 m g, 47.6%, m .p. 89–91 °C) was obtain ed by repeated colum n
ch rom atograph y (SiO2, first colum n len gth 6 cm , diam eter 1.5 cm ; secon d colum n len gth
3
25 cm , diam eter 2.5 cm , eluen t petroleum eth er). 1H NMR: 2.63 d, 3 H, JHF = 18.5; 7.33 m ,
3
3
4
4 H; 7.60 d, 1 H, JHH2 = 8.0; 7.76 m , 2 H; 8.00 dd, 1 H, JHH = 5.6, JHH = 2.6. 13C NMR: 18.4 d,
2JCF = 29; 117.9 d, JCF = 12; 119.4 s; 120.0 s; 122.7 s; 125.6 d, JCF = 15; 126.8 s; 127.0 s;
127.3 s; 127.4 s; 136.1 d, JCF = 12; 136.4 s; 138.1 s; 140.0 s; 159.7 d, JCF = 265. 19F NMR:
1
3
–74.4 q, JHF = 19. IR: 938 s, 1 200 s, 1 212 s, 1 442 s, 1 665 s, 3 080 m . For C15H11F (210.3)
calculated: 85.7% C, 5.3% H; foun d: 85.6% C, 5.1% H.
Reaction of Fluoroeth en yllith ium 1a with Ch loro(trim eth yl)silan e (8)
To th e solution of (9H-fluoren -9-yliden e)fluorom eth yllith ium (1a), form ed by reaction of 2a
with 2.28 M Bu Li solu tion (0.48 m l) at –110 °C, ch loro(trim eth yl)silan e (8; 130 m g,
1.20 m m ol) in THF (1 m l) was dropwise added. Th e m ixture was th en stirred at –90 °C for
30 m in an d allowed to warm slowly to room tem perature (2 h ). Evaporation of solven ts
followed by repeated colum n ch rom atograph y (SiO2, first colum n len gth 6 cm , diam eter
1.5 cm ; secon d colum n len gth 25 cm , diam eter 2.5 cm , eluen t petroleum eth er) afforded
[(9H-fluoren-9-ylidene)fluoromethyl]trimethylsilane (18; 107 m g, 39.9%, m .p . 60–64 °C).
3
3
1H NMR: 0.49 s, 9 H; 7.16 t, 1 H, JHH = 7.6; 7.33 m , 3 H; 7.63 d, 1 H, JHH = 7.7; 7.71 m ,
2 H; 8.13 d, 1 H, JHH = 7.5. 13C NMR: –1.6 d, JCF = 4; 119.3 s; 120.0 s; 123.3 s; 126.4 s;
127.3 d, JCF = 18; 127.6 s; 127.7 s; 128.2 s; 132.6 d, JCF = 7; 136.0 d, JCF = 15; 136.5 s; 138.7 s;
140.3 s; 173.2 d, JCF = 297. 19F NMR: –94.0 bs. IR: 865 s, 931 s, 1 077 s, 1 254 s, 1 442 s,
3
3
2
1
1 473 m , 1 592 m , 1 612 m , 1 712 m , 2 910 m , 2 970 m , 3 060 m . For C17H17FSi (268.4) cal-
culated: 76.1% C, 6.4% H; foun d: 76.3% C, 6.3% H.
