Aromatic Stabilized Cations and Dications

If a carbocation or a dication at the same time is also a Hiickeloid An + 2)jt aromatic system, resonance can result in substantial stabilization. The simplest 2jt aromatic system is the Breslow s cyclopropenium ion 206.434 439 Recently, electronic and infrared spectra of the parent ion cyclo-C3H3+ (206, R = H) in neon matrices440 and the X-ray characterization of the tris(trimethylsilyl) derivative were reported.441 The destabilizing effect of the silyl groups was found to be significantly smaller than in vinyl cations. The ion was computed to be more stable than the parent cyclopropenium ion by 31.4 kcal mol1 [MP3(fc)/6-31 lG //6-31G +ZPVE level]. The alkynylcy-clopropenylium ions 207 have been reported recently.442 

Among 6ji aromatic systems, tropylium ion 213455 456 and numerous substituted derivatives have been prepared and characterized (214,457215,458216,459 and 217459). The aromatic stabilization energy of tropylium ion 213 was found to be 

Crystallographic characterization460 of cation 215 revealed that the seven-mem-bered ring adopts a shallow boat conformation whereas the phenyl substituents possess an average dihedral angle of 80° relative to the plane of the tropylium skeleton. Since the phenyl rings can adopt either clockwise or anticlockwise orientations, cation 215 is chiral. 

Alkylazuelenes are protonated at C( 1) or C(3) position to form the stable azulenium cations 220,464 which can be viewed as vinyl-substituted tropylium ions [Eq. (3.52)]. 

The highly reactive bridged homoazulene could be easily protonated to yield the homotropylium ion 221 [Eq. (3.53)].465 

---

FIGURE 2 Aromatically stabilized cations and dications and some bridgehead dications. 

---