Acid-promoted isomerization of a tungsten-η1-2,5-dihydropyrrolyl complex to its tungsten-η1-2,3-dihydropyrrolyl isomer

Article abstract of DOI:10.1021/om970563s

Treatment of the tungsten-η¹-2,5-dihydropyrrolyl complex 3 with a mixture of Ph₃CBF₄ and water in cold CH₂Cl₂ produced the tungsten-η¹-2,3-dihydropyrrolium cation 7; this cation gradually lost one proton in CH₂Cl₂ at 23 °C, to yield the tungsten-η¹-2,3-dihydropyrrolyl species 5.

Full text of DOI:10.1021/om970563s

4232
Organometallics 1997, 16, 4232-4234
Acid -P r om oted Isom er iza tion of a
Tu n gsten -η1-2,5-Dih yd r op yr r olyl Com p lex to Its
Tu n gsten -η1-2,3-Dih yd r op yr r olyl Isom er
J ang-Shyang Fan,† Gene-Hsian Lee,‡ Shie-Ming Peng,‡ and Rai-Shung Liu*,†
Departments of Chemistry, National Tsing Hua University,
Hsinchu 30043, Taiwan, ROC, and National Taiwan University, Taipei 10764, Taiwan, ROC
Received J uly 3, 1997X
Summary: Treatment of the tungsten-η1-2,5-dihydro-
pyrrolyl complex 3 with a mixture of Ph3CBF4 and water
in cold CH2Cl2 produced the tungsten-η1-2,3-dihydro-
pyrrolium cation 7; this cation gradually lost one proton
in CH2Cl2 at 23 °C, to yield the tungsten-η1-2,3-
dihydropyrrolyl species 5.
Transition-metal five-membered η1-heterocyclics such
as η1-pyrrolyl, η1-furyl, and η1-thienyl complexes1-3 have
attracted considerable attention due to their interesting
reaction chemistry. Most of them are prepared with the
heterocyclic ring linked to the metal center at the C(2)
carbon; few η1-3-metalated heterocyclics have been
reported.1a,2b,3 Previously, we reported an unusual acid-
catalyzed isomerization of tungsten-η1-3-furyl to η1-2-
furyl species;3 the reaction was proposed to proceed via
a ring-opening mechanism involving a tungsten-η2-
alkyne tungsten-η1-allenylidene rearrangement. In
this paper, we report another atypical isomerization of
a tungsten-η1-2,5-dihydropyrrolyl complex to its η1-2,3-
dihydropyrrolyl isomer; the reaction intermediate has
been successfully isolated in order to elucidate the
isomerization mechanism.
F igu r e 1. ORTEP drawing of tungsten-η1-pyrrolyl com-
plex 5. Pertinent distances (Å): W-C(4) ) 2.236(4), C(4)-
C(7) ) 1.327(6), C(7)-N ) 1.446(6), C(6)-N ) 1.490(6),
C(4)-C(5) ) 1.327(6).
Shown in Scheme 1 is the reaction between CpW-
(CO)3Na and bromopropargyl amine 1 in cold THF (0
°C, 4 h) to afford the tungsten-η1-propargyl species 2
in 88% isolated yield; slight warming of a THF solution
of 2 at 40 °C for 8 h led to intramolecular cyclization to
give η1-2,5-dihydropyrrole 3 and η3-anti-γ-lactam 4 in
51 and 14% yields, respectively, after separation on a
silica column. Complex 4 has an anti configuration, as
characterized by the coupling constant J 34 ) 0 Hz. The
syn isomer is expected to have the coupling constant
value J 34 ) 4-5 Hz according to the NMR data for
tungsten-η3-γ-lactonyl species.4
Treatment of 3 with 1 equiv of CF3CO2H in CH2Cl2
(23 °C) effected the isomerization of 3 to its η1-2,5-
