1-Vinyl-1,2,2,3,3-pentafluorocyclopropane: An extraordinarily rapid vinylcyclopropane rearrangement

Article abstract of DOI:10.1016/S0022-1139(02)00187-2

The synthesis of 1-vinyl-1,2,2,3,3-pentafluorocyclopropane (1) and the kinetics of its thermal, unimolecular rearrangement to 1,4,4,5,5-pentafluorocyclopentene (2) are reported. Kinetic data were obtained by gas-phase ¹⁹FNMR at 80-120 °C, and the activation parameters for the rearrangement of 1 to 2 are: ΔG?(100 °C) = 28.7 kcal/mol, ΔH? = 26.7 kcal/mol, and ΔS? = -5.5 eu (log A = 12.7, E_a = 28.4 kcal/mol). Vinyl(pentafluorocyclopropane) (1) with a half-life of only 38 min at 110 °C, is one of the most thermally labile vinylcyclopropanes known. The isomeric E- and Z-1-propenyl-1,2,2,3,3-pentafluorocyclopropanes (3) similarly have relatively low activation parameters: ΔG?(E) ? 28.5 kcal/mol and ΔG?(Z) ? 31.1 kcal/mol. A favorable, polar biradical transition state for rearrangement is proposed to account the exceptional reactivity of these alkenyl pentafluorocyclopropanes.

Full text of DOI:10.1016/S0022-1139(02)00187-2

Journal of Fluorine Chemistry 117 (2002) 199–205
1-Vinyl-1,2,2,3,3-pentafluorocyclopropane: an extraordinarily
rapid vinylcyclopropane rearrangement$
Bruce E. Smart*, Paul J. Krusic, D. Christopher Roe, Zhen-Yu Yang
Dupont Central Research and Development, Experimental Station, Wilmington, DE 19880, USA
Received 28 June 2002; received in revised form 12 July 2002; accepted 15 July 2002
Dedicated to Professor Lev M. Yagupolskii on the occasion of his 80th birthday.
Abstract
The synthesis of 1-vinyl-1,2,2,3,3-pentafluorocyclopropane (1) and the kinetics of its thermal, unimolecular rearrangement to 1,4,4,5,5-
pentafluorocyclopentene (2) are reported. Kinetic data were obtained by gas-phase 19F NMR at 80–120 8C, and the activation parameters for
the rearrangement of 1 to 2 are: DGzð100 ꢀCÞ ¼ 28:7 kcal/mol, DHz ¼ 26:7 kcal/mol, and DSz ¼ À5:5 eu (log A ¼ 12:7, Ea ¼ 28:4 kcal/
mol). Vinyl(pentafluorocyclopropane) (1) with a half-life of only 38 min at 110 8C, is one of the most thermally labile vinylcyclopropanes
known. The isomeric E- and Z-1-propenyl-1,2,2,3,3-pentafluorocyclopropanes (3) similarly have relatively low activation parameters:
DGzðEÞ ffi 28:5 kcal/mol and DGzðZÞ ffi 31:1 kcal/mol. A favorable, polar biradical transition state for rearrangement is proposed to account
the exceptional reactivity of these alkenyl pentafluorocyclopropanes.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Vinylcyclopropane rearrangement; Fluorine substitution; Biradical; Gas-phase 19F NMR; Kinetics
1. Introduction
[11–13]. It might be expected that perfluoro(vinylcyclopro-
pane) would show the largest effect of fluorination whereby
the increased cyclopropane ring-strain and decreased C=C
p-bond energy work in concert to lower the activation energy.
In fact, new partially fluorinated vinylcyclopropanes reported
in this paper are notably more thermally labile than perfluoro
(vinylcyclopropane) or any other simple substituted vinylcy-
clopropanes [7].
Gas-phase 19F NMR has proved to be a convenient, reliable
method for determining rate constants and activation para-
meters for both first- [14–16] and second-order [17] reaction
kinetics. In the present work, it is used to determine the
kinetics of the rearrangement of 1-vinyl-1,2,2,3,3-pentafluor-
ocyclopropane (1) to 1,4,4,5,5-pentafluorocyclopropane (2),
and the isomeric E,Z-1-propenyl-pentafluorocyclopropane
(3) rearrangements to 4 (Scheme 1).
