Cyclopentadiene cation formation

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

Among the earliest examples of symmetrical bifunctional radical cations, the distonic trimethylene species (103) invoked by Williams and coworkers [293, 296, 297] are stabilized solely by hyperconjugation. The main rationale for their formation would be the relief of ring strain. On the other hand, the non-vertical radical cations 137 derived from cyclopentadiene dimers [386-389] are favored by two elements of allylic stabilization. This radical cation has three eonformat-... 

Formation of stable anhydro-bases. Compounds containing methylene groups activated by both a cationic ring and another electron-withdrawing group easily form stable anhydro-bases, e.g., 673 674, 675 676. Stabilization can also be achieved by utilization of the aromatic character of the cyclopentadiene anion or the pyrrole anion compounds of types 677 (Z = NR, O, S) and 679 readily lose protons to give the anhydro-bases, e.g., 678. 

A mechanistically relevant dimerization of the propargyl alcohol derivative 69 resulting in 70 was also promoted by a cationic complex 68 (Scheme 4.25) [58]. The reaction was considered to start with the regioselective formation of the ruthena-cyclopentadiene 71 from 68 and 69, and the subsequent migration of the hydroxy... 

This dichotomy has been rigorously established by both theoretical methods (semi-empirical and ab initio calculations) and by experimental measurements for both cation radical and carbocation formation (Scheme 32) [66]. The kinetics of aminium salt-induced cation radical Diels-Alder reactions of these substrates were then studied, and an excellent correlation with the Hammett a values was observed for both the enol ethers and the enol sulfides. The reactions of the aryl propenyl ethers involved 2,3-dimethyl-l,3-butadiene as the dienic component [64], while the reactions of the aryl vinyl sulfides involved cyclopentadiene as the dienic component (Scheme 33) [65]. 

The El mass spectra of methyl-substituted cyclopentadienes were studied by Harrison and coworkers" and their fragmentation behaviour was found to be very similar to that of the isomeric cyclohexadienes. Major fragmentation paths were suggested to lead to protonated alkylbenzenes such as benzenium (CeKv+j and toluenium (C7H9+) ions. Obviously, formation of antiaromatic cyclopentadienyl cations is circumvented however, other isomers may also be formed along with ihe (most stable) arenium-type fragment ions (see below). Open-chain 1,3,5-hexatriene isomers were also found to give similar El mass spectra. 

A related rhodium catalyzed enantioselective reductive coupling of acetylene to N arylsulfonyl imines leads to the formation of (Z) dienyl allylic amines (Scheme 1.28) [105]. The scope of the reaction is comparable to that demonstrated for the analogous iridium catalyzed process. The reaction between the acetylene and rhodium leads to the oxidative dimerization of acetylene to form a cationic rhoda cyclopentadiene that then reacts with the imine to generate the product after the protolytic cleavage and reductive elimination. 

Cationic copper(II) complex 37 derived from a chiral bis(imine) ligand has also been shown to be an effective catalyst for reactions between cyclopentadiene and acylated thiazolidine-2-thione dienophiles, albeit with slightly lower se-lectivities than for the bis(oxazoline) complex 31 (Scheme 30) [93]. The bis(2,6-dichlorophenylimine) was found to be optimal among a number of electron-rich and -poor aryl imines screened. The reaction exhibits a positive non-linear effect which suggests that the minor ligand enantiomer can be sequestered by the formation of a catalytically less active (l ,l )/(S,S)Cu(II) dimer. 

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