Article pubs.acs.org/Organometallics
Formation of Hydroxyindenyl and Vinylidene Ligands by Reaction of Internal Alkynes with Cp*Fe(CO)(NCMe)Ph Steven E. Kalman,† T. Brent Gunnoe,*,† and Michal Sabat‡ †
Department of Chemistry and ‡Nanoscale Materials Characterization Facility, Materials Science and Engineering Department, University of Virginia, Charlottesville, Virginia 22904, United States S Supporting Information *
ABSTRACT: The reactivity of Cp*Fe(CO)(NCMe)Ph (1) (Cp* = pentamethylcyclopentadienyl) with internal alkynes has been studied. Reactions of 1 with dihydrocarbyl-substituted alkynes result in the formation of pentamethylcyclopentadienyl hydroxyindenyl sandwich complexes. A divergent reaction pathway is observed in the case of bis(trimethylsilyl)acetylene, where a neutral Fe vinylidene is isolated. The reactions of hydrocarbyl trimethylsilyl-substituted alkynes with complex 1 result in the formation of the pentamethylcyclopentadienyl hydroxyindenyl sandwich complexes. This reactivity suggests the necessity of two trimethylsilyl groups for vinylidene formation.
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INTRODUCTION Alkynes are desirable synthons due to their unsaturation, which provides a means for making complex molecules.1−3 The insertion of alkynes into transition metal−hydrocarbyl bonds is a key step in stoichiometric and catalytic formation of new C− C bonds, including the polymerization and hydroarylation of alkynes.1−9 Additionally, transition metals mediate a variety of cycloaddition reactions with alkynes, such as the trimerization of alkynes to give substituted benzenes and the Pauson−Khand reaction for the synthesis of cyclopentanones.2,10−15 In one application, alkynes have been used for the synthesis of indenols (Figure 1).16−29 Indenols have been shown to have
prefunctionalization but rather proceeds via aromatic C−H activation.16−18 One can envision another strategy for indenol synthesis that involves the coupling of benzene, CO, and an alkyne (Scheme 1). Germane to this strategy is an example of the synthesis of Scheme 1. Retrosynthesis of Indenol to Benzene, Alkyne, and Carbon Monoxide
indenones by the high-temperature reaction of CpFe(CO)2Ph and alkynes reported by Butler (Scheme 2a).34,35 In addition, Allison and co-workers have shown that CpFe(CO)2(η1-1,3Figure 1. Structure of substituted indenol.
Scheme 2. Previous Work on Fe-Mediated Cyclizations to Give Indenones and Hydroxyferrocenes34−36
analgesic and anti-inflammatory properties,30,31 and they are intermediates in the synthesis of compounds with insecticidal properties.32,33 Due to the important applications of indenols, there has been interest in developing new methods for the preparation of these compounds. Among the variety of synthetic strategies, transition metal-mediated reactions have shown promise, with many methodologies involving the carbocyclization of aryl ketones and alkynes.16−19,21,22,24,26,27,29 One strategy involves the prefunctionalization of an aryl ketone with a halide or boronic acid, and catalytic systems based on palladium, cobalt, and nickel have been reported.19,21,22,24−27 The groups of Cheng and Glorious have independently developed Rh catalysts and Jeganmohan has developed a Ru catalyst for a reaction that does not require © 2014 American Chemical Society
Received: July 21, 2014 Published: September 4, 2014 5457
dx.doi.org/10.1021/om500748v | Organometallics 2014, 33, 5457−5463
Organometallics
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butadienyl) complexes undergo electrocyclic ring closures upon photolysis to give hydroxyferrocenes (Scheme 2b).36 On the basis of these reports, we considered whether a similar reaction could be used to synthesize coordinated indenols. We recently reported that the complex Cp*Fe(CO)(NCMe)Ph (1) (Figure 2) initiates the activation of aromatic
Figure 3. ORTEP of Cp*Fe(η5-1-hydroxy-2,3-dimethylindenyl) (2) (50% probability ellipsoids). Most H atoms are omitted and one independent molecule is shown for clarity. Selected bond lengths (Å): Fe2−C22 2.058(2); Fe2−C23 2.088(2); Fe2−C28 2.070(2); Fe2− C29 2.051(2); Fe2−C30 2.067(2); C22−O2 1.374(2). Selected bond angles (deg): O2−C22−C23 122.8(2); O2−C22−C30 128.1(2).
Figure 2. Cp*Fe(CO)(NCMe)Ph (1).
C−H bonds under mild conditions.37 Herein, we study the reactivity of complex 1 with internal alkynes to give hydroxyindenyl ligands. Moreover, the impact of the identity of the alkyne substituents toward this cyclization reaction is also addressed, including a divergent reaction pathway for bis(trimethylsilyl)acetylene that results in a neutral Fe vinylidene complex.
