Epimerization of cyclic vinylphosphirane complexes: The intermediacy of biradicals

Article abstract of DOI:10.1021/ja953696i

The phosphinidene complex Ph-P-W(CO)₅ reacts with cyclopentadiene, 1,3-cyclohexadiene, 1,3-cycloheptadiene, and 1,3-cyclooctadiene to give in each case isomeric mixtures of syn- and anti-vinylphosphiranes. With longer reaction times and/or higher reaction temperatures, the anti adducts isomerize to the syn adducts. These epimerizations are likely to proceed via biradical intermediates. Only the syn-vinylphosphirane of 1,3-cyclohexadiene undergoes a [1,3]-sigmatropic shift to yield a syn-phopholene. The dichotomy of biradical and concerted mechanisms is discussed in relationship with the analogous mechanism for the vinylcyclopropane → cyclopentene rearrangement.

Full text of DOI:10.1021/ja953696i

1690
J. Am. Chem. Soc. 1996, 118, 1690-1695
Epimerization of Cyclic Vinylphosphirane Complexes: The
Intermediacy of Biradicals
Bing Wang, Charles H. Lake,† and Koop Lammertsma*
Contribution from the Department of Chemistry, UniVersity of Alabama at Birmingham,
UAB Station, Birmingham, Alabama 35294
ReceiVed NoVember 2, 1995X
Abstract: The phosphinidene complex Ph-P-W(CO)5 reacts with cyclopentadiene, 1,3-cyclohexadiene, 1,3-
cycloheptadiene, and 1,3-cyclooctadiene to give in each case isomeric mixtures of syn- and anti-vinylphosphiranes.
With longer reaction times and/or higher reaction temperatures, the anti adducts isomerize to the syn adducts. These
epimerizations are likely to proceed via biradical intermediates. Only the syn-vinylphosphirane of 1,3-cyclohexadiene
undergoes a [1,3]-sigmatropic shift to yield a syn-phopholene. The dichotomy of biradical and concerted mechanisms
is discussed in relationship with the analogous mechanism for the vinylcyclopropane f cyclopentene rearrangement.
Introduction
to validate the Woodward-Hoffmann rules for [1,3]-sigmatropic
shifts. Examples have been reported that support either a
concerted or a biradical pathway, while this distinction is not
clear in some cases. Inversion of the apical cyclopropane carbon
(si, suprafacial-inversion) is considered to be the mechanistic
evidence for the one-step concerted process of which the
reactions shown in eqs 2 and 3 are representative.5
Recently, we reported on the syntheses of vinylphosphiranes
and their subsequent rearrangements to phospholenes.1,2
A
remarkably high stereoselectivity was observed for both the Ph-
P-W(CO)5 phosphinidene addition to 1-methoxy-1,3-cyclo-
hexadiene and the rearrangement of the resulting vinylphos-
phirane (eq 1).1 Of particular interest is the formal [1,3]-
H
H
CH3
(OC)5W
CH3
W(CO)5
120°
P
Ph
Ph
(2)
P
[1,3]
[PhPW(CO)5]
(1)
CH2
MeO
H
Me
H
H
MeO
MeO
tBu
(3)
Me
tBu
tBu
2
1
Me
CH2
H
R (85%) inversion
S (15%) retention
S,S
sigmatropic shift which was shown to occur exclusively with
inversion of the P-center. The observed high stereoselectivity
suggests a concerted mechanism. This result is surprising
because the steric crowding at the P-center is expected to hinder
an inversion. A sterically less demanding process would involve
a ring-opening and ring-closure sequence. However, such a
process is expected to favor retention of the P-configuration,
which contrasts with the observed data.
Insight may be obtained from the related hydrocarbon
rearrangements for which the issue of concerted versus biradical
mechanisms has been intensely studied. The rich literature on
the [1,3]-sigmatropic shift of vinylcyclopropanes to cyclopentene
and related cis-trans isomerizations, starting as early as 1959,3
is particularly informative.4 Numerous reactions were studied
However, reactions have also been reported that show the [1,3]-
shift of isotopically labeled vinylcyclopropanes to occur with
si, sr (r ) retention), ar (a ) antarafacial), and ai stereochem-
istry.6 An example is shown in eq 4. In these cases the
D
D
D
D
D
D
D
D
D
D
(4)
D
D
D
D
D
(D)
syn-E (Z)
40% si
24% ai
13% ar
23% sr
mechanistic pathway is considered to involve a biradical
intermediate. An elegant illustration of a biradical process is
the thermally or photochemically induced “walk” or circumam-
bulatory rearrangement of bicyclo[4.1.0]hepta-2,4-diene which
occurs though mainly with inversion of the apical cyclopropane
carbon (eq 5).7 Using an optically active diazo precursor to
† Author to whom correspondence regarding the X-ray crystallographic
data should be addressed.
X Abstract published in AdVance ACS Abstracts, February 1, 1996.
(1) Lammertsma, K.; Hung, J.-T. J. Org. Chem. 1992, 57, 6557.
(2) Lammertsma, K.; Hung, J.-T. J. Org. Chem. 1993, 58, 1800.
(3) Neureiter, N. P. J. Org. Chem. 1959, 24, 2044.
(4) For representative reviews, see: Hudlicky, T. Chem. ReV. 1989, 89,
165. Goldschmidt, Z.; Crammer, B. Chem. Soc. ReV. 1988, 17, 229.
(5) Roth, W. R.; Friedrich, A. Tetrahedron Lett. 1969, 2607. Gajewski,
J.; Olson, L. P. J. Am. Chem. Soc. 1991, 113, 7432. Gajewski, J.;
Squicciarini, M. P. J. Am. Chem. Soc. 1989, 111, 6717.
