[3+2+1] Cycloaddition involving alkynes, CO and bridging vinyliminium ligands in diiron complexes: A dinuclear version of the Doetz reaction?

Article abstract of DOI:10.1039/b925207j

The vinyliminium complex [Fe₂{μ-η¹: η³-C(SiMe₃)CHCN(Me)₂}(μ-CO)(CO)(Cp) ₂][SO₃CF₃] reacts with HCCR (R = CPh ₂OH), affording a mixture of the 2,4,6-trisubstituted oxo-η⁵-cyclohexadienyl complex [Fe{η⁵-C ₆H₂O(NMe₂)(SiMe₃)(R)}(Cp)], the 2,6-disubstituted phenol C₆H₃R′(NMe₂)OH (R′ = CHPh₂) and 1,2,4-trisubstituted ferrocene [1-NMe ₂-2-R-4-SiMe₃-Fc]. The corresponding reaction with HCCR (R = CMe₂OH) yields analogous products: [Fe{η⁵-C ₆H₂O(NMe₂)(SiMe₃)(R′)}(Cp)] (R′ = CMeCH₂), the phenol C₆H₃R′ (NMe₂)OH together with [1-NMe₂-2-R-4-SiMe₃-Fc].

Full text of DOI:10.1039/b925207j

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[3+2+1] cycloaddition involving alkynes, CO and bridging vinyliminium
¨
ligands in diiron complexes: a dinuclear version of the Dotz reaction?w
Luigi Busetto,a Fabio Marchetti,b Rita Mazzoni,a Mauro Salmi,a Stefano Zacchinia and
Valerio Zanotti*a
Received 1st December 2009, Accepted 19th March 2010
First published as an Advance Article on the web 14th April 2010
DOI: 10.1039/b925207j
The vinyliminium complex [Fe2{l-g1:g3-C(SiMe3)QCHCQ
N(Me)2}(l-CO)(CO)(Cp)2][SO3CF3] reacts with HCRCR
(R = CPh2OH), affording a mixture of the 2,4,6-trisubstituted
oxo-g5-cyclohexadienyl complex [Fe{g5-C6H2O(NMe2)(SiMe3)-
(R)}(Cp)], the 2,6-disubstituted phenol C6H3R0(NMe2)OH
(R0 = CHPh2) and 1,2,4-trisubstituted ferrocene [1-NMe2-2-
R-4-SiMe3-Fc]. The corresponding reaction with HCRCR
(R = CMe2OH) yields analogous products: [Fe{g5-C6H2O-
(NMe2)(SiMe3)(R0)}(Cp)] (R0 = CMeQCH2), the phenol
C6H3R0(NMe2)OH together with [1-NMe2-2-R-4-SiMe3-Fc].
complexes of the type [Fe2{m-Z1:Z3-C(R)QCHCQNMe2}-
(m-CO)(CO)(Cp)2][SO3CF3] (4) (R = alkyl, aryl, SiMe3),5
mostly because the less saturated bridging C3 chain, has the
appropriate number of substituents to give cyclopentadienyls,
by reaction with alkynes, without requiring any C–N or
C–H bond cleavage, as for 1. Unfortunately, vinyliminium
complexes were rather unreactive toward alkynes, with a
relevant exception concerning propargyl alcohols, as shown
in Scheme 2. We have found that complex 4 reacts with
HCRCPh2OH and HCRCMe2OH to give a mixture of
products that include the trisubstituted oxo-Z5-cyclohexadie-
nyl complexes 6 and 9, the phenols 7, 10, and the trisubstituted
ferrocenyl complexes 5 and 8, respectively (Scheme 2).z
Complex 5 was identified by comparison of its spectroscopic
properties with those reported in the literature,3 whereas the
other products have been characterized by spectroscopy, ele-
mental analysis and high-resolution mass spectrometry.z
Moreover, the molecular structure of 6 has been determined
by X-ray diffraction studies.6 In the structure of 6 (Fig. 1), the
iron center is Z5-coordinated to a Cp ligand [average Fe–C
distances 2.041(7) A] and also Z5-coordinated to a trisubsti-
tuted oxo-cyclohexadienyl ligand Z5-C6H2O(NMe2)(SiMe3)-
(CPh2OH). The Fe–C interactions with this Z5-ligand are
sensibly spread [range 2.049(3)–2.116(3) A, average 2.098(7) A]
whereas Fe(1)–C(1) [2.275(3) A] is rather long (sum of the
covalent bonds for Fe and C 2.02 A). The Z5-coordination of
the ligand is also in agreement with the fact that the C(3)–C(2)
[1.413(4) A], C(3)–C(4) [1.419(4) A], C(6)–C(5) [1.414(3) A]
and C(5)–C(4) [1.413(4) A] interactions all display a con-
siderably delocalised p-character, whereas C(2)–C(1) [1.457(4) A]
and C(1)–C(6) [1.446(4) A] are essentially single bonds. More-
over, the C(1)–O(1) distance [1.227(3) A] is indicative of a
ketonic nature of this group. A significant hydrogen bond is,
then, found between the hydroxy O(2)–H(40) group and O(1)
[O(2)–H(40) 0.862(17) A; H(40)ꢀ ꢀ ꢀO(1) 1.78(2) A; O(2)ꢀ ꢀ ꢀO(1)
2.578(3) A; O(2)–H(40)ꢀ ꢀ ꢀO(1) 153(3)1].
