Cations tropylium

Write resonance structures for tropylium cation sufficient to... 

Although this cleavage is probably driven by the stability of benzyl cation, evidence has been obtained suggesting that tropylium cation, formed by reanangement of benzyl cation, is actually the species responsible for the peak. 

Electrostatic potential map for tropylium cation shows most positively-charged regions (in blue) and less positively-charged regions (in red). 

The framework for the tropylium cation, C7H7f, is a seven-membered ring of carbon atoms with a hydrogen atom... 

Quasi-aromatic structures are also known in which the stabilised cyclic species is an ion, e.g. the cycloheptatrienyl (tropylium) cation (15, cf. p. 106), the cyclopentadienyl anion (16, cf. p. 275), both of which have 67te (n = 1), and even more surprisingly the cyclopropenyl cation (17, cf. p. 106) which has 2ne (n = 0) ... 

New synthetic transformations are highly dependent on the dynamics of the contact ion pair, as well as reactivity of the individual radical ions. For example, the electron-transfer paradigm is most efficient with those organic donors yielding highly unstable cation radicals that undergo rapid unimolecular reactions. Thus, the hexamethyl(Dewar)benzene cation radical that is generated either via CT activation of the [D, A] complex with tropylium cation,74... 

While benzene was the first aromatic system studied, the formulation of HtickePs rule and the theory behind it created an impetus to prepare non-benzenoid species such as the tropylium cation and cyclopentadienyl anion that also obeyed Huckel s rule to see if these species were also aromatic. This required that the properties of aromatic compounds be defined. 

Cathodic Reduction of the Tropylium Cation in Methylene Dichloride, P.H. Plesch and A. Stasko, Journal of the Chemical Society (B), 1971, 2052-2053. 

Fig. 4 Charge-transfer absorption bands of the EDA complexes of tropylium cation with various donors (A) benzenes, (B) naphthalenes and (C) anthracenes.

A further group of nonbenzenoid aromatics is the series of odd-membered cations and anions such as cycloprope-nium (14) and tropylium cations (15) as well as cyclopentadienyl (16) and cyclononatetracenyl anions (17). Regarding the arguments for the properties of Hiickel-like 4 + 2 jr-systems, all these molecules should be energetically stabilized. Obviously, this is not fulfilled in all cases. The tropylium cation (15) can be reduced in a one-electron step to the tropyl radical even at A = +0.06 V vs. SCE [85, 86]. The radical is unstable and rapidly dimerizes to bitropyl. The hep-taphenyl tropylium radical is stable on the voltammetric timescale, but decays... 

The photo-oxidation of the aryl-substituted cycloheptatrienes 7-(/ -methoxy-phenyl)cycloheptatriene and 7-, 1- and 3-(/ -dimethylaminophenyl)cycloheptatrienes to the corresponding radical cations in de-aerated acetonitrile solution was accomplished by electron transfer to the electronically excited acceptors 9,10-dicyanoanthracene, iV-methylquinolinium perchlorate, iV-methylacridinium perchlorate and l,T-dimethyl-4,4-bipyridinium dichloride. In the case of l- p-methoxyphenyl)cycloheptatriene (62), deprotonation of the radical cation occurs successfully, compared with back electron transfer, to give a cycloheptatrienyl radical (63) which undergoes a self-reaction forming a bitropyl. If the photooxidation is done in air-saturated acetonitrile solution containing HBF4 and one of the acceptors, the tropylium cation is formed. Back electron transfer dominates in the / -dimethylaminocycloheptatrienes and the formation of the cycloheptatrienyl radical is prevented. 

Mechanistic insight into the PET reaction of aryl-substituted tropylium cations to aryl-substituted bitropyls via reduction as well as the oxidative counterpart... 

The tropylium cation (274) first observed 1891 and rediscovered in 1957 is perfectly stable and isolable. Cyclopropenyl cations have been observed in solution a long time ago, but 273 remained elusive until very recently. Benzocyclo-propene (1) reacts with triphenylfluoroborate via hydride transfer some 5 times less rapidly than cycloheptatriene. The reaction of deuterated 1 exhibits a kinetic isotope effect of 7.0. However, only a low yield of benzaldehyde (277), the expected hydrolysis product of 273, could be isolated from the reaction mixture. ... 

The MP2/4-31G/STO-3G calculations show (83TL1863) the tropylium cation to be 112.7 kcal/mol more stable than the hypothetical bicyclo [4.1.0]hepta-2,4-dien-7-yl cation. This difference, attributed to the effects of aromatic stabilization, should be compared with the value of AE (172) - (124) = 44.8 kcal/mol (at the same computational level). Therefore, the energy of aromatic stabilization of borepin comes to about 40% of that of the tropylium cation. The 6-31G /6-31G calculated AE (172) -(124) = 37.6 kcal/mol, whereas nonaromatic cycloheptatriene is a mere... 

Other li acetylides Li-C=C-R with R = hexyl [21] or benzylether dendrons [22, 23] (up to the fourth generation) have also been attached to (Figure 3.3), and various different electrophiles have been used to complete the reaction with the intermediate li-fuUeride (Scheme 3.2 and Figure 3.3). Besides the protonation, alkyl-, benzyl-, cycloheptatrienyl-, benzoyl- or vinylether-derivatives or formaldehyde and dichloro-acetylene were used as electrophiles [12,20]. Most of these electrophiles are attached to the anion in the expected C-2 position. The 1,4-adducts are available by quenching the anion with the tropylium cation or benzoyl chloride [12]. The fuUerene anion can be stabilized by introduction of benzylether dendrons. The lifetimes of the anions change with the size of the dendrons [22]. 

The first reaction provides a route for the reduction of alkyl halides since the carbo-cation (isopropyl, in Rl) may be prepared from action of AICI3 on the corresponding alkyl halide. Reactions of the type Rl are also important in the process, catalytic cracking, in the manufacture of gasoline. They have also been studied in mass spectro-metric experiments [235]. Reaction R2 is one route to the preparation of carbocations under stable ion conditions. Reaction R3 is employed in the laboratory synthesis of the tropylium cation. Reaction R4, the (crossed) Cannizzaro reaction, is unusual in that it takes place under strongly basic conditions. The oxy dianion is an intermediate in the reaction of concentrated hydroxide with the aldehyde, R HO. None of R1, R2, or R3 may have hydrogen atoms a to the carbonyl groups. Formaldehyde (R1 = H) is readily... 

Use orbital interaction analysis to suggest a reason that tropylium cation C7H7 is such a stable cation. This may be done in either of two ways by considering the interaction of a simple carbocation with 1,3,5-hexatriene or the interaction of an allyl cation with butadiene, both held in the 7-membered planar ring geometry of tropylium. In either case, attention must be paid to orbital symmetry. 

At present, quite a number of derivatives of analogues of the tropylium cation containing two sulfur atoms are known. Both 1,2-85 and 1,3-dithiolium cations86,87 (69 and 70, respectively) have been synthesized recently the possible existence of the cation 70 had been... 

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