Antiaromatic compounds, and

Borole (IV in Figure 14) has been used to synthesize a plethora of sandwich and half-sandwich stmctures it is the bora analog of cyclopentadiene. Borole is an antiaromatic system see Antiaromatic Compound), and only the blue pentaphenyl derivative has been prepared. The general route to borole complexes consists of complexation under dehydrogenation of 2- or 3-borolenes, which are prepared from [Mg(butadiene)]x and RBX2. Thus, the heating of either isomer of borolene with Co2(CO)g produces [(borole)Co(CO)2]2 (47) (equation 60) from which further cobalt borole complexes are prepared (equation 61). CpCo(borole) (48) can be made from (47) by Cp transfer from the labile 20-electron nickelocene (equation 61). Pyrolysis of (48) at 160-180 °C gives a triple-decker complex (49) in nearly quantitative yield, which in turn can be cleaved by Cp into (48) and the bis(borole)cobaltate anion (50 ) (Scheme 34). 

Yet, compared to the other x-systems, the acyclic linear polyenes, and the cyclic oligoenes (i.e., the annulenes), cross-conjugated molecules have lived in the shadows so far. Acyclic linear polyenes are encountered widely in fatty acid, polyketide, and terpenoid natural product domains and the ultimate polyene, polyacetylene is a conductive polymer that was the focus of the 2000 Chemistry Nobel Prize. The annulenes played crucial roles in the development of practical and theoretical organic chemistry (aromatic and antiaromatic compounds), and serve as pivotal bulk chemicals in industry (benzene, toluene, the xylenes, styrene, etc.). 

Shetty, S. Kar, R. Kanhere, D. G. Pal, S. Intercluster reactivity of metalloaromatic and antiaromatic compounds and their applications in molecular electronics A theoretical investigation. J. Phys. Chem. A. 2006,110, 252-256. 

Since antiaromaticity is related to aromaticity, it should be defined by many of the same criteria (31). That is, antiaromatic species should be less stable in comparison to a localized reference system, should demonstrate paratropic shifts in the H NMR spectrum, should have positive NICS values, and positive values of magnetic susceptibility exaltation, A. While the presence of enhanced bond length alternation has been considered as evidence of antiaromaticity (31), the deformation of square cyclobutadiene to rectangular cyclobutadiene to reduce its antiaromaticity suggests that the lack of bond length alternation is also a characteristic of antiaromatic compounds. 

Anti-aromatic 1,2-dithiins 179 display properties opposite to those of 1,4-dithiins 180, whose dications show aromatic stabilization. Unlike other antiaromatic compounds, the 1,2-dithiin derivatives, with eight jr-electrons (such as 181 and 182), appear in... 

A property associated with compounds that are destabilized by a closed loop of electrons. Antiaromatic compounds are typically planar and contain An electrons, where n is a positive integer, in overlapping parallel p orbitals. These compounds also have a paramagnetic ring current. Thus, protons on the outside of the ring will exhibit an upheld NMR chemical shift. See Aromatic... 

The contributions of the first three types are practically local in character they are close in value for two protons with similar structural environment, such as the ethylenic- and aromatic-type protons. It is only the last term in Eq. (35) that defines the values of the chemical shifts characteristic of aromatic or antiaromatic compounds. 

The material presented in Section II warrants, apparently, the conclusion that the main test of aromaticity and antiaromaticity is represented by the energetic criterion realizable within the framework of various schemes for calculating resonance energies. In most cases it correlates with structural and magnetic criteria moreover, it often accords well with a manifestation of numerous properties of compounds, which, being regarded as attributes of aromaticity, make its very concept substantially broader. Indeed, the concept of aromaticity claims an increasing number of types of compounds and requires a more and more sophisticated classification. 

The scheme for treating these subjects is as follows. We compare typical aromatic and antiaromatic compounds. Similar to the comparison between benzene and cyclobutadiene, we concern ourselves here with pyridine (34) and azete (57). Such an approach provides, apart from other advantages,... 

The MNDO calculations on sila-, germa-, and stannacyclopentadienyli-denes have shown that whereas for cyclopentadienylidene (272) the energies of the antiaromatic 47t- and the aromatic 6ir-electron structures are close in value (89UK1067), in the (273)-(275) series the 67r-electron structures are quite noticeably destabilized (Table XXIII). Unlike (272), the electronic ground state of compounds (273)—(275) correspond to minima on the PES. These results point to the diminished role of antiaromatic destabilization in the 47r-electron structure (273)—(275), as opposed to (272). It should therefore be expected that these molecules would be more stable than (272). This has indeed been confirmed by our calculation on the heats of the isodesmic reaction (85) (Table XXIII). 

