The 2-vinylphosphirane-3-phospholene rearrangement: Biradicaloid and concerted features

Article abstract of DOI:10.1021/ja993829q

Activation parameters have been determined for the 2-vinylphosphirane-3- phospholene rearrangement in the cycloheptane annellated series 7 → 8. For the thermal reaction, the mean activation parameters (ΔH(+) = 125 ± 5 kJ mol^-1 and ΔS(+) = -1 ± 14 J K^-1 mol^-1) are as expected for a biradicaloid character of the transition structure. In the presence of Cu(I) as a catalyst, the activation parameters for this conversion (ΔH(+) = 80.2 ± 3 kJ mol^-1 and ΔS(+) = -100 ± 7 J K^-1 mol^-1) are strikingly different and more in line with a concerted pathway.

Full text of DOI:10.1021/ja993829q

J. Am. Chem. Soc. 2000, 122, 3033-3036
3033
The 2-Vinylphosphirane-3-Phospholene Rearrangement: Biradicaloid
and Concerted Features
Maurice J. van Eis, Tom Nijbacker, Franciscus J. J. de Kanter, Willem H. de Wolf,
Koop Lammertsma,* and Friedrich Bickelhaupt*
Contribution from the Scheikundig Laboratorium, Vrije UniVersiteit, De Boelelaan 1083,
NL-1081 HV, Amsterdam, The Netherlands
ReceiVed October 27, 1999
Abstract: Activation parameters have been determined for the 2-vinylphosphirane-3-phospholene rearrangement
in the cycloheptane annellated series 7 f 8. For the thermal reaction, the mean activation parameters (∆Hq )
125 ( 5 kJ mol-1 and ∆Sq ) -1 ( 14 J K-1 mol-1) are as expected for a biradicaloid character of the
transition structure. In the presence of Cu(I) as a catalyst, the activation parameters for this conversion (∆Hq
) 80.2 ( 3 kJ mol-1 and ∆Sq ) -100 ( 7 J K-1 mol-1) are strikingly different and more in line with a
concerted pathway.
Introduction
Scheme 1a
Since its discovery nearly 4 decades ago,1 the structural
rearrangement of vinylcyclopropane (1a) to cyclopentene (2a)
has been subject to controversy.2 Two limiting pathways have
been proposed: (a) a rearrangement involving a biradical
intermediate 3a and (b) a concerted [1,3] sigmatropic shift
(Scheme 1).
Interestingly, the reaction shows characteristics of both. There
is ample evidence for the involvement of a biradical.3 However,
the rearrangement of, e.g., a tert-butyl substituted vinylcyclo-
propane proceeds with high stereospecificity under inversion
at the migrating carbon in support of a Woodward-Hoffmann
allowed suprafacial [1,3] sigmatropic shift.4 Recent studies5
suggest that these seemingly contradictory results may be
reconciled by recognizing that the transition state structure lies
on a rather flat plateau and has considerable biradical character,
with dynamic effects playing a decisive role in the stereochem-
ical course of the reaction.
a a: X ) CH2. b: X ) RP. c: X ) PhPW(CO)5.
occurs via a biradical intermediate.6 Mathey et al.7 reported the
addition of terminal phosphinidene complex PhPW(CO)5 (4)
to 1,3-butadiene to give a 2-vinylphosphirane complex of type
1c, which rearranges to the corresponding 3-phospholene (2c),
but stereochemical details were not provided. Lammertsma et
al.8 studied the addition of 4 to cyclic dienes and reported a
vinylphosphirane-phospholene rearrangement with inversion
at the phosphorus center but provided no kinetic data. Recently,
we observed the formation of phospholenes both by a direct
1,4-addition of 4 to cisoid 1,3-dienes and by a stepwise process
via the corresponding 1,2-adducts.9 In the present study we
report on the kinetics of this latter conversion, i.e., the
rearrangement of a vinylphosphirane to the (formal) 1,4-adduct,
a phospholene.
