Facile α-β isomerisation of the σ-π phenylethenyl ligand at the diiron centre

Article abstract of DOI:10.1039/a804403a

At 110 °C α-substituted [Fe₂(CO)₆(n-PhC=CH₂)(μ-PPh₂)] 1 is cleanly converted into the β-isomer [Fe₂(CO)₆(μ-HC=CHPh)(μ-PPh₂)] 2, a process which is accelerated in the presence of arylphosphines; with P(OMe)₃ mono- and disubstituted adducts of 1 are isolated at 70 °C, which cleanly isomerise at higher temperatures.

Full text of DOI:10.1039/a804403a

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Facile a-b isomerisation of the s-p phenylethenyl ligand at the diiron centre
Simon Dohertya and Graeme Hogarth*b†
a Department of Chemistry, University of Newcastle, Bedson Building, Newcastle-upon-Tyne, UK NE1 7RU
b Department of Chemistry, University College London, 20 Gordon Street, London, UK WC1H 0AJ
At 110 °C a-substituted [Fe2(CO)6(m-PhC§CH2)(m-PPh2)] 1
is cleanly converted into the b-isomer [Fe2(CO)6(m-
HC§CHPh)(m-PPh2)] 2, a process which is accelerated in
the presence of arylphosphines; with P(OMe)3 mono- and di-
substituted adducts of 1 are isolated at 70 °C, which cleanly
isomerise at higher temperatures.
The alkenyl ligand has been proposed as a key intermediate for
the Fischer–Tropsch process1 and as a result, the chemistry of
this important ligand has been studied at a variety of metal
centres, the emphasis of this work being based on carbon–
carbon bond forming reactions.2–4 Monosubstituted alkenyl
complexes can adopt isomeric forms I–III with the substituent
Scheme 1
P(o-tolyl)3 did not result in accelerated isomerisation, 2 being
the sole product, and suggesting that it is carbonyl substitution
which accelerates alkenyl isomerisation.
Further insight was gleaned from the reaction of P(OMe)3
with 1 (Scheme 2). This proceeded rapidly (10 min) at 70 °C and
on either the a- or b-carbon. Interconversion of b-isomers II and
led to a mixture of [Fe2(CO)5{P(OMe)3}(m-PhC§CH2)(m-
PPh2)] 4d and [Fe2(CO)4{P(OMe)3}2(m-PhC§CH2)(m-PPh2)]
4e.‡ Pertinently, the a-substituted alkenyl ligand is maintained
throughout. Warming both 4d and e to reflux in toluene resulted
in their rapid ( < 10 min) conversion into the b-substituted
isomers [Fe2(CO)5{P(OMe)3}(m-HC§CHPh)(m-PPh2)] 3d and
[Fe2(CO)4{P(OMe)3}2(m-HC§CHPh)(m-PPh2)] 3e, respec-
tively, also obtained from the direct reaction of P(OMe)3 with 2,
while reaction of the latter with PPh3 afforded 3a in a similar
manner.
Conversion of 3 to 4 involves both carbonyl substitution and
alkenyl isomerisation. For the arylphosphines utilised to date,
carbonyl substitution is rate-determining, being followed by
rapid alkenyl isomerisation. In contrast, with the less sterically
demanding P(OMe)3, carbonyl substitution is more facile and
alkenyl isomerisation becomes rate-determining. While the
III has been noted in a number of instances, and results from
rotation about the carbon-carbon bond.5,6 In contrast, as far as
we are aware, and despite the large volume of literature
concerning alkenyl complexes, a,b-isomerisation of alkenyl
complexes has not been reported. Herein we describe the clean
conversion of an a-substituted alkenyl ligand (I) to its b-isomer
(II) at the diiron centre, a process which is accelerated in the
presence of arylphosphines.
A convenient route to alkenyl complexes involves the
hydrometalation of alkynes, primary alkynes giving both a- and
b-substituted isomers depending upon the regioselectivity of the
process. Hydrodimetalation of PhC°CH by [Fe2(CO)7(m-H)(m-
PPh2)] is reported to afford a mixture of a-[Fe2(CO)6(m-
PhC§CH2)(m-PPh2)]
1 and b-[Fe2(CO)6(m-HC§CHPh)(m-
PPh2)] 2 substituted isomers in a 4:1 ratio.7 In our hands, and
provided that the temperature is not raised during work-up, the
a-isomer 1 is the sole product as shown by 1H NMR
spectroscopy. However, when a toluene solution of 1 was
heated at 110 °C for 1 h clean conversion to b-isomer 2 was
noted, being isolated in > 90% yield after chromatography.
