Cyclic electron delocalization

Here liberties have been taken with the Robinson symbol Instead of restricting its use to a sextet of electrons organic chemists have come to adopt it as an all purpose sym bol for cyclic electron delocalization... 

In 1991 Arduengo and co-workers42 reported the isolation and characterization of the first stable carbene, l,3-di(l-adamantyl)-2,3-dihydroimidazol-2-ylidene. Since that time many substituted nucleophilic carbenes have been prepared.43 From ab initio studies it can be shown that cyclic electron delocalization is partly responsible for the extraordinary stability of 2,3-dihydroimidazol-2-ylidenes.44... 

Carbon and nitrogen are the most common elements from the first row of the periodic table to form aromatic compounds, characterized by cyclic electron delocalization. The bonding of these elements in the conjugated systems shows a large variety. Carbon can be a divalent (carbene), sp carbon with one jT-electron, but also sp carbon can be part of hyperconjugate aromatic systems, provided that it is properly substituted. The pyrrole- and pyridine-type nitrogens also allow the formation of cyclic electron delocalization in a large variety of aromatic systems. 

The reason for this behavior in the case of the double-bonded structures ( A -type phosphorus) is the easy dimerization of the P=C bond. Tricoordinate planar (or nearly planar) phosphorus ( B - and C -type bonding) can be stabilized by repyramidalization when the cyclic electron delocalization is disturbed or lost (e.g., in a chemical reaction). The fine balance between these energetic effects cannot easily be predicted by using analogies or other simple models. Such predictions, however, can be made by using the sophisticated methods of computational chemistry, leaving the field of the chemistry of the aromatic phosphorus compounds an interesting research area with unexpected results in the future. 

This conclusion, nevertheless, should not be considered categorical but it points to the necessity of careful consideration of the correlation between the AEdis value and the part of it that relates to cyclic electron delocalization. It has been shown by use of TRE calculations of aromatic benzene and antiaromatic cyclobutadiene molecules that in the case of benzene the distortion into a Kekule-type structure is characterized by a change of the aromatic cyclic Tr-electron delocalization energy in an opposite direction... 

The theory of molecular structure based on the topology of molecular charge distribution, developed by Bader and co-workers (83MI2 85ACR9), enables certain features to be revealed that are characteristic of the systems with aromatic cyclic electron delocalization. To describe the structure of a molecule, it is necessary to determine the number and kind of critical points in its electronic charge distribution, i.e., the points where for the gradient vector of the charge density the condition Vp = 0 is fulfilled. 

Such a substitution may involve substantial changes in the structure and disturb the cyclic electron delocalization. Thus, 1,4-dihydropyrazine (25), as judged from X-ray data on its N-substituted derivatives [83AG(E)171] as well as from the results of MNDO [84JMS( 109)277] and ab initio (6-31G) (88JOC2127) calculations, has a slightly bent boat structure with pyramidalized nitrogen atoms. 

In the case of 18, the various C—C bond distances in the basal seven-membered ring are consistent with there being a significant degree of cyclic electron delocalization and the ion being classified as homoaromatic. The C(1),C(7) bond order of 18 was estimated to be 0.56 on the basis of the measured internuclear distance91. It is interesting to note that the bond distances found for 18 are substantially different from those reported for the proto-nated cyclopropyl ketones discussed above. 

A semiquantitative study by Nazzal and Mueller-Westerhoff131 of the extent of cyclic electron delocalization ( aromaticity ) within each ligand ring in both classes was based on the NMR chemical shift differences observable in mesityl derivatives of these compounds. If a delocalized cyclic structure carries a mesityl substituent, the methyl groups in the ortho position will experience an upheld shift, while those in the para position do not. Their chemical shift difference A<5 can be used as a measure of delocalization. Compounds with Ad > 0.30 Hz are considered to be aromatic , i.e. highly delocalized. 

Although the ptt complexes are deeply colored,150 they lack the intense absorption at low energy found in the class of dithiolenes and which is assigned to a n-% transition of the delocalized system. The absence of such a transition in the ptt complexes is not a contradiction of the above results on the cyclic electron delocalization. It rather points out that the dithiolenes show delocalization throughout the entire complex unit as shown in (81), but the delocalization in the ptt complexes apparently involves the two ring systems independently, as shown in (82) the cyclic electron delocalization does involve the metal, but there is no crossover of delocalization from one half of the molecule to the other. 

An increase of stability related to the system without cyclic -electron delocalization... 

Hiickel s rule has been abundantly verified [17] notwithstanding the fact that the SHM, when applied without regard to considerations like the Jahn-Teller effect (see above) incorrectly predicts An species like cyclobutadiene to be triplet diradicals. The Hiickel rule also applies to ions for example, the cyclopropenyl system two n electrons, the cyclopropenyl cation, corresponds to n 0. and is strongly aromatic. Other aromatic species are the cyclopentadienyl anion (six n electrons, n = 1 Hiickel predicted the enhanced acidity of cyclopentadiene) and the cyclohep-tatrienyl cation. Only reasonably planar species can be expected to provide the AO overlap need for cyclic electron delocalization and aromaticity, and care is needed in applying the rule. Electron delocalization and aromaticity within the SHM have recently been revisited [43]. 

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 ... 

It has been suggested that this attenuation of A<5 with substitution is attributable to a reduction in cyclic electron delocalization in the homoaromatic ring and a consequent attenuation in the induced ring current. However, as will be shown later, substitution also results in some fairly major changes in the structure of the homotropenylium ion and the impact of these structural changes on the relative position of the C(8) protons and/or ring current have not been disentangled. 

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