Ferrocenyl acetate

Benzylic acetates are unreactive toward organozinc compounds. However, various ferrocenyl acetates, such as 236, react with dialkylzinc halides in the presence of BF3 OEt2 with retention of configuration leading to the chiral ferrocenyl derivatives like 237 (Scheme 68) . 

Another important reaction of XXXIII and of XXXIV consists of their facile conversion to ferrocenyl acetate on treatment with cupric acetate. Ferrocenyl acetate in turn has been hydrolyzed to hydroxyferrocene, the ferrocene analog of phenol (74). 

One of the most fascinating areas of metal-cyclopentadienyl chemistry in recent years concerns the formation and relative stabilities of metal-locenyl-carbonium ions. The ability of iron to stabilize cationic centers in certain ferrocene compounds was noted by Weliky and Gould 243) in 1957. In 1959 Richards and Hill 218y 214, ) reported the results of some kinetic studies relating to the relative rates of solvolysis of metal-locenylmethylcarbinyl acetates. They determined that these solvolyses proceeded via a carbonium ion mechanism and found that the ferrocenyl acetate, for example, solvolyzed nearly seven times faster than did tri-phenylmethyl acetate under the same conditions. 

For instance, organometaUic compounds such as hydrolytically labile chromium-tricarbonyl complexes, which are of interest as chiral auxiliary reagents for asymmetric synthesis [201], were easily resolved by PSL (Scheme 3.11) [202, 203]. A remarkable enhancement in selectivity was obtained when the acyl moiety of the vinyl ester used as acyl donor was varied. This concept was successfully employed for the resolution of 1-ferrocenylethanol, which cannot be well resolved via enzymatic hydrolysis due to the lability of 1-ferrocenyl acetate in aqueous systems [204, 205]. 

Asymmetric palladacyclizations resulting in six-membered rings, for example, 75, were carried out by Burke and Overman.A series of reactions of [Pd(/x-Gl)(G6H4NMe2)]2 with r< < - 2-(diphenylphosphino)ferrocenyl acetic acid and related compounds were carried out. ... 

To overcome this problem the ferrocenyl acetal 20 was developed, in which the acetal functionality directs the formation of a single diastereoisomer during the lithiation and electrophile quench sequence. Subsequent addition of aqueous acid then releases enantiomerically pure 2-substituted ferro-cenealdehyes 21a-d. A further advantage of this method is that 20 and its corresponding enantiomer are synthesised from commercially available (5)-or (i )-l,2,4-butanetriol respectively, thus avoiding the need to perform a classical resolution. 

This tethered ferrocenyl-based Pd complex on MCM-41 (17) was then used for the catalytic amination reaction between cinnamyl acetate and benzylamine (40 °C, THF) [59]. In this case, confinement of the catalyst results in profound changes in regio- and enantioselectivity. When the homogeneous equivalent is used to catalyze the reaction, the straight chained derivative is the sole product. Similar results (only 2% of the branched product) were obtained when the catalyst was tethered to the surface of the non-porous silica Cabosil. When tethered inside the pores of MCM-41 a major change occurred in that now the branched product accounts for about 50% and a change in e.e. from 49% e.e. when anchored to the Cabosil support to +99% when anchored inside the MCM-41 pore could be observed. If the catalyst s chirality was reversed in the MCM-41 immobihzed case, so was the chirality of the product (measured at 93% e.e.) [60]. 

Chinese researchers reported a synthetic route to photochrome 16 (Scheme 7), in which the aldehyde functions are then transformed into cyclic acetals and thioacetals, methylol and dicyanoethylene groups (07T5437, 08T2576). Scheme 7 also gives (at the bottom) symmetrical photochrome 17 (where R are ferrocenyl substituents), which does not exhibit fluorescence in the initial state, but shows fluorescence in the cyclic form and which is also synthesized starting from dialdehyde 16 (08AFM302). 

Addition of water (36) or alcohols (37—39) direcdy to butadiene at 40—100°C produces the corresponding unsaturated alcohols or ethers. Acidic ion exchangers have been used to catalyze these reactions. The yields for these latter reactions are generally very low because of unfavorable thermodynamics. At 50°C addition of acetic acid to butadiene produces the expected butenyl acetate with 60—100% selectivity at butadiene conversions of 50%. The catalysts are ion-exchange resins modified with quaternary ammonium, quaternary phosphonium, and ammonium substituted ferrocenyl ions (40). Addition of amines yields unsaturated alkyl amines. The reaction can be catalyzed by homogeneous catalysts such as Rh[P(C(5H5)3]3Q (41) or heterogeneous catalysts such as MgO and other solid bases (42). 

S-Ketoesters, /3-ketophospbonates, and /3-ketosulfones have been used to alkylate ferrocene to afford the corresponding /3-ferrocenyl-a,/3-unsaturated derivatives in excess triflic acid262 263 [Eq. (5.98)]. The transformations are highly stereoselective, giving exclusively the (El-isomers this was explained by the exo-deprotonation of carbenium ion 71a of more stable conformation. Acetals of formylphosphonates and formylsulfones react in a similar manner. 

Using a chiral 4-dimethylaminopyridine-ferrocenyl catalyst, acyclic silyl ketene acetals react with anhydrides to furnish 1,3-dicarbonyl compounds containing allcarbon quaternary stereocentres in good yield and ee.144 Evidence for dual activation (anhydride -> acylpyridinium, and acetal -> enolate) is presented. 

Scheme 8.21 Fu s planar chiral ferrocenyl PPY-catalyzed C-acetylation of silyl ketene acetals [60, 64].

The Zhou group reported the use of silver acetate and /V,P-ferrocenyl ligand 97 in the azomethine cycloaddition with dimethyl maleate (76) as shown in Scheme 2.27... 

Naturally, it is possible to synthesise a similar ligand system without central chirality and in fact without the unnecessary methylene linker unit. A suitable synthesis starts with planar chiral ferrocenyl aldehyde acetal (see Figure 5.30). Hydrolysis and oxidation of the acetal yields the corresponding carboxylic acid that is transformed into the azide and subsequently turned into the respective primary amine functionalised planar chiral ferrocene. A rather complex reaction sequence involving 5-triazine, bromoacetal-dehyde diethylacetal and boron trifluoride etherate eventually yields the desired doubly ferrocenyl substituted imidazolium salt that can be deprotonated with the usual potassium tert-butylate to the free carbene. The ligand was used to form a variety of palladium(II) carbene complexes with pyridine or a phosphane as coligand. 

Ferrocene derivatives coupled with heterocyclic systems have attracted special attention in recent years because of their interesting organic and inorganic properties. Recently, an efficient and rapid route for the synthesis of 4-aryl-2-ferrocenyl-quinolines 70 has been described by Tu and co-workers [116] through a microwave-assisted MCR of acetylferrocene with an aromatic aldehyde and dimedone in the presence of ammonium acetate in DMF. This novel procedure provides the target hetero-metallic compounds in excellent yields without the need of any purification (Scheme 54). 

---