The vinylcyclopropane–cyclopentene rearrangement is an
extensively studied, prototypical small hydrocarbon thermal
reactionthathascharacteristicsofbothstepwise,biradicaland
concerted mechanisms [1,2]. The kinetics of the parent vinyl-
cyclopropanerearrangementinthegas-phasehavebeendeter-
mined by several laboratories, and essentially identical
unimolecular Arrhenius parameters have been found: log A
¼ 13.5–13.6 and Ea ¼ 49:6–49.7 kcal/mol [3–5], although
somewhat higher values (log A ¼ 14:3, Ea ¼ 51:7 kcal/mol)
have been reported more recently [6]. Rate constants and
Arrhenius parameters for several substituted vinylcyclopro-
pane rearrangements have been measured [7], including some
fluorinated derivatives [8–10]. Gem-difluorination lowers the
activation energy for rearrangement by about 8 kcal in 1-
vinyl-2,2-difluorocyclopropane (Ea ¼ 41:5 kcal/mol) [9,10],
and by about 15 kcal/mol in perfluoro(vinylcyclopropane)
(Ea ¼ 34:6 kcal/mol) [8]. These results are consistent with
the well-known effects of fluorination on increasing cyclo-
propane ring-strain and weakening its C–C bond strengths
2. Experimental
2.1. Materials
1
Solution H and 19F NMR spectra were recorded on a
$ Dupont contribution no. 8334.
* Corresponding author. Tel.: þ1-302-695-7482; fax: þ1-302-695-9799.
QE-300 (General Electric, 300 Hz) or a Bruker DRX-400
instrument using tetramethylsilane (TMS) and trichloro-
E-mail address: bruce.e.smart@usa.dupont.com (B.E. Smart).
0022-1139/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 0 2 ) 0 0 1 8 7 - 2

200
B.E. Smart et al. / Journal of Fluorine Chemistry 117 (2002) 199–205
at 80 8C in an autoclave for 8 h and distilled to give 367.4 g
of recovered BrCF2CFClBr and 367.5 g (97%) of 6, bp 82–
86 8C (25 mm), literature [18] bp 66 8C (11 mm). 19F NMR:
d À62.2 (s), À62.23 (s) (total 2F), À116.8 (m) and À119.7
(m) (total 1F). 1H NMR: d 4.61–4.41 (m, 1H), 3.10–2.56 (m,
2H), 1.87 (m, 3H).
2.1.4. 4-Bromo-1,1,2-trifluoropentene (7)
To a stirred mixture of Zn powder (22.7 g) and 180 ml of
i-PrOH at 70 8C was slowly added BrCH2CH2Br (2.0 g), and
the mixture was stirred at 70 8C for 20 min.
BrCF2CFClCH2CHBrCH3 (6) (95.5 g, 0.30 mol) was then
slowly added over 30 min. The resulting reaction mixture
was stirred for 6.5 h. All volatiles were distilled and col-
lected in a À78 8C trap at 30 mmHg. The distillate was
poured into water and the lower layer was separated, washed
with water and distilled to give 41.6 g (90% conversion, 76%
yield) of 7, bp 110–114 8C and 10.0 g (10%) of recovered
starting material. 19F NMR: d À150.9 (dd, J ¼ 196:5,
J ¼ 7:0, 1F), À152.8 (dd, J ¼ 196:5, J ¼ 7:4, 1F)
Scheme 1.
fluoromethane (CFCl3) as respective internal standards. All
NMR spectra were taken in CDCl3 solvent. IR spectra were
recorded on Perkin-Elmer-1600 FT spectrometer.
4-Bromo-1,1,2-trifluoro-1-butene (CF2=CFCH2CH2Br)
was obtained from Lancaster Synthesis Inc., and 1-
chloro-1,2-dibromo-1,2,2-trifluoroethane (BrCF2CFClBr)
from SynQuest Laboratories Inc., or Fluorochem USA.
