Aromatics five-membered carbocycles

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

The chemistry of oligounsaturated five-membered carbocycles has fascinated chemists for more than 100 years, since Thiele published the first synthesis of the aromatic cyclopentadienyl (Cp) anion 1 [1], In the last couple of decades, knowledge about fully unsatu-... 

Draw one canonical form for each of the following aromatic compounds a three-membered carbocyclic cation with 2 electrons (n=0, 3 p sub-orbitals) a five-membered carbocyclic anion with 6 electrons (n= 1, 5 p sub-orbitals) and a six-membered neutral ring containing one nitrogen atom and five carbon atoms (n= 1, 6 p sub-orbitals). 

Intramolecular arylation of G-H bonds gives cyclic aromatic compounds. In this intramolecular arylation, the carbon-palladium cr-bond is first formed by the oxidative addition of Pd(0) species and then the resulting electrophilic Pd(n) species undergoes the intramolecular G-H bond activation leading to the formation of the palladacycle, which finally affords the cyclic aromatic compounds via reductive elimination.87 For example, the fluoroanthene derivative is formed by the palladium-catalyzed reaction of the binaphthyl triflate, as shown in Scheme 8.88 This type of intramolecular arylation is applied to the construction of five- and six-membered carbocyclic and heterocyclic systems.89 89 89 ... 

In many ways, the electron-rich five-membered aromatic heterocycles behave very much like carbocyclic aromatic compounds when it comes to lithiation. Lithiation a to O or S of furan and thiophene is straightforward (Scheme 130) . The usual selection of orf/io-directing groups allows lithiation at other positions and some examples... 

We will take a semiempirical approach using numerous molecules, models, assumptions, and estimates rather than doing new calorimetric experiments and/or quantum chemical calculations. Indeed, we will also test what is probably the simplest assumption - that (4n + 2) n electrons found within a conjugated ring species is expected to result in enhanced stability and that this compound is called aromatic. We will consider the dihydroindene (indane) skeleton composed of a benzene ring fused to a nonaromatic five-membered ring that lacks additional double bonds, and will use this carbocyclic hydrocarbon with X = Y = Z = CH2 as a paradigm for many heterocyclic derivatives for which the possible aromaticity is of relevance to the current chapter. Similarly we use indene with -X-Y- = -CH=CH-, Z = CH2 for a variety of unsaturated heterocycles of interest here. 

Aromatic protons of five-membered heterocyclic systems absorb in the same region as the protons of carbocyclic analogs, namely, 6.9to 8.5 S (1.5-3.1 r). Differences, however, between chemical shifts of the protons located at different positions of the heterocyclic rings are greater than those in benzene derivatives. The differences between the chemical shifts of the a and /3 protons in furan, thiophene, and selenophene are 1.05,20 0.12,21 and 0.57 ppm,20,22 respectively the difference is least for thiophene, thus confirming its higher aromaticity as compared with furan or selenophene. 

The Pd-catalysed aerobic 2- -2- -2-cycloaddition of 1,6-diynes and acrylates produced polysubstituted five-membered aromatic carbocycles/heterocycles in good to... 

As an aromatic compound, thiophene is stabilised by resonance energy - actual values for the stabilisations of five-membered aromatic heterocycles (Fig. 2 kJ mol ) vary according to the assumptions made, but are always in the same relative order benzene (150), thiophene (122), pyrrole, (90), and furan (68) -thiophene is the most aromatic of the five-membered heterocyclic trio, i.e. it is the least like a diene. In its reactions, too, thiophene behaves very much like a carbocyclic aromatic compound, exceptions being associated with the presence of the heteroatom. 

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