[3,3] Sigmatropic rearrangement of some fluorinated 1,5-hexadienes

Article abstract of DOI:10.1135/cccc20021517

Fluorine atoms incorporated into 1,5-hexadiene molecule should influence the kinetic as well as the thermodynamic parameters of [3,3] sigmatropic rearrangement (Cope rearrangement). Within few decades is has been documented that this transformation proceeds in a concerted manner, rather than stepwise with some radical intermediates involved. Few new terminally fluorinated 1,5-hexadienes (compounds 3, 5A, 7, 9 and 5B) have been synthesized. The activation parameters of rearrangement have been determined and compared with those known for hydrocarbon analogues. While systems developing chair-like transition states (compounds 3 and 5) showed close similarity with hydrocarbon analogues (compound 1), those developing boat-like transition states (compounds 7, 9 and 5B) may proceed through radical stepwise mechanism. Computational studies of the transition states were carried out, showing that only ab initio methods (MP2 and especially DFT) can give approximate correlation with experimental data, whereas in the case of hydrocarbon analogues even simple semiempirical methods (AM1) were reliable enough to reproduce experimental results.

Full text of DOI:10.1135/cccc20021517

Fluorinated 1,5-Hexadienes
1517
[3,3] SIGMATROPIC REARRANGEMENT OF SOME FLUORINATED
1,5-HEXADIENES
a
b1
William R. DOLBIER, Jr.a1, Keith W. PALMER , Feng TIANa2, Piotr FIEDOROW
,
b
b2,
Andrzej ZAGANIACZYK and Henryk KORONIAK
*
a
University of Florida, Department of Chemistry, Gainesville, FL 32611, U.S.A.;
1
2
e-mail: wrd@chem.ufl.edu, ftian@scripps.edu
b
Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznań, Poland;
e-mail: piotrfie@amu.edu.pl, koroniak@amu.edu.pl
1
2
Received May 2, 2002
Accepted October 17, 2002
In Memory of Professor Miloš Hudlický.
Fluorin e atom s in corporated in to 1,5-h exadien e m olecule sh ould in fluen ce th e kin etic as
well as th e th erm odyn am ic param eters of [3,3] sigm atropic rearran gem en t (Cope rearran ge-
m en t). With in few decades is h as been docum en ted th at th is tran sform ation proceeds in a
con certed m an n er, rath er th an stepwise with som e radical in term ediates in volved. Few n ew
term in ally fluorin ated 1,5-h exadien es (com poun ds 3, 5A, 7, 9 an d 5B) h ave been syn th e-
sized. Th e activation param eters of rearran gem en t h ave been determ in ed an d com pared
with th ose kn own for h ydrocarbon an alogues. Wh ile system s developin g ch air-like tran si-
tion states (com poun ds 3 an d 5) sh owed close sim ilarity with h ydrocarbon an alogues (com -
poun d 1), th ose developin g boat-like tran sition states (com poun ds 7, 9 an d 5B) m ay
proceed th rough radical stepwise m ech an ism . Com putation al studies of th e tran sition states
were carried out, sh owin g th at on ly ab initio m eth ods (MP2 an d especially DFT) can give ap-
proxim ate correlation with experim en tal data, wh ereas in th e case of h ydrocarbon an alogues
even sim ple sem iem pirical m eth ods (AM1) were reliable en ough to reproduce experim en tal
results.
Keyw ord s: Sigm atropic rearran gem en ts; Cope rearran gem en t; Dien es; Fluorin ated com -
poun ds; Tran sition states; Ab initio calculation s; Con form ation an alysis.
It is gen erally un derstood th at [3,3] sigm atropic rearran gem en t (Cope rear-
ran gem en t) is a pericyclic, con certed reaction . Th is gen eral un derstan din g
com es from th e con cept th at th e altern ative m ech an ism s – in particular
stepwise biradical m ech an ism – do n ot fulfill en ergetical requirem en ts.
Moreover, th e con certed path way is con sidered to follow th e Woodward
an d Hoffm an rules. After origin al reports by Hurd1 an d Cope2 about fifty
Collect. Czech. Chem. Commun. (Vol. 67) (2002)
doi:10.1135/cccc20021517

1518
Dolbier et al.:
years ago, Cope rearran gem en t was em ployed as th e very useful syn th etic
strategy leadin g to n ew polycyclic system s.
Recen tly Cope rearran gem en t h as attracted som e n ew in terest of experi-
m en tal as well as th eoretical ch em ists, tryin g to explore th e ideas about th e
tran sition states (TS) of th ese reaction s3,4 (Sch em e 1). Th e two im portan t as-
pects of th ese studies are (i) structure an d en ergy requirem en t of th e tran si-
tion state3,4 an d (ii) usefuln ess of com putation al m eth ods for reaction
m odelin g5–9. Th ere are two possible structures of th e tran sition state for
Cope rearran gem en t: ch air-like an d boat-like with th e latter on e disfavored
en ergetically by about 10 kcal/m ol 8,9
.
R
R
chair
R
H
H
H
R
R
boat
SCHEME 1
Alth ough [3,3] sigm atropic processes are well kn own in literature an d
quite well un derstood, th e studies of th erm al rearran gem en t of fluorin ated
1,5 dien es h ad received very little atten tion . Th e paren t com poun d:
1,5-h exadien e un dergoes [3,3] sigm atropic rearran gem en t, but due to th e
degen eracy in th e system , th e 1,1-dideutero-1,5-h exadien e was studied10. In
th is fun dam en tal research th e basic kin etic param eters were determ in ed.
Partially fluorin ated (E)- an d (Z)-1-fluoro-1,5-h exadien es h ave been re-
ported to un dergo th e Cope rearran gem en t with alm ost iden tical activation
param eters as th e paren t 1,5-h exadien e15. Th is fin din g is som eh ow surpris-
in g in term s of th e expected fluorin e in fluen ce on con strain of th e tran si-
tion state. In our prelim in ary report we presen ted th e results sh owin g
sign ifican t fluorin e steric effect bein g im portan t factor in kin etic con trol of
th e rearran gem en t16. It was also sh own th at Cope rearran gem en t of h igh ly
fluorin ated 1,5-h exadien es m igh t proceed via radical, n ot con certed m ech a-
n ism 17. In th is study we would like to determ in e th e in fluen ce of fluorin e
atom s on [3,3] sigm atropic rearran gem en t: th e kin etic as well as th erm ody-
n am ic aspects of th ese reaction s. Th e even m ore im portan t issue was to get
an idea about structure of th e tran sition state an d to support con certed or
radical m ech an ism applyin g also com putation al m eth ods.
Collect. Czech. Chem. Commun. (Vol. 67) (2002)

