Cyclopentadienyl salts

Cyclopentadiene reacts with alkaline oxides and hydroxides forming the alkaline cyclopentadienyl salts. Alkaline hydroxides supported on high surface materials such as kiesclguhr and alumina are good absorbents for trace amounts of CPD contaminant in other hydrocarbons (27). 

Perhaps because of inadequate or non-existent back-bonding (p. 923), the only neutral, binary carbonyl so far reported is Ti(CO)g which has been produced by condensation of titanium metal vapour with CO in a matrix of inert gases at 10-15 K, and identified spectroscopically. By contrast, if MCI4 (M = Ti, Zr) in dimethoxy-ethane is reduced with potassium naphthalenide in the presence of a crown ether (to complex the K+) under an atmosphere of CO, [M(CO)g] salts are produced. These not only involve the metals in the exceptionally low formal oxidation state of —2 but are thermally stable up to 200 and 130°C respectively. However, the majority of their carbonyl compounds are stabilized by n-bonded ligands, usually cyclopentadienyl, as in [M(/j5-C5H5)2(CO)2] (Fig. 21.8). 

The lithium salt of 2-(di-wo-propylamino)-l,2-thiaborolide with [( -Cp ) RuC1]4 or [(i -Cp )ZrCl3] yields sandwiches similar to 39 (M = Ru, ZrCl2) (OOOM4935). The same anionic ligand enters a sequence of reactions with dimethylchlorosilane, lithium cyclopentadienyl, lithium di-wo-propylamide, and zirconium(IV) chloride to give sandwich 41. 

Diphosphastibolyl anion reacts with [( -codjMCli] to give 166 (M = Rh, Ir) (97JOM(534)89). The same salt mixed with lithium cyclopentadienyl and C0CI2 gives a mixture of two sandwiches 167 and 168. 

The planar cyclic Pj anion isoelectronic with cyclopentadienyl anion has been prepared in the form of M Pj" salts (M = Li, Na) by Handler et al. [49], The penta-phosphole anion Pj" favors planar geometry [50] while the most stable structure of P/ is square-pyramidal [51], The negatively charged pentamers Sb and Bi are planar rings [52, 53],... 

The fourth chapter gives a comprehensive review about catalyzed hydroamina-tions of carbon carbon multiple bond systems from the beginning of this century to the state-of-the-art today. As was mentioned above, the direct - and whenever possible stereoselective - addition of amines to unsaturated hydrocarbons is one of the shortest routes to produce (chiral) amines. Provided that a catalyst of sufficient activity and stabihty can be found, this heterofunctionalization reaction could compete with classical substitution chemistry and is of high industrial interest. As the authors J. J. Bmnet and D. Neibecker show in their contribution, almost any transition metal salt has been subjected to this reaction and numerous reaction conditions were tested. However, although considerable progress has been made and enantios-electivites of 95% could be reached, all catalytic systems known to date suffer from low activity (TOP < 500 h ) or/and low stability. The most effective systems are represented by some iridium phosphine or cyclopentadienyl samarium complexes. 

In most cases oxidation of uncharged borabenzene complexes produces cations which can only be observed electrochemically. The iron compounds 62 and 63 may serve as an example. Oxidation is fully reversible in rigorously dried CH2C12 but irreversible in more basic solvents such as THF and acetonitrile (62). Preparative oxidation with Ce(IV) salts cleanly produces monosubstituted ferricenium cations 64 (Scheme 10) (66). In contrast to the above mentioned boranediyl extrusions, the substituent at boron is retained here in the newly formed cyclopentadienyl ring. 

A carbon-iron bond is also formed by the reaction of the cyclopropenium salt 185 with dicarbonyl(i/5-cyclopentadienyl)(trimethylsilyl)iron [92], (Scheme 69) In the reaction with benzocyclobutenylidene- 5-cyclopentadienyliron(II) hexafluorophosphate 186, CpFe(CO)2R (R=cyelo-C3H5, CH2-cyclo-C3H5) is converted to the allene and butadiene complexes, 187 and 188, respectively [93]. (Scheme 70)... 

Analogously the ethoxy cation 75 was found to be valuable for the synthesis of calicenes when combined with cyclopentadienyl anions93 Hexaphenylcalicene (100) was prepared by this route from tetraphenylcyclopentadienyl-Li94,76) and the highly polar dicyanocalicene 10195 from the tetramethylammonium salt of dicyano-cyclopentadiene. 

The lithium salt of a substituted cyclopentadienyl anion has been used in reaction with phosphorus trichloride for carbon-phosphorus bond formation.70 The resultant simple displacement product ultimately undergoes dimerization and loss of four (from the dimer) equivalents of HC1 (Equation 4.25). 

Trimethylpyrylium perchlorate is a very versatile and useful starting material. Thus its reaction with cyclopentadienyl-sodium has made 4,6,8-trimethylazulene 12 easily available for general studies of the properties of azulenes 18 and for the synthesis of related compounds.14 In addition, pyrylium salts are readily converted to a variety of pyridine derivatives 9 15 as well as to derivatives of nitrobenzene16 and phenol.9 17,18 It is clear that its value as a starting material is such that it is receiving wide use. 

Loosely bound fj -cyclopentadienyl anions can also serve as the base to deproto-nate imidazolium salts. When chromocene is reacted with an imidazolium chloride in THF the metal precursor loses one molecule of cyclopentadiene to form the 14-electron complex [( 7 -C5H5)Cr(NHC)Cl] [Eq. (13)]. This complex can be further oxidized by CHCI3 to give [( 7 -C5H5)Cr(NHC)Cl2]. This route also works with nickelocene to generate the corresponding [( -C5H5)Ni(NHC)Cl] complex. ... 

Ferrocenyl-based polymers are established as useful materials for the modification of electrodes, as electrochemical biosensors, and as nonlinear optical systems. The redox behavior of ferrocene can be tuned by substituent effects and novel properties can result for example, permethylation of the cyclopentadienyl rings lowers the oxidation potential, and the chaige transfer salt of decamethylfer-rocene with tetracyanocthylene, [FeCpJ]" (TCNE], is a ferromagnet below = 4.8 K, and electrode surfaces modified with a pentamethylferrocene derivative have been used as sensors for cytochrome c These diverse properties have provided an added impetus to studies on ferrocene dendrimers. 

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