Aromatic systems cyclobutadiene

In this and similar compounds the acetylene bond is supposed to donate only two jt-electrons to the conjugated system while the other jt-bond is located in the plane of the molecule and does not participate in the conjugation. Consequently, this compound satisfies the Hiickel rule for = 4. It indeed possesses aromatic properties. Anti-aromaticism. When a cyclic polyene system is studied it is important to know whether this system is nonaromatic, i.e.not stabilized by conjugation and sufficiently reactive due to the internal tension and other causes, or destabilized by conjugation, i.e. the cyclic delocalization increases the total energy of the system. In the latter case the molecule is called anti-aromatic. Here are typical examples of anti-aromatic systems cyclobutadiene, a cyclopropenyl anion, a cyclopentadi-enyl cation, and others. 

Catenated Organic Compounds of the Group IV Elements, 4,1 Conjugate Addition of Grignard Reagents to Aromatic Systems, 1, 221 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365 Diene-Iron Carbonyl Complexes, 1, 1... 

Secondly, the carbon framework holding the exocyclic double bonds could be extended. This is demonstrated by naphtharadialene 5, a highly reactive intermediate which has been generated by thermal dehydrochlorination from either the tetrachloride 178 or its isomer 179106. Radialene 5 has not been detected as such in these eliminations rather, its temporary formation was inferred from the isolation of the thermolysis product 180 which was isolated in 15% yield (equation 25). Formally, 5 may also be regarded as an [8]radialene into whose center an ethylene unit has been inserted. In principle, other center units—cyclobutadiene, suitable aromatic systems—may be introduced in this manner, thus generating a plethora of novel radialene structures. 

Common examples of systems often mistaken as being aromatic (because of their alternating double and single bonds) are cyclobutadiene and cyclo-octatetraene (shown in Figure 6-9). In the case of cyclobutadiene, 4n + 2 = 4, giving n = 0.5, while for cyclooctatetraene, 4n + 2 = 8, so that n = 1.5. In these two compounds, n is not an integer, so these systems are anti-aromatic (nonaromatic). Anti-aromatic systems (non-Hilckel systems) are less stable than aromatic or normal systems. 

Benzoquinones owe their unusual properties as a,/3-unsaturated ketones to the ease by which they are transformed to stable aromatic systems. How would these properties change if the quinone were derived from nonaromatic structures, such as cyclobutadiene, cyclooctatetraene, or pentalene There is no final answer to this question because few such substances have been prepared, the best known so far being the mono- and diphenylcyclobulenediones ... 

In a series of classic studies, Pettit et al. reported the synthesis of (cyclobutadiene)iron tricarbonyl together with a variety of electrophilic substitution reactions of this aromatic system (Scheme S).25-27... 

Minkin VI, Glukhovtsev MN, Simkin B Ya (1994) Aromaticity and antiaromaticity electronic and structural aspects. Wiley, New York, pp 157-161. (b) Krogh-Jespersen K, Schleyer PvR, Pople JA, Cremer D (1978) J Am Chem Soc 100 4301. (c) The cyclobutadiene dianion, another potentially aromatic system, has been prepared Ishii K, Kobayashi N, Matsuo T, Tanaka M, Sekiguchi A (2001) J Am Chem Soc 123 5356... 

To explain the increase in the rate of the cyclobutene opening 6.292 —> 6.293, we need to remember that the conrotatory pathway will have a Mobius-like aromatic transition structure, not the anti-aromatic Hiickel cyclobutadiene that we saw in Fig. 1.38. We have not seen the energies for this system expressed in 8 terms, nor can we do it easily here, but the numbers are in Fig. 6.42, where we can see that a... 

Clearly, neither cyclobutadiene nor (even plane) cyclooctatetraene can be aromatic systems since they possess An p -electrons. 