Reaction of Fluoroeth en yllith ium 1a with Ben zaldeh yde (6)
To th e solution of (9H-fluoren -9-yliden e)fluorom eth yllith ium (1a), form ed by reaction of 2a
with 1.98 M BuLi solution (0.51 m l) at –110 °C, ben zaldeh yde (6; 106 m g, 1.00 m m ol) in
THF (1 m l) was dropwise added. Th e m ixture was th en stirred at –90 °C for 30 m in an d al-
lowed to warm slowly to room tem perature (2 h ). Evaporation of solven ts followed by col-
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

1514
Kvíčala, Pelter:
um n ch rom atograph y (SiO2, colum n len gth 6 cm , diam eter 1.5 cm , eluen t petroleum
eth er–dich lorom eth an e 4 : 1) afforded 2-(9H-fluoren-9-ylidene)-2-fluoro-1-phenylethanol (19;
3
3
178 m g, 58.9%, m .p. 98–104 °C). 1H NMR: 2.76 d, 1 H, JHH = 6.8; 6.40 dd, 1 H, JHH = 6.4,
3
4
3
4
3JHF = 25.3; 7.26 dt, 1 H, JHH = 7.7, JHH = 1.2; 7.30 dt, JHH = 7.5, JHH = 1.3; 7.32 dt, 1 H,
3JHH = 7.7, JHH = 1.2; 7.32 dt, 1 H, JHH3 = 7.4, JHH = 1.4; 7.36 m , 4 H; 7.56 d, 2 H, JHH
=
=
4
3
4
3
3
3
3
7.3; 7.66 d, 1 H, JHH = 7.8; 7.72 d, 1 H, JHH = 7.5; 7.76 d, 1 H, JHH = 7.6; 8.01 d, 1 H, JHH
7.7. 13C NMR: 70.6 d, JCF = 26; 119.5 d, JCF = 15; 119.5 s; 120.3 s; 126.3 d, JCF = 17; 126.3 s;
2
3
4
2
127.3 s; 127.8 s; 128.2 s; 128.4 d, JCF = 2; 128.6 s; 128.8 s; 134.7 d, JCF = 11; 136.2 s; 138.4 s;
1
3
139.0 s; 140.7 ; 159.1 d, JCF = 275. 19F NMR: –108.0 d, JHF = 26. IR: 942 s, 1027 s, 1 173 s,
1 219 s, 1 442 s, 1 473 m, 1 488 m, 1 542 s, 1 658 s, 2 940 m, 3 050 m, 3 070 m, 3 450 m, 3 610 m .
For C21H15FO (302.4) calculated: 83.4% C, 5.0% H; foun d: 83.2% C, 5.0% H.
Reaction of Fluoroeth en yllith ium 1a with Tributyl(ch loro)stan n an e (20)
To th e solution of (9H-fluoren -9-yliden e)fluorom eth yllith ium (1a), form ed by reaction of 2a
with 1.85 M BuLi solution (0.57 m l) at –110 °C, tributyl(ch loro)stan n an e (20; 395 m g,
1.21 m m ol) in THF (1 m l) was dropwise added. Th e m ixture was stirred at –90 °C for 1 h
an d at –70 °C for 1 h an d th en allowed to warm slowly to room tem perature (2 h ).
Evaporation of solven ts followed by repeated colum n ch rom atograph y (SiO2, first colum n
len gth 6 cm , diam eter 1.5 cm ; secon d colum n len gth 25 cm , diam eter 2.5 cm , eluen t petro-
leum eth er) afforded tributyl[(9H-fluoren-9-ylidene)fluoromethyl]stannane (21; 231 m g, 47.6%).
3
3
1H NMR: 0.89 t, 9 H, JHH = 7.3; 1.29 m , 6 H; 1.36 sextet, 6 H, JHH = 7.4; 1.62 m , 6 H;
3
4
3
3
7.24 dt, 1 H, JHH = 7.4, JHH = 1.6; 7.33 t, 1 H, JHH = 6.9; 7.34 t, 1 H, JHH = 7.3; 7.49 d,
1 H, JHH = 7.7; 7.73 dd, JHH = 7.5, JHH = 0.9; 7.74 dd, 1 H, JHH = 7.5, JHH = 0.9; 8.14 d,
1 H, JHH = 7.1. 13C NMR: 10.9 s; 13.6 s; 27.2 s; 28.9 s; 119.3 s; 120.0 s; 120.5 s; 126.2 s;
3
3
4
3
4
3
2
126.7 d, JCF = 16; 127.2 s; 127.5 s; 127.7 s; 133.5 d, JCF = 3; 135.8 d, JCF = 6; 137.3 d, JCF
=
17; 138.1 s; 139.8 d, JCF = 4; 181.0 d, JCF = 334. 19F NMR: –82.0 s. IR: 931 s, 1 035 s, 1 046 s,
1 442 s, 1 461 s, 1 592 s, 1 608 s, 2 860 s, 2 920 s, 2 970 s, 3 060 m . For C26H35FSn (485.3)
calculated: 64.4% C, 7.3% H; foun d: 64.6% C, 7.5% H.