Fluorinated 1,5-Hexadienes
1519
RESULTS AND DISCUSSION
Sin ce on ly few exam ples of term in ally fluorin ated dien es were studied,
th e syn th esis of com poun ds 3, 5A, 7, 9 an d 5B was accom plish ed. Fluori-
n ated dien es 3 an d 5A sh ould un dergo Cope rearran gem en t with ch air-like
tran sition states (Sch em e 2). Th e h ydrocarbon an alogues of th ese com -
poun ds are kn own (see Table I) an d logically it was worth to com pare th ese
two system s.
CD2
CD2
1
3
2
4
CF2
CF2
CF2
CF2
H
H
CF2
CF2
CF2
CF2
6
5A
SCHEME 2
TABLE I
Kin etic param eters for [3,3] sigm atropic rearran gem en t of som e fluorin ated 1,5-h exadien es
Reaction a
∆H#, kcal/m ol
∆S#, cal/m ol deg
Referen ce
1→2
33.5 ± 0.5
29.9 ± 0.2
28.0 ± 1.1
22.4 ± 0.2
44.4 ± 1.8
40.8 ± 0.5
40.7 ± 0.5
41.6 ± 0.5
49.5 ± 1.0
–13.8 ± 1.0
–18.5 ± 0.5
–11.3 ± 2.6
–17.5 ± 0.4
–3.7 ± 3.2
–6.1 ± 0.9
–10.1 ± 0.8
–0.7 ± 1.0
+8.1 ± 1.7
10
3→4
16
5A→6(H)
5A→6
7→8(H)
7→8
12
16
11
th is work
th is work
12
9→10
5B→6(H)
5B→6
16
a
H in dicates h ydrocarbon an alogues.
Collect. Czech. Chem. Commun. (Vol. 67) (2002)

1520
Dolbier et al.:
Sim ilarly, th ere are kn own h ydrocarbon system s of Cope rearran gem en t
in wh ich boat-like tran sition states h ave to be developed to proceed th e re-
arran gem en t, an d th eir fluorin ated an alogues were com pared with th em as
well (com poun ds 7, 9 an d 5B, see Table I an d Sch em e 3).
CH2
CF2
CF2
CF2
CH2
CH2
8
7
CH2
CF2
CF2
CF2
CH2
10
CF2
9
H
H
CF2
CF2
CF2
CF2
6
5B
SCHEME 3
Th erm al Cope rearran gem en t of th ese species (com poun ds 3, 5A, 7, 9
an d 5B) was studied in a gas ph ase (com poun ds 3, 7 an d 9) or by NMR ki-
n etic studies (low volatility of dien es 5A an d 5B did n ot allow to do direct
gas ph ase studies). Kin etic param eters (∆H≠ an d ∆S≠) of th ese processes h ave
been determ in ed an d are presen ted in Table I.
On e can con clude from th e above data th at fluorin ated an d n on -
fluorin ated system s sh ow sim ilar activation param eters with a sign ifican t
n egative ∆S≠ ch aracteristic of a well organ ized tran sition state (ch air-like
tran sition state). Differen t activation param eters describe an d illustrate th e
process un dergoin g th rough th e boat-like tran sition state. In th is case ∆H≠
is about 10 kcal/m ol h igh er in com parison with th e ch air-like tran sition
state. Sm all, but still n egative ∆S≠ m ay support th e idea about con certed
path way with th e exception of com poun d 11, wh ere both relatively h igh
∆H≠ an d h igh positive ∆S≠ stron gly suggest a radical m ech an ism .
In all studied fluorin ated system s th e rearran gem en t led irreversibly to
products (com poun ds 4, 6, 8 an d 10) an d equilibria were n ot observed. It is
well kn own th at alken es bearin g fluorin e atom s on sp2 h ybridized carbon
Collect. Czech. Chem. Commun. (Vol. 67) (2002)

Fluorinated 1,5-Hexadienes
1521
atom s are usually less stable th an th e products with fluorin e atom s located
on sp3 carbon s.
Com putation al m eth ods were em ployed to get an in sigh t in to th e struc-
ture of th e tran sition states of th e above processes: to determ in e th eir ge-
om etry an d to estim ate en ergy barrier between th e substrate an d tran sition
states. Origin ally sim ple sem iem pirical (AM1) calculation s were carried out.
Th is m eth od - successfully used by Dewar et al.8,13 – in th e case of fluori-
n ated species gave very poor agreem en t with experim en tal kin etic data.
Also th e th erm odyn am ic equilibria between th e substrates an d products
were un reliable in term s of experim en tal results. Th e problem of a calcu-
lated structure an d an en ergy of tran sition states of pericyclic processes (in -
cludin g [3,3] sigm atropic rearran gem en t) h as been discussed in detail by
Houk et al.14. It seem s th at on ly advan ced ab initio calculation could – to
som e exten t – reproduce experim en tal results. As an illustration of th e
problem , activation en ergies of tran sition states of a few sim ple fluorin ated
alken es were determ in ed, com parin g reliability of com putation al m eth ods
(Table II).
TABLE II
Ab initio calculation s of ∆H≠ (in kcal/m ol) for [3,3] sigm atropic rearran gem en ts of som e fluo-
rin ated 1,5-h exadien es
MP2(fc)6-31G*//R
Substrate
RHF/4-31G
RHF/6-31G*
Experim en tal
HF/6-31G*
ch air: 48.2
boat: 58.6
ch air: 54.7
boat: 65.2
ch air: 30.7
boat: 41.68
34.0
CF2
CF2
boat: 59.3
boat: 63.6
boat: 64.9
boat: 68.2
boat: 30.0
boat: 31.2
40.8
40.7
F2C
CF2
Collect. Czech. Chem. Commun. (Vol. 67) (2002)

1522
Dolbier et al.:
Neverth eless, th e m ost accurate n um erical results h ave been obtain ed us-
in g DFT m eth od. It seem s th at th e com putation results dealin g with th e
ch air TS are in good agreem en t with experim en tal on es. Con trary to th ese,
calculated results for rearran gem en t un dergoin g th rough th e boat TS are
again dram atically off expectation s, wh ich m ay suggest a radical, n ot con -
certed m ech an ism of tran sform ation s (Table III).
Th e in fluen ce of term in al gem in al difluorin ation in [3,3] sigm atropic re-
arran gem en t in volvin g ch air vs boat tran sition states is fun dam en tally dif-
feren t. Th e ch air-like tran sition states accom m odate term in al fluorin es to
an y degree with out detrim en tal kin etic effects. Th e ch an ges in th e activa-
tion param eters in th is case can be attributed to ch an ges in th e relative
th erm odyn am ics of th e 1,5-dien e system s as th e degree of term in al gem in al
difluorin ation is in creased. Th is is con sisten t with a m ech an ism in volvin g a
h igh degree of C1–C6 bon d form ation with little C3–C4 bon d cleavage. Th e
boat-like tran sition state un dergoes th e sam e relative ch an ges in th erm ody-
n am ics upon in creased fluorin ation but th e twistin g of term in al fluorin e at-
om s in th is con form ation leads to h igh ly repulsive in teraction s. Th is gives
rise to activation param eters con sisten t with a tran sition state m ore resem -
blin g a pair of allyl radicals in volvin g a h igh degree of C3–C4 bon d cleav-
age with out a sim ilar degree of C1–C6 bon d form ation .
TABLE III
DFT calculation s of en th alpy of activation , ∆H≠ an d en tropy of activation ∆S≠ for boat an d
ch air tran sition states of som e fluorin ated 1,5-h exadien es at 298.15 K an d
1 atm
(B3LYP/6-311+G(2df,2p)//B3LYP/6-31G(d)). Th erm al en ergy were corrected by factor 0.9804.
∆∆Hf represen ts th erm odyn am ic stability differen ce between product an d substrate in th e re-
action
∆H≠
kcal/m ol
∆S≠
cal/m ol
∆H≠
kcal/m ol
∆S≠
ch air
cal/m ol
∆∆Hf
kcal/m ol
boat
boat
ch air
Com poun d
1
41.1
41.6
41.2
44.3
47.9
44.7
46.5
–6.6
–7.7
–8.2
–9.7
–2.2
–5.9
–3.6
34.8
35.2
33.0
34.2
–8.6
–10.1
–10.9
–12.8
(Z)-1-Fluoro-1,5-h exadien e
–0.92
–5.6
1,1-Difluoro-1,5-h exadien e
3
–5.2
7(H)a
7
9
–7.3
–6.2
a
H in dicates h ydrogen an alogues.
Collect. Czech. Chem. Commun. (Vol. 67) (2002)

