Cyclopropenium cation studies

The cyclopropenium cation (45, M = C) is a well established aromatic system with a high aromatic stabilization energy of 58.7 kcalmol-1 [equation 18, M = C, at B3LYP/6-311++G(2d,2p)]70. What happens when one carbon atom is substituted by silicon At HF/STO-2G the monosilacyclopropenium cation, 46, has a rather delocalized structure with r(C-C) = 1.393 A, r(C-Si) = 1.722 A and with CSiC and SiCC bond angles of 46.3° and 66.9°, respectively413. However, according to equation 19 which measures the effect of delocalization, 46 is destabilized by 11 kcalmol-1 (HF/3-21G//HF/STO-2G) while 45, M = C is stabilized by 36 kcalmol-1. Unfortunately, this study used a very low computational level and should be repeated using more reliable methods. 

The radical cation derived from bic.yclopropenyl (129) is of interest a) because it is the only benzene isomer with any degree of conformational mobility, b) as a potential intermediate in the rearrangement of prismane radical cation (cf., the 2B2 state, Scheme 12), and c) as a potential adduct between cyclopropenium cation and cyclopropenyl radical. It has proved practical to study a simple (3,3 -dimethyl-) derivative (131) of this species, because this precaution eliminates the prototropic rearrangement to the conjugated isomer. 

The expansion of the concept to encompass cyclic electron delocalization or homoaromaticity occurred in the late 1950s. In 1956 Applequist and Roberts pointed out that the cyclobutenyl cation resembles the cyclopropenium cation . Doering and colleagues suggested that the cycloheptatriene carboxylic acids could be regarded as planar pseudoaromatic type structures with a homoconjugative interaction between C(l) and C(6) . Based on the results of solvolytic studies on the bicyclo[3.1. OJhexyl system, Winstein set out the general concept of homoaromaticity in 1959 ... 

In this context, we mention the formation of a derivative of 61 by rearrangement of 3,3 -dimethylbicyclopropenyl (62). The electron-transfer chemistry of bicyclo-propenyl was of interest, because its radical cation might be an intermediate in the rearrangement of prismane (vide infra), and also as a potential adduct between cyclopropenium cation and cyclopropenyl radical. For reasons of practicality the 3,3 -dimethyl-derivative, 62, was chosen for study because it is not subject to prototropic rearrangements. 

Data on the radical cations are much more definitive because the parent is well established as a fragment formed by the photoionization of benzene and its isomers, and its hetero derivatives such as pyridine and the three diazines. (This ion, its formation and thermochemistry, is reviewed in Ref. 76.) Unequivocally, the most stable C4H4 isomer is the radical cation of methylenecyclopropene. This last species is perhaps more explicitly and sensibly described as the -CH 2 derivative of cyclopropenium cation and so renamed (cyclopropenio)methyl radical (15). From these studies, the heat of formation of this... 

The interaction of transition metal complexes with cyclopropenium cations 2C has been widely studied [48-50]. Besides the expected if (an example of if ligation has also been observed by Nixon et al. with phosphirenylium H [24 c])... 

A natural extension of the above studies is to charged aromatic molecules containing silicon. The potential for aromaticity in the silacyclopentadienyl anion 101 and the silacyclopropenium cation 102, both formally possessing an aromatic (4n + 2) -electron network, was studied by Gordon and coworkers2463. 101 is isoelectronic with the cyclopentadienyl anion and 102 with the cyclopropenium cation, both well established aromatic systems with high delocalization energies. 

During recent studies of the upper atmosphere of Titan - the largest moon of Saturn - the Cassani spacecraft detected numerous carbocationic species. In particular, mass spectrum studies detected significant quantities of cyclopropenium cation (35) and its methylated analogs. The formation of these ions has been studied and a mechanism... 

Freeman used DFT methods (B3LYP/6-31G ) to study the carbene analogues 120—122 of the iso-electronic homoaromatic cyclopropenium cations 1, 5 (vide supra), and 124. Isodesmic reactions (e.g.. 

Aspects of electronic theory like aromaticity (Figure 4) (and studies of orbital symmetry control, see later) have been probed through the synthesis of molecules like cyclobutadiene 44, 45 ), bridged armulenes ( 46), the cyclopropenium cation (47), and the cyclooctadiene dianion 48). 

The 1,2,3-tri-f-butylcyclopropenium cation is so stable that the perchlorate salt can be recrystallized from water. An X-ray study of triphenylcyclopropenium perchlorate has verified the existence of the carbocation as a discrete ion. ° Quantitative estimation of the stability of the unsubstituted cyclopropenium ion can be made in terms of its pXjj+ value of —7.4, which is intermediate between such highly stabilized ions as triphenylmethyl cation and the M-(4-methoxyphenyl)methyl cation. (See Section 4.4.1 for the definition of pXr+). An HF/6-31G MO calculation on the following isodesmic reaction ... 

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