Dithiins are the only biomolecules found in nature that are formally nonaromatic living organisms tend to avoid synthesizing antiaromatic compounds because of their thermodynamic and kinetic instability. Ab initio calculations of... 

X and Y -CH=CH-). This suggests that the geometry distortion around the C-F bonds in [27](X and Y CH=CH-) when compared with the [27](X = Y = H) geometry, increases the corresponding As values between the fluorine lone pair orbitals and the (C-F) antibonding orbitals. It should be recalled that the d(F-F) distance in [27](X and Y -CH=CH-) is notably larger than in [27](X = Y = H). There is also about 2 Hz difference between the C-C contributions. Probably, this difference comes from the difference in aromaticity between these two compounds in fact while [27](X = Y = H) is an aromatic compound, [27](X and Y CH=CH-) is an antiaromatic compound. This seems to indicate that the outlier condition of [27]... 

Sumonja, M. Tidwell, T. T., /. Am. Chem. Soc., 1997, 119, 2371-2375, and references therein). It has the properties expected of an antiaromatic compound. It is very difficult to generate 1 under solvolytic (SnI) conditions from the precursor iodide 2. Nevertheless, 2 undergoes SN2 reaction unusually rapidly (2 reacts with bromide ion about 10 times faster than 3). 

The most obvious compound in which to look for a closed loop of four electrons is cyclobutadiene (44).135 Hiickel s rule predicts no aromatic character here, since 4 is not a number of the form 4n + 2. There is a long history of attempts to prepare this compound and its simple derivatives, and, as we shall see, the evidence fully bears out Hiickel s prediction— cyclobutadienes display none of the characteristics that would lead us to call them aromatic. More surprisingly, there is evidence that a closed loop of four electrons is actually ami-aromatic.1 If such compounds simply lacked aromaticity, we would expect them to be about as stable as similar nonaromatic compounds, but both theory and experiment show that they are much less stable.137 An antiaromatic compound may be defined as a compound that is destabilized by a closed loop of electrons. 

Cyclazinylium salts (233), which following MO considerations should be unstable antiaromatic compounds, have not been described so far. Topologically equivalent -systems containing two additional electrons, however, appear to be closed shell systems and some compounds corroborating this expectation have been prepared. 

In contrast to aromatic molecules which have An + 2 n electrons, cyclobutadiene and cyclooctatetraene do not have An + 2 7r electrons and are not aromatic. In fact, diese molecules, which contain An jt electrons (n is an integer), are less stable than die planar model compounds and are termed antiaromatic. Bodi of these molecules adopt shapes that minimize interactions of die n orbitals. 

The facially perturbed enantiopure (.S, .S )-2-(p-tolylsulfinyl)norborncno-/7-bcnzoquin-ones (119), undergo asymmetric Diels-Alder additions with cyclopentadiene to yield the four possible adducts (120) and (121). The endo-syn cycloadducts (121) can be used in the synthesis of the cage compound garudane (122) (Scheme 44).234 The antiaromatic compound 1,4-biphenylenequinone (123) has been synthesized and trapped by Diels-Alder reaction with cyclopentadiene (Scheme 45).235 The 4 + 2-cycloadditions of 4-methylene-5-(bromomethylene)-4,5-dihydrothiazole with 2- and 3-bromonaphtha-quinones are highly regiospecific.236... 

It is clear, as Katritzky et al. [7, 8] and ourselves [9] have pointed out, that aromaticity cannot be described with a single parameter. It is possible to select a parameter and classify aromatic compounds according to it and this approach is correct if one bears in mind that the aromaticity scale thus obtained is valid only for the chosen parameter. One of the most successful is Schleyer s NICS (nuclear independent chemical shifts) [10-12], a criterion we have used to separate aromatic and antiaromatic compounds [13], Cyranski et al. [14] as well as Sadlej-Sosnowska [15] have tried, with moderate success, to find an agreement between these different points of view. 

Oligounsaturated Five-Membered Carbocycles -Aromatic and Antiaromatic Compounds in the Same Family... 

I 2 Oiigounsaturated Five-Membered Carbocycies - Aromatic and Antiaromatic Compounds... 

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