(6) Baldwin, J. E.; Villarica, K. A.; Freedberg, D. I.; Anet, F. A. L. J.
Am. Chem. Soc. 1994, 116, 10845. Baldwin, J. E.; Bonacorsi, S., Jr. J. Am.
Chem. Soc. 1993, 115, 10621. Baldwin, J. E.; Ghatlia, N. D. J. Am. Chem.
Soc. 1991, 113, 6273. Gajewski, J. J. J. Am. Chem. Soc. 1984, 106, 802.
Andrews, G. D.; Baldwin, J. E. J. Am. Chem. Soc. 1976, 98, 6705.
(7) Kla¨rner, F.-G.; Band, R.; Glock, V.; Ko¨nig, W. A. Chem. Ber. 1992,
125, 197.
Me
Me
CH2OMe
Me
MeOCH2
si
CH2OMe
Me
C
C
Me
Me
Me
CH2OMe
Me
(5)
0002-7863/96/1518-1690$12.00/0 © 1996 American Chemical Society

Epimerization of Cyclic Vinylphosphirane Complexes
J. Am. Chem. Soc., Vol. 118, No. 7, 1996 1691
generate the biradical intermediate, Kla¨rner showed that its ring
closure occurs faster than the rotation of the single bond.
Carpenter8 has highlighted cases where the distinction between
biradical and concerted processes is less clear, such as for the
thermal isomerization of 1-phenylbicyclo[2.1.1]hex-2-ene-endo-
5-d. Its temperature-independent 10:1 product ratio for the
formation of the preferred (concerted) endo over the exo
(biradical) epimer (eq 6) defies differences in activation en-
thalpies for product formation via the concerted as well as the
biradical pathway.
(OC)5W
(OC)5W
Ph
Ph
P
Me
Me
Ph
P
105 °C
P
W(CO)5
Me
(9)
H2C
52 °C
Me
Me
Me
were provided. The same group16 reported later on the similar
addition of NEt2-P-W(CO)5 at 70 °C, in which case only the
phospholene was isolated and not the vinylphosphirane. Our
noted study on a cyclic W(CO)5-complexed vinylphosphirane,
summarized in eq 1, suggests a concerted isomerization pathway.
While this system had the advantage of restricted syn h anti
isomerization due to the bicyclic structure, it is clear that the
nature of these [1,3]-sigmatropic shifts warrants closer scrutiny.
We report here on the Ph-P-W(CO)5 addition to other
conjugated cyclodienes and their subsequent isomerizations.
Because of their importance, the structural assignments are
summarized in the Results section prior to discussing the
isomerizations.
H
D
D
H
D
H
(6)
+
Ph
Ph
(10)
Ph
(1)
If biradical intermediates are prominent in cyclopentene h
vinylcyclopropane rearrangements, they can also be expected
in analogous heterocyclic isomerizations. In fact, this has been
demonstrated for the 1,5-electrocyclization of homophospholes
(eq 7), homopyrroles, homothiophenes, and homofurans.9
Thermal isomerizations of vinylaziridines have been shown to
yield 3-pyrrolines,10 but no mechanistic detail was provided in
these studies. The same applies to the Cope rearrangements of
divinylaziridines11 and the thermal isomerizations of vinylacety-
loxiranes.12
Results
In our approach, we first conducted 1,2-cycloadditions to 1,3-
cycloalkadienes (5) with the terminal phosphinidene complex
Ph-P-W(CO)5, which is generated in situ from thermal
degradation of the corresponding 7-phosphanorbornadiene
precursor 4. The resulting vinyl phosphiranes (obtained in ca
(OC)5W
Ph
P
•
H3C
H3C
CO2CH3
CO2CH3
(7)
+
P
P
P
Ph
Ph
Ph
4
Even less information is available on the vinylphosphirane
h phospholene rearrangement. In an elegant kinetic study,
Richter postulated that the thermal isomerization of (Z)-1-tert-
butyl-2-vinylphosphirane to the corresponding phospholene
involves sequential first-order reactions via biradical intermedi-
ates (eq 8).13 A key component in this process is the observed
60-80% isolated yields) were then subjected to thermal
isomerizations. The product assignments are based on 1H, 13C,
and 31P NMR spectroscopic data, except for the anti isomer 5b
for which also a single-crystal X-ray structure was determined.
