Carbocations cyclopentadienyl

In contrast, the cyclopentadienyl carbocation, which has four pi electrons and is antiaromatic, is quite unstable. Thus, 5-iodo-l,3-cyclopentadiene is unreactive under conditions in which iodocyclopentane reacts rapidly by an SN1 mechanism. 

Although live equivalent resonance structures can be drawn for all three species, Huckel s rule predicts that only the six-ir-electron anion. shouldbe aromatic. The four-TT-electron cyclopentadienyl carbocation and tlic- five-7r-electron cyclopentadienyl radical are predicted to be unstable and antiaromatic. 

The synthesis of a cyclopentadienyl-annulated imidazolium salt 282 was accomplished through a Nazarov-type cyclization as a key transformation. This annulation step was affected by toluenesulfonic acid via protonation-dehydration of the tertiary allylic alcohol 278 to form a three-centered carbocation, which was then annulated, in an electrophilic fashion, onto the C-4 position of the imidazole to form 279. The formation of the alcohol 278 was achieved via lithiation of imidazole 276 and then quenching with ketone 277 to give the 1,2-addition product (Scheme 70) <2005TL6847>. 

One fascinating aspect of ferrocene chemistry is its extraordinary ability to stabilize carbocations that formally should have their positive charge in a position adjacent to the cyclopentadienyl ring (a-ferrocenylalkyl carbocations). Such cations are so stable that they form quantitatively from appropriate precursors (e.g., alcohols) on treatment with acid and many of them remain unchanged in solution for days and... 

The reactivity of water with both carbanion and carbocation intermediates is well known and recognised, but until recently it was generally believed that water is inert towards free radicals. Some years ago, Cuerva et al. by chance observed that tertiary radicals were reduced effectively in the presence of bis(cyclopentadienyl)titanium(III) chloride and water. Now the authors have solid evidence to show that water really acts as a complete hydrogen atom source rather than a simple proton donor for radical reductions mediated by Ti(III) and, presumably, other metals that react by single electron transfer (Scheme 8.7).6... 

Both the carbanion and carbocations are stable provided they contain [An+ 2) K electrons. For example, cyclopentadienyl anion, cyclopropenium cation, and tropyhum cation exhibit unusual stability. Stable carbanions do, however, exist. In 1984 Ohnstead presented the lithium crown ether salt of the diphenylmethyl carbanion from diphenylmethane, butyllithium, and 12-crown-4 at low temperatures. Addition of n-butyUithium to triphenyhnethane in THF at low temperatures followed by 12-crown-4 resulted in a red solution and the salt complex precipitated at —20°C. The central C-C bond lengths are 145 pm with the phenyl ring propelled at an average angle of 31.2° (Scheme 3.11). 

Figure 4.26 shows the FfMOs for cyclopropenyl carbocation, square planar cyclobutadiene, cyclopentadienyl anion, benzene, cycloheptatrienyl carbocation, and planar (Ds/,) cyclooctatetraene. If we place electrons into the molecular orbitals of each species according to the aufbau principle, we notice an important relationship between the stability of the systems and the... 

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