Article pubs.acs.org/Organometallics
Inter- versus Intraannular Ring-Closing Metathesis of Polyallylferrocenes: Five-Fold RCM within a Single Molecule Masamichi Ogasawara,*,† Wei-Yi Wu,† Sachie Arae,† Kiyohiko Nakajima,‡ and Tamotsu Takahashi*,† †
Catalysis Research Center and Graduate School of Life Science, Hokkaido University, Kita-ku, Sapporo 001-0021, Japan Department of Chemistry, Aichi University of Education, Igaya, Kariya, Aichi, 448-8542, Japan
‡
S Supporting Information *
ABSTRACT: Treatment of decaallylferrocene (1) with the Grubbs-II catalyst in refluxing dichloromethane has promoted the 5-fold ringclosing metathesis in 1 to afford single-bridged bis(1,4,5,8tetrahydrofluorenyl)iron(II) species 3 in excellent yield. The preferential formation of 3 over quintuply bridged “superferrocenophane” 2 was explained as the intraannular RCM process being preferred over the interannular one in the reactions of polyallylferrocenes.
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ve4,5a,8b and enantioselective fashions.8,9 The same strategy was found to be applicable to preparation of the doubly bridged metallocenes by the 2-fold interannular RCM reactions.5b,10 In 2013, we also showed that the metathesis reaction on 1,2diallylferrocenes afforded the intraannular RCM products, i.e., (η5-4,7-dihydroindenyl)Fe species, in good yields (eq 2).9 The reactions shown in eqs 1 and 2 take place with high efficiency, and thus we are interested in applying the RCM reaction to the modulation of decaallylferrocene (1).11 Whereas ferrocene 1 possesses multiple (and identical) reaction sites within a single molecule, its RCM reaction would provide a complex mixture of multiple products, or the reversible nature of the olefin metathesis reaction may afford a single (or a single dominant) product by thermodynamic control. If the latter was the case, we envisioned that the RCM reaction of 1 might lead to quintuply bridged “superferrocenophane 2”12 by the simple operation via 5-fold interannular ring-closing metathesis. Indeed, the 5-fold RCM reaction of 1 proceeded as we expected; however, the major product from the reaction was not 2 but the isomeric single-bridged bis(tetrahydrofluorenyl)Fe(II) species 3 (Scheme 1). It should be pointed out that the multifold RCM reactions have been utilized for assembling/ modulating various transition-metal complexes13−17 and topologically unique organic compounds.18−20 The preferential formation of 3 over the superferrocenophane in the 5-fold RCM reaction postulates that the intraannular RCM process is preferred over the interannular process during the RCM reaction of polyallylferrocenes, which was confirmed by controlled experiments using the 1,1′,2triallylic ferrocene substrate. Here we would like to report the results of the RCM studies on the polyallylic ferrocenes.
INTRODUCTION Metal-catalyzed structural modification of transition-metal complexes within their coordination spheres has been an interesting subject in inorganic/organometallic synthesis.1 Since the discovery of Schrock’s Mo-2 and the Grubbs’ Ru-alkylidene3 complexes, the olefin metathesis reaction has proven to be a powerful tool in synthetic organic chemistry due to the excellent functional group tolerance of these complexes. These well-defined metathesis catalysts have been demonstrated to be persistent to various organometallic functional groups as well and are applicable to the modulation of organometallic substrates.1a In 2001, Richards and co-workers reported a novel synthetic method of ferrocenophanes (bridged ferrocenes) by the interannular ring-closing metathesis (RCM) reaction of 1,1′diallylferrocenes using Grubbs’ catalyst (eq 1).4 Shortly after
this report, we independently found analogous reactions and reported that the RCM route could be applied to the preparation of a variety of bridged metallocenes of Fe(II), Ru(II), Zr(IV), and Hf(IV).5 In the same period, Erker described application of the metathesis method for preparing ansa-zirconocene species.6,7 The interannular RCM process was extended into stereoselective counterparts, and diverse planarchiral bridged metallocenes were prepared in diastereoselecti© XXXX American Chemical Society
Received: September 19, 2013
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dx.doi.org/10.1021/om400936b | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Article
olefin moieties (CH2CH−) are detected at higher field (δ ca. 4.9) compared to the signals for the internal olefinic hydrogens (CH-CH2−; δ 5.7−6.1). Accordingly, the progress of the metathesis transformations was evaluated as the consumption of the terminal CH2CH− moieties, which could be estimated by the integration between the CH2CH− and the CHCH2− signals in the 1H NMR spectra, since the intensities of the signals for the latter olefinic hydrogens stayed constant during the metathesis transformations (the allyl to propenyl isomerization could be excluded under the reaction conditions employed for this study). The results of optimizing the reaction conditions are summarized in Table 1. Treatment of 1 (initial concentration of 1 = 10 mmol/L) with the Grubbs-II catalyst3c,21 (A; 10 mol %) for 18 h in refluxing dichloromethane consumed >99% of the terminal allyl groups, and the orange powders precipitated (entry 1). This insoluble solid material seemed to be the polymeric materials developed by the ADMET-type polymerization.5a,22 The intermolecular reaction was effectively suppressed under the high-dilution conditions (initial concentration of 1 = 0.5 mmol/L), and 99% of the allyl groups were consumed in 18 h in the presence of A (20 mol %). The GCMS analysis revealed that the major product from the reaction was a mononuclear 5-fold RCM species, of which GC yield was 93% (entry 2; see the next section for the characterization of the product). The lower catalyst loading (5 mol %; entry 3) or the lower temperature (23 °C; entry 4) resulted in incomplete reactions. A relatively high catalyst loading (∼20 mol %) was
Scheme 1. Five-Fold Ring-Closing Metathesis Reaction of Decaallylferrocene 1
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RESULTS AND DISCUSSION Ring-Closing Metathesis of Decaallylferrocene. Decaallylferrocene (1), which was reported by Jutzi and his coworkers in 1996,11 was subjected to the RCM reactions under various conditions. There are three plausible processes for the metathesis transformation of 1: intra- and interannular RCM and bimolecular cross-metathesis (see the scheme in Table 1). In any case, metathesis reactions convert the terminal allyl groups in 1 into internal olefins (with released ethylene). In the 1 H NMR spectra of 1 and the reaction mixtures recorded in CDCl3 or C6D6, the resonances of the remaining terminal Table 1. Ring-Closing Metathesis of Decaallylferrocene 1a
entry
precatalyst (mol %)
solvent
initial conc of 1 (mmol/L)
temp
conversionb (%)
productsc
1 2 3 4 5 6 7 8
A (10) A (20) A (5) A (20) A (20) B (20) C (20) D (20)
CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 benzene CH2Cl2 CH2Cl2 CH2Cl2
10 0.5 0.5 0.5 0.5 0.5 0.5 0.5
reflux reflux reflux 23 °C reflux reflux reflux reflux
>99 99 69 61 >99 96 69