1,3-Cycloalkadienes. Reaction of 4 with cyclopentadiene
in toluene at 55 °C with 10% of CuCl as catalyst gave a 4:1
isomeric mixture of the vinylphosphiranes 5a and 5b, respec-
tively. The major product is syn isomer 5a; syn is defined as
having the P-W(CO)5 group over the hydrocarbon ring. The
structural assignment of the anti isomer 5b was ascertained by
a single-crystal X-ray structure determination, which is shown
in Figure 1. The assignment of the syn and anti isomers are
further supported by their 1H, 13C, and 31P NMR spectroscopic
data. For example, the chemical shift difference between the
olefinic protons of 5b (δ 5.17 and 5.73 ppm) and those of isomer
5a (δ 5.80 and 6.45 ppm) is attributed to the shielding effect of
the bridging P-phenyl; this group is located over the hydrocarbon
ring in the anti isomer 5b. A distinction between the syn and
anti isomers is also found in the 31P NMR chemical shifts and
tBu
tBu
tBu
tBu
tBu
P
•
(8)
P
P
P
P
•
•
•
initial P-epimerization of the phosphirane, which is best
explained via a ring-opening-ring-closure sequence. Such cis-
trans isomerizations, which obviously make mechanistic inter-
pretations more difficult, are also well-known in the noted
vinylcyclopropanes.6,14 Marinetti and Mathey15 reported in an
exploratory study that Ph-P-W(CO)5 reacts at 55 °C with
conjugated dienes to give 1,2-adducts, which in the case of 2,3-
dimethyl-1,3-butadiene was shown to rearrange to 3-phos-
pholene at temperatures g95 °C (eq 9). No mechanistic details
1
the J(P-W) coupling constants of all the vinylphosphiranes
of this and earlier studies. The 1J(P-W) coupling constant of
the syn isomer is in all cases significantly larger than that of
the anti isomer (Vide infra) with a difference of 17.5 Hz for the
vinylphosphiranes 5a and 5b. The difference in 31P NMR
chemical shifts is less clear, although the anti isomer has in all
cases the more shielded resonance. In the 13C NMR spectrum
(8) Carpenter, B. K. Acc. Chem. Res. 1992, 25, 520. Carpenter, B. K. J.
Org. Chem. 1992, 57, 4645. Newman-Evans, R. H.; Simon, R.; Carpenter,
B. K. J. Org. Chem. 1990, 55, 695.
(9) Kla¨rner, F.-G.; Oebels, D.; Sheldrick, W. S. Chem. Ber. 1993, 126,
473. Oebels, D.; Kla¨rner, F.-G.; Tetrahedron Lett. 1989, 30, 3525. Kla¨rner,
F.-G.; Schro¨er, D. Angew. Chem., Int. Ed. Engl. 1987, 26, 1294.
(10) Sauleau, J.; Sauleau, A.; Huet, J. Bull. Soc. Chim. Fr. 1978, Part 2,
97. Scheiner, P. J. Org. Chem. 1967, 32, 2628.
(11) Pommelet, J. C.; Chuche, J. Can J. Chem. 1976, 54, 1571.
(12) Manisse, N.; Chuche, J. Tetrahedron 1977, 33, 2399.
(13) Richter, W. J. Chem. Ber. 1983, 116, 3293. Richter, W. J. Chem.
Ber. 1985, 118, 1575.
(14) For example, see: Baldwin, J. E.; Selden, C. B. J. Am. Chem. Soc.
1993, 115, 2239.
2
the magnitude of the J(C-P) coupling constant of the CH2
group is characteristic for the identification of the syn vs anti
form with the anti isomer having the larger coupling. In accord
with our previous work and Mathey’s earlier studies on the
(16) Mercier, F.; Deschamps, B.; Mathey, F. J. Am. Chem. Soc. 1989,
111, 9098.
(15) Marinetti, A.; Mathey, F. Organometallics 1984, 3, 456.

1692 J. Am. Chem. Soc., Vol. 118, No. 7, 1996
Wang et al.
appearance of the resonance of the corresponding syn form (i.e.,
-146 (5a), -142 (7a), and -153 ppm (8a)) in addition to
formation of some side products.19 For example, the syn:anti
ratio changed for 5ab from 0.4 (1 h) to 0.8 (2 h), for 7ab from
2 (1 h) to 7 (4 h), and for 8ab from 0.1 (45 min) to 0.4 (1.5 h)
to 1 (4 h). These anti f syn epimerizations occur faster at
higher temperatures. After 4 h at 110 °C the conversions were
nearly complete with syn:anti ratios of ca. 10. Under these
conditions the 31P NMR spectra did also show, among various
decomposition products, indications of minute amounts (too
small to integrate) of products attributable to [1,3]-sigmatropic
shifts. Thus, resonances were observed at δ 51.1 and 95.8 ppm
for 1,3-cyclopentadiene, at δ 58.7 and 93.8 ppm for 1,3-
cyclooctadiene, and at δ 55.0 and 97.3 ppm for 1,3-cycloocta-
diene. These high-field resonances at δ 50-60 ppm, which
compare with the δ 65.7 ppm for 3, could be from the syn-
phospholenes 5c, 7c, and 8c, respectively. It is noted that
overnight heating of the syn-phosphiranes at 90 °C did not lead
to increased formation of the 1,3-shift products but instead to
increased decomposition.
Figure 1. ORTEP representation of 5b.
phosphiranes of 1,4-cyclohexadiene,18 we find that the syn
isomer shows no 2J(C-P) coupling with the CH2 group, while
the coupling constant of the anti isomer is 8.4 Hz.2,17 Finally,
we note the unexpectedly large coupling constants of 36.9 and
30.8 Hz for the two olefinic hydrogens in 5a. These are
tentatively assigned to long-range P-H couplings. In earlier
work on phenyl-substituted phosphiranes we have shown the
presence of a W-type conjugation between the P-phenyl group
and a trans-phenyl group of the 3-membered phosphirane ring.
Such an electronic effect may underlie these large couplings.