Examination of the cyclobutadiene system indicates that it possesses four 7c-electrons and is thus an unstable 4 system. Cyclobutadiene itself only exists at very low temperatures, though some of its derivatives are stable to some extent at room temperature. Cyclobutadiene is a rectangular diene. Loss of two electrons through the departure of two chloride ions from the 3,4-dichlorocyclobutene derivative creates a 2jc-electron aromatic system, the square, stable cyclobutenyl dication (Scheme 1.2). 

The mechanism of these substitution reactions can be readily rationalized in a manner which completely parallels the accepted electrophilic mechanism of benzene and other aromatic systems. The electrophile, R", adds to the cyclobutadiene ligand to produce the 7r-allyl-Fe(CO)3 cationic intermediate (XVI) loss of a proton from this intermediate generates the substituted cyclobutadiene -Fe(CO)3 complex. We have previously isolated salts of the 7r-allyl-iron tricarbonyl cation (XVII), as well... 

For monocyclic conjugated polyenes, high stabilization is found for systems with (4n + 2) TT electrons but not for systems with (4n) tt electrons. The relationship is formulated as Hiickel s rule, which states that completely conjugated planar hydrocarbons are strongly stabilized (aromatic) when they have 4n -I- 2) tt electrons. Benzene (6 tt electrons) is aromatic but cyclobutadiene (4 tt electrons) and cyclooc-tatetraene (8 tt electrons) are not. 

The size of the K-system chosen has important implication on the structural and functional aspects of metal binding. To explore the size effect calculations were performed on the cation-ir complexes of Li+ and Mg + with the Jt-face of linear and cyclic unsaturated hydrocarbons [45]. In the case of the acyclic Jt-systems, we started with the simplest system, e.g. ethylene followed by buta-1,3-diene, hexa-l,3,5-triene, and octa-1,3,5, 7-tetraene with 2, 3 and 4, conjugated jt units, respectively. These linear systems with two and more number of jt units can have various conformations wherein the jt units can have cis, trans or a combination of both cis and trans orientations. Similarly for cyclic systems cyclobutadiene, benzene, cyclooctateraene, naphthalene, anthracene, phenanthrene and naphthacene have been included. Thus a wide range of sizes for aromatic systems have been covered. 

Whereas aromatic systems are defined by a positive resonance energy, anti-aromatic systems are characterized by a negative resonance energy. As a rule, antiaromatic compounds are unstable and contain 4n 7i-electrons in a cyclic planar, completely conjugated arrangement. Cyclobutadiene belongs to this category and is stable only in a solid matrix at very low temperatures (20 K). 

A rare example of an isolable cyclobutadiene which is not stabilized by conjugation to an aromatic system is (815), wWch has a rectangular four-membered ring. ... 

It suggests that it is not the size of the ring but the number of electrons present in it determines whether a molecule would be aromatic or antiaromatic. In fact the molecules with An+ 2) n electrons are aromatic whereas with (An, 0) n electrons are antiaromatic. Thus, benzene, cyclopropenyl cation, cyclobutadiene dication (or dianion), cyclopentadie-nyl anion, tropylium ion, cyclooctatetraene dication (or dianion), etc. possess (4 + 2) ti electrons and hence aromatic whereas cyclobutadiene, cyclopentadienyl cation, cycloheptatrienyl anion, cyclooctatetraene (non-planar) etc. have An n electrons which make them antiaromatic . Systems like [10] annulene are forced to adopt a nonplanar conformation due to transannular interaction between two hydrogen atoms and hence their aromaticity gets reduced even if they have (An + 2)n electrons. On the other hand the steric constraints in systems like cyclooctatetraene force it to adopt a tube-like non-planar conformation which in turn reduces its antiaromaticity. Various derivatives of benzene like phenol, toluene, aniline, nitrobenzene etc. are also aromatic where the benzene ring and the n sextet are preserved. In homoaromatic " systems, like cyclooctatrienyl cation, delocalization does not extend over the whole molecule. 

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