1
À157.2 (dm, J ¼ 197:6, 1F), À211.2 (m, 1F). H NMR:
d 4.26 (hext, J ¼ 7:4, 1H), 2.70–2.90 (m, 2H), 1.77 (dd,
J ¼ 6:7, J ¼ 0:5, 3H). HRMS: calcd for C5H6F3Br:
201.9605. Found: 201.9622.
2.1.1. 2-Bromo-1-(1,2,2,3,3-pentafluorocyclopropyl)ethane
(5)
2.1.5. 2-Bromo-1-
(1,2,2,3,3-pentafluorocyclopropyl)propane (8)
A 1 l autoclave was charged with CF2=CFCH2CH2Br
(283 g, 1.50 mol) and hexafluoropropylene oxide (415 g,
2.50 mol) and heated at 190 8C for 6 h. After an additional
166 g (1.00 mol) of HFPO was added, the mixture was
heated for another 6 h. Crude product (360.5 g) was distilled
to give 7.3 g of forecut, bp 87–106 8C, and 329.8 g (92%) of
pure 5, bp 111–112 8C. 19F NMR: d À152.5 (dt, J ¼ 197
(Hz), J ¼ 7:2, 2F), À157.1 (dm, J ¼ 197, 2H), À212.3 (tt,
A mixture of 7 (40.0 g, 0.20 mol) and hexafluoropropy-
lene oxide (80.0 g) was heated at 185 8C for 6 h, and then an
additional 40 g of HFPO was added and the resulting
mixture was heated at 185 8C for another 6 h. After the
gas was vented, the residue was distilled to give 40.0 g
(80%) of 8, bp 121–125 8C. NMR indicated a diastereoiso-
meric mixture. 19F NMR: d À150.9 (dd, J ¼ 196:5, J ¼ 6:8,
2F), À157.3 (dm, J ¼ 197, 2F), À211.2 (m, 1F) and another
isomer: d À152.8 (dd, J ¼ 196:7, J ¼ 7:4, 2F), À157.6 (dt,
1
J ¼ 21:5, J ¼ 7:1, 1F). H NMR: d 3.53 (t, J ¼ 7:1, 2H),
2.55 (dm, J ¼ 21:5, 2H).
1
J ¼ 197:6, J ¼ 7:1, 2F), 211.2 (m, 1F). H NMR: d 4.30
2.1.2. 1-Vinyl-1,2,2,3,3-pentafluorocyclopropane (1)
(hext, J ¼ 6:8, 1H), 2.51 (m, 2H), 1.81 (d, J ¼ 6:8, 3H).
Anal: calcd for C6H5F5Br: H, 2.22; Br, 29.38. Found: H,
2.55; Br, 30.36. HRMS: calcd for C6H6F5Br (–Br):
173.0389. Found: 173.0287.
To a stirred solution of KOH (18.0 g) in 15 ml of ethanol
and 20 ml of water at 70 8C was slowly added 2-bromo-1-
(pentafluorocyclopropyl)ethane 5 (24.0 g, 0.10 mol). During
the addition, the volatiles were collected in a À78 8C
trap. After the addition was complete, the reaction mixture
was stirred at 70 8C for an additional 2 h until no more
volatiles were collected. The material collected in the
À78 8C trap was distilled to give 9.1 g (58%) of 1, bp
30 8C. 19F NMR: d À152.3 (dt, J ¼ 195:4, J ¼ 9:8, 2F),
2.1.6. E,Z-1-(1,2,2,3,3-Pentafluorocyclopropyl)propene (3)
To a stirred solution of KOH (15 g) in 50 ml of ethanol
and 50 ml of water at 70–75 8C was slowly added 8 (35.4 g,
0.14 mol), during which volatiles were distilled out and
collected in a À78 8C receiver. The distillate was poured
into water and the lower layer was separated to give 19.8 g of
crude product, which was distilled to give 17.7 g (76%) of
pure 3 as a 5:1 mixture of E:Z isomers 19F NMR for E-3: d
À152.2 (d, J ¼ 194, 2F), À156.3 (dm, J ¼ 194, 2F), À202.6
(d, J ¼ 8:8, 1F); for Z-3: d À151.9 (dm, J ¼ 192, 2F),
1
À156.0 (dm, J ¼ 195:6, 2F), À210.7 (t, J ¼ 7:7, 1F). H
1
NMR: d 5.86–5.73 (m, 2H), 5.70–5.65 (m, H). IR (gas)
(cmÀ1): 3110 (w), 1650 (w), 1515 (m), 1291 (s), 1189 (s),
933 (s). The product 1 is unstable at room temperature and is
best stored cold.