Fluorinated 1,5-Hexadienes
1523
EXPERIMENTAL
All n uclear m agn etic reson an ce ch em ical sh ifts are reported in ppm (δ-scale) usin g in tern al
referen ce TMS for 1H an d 13C NMR spectra an d CFCl3 for 19F NMR spectra. Couplin g con -
stan ts (J) are given in Hz. All NMR spectra were obtain ed on Varian VXR 300 or Varian
XL-200 in strum en ts. Gas ch rom atograph ic separation s were perform ed by gas-liquid ph ase
ch rom atograph y on packed colum n s. Quan titative GLPC an alyses were run on Hewlett
Packard 5890 Series II gas ch rom atograph with a flam e ion ization detector. Preparative
GLPC was accom plish ed on Varian Aerograph A-90 ch rom atograph with th erm al con ductiv-
ity detector. Con dition s an d colum n s used are described in each case in relevan t experi-
m en ts. Low an d h igh resolution m ass spectra were determ in ed on Kraytos/AEI-30
spectrom eter at 70 eV.
Syn th esis of 1,1,6,6-Tetrafluoro-1,5-h exadien e (3)
Com poun d 3 was accom plish ed accordin g to Sch em e 4.
O
CF2
CF2
OTHP
OTHP
OH
H
H
H
13
12
11
CF2
CF2
CF2
O
H
3
14
SCHEME 4
4-[(Tetrahydro-2H-pyran-2-yl)oxy]-1-butanol was prepared accordin g to th e m eth od of
Hoffm an n an d Rabe18. 1,4-Butan ediol (60.0 m l, 0.68 m ol) was placed in a roun d-bottom
flask equipped with a stron g m agn etic stirrer. Dieth yl eth er (50 m l) an d 5 drops of con cen -
trated HCl were th en added. 3,4-Dih ydro-2H-pyran (DHP; 41.1 m l, 37.9 g, 0.45 m ol) was dis-
solved in 50 m l of dieth yl eth er an d added dropwise at room tem perature with rapid stirrin g
to th e h eterogen eous diol/eth er m ixture. Th e reaction m ixture becam e h om ogen eous after
addition of approxim ately 10 m l of DHP/eth er solution . Upon com pletion of addition of th e
DHP solution , th e reaction m ixture was stirred at room tem perature for 3 h , th en extracted
with 2 × 50 m l of 10% aqueous KOH. Th e aqueous solution was extracted with 50 m l of
dieth yl eth er, th en eth er extracts were com bin ed an d dried over MgSO4. Th e eth er was th en
rem oved on a rotary evaporator an d th e rem ain in g clear colorless liquid was fraction ally
distilled th rough an 12-cm Vigreaux colum n . 4-[(Tetrah ydro-2H-pyran -2-yl)oxy]-1-butan ol
(52.8 g, 67%) was obtain ed as an early fraction boilin g at 100–109 °C at 0.1 m m Hg as a
colorless viscous liquid. 1H NMR (200 MHz, CDCl3, TMS): 1.45–1.85 (m , 10 H); 3.22 (s, 1 H);
3
3.37 (m , 6 H); 4.60 (t, 1 H, JHH = 3). 13C NMR (50 MHz, CDCl3, CHCl3): 19.3, 25.2, 26.1,
29.6, 30.4, 62.0, 62.1, 67.2, 98.6.
4-[(Tetrahydro-2H-pyran-2-yl)oxy]butanal (11) was prepared by adapted procedure as de-
scribed by Corey an d Sch m idt19. A 1-liter roun d-bottom flask was flam e-dried un der argon
purge an d equipped with a stron g m agn etic stirrer an d septum . 4-[(Tetrah ydro-2H-pyran -
Collect. Czech. Chem. Commun. (Vol. 67) (2002)