The same 1,2-addition reaction with 1,3-cycloheptadiene gave
a ca. 7:1 mixture of the syn and anti vinylphosphiranes 7a and
7b, respectively (eq 10). Only for the syn isomer could all NMR
W(CO)5
W(CO)5
W(CO)5
W(CO)5
Ph
Ph
Ph
Ph
P
P
P
P
5c
6c
7c
8c
In earlier work on 1,3-cyclohexadiene, we reported on the
isomerization of the syn-vinylphosphirane 6a to the bicyclic
phospholene 6c. The anti isomer 6b was thought to decompose;
it diminished during reaction. In light of the results on the 5-,
7-, and 8-membered 1,3-cycloalkadienes, we repeated this earlier
work, using both the isolated syn and anti isomers 6a and 6b
in separate experiments. By monitoring the 31P NMR spectra,
these experiments showed unequivocally that the anti-vi-
nylphosphirane 6b (δ -139.9 ppm) epimerizes to the syn isomer
6a (δ -137.1 ppm) and that this syn isomer rearranges to the
syn-phospholene 6c (δ 66.7 ppm) and possibly some of its anti
form (δ 97.9 ppm). The syn-phosphirane 6a does not epimerize
to its anti form 6b.
Ph
(OC)5W
P
Ph
P
W(CO)5
(10)
(CH2)n
syn
(CH )
2 n
Ph
P
W(CO)5
(CH2)n
5: n = 1
6: n = 2
7: n = 3
+
anti
5a: n = 1
6a: n = 2
7a: n = 3
5b: n = 1
6b: n = 2
7b: n = 3
Structures. The conformational assignments in this study
are of imminent importance. To make these unequivocal, a
single-crystal X-ray structure was determined for the anti-
phosphirane isomer 5b (Figure 1) with subsequent verification
of its 31P NMR chemical shift (δ ) -149.0 ppm) using the
same crystal. Selected interatomic distances and angles are
summarized in Table 1 (supporting information). Structural
features are highlighted in relationship with those of syn 1.1
The anti conformation of 5b is confirmed. The cyclopentene
ring is planar with an average deviation from the plane of 0.007
Å. The plane formed by this ring intercepts the one formed by
the syn P-phenyl group with an angle of 37.4°. This contrasts
with the parallel orientation in syn 1, in which these groups
have an antiperiplanar arrangement. There is no apparent
repulsion between the P-phenyl substituent and the 5-membered
cyclopentene ring in 5b. The angle of the intercept between
the phosphirane and cyclopentene rings of 117.1° is normal and
also the P-bonding angles are as expected, i.e., the phosphirane
CPC angle is 50.1(4)° and the WPC(phenyl) angle is 120.1-
(3)°.1,20
2
spectroscopic data be obtained. The small J(C-P) coupling
constant of 4.3 Hz for the CH2 group substituted at the
phosphirane ring and the larger 1J(P-W) coupling constant of
265.5 Hz are assigned to isomer 7a. The difference in the P-W
coupling constants between 7a and 7b is 15.9 Hz.
A ca. 2:3 anti-syn isomeric mixture 8a,b is obtained for the
phosphinidene addition to 1,3-cyclooctadiene. The assignments
are again based on the NMR characteristics, which are (1) the
more shielded 1H NMR chemical shifts for the olefinic
hydrogens, (2) the absence of a 2J(C-P) coupling constant for
the R-CH2 of the syn isomer (versus 12.3 Hz for 8b), and (3)
1
the 9.1 Hz larger J(P-W) coupling constant of 261.5 Hz for
the syn isomer.
P-Epimerizations. Heating of the isomeric mixtures of the
vinylphosphiranes 5ab, 7ab, and 8ab, obtained from the
phosphinidene additions, for up to 4 h in toluene at 60 °C led
in each case to a slow conversion of the anti isomer b into the
syn isomer a. These P-epimerizations were also conducted for
the isolated anti isomers 5b and 8b under the same reaction
conditions. The isomerizations were monitored by 31P NMR
spectroscopy using reaction aliquots. They showed the disap-
pearance of the 31P chemical shift of the anti form (i.e., -149
(5b), -157 (7b), and -168 ppm (8b)) with the concurrent
Examination of the bond distances and angles in the crystal
structure of 5b shows that it exhibits static disorder. This is
(19) The epimerizations of 5-8 constitute 70-90% of the total isomer-
izations, as based on 31P NMR data, with 60-80% isolated yields for the
epimers.
(20) Hung, J.-T.; Yang, S.-W.; Gray, G. M.; Lammertsma, K. J. Org.
Chem. 1993, 58, 6786.
(17) Verkade, J. G.; Quin, L. D. Phosphorus-31 NMR Spectroscopy in
Stereochemical Analysis; VCH: Deerfield Beach, FL, 1987.
(18) Marinetti, A.; Mathey, F. J. Am. Chem. Soc. 1985, 107, 4700.

Epimerization of Cyclic Vinylphosphirane Complexes
J. Am. Chem. Soc., Vol. 118, No. 7, 1996 1693
apparent from the identical bond lengths of 1.378(16) Å for
C(13)-C(14) and 1.381(20) Å for C(14)-C(15), as they both
represent an average value for a single and a double bond. The
structure has also two equivalent C-C bonds connected to the
CCP ring (i.e., 1.479(14) Å for C(12)-C(13) and 1.503(13) Å
for C(11)-C(15)), and the two phosphirane P-C bonds of equal
length (i.e., 1.836(9) Å for P-C(11) and 1.827(9) Å for
P-C(12)). Apparently, there are two conformations of 5b
randomly distributed at equivalent sites in the crystal. This
results in an averaging of the two structures and explains the
large estimated standard deviations despite the small residual
index of 2.85%. The static disorder is also apparent from a
comparison with the crystallographic data of 1. For example,
the double bond of 1.319(9) Å in structure 1 is separated by a
short C-C bond of 1.469(9) Å from the phosphirane ring, which
has two different P-C ring bonds of 1.816(6) and 1.851(5) Å.