Surprisingly, the related vinylphosphirane-phospholene rear-
rangement has received little attention. On the basis of an
exploratory kinetic study, Richter proposed that the thermal
rearrangement of 1-tert-butyl-2-vinylphosphirane (1b, R ) t-Bu)
Results and Discussion
(1) (a) Neureiter, N. P. J. Org. Chem. 1959, 24, 2044. (b) Vogel, E.
Angew. Chem. 1960, 72, 4, ref 162. (c) Overberger, C. G.; Borchert, A. E.
J. Am. Chem. Soc. 1960, 82, 1007, 4896.
Reaction of 1,2-dimethylenecycloheptane (6) with 4, gener-
ated by the CuCl-catalyzed cheletropic elimination from 5 at
50 °C in xylene, affords a mixture of 1,2-adducts 7a and 7b
and 1,4-adduct 8 in 53, 28, and 19% yield, respectively (Scheme
2).9 Heating to 100 °C converts both vinylphosphiranes 7a,b to
phospholene 8; the latter was characterized by an X-ray crystal
structure determination.9 The 1,2-adducts 7a,b could not be
isolated separately, but were identified in the mixture by 31P
NMR spectroscopy.
(2) (a) For a review of the extensive early literature see: Gajewski, J. J.
Hydrocarbon Thermal Isomerizations; Academic Press: New York, 1980;
pp 81-87. (b) For a recent review see: Baldwin, J. E. J. Comput. Chem.
1998, 19, 222.
(3) For a viewpoint in favor of diradicals see: (a) Baldwin, J. E.;
Bonacorsi, S. J., Jr. J. Am. Chem. Soc. 1994, 116, 10845. (b) Baldwin, J.
E.; Bonacorsi, S. J., Jr. J. A. Chem. Soc. 1996, 118, 8258.
(4) For experiments in favor of a concerted mechanism see: (a) Gajewski,
J. J.; Olson, L. P. J. Am. Chem. Soc. 1991, 113, 7432. (b) Gajewski, J. J.;
Olson, L. P.; Willcott, M. R., III J. Am. Chem. Soc. 1996, 118, 299.
(5) (a) Davidson, E. R.; Gajewski, J. J. J. Am. Chem. Soc. 1997, 119,
10543. (b) Houk, K. N.; Nendel, M.; Wiest, O.; Storer, J. W. J. Am. Chem.
Soc. 1997, 119, 10545. (c) Sperling, D.; Fabian, J. Eur. J. Org. Chem. 1999,
1, 215. (d) Doubleday, C.; Nendel, M.; Houk, K. N.; Thweatt, D.; Page,
M. J. Am. Chem. Soc. 1999, 121, 4720. (e) Baldwin, J. E.; Bonacorsi, S. J.,
Jr.; Burell, R. C. J. Org. Chem. 1998, 63, 4721. (f) Baldwin, J. E.; Burell,
R. C. J. Org. Chem. 1999, 64, 3567. (g) Baldwin, J. E.; Shukla, R. J. Am.
Chem. Soc. 1999, 121, 11018.
(6) (a) Richter, J. W. Chem. Ber. 1983, 116, 3293. (b) Richter, J. W.
Chem. Ber. 1985, 118, 97. (c) Richter, J. W. Chem. Ber. 1985, 118, 1575.
(7) Marinetti, A.; Mathey, F. Organometallics 1982, 3, 456.
(8) Lammertsma, K.; Hung, J.; Chand, P.; Gray, G. M. J. Org. Chem.
1992, 57, 6557.
(9) van Eis, M. J.; de Kanter, F. J. J.; de Wolf, W. H.; Lammertsma, K.;
Bickelhaupt, F. Tetrahedron 2000, 56, 129.
10.1021/ja993829q CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/21/2000

3034 J. Am. Chem. Soc., Vol. 122, No. 13, 2000
Van Eis et al.