In the expanding field of transition-metal mediated transfor-
mations of organic molecules, dinuclear complexes might have
a special role in consideration of the cooperative effects
associated with the presence of adjacent metal centres, and
of the activation modes offered by multisite coordination.1
These possibilities have been, so far, under-exploited.
We have been interested in the assembly, functionalization
and transformation of bridging organic frames in diiron
complexes which offer, as further advantage, the use of a
non-toxic, readily available and inexpensive transition metal,
better responding to the growing need for a ‘‘sustainable metal
catalysis’’.2 Recently, we reported that bridging vinylalkyl-
idene ligands in diiron complexes can be involved in a [3+2]
cycloaddition with alkynes resulting in the formation of new
polysubstituted ferrocenyl products.3 One example is reported
in Scheme 1.
A mixture of different ferrocenes is formed since, in the
cyclization reaction, the bridging vinylalkylidene ligand loses
one of the two substituents: H or NMe2, generating the tri-
and di-substituted ferrocenes, respectively (i.e. 2 and 3 in
Scheme 1). Nevertheless, the reaction shows that is possible
to involve bridging C3 frames in cycloaddition with alkynes,
reminiscent of the well known cycloaddition reactions of
mononuclear Fischer-type vinylcarbene complexes.4
Then, we became interested in extending these studies to
other related diiron complexes containing different bridging
C3 ligands. In particular we investigated vinyliminium
The differences between the two series of products, obtained
from HCRCPh2OH and HCRCMe2OH, respectively
a
`
Dipartimento di Chimica Fisica e Inorganica, Universita di Bologna,
Viale Risorgimento 4, 40136 Bologna, Italy.
E-mail: valerio.zanotti@unibo.it; Fax: +390512093690;
Tel: +390512093695
`
Dipartimento di Chimica e Chimica Industriale, Universita di Pisa,
Via Risorgimento 35, I-56126 Pisa, Italy.
E-mail: fabmar@dcci.unipi.it; Tel: +390502219245
b
w CCDC 756445. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b925207j
Scheme 1
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Scheme 2
and is one of the most powerful protocols for the synthesis of
phenols and other organic products.7 These include cyclo-
hexadienones, which in addition, are also indicated as possible
reaction intermediates.8
On the other hand, cyclization reactions analogous to the
benzannulation, but involving bridging vinylalkylidenes, are
almost unknown. This is believed to be the consequence of the
higher stability and inertness associated with vinylalkylidenes
in the bridging Z1:Z3-coordination. To the best of our know-
ledge, there is only one example of cyclohexadienone forma-
tion at a dinuclear centre, by assembling of a bridging
vinylalkylidene, an alkyne and a CO, that is the reaction of
the dimolybdenum complex [Mo2(m-CHCHCMe2)(CO)4(Cp)2]
with HCRCBut.9 In this case, the cyclohexadienone remains
coordinated as bridging ligand, which is a remarkable difference
with respect to our results. Indeed, the reaction of 4 with
alkynes leads to fragmentation of the diiron species and
formation of mononuclear complexes. Therefore, it will be
of great interest understanding to what extent the cyclizations
shown in Schemes 1 and 2, result from dinuclear activation, or
are simply associated to processes occurring at mononuclear
centres.