1524
Dolbier et al.:
2-yl)oxy]-1-butan ol (17.9 g, 1.03 m m ol) was syrin ged in to th e flask followed by 400 m l of
dry CH2Cl2. Pyridin ium dich rom ate (PDC; 58.3 g, 0.155 m ol) was quickly added an d rapid
stirrin g was started. Th e solution turn ed dark brown -black with in m in utes of addition of
PDC. Stirrin g was con tin ued at room tem perature for 15 h an d th en th e reaction m ixture
was diluted with 400 m l of dieth yl eth er. Th e black solid residue was gravity-filtered an d th e
resultan t brown -black solution filtered th rough
a bed of 150 m esh , basic activated
(Brockm an n I) alum in um oxide. Th e resultan t solution was con cen trated on a rotary evapo-
rator to yield a colorless sligh tly viscous liquid. Th is m aterial was fraction ally distilled
th rough a 6-cm Vigreaux colum n an d 12.7 g (72%) of colorless, pleasan tly sm ellin g alde-
h yde 11 was obtain ed as an early fraction boilin g at 85–90 °C at 0.05 m m Hg. 1H NMR
3
(200 MHz, CDCl3, TMS): 1.45–1.85 (m , 6 H); 1.94 (quin tet, 2 H, JHH = 6.7); 2.54 (dt, 2 H,
3
3JHH = 7.0, 1.7); 3.35–3.55 (m , 2 H); 3.73–3.88 (m , 2 H); 4.56 (t, 1 H, JHH ≈ 3); 9.78 (t, 1 H,
3JHH = 1.7). 13C NMR (50 MHz, CDCl3, CHCl3): 19.5, 22.7, 25.5, 30.6, 41.1, 62.2, 66.4,
98.8, 202.2.
1,1-Difluoro-5-[(tetrahydro-2H-pyran-2-yl)oxy]-pent-1-ene (12) was prepared by adapted pro-
cedure as described by Naae an d Burton 20. A th ree-n eck 1-liter flask was assem bled un der
argon purge with a m ech an ical stirrer, septum an d two addition fun n els. Th e system was
flam e-dried un der argon purge. Dibrom odifluorom eth an e (29.1 g, 0,139 m ol) was tran sferred
to a Rotaflow tube, dissolved in 60 m l of dry THF an d th en syrin ged in to th e reaction vessel
followed by an addition al 160 m l of dry THF. Th is solution was cooled to –5 °C by an
ice/salt bath . Dry THF (140 m l) was placed in an addition fun n el followed by 50.0 m l
(44.9 g, 0.275 m ol) of P(N(CH3)2)3. Rapid stirrin g was started an d th e P(N(CH3)2)3 was added
dropwise to th e CF2Br2 solution over 1 h with coolin g. A fin e wh ite precipitate form ed as
addition con tin ued. Upon com pletin g addition of th e P(N(CH3)2)3, th e m ixture was stirred
at –5 °C for 1 h an d th en brough t to room tem perature. Aldeh yde 11 (16.0 g, 92.9 m m ol)
was dissolved in 50 m l of dry THF an d placed in th e secon d addition fun n el. Th e solution of
11 was th en added to th e ylide solution at room tem perature over 15 m in an d th en stirred
at room tem perature for 16 h . At th is tim e th e reaction m ixture was a yellow-brown solu-
tion with a fin e yellow precipitate settlin g on th e bottom of th e reaction vessel. Th e solids
were gravity-filtered an d rin sed with 100 m l of dieth yl eth er. Th is solution was con cen trated
on a rotary evaporator to about 25 m l volum e wh ich was taken up in 200 m l of dieth yl
eth er an d exh austively extracted with water. Th e eth er solution was th en dried over MgSO4
an d th e eth er was rem oved by distillation . Alken e 12 (11.1g, 58% by 19F NMR; about 80%
pure, th e rem in der bein g dieth yl eth er or traces of THF) was obtain ed as a clear, ligh t brown
liquid. It seem s, th at th e com poun d 12 form s azeotropic m ixtures with dieth yl eth er an d/or
THF an d as an 80% pure m aterial was used for furth er syn th esis. 1H NMR (200 MHz, CDCl3,
TMS): 1.45–1.85 (m , 8 H); 2.03–2.17 (m , 2 H); 3.33–3.57 (m , 2 H); 3.75–3.92 (m , 2 H);
3
3
3
3
4.17 (dtd, 1 H, JtransHF = 25.2, JHH = 7.94, JcisHF = 2.54); 4.55 (t, 1 H, JHH ≈ 3). 13C NMR
3
4
(50 MHz, CDCl3, CHCl3): 18.9 (d, 1 C, JCF = 4.3); 19.4, 25.3, 29.3 (t, 1 C, JCF ≈ 2); 30.6,
62.1, 66.3, 77.6 (t, 1 C, JCF = 21.2); 98.7, 156.2 (t, 1 C, JCF = 283.9). 19F NMR (188 MHz,
CDCl3, CFCl3): –89.8 (d, 1 F, JFF = 48.4); –92.3 (dd, 1 F, JFF = 48.4, JHF = 25.2).
2
1
2
2
3
5,5-Difluoro-4-penten-1-ol (13). Com poun d 12 was deprotected to alkoh ol 13 by adaptation
of th e procedure described by Beier an d Mun dy21. Alken e 12 (11.1 g, 43 m m ol, 80% pure,
im purities bein g dieth yl eth er or traces of THF) was dissolved in 70 m l of 1,4-butan ediol.
Acid activated Dowex 50X80–200 ion exch an ge resin (5.0 g) was added an d th e m ixture
stirred rapidly for 2.5 h . Th e reaction was flash -distilled down to pressure of 0.5 m m Hg. Ap-
proxim ately 2 m l of THF an d eth er was collected upon pum p down of th e system . At th is
Collect. Czech. Chem. Commun. (Vol. 67) (2002)

Fluorinated 1,5-Hexadienes
1525
stage it was possible to rem ove traces of dieth yl eth er an d THF in troduced as im purities with
alken e 12. Th e fraction boilin g between 55 an d 110 °C (bath tem perature) was collected to
yield about 5 m l of clear, colorless liquid. Bein g sligh tly acidic, th is m aterial was distilled off
from 2 g of dry NaHCO3 to yield 4.37 g (83%) of alcoh ol 13, b.p. 74–78 °C at 115 m m Hg.
3
1H NMR (200 MHz, CDCl3, TMS): 1.63 (tt, 2 H, JH3H = 7.42, 6.36); 2.07 (m , 2 H); 2.78
3
3
3
(bs, 1 H); 3.62 (t, 2 H, JHH = 6.34); 4.16 (dtd, 1 H, Jtr3ansHF = 25.38, JHH = 7.78, JcisHF
=
2.54). 13C NMR (50 MHz, CDCl3, CHCl3): 18.5 (d, 1 C, JCF = 4.2); 32.1 (t, 1 C, JCF = 2.1);
61.6, 77.3 (t, 1 C, JCF = 21.3); 156.3 (dd, 1C, JCF = 284.9, 285.1). 19F NMR (188 MHz,
CDCl3, CFCl3): –89.6 (d, 1 F, JFF = 48.6); –92.2 (dd, 1 F, JFF = 48.6, JtransHF = 25.5). MS EI,
m/z (% rel. in t.): 104 (100), 84 (16), 77 (69).
4
2
1
2
2
3
5,5-Difluoro-4-pentenal (14) was prepared accordin g to th e procedure described by Corey
an d Sch m idt19. A roun d-bottom flask was flam e dried un der argon purge an d 150 m l of dry
CH2Cl2 was added. Alcoh ol 13 (4.37 g, 35.8 m m ol) was th en added followed by 20.17 g
(53.6 m m ol) of pyridin ium dich rom ate. Th e reaction m ixture was stirred at room tem pera-
ture for 18 h th en diluted with 250 m l of dieth yl eth er. Th e m ixture was gravity-filtered to
rem ove th e black solids wh ich were rin sed with dieth yl eth er an d th e resultan t brown -black
solution was filtered th rough th e bed of 150 m esh base-activated (Brockm an n I) alum in um
oxide. Con tam in ation with pyridin e was detected by GLPC, so th e solution was extracted
with 2 × 50 m l of h alf-saturated CuSO4 solution , th en with 2 × 50 m l of water. By GLPC
pyridin e h ad been rem oved an d th e solution was dried over MgSO4. Eth er an d CH2Cl2 were
carefully distilled off th rough a 6-cm Vigreaux colum n an d th e residue was tran sferred to a
m icrodistillation apparatus. As th e h eatin g bath tem perature raised to about 40 °C with th e
applied pressure of 100 m m Hg, th e rest of m aterial bum ped over. Due to sm all am oun t of
m aterial, distillation was n ot attem pted again . A clear ligh t yellow liquid was 87% pure alde-
h yde 14 (3.37 g, 70%) con tam in ated with dieth yl eth er an d CH2Cl2. 1H NMR (200 MHz,
3
3
CDCl3, TMS): 2.22–2.35 (m , 2 H); 2.53 (tm , 2 H, JHH = 6.92); 4.22 (dtd, 1 H, JtransHF
=
25.10, JHH = 7.78, JcisHF = 2.38); 9.76 (t, 1 H, JHH = 1.2). 13C NMR (50 MHz, CDCl3,
CHCl3): 14.9 (d, 1 C, JCF = 4.9); 43.0 (t, 1 C, JCF = 2.6); 77.0 (t, 1 C, JCF = 31.9); 156.2 (dd,
1 C, JCF = 285.6, 286.2); 200.6. 19F NMR (188 MHz, CDCl3, CFCl3): –88.9 (d, 1 F, JFF
3
3
3
3
4
2
1
2
=
2
3
46.4); –91.0 (dd, 1 F, JFF = 46.4, JtransHF = 25.0). MS EI, m/z (% rel. in t.): 120 (10), 100 (11),
91 (7), 7 7(66), 69 (100), 64 (19). HRMS, calculated for C5H6F2O: 120.0386; foun d: 120.0356.
1,1,6,6-Tetrafluoro-1,5-hexadiene (3) was prepared by th e m eth od described by Naae an d
Burton 20. A th ree-n ecked roun d-bottom flask was assem bled with two septa, a pressure-
equalizin g addition fun n el, m agn etic stirrer an d flam e dried un der argon purge.
Dibrom odifluorom eth an e (1.55 g, 7.39 m m ol) was tran sferred to a Rotaflow tube, th en dis-
solved in 15 m l of dry triglym e an d added to th e reaction vessel with a syrin ge. Th is solu-
tion was cooled to –5 °C with an ice/salt bath . P(N(CH3)2)3 (2.6 m l, 2.3 g, 14 m m ol) was
dissolved in 10 m l of dry triglym e in th e addition fun n el an d th en added to th e CF2Br2 so-
lution over 15 m in with good stirrin g an d coolin g at –5 °C. Th e resultan t cloudy solution ,
th ick with wh ite precipitate, was stirred at –5 °C for 1 h an d th en brough t to room tem pera-
ture. Aldeh yde 14 (0.90 g, 7.5 m m ol) was dissolved in 5 m l of dry triglym e an d added to th e
ylide solution at room tem perature over 5 m in . Th is m ixture was allowed to stir at room
tem perature for 18 h . Th e reaction m ixture was th en flash -distilled in to a two-n ecked
Rotaflow trap up to a bath tem perature of 40 °C at 0.5 m m Hg. 1,1,6,6-Tetrafluoro-
1,5-h exadien e (0.44 g, 38%) was obtain ed as a clear, colorless liquid con tain in g traces of
dieth yl eth er, m eth ylen e ch loride an d OP(N(CH3)2)3. 1H NMR (300 MHz, CDCl3, TMS):
3
2.05–2.08 (m , 4 H); 4.06–4.20 (dm , 2 H, JtransHF = 25.5). 13C NMR (75 MHz, CDCl3, CHCl3):
Collect. Czech. Chem. Commun. (Vol. 67) (2002)