This comparison between anti 5b and syn 1 suggests that these
vinylphosphiranes have indeed the same structural features.
To further examine the structural characteristics of 5b, an ab
initio molecular orbital geometry optimization at MP2/6-31G*,
using the Gaussian 94 suite of programs,21 was conducted on
the parent system 9. This structure differs from 5b in that it
The epimerizations are indicative of a biradical process.22
That is, the phosphirane undergoes a ring opening to a biradical
intermediate which closes again to return to the phosphirane
ring. In this process, the syn and anti isomers interconvert by
C-P bond rotation in the biradical species. Because the
isomerizations yield in all cases the syn-vinylphosphiranes, these
appear to be the thermodynamically preferred products except
for 1,3-cyclohexadiene in which case the final product is
phospholene 6c. A similar biradical process was previously
proposed by Richter to explain the syn f anti epimerization of
the uncomplexed (Z)-1-tert-butyl-2-vinylphosphirane; anti is
here defined as having the ring hydrocarbon substituents in a
trans configuration (eq 8).
Surprising in these results is that seemingly both biradical
and concerted pathways are available for the rearrangement of
vinylphosphiranes. Thus, in the case of the 5-, 7-, and
8-membered 1,3-cyclodienes the anti f syn epimerization is
evidence for the exclusive biradical reaction channel, while for
the 1,3-cyclohexadiene the formation of a syn-phospholene
suggests a concerted [1,3]-sigmatropic shift. The latter reaction
does obscure the concurrent anti f syn epimerization of the
vinylphosphiranes. Analogous to the Ph-P-W(CO)5 addition to
1-methoxy-1,3-cyclohexadiene, which gives in g95% the syn
adduct (within 45 min at 55 °C), the same addition with the
nonactivated, unsubstituted system (6) affords a mixture of the
anti- and syn-phosphiranes of which the syn isomer becomes
the dominant product with longer reaction times and higher
temperatures. In fact, the anti isomer 6b is no longer present
in the reaction mixture after 5 h at 90 °C while the conversion
of the syn isomer 6a to the phospholene 6c is still incomplete,
i.e., the 6a:6c ratio is 4. It is then evident that the vinylphos-
phiranes of all 1,3-cycloalkadienes 5-8 undergo anti f syn
epimerizations which must proceed via biradical intermediates.
H
1.873 P
1.889
1.529α
1.511
1.501
1.484
1.345
α = 113.8°
9
(a) lacks the W(CO)5 group and (b) has a P-H substituent
instead of a P-Ph group. Its main geometrical features are
given in the displayed structure. These show a relatively short
C-C bond of 1.484 Å between the 1.345 Å double bond and
the phosphirane ring, thereby underscoring the disorder in the
crystal structure of 5b. The geometrical parameters of unsub-
stituted 9 are similar to those of the crystal structure of the larger
ring system, 1.
The syn isomers are the thermodynamically preferred products
in these epimerizations. Their configuration enables a W-type
conjugation between the vinyl group and the anti P-phenyl
group. The presence of such a stabilizing effect has been
demonstrated for phosphiranes with two trans phenyl substit-
uents.25 Apparently, this electronic stabilization overcomes any
steric interaction that may be attributed to the bulky transition
metal group that is located in the endo position. In fact, it may
be argued that since the P-W bond is much longer than the
P-C(phenyl) bond, the W(CO)5 group in 5a might interact less
with the cyclopentene ring than the P-phenyl substituent in 5b.
Discussion
Addition of the phosphinidene Ph-P-W(CO)5 to 1,3-cy-
cloalkadienes readily provides isomeric mixtures of anti- and
syn-vinylphosphiranes of varying ratios. The exciting result of
the present study is the isomerization of the anti isomer to the
syn isomer. This process represents an epimerization at the
phosphorus center. These results are consistent for the 5-, 6-,
7-, and 8-membered ring systems with the exception that the
syn-vinylphosphirane of 1,3-cyclohexadiene undergoes a sub-
sequent [1,3]-sigmatropic shift to a phospholene.
If biradicals are prevalent in the epimerizations, then shouldn’t
they also be present in the conversion of the vinylphosphiranes
to the phospholenes? As noted, biradicals have been implicated
in vinylcyclopropane f cyclopentene rearrangements. How-
ever, the [1,3]-sigmatropic shifts that take place in 1 f 2 and
6a f 6c seem to be best explained as pericyclic reactions.
Firstly, these transformations occur with apparent inversions of
the P-center in accord with such a si process. Secondly, a
biradical process would be expected to give significant amounts
of the anti isomers of 2 and 6c assuming that the P-C bond
breaking/making process is faster than the P-C bond rotation.
The question can be raised, however, whether such anti isomers
are in fact the primary rearrangement products, which rapidly
undergo an anti f syn epimerization under the reaction
conditions to give the thermodynamically more stable syn
products. Whereas this would explain the observed products,
such a multistep process is an unlikely event. Firstly, the anti
f syn epimerization of the phospholenes and that of the
(21) Gaussian 94 (Revision B), Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.;
Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-
Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski,
J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart,
J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian Inc., Pittsburgh,
PA, 1995.