Table 1. Kinetic Data for the Thermal Rearrangement of 7 to 8
entry
T (K)
106k (7a) (s-1
)
106k (7b) (s-1
)
1
2
3
4
5
403.0
393.0
373.0
363.0
348.0
428.1 ( 6.4
167.4 ( 3.2
20.7 ( 0.45
8.62 ( 0.32
0.77 ( 0.09
594.8 ( 14.8
271.2 ( 8.6
29.67 ( 1.36
12.49 ( 0.54
1.62 ( 0.06
mol-1 12 Strain relief of the phosphirane further reduces ∆Hq;
.
for the parent C2H5P system, a ring strain of 84 kJ mol-1 was
found at G2 theory.13 Combining these energies gives ∆Hq )
117 kJ mol-1 for generating biradical intermediate 3b from 1b
(R ) H). This estimate does, of course, not consider the effect
of W(CO)5 complexation on the P-C bond strength, on the
phosphirane ring strain, or on the resonance stabilization of the
biradical.13 Moreover, it is reasonable to assume that the allylic
delocalization is not yet fully developed in the transition state
and that the ring strain has not been released completely. For
example, based on kinetic data, it was suggested that the
transition state for the vinylcyclopropane-cyclopentene rear-
rangement retains 21 kJ mol-1 of ring and torsional strain.10c
Therefore, one may conclude that the actual activation enthalpy
for the formation of biradical 3c is higher than the 117 kJ mol-1
mentioned above; indeed, this is what we find, and the
differences are seductively small (7a f 8, ∆∆Hq ) 10.4 kJ
mol-1; 7b f 8, ∆∆Hq ) 4.6 kJ mol-1). The low activation
entropies of 5.0 ( 16.0 (7a) and -6.8 ( 9.6 J K-1 mol-1 (7b)
are also in line with a biradicaloid transition structure. A
similarly low ∆Sq ) 6 ( 5 J K-1 mol-1 has been reported for
the rearrangement of 1-alkoxy-1-vinylcyclopropane to the
corresponding 1-alkoxycyclopentene.14 It was suggested to
follow a biradicaloid pathway; the relatively small value of ∆Sq
was attributed to cancellation of positive contributions (cyclo-
propane ring opening) and negative ones (hindered rotation of
the allyl radical).14 Analogous entropy effects may be expected
for a biradicaloid rearrangement 7 f 8.
Conducting the vinylphosphirane rearrangement in the pres-
ence of the N-tert-butylphenylnitrone spin trap gave no detect-
able amounts of radicals. This testifies to the at best fleeting
existence2b,5g,15 of a biradical intermediate such as 3.
Thus it appears that the thermal vinylphosphirane-phos-
pholene rearrangement 7 f 8 has kinetic parameters which place
it more at the biradicaloid end of the flat mechanistic plateau
that is well-known for the vinylcyclopropane-cyclopentane
rearrangement.
During this investigation, we found the rearrangement to
proceed markedly faster in crude reaction mixtures, presumably
due to the presence of traces of CuCl. Again, the disappearance
of 7 followed first-order kinetics, but the reproducibility for
different runs was somewhat less satisfactory than that for the
uncatalyzed reaction because of presumed decomposition (31P
NMR: 10-20%, especially in the higher temperature range).
Interestingly, the reaction rates for the conversion to 8 are the
same for 7a and 7b within the limits of detection over the
temperature domain of 50-100 °C (Table 2). As a result, the
ratio of 7a and 7b remains constant and the linear relationship
between ln(k/T) and 1/T is essentially identical for both isomers
[R ) 0.998] (Figure 2).
Figure 1. Eyring Plot for the uncatalyzed thermal rearrangement of 7
to 8. Squares and circles represent the data from Table 1 for 7a and
7b, respectively. The straight lines are unweighted linear regressions.
Scheme 2
The mixtures of 7a,b and 8 employed in the present study
were freshly prepared from 5 and 6 and carefully purified by
TLC (2-3 times) to completely remove CuCl. We determined
rate constants k for the conversion of 7a,b to 8 at 75-130 °C
in 10-15 deg intervals by monitoring the disappearance of their
31P NMR chemical shifts at δ -136.1 and -132.3 ppm.
