Cyclopentadiene reaction: with basic metals

Reaction of Cyclopentadiene with Basic Metal Compounds (Method 2)... 

The general equation for the reaction of basic metal compounds with cyclopentadiene is... 

Early-transition-metal amides are extremely reactive toward protic adds and are therefore useful precursors to a variety of M-X complexes (in which X is an anionic ligand less basic than an amide) by reaction of the amido complex with H-X (Equation 4.13). Common exchange processes that favor elimination of amine involve the reaction with alcohols, amides, amidines, and guanidines. However, weakly acidic hydrocarbons, such as cyclopentadiene derivatives, also react to form Cp complexes (Equation 4.14). ... 

The strong acidity of cyclopentadiene, estimated as approximately = 17 (3), was first recognized by Thiele, who took advantage of this property in the preparation of potassium cyclopentadienide from potassium metal and cyclopentadiene (4). While direct reaction with cyclopentadiene is limited in scope to the elements of Groups lA and IIA, to their strongly basic compounds, and to a few other elements, it is the only approach for the synthesis of the ionic cyclopentadienides of elements of Groups lA and IIA. Direct reaction of cyclopentadiene is also applicable to the preparation of certain compounds of the (C5H5) M(CO), class from metal carbonyls and is often the preferred method. 

Solvents of high dielectric constant with good solvent properties for the reacting basic metal salt are most satisfactory media for the reaction of cyclopentadiene with metal salts in the presence of base. Most frequently used solvents are tetrahydrofuran and 1,2-dimethoxyethane less frequently used solvents are dioxane, ethanol, ammonia, and diethylamine. 

Diels-Alder reaction of 2-bromoacrolein and cyclopentadiene using 10 mol% of titanium catalyst 74 gave the synthetically versatile (R)-bromoaldehyde adduct 75 in 94% yield, 67 1 exo. endo diastereoselectivity, and 93% ee. The absolute stereochemical outcome of the reaction is consistent with the proposed transition state assembly 76 in which the dienophile coordinates at the axial site of the metal, proximal to the indane moiety through Ji-attractive interactions. In this complex, the 7t-basic indole and the Ji-acidic dienophile can assume a parallel orientation facilitated by the octahedral geometry of the transition metal. The aldehyde would then react through a preferential s-cis conformation (Scheme 17.27).54... 

In the course of discovery research, reexamination of the benzyne chemistry led to an easily and safely operated benzyne reaction (Scheme 4).38 Solvent was found to affect the rate of halogen-metal exchange processes and the basicity of RLi reagents toward cyclopentadiene, a compound with a low pKa ( 14). Noncoordinating solvents... 

Bis(cyclopentadienyl)iron derivatives, ferrocenes, are remarkably stable against heat and air and undergo various kinds of chemical reactions. They are usually prepared by the reaction of FeCh with an alkali metal salt of cyclopentadienyl or cyclopentadiene in the presence of base [51]. Although ferrocene derivatives basically have chemical reactivities similar to those of aromatic compounds, they have found only limited applications to organic synthesis so far. However, because chiral ferrocenylphosphines are capable of having both planar chirality and an asymmetry in the side chain in their rigid framework, they have been used recently in a number of asymmetric reactions. In this section, synthesis and developments of monocyclopentadienyl complexes and ferrocenylphosphines are described. The general chemistry of ferrocenes and half metallocene complexes is reviewed elsewhere [52-53]. 

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