(22) A concerted epimerization is unlikely. The inversion barrier for the
parent uncomplexed phosphirane (C2PH5) is estimated to be 68 kcal/mol
at MP2/6-31G*.23 With an additional W(CO)5 group such a C2V symmetrical
transition is not possible and a “Cs symmetrical” transition is required, which
is a much higher energy process. Such an edge inversion for the parent
phosphirane is estimated at 127 kcal/mol (MP2/6-311G**). A biradical
process is much less energy demanding. The average P-C bond strength
is estimated to be only 63 kcal/mol.24 Moreover, homolytic P-C bond
cleavage is aided by the relief of ca. 20 kcal/mol in strain energy from the
phosphirane23 and by the additional allylic stabilization in the resulting
diradical.
(23) Bachrach, S. M. J. Phys. Chem. 1989, 93, 7780.
(24) Huheey, J. E. Inorganic Chemistry; Harper & Row: New York.
(25) Hung, J.-T.; Lammertsma, K. J. Organomet. Chem. 1995, 489, 1.

1694 J. Am. Chem. Soc., Vol. 118, No. 7, 1996
Wang et al.
2.76 (m, 2H, CH2), 2.44 (dd, 2J(H-H) ) 7.5 Hz, 2J(H-P) ) 14.8 Hz,
vinylphosphiranes would formally have one common biradical
intermediate. In the absence of memory effects, the anti-
phosphirane 6b is then expected to directly yield syn-phosp-
holene 6c. This is not corroborated by the facts. Secondly,
the epimerization of the thermodynamically preferred phosp-
holenes is expected to be slower than that of the vinylphos-
phiranes.
2
1H, CHP), 2.19 (d, br, J(H-P) ) 16.4 Hz 1H, CHP).
(8-Phenyl-8-phosphabicyclo[5.1.0]octa-3-ene)pentacarbonyltung-
sten (7a,b). The same reaction of 4 with 1,3-cycloheptadiene, as
described for 5, gave a mixture of syn-7a and anti-7b in a 7:1 ratio
(68% yield), based on integration of their 31P NMR resonances.
Fractional crystallization from hexanes gave 7a as a colorless solid:
mp 82-84 °C; 31P NMR (C6H6) δ -142 (1J(31P-183W) ) 265.5 Hz);
2
13C NMR (CDCl3) δ 197.1 (d, J(C-P) ) 21.9 Hz, trans CO), 194.5
These data on the isomerizations of phosphinidene-diene
adducts show that biradicals are viable intermediates. This
added dimension is currently being explored in our laboratories.
2
(d, J(C-P) ) 7.5 Hz, cis CO), 127.3-130.5 (m, phenyl), 131.7 (d,
3
2J(C-P) ) 12.0 Hz, CdCH), 122.4 (d, J(C-P) ) 2.0 Hz, HCdC),
30.0 (d, 4J(C-P) ) 3.0 Hz, CH2), 29.0 (d, 1J(C-P) ) 14.3 Hz, CHP),
Conclusion. The important observation made in this study
is that W(CO)5-complexed vinylphosphiranes are subject to
epimerization at the P-center. Anti isomers are converted to
the syn forms, likely by way of a biradical intermediate. Only
the syn adducts of 1,3-cyclohexadienes isomerize to phosp-
holenes, and this process may proceed via a concerted pathway.
28.5 (d, 3J(C-P) ) 15.1 Hz, CH2), 26.7 (d, 2J(C-P) ) 4.3 Hz, CH2),
1
1
23.5 (d, J(C-P) ) 13.5 Hz, CHP); H NMR (C6D6) δ 6.9-7.1 (m,
5H, phenyl), 5.72 (m, 1H, CHdCH), 5.45 (m, 1H, CHdCH), 2.0-2.1
2
3
(m, 2H, CH2), 1.71 (dd, J(H-P) ) 6.6 Hz, J(H-H) ) 1.5 Hz, 1H,
2
CHP), 1.8-1.9 (m, 2H, CH2), 1.2-1.5 (m, 2H, CH2), 1.39 (d, J(H-
P) ) 4.8 Hz, 1H, CHP); MS (184W) m/e (relative intensity) for 7a,b:
526 (M+, 10), 442 (M - 3CO, 18), 404 (PhPW(CO)4, 45), 376 (PhPW-
(CO)3, 22), 348 (PhPW(CO)2, 95), 320 (PhPW(CO), 60), 292 (PhPW,
100). Anal. Calcd for C18H15O5PW: C, 41.06; H, 2.85. Found: C,
40.99; H, 2.86. Minor isomer 7b: colorless, mp 68-70 °C; 31P NMR
Experimental Section
NMR spectra were recorded on a GE NT-300, wide-bore FT-NMR
spectrometer. Chemical shift are referenced in ppm to internal (CH3)4-
(C6H6) δ -157 (1J(31P-183W) ) 249.6 Hz); H NMR (C6D6) δ 6.8-
1
1
Si for the H and 13C NMR spectra and external 85% H3PO4 for the
7.3 (m, 5H, phenyl), 5.55 (s, 1H, CHdCH), 5.42 (m, 1H, CHdCH),
2.5-2.8 (m, 4H, CH2), 2.38 (d, J(H-P) ) 16 Hz, 1H, CHP), 2.12
(br, 2H, CH2), 1.48 (m, 1H, CH2).
2
31P NMR spectra. Downfield shifts are reported as positive. Product
compositions were determined from integration of 31P NMR spectra.
