Synthesis of Aryliron Complexes [CpFe(CO)2Ar] by Palladium-Catalyzed Reactions of [CpFe(CO)2I] with Arylzinc, -Boron, or -Indium Reagents

Transmetalation between [CpFe(CO)2I] and arylzinc iodide-lithium chloride complexes proceeds in the presence of catalytic amounts of palladium acetate and N,N,N’,N’-tetramethyl-1,2-cyclohexanediamine to yield the corresponding aryliron complexes [CpFe(CO)2Ar]. Phenylation of [CpFe(CO)2I] also takes place when triphenylindium is used under similar conditions. Arylboronic acids undergo arylation in the presence of cesium carbonate and a palladium-N-heterocyclic carbene complex, PEPPSI. The present methods are useful for the facile synthesis of various functionalized [CpFe(CO)2Ar]. The products [CpFe(CO)2Ar] represent an interesting class of aryl metals that undergo several transformation.


Introduction
Aryldicarbonylcyclopentadienyliron complexes [CpFe(CO) 2 Ar] are important as typical 18-electron organometallics [1], reagents in organic synthesis [2], and functional organic materials [3][4][5]. Despite their importance, there had been few reports of a concise and general synthesis of [CpFe(CO) 2 Ar] [6-OPEN ACCESS 10]. Recently we reported catalytic reactions for the synthesis of [CpFe(CO) 2 Ar]. Our first report showed that transmetalation between [CpFe(CO) 2 I] and arylmagnesium reagents proceeds smoothly under palladium catalysis [11]. However, the scope of the reaction is limited due to the high reactivity of arylmagnesium reagents. Subsequently, we reported the use of arylzinc or arylboron reagents for palladium-catalyzed arylation reactions of [CpFe(CO) 2 I], taking advantage of the mild reactivities of these reagents [12]. The reactions with arylzinc or arylboron reagents showed excellent functional group compatibility and allowed us to prepare a wide range of [CpFe(CO) 2 Ar]. Here we report the full details of the improved method and the utility of the products.

Reactions of [CpFe(CO) 2 I] with Arylzinc Reagents
We initially examined the reaction of [CpFe(CO) 2 I] (1) with commercially available PhZnI (Aldrich) in the presence of 0.25 mol% of palladium acetate and 0.50 mol% of trans-N,N,N',N'-tetramethyl-1,2-cyclohexanediamine ( Table 1, entry 1). The reaction proceeded to yield the corresponding product 2a in only 46% yield. A 29% of starting material 1 was recovered, and dimer 3 was detected as the only identifiable byproduct. We then tried to use the more reactive arylzinc reagent reported by Knochel [13]. Treatment of iodobenzene with zinc powder in the presence of lithium chloride at 50 °C for 24 h afforded PhZnI•LiCl. The reaction with PhZnI•LiCl was indeed successful, affording 2a quantitatively (90% isolated yield) (entry 2). It is worth noting that the reaction of 1 with PhZnI•LiCl proceeded less efficiently in the absence of the palladium catalyst (25 °C, 30 min) to provide 2a in only 52% yield. The use of Knochel's arylzinc reagent was indispensable. The yield of 2a was low when we used a phenylzinc reagent prepared by mixing commercially supplied LiCl-free phenylzinc iodide and LiCl just prior to use (entry 3). The complexation of PhZnI with LiCl would not proceed efficiently at ambient temperature, but it proceeded to near completion upon heating a mixture of PhZnI and LiCl at 50 °C for 24 h to ensure sufficient reactivity (entry 4). The use of diphenylzinc prepared from ZnCl 2 and 2 equiv of PhMgBr was also effective (entry 5), although we could not prepare functionalized diarylzinc by this method. The scope of arylzinc reagents is wide, as shown in Table 2. Although the reaction of 1 with sterically demanding aryl Grignard reagent had failed previously [11], the present method allowed us to introduce aryl groups bearing a substituent at the 2 position (entries 2, 3, and 6). The electronic nature of arylzinc reagents had little effect on the yield of arylirons (entries 4-10). The modest reactivity of organozinc reagents opened the way for facile preparation of aryliron complexes having bromo, cyano, and ethoxycarbonyl groups (entries 7-10). However, the reaction with 2ethoxycarbonylphenylzinc reagent or 4-acetylphenylzinc reagent resulted in recovery of 1 (entries 11 and 12). Thienyliron complex 2m was obtained in high yield, while no reaction took place with 3pyridylzinc reagent (entries 13 and 14). A mechanism similar to the conventional cross-coupling reaction would operate [14] in the phenylation, i.e., oxidative addition of 1 to palladium that generates [Cp(CO) 2 Fe-Pd-I], transmetalation with PhZnI•LiCl, and reductive elimination to yield 2a.