dihydropyrrolyl isomer 5 (Scheme 1, eq ii). The reac-
tion, however, required a prolonged time (ca. 96 h) for
completion, giving a 55% yield of 5 with some insoluble
demetalated species persisting. The crystal structure
of 5 is shown in Figure 15 to confirm the isomerization,
which involves a 1,2-shift of the double bond of the
pyrrolyl ring. We employed the stronger acid CF3SO3H
to increase the reaction rate in an attempt to isolate
the reaction intermediate; unfortunately, we obtained
the tungsten-η1(N)-2,3-dihydropyrrole 6 salt exclusively
(92% yield), which was fully characterized by IR, 1H and
13C NMR, and elemental analyses. We later discovered
that treatment of 3 with a mixture of purified Ph3CBF4
(1.5 equiv) and water (1.0 equiv) in cold CH2Cl2 (0 °C,
2 h) generated the cationic tungsten species 7, which
was purified by rapid crystallization twice in cold CH2-
Cl2/diethyl ether (-30 °C); the yield was 56%. This salt
could also be prepared from unpurified Ph3CBF4 (2.0
equiv) in 46% yield. Attempts to grow single crystals
of 7 were hampered by its kinetic instability as well as
its poor crystallinity. If the NMR sample (CDCl3) was
allowed to stand at 23 °C for 36 h, species 7 gradually
† National Tsing Hua University.
‡ National Taiwan University.
X Abstract published in Advance ACS Abstracts, September 1, 1997.
(1) For transition-metal-η1-furyl compounds, see: (a) J ones, W. D.;
Dong, L.; Myers, A. W. Organometallics 1995, 14, 855. (b) Selnau, H.
E.; Merola, J . S. Organometallics 1993, 12, 1583. (c) Selnau, H. E.;
Merola, J . S. Organometallics 1993, 12, 3800. (d) Pannell, K. H.; Cea-
Olivares, R.; Toscano, R. A.; Kapoor, R. N. Organometallics 1987, 6,
1821.
(2) For transition metal-η1-pyrrolyl and -η1-thienyl compounds,
see: (a) J ohnson, T. D.; Arif, A. M.; Gladysz, J . A. Organometallics
1993, 12, 4728. (b) Robertson, M. J .; White, C. J .; Angelici, R. J . J .
Am. Chem. Soc. 1994, 116, 5190. (c) Dong, L.; Duckett, S. B.; Ohman,
K. F.; J ones, W. D. J . Am. Chem. Soc. 1992, 114, 151. (d) Ng, M. M.
P.; Roper, W. R.; Wright, L. J . Organometallics 1994, 13, 2563. (e)
Erker, G.; Petrenz, R.; Kruger, C.; Lutz, F.; Weiss, A.; Werner, S.
Organometallics 1992, 11, 1646.
(3) (a) Shu, H.-G.; Shiu, L.-H.; Wang, S.-H.; Wang, S.-L.; Lee, G.-
H.; Peng, S.-M.; Liu, R.-S. J . Am. Chem. Soc. 1996, 118, 530. (b) Wang,
S.-H.; Shiu, L.-H.; Shu, H.-G.; Wang, S.-L.; Lee, G.-H.; Peng, S.-M.;
Liu, R.-S. J . Am. Chem. Soc. 1994, 116, 5967.
(4) Chen, C.-C.; Fan, J .-S.; Shieh, S.-J .; Lee, G.-H.; Peng, S.-M.;
Wang, S.-L.; Liu, R.-S. J . Am. Chem. Soc. 1996, 118, 9279.
(5) Compound 5 crystallizes in the triclinic space group P1h, with a
) 10.1136(13) Å, b ) 10.999(2) Å, c ) 12.543(5) Å, R ) 67.33(3)°, â )
71.770(21)°, γ ) 72.78(3)°, V ) 1198.1(7) Å3, and Z ) 2. Final R )
0.0240 and Rw ) 0.026 for 3569 reflections >2.0σ(I), out of 4192 unique
reflections.
S0276-7333(97)00563-3 CCC: $14.00 © 1997 American Chemical Society