Excellent mass recovery of >90% was observed for all
conversions. First-order kinetics was confirmed by the linear
relationship between ln([7]) and time (t). Interestingly, the
reaction rates are slightly different for 7a and 7b. Each isomer
gives a good linear relationship between ln(k/T) and 1/T [7a, R
) 0.997; 7b, R ) 0.999] (Figure 1). The activation parameters
for the rearrangement 7a f 8 are ∆Hq ) 127.4 ( 6.0 kJ mol-1
and ∆Sq ) 5.0 ( 16.0 J K-1 mol-1 (Arrhenius parameters (T
) 373 K): Ea ) 130.5 ( 6.1 kJ mol-1 and ln A ) 31.3 ( 1.9).
For the process 7b f 8, these values are ∆Hq ) 121.6 ( 3.6
kJ mol-1 and ∆Sq ) - 6.8 ( 9.6 J K-1 mol-1 (Arrhenius
parameters (T ) 373 K): Ea ) 124.8 ( 3.7 kJ mol-1 and ln A
) 29.9 ( 1.2). The average values for 7 f 8 are ∆Hqav ) 125
( 5 kJ mol-1 and ∆Sq ) -1 ( 14 J K-1 mol-1
.
av
Even though the activation enthalpies are considerably smaller
than those for the analogous vinylcyclopropane-cyclopentene
rearrangement (216 kJ mol-1),10 they are in line with expecta-
tions for a biradicaloid pathway. A limiting ∆Hq value for such
a pathway can be deduced as follows. Homolytic cleavage of a
P-C bond is estimated at 264 kJ mol-1 11
Stabilization of the
.
resulting biradical by allylic resonance is estimated at 63 kJ
(12) Korth, H.-G.; Trill, H.; Sustmann, R. J. Am. Chem. Soc. 1981, 103,
4483.
(13) Lammertsma, K.; Wang, B.; Hung, J.-T.; Ehlers, A. W.; Gray, G.
M. J. Am. Chem. Soc. 1999, 121, 11650.
(14) McGaffin, G.; de Meijere, A.; Walsh, R. Chem. Ber. 1991, 124,
939.
(15) Willcott, M. R.; Cargill, R. L.; Sears, A. B. Prog. Phys. Org. Chem.
1972, 9, 25 and references cited.
(10) (a) Flowers, M. C.; Frey, H. M. J. Chem. Soc. 1961, 3547. (b)
Wellington, B. J. Phys. Chem. 1962, 66, 1671. (c) Lewis, D. K.; Charney,
D. J.; Kalra, B. L.; Plate, A.; Woodard, M. H. J. Phys. Chem. A 1997, 101,
4097.
(11) Gilheany, D. G. In The Chemistry of Organophosphorus Com-
pounds; Hartley, F. R., Ed.; John Wiley & Sons Ltd: London, 1990; Vol.
1, pp 9-49.

The 2-Vinylphosphirane-3-Phospholene Rearrangement
J. Am. Chem. Soc., Vol. 122, No. 13, 2000 3035
8 (∆Hqav ) 125 kJ mol-1 and ∆Sqav ) -1 J K-1 mol-1) suggest
a mechanism with biradicaloid features, in line with the
analogous vinylcyclopropane rearrangement. Interestingly, the
rearrangement is catalyzed by CuCl, and according to the
activation parameters (∆Hq ) 80 kJ mol-1 and ∆Sq ) - 100
J K-1 mol-1), this apparently shifts the mechanism toward a
concerted process.
Table 2. Kinetic Data for the Cu(I)-Catalyzed Thermal
Rearrangement of 7 to 8
entry
T (K)
106k (7a) (s-1
)
106k (7b) (s-1
)
1
2
3
4
5
373.0
353.0
342.4
332.8
323
305 ( 17
56.3 ( 0.2
26.1 ( 1.3
11.8 ( 1.5
4.6 ( 0.4
303 ( 48
59.7 ( 4.3
24.5 ( 2.2
13.5 ( 1.5
4.3 ( 0.4
Experimental Section
General Procedures. 1H, 13C, and 31P NMR spectra were recorded
on a Bruker MSL 400 spectrometer at 400.13, 100.62, and 162.0 MHz,
respectively. 1H NMR spectra were referenced to CHCl3 (δ 7.27 ppm),
13C NMR spectra to CDCl3 (δ 77.0 ppm), and 31P NMR spectra to
external H3PO4. High-resolution mass spectrometry (HRMS) was
performed on a Finnigan MAT-90 mass spectrometer operating at an
ionization potential of 70 eV. Melting points were measured on samples
in unsealed capillary tubes and are uncorrected. Microanalyses were
performed by Microanalytisches Labor Pascher. Xylene (mixture of
isomers) and toluene were purchased from Aldrich, distilled from
sodium, and stored on sodium under nitrogen. CuCl (99.99% purity)
was purchased from Acros and stored under nitrogen. Preparative thick-
layer chromatography was carried out on alumina plates. Diene 618
and 7a, 7b, and 8 were prepared as described previously;9 7a and 7b
were obtained as an inseparable mixture with the corresponding 1,4-
adduct 8, and have not been isolated.9 Their identity is based on 31P
NMR spectroscopy and on their (close to) quantitative conversion to
8.