Fig. 1 ORTEP drawing of 6. Thermal ellipsoids are at the 30%
probability level. Selected bond lengths (A): Fe(1)–C(1) 2.275(3),
Fe(1)–C(2) 2.166(3), Fe(1)–C(3) 2.078(3), Fe(1)–C(4) 2.073(3),
Fe(1)–C(5) 2.049(3), Fe(1)–C(6) 2.126(3), Fe(1)–Cp(av.) 2.041(7),
C(1)–C(2) 1.457(4), C(2)–C(3) 1.413(4), C(3)–C(4) 1.419(4),
C(4)–C(5) 1.413(4), C(5)–C(6) 1.414(3), C(1)–C(6) 1.446(4),
C(2)–N(1) 1.408(3), C(1)–O(1) 1.277(3), C(6)–C(17) 1.542(4),
C(4)–Si(1) 1.883(3), C(17)–O(2) 1.427(3).
The formation of phenols 7 and 10 takes place in too low
yields to be useful as a synthetic approach. Moreover, it
should be noted that 7 and 10 are not simply the result of a
[3+2+1] cycloaddition, but also involve the replacement of a
C–SiMe3 bond with a C–H bond, and the replacement of OH
with H, in the case of 7. Thus, the reaction sequence leading
to the observed products must include a number of rearrange-
ments and transformations that, at this stage, can be hardly
formulated in a reliable mechanism. Beside these limitations, it
is worth noting that the phenols 7 and 10 exhibit an unusual
substitution pattern, and neither 7 nor 10 have been described
previously.
(i.e. 5, 6, 7 vs. 8, 9, 10) is not limited to the presence of Ph
groups in the place of Me. A further difference between 8, 10
and 5, 7 is that the cyclopentadienyl substituent CMe2OH
undergoes H2O elimination to form the –C(Me)QCH2 group.
This additional transformation is reasonable in consideration
of the reaction conditions: prolonged heating and subsequent
workup by chromatography on deactivated alumina.
The cycloaddition reactions reported in Scheme 2 deserve
further comments. First, the expected ferrocenes 5 and 8 are
obtained only in low yields. Conversely, other products are
formed: the oxo-Z5-cyclohexadienyl complex 6 and 9, and the
phenols 7 and 10. In these compounds a six-membered ring
(i.e the oxo-Z5-cyclohexadienyl ligand and the phenol) is most
likely originated by a [3+2+1] cycloaddition that involves the
bridging C3 ligand of the parent complex 4, the alkyne and a
CO. The incorporation of CO makes the cycloaddition sub-
stantially different from that shown in Scheme 1, and closer to
As concluding remark, our results evidence that bridging
vinyliminium ligands in diiron complexes can undergo a [3+2+1]
cycloaddition which resembles the classic Dotz benzannu-
¨
lation. Therefore, this type of cyclization is not necessarily
restricted to vinylalkylidene ligands, but can be extended to
other bridging unsaturated C3 fragments.10 A further interesting
aspect is that iron complexes are involved, whereas the classic
benzannulation mostly concern Group 6 metals, with a limited
extension to Mn and Ru.11 Our findings add a new piece
of evidence that bridging ligands in diiron complexes offer
the classic Dotz benzannulation, that consists in the assembly
¨
of a a,b-unsaturated carbene ligand, an alkyne and a carbonyl,
3328 | Chem. Commun., 2010, 46, 3327–3329
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potential for new transformations and C–C bond forming
reactions, unattainable in mononuclear species.
We thank the Ministero dell’Universita e della Ricerca
Scientifica e Tecnologica (M.U.R.) and the University of
Bologna for financial support.
1 (a) P. Braunstein and J. Rose, Metal Clusters in Chemistry,
ed. P. Braunstein, L. A. Oro and P. R. Raithby, Wiley–VCH,
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Chemistry and Catalysis, Wiley–VCH, Weinheim, 2007;
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Camps, A. Volbeda, C. Cavazza and Y. Nicolet, Chem. Rev., 2007,
107, 4273.
Notes and references
z Synthesis of 5, 6 and 7: Complex 4 (0.610 g, 1.01 mmol), in toluene
(15 ml), was treated with HCRCCPh2OH (457 mg, 2.19 mmol) and
the resulting mixture was heated at reflux temperature for 10 h.
Removal of the solvent and chromatography of the residue on a
alumina column, with petroleum ether (bp 40–60 1C) as eluent, gave a
first yellow fraction containing ferrocene [FeCp2] (2.4%), 5 (4.2%) and
7 (19.8%; yields calculated by 1H NMR integration). Elution with
Et2O gave a second orange fraction containing 6 (33.0% yield).