1
À157.4 (dm, J ¼ 192, 2F), À193 (m, 1F). H NMR for
2.1.3. 2-Chloro-1,4-dibromo-1,1,2-trifluoropentane (6)
A mixture of BrCF2CFClBr (749 g, 2.71 mol), benzoyl
peroxide (10 g) and propylene (50.0 g, 1.19 mol) was heated
E-3: d 6.28 (dqd, J ¼ 15:8, J ¼ 6:7, J ¼ 2:1, 1H), 5.53 (dm,
J ¼ 15:8, 1H), 1.88 (m, 3H). Z-3: d 6.38 (m, 1H), 5.44 (m,
1H), 1.86 (m, 3H).

B.E. Smart et al. / Journal of Fluorine Chemistry 117 (2002) 199–205
201
Fig. 2. First-order kinetic plots for 1 to 2 rearrangement.
Fig. 1. Gas-phase 19F NMR of 1 and 2.
Table 1
Rates of thermal isomerization of 1 to 2
Temperature (8C)
80.0
0.050
1.39 ꢂ 0.01
90.0
0.147
4.08 ꢂ 0.04
100.0
0.400
11.1 ꢂ 0.1
110.0
1.11
30.8 ꢂ 0.3
120.0
3.13
86.9 ꢂ 0.9
k (hÀ1
)
k (ꢁ105 sÀ1
)
2.1.7. Rearrangement of E,Z-3: 1,2,2,3,3-pentafluoro-
4-methylcyclopentene (4)
the automated acquisition of spectra have been described in
detail [14,17]. The gas-phase NMR ampules are available
commercially from New Era Enterprises, Vineland, NJ. All
spectra were obtained with a GE NMR Instruments Omega
360 spectrometer. As before, no field lock is used, and the
ampules are not spun.
A 5-ml tube was charged with 3 (0.23 g, E:Z ¼ 5:1) and
then the tube was cooled in liquid N2 and then evacuated.
The content of the tube was stirred at 115 8C for 3.5 h, and
then at 131 8C for 1.5 h. NMR analysis indicated a 19.2:1
mixture 4 and unreacted Z-3. 19F NMR (4): d À116.1 (dm,
J ¼ 239, 1F), À117.8 (dm, J ¼ 255:9, 1F), À122.8 (dd,
J ¼ 255:8, J ¼ 14:5, 1F), À127.3 (dm, J ¼ 239, 1F),
2.2.1. 1-Vinyl-pentafluorocyclopropane
(1) – cyclopentene (2) rearrangement
1
À139.9 (m, 1F). H NMR: d 5.63 (m, 1H), 3.04 (m, 1H),
The gas-phase NMR spectrum of 1 at the start of a
kinetic run (80 8C) and the spectrum of the rearranged
product 2 at the end of the run are shown in Fig. 1. The
integrated area of the C–F resonance in 1 at d À212 ppm
was monitored with time. Clean first-order kinetics were
observed from 80 to 120 8C (Fig. 2), from which the first-
order rate constants at 80, 90, 100, 110, and 120 were
obtained (Table 1).
1.21 (dd, J ¼ 8:3, J ¼ 1:0, 3H). HRMS: calcd for C6H5F5:
172.0311. Found: 172.0324.
2.2. Gas-phase 19F NMR kinetics
The NMR reaction vessel made from a short section of a
10-mm NMR tube, the procedure for loading the tube, and
Table 2
Rates of thermal isomerization of E-3 to 4
Temperature (8C)
90.0
0.565 ꢂ 0.01
100.0
1.44 ꢂ 0.01
110.0
3.58 ꢂ 0.04
120.0
9.02 ꢂ 0.1
k (ꢁ104 sÀ1
)
Table 3
Rates of thermal isomerization of Z-3 to 4
Temperature (8C)
110.0
0.115 ꢂ 0.01
120.0
0.233 ꢂ 0.02
130.0
0.541 ꢂ 0.05
140.0
1.26 ꢂ 0.1
k (ꢁ104 sÀ1
)

202
B.E. Smart et al. / Journal of Fluorine Chemistry 117 (2002) 199–205
Fig. 4. Arrhenius plot for 1 to 2 rearrangement.
Fig. 3. Eyring plot for 1 to 2 rearrangement.