1
Reaction of Fluoroeth en yllith ium 1a with Trifluoroacetic An h ydride (22)
To th e solution of (9H-fluoren -9-yliden e)fluorom eth yllith ium (1a), form ed by reaction of 2a
with 1.85 M BuLi solution (0.57 m l) at –110 °C, trifluoroacetic an h ydride (22; 252 m g,
1.20 m m ol) in THF (1 m l) was dropwise added. Th e m ixture was stirred at –90 °C for 1 h
an d th en allowed to warm slowly to room tem perature (2 h ). Evaporation of solven ts fol-
lowed by colum n ch rom atograph y (SiO2, colum n len gth 6 cm , diam eter 1.5 cm , eluen t
petroleum eth er, th en petroleum eth er–CH2Cl2, fin ally CH2Cl2) afforded 1,3-di(9H-fluoren-
9-ylidene)-1,3-difluoro-2-(trifluoromethyl)propan-2-ol (23; 130 m g, 53.3%, m .p. 190–194 °C).
3
4
3
1H NMR: 4.60 s, 1 H; 7.06 dt, 2 H, JH3H = 7.7, JHH = 1.2; 7.25 t, 2 H, JHH = 7.5; 7.28 dt, 2 H,
3JHH = 7.7, JHH = 1.1; 7.35 dt, 2 H, JHH = 7.5, JHH = 1.0; 7.61 d, 2 H, JHH = 7.5; 7.63 d,
2 H, JHH = 7.0; 8.01 d, 2 H, JHH = 7.7; 8.16 d, 2 H, JHH = 8.0. 13C NMR: 79.0 sextet, JCF
4
4
3
3
3
3
2
=
1
28; 119.4 s; 119.6 s; 124.1 q, JCF = 285; 125.1 d, JCF = 16; 127.0 s; 127.3 s; 127.6 s; 128.0 s;
1
129.0 d, JCF = 2; 129.3 s; 132.9 d, JCF = 9; 136.2 s; 139.5 s; 141.2 d, JCF = 4; 152.3 d, JCF
=
273. 19F NMR: –76.1 t, 3 F, JFF = 14; –97.8 q, 2 F, JFF = 14. IR: 1 104 s, 1 119 s, 1 150 s, 1 208 s,
1 258 s, 1 450 s, 1 473 m , 1 596 m , 1 642 m , 3 470 m . For C30H17F5O (488.5) calculated:
73.8% C, 3.5% H; foun d: 72.6% C, 3.4% H.
4
4
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Stabilized Fluoroalkenyl Carbanion
1515
NMR Observation s of Fluoroeth en yllith ium 1
A W ilm ad n arrow-m ou th NMR tu be was ch arged with brom oflu oroalken e 2 (13.8 m g,
50 µm ol) an d closed with a Wilm ad Om n ivalve adapter. THF (0.5 m l) an d THF-d8 (dried,
150 µl) were syrin ged to th e tube, th e tube was sh aken an d cooled to –110 °C. BuLi (65 µl)
was carefully syrin ged to th e tube an d th e tube was altern atively cooled an d quickly several
tim es reverted to m ix th e com pon en ts wh ile preven tin g overh eatin g of th e reaction m ix-
ture. Wh en h om ogen ization was ach ieved, th e NMR observation s were started.
RESULTS AND DISCUSSION
Preparation of Bromo(9H-fluoren-9-ylidene)fluoromethane (2a )
Brom o(9H-fluoren -9-yliden e)fluorom eth an e (2a) was origin ally prepared by
reaction of 9-diazo-9H-fluoren e with brom ofluorocarben e22. We preferred a
m eth odology developed by Eddarir an d coworkers23, wh ich em ployed th e
Wittig–Horner reaction of lithium salt of ethyl (diethoxyphosphoryl)fluoro-
acetate (4) with fluoren -9-on e (3) to form substituted fluoropropen oate 10a,
wh ich on brom in ation followed by debrom odecarboxylation afforded th e
target brom ofluoroalken e 2a (Sch em e 1). Wh at m akes th e procedure used
attractive is th at on ly th e fin al product h as to be purified an d all previous
steps can be perform ed with crude reaction products.