Crystals of 6 suitable for X-ray analysis were obtained from a
MeOH–H2O solution.
3 L. Busetto, F. Marchetti, R. Mazzoni, M. Salmi, S. Zacchini and
V. Zanotti, Organometallics, 2009, 28, 3465.
6. Anal. Calc. for C29H33FeNSiO2: C, 68.10; H, 6.46; N, 2.74.
4 (a) K. H. Dotz and J. Stendel, Jr., Chem. Rev., 2009, 109, 3227;
¨
Found: C, 67.95; H, 6.48; N, 2.70%. IR (CH2Cl2) n(CO) 1512 (s) cmꢂ1
.
(b) J. Barluenga, J. Santamaria and M. Tomas, Chem. Rev., 2004,
104, 2259; (c) J. Barluenga, Pure Appl. Chem., 2002, 74, 1317–1325;
(d) A. De Meijere, H. Schirmer and M. Duetsch, Angew. Chem.,
Int. Ed., 2000, 39, 3964.
5 V. G. Albano, L. Busetto, F. Marchetti, M. Monari, S. Zacchini
and V. Zanotti, Organometallics, 2003, 22, 1326.
6 Crystal data for 6: C29H33FeNO2Si, M = 511.50, T = 293(2) K,
monoclinic, space group P21/n, a = 10.3930(7), b = 17.8941(12),
c = 13.7491(9) A, b = 92.8090(10)1, V = 2553.9(3) A3, Z = 4,
Dc = 1.330 g cmꢂ3, m = 0.664 mmꢂ1, graphite-monochromatized
1H NMR (CDCl3) d 9.12 (s, 1 H, OH); 7.58–7.10 (m, 10 H, Ph);
5.25 (d, 4JHH = 1.2 Hz, 1 H, C3H); 4.92 (d, 4JHH = 1.2 Hz, 1 H, C5H);
4.45 (s, 5 H, Cp); 3.00 (s, 6 H, NMe2); 0.24 (s, 9 H, SiMe3). 13C{1H}
NMR (HSQC, HMBC) (CDCl3) d 148.2 (C1); 143.9, 143.5 (ipso-Ph);
128.4–126.3 (Ph); 115.35 (C2); 97.47 (C6); 84.07 (C5); 80.93 (CPh2OH);
77.11 (C3) 72.52 (C4); 71.37 (Cp); 41.91 (NMe2); ꢂ1.10 (SiMe3).
ESI-MS(+) m/z: [M]+ + H = 512; [M]+ + Na = 534.
1
3
7. H NMR (CDCl3) d 7.29–7.13 (m, 10 H, Ph); 7.08 (dd, JHH
=
7.6 Hz, 4JHH = 2 Hz, 1 H, C5H); (6.76, t, 3JHH = 7.6 Hz, 1 H, C4H);
6.65 (dd, JHH = 7.6 Hz, JHH = 2 Hz, 1 H, C3H); 5.90 (s, 1 H,
CHPh2); 2.63 (s, 6 H, NMe2). 13C{1H} NMR (HSQC, HMBC)
(CDCl3) d 149.4 (C1); 143.6 (ipso-Ph); 140.3 (C2); 129.4–126.1 (Ph);
(126.9) (C3); 118.92 (C4), 118.70 (C5); 50.17 (CHPh2); 45.3 (NMe2).
ESI-MS: [M]+ + H = 304 m/z; [M]+ + Na = 326 m/z. HRMS m/z
Calc. for [M]+ C21H21N1O1 303,1623, found 303,1623.
Mo-Ka radiation (l = 0.71073 A). Final R indices were R1 =
3
4
0.0452 and wR2 = 0.0842 for 6061 independent reflections
having I > 2s(I) (Rint = 0.0870). The diffraction experiments
were carried out on a Bruker APEX II diffractometer equipped
with a CCD detector and using Mo-Ka radiation. H(40) bonded to
O(2) was located in the Fourier map and refined isotropically with
the O(2)–H(40) distance restrained to 0.87 A (s.u. 0.02).
Compounds 8, 9 and 10 were obtained by an analogous procedure,
upon reaction of 4 (490 mg, 0.80 mmol) and HCRCCMe2OH
(0,30 mL, 3.1 mmol)
8 (12%). Anal. Calc. for C18H27FeNSi: C, 63.34; H, 7.97; N, 4.10.