Reactions of [CpFe(CO) 2 I] with Triphenylindium or -Aluminum
Triphenylindium, prepared from InCl 3 and 3 equiv. of PhMgBr, could transfer the phenyl group to 1 under similar conditions ( Table 3, entry 1) [15]. The amount of Ph 3 In could be reduced to 0.50 equiv (entry 2). However, the yield of 2a was modest when 0.33 equiv of Ph 3 In was used (entry 3). Thus, two of the three phenyl groups on indium would be efficiently transferred [16]. The reaction of 1 with triphenylaluminum prepared from AlCl 3 and 3 equiv of PhMgBr afforded 2a sluggishly (entry 4).

Reactions of [CpFe(CO) 2 I] with Arylboronic Acids
Initially, a number of attempts to perform Suzuki-type arylation of 1 with phenylboronic acid failed to afford the corresponding aryliron 2a when various phosphine ligands, amine ligands and bases were screened. We then found that bulky N-heterocyclic carbenes were good ligands. Especially, a combination of a palladium complex PEPPSI and cesium carbonate proved to provide marked improvement [17,18] (Table 4, entry 1). A variety of arylboronic acids underwent the arylation with high efficiency. In case that the arylation was not efficient enough, the addition of copper(I) iodide promoted the reactions (entries 5,9,11,12,14) [19][20][21][22]. We assume that arylcopper species generated from CuI and arylboronic acids would undergo more efficient transmetalation with the iodopalladium intermediate [23,24]. Notably, the combination of PEPPSI and copper(I) iodide allowed the synthesis of 2k (entry 12), which could not be prepared from the corresponding arylzinc reagent. Styrene derivative 2r was prepared from 4-vinylphenylboronic acid in high yield (entries 13 and 14). Unfortunately, the reactions with arylboronic acids having a hydroxy or amino group were sluggish (entries 15 and 16).

Scheme 1.
(Vinylphenyl)iron 2r underwent ruthenium-catalyzed metathesis to expand the diversity of available aryliron complexes (Table 7) [28,29]. Self-metathesis of 2r afforded (E)-stilbene derivative 2w in excellent yield (entry 1). A cross-metathesis reaction of 2r with 3 equiv of ethyl acrylate proceeded smoothly (entry 2). A cross-metathesis reaction with 1-octene or allyltrimethylsilane required a large excess of the alkene to achieve reasonable efficiency (entries 3 and 4). All the reactions proceeded with exclusive E selectivity. The aryl-iron bonds were tolerant under the metathesis conditions.

Chemicals
Unless otherwise noted, materials obtained from commercial suppliers were used without further purification. THF was purchased from Kanto Chemical Co., stored under argon, and used as-is. Dioxane was obtained from Wako Pure Chemicals Co., and stored over slices of sodium. Palladium acetate, cesium carbonate, ceric ammonium nitrate, and copper(I) iodide were obtained from Wako Pure Chemicals Co. PEPPSI and 2nd generation Grubbs catalyst were purchased from Aldrich.
Arylzinc iodide-lithium chloride complexes were prepared according to the literature [13] and stored under argon. [CpFe(CO) 2 I] was prepared according to the literature [11].
Typical Procedure for Arylation with Arylzinc Reagents (Table 2, trans-N,N,N',N'-tetramethyl-1,2-cyclohexanediamine (0.050 M THF solution, 0.050 mL, 0.0025 mmol), and phenylzinc iodide-lithium chloride complex (0.66 M THF solution, 1.14 mL, 0.75 mmol) were sequentially added at 0 °C. After the mixture was stirred for 15 min, a saturated ammonium chloride solution (1 mL) was added, and the product was extracted with ethyl acetate (10 mL × 3). Combined organic layer was passed through a pad of anhydrous sodium sulfate/Florisil and concentrated. 1 H-NMR analysis of the crude product by using 1,1,2,2-tetrabromoethane as an internal standard indicated that 2a was quantitatively formed. The crude oil was purified in air on silica gel by using carbon disulfide as an eluent to yield 2a (114 mg, 0.45 mmol, 90%). (Table 3,  were sequentially added at 0 °C. After the mixture was stirred for 1 h, a saturated ammonium chloride solution (0.6 mL) was added, and the product was extracted with ethyl acetate (10 mL × 3). Combined organic layer was passed through a pad of anhydrous sodium sulfate/Florisil and concentrated. 1 H-NMR analysis of the crude product by using 1,1,2,2tetrabromoethane as an internal standard indicated that 2a was obtained in 92% yield. Chromatographic purification using carbon disulfide as an eluent yielded 2a (66 mg, 0.26 mmol, 87%). (Table 4,

Conclusions
The palladium-catalyzed arylation of [CpFe(CO) 2 I] with arylzinc or arylboron reagents offers an efficient method for the synthesis of various functionalized iron complexes. Triphenylindium transfers the phenyl groups under palladium catalysis to arylate [CpFe(CO) 2 I]. The functionalized aryliron complexes [CpFe(CO) 2 Ar] undergo carbon-carbon bond formations with cleaving the carbon-iron bonds as well as functional group transformations without cleaving the carbon-iron bonds. The iron complexes thus synthesized can find many applications in material chemistry as well as in organic synthesis.