with C−O bond lengths for other hydroxycyclopentadienyl and hydroxyindenyl ligands.38,42,43 The mechanism of the formation of the hydroxyindenyl ligand is not clear. However, the overall transformation bares similarities to the work of Butler et al. on indenone synthesis from CpFe(CO)2Ph and alkynes and Allison and co-workers’ study of electrocyclic ring closures in CpFe(CO)2(η1-1,3butadienyl) complexes to give hydroxyferrocenes.34−36 Thus, a possible reaction pathway is shown in Scheme 3. After initial
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RESULTS AND DISCUSSION As an initial probe into the reactivity of Cp*Fe(CO)(NCMe)Ph (1) with internal alkynes, complex 1 was treated with excess 2-butyne in THF at room temperature (eq 1). Upon addition
Scheme 3. Possible Mechanism for the Conversion of Cp*Fe(CO)(NCMe)Ph (1) and 2-Butyne to Cp*Fe(η5-1hydroxy-2,3-dimethylindenyl) (2)
of the alkyne, an immediate color change from red to deep purple was observed. The absence of any terminal or bridging carbonyl stretches in the infrared spectrum of the product indicates that the CO ligand was consumed in the reaction. After workup, the complex Cp*Fe(η5-1-hydroxy-2,3-dimethylindenyl) (2) was isolated in 60% yield as a purple crystalline solid. The aromatic region of the 1H NMR spectrum (acetoned6) of complex 2 exhibits a pair of doublets at 7.43 and 7.23 ppm as well as a complex multiplet at 6.90 ppm that integrates for two protons, consistent with the portion of the indenyl fragment that is not directly interacting with Fe. The hydroxyl proton is visible as a sharp singlet at 6.33 ppm. A crystal suitable for a single-crystal X-ray diffraction study was grown from the slow evaporation of a saturated pentane solution of 2 (Figure 3). To the best of our knowledge, complex 2 is only the second example of a structurally characterized transition metal ligated by a hydroxyindenyl ligand. Previously, Jones and co-workers isolated and structurally characterized a related Ru sandwich compound from their studies on carbene migratory insertion.38 Notably, the −OH of complex 2 is in the α-position relative to the indenyl ring junction, while the −OH group is in the β-position for the previously reported Ru complex. The average Fe− indenyl bond distance is ∼2.07 Å, which is consistent with other structurally characterized Fe−indenyl fragments.39−41 The C22−O2 bond length is 1.374(2) Å, which is consistent
ligand exchange between NCMe and 2-butyne, the alkyne likely inserts into the Fe−Ph bond to give a vinyl intermediate. For simplicity, a cis insertion is shown, which has been demonstrated to be more likely based on experiment and theoretical predictions.2,44 We have previously shown that Cp*Fe(CO)(NCMe)Ph (1) performs facile aromatic C−H activation; thus cyclometalation via intramolecular aromatic C− H activation appears viable, which would transfer the proton to the vinyl group.37 The overall C−H activation could occur by a σ-bond metathesis, an oxidative addition/reductive coupling sequence, or a concerted metalation−deprotonation pathway. Subsequent CO insertion, ring closure, and tautomerization would give the final product 2. Allison and co-workers have observed a similar pathway in their work.36 From the pentadienoyl intermediate, ring closure and subsequent tautomerization would give complex 2. As indicated in Scheme 5458
dx.doi.org/10.1021/om500748v | Organometallics 2014, 33, 5457−5463
Organometallics
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3, CO insertion into the Fe-vinyl complex prior to phenyl C−H bond rupture is also possible. With current data, the intimate details following alkyne insertion cannot be known. To assess the regioselectivity of this reaction, we next turned our attention to asymmetric internal alkynes. The reaction of complex 1 with excess 1-phenylpropyne in THF gave the expected hydroxyindenyl product 3a in 49% isolated yield (eq 2). Monitoring the reaction by 1H NMR spectroscopy revealed
spectrum (acetone-d6) for the major isomer has two doublets at 7.39 and 7.31 ppm with a multiplet at 7.06 ppm for the unbound ring and a singlet at 7.00 ppm for the hydroxyl proton. In order to determine the regiochemistry of the major isomers for complexes 3 and 4, two-dimensional NOESY spectra were obtained. Figure 4 shows representations of the the formation of one major product with a small amount (∼15%) of a second product, presumably the regioisomer (3b) (Table 1). Upon workup by washing with cold pentane or Table 1. Ratio of Regioisomers Observed in the Crude Reaction Mixtures for the Reactions of Cp*Fe(CO)(NCMe)Ph (1) with Asymmetric Alkynes
no.
R1
R2
A:B(%)
3 4 6 7
Ph COOMe TMS TMS
Me Me Me Ph
85:15 80:20 75:25 50:50
Figure 4. Representations showing important cross-peaks from NOESY spectra of Cp*Fe(1-hydroxy-2-phenyl-3-methylindenyl) (3a) and Cp*Fe(1-hydroxy-2-methyl ester-3-methylindenyl) (4a).
cross-peaks observed (see Supporting Information for spectra). For complex 3a, cross-peaks are observed between the ortho phenyl resonances and the resonance for the hydroxyl proton as well as between the methyl resonance and a resonance associated with the unbound indenyl ring. This correlation suggests that the phenyl ring is proximal to the alcohol, as shown in Figure 4. For complex 4a, a cross-peak was observed between the methyl group directly attached to the indenyl ligand and a proton on the unbound ring. No cross-peaks were observed between the hydroxyl proton and the methyl group of the ester; however, that interaction may be too weak to be observed. Nonetheless, these data provide evidence that the ester functionality is adjacent to the alcohol group. To determine whether the insertion/cyclization was compatible with large substituents on the alkyne, complex 1 was treated with excess bis(TMS)acetylene (TMS = trimethylsilyl) at 60 °C in THF (eq 4). Contrary to the
hexanes, a single isomer can be isolated; however, trace amounts (
