Dienyl cations

Allenyl cations 1 are a stabilized form of vinyl cations1-3 in which the /1-carbon atom of the vinylic structure is part of the substituent which effects the stabilization of the ion via its electron-donating ability. This leads to a resonance hybrid having formally the alkynyl cation structure 2. Allenyl cations should be distinguished from the allenyl substituted carbenium ions 3 formulated as the mesomeric structures of the vinyl cations 4 (dienyl cations) stabilized by an w-vinyl group (equation 1). 

The GIAO-MP2/TZP calculated 13C NMR chemical shifts of the cyclopropylidene substituted dienyl cation 27 show for almost all carbon positions larger deviations from the experimental shifts than the other cations 22-26. The GIAO-MP2/TZP method overestimates the influence of cr-delocalization of the positive charge into the cyclopropane subunit on the chemical shifts. Electron correlation corrections for cyclopropylidenemethyl cations such as 27 and 28 are too large to be adequately described by the GIAO-MP2 perturbation theory method and higher hierarchies of approximations such as coupled cluster models are required to rectify the problem. 

This indicates that the deviations are due to systematic errors, for example deficiencies of the applied methods and basis sets. DFT-based methods, such as GIAO/DFT calculations are known to overestimate paramagnetic contributions to the chemical shielding. This results, for critical cases with small HOMO/LUMO separations, in overly deshielded competed chemical shifts. Notorious examples for these deficiencies are 29Si or 13C NMR chemical shift computations of silylenes, silylium ions or dienyl cation .(5/-54) Taking into account the deficiencies of the applied method, and bearing in mind very reasonable correlations shown in Figures 4 and 5, the computational results do support the structural characterization of the synthesized vinyl cations. 

The presence of a 4-methoxy substituent on the 2-phenylethyloxyl or 3-phenylpropyl-oxyl side chains radically altered the course of these cyclizations (Scheme 4). 31a and 31b afforded the spiro-fused ring systems 32 and 33 in 26 and 69% yields, respectively, as the only cyclization products. With this substituent, cyclization onto the activated ipso positions was favoured over direct attack, even where the strained transition state for Atj -5 cyclization of 31a to 32 was involved. Demethylation of the intermediate spirocyclohexa-dienyl cation is favoured over rearrangement in these cases. Kikugawa and coworkers effected the formation of 32 (82%) and 33 (39%) with reverse efficiencies using AgaCOs in TFA . 

However, comparison of the l3C NMR spectrum of 75 with the model compounds raises in our minds serious questions as to the validity of these claims. Apart from the l3C chemical shifts of the C(l)/C(5) carbon resonances (137.0 ppm) the rest of the resonances are in positions typical of a dienyl cation and an isolated double bond [C(6) and C(7)]. The large spread in the l3C chemical shifts reported for 75 indicated a very uneven charge distribution in the ion. 

An a-cyclopropylvinyl cation, l-cyclopropyl-3-methylbuta-l,2-dienyl cation 38, has been prepared by Siehl by ionizing the corresponding acetylenic alcohol with SbF5 (equation 32)68. 

The dienylic cations 44 with cyclopropyl and phenyl groups were also prepared and characterized by the protonation of respective fulvenes74. Other cyclopropyl substituted allyl cations include acyclic 1,3- and 1,4-disubstituted allyl cations 45 and 46. The charge in these cations is localized mainly on the carbon adjacent to the cyclopropyl group. The rearrangement of these cations at higher temperatures was also studied76. 

When the 13 C NMR chemical shift of the Cy carbon in dienyl cations 349, 350, 352, 353, 355 and 356 is used as a probe for the ability of /J-substituents to hyperconjugatively donate electrons to the formally empty 2p(7t) orbital of the vinyl cation C , the order in Scheme 35 is obtained ... 

Eq. (3.59)]. and 13C NMR data are consistent with cation 242 with the cationic center observed at 813C+173.74 (Ar = Ph) and 813C+ 171.66 (Ar = 4-FC6H4). This is accounted for by the transformation of the initially formed antiaromatic cyclopenta-dienyl cation 241 through 1,2-aryl migration. 

Removal of the 0-substituted Fp group can be achieved by conversion into the cationic alkene-Fp complex using Ph3CPF6 and subsequent treatment with iodide, bromide or acetonitrile. Oxidative cleavage with ceric ammonium nitrate in methanol provides the methyl esters via carbon monoxide insertion followed by demetallation. The [3 + 2]-cydoaddition has been successfully applied to the synthesis of hydroazulenes (Scheme 1.11) [34]. This remarkable reaction takes advantage of the specific nucleophilic and electrophilic properties of V-allyl-, cationic t 5-dienyl-, cationic ri2-alkene- and ti4-diene-iron complexes, respectively. 

Another group of unstable carbanions are those with antiaromatic character (Scheme 5.71). Thus, cyclopropenyl anions or oxycyclobutadienes, generated by deprotonation of cyclopropenes or cyclobutenones, respectively, will be highly reactive and will tend to undergo unexpected side reactions. Similarly, cyclopentenediones are difficult to deprotonate and alkylate, because the intermediate enolates are electronically related to cyclopentadienone and thus to the antiaromatic cyclopenta-dienyl cation. 

Recent applications of the Nazarov reaction, the cyclization of a 3-hydroxyphenyl-penta-l,4-dienyl cation, were reviewed.142 Tandem processes and asymmetric cycliza-tions were a particular focus of attention. Irradiation of (40) in aqueous base results in regioselective and stereoselective formation of (42).143 The allylic cation (41) is proposed as the key intermediate. A computational investigation was performed into the cofacial intermolecular n-n orbital interaction between n-conjugated main chains (C H +2) and allylic cations C3H54".144... 

Protonation of cyclooctatetreneiron tricarbonyl by strong acids produces a bicyclic dienyl cation (II) (64), rather than the direct proton adduct (III) as initially proposed (221). 

Dienyl cations and dienyl anions both undergo electrocyclic ring closure—a nice example occurs when cyclooctadiene is deprotonated with butyllithium. 

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