Conversion of 1 to 2 is easily followed by 1H NMR
spectroscopy. Under these conditions conversion was quantita-
tive, the appearance of a low-field multiplet being characteristic
of the a-proton of an alkenyl ligand. Following the reaction by
NMR and IR spectroscopy failed to reveal any intermediates.
In the presence of arylphosphines, alkenyl isomerisation was
accelerated. Heating a toluene solution of 1 and PPh3 to 110 °C
for 10 min lead to the quantitative formation of the b-substituted
phosphine adduct [Fe2(CO)5(PPh3)(m-HC§CHPh)(m-PPh2)]
3a.‡ The 1H NMR spectrum clearly showed that both carbonyl
substitution and alkenyl isomerisation had occurred, the
a-proton appearing at d 8.35. Monitoring the reaction by 31P
NMR spectroscopy again failed to reveal any intermediates.
The acceleration of alkenyl isomerisation was found for other
phosphines, including P(p-tolyl)3 and P(m-tolyl)3 (Scheme 1).
In contrast, addition of the more sterically demanding isomer
Scheme 2
Chem. Commun., 1998
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Ph), 4.12 (dd, J 12.9, 5.4, 1H, Hb); dP(CDCl3) 181.2 (d, J 93.4, m-PPh2), 73.0
(d, J 93.4, PPh3). For 3b: n(CO)(CH2Cl2)/cm21 2030m, 1982vs, 1946s,
1917m, 1885w; dH(CDCl3) 8.35 (ddd, J 27.8, 12.9, 6.5, 1H, Ha), 7.8–7.0
(m, 25H, Ph), 6.62 (d, J 7.5, 2H, Ph), 4.13 (dd, J 12.9, 5.3, 1H, Hb), 2.36 (s,
9H, Me); dP(CDCl3) 180.6 (d, J 93.9, m-PPh2), 72.0 (d, J 93.9, PPh3). For
3c: n(CO)(CH2Cl2)/cm21 2033m, 1982vs, 1970sh, 1949m, 1915m;
dH(CDCl3) 8.31 (ddd, J 27.8, 12.9, 6.5, 1H, Ha), 7.8–7.0 (m, 25H, Ph), 6.58
(d, J 7.1, 2H, Ph), 4.08 (dd, J 12.8, 5.4, 1H, Hb), 2.37 (s, 9H, Me); dP(CDCl3)
180.6 (d, J 94.4, m-PPh2), 70.6 (d, J 94.4, PPh3). For 3d (two isomers A:B
in 2:1 ratio): n(CO)(CH2Cl2)/cm21 2035s, 1987vs, 1971m, 1958s;
dH(CDCl3) 7.8–7.1 (m, Ph), 3.65 (d, J 11.2, Me, 9H, A), 3.60 (d, J 11.2, Me,
9H, B), 3.16 (d, J 15.2, 1H, B), 3.01 (dt, J 15.4, 3.3, 1H, A), 2.28 (dt, J 12.1,
3.3, 1H, A), 2.24 (d, J 10.5, 1H, B); dP(CDCl3, 213 K) 188.1 [d, J 52.7,
P(OMe3), B], 186.6 [d, J 70.2, P(OMe3), A], 165.0 (d, J 70.2, PPh2, A),
152.5 (d, J 52.7, PPh2, B). For 3e: n(CO)(CH2Cl2)/cm21 1997vs, 1962s,
1932m; dH(CDCl3) 7.8–7.1 (m, 15H, Ph), 3.55 (d, J 11.2, 9H, Me), 3.51 (d,
J 11.2, 9H, Me), 3.00 (d, J 16.1, 1H), 2.15 (d, J 10.8, 1H); dP(CDCl3, 213
K) 189.8 [d, J 47.0, P(OMe)3], 189.1 (d, J 71.6), 145.9 (br, PPh2). For 4d:
n(CO)(CH2Cl2)/cm21 2037s, 1989vs, 1976s, 1956s, 1928m; dH(CDCl3,
223K) 8.73 (ddd, J 31.8, 12.7, 8.1, 1H, Ha), 7.8–6.8 (m, 15H, Ph), 3.79 (d,
J 11.1, 9H, Me), 3.73 (t, J 14.0, 1H, Hb); dP(CDCl3, 223 K) 182.6 [d, J
154.7, P(OMe)3], 177.7 (d, J 154.7, PPh2). For 4e (two isomers A:B in 6:1
ratio): n(CO)(CH2Cl2)/cm21 1995s, 1963vs, 1930s, 1910m; dH(CDCl3, 223
K) 8.74 (ddd, J 30.2, 13.0, 8.1, 1H, Ha, A), 8.63 (br, 1H, Ha, B), 7.85–6.8
(m, Ph), 3.96 (br, 1H, Hb, B), 3.79 (d, J 11.2, 9H, A), 3.73 (d, J 11.2, 9H,