IR spectra were recorded on a Nicolet IR44 spectrometer. Mass spectra
were recorded on a HP 5985 at 70 eV. Elemental analyses were
performed by Atlantic Microlab, Inc., Norcross, GA. All materials
were handled under an atmosphere of nitrogen. Reagents and solvents
were used as purchased, except for THF, which was distilled from
sodium benzophenone prior to use. Chromatographic separations were
performed on silica gel columns (230-400 mesh, EM Science). The
synthesis of [5,6-dimethyl-2,3-bis(methoxycarbonyl)-7-phenyl-7-phos-
phanorbornadiene]pentacarbonyltungsten, 4, is described in ref 26. The
synthesis of syn- and anti-(6-phenyl-6-phosphabicyclo[4.1.0]hepta-3-
ene)pentacarbonyltungsten 6a,b has been described previously.2,27
(9-Phenyl-9-phosphabicyclo[6.1.0]nona-3-ene)pentacarbonyltung-
sten (8a,b). The same reaction of 4 with 1,3-cyclooctadiene, as
described for 5, gave a mixture of syn-8a and anti-8b in a 3:2 ratio
(71% yield), based on integration of their 31P NMR resonances.
Fractional crystallization from hexane gave 8a as a colorless crystals:
mp 67-69 °C; 31P NMR (C6H6) δ -153 (1J(31P-183W) ) 261.5 Hz);
2
13C NMR (CDCl3) δ 196.7 (trans CO), 194.4 (d, J(C-P) ) 8.5 Hz,
cis CO), 127.6-130.5 (m, phenyl), 137.1 (d, 2J(C-P) ) 13.3 Hz,
1
CdCH), 120.0 (s, HCdC), 30.6 (d, J(C-P) ) 12.4 Hz, CHP), 29.9
(d, 1J(C-P) ) 14.7 Hz, CHP), 28.7 (s, CH2), 27.7 (d, 3J(C-P) ) 13.0
1
Hz, CH2), 24.7 (s, CH2), 23.1 (s, CH2); H NMR (CDCl3) δ 7.17-
3
7.47 (m, 5H, phenyl), 5.54 (d, J(H-P) ) 12.6 Hz, 1H, CHd), 5.85
(6-Phenyl-6-phosphabicyclo[3.1.0]hexa-3-ene)pentacarbonyltung-
sten (5a,b). Complex 4 (1.00 g, 1.53 mmol) and 1,3-cyclopentadiene
(0.40 g, 6.12 mmol) were heated at 60 °C in toluene with CuCl (100
mg, 1.0 mmol) for 1.5-2 h. The reaction mixture was filtered,
evaporated to dryness, and chromatographed on silica gel with hexanes
to yield 0.46 g (60.4%) of a 4:1 mixture of syn-5a and anti-5b, based
on integration of their 31P NMR resonances. Fractional crystallization
from hexanes gave 5a as colorless crystals: mp 58-60 °C; 31P NMR
(C6H6) δ -146 (1J(31P-183W) ) 268.5 Hz); 13C NMR (CDCl3) δ 135.1
(d, 2J(C-P) ) 10.1 Hz, CdCH), 130.5 (d, 3J(C-P) ) 2.0 Hz, CdCH),
36.4 (s, CH2), 37.6 (d, 1J(C-P) ) 14.0 Hz, CHP), 27.6 (d, 1J(C-P) )
3
(m, 1H, CHd), 2.71 (t, J(H-P) ) 9.8 Hz, 2H, CH2), 1.2-2.47 (m,
8H, 3CH2, 2CHP); MS (184W) m/e (relative intensity) for 8a,b: 540
(M+, 20), 456 (M - 3CO, 10), 404 (PhPW(CO)4, 70), 376 (PhPW-
(CO)3, 25), 348 (PhPW(CO)2, 100), 320 (PhPW(CO), 65), 292 (PhPW,
90); Isomer 8b: colorless crystals, mp ) 84-86 °C; 31P NMR (C6H6)
δ -168 (1J(31P-183W) ) 252.4 Hz); 13C NMR (CDCl3) δ 195.9 (trans
CO), 195.5 (d, 2J(C-P) ) 11.0 Hz, cis CO), 128.7-133.3 (m, phenyl),
137.2 (d, J(C-P) ) 12.1 Hz, CdCH), 119.5 (s, HCdC), 31.2 (d, 1J(C-
P) ) 17.4 Hz, CHP), 30.1 (s, CH2), 28.5 (d, 1J(C-P) ) 22.7 Hz, CHP),
25.9 (s, CH2), 25.6 (d, J(C-P) ) 12.1 Hz, CH2), 24.2 (d, J(C-P) )
1
2
12.3 Hz, CH2); H NMR (CDCl3) δ 7.17-7.47 (m, 5H, phenyl), 5.30
13.7 Hz, CHP), 129.0-132.1 (m, phenyl), 195.9 (d, J(C-P) ) 8.0
(d, 3J(H-P) ) 8.4 Hz, 1H, CHd), 5.25 (m, 1H, CHd), 2.31 (d, 2J(H-
P) ) 9.0 Hz, 1H, CHP), 1.78-0.76 (m, 9H, 4CH2, CHP). Anal. Calcd
for C19H12O5PW: C, 42.22; H, 3.15. Found: C, 42.21; H, 3.22.