1526
Dolbier et al.:
2
1
22.3 (m , 2 C); 76.8 (t, 2 C, JCF = 21.5); 156.6 (dd, 2 C, JCF = 285.6, 285.7). 19F NMR
2
2
3
(282 MHz, CDCl3, CFCl3): –89.0 (d, 2 F, JFF = 46.4); –91.2 (dd, 2 F, JFF = 46.4, JtransHF
=
25.5). MS EI, m/z (% rel. in t.): 154 (1), 134 (1), 115 (2), 95 (2), 85 (23), 77 (100), 64 (2).
HRMS, calculated for C6H6F4: 154.0405; foun d: 154.0410.
Gas Ph ase Th erm olysis of 1,1,6,6-Tetrafluoro-1,5-h exadien e (3)
An about 100 m g sam ple of dien e 3 was obtain ed 98% pure by preparative GLPC separation
from NMR solution in C6D6 (colum n 8 m m × 4 m , 20% SE-30 on Ch rom osorb P 80/100).
Vapor of dien e 3 was expan ded in to th e gas kin etic vessel to th e pressure of about 1 kPa,
an d con version to 3,3,4,4-tetrafluoro-1,5-h exadien e (4) was followed by GLPC (colum n
4 m m × 4 m , 30% of AgNO3/ C6H5CH2CN on Ch rom osorb Z 80/ 100, in tern al stan dard
n-octan e). Th e yield of th is tran sform ation was >99%. Reversibility of 4 to 3 was n ot ob-
served.
3,3,4,4-Tetrafluoro-1,5-hexadiene (4). Solution (30%) of dien e 3 in C6D6 was sealed in a
Carius tube an d th erm olyzed at 185 °C for 42 h . 19F NMR spectrum sh owed on ly presen ce of
dien e 4 wh ich was isolated by preparative scale GLPC from th is solution (colum n 8 m m ×
6.5 m , 20% Triton X-305 on Ch rom osorb W 80/100). 1H NMR (300 MHz, CDCl3, TMS):
5.66–5.72 (m , 2 H); 5.81–6.08 (m , 4 H). 13C NMR (75 MHz, CDCl3, CHCl3): 114.9 (tt, 2 C,
2
2
1JCF = 247.1, JCF = 35.7); 124.0 (m , 2 C); 126.5 (t, 2 C, JCF = 24.6). 19F NMR (282 MHz,
3
CDCl3, CFCl3): –115.6 (dm , 4 F, JHF = 12.1). MS EI, m/z (% rel. in t.): 154 (1), 135 (5), 134
(2), 115 (8), 85 (13), 78 (3), 77 (100), 57 (3), 51 (16). HRMS, calculated for C6H6F4:
154.0405; foun d: 154.0400.
1-(Difluorom eth ylen e)-4-m eth ylen ecycloh exan e (7)
Preparation of dien e 7 was accom plish ed accordin g to th e Sch em e 5.
O
O
O
O
H2C
H2C
O
H2C
CF2
O
15
SCHEME 5
17
16
7
4-Methylene-1-cyclohexanone ethylene ketal (16) was prepared by adaptation of a gen eral
procedure22. A 250-m l th ree-n ecked, roun d-bottom flask was assem bled with a m agn etic stir-
rer an d two pressure equalizin g addition fun n els with septa. Th e system was th en flam e
dried un der argon purge. Meth yltriph en ylph osph on ium brom ide (22.6 g, 63.3 m m ol) was
added an d a slurry was created by th e addition of 80 m l of dry THF. Th is slurry was cooled
to 0 °C by ice-water bath . Butyl lith ium (BuLi) in pen tan e (31.7 m l of 2.0 M solution ,
63.4 m m ol) was added to an addition fun n el with a Teflon stopcock. Wh ile vigorously stir-
rin g th e slurry, BuLi was added over 40 m in . Th e solution turn ed can ary yellow. In th e
course of 30 m in of th e addition , th e m ixture becam e a clear, deep yellow-red h om ogen eous
solution . Upon com pletion of th e addition , th e solution was warm ed to room tem perature
an d stirred for 1 h . Next th e ylide was cooled to 5 °C an d 9.0 g (57.6 m m ol) of 1,4-cyclo-
h exan edion e m on oeth ylen e ketal 15 in 20 m l of dry THF was added dropwise from th e
secon d addition fun n el over 20 m in . Th e reaction m ixture was th en stirred at room tem per-
ature for 15 h to yield a clear, ligh t yellow solution con tain in g m uch fin e wh ite precipitate.
Collect. Czech. Chem. Commun. (Vol. 67) (2002)