Me, B), 3.50 (d, J 11.5, 1H, Hb, A), 3.32 (d, J 11.0, 9H, A), 3.28 (br, 9H, Me,
B); dP(CDCl3, 223 K) 196.8 [d, J 45.5, m-P(OMe)3, B], 188.6 [d, J 145.7,
P(OMe)3, A], 181.0 [d, J 150.7, P(OMe)3, B], 180.3 [d, J 59.8 P(OMe)3, A],
165.4 (dd, J 145.7, 59.8, PPh2, A), 159.0 (dd, J 150.7, 45.0, PPh2, B).
Scheme 3
origin of the rate acceleration of alkenyl isomerisation is not
clear, since the b-phenylethenyl ligand is generated in all cases
it must be thermodynamically preffered. It is difficult to see how
this can be a result of steric effects in the hexacarbonyl
complexes and we believe that their must be an electronic
preference for the adoption of the b-isomer, while the
hydrodimetalation reaction affords the a-isomer preferentially
as a result of Markovnikov addition.
The precise manner in which alkenyl isomerisation occurs is
as yet unknown. It may simply occur via a direct 1,2-proton
shift, although it is difficult to see how the rate of such a process
would be strongly affected by ligand substitution. A second
possibility is that it results from a reversible C–H addition to the
diiron centre (Scheme 3). Such a process would afford a
hydrido–alkyne intermediate, with the alkyne lying parallel to
the diiron vector and acting as a two-electron donor in order to
preserve the EAN count of 34. Carbon-hydrogen bond forma-
tion from this intermediate would either regenerate the a-isomer
or irreversibly afford the thermodynamically favoured
b-isomer. Both oxidative addition and reductive elimination are
likely to be sensitive to the steric and electronic nature of the
other ligands, and this may account for the observed changes in
the rate of alkenyl isomerisation.
1 P. M. Maitlis, H. C. Long, R. Quyoum, M. L. Turner and Z-Q. Wang,
Chem. Commun., 1996, 1.
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Garner, S. A. R. Knox, A. G. Orpen and A. J. Phillips, J. Chem. Soc.,
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We are currently investigating whether a–b alkenyl iso-
merisation is general by studying analogous reactions of other
a-substituted complexes [Fe2(CO)6(m-RC§CH2)(m-PPh2)].
Further we are trying to gain more mechanistic insight into this
transformation via reaction of 1 with a wider range of
phosphines, phosphites and related reagents, while we are
looking for alternative low-temperature routes to phosphine
substituted a-alkenyl complexes in order to obtain kinetic
information concerning the rate acceleration.
Notes and References
† E-mail: g.hogarth@ucl.ac.uk
7 S. A. MacLaughlin, S. Doherty, N. J. Taylor and A. J. Carty,
Organometallics, 1992, 11, 4315.
‡ All compounds exhibit satisfactory spectroscopic and analytical data.
Selected data for 3a: n(CO)(CH2Cl2)/cm21 2034m, 1983vs, 1947s, 1917m,
1884w; dH(CDCl3) 8.35 (dd, J 27.9, 12.9, 6.5, 1H, Ha), 7.9–6.4 (m, 30H,
Received in Cambridge, UK, 10th June 1998; 8/04403A
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Chem. Commun., 1998