X-ray Structure Determination of 5b. Structure Solution and
Refinement. A suitable crystal of 5b was sealed into a thin-walled
capillary and was mounted and aligned on an Enraf Nonius CAD4
diffractometer. The crystal belongs to the orthorhombic crystal system,
with the space group uniquely defined as the most common noncen-
trosymmetric space group P212121 (No. 19) by the systematic absences
h00 for h ) 2n + 1, 0k0 for k ) 2n + 1 and 00l for l ) 2n + 1. All
data were collected by a coupled ω-2θ scan. A total of 3059
reflections were colllected with Mo KR radiation. An empirical
absorption correction was applied to all data. These were then merged
to produce 2216 independent reflections.
Hz, cis CO), 197.9 (d, 2J(C-P) ) 31.4 Hz, trans CO); 1H NMR (CDCl3)
δ 7.1-7.5 (m, 5H, phenyl), 6.45 (dd, 3J(H-P) ) 36.9 Hz, 2J(H-H) )
4
4.5 Hz, 1H, CHd), 5.80 (d, J(H-P) ) 30.8 Hz, 1H, CHd), 3.07-
2.83 (m, 2H, CH2), 2.66 (t, 2J(H-P) ) 16 Hz, 1H, CHP), 2.45 (t, 2J(H-
P) ) 7.5 Hz, 1H, CHP); MS (184W) m/e (relative intensity) for 5a,b:
498 (M+, 20), 414 (M - 3CO, 5), 404 (PhPW(CO)4, 47), 376 (PhPW-
(CO)3, 18), 348 (PhPW(CO)2, 100), 320 (PhPW(CO), 60), 292 (PhPW,
100). Anal. Calcd for C16H11O5PW: C, 38.50; H, 2.21. Found: C,
38.61; H, 2.31. Minor isomer 5b: colorless, mp 133-135 °C; 31P NMR
(C6H6) δ -149.0 (1J(31P-183W) ) 251.0 Hz); 13C NMR (CDCl3) δ
133.9 (d, 2J(C-P) ) 10.5 Hz, CdCH), 130.3 (d, 3J(C-P) ) 1.80 Hz,
1
1
CdCH), 35.9 (d, J(C-P) ) 8.4 Hz, CH2), 39.2 (d, J(C-P) ) 14.7
Hz, CHP), 29.0 (d, 1J(C-P) ) 14.7 Hz, CHP), 129.0-132.1 (m,
2
2
phenyl), 196.2 (d, J(C-P) ) 8.1 Hz, cis CO), 199.1 (d, J(C-P) )
1
31.5 Hz, trans CO); H NMR (CDCl3) δ 7.1-7.3 (m, 5H, phenyl),
All crystallographic calculations were carried out with the aid of
the SHELXTL-PC program package. The analytical form of the
scattering factors for neutral atoms was used with both of the real (∆f′)
and imaginary (i∆f′′) components of anomalous dispersion included
in the calculations. The positional and anisotropic thermal parameters
were refined for all non-hydrogen atoms. All hydrogen atoms were
included in calculated positions (d(C-H) ) 0.96 Å) with the appropriate
staggered geometry. The isotropic thermal parameters of the hydrogen
atoms were fixed equal to the Ueq of the carbon atoms to which they
3
5.73 (s, 1H, CHd), 5.17 (d, J(H-P) ) 2.5 Hz, 1H, CHd), 2.95-
(26) Marinetti, A.; Mathey, F J. Am. Chem. Soc. 1982, 104, 4484.
Marinetti, A.; Mathey, F. Organometallics 1982, 1, 1488. Marinetti, A.;
Mathey, F.; Fischer, J.; Mitschler, A. J. Chem. Soc., Chem. Commun. 1982,
667.
(27) On the basis of the NMR spectroscopic arguments presented here,
the assignment of 6a and 6b in the Experimental Section of ref 2 must be
reversed.

Epimerization of Cyclic Vinylphosphirane Complexes
J. Am. Chem. Soc., Vol. 118, No. 7, 1996 1695
are bonded. The absolute structure was determined by the η refinement
procedure. The final value of η ) 1.06(4) confirms that the correct
conformation of the molecule was chosen.
Acknowledgment is made to the National Science Founda-
tion (CHE-9500344) for support. We thank Dr. Jui-Te Hung
for his contributions in the early stages of this work.
Crystal and Intensity Data. C16H11O5PW, MW ) 498.1 g/mol,
orthorombic crystal system, a ) 6.6763(9) Å, b ) 13.5269(24) Å, c )
18.8822(27) Å, V ) 1705.2(9) Å3, space group P212121, Z ) 4, Dc )
1.940 mg/m3, crystal dimensions 0.20 × 0.20 × 0.10 mm, absorption
coefficient µ ) 6.888 mm-1. A total of 3059 reflections were collected
with index ranges of 0 e h e 7, 0 e k e 14, and -20 e l e 20,
which were merged into a unique set of 2216 reflections (Rint ) 1.81%).
Their refinement against 210 parameters converged (largest ∆/σ )
0.002) with R ) 2.85% and Rw ) 3.77%. R ) 2.47% and Rw ) 2.83%
for those 1964 reflections with |F| > 6.0σ(|F|). Largest features left
on the difference Fourier map were a peak of 0.67 e Å-3 and a hole of
-0.87 e Å-3. Absolute structure was verified η ) 1.07(3).
Supporting Information Available: Positional and thermal
parameters, a complete listing of bond lengths and angles, and
details of the X-ray structure determination (4 pages). This
material is contained in many libraries on microfiche, im-
mediately follows this article in the microfilm version of the
journal, can be ordered from the ACS, and can be downloaded
from the Internet; see any current masthead page for ordering
information and Internet access instructions.
JA953696I