Kinetic Measurements. (a) Uncatalyzed Rearrangement. Double
and more runs were performed with slightly different amounts of
starting materials 7a,b and 8 which were obtained in several prepara-
tions. In all cases, identical k values were obtained. A representative
experiment is described below.
Figure 2. Eyring Plot for the CuCl-catalyzed thermal rearrangement
of 7 to 8. Squares and circles represent the data from Table 2 for 7a
and 7b, respectively. The straight line is an unweighted linear regression.
In a one-necked flask, 5 (243 mg, 0.37 mmol) and CuCl (5 mg)
were 3 times evacuated and flushed with nitrogen. Then toluene (3
mL) and 6 (74 mg, 0.6 mmol) were added and the mixture was stirred
under nitrogen for about 30 h at 23 °C. At this stage, 31P NMR revealed
the presence of a mixture of 7a,b, and 8 in a ratio of approximately
2:1:1. The reaction mixture was concentrated at reduced pressure and
the residue was purified by column chromatography (Al2O3/pentane)
to afford a yellow oil. Further purification was achieved by repetitive
TLC (Al2O3/pentane) (2-3 times) to yield a pale yellow oil consisting
of 7a, 7b, and 8 in a 2:1:1 ratio. Yield: 137 mg (0.25 mmol, 67%).
In an NMR tube, 10-20 mg samples of this mixture were dissolved
in 0.50 mL of dry toluene under nitrogen. After tightly capping the
NMR tube with a Teflon cap, the conversion of 7a and 7b to 8 was
monitored by 31P NMR spectra which were acquired at 20-60 min
intervals depending on the temperature at which the reaction was
performed. Normally 10-30 data points were collected. The 31P NMR
spectra were recorded separately (7a + 7b; 8) and successively in
unlocked mode with a SW of 4000 Hz and decoupling only during
aquisition. Multipulse decoupling with WALTZ-16 has been applied
to suppress heating effects during acquisition. In each case, 32 transients
were acquired over 5 min (pulse repetition time 9.5 s, acquisition time
2.05 s, relaxation delay 7.5 s). After careful phasing of the signals,
their areas were accurately integrated. The temperature of the NMR
probe was calibrated using the standard ethylene glycol method of Geet
before acquisition.19 Temperatures were shown to vary by no more
than 0.1 °C during the course of each experiment, but the accuracy is
assumed to be not better than (1 °C.
The activation parameters for the CuCl catalyzed rearrange-
ment 7 f 8 are ∆Hq ) 80.2 ( 2.5 kJ mol-1 and ∆Sq ) -100
( 7 J K-1 mol-1 (Arrhenius parameters (T ) 373 K): Ea )
83.9 ( 3.1 kJ mol-1 and ln A ) 18.9 ( 0.9). The activation
enthalpy of 80 kJ mol-1 is significantly lower than that for the
uncatalyzed biradicaloid reaction for 7a (127 kJ mol-1) and 7b
(122 kJ mol-1). This fact as well as the strongly negative
activation entropy of -100 J K-1mol-1 point to a higher degree
of concertedness of this reaction which must be attributed to
the presence of CuCl. The transition-metal (Ni0, Pd0) catalyzed
rearrangement of substituted 2-vinylphosphiranes reported by
Richter6b,c may be considered as a precedent for such a
transformation. While the role of copper(I) as a catalyst remains
to be elucidated, we speculate that initially, Cu(I) coordinates
to the double bond16 of 7 and subsequently stabilizes the
transition structure while shifting it toward the concerted end
of the mechanistic spectrum. However, it must be pointed out
that the 1H NMR spectrum of 7 remains unchanged upon
addition of CuCl; therefore the initial activating copper complex
must be present in a low concentration beyond the detection
limit of 1%. We consider cleavage of the C-P bond by Cu(I)
to form a four-membered metallacycle17 prior to rearrangement
to 8 a less likely alternative.