O
C
F
OEt
Br
F
(iv), (v), (vi)
(i)
C
X
C
O
C
X
3
10
2
O
(iii)
OH
C
F
OEt
(ii)
2,10
a
X
C
13C
H
C
13C
b
14
(i) (EtO)2POCHFCO2Et (4), BuLi, THF, -30 °C, 30 min; (ii) 13CH2FCO2Et (13), HMDS,
MeLi, LiBr,THF, HMPA, -110 °C, 2 h; (iii) TsOH, toluene, reflux, 48 h; (iv) NaOH, dioxane
or THF, r.t., 24 h, then HCl; (v) Br2, CHCl3, irradiation, r.t., 6 h; (vi) NaHCO3, acetone,
reflux, 16 h
SCHEME 1
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

1516
Kvíčala, Pelter:
Preparation of Bromo(9H-fluoren-9-ylidene)fluoro(13C)methane (2)
Due to low in ten sity of sign als, large h alf-lin e width an d splittin g of carbon
atom by fluorin e an d lith ium atom s, low-tem perature 13C NMR spectra of
fluorocarben oids h as to be taken with 13C labeled com poun ds. Neith er
labeled dibrom ofluorom eth an e, n or labeled eth yl (dieth oxyph osph oryl)-
fluoroacetate (4) are easily accessible com poun ds an d we h en ce h ad to
foun d an oth er reaction path way. Fortun ately, labeled eth yl brom o-
(2-13C)acetate (12) is com m ercially available an d can be easily con verted to
th e correspon din g labeled fluoro ester 13 (ref.24). Directed aldol con den sa-
tion of lith ium 1-eth oxy-2-fluoro(2-13C)eth en -1-olate (5) with various ke-
ton es h as been publish ed25. We obtain ed excellen t yield of labeled
fluoroacetate 13 by fluorin ation with an h ydrous potassium fluoride un der
PTC con dition s (18-crown -6 eth er) with out an y solven t. Directed aldol con -
den sation of th e fluoroen olate 5 with fluoren -9-on e (3) un der sligth ly m od-
ified con dition s afforded labeled fluoro(h ydroxy)propan oate 14, wh ich was
deh ydrated un der m ild con dition s to labeled fluoropropen oate 10b an d, in
an alogy to un labeled ester 10a, furth er tran sform ed to labeled brom o-
fluoroeth en e 2b.
Formation of (9H-Fluoren-9-ylidene)fluoromethyllithium (1a ) and Its
Reaction with Electrophiles
We prepared fluorocarben oid 1a by lith ium –h alogen exch an ge with BuLi in
THF at –110 °C. We decided for th is way for two reason s. First, lith ium –
h alogen exch an ge is m ore rapid an d selective th an lith iation of a carbon –
h ydrogen bon d1, secon d, n on brom in ated an alog 15 of brom ofluoroeth en e
2a is n ot lon g stable an d ten ds to polym erize spon tan eously. Th e form ation
of fluorocarben oid 1a h as to be perform ed slowly, as th e m ixture is h igh ly
sen sitive to overh eatin g followed by decom position of flluorocarben oid 1a.
Th e in situ preform ed fluorocarben oid was reacted with a series of electro-
ph iles to fin d scope an d lim itation s of its use (Sch em e 2). In m ost cases, th e
reaction s with electroph iles were perform ed at –90 °C. Tributyl-
(chloro)stannane (20) had to be reacted at slightly higher tem perature, –70 °C,
to com plete th e con version . Th e less reactive electroph ile successfully used
was m eth yl iodide (7). To obtain acceptable yields, still h igh er tem perature
(–40 °C) an d a large excess of electroph ile 7 h ad to be em ployed. Moreover,
th e reaction m ixture h ad to be acidified with trifluoroacetic acid before
work-up. Th e reaction with trifluoroacetic an h ydride (22) afforded m ostly
th e 2 : 1 product, fluorin ated diol 23, an d on ly traces of trifluorom eth yl
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Stabilized Fluoroalkenyl Carbanion
1517
keton e were iden tified in th e crude reaction m ixture even if large excess of
fluoro an h ydride 23 was applied. Attem pts to react fluorocarben oid 1a with
ben zyl brom ide or ben zyl iodide failed com pletely.
