Cations cyclopropenyl

Other aromatic ions include cyclopropenyl cation (two rr electrons) and cycloocta tetraene dianion (ten tt electrons)... 

The tropylium and the cyclopropenyl cations are stabilized aromatic systems. These ions are arumatic according to Hiickel s rule, with the cyclopropeniiun ion having two n electrons and the tropyliiun ion six (see Section 9.3). Both ring systems are planar and possess cyclic conjugation, as is required for aromaticity. 

Alkylation of enamines with epoxides or acetoxybromoalkanes provided intermediates for cyclic enol ethers (668) and branched chain sugars were obtained by enamine alkylation (669). Sodium enolates of vinylogous amides underwent carbon and nitrogen methylation (570), while vicinal endiamines formed bis-quaternary amonium salts (647). Reactions of enamines with a cyclopropenyl cation gave alkylated imonium products (57/), and 2-benzylidene-3-methylbenzothiazoline was shown to undergo enamine alkylation and acylation (572). A cyclic enamine was alkylated with methylbromoacetate and the product reduced with sodium borohydride to the key intermediate in a synthesis of the quebrachamine skeleton (57i). 

The di-n-propyl cyclopropenyl cation failed to photolyze either in aqueous acid or organic solvents, with or without sensitizers. A possible explanation in the discrepancy between the triphenyl system and this one lies in the calculated energy differences between the cations and their corresponding radicals. In the triphenyl system this energy difference is 0-5 3 or 16 kcal/mol, while in the di-n-propyl case it is 1 00)3 or 32 kcal/ mol, based on calculated delocalization energies for the two species. 

It is strong evidence for Hiickel s rule that 59 and 60 are not aromatic while the cyclopropenyl cation (55) and the cyclopentadienyl anion (39) are, since simple resonance theory predicts no difference between 59 and 55 or 60 and 39 (the same number of equivalent canonical forms can be drawn for 59 as for 55 and for 60 as for 39). 

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) ... 

A particularly interesting case of carbocation stabilisation occurs with Hiickel 4n + 2 systems when n = 0, i.e. cyclic systems with 2-7re (p. 18). Thus derivatives of 1,2,3-tripropylcyclopropene (8) are found to yield ion pairs containing the corresponding cyclopropenyl cation (9) extremely readily,... 

Tapia, O., Andres, J. and Cardenas, R. Transition structure for the hydride transfer reaction from formate anion to cyclopropenyl cation a simple theoretical model for the reaction catalyzed by formate dehydrogenase, Chem. Phys. Lett, 189 (1992), 395-400... 

M. R. Wasielewski, R. Breslow. Thermodynamic Measurements on Unsubstituted Cyclopropenyl Radical and Anion, and Derivatives, by Second Harmonic Alternating Current Voltammetry of Cyclopropenyl Cations. J. Am. Chem. Soc. 1976, 98, 4222—4229. 

Structures that are also aromatic are the cyclopropenyl cation (2 jt electrons n = 0) and the cyclopentadienyl anion (6 n electrons n = 1). Although we do not wish to pursue these examples further, they are representative of systems where the number of jr electrons is not the same as the number of carbon atoms in the ring. 

In 1997, Jemmis, Schleyer and coworkers studied the structure and energetics of lithi-ated cyclopropenyl cation and their acyclic isomers using ab initio MO (HF/6-31G ) and density functional theory (DFT, B3LYP/6-31G ) methods . Successive lithiation results in... 

Simple HUckel calculations predict the resonance energy of the benzocyclopropenyl cation (273) (3.65 P) to be higher than that of the tropylium ion (274) (2.99 P) and that of the cyclopropenyl cation (275) (2.00 P). Recent ab initio calculations at the 3.2 IG level attribute the same stability to 273 and 275 with respect to their hydrocarbon precursors The enthalpy change in going from 1 to... 

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. ... 

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