Conclusions
The rearrangement of 7 was studied at five temperatures in the range
from 75 to 130 °C. First-order graphs were obtained by plotting the
natural logarithm of the integrals, ln([7]), vs time (t). Rate constants
were obtained from the slope of the linear regression line to the
experimental data using a 95% confidence level with errors of
approximately 2 standard deviations (Table 1).
The kinetic data for the thermal rearrangement of the W(CO)5
complexed vinylphosphirane 7 to the corresponding phospholene
(16) Complexes of Cu(I) with olefins have been well documented. For
a review see: Van Koten, G.; Noltes, J. G. In ComprehensiVe Organome-
tallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;
Pergamon: Oxford, 1982; Vol. 2, p 709.
(17) Precedents for the formation of four-membered metallaphos-
phacycles from Pd or Pt complexes and complexed phosphiranes have been
reported, see: Carmichael, D.; Hitchcock, P. B.; Nixon, J. F.; Mathey, F.;
Richard, L. J. Chem. Soc., Dalton Trans. 1993, 1811.
(18) Van Straten, J. W.; Van Norden, J. J.; Van Schaik, T. A. M.; Franke,
G. Th.; de Wolf, W. H.; Bickelhaupt, F. Recl. TraV. Chim. Pays-Bas 1978,
97, 105.
(19) Geet, A. L. Anal. Chem. 1970, 42, 672.

3036 J. Am. Chem. Soc., Vol. 122, No. 13, 2000
Van Eis et al.
The Eyring plots (ln(k/T) vs 1/T) are shown in Figure 1. Activation
parameters were obtained by a linear least-squares fit to the data: (7a
and 7b to 8 was monitored by 31P NMR spectroscopy. For additional
details see the uncatalyzed reactions.
f 8) ∆Hq ) 127.4 ( 6.0 kJ mol-1 and ∆Sq ) 5.0 ( 16.0 J K-1 mol-1
;
The rearrangement of 7 was studied at five temperatures in the range
from 50 to 100 °C. Rate constants (Table 2) and Eyring plots (Figure
2) were obtained as described for the uncatalyzed rearrangement giving
(7b f 8) ∆Hq ) 121.6 ( 3.6 kJ mol-1 and ∆Sq ) - 6.8 ( 9.6 J K-1
mol-1. Errors are 95% confidence regions and correspond to 2 standard
deviations.
∆Hq ) 80.2 ( 2.5 kJ mol-1 and ∆Sq ) -100 ( 7 J K-1 mol-1
.
(b) Copper-Catalyzed Rearrangement. Solutions containing a
mixture of 7a, 7b, and 8 in a 2:1:1 ratio were freshly prepared as
follows. In a 5 mm NMR tube (507 pp), 5 (39.49 mg, 0.060 mmol), 6
(14.64 mg, 0.012 mmol), CuCl (5-6 mg, about 10 mol % of 5), and
xylene (0.5 mL) were introduced. The NMR tube was flushed with
nitrogen and capped. After wrapping the cap with Teflon tape, the tube
was transferred to the preheated NMR probe. The progress of the
reaction was monitored by 31P NMR spectroscopy. After the starting
material 5 had disappeared (approximately 10 h), the conversion of 7a
Acknowledgment. We thank Dr. H. Zappey for performing
the HRMS measurements. This investigation was supported by
the Council of Chemical Sciences (GCW) with financial aid
from The Netherlands Organization for Scientific Research
(NWO).
JA993829Q