Fluorinated 1,5-Hexadienes
1527
Water (3 m l) was th en added to decom pose an y possible rem ain in g ylide. Th e solids were
gravity-filtered an d rin sed with 40 m l of THF an d 50 m l of h exan es. Th e solution was th en
con cen trated on a rotary evaporator to yield an oil with som e wh ite solid. Th is m aterial was
dissolved in 100 m l of m eth an ol an d extracted with 3 × 100 m l of h exan es. Th e com bin ed
h exan es extracts were th en wash ed with 3 × 100 m l of saturated aqueous NaCl. Th e h exan e
solution was dried over MgSO4 an d con cen trated on a rotary evaporator to yield about 5 m l
of clear oil con tain in g som e wh ite crystals. Th is m aterial was dissolved in 25 m l of h exan es
an d set aside in a freezer at –5 °C for 18 h . Th e solution was vacuum filtered (wh ile cold) to
rem ove th e wh ite crystallin e OP(C6H5)3. Th e resultan t clear, colorless solution was con cen -
trated on a rotary evaporator to yield 6.2 g (69%) of ketal 16. 1H NMR (200 MHz, CDCl3,
3
3
TMS): 1.69 (t, 4 H, JHH = 6.2); 2.27 (t, 4 H, JHH = 6.2); 3.96 (s, 4 H); 4.66 (s, 2 H). 13C NMR
(50 MHz, CDCl3, CHCl3): 31.9, 35.8, 64.2, 108.1, 108.5, 147.2.
4-Methylenecyclohexanone (17) was prepared by adaptation of a procedure described by
Huet et al.23. Silica gel (73 g) was suspen ded in 180 m l of CH2Cl2 by rapid stirrin g in a
roun d-bottom flask. Sulfuric acid (15%, 7.3 g) was dropwise added to th e silica gel slurry. Af-
ter 5 m in th e aqueous acid h ad been adsorbed in to th e silica gel. Ketal 16 (6.2 g, 40 m m ol)
was th en added an d rapid stirrin g con tin ued. After 3 h , GLPC an alysis sh owed a 4 : 1 ratio
of deprotected to protected keton e. Th e solution was filtered an d th en subjected to a secon d
treatm en t as above (38 g of silica gel, 4.8 g of 15% H2SO4). After 3 h of stirrin g, GLPC
sh owed 10.2 : 1 ratio of com poun ds 17 an d 16. Th e solution was th en filtered an d th e silica
gel was rin sed with 2 × 100 m l of CH2Cl2. Com bin ed solution s were con cen trated on a ro-
tary evaporator to yield 4.6 g of ligh t yellow oil. Th is m aterial was distilled yieldin g 3.9 g
(90%) of clear, colorless keton e 17 boilin g at 99–102 °C at 115 m m Hg. 1H NMR (200 MHz,
CDCl3, TMS): 2.35–2.60 (m , 8 H); 4.89 (s, 2 H). 13C NMR (50 MHz, CDCl3, CHCl3): 32.9,
41.4, 110.5, 144.2, 210.3.
1-(Difluoromethylene)-4-methylenecyclohexane (7) was prepared by adaptation of th e proce-
dure as described by Naae an d Burton 20. A 250-m l th ree-n eck flask was assem bled un der ar-
gon purge with a m ech an ical stirrer, septum an d pressure equalizin g fun n el. Th e system was
flam e dried un der argon purge. Dibrom odifluorom eth an e (5.8 g, 28 m m ol) was dissolved in
75 m l of dry THF, th en tran sferred to th e reaction vessel by syrin ge. Th is solution was
cooled to –5 °C by an ice/salt bath . P(N(CH3)2)3 (10.2 m l, 9.1 g, 56 m m ol) was tran sferred to
th e addition fun n el, dissolved in 40 m l of dry THF an d th en dropwise added to th e CF2Br2
solution over 30 m in with coolin g. Th e resultan t slurry of wh ite precipitate was stirred for
1 h an d th en brough t to room tem perature. 4-Meth yliden ecycloh exan on e (17; 1.92 g,
17 m m ol) was dissolved in 5 m l of dry THF an d added dropwise to th e ylide solution . Th e
m ixture was stirred at room tem perature for 14 h . Th e yellow solid was gravity filtered. Th e
liquid was th en dissolved in 200 m l of dieth yl eth er an d exh austively extracted with water.
Th e eth er solution was th en dried over MgSO4, filtered an d distilled th rough a 6-cm
Vigreaux colum n . Th e rem in der of th e m aterial was tran sferred to a m icrodistillation appara-
tus an d distilled un der vacuum to yield 1.5 g (60%) of dien e 7 boilin g at 55–59 °C at 115
m m Hg. 1H NMR (300 MHz, CDCl3, TMS): 2.18 (m , 8 H); 4.70 (s, 2 H). 13C NMR (50 MHz,
3
2
CDCl3, CHCl3): 25.5, 34.4 (t, 2 C, JCF = 2); 87.1 (t, 1 C, JCF = 18.9); 108.6, 147.3, 150.9 (t,
1 C, JCF = 279.5). 19F NMR (282 MHz, CDCl3, CFCl3): –98.4 (m , 2 F). MS EI, m/z (% rel.
1
in t.): 144 (100), 129 (65), 127 (12), 115 (20), 109 (40), 97 (9), 79 (15), 77 (18). HRMS, calcu-
lated for C6H10F2: 144.0750; foun d: 144.0753.
Collect. Czech. Chem. Commun. (Vol. 67) (2002)

1528
Dolbier et al.:
Gas Ph ase Th erm olysis of 1-(Difluorom eth ylen e)-4-m eth ylen ecycloh exan e (7)
An about 100 m g sam ple of dien e 7 was obtain ed >99% pure by preparative GLPC (colum n
8 m m × 3.5 m , 20% SE-30 on Ch rom osorb P 80/100). Vapor of dien e 7 was expan ded in to
th e gas kin etics vessel to th e pressure of about 1 kPa (8 m m Hg). Th e con version of dien e 7
to 1,1-difluoro-2,5-dim eth ylen ecycloh exan e 8 was followed by GLPC (colum n 4 m m
×
3.5 m , 20% DNP on Ch rom osorb 60/80, in tern al stan dard n-octan e). Th e yield of th e tran s-
form ation was m ore th an 98%. Reversibility of com poun d 8 to 7 was n ot observed. 1,1-Di-
fluoro-2,5-dim eth ylen ecycloh exan e (8) was isolated by repeated preparative gas ph ase
th erm olysis (at vapor pressure of about 1.5 kPa each run ). 1H NMR (300 MHz, CDCl3, TMS):
3
4
2.25–2.30 (m , 2 H); 2.36–2.41 (m , 2 H); 2.67 (tt, 2 H, JHF = 13.7, JHH = 1.2); 4.87 (m , 1 H);
4.92 (m , 1 H); 5.09 (m , 1 H); 5.39 (m , 1 H). 19F NMR (282 MHz, CDCl3, CFCl3): –100.9 (t,
3
2 F, JHF = 13.7). MS EI, m/z (% rel. in t.): 144 (100), 129 (88), 115 (27), 109 (86), 97 (29).
HRMS, calculated for C8H10F2: 144.0750; foun d: 144.0748.
1,4-Bis(difluorom eth ylen e)cycloh exan e (9)
It was prepared accordin g to Sch em e 6.
O
O
F2C
CF2
9
SCHEME 6
Dien e 9 was prepared by adaptation of th e procedure described by Naae an d Burton 20
.
A 500-m l th ree-n ecked roun d-bottom flask was equipped with a m ech an ical stirrer, septum
an d pressure-equalizin g addition fun n el an d flam e dried un der argon purge. Dibrom o-
difluorom eth an e (16.93 g, 80.7 m m ol) was tran sfered to a Rotaflow tube an d th en dissolved
in 70 m l of dry THF. Th is solution was tran sferred to th e reaction vessel by syrin ge followed
by an addition al 70 m l of dry THF. Th is solution was cooled to –5 °C by an ice/salt bath .
P(N(CH3)2)3 (29.5 m l, 26.5 g, 0.162 m ol) was dissolved in 70 m l of dry THF in th e addition
fun n el. Th e CF2Br2 solution was cooled to about –5 °C with an ice/salt bath an d added
dropwise over 1 h with coolin g. Th is m ixture was stirred at –5 °C for 2 h an d th en brough t
to room tem perature. 1,4-Cycloh exan edion e (4.5 g, 40 m m ol) was dissolved in 40 m l of dry
THF an d added to th e ylide solution over 35 m in . Th e reaction m ixture was th en stirred at
room tem perature for 18 h . Th e pale yellow solids were filtered off from yellow-brown solu-
tion . Th e solution was th en con cen trated on a rotary evaporator to a volum e of about 15 m l
an d was taken up in 200 m l of dieth yl eth er. Th e eth er solution was exh austively extracted
with water an d th en dried over MgSO4. Th e eth er solution was distilled th rough a 6-cm
Vigreaux colum n to leave about 3 m l of clear brown liquid. Th is m aterial was tran sferred to
a m icrodistillation apparatus an d distilled un der reduced pressure. After in itial rem oval of
eth er, th e fraction of dien e 9 (2.5 g, 36%) boilin g at 60–62 °C at 1.3–1.6 kPa was collected.
4
1H NMR (300 MHz, CDCl3, TMS): 2.15 (m , 8 H, JHF = 1.3). 13C NMR (75 MHz, CDCl3,
2
1
CHCl3): 23.9 (m , 4 C); 86.5 (t, 2 C, JCF = 19.2); 151.2 (t, 2 C, JCF = 280.2). 19F NMR
4
(282 MHz, CDCl3, CFCl3): –97.7 (septet, 4 F, JHF = 1.3). MS EI, m/z (% rel. in t.): 180 (100),
165 (12), 145 (19), 127 (15), 111 (61), 109 (26), 103 (19), 77 (24). HRMS, calculated for
C8H8F4: 180.0562; foun d: 180.0571.
Collect. Czech. Chem. Commun. (Vol. 67) (2002)

