Huckel aromatics

Trifluoromethanesulfonic (triflic) anhydride is commercially available or can be prepared easily by the reaction of triflic acid with phosphorus pentoxide [66] This moderately hygroscopic colorless liquid is a useful reagent for the preparation of various organic derivatives of triflic acid A large variety of organic ionic triflates can be prepared from triflic anhydride A recent example is the preparation of unusual oxo-bridged dicatiomc salts of different types [SS, 89, 90, 91, 92, 93] (equations 38-44) Stabilized dication ether salts of the Huckel aromatic system and some other systems (equations 38 and 39) can be prepared in one step by the... 

The origin of cyclopropenone chemistry goes back to the successful preparation of stable derivatives of the cyclopropenium cation <5 3), the first member of a series of Huckel-aromatic monocyclic carbo-cations possessing a delocalized system of (4n + 2)-7r-electrons. This experimental confirmation of LCAO-MO theory stimulated efforts to prepare other species formally related to cyclopropenium cation by a simple resonance description of electron distribution, namely cyclopropenone 7 and methylene cyclopropene (triafulvene) 8 ... 

One could expect diazadiboretidines to be converted into Huckel aromatic systems either by adding or by subtracting one pair of n-electrons. The addition of two electrons to diazadiboretidines of the type (RBNalkali metals. The dianions [(RBNiBu)2] are stable in solution and can be reconverted into the diazadiboretidines by oxidants. Because they contain six n-electrons, aromatic character may be attributed to the dianions (19). Cyclodimers of the type (R BNR)2 are also readily oxidized, but the adoption of an aromatic dication [(R BNR)2] as a product would be mere speculation at present. 

In this method, the orbital symmetry rules are related to the Huckel aromaticity rule discussed in Chapter 2. Huckel s rule, which states that a cyclic system of electrons is aromatic (hence, stable) when it consists of 4n + 2 electrons, applies of course to molecules in their ground states. In applying the orbital symmetry principle we are not concerned with ground states, but with transition states. In the present method we do not examine the molecular... 

There is, however, an important difference between examples 27 and 41. The later compound forms a Huckel-aromatic orbital system in 41b while the former compound adopts a Mobius orbital system with 4q + 2 electrons, i.e. 27 is Mobius antiaromatic although six electrons participate in cyclic delocalization (see Section III. B). This is in line with a destabilizing resonance energy of 9.9 kcalmol"1 (Table 2) calculated with the MM2ERW method41-42. 

Huckel aromaticity upon reduction, whereas 26b gains aromaticity (4n + 2, n — 3) when converted to its dianion. The effect of substituting the peripheral silyl groups by aromatic groups still needs to be explored for these perethynylated dehydroannulenes. 

For a homoaromatic system, surface delocalizjktion in the cyclopropyl ring is perpendicular to the bridging bond, thus forming a Huckel aromatic electron ensemble which is delocalized in just one part of the bi(poly)cyclic system. 

Dewafr-Huckel aromaticity rule. In a polycyclic alternant hydrocarbon a cycle with (4n + 2) n-centres is aromatic whereas a cycle with An jt-centres is anti-aromatic. 

Huckel aromatics The benzene molecule has the suprafacial topology this means that the TT - electron density in benzene is continuous along the top or bottom face of the molecule. If the transition state for the pericyclic reaction has the same topology, it is said to resemble Huckel topology (Fig. 8.17). 

Day has given a careful account of the relationship between the Woodward-Hoffmann rules and Mobius/HUckel aromaticity, and has defined the terms supra-facial and antarafacial in terms of the nodal structure of the atomic basis functions. His approach makes quite explicit the assumption that the transition state involves a cyclic array of basis functions. Thus the interconversion of prismane (10) and benzene, apparently an allowed (B2g-t- 2s + 2s) process, is in fact forbidden because there are additional unfavourable overlaps across the ring. ... 

It has been shown experimentally (heats of hydrogenation, heats of formation) that there is significant homoaromatic stabilization in cycloheptatrienes such as 1, 7, 8, 9, 12 (in the latter three there is additional classical Huckel aromaticity), as well as in norcaradienes such as 2, 3, 10, 11. In cases where other energetic factors of the pair of valence tautomers 1 2 are very similar, the relative homoaromaticity determines the equilibrium position. ... 

In contrast, cyclobutadiene dianion 38, a. Huckel aromatic compound, was shown calculationally by Schleyer to be — due to coulombic repulsion — far less stable than Y-delocalized spezies with more favorable n charge distributions. This was confirmed by Boche and Thiel Although JS itself could not be prepared until now, phenyl substituted derivatives show no special charge delocalization within the cyclobutadiene ring. 

The Huckel aromaticity of the 2 -phosphinins was supported by x-ray crystal structure analysis combined with ... 

The spectroscopic data of (104) confirm the Huckel aromaticity of this system <88TL535>. The H NMR data of H-5 are comparable with those of phosphinines and nearly identical to those of the corresponding 1,3-azaphosphinines (Table 1) <87TL1093>. 

Considering the results of the theoretical investigations on smaller C clusters summarized in this chapter, it is clear why this statement caused some controversy. Parasuk et al. found the conclusions drawn from the SCF calculations surprising, since they claimed that this molecule, with its (4n -(- 2) Ji-electron system, was a candidate for Huckel aromaticity and expected to adopt the delocalized structure 13 [266]. They confirmed the earlier SCF results by redoing these calculations with larger basis sets, but found the Bjg/, symmetrical form 13 to be most stable at the MP2 level. However, they were faced with considerable symmetry-breaking effects at the SCF level which made it impossible to treat this structure consistently at the MP2 level. 

Tetraphenyl-1,4-dioxin, with SbCl5, forms a blue-violet radical cation. On further oxidation, a green dication is obtained. However, tetraphenyl-1,4-dithiin with SbCl5 yields a violet dication as a hexachloroantimonate. The dications 8 can be regarded as Huckel aromatic 67c-systems, in contrast to the antiaromatic parent compounds 1-3. 

Huckel aromaticity An aromatic annulene composed of a continuous circuit of p -orbitals oriented along the same axis (or plane) and possessing 4/H-2 electrons. 

Huckel aromatic transition structure, while systems with 4 electrons prefer... 

The Mobius-Huckel aromatic Iransition state method... 

Cyclobutadienes. Cyclobutadiene dianion is a Huckel aromatic compound, and evidence has been reported that it can be generated from cis-3,4-dichIoro-cyclobutene with sodium naphthalenide in THF at — 40 °C. The greater instability of this dianion than that of cyclo-octatetraene is probably largely ascribable to ring strain. 

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