Communications
Organometallics, Vol. 16, No. 20, 1997 4233
Sch em e 1a
a
Legend: W ) CpW(CO)2; (i) THF, 0 °C, 4 h, (ii) 40 °C, 8 h; (iii) CF3CO2H (1.0 equiv), 23 °C, 96 h; (iv) CF3SO3H (1.0 equiv),
-40 °C, 2 h; (v) Ph3CBF4 (1.5 equiv) H2O (1.0 equiv), (vi) LiAlH4 (2.0 equiv); (vii) 23 °C.
Ch a r t 1
Sch em e 2a
a
W ) CpW(CO)3.
of 7 can be best described as an intermediate between
η1-iminium A and η2-pyrrole B.
The CâH proton of the cation 7 was readily deproto-
nated by Et3N (2.0 equiv) in CH2Cl2 to afford 5 in
quantitative yield, as shown in Scheme 1. Reduction
of 7 with LiAlH4 in CH2Cl2 yielded the η1-tetrahydro-
pyrrolyl species 8 as a single diastereomer in 68% yield
in addition to 5 (21%). The stereochemistry of 8 was
determined by proton NOE difference spectra; the NOE
map in part b of Chart 1 concludes that the CpW(CO)3
and phenyl groups are on opposite sides relative to the
pyrrolyl plane.
lost one proton to generate 5 at a 42% conversion level.
1
Assignment of the H and 13C NMR signals of 7 were
Clearly, the cation 7 is the intermediate in the
isomerization of η1-2,5-dihydropyrrole 3 to η1-2,3-dihy-
dropyrrole 5. Scheme 2 depicts a plausible mechanism
for this isomerization. The action of Ph3CBF4 with
water is expected to release one proton. The isomer-
ization is initiated by addition of this proton at the dCγ
carbon7 of 3 to yield the tungsten-η1-3-pyrrolylidenium
species D, which subsequently undergoes a 1,2-hydro-
gen shift8 to yield the η1-pyrrolium cation 7. The CpW-
(CO)3 fragment of 7 preferably coordinates to the Câ
carbon opposite the phenyl group to avoid steric interac-
tions; this stereochemistry is inferred from the structure
of its related derivative 8. Further loss of the Câ proton9
of 7 yields the η1-2,3-dihydropyrrole species 5. The role
of Ph3CBF4 is to generate a proton with suitable acidity
to effect the formation of the η1-iminium intermediate
7. If CF3CO2H was employed, the formation rate of 7
made on the basis of proton NOE effects and 1H-13C
NMR correlation spectra. Key NMR data for 7 are
depicted in Chart 1. Spectral data for 7 suggest that
the structure is more consistent with the η1-iminium
salt A rather than the η2-pyrrole species6a B. Diagnostic
for the structure are the large 1H and 13C NMR chemical
shifts of CRH1 (CR, δ 166.8; H1, δ 8.89) characteristic of
an iminium group. The Câ carbon has a small chemical
shift at δ 23.5 ppm characteristic of a sp3-hybridized
carbon. Moreover, the proton coupling constant J 12
)
0-1 Hz shows a large dihedral angle (75-90°) between
the C-H1 and C-H2 bonds. The proposed structure is
also supported by proton NOE effects; irradiation of the
H2 signal gave a 3.1% intensity increase in the H3′
signal, whereas the intensity of the H3′′ signal is only
increased by 0.9%. One discrepancy here is the absence
of the υ(CdN) vibration in the 1600-1750 cm-1 region;
we envisage that this η1-bonding mode is probably
mixed with some signals of the η2-bonding B with a
weak W-CR interaction. In this manner, the structure
(7) Bodner, G.-S.; Smith, D. E.; Hatton, W. G.; Heah, P. C.; Georgiou,
S.; Rheingold, A. L.; Geib, S. J .; Hutchinson, J . P.; Gladysz, J . A. J .
Am. Chem. Soc. 1987, 109, 7689.
(8) (a) Kremer, K. A. M.; Kuo, G.-H.; O’Connor, E. J .; Helquist, P.;
Kerber, R. C. J . Am. Chem. Soc. 1982, 104, 6119. (b) Casey, C. P.;
Miles, W. H.; Kukada, H. J . Am. Chem. Soc. 1985, 107, 2924.
(9) For formation of a metal-η1-vinyl complex via deprotonation of
a metal-η2-olefin cation, see the representative example: Kowalczyk,
J . J .; Arif, A. M.; Gladysz, J . A. Chem. Ber. 1991, 124, 729.
(6) (a) Hodges, L. M.; Moody, M. W.; Harman, W. D. J . Am. Chem.
Soc. 1994, 116, 7931. (b) Myers, W. H.; Koontz, J . L.; Harman, W. D.
J . Am. Chem. Soc. 1992, 114, 5684. (c) Ofial, A.; Mazyr, H., J . Org.
Chem. 1996, 61, 5823.

4234 Organometallics, Vol. 16, No. 20, 1997
Communications
Su p p or tin g In for m a tion Ava ila ble: Text giving spectral
data for the new compounds 2-8 and tables of crystal data,
atomic coordinates, and bond distances and angles for com-
pound 5 (12 pages). Ordering information is given on any
current masthead page.
would be too slow compared to the loss of the CâH proton
of 7; a stronger Brønsted acids such as CF3SO3H and
HBF4‚Et2O cleaved the σ W-C bond to yield the η1(N)-
pyrrole cation 6 instead.
Ack n ow led gm en t. We thank the National Science
Council of the Republic China for financial support of
this work.
OM970563S