The activation parameters for the rearrangement and the
corresponding 95% confidence limits were obtained by non-
least squares fitting of the Eyring (Fig. 3) and Arrhenius
(Fig. 4) equations. For 1 to 2 (Fig. 4), DGzð100 ꢀCÞ ¼ 28:7ꢂ
0:1 kcal/mol, DHz ¼ 26:7 ꢂ 0:3 kcal/mol, andDSz ¼À5:5ꢂ
0:8 eu; log A ¼ 12:7 ꢂ 0:05, Ea ¼ 28:4 ꢂ 0:3 kcal/mol.
versus temperature gave the activation parameters: (E-3 to 4,
Fig. 7) DGzð100 ꢀCÞ ffi 28:5 kcal/mol, DHz ¼ 25:4 ꢂ 1:7
kcal/mol, and DSz ¼ À8 ꢂ 4 eu; (Z-3 to 4, Fig. 8) DGz
ð100 ꢀCÞ ffi 31:1 kcal/mol, DHz ¼ 24:5 ꢂ 4:9 kcal/mol, and
DSz ¼ À18 ꢂ 12 eu. The parameters are less precise than
those for the rearrangement of 1, particularly for the Z-3
isomer.
2.2.2. E,Z-1-Propenyl-pentafluorocyclopropane
(3) – cyclopentene (4) rearrangements
The gas-phase 19F NMR spectra of the starting 5:1
mixture of E,Z-3 at 100 8C and the mixture of E,Z-3 and
rearranged product 4 during a kinetic run (120 8C) are
displayed in Figs. 5 and 6, respectively. The CÀF resonances
in E,Z-3 varied slightly with temperature but the integrands
of E-3 C–F (dave ¼ À203:9 for 90–120 8C) and Z-3 C–F
(dave ¼ À198:4 for 110–140 8C) were easily obtained for the
kinetic plots. Good first-order kinetics were obtained
(Tables 2 and 3). Least-squares plots of the rate constants
3. Results and discussion
The three alkenyl pentafluorocyclopropanes for the
kinetic studies were easily synthesized from commercially
available starting materials using conventional methods
(Schemes 2 and 3). The 1-propenyl-pentafluorocyclopro-
pane was prepared as a 5:1 mixture of E,Z-3 isomers, which
was not separated but used directly in the gas-phase kinetic
runs where the well-resolved methine C–F resonances of the
Fig. 5. Gas-phase 19F NMR of E,Z-3 (100 8C).

B.E. Smart et al. / Journal of Fluorine Chemistry 117 (2002) 199–205
203
Fig. 6. Gas-phase 19F NMR of E,Z-3 þ 4 (120 8C).
isomers (dave ¼ À203:9 and À198.4 ppm) were easily mon-
itored (see Section 2). All three vinylcyclopropanes cleanly
rearranged to the expected cyclopentenes and no byproducts
were detected by 19F NMR in any of the kinetic runs.
Excellent first-order kinetics were obtained for all three re-
arrangements, although the data for the slower rearrangement
of the minor Z-3 isomer is the least precise. The activation
energies for the rearrangements of 1, E-3, and Z-3 are com-
pared with those for related vinylcyclopropanes in Table 4.
Fig. 7. Eyring plot for E-3 to 4 rearrangement.
Scheme 2.
Fig. 8. Eyring plot for Z-3 to 4 rearrangement.
Scheme 3.

204
B.E. Smart et al. / Journal of Fluorine Chemistry 117 (2002) 199–205
Table 4
Activation energies for vinylcyclopropane–cyclopentene rearrangements
Vinylcyclopropane
Ea (kcal/mol)
Reference
49.6–49.7, 51.7
[3–5,6]
Fig. 9. Polar biradical transition states for rearrangements of alkenyl
pentafluorocyclopropanes.
41.5
[9,10]
In the conventional biradical transition states for the
rearrangements of 1, E-3, and Z-3, there is a favorable,
stabilizing polar contribution which is depicted in Fig. 9.