Found: C, 63.51; H, 7.90; N, 4.15%. 1H NMR (CDCl3) d 5.36, 5.07
(m, 2 H,QCH2); 4.16 (s, 5 H, Cp); 3.95 (m, 2 H, C5H and C3H); 2.60
(s, 6 H, NMe2); 2.11 (s, 3 H, Me); 0.21 (s, 9 H, SiMe3). 13C{1H} NMR
7 (a) K. H. Dotz, Angew. Chem., Int. Ed. Engl., 1975, 14, 644;
¨
187; (c) K. H. Dotz and J. Stende1 Jr, in Modern Arene Chemistry,
¨
(b) K. H. Dotz and P. Tomuschat, Chem. Soc. Rev., 1999, 28,
¨
ed. D. Astruc, Wiley-VCH, Weinheim, Germany, 2002, p. 250;
(d) A. Minatti and K. H. Dotz, Top. Organomet. Chem., 2004,
¨
13, 123; (e) M. L. Waters and W. D. Wulff, Org. React., 2008,
70, 121.
8 (a) P.-C. Tang and W. D. Wulff, J. Am. Chem. Soc., 1984, 106,
1132; (b) W. D. Wulff, B. M. Bax, T. A. Brandvold, K. S. Chan,
A. M. Gilbert and R. P. Hsung, Organometallics, 1994, 13,
102.
(HSQC, HMBC) (CDCl3)
d
141.8 (CQCH2); 114.8 (C1); 112.8
(QCH2); 81.2 (C2); 70.6 (C3). 9 (30%). Anal. Calc. for C19H29Fe2NO2-
Si: C, 58.91; H, 7.55; N, 3.62. Found: C, 58.84; H, 7.60; N, 3.54%. IR
(CH2Cl2) n(CO) 1519 (s) cmꢂ1. 1H NMR (CDCl3) d 5.23 (d, 1 H, CH,
4JHH = 1.2 Hz); 5.08 (d, 1 H, CH, JHH = 1.2 Hz); 4.55 (s, 5 H, Cp);
4
2.98 (s, 6H, NMe2); 1.57 (s, 3H, Me); 1.54 (s, 3H, Me); 0.36 (s, 9H,
SiMe3); OH not observed. 13C{1H} NMR (HSQC, HMBC) (CDCl3) d
148.4 (C1); 114.8 (C2); 99.0 (C6); 81.3 (CMe2); 78.4 (CH); 73.8 (C4); 72.5
(CH); 71.5 (Cp); 42.2 (NMe2); 31.2, 27.8 (Me); 1.3 (SiMe3). ESI-MS
m/z: 389 [M]+ + H; 411 [M]+ + Na. 10 (17%).1H NMR (CDCl3,
400 MHz) d 7.09 (dd, 1 H, 3JHH = 7.8 Hz, 4JHH = 1.5 Hz, C5H); 7.03
9 (a) M. Green, A. G. Orpen, C. J. Schaverien and I. D. Williams,
J. Chem. Soc., Chem. Commun., 1983, 181; (b) M. Green,
R. J. Mercer, A. G. Orpen, C. J. Schaverien and I. D. Williams,
J. Chem. Soc., Dalton Trans., 1986, 1971.
10 L. Busetto, P. M. Maitlis and V. Zanotti, Coord. Chem. Rev., 2010,
254, 470–486.
11 (a) B. L. Balzer, M. Cazanoue, M. Sabat and M. G. Finn,
Organometallics, 1992, 11, 1759; (b) B. L. Balzer, M. Cazanoue
and M. G. Finn, J. Am. Chem. Soc., 1992, 114, 8735;
(c) K. E. Stockman, M. Sabat, M. G. Finn and R. N. Grimes,
J. Am. Chem. Soc., 1992, 114, 8733.
3
4
3
(dd, 1H, JHH = 7.8 Hz, JHH = 1.5 Hz, C3H); 6.81 (t, 1 H, JHH
=
7.8 Hz, C4H); 5.32–5.20 (m, 2 H, QCH2); 2.66 (s, 6 H, NMe2); 2.17
(s, 3 H, CH3); OH not observed. 13C{1H} NMR (HSQC, HMBC)
(CDCl3) d 148.5 (C1); 143.4 (C6); 140.2 (C2); 128.3 (CQCH2); 125.7
(C3), 119.8, (C4), 119.4 (C5); 115.6 (QCH2); 45.3 (NMe2); 23.1 (CH3).
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