D
C
C
CH3
F
H
F
F
C
C
C
C
16
17
15
(iii)
(ii)
(iv)
SiMe3
F
(v)
C
C
Br
F
Li
(i)
C
C
C
C
18
F
(vi)
1a
2a
HO
C
(vii)
(vi)
C
F
19
HO CF3
C
SnBu3
F
C
C
C
C
C
C
F
F
23
21
(i) BuLi, THF, -110 °C, 30 min; (ii) CF3COOH, THF, -110 °C, 5 min; (iii) CF3COOD, THF,
-110 °C, 5 min; (iv) CH3I (7), THF, -70 to -40 °C, 1 h, then CF3COOH; (v) Me3SiCl (8),
THF, -90 °C, 30 min; (vi) PhCHO (6), THF, -90 °C, 30 min; Bu3SnCl (20), THF, -70 °C, 30
min; (vii) (CF3CO)2O (22), THF, -90 °C, 30 min.
SCHEME 2
NMR Observations of Fluoroethenyllithium 1
After preparin g fluorocarben oid 1a in th e NMR tube at –110 °C, we gradu-
ally rose th e 19F NMR observation tem perature to fin d th e lim its of th e tem -
perature stability of fluorocarben oid 1a. Wh ereas n o ch an ge was observed
at –40 °C, at –30 °C fluorocarben oid 1a slowly started to decom pose with a
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

1518
Kvíčala, Pelter:
h alf-life of 12 h . Th e decom position was reason ably faster at –20 °C
(h alf-life 0.8 h ). Wh en two-fold m olar excess of N,N,N′,N′-tetram eth yl-
eth ylen ediam in e (TMEDA) was added to th e reaction m ixture, two sign als
could be observed at –110 °C (Fig. 1). Th e first on e, at –40.6 ppm , corre-
spon ds to th e sign al of fluorocarben oid 1a from experim en t with out
TMEDA addition wh ich is coordin ated accordin g to calculation s26 to th ree
m olecules of THF. Th e secon d on e, at –38.9 ppm , arises probably from coor-
din ation to on e m olecule of TMEDA an d on e of THF in agreem en t with
publish ed16 X-ray of substituted ch loroeth en yllith ium . Both sign als co-
alesce at –70 °C.
In 13C NMR spectra of labeled fluorocarbenoid 1b, broad doublet (188.8 ppm)
of th e 13C-1 carbon arisin g from th e carbon -fluorin e couplin g could be ob-
served at –70 °C. Its h alf-lin e width grew as th e tem perature was lowered to
–90 °C an d fin ally th e sign al was splitted to a doublet of quartet at –110 °C
as a result of carbon –lith ium couplin g (Fig. 1). Th is is in agreem en t with
13C NMR spectra of oth er h alocarben oids observed by Seebach an d cowork-
ers15 an d sh ows th at fluorocarben oid 1b form s eith er solvated m on om er in
solution as is supported by th e X-ray of an alogous ch lorocarben oid16, or a
dim er with a C–F–Li–C lin kage, wh ich was com puted17 to be th e preferred
structure for fluorom eth yllith ium .
–70 °C
–70 °C
–90 °C
–110 °C
–110 °C
–35
–40
–45
192 191 190 189 188 187 186
ppm
FIG. 1
19F an d 13C NMR spectra of (9H-fluoren -9-yliden e)fluorom eth yllith ium (1a)
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Stabilized Fluoroalkenyl Carbanion
1519
We th us syn th esized th e first fluorocarben oid wh ich is n ot stabilized by
h eteroatom . We for th e first tim e observed low-tem perature 13C an d 19F NMR
spectra of fluorocarben oid with th e aim to fin d lim its of its stability an d th e
level of aggregation in solution . We also studied its reactivity with th e aim
to fin d scope an d lim itation s of its use.
We thank the British Council, the Grant Agency of the Czech Republic (grant No. 203/00/1232)
and the Royal Society for financial support of this project.
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Perkin Trans. 1 1995, 2681.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

1520
Kvíčala, Pelter:
19. Preparation and
a
low-temperature 13C NMR spectrum of 13C-1 labeled
(9H-fluoren-9-ylidene)fluoro(13C)methyllithium (1b) have been published as
preliminary note: Pelter A., Kvíčala J.: Tetrahedron Lett. 1997, 38, 4877.
a
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2000, 65, 772.
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26. Kvíčala J., Czernek J., Böhm S., Paleta O.: Unpublished results.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)