Similar polar effects have been invoked to rationalize
somewhat related cyclopropylcarbinyl free-radical rearran-
gements [20,21], although more recent analysis indicates
the effect is not important in difluorocyclopropylcarbinyl
or difluorooxiranylcarbinyl radicals [22,23]. For the much
more electron deficient pentafluorocyclopropyl systems in
this study, however, there is greater likelihood that polar
effects will be important. Notably, E-3 with its more
electron rich double bond compared to that in 1 would
be expected to have an even more favorable polar transition
state (Fig. 9, E-3ts versus 1ts), and thus should be more
reactive than 1. This indeed is the case (Table 4) where
t1=2 ¼ 32 min at 110 8C for E-3 compared with 38 min for 1
at 110 8C. For the Z-3 isomer, however, there is complicat-
ing, unfavorable steric repulsion between the syn-CH3
group and the cyclopropane ring in the transition state
Z-3ts that will result in decreased reactivity. In fact, Z-3
(t1=2 ¼ 3:6 h at 110 8C) is less reactive than 1 (the uncer-
tainty in the activation parameters for Z-3 makes tenuous
any direct comparisons of the data in Table 4, although the
comparative rate constants from Tables 1–3 unequivocally
show an order of reactivity E-3 > 1 > Z-3).
There is another interesting possible ‘‘polar’’ effect invol-
ving a stabilizing charge-transfer interaction between the
electron deficient cyclopropane ring and the alkenyl double
bond which would favor the syn-conformation approaching
the transition state (Fig. 10). Again, the same electronic and
steric effects discussed above would favor the syn-E-3
versus syn-1 conformer and relatively disfavor the syn-Z-
3 one. Notably, the activation entropies are negative for the
alkenyl pentafluorocyclopropane rearrangements compared
with positive values for the other vinylcyclopropane rear-
rangements in Table 4 (DSz ¼ À5:5 eu for 1 versus 2.5 eu
for perfluoro(vinylcyclopropane), for example), and like-
wise their log A values are comparatively smaller
(log A ¼ 12:7 for 1 versus 13.5–14.3 for vinylcyclopropane
itself and 13.9 for perfluoro(cyclopropane)). These entropy
differences imply more ordered transition states for the
alkenyl pentafluorocyclopropane rearrangements, which is
consistent with either a ‘‘tighter’’ polar biradical transition
state or one involving charge-transfer complexation.
34.6
[8]
28.4 ꢂ 0.3, (27.4 ꢂ 0.3)a,b
This work
This work
This work
26.2 ꢂ 1.7a,c
25.3 ꢂ 4.9a,d
a Calculated Ea ¼ DHz þ RTm.
b Tm ¼ 373 K.
c Tm ¼ 378 K.
d Tm ¼ 398 K.
The relatively low activation energies for 1 and E,Z-3, all
of which are over 21 kcal/mol less than that for the parent
hydrocarbon and even over 8 kcal/mol less than that for the
perfluoro(vinylcyclopropane), are truly striking. Vinyl(pen-
tafluorocyclopropane) (1) with a half-life of about 38 min at
110 8C compared with 135 h for perfluoro(vinylcyclopro-
pane) at the same temperature [8], is one of the most
thermally labile vinylcyclopropanes known. In fact, we
are not aware of any monocyclic vinylcyclopropanes with
Ea or DGz values for rearrangement as low as those for 1 and
E,Z-3. A practical consequence of this unusual reactivity is
that 1 can be free-radically polymerized with a wide variety
of fluorinated monomers to produce copolymers containing
the –CF2CF2CF=CHCH2– repeat unit that is suitable for
subsequent grafting or cross-linking [19].
The expected effects of fluorination on cyclopropane ring-
strain and its bond strengths alone are clearly not enough to
explain the extraordinary reactivity of the alkenyl penta-
fluorocyclopropanes (see Section 1). A special stabilizing
‘‘polar effect’’ in the transition states, and possibly one
favoring transition state-like conformations, that are pecu-
liar to the semi-fluorinated systems with a very electron
deficient cyclopropane ring combined with a relatively
electron-rich alkene are postulated to explain their unusual
kinetic reactivities.
Computational modeling of the alkenyl pentafluorocyclo-
propane conformations and their transition states for rear-

B.E. Smart et al. / Journal of Fluorine Chemistry 117 (2002) 199–205
205
demonstrates the dramatic and unexpected effect that fluor-
ine can have on a well-studied, classical small-molecule
reaction.
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date the electronic effects underlying the observed rate
accelerations, but this is beyond the scope of the present
work. Despite the lack of a thorough understanding of
mechanism and the substituent effects, this work once again
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