Fluorinated 1,5-Hexadienes
1529
Gas Ph ase Th erm olysis of 1,4-Bis(difluorom eth ylen e)cycloh exan e (9)
An about 100 m g sam ple of 9 was obtain ed >99% pure by preparative GLPC (colum n 8 m m ×
3.5 m , 20% SE-30 on Ch rom osorb P 80/100). Vapor of dien e 9 was expan ded in to th e gas ki-
n etics vessel to th e pressure of about 1 kPa (8 m m Hg) an d con version of 9 to 1,1,2,2-tetra-
fluoro-3,6-dim eth ylen ecycloh exan e 10 was followed by GLPC (colum n 4 m m × 3.5 m , 20%
QF-1 on Ch rom osorb 100/120, in tern al stan dard n-octan e). Th e yield of th e tran sform ation
was m ore th an 99%. Reversible tran sform ation of 10 to 9 was n ot observed. 1,1,2,2-Tetra-
fluoro-3,6-dim eth ylen ecycloh exan e (10) was isolated by double th erm olysis of dien e 9 (at
vapor pressure of about 1.5 kPa) at 360 °C for 200 m in . 1H NMR (300 MHz, CDCl3, TMS):
2.42 (bs, 4 H); 5.35 (s, 1 H); 5.63 (s, 1 H). 19F NMR (282 MHz, CDCl3, CFCl3): –123.1 (s, 4 F).
MS EI, m/z (% rel. in t.): 180 (60), 165 (17), 145 (19), 111 (100), 77 (23). HRMS, calculated
for C8H10F2: 180.0562; foun d: 180.0564.
Preparation of 1,1′-Bi(cyclopen tan e)-2,2′-dion e (19)
It was accom plish ed accordin g to Sch em e 7.
CF2
CF2
O
O
O
O
O
H H
5A and 5B
H H
19A and 19B
18
SCHEME 7
Diketon e 19 (m ixture of diastereoisom ers 19A an d 19B) was prepared by th e procedure of
Paul24. A th ree-n ecked roun d-bottom flask was equipped with a con den ser, m agn etic stirrer
an d pressure-equalizin g addition fun n el an d th en flam e dried un der argon purge. Fresh ly
cut sodium m etal (0.75 g, 33 m m ol) was placed in th e flask followed by 20 m l of dry tolu-
en e. Th e system was h eated to reflux an d stirred rapidly breakin g sodium in to sm all sph eres.
Eth yl 2-oxocyclopen tan ecarboxylate (18; 5.0 g, 32 m m ol) was dissolved in 5 m l of dry tolu-
en e an d added dropwise to th e sodium in refluxin g toluen e wh ile stirrin g. Upon com pletion
of addition th e m ixture was h eated to reflux for 1 h after wh ich all sodium m etal h ad been
con sum ed. 2-Ch lorocyclopen tan on e (3.97 g, 33 m m ol) was dissolved in dry toluen e an d
added dropwise to th e refluxin g en olate solution . Upon com pletion of addition th e m ixture
was h eld at a gen tle reflux for 9 h . Th e resultan t solids were filtered off, th e reaction solu-
tion was wash ed with 3 × 50 m l of water an d th en dried with MgSO4. Toluen e was rem oved
on a rotary evaporator to yield 3.4 g (66%) of crude product wh ich was purified by
kugelroh r distillation to give 2.4 g (47%) of wh ite solid eth yl 2,2′-dioxo-1,1′-bi(cyclopen -
tan e)-1-carboxylate. Th is m aterial was added to a roun d-bottom flask con tain in g 15 m l of
20% HCl, assem bled with a con den ser. Th e m ixture was brough t to a gen tle reflux an d eth a-
n ol was added (about 5 m l) un til th e m ixture becam e h om ogen eous. Th e solution was
h eated to a gen tle reflux for 24 h at wh ich tim e GLPC an alysis sh owed th e decarboxylation
to be about 90% com plete. Th e reaction m ixture was extracted with 4 × 50 m l of dieth yl
eth er, wash ed with water an d dried with MgSO4. Dieth yl eth er was rem oved on a rotary
evaporator an d th e resultan t m aterial was purified by kugelroh r distillation to yield 0.91 g
(55%) of wh ite solid bein g a m ixture of diastereoisom ers 19A an d 19B. 1H NMR (300 MHz,
CDCl3, TMS): 1.52–1.88 (m , 8 Hmeso an d rac); 1.98–2.39 (m , 16 Hmeso an d rac); 2.53 (t, 2 Hmeso or rac
,
Collect. Czech. Chem. Commun. (Vol. 67) (2002)

1530
Dolbier et al.:
3JHH = 8.9); 2.63 (t, 2 Hmeso or rac
,
3JHH = 9.1). 13C NMR (75 MHz, CDCl3, CDCl3): 20.2, 20.4,
24,9, 26.3, 37.4, 37.6, 48.0, 48.7, 218.3, 219.2.
Preparation of (1R*,1′R*)- an d (1R*,1′S*)-2,2′-bis(difluorom eth ylen e)-
1,1′-bi(cyclopen tan e) (5A an d 5B)
A m ixture of diastereoisom ers 5A an d 5B was prepared by adaptation of th e procedure de-
scribed by Naae an d Burton 20. A 500-m l th ree n ecked roun d-bottom flask was equipped with
a m ech an ical stirrer, septum an d pressure-equalizin g addition fun n el an d flam e dried un der
argon purge. Dibrom odifluorom eth an e (21.25 g, 0.101 m ol) was tran sferred to a Rotaflow
tube an d th en dissolved in 80 m l of dry THF. Th is solution was th en tran sferred to th e reac-
tion vessel with a syrin ge followed by an addition al 150 m l of dry THF. Th is solution was
cooled to –5 °C with an ice/salt bath . P(N(CH3)2)3 (36.0 m l, 32.3 g, 0.198 m ol) was dissolved
in 100 m l of dry THF, tran sferred to th e addition fun n el an d added dropwise over 1.5 h to
th e cooled CF2Br2 solution . Th e resultan t slurry of wh ite solid was stirred at –5 °C for 1.5 h
after wh ich it was h eated to 40 °C. Dion e 19 (4.27 g, 25.7 m m ol) was dissolved in 20 m l of
dry THF an d added dropwise to ylide m ixture at 40 °C over 1 h with vigorous stirrin g. Th e
m ixture was th en stirred at 40 °C for 17 h . At th is tim e th e yield of m ixture of diastereo-
isom ers 5A an d 5B is 31% as ch ecked by 19F NMR vs in tern al stan dard C6H5CF3. Th e resul-
tan t reaction m ixture was filtered to rem ove tan solids an d th e solution was con cen trated
on a rotary evaporator to yield about 3 m l of a dark brown viscous liquid. Dieth yl eth er
(250 m l) was added an d th is solution was exh austively extracted with water. Th e eth er
ph ase was th en dried over MgSO4 an d con cen trated on a rotary evaporator to yield 1.9 g
(28%) of ligh t brown oil bein g a m ixture of diastereoisom ers 5A an d 5B (approxim ately ra-
tio 5.4 : 1 based on in tegration of 19F NMR spectrum )16, con tam in ated with THF, dieth yl
eth er an d OP(N(CH3)2). Mixture of 5A an d 5B: 1H NMR (200 MHz, CDCl3, TMS): 1.43 (m ,
8 H5A an d H5B); 2.78 (bs, 2 H5B); 2.96 (bs, 2 H5A). 13C NMR (50 MHz, CDCl3, CHCl3): dia-
2
stereoisom ers 5A an d 5B: 23.9, 24.6, 26.0, 26.6, 29.3, 30.9, 41.0 (m , 2 C), 93.0 (t, 2 C, JCF
=
=
1
2
19.2); 151.2 (t, 2 C, JCF = 280.9). 19F NMR (188 MHz, CDCl3, CFCl3) of 5A: –91.1 (d, 1 F, JFF
2
63.0); –92.8 (d, 1 F, JFF = 63.0). 19F NMR (188 MHz, CDCl3, CFCl3) of 5B: –91.2 (d, 1 F,
2
2JFF = 61.6); –91.5 (d, 1 F, JFF = 61.6). MS EI, m/z (% rel. in t.) of 5A: 234 (30), 214 (4), 118
(7), 117 (100), 116 (12), 115 (5), 97 (22), 77 (4). MS EI, m/z (% rel. in t.) of 5B: 234 (21), 214
(6), 118 (7), 117 (100), 116 (12), 115 (5), 97 (22), 77 (4). HRMS, calculated for C12H14F4:
234.104; foun d: 234.103.
Solution Ph ase Th erm olysis of (1R*,1′R*)- an d (1R*,1′S*)-2,2′-Bis(difluorom eth ylen e)-
1,1′-bi(cyclopen tan e) (5A an d 5B)
A 0.160 g sam ple of a m ixture of diastereoisom ers 5A an d 5B was obtain ed 97% pure by pre-
parative GLPC (con tam in ated on ly with 3% of 1,2-di(cyclopen t-1-en -1-yl)-1,1,2,2-tetra-
fluoroeth an e (6) (colum n 8 m m × 1.8 m , 20% OF-1 on Ch rom osorb WHP 80/100). Th is
m aterial was dissolved in 1.47 m l of dry, degassed dodecan e an d 0.033 g of C6H5F was
added as 19F NMR in tern al stan dard. 0.25-m l aliquots were drawn with syrin ge an d placed
in th ick wallet NMR tubes (wash ed successively with base, deion ized water, aceton e an d
flam e dried) an d n ext flam e sealed un der N2. Lower tem perature rearran gem en t of
diastereoisom er 5A to dien e 6 (94–124 °C) was carried out by th erm olysin g th e sam ple for a
specified tim e in a Statin th erm ostated oil bath , th en rapidly cooled an d stored in an ice
bath un til 19F NMR an alysis was accom plish ed. High er tem perature rearran gem en t of
Collect. Czech. Chem. Commun. (Vol. 67) (2002)

Fluorinated 1,5-Hexadienes
1531
diastereoisom er 5B to dien e 6 (274–293 °C) was carried out an alogously by th erm olysin g th e
sam ple for a specified tim e in th e th erm ostat with m olten salt bath . Th e relative con cen tra-
tion s of all reaction com pon en ts were m on itored by in tegration of th e 19F NMR sign als vs
in tern al stan dard C6H5F. Each sealed sam ple yielded on e rate con stan t for each diastereo-
isom eric com pon en t: on e from followin g th e rearran gem en t of diastereoisom er 5A at th e
lower tem perature (94–124 °C), an d on e from followin g th e reaction at elevated tem perature
(274–293 °C). After collectin g sufficien t data from th e sam ple for th erm olysis of
diastereoisom er 5A at a given tem perature th e sam ple was furth er th erm olyzed at a lower
tem perature un til n o 5A rem ain ed before begin n in g th erm olysis an d data collection for
diastereoisom er 5B at an elevated tem perature. Rearran gem en t of diastereoisom er 5A to
dien e 6 was foun d to follow th e first-order kin etics an d also was quan titative an d irrevers-
ible toward form ation of dien e 6 over th e tem perature ran ge 94.0–124.1 °C. In th erm olysis
of diastereoisom er 5B traces (less th an few percen t by in tegration after prolon ged tim e of re-
arran gem en t) of oth er m aterials were detected by 19F NMR. Th e product 6 was obtain ed for
ch aracterization by th erm olysin g
a crude sam ple (about 90% pure) of m ixture of
diastereoisom ers 5A an d 5B at 140 °C for 2 h an d th en isolatin g it from un reacted com -
poun d 5B by preparative GLPC (colum n 8 m m × 1.8 m , 20% QF-1 on Ch rom osorb WHP
3
80/100). 1H NMR (300 MHz, CDCl3, TMS): 1.96 (quin tet, 4 H, JHH = 7.54); 2.40–2.57 (m ,
3
8 H); 6.19 (bs, 2 H). 13C NMR (75 MHz, CDCl3, CHCl3): 23.3, 31.4 (m , 2 C, JCF = 1.9); 32.7,
3
135.9 (m , 2 C, JCF = 4.1). 19F NMR (282 MHz, CDCl3, CFCl3): –91.1 (d, 1 F5A
,
2JFF = 63.0);
–91.2 (d, 1 F5B,
2JFF = 61.6); –110.5 (s, 4 F). MS EI, m/z (% rel. in t.): 234 (38), 118 (7), 117
(100), 116 (11), 115 (5), 97 (18), 77 (6), 67 (5). HRMS, calculated for C12H14F4: 234.104;
foun d: 234.102.
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