7t systems

T system, the PPP a—-tt method (133). The -tt and a net charges and bond orders of thiophene and thiazole are compared in Table 1-5. Whatever the method considered the variation of the indices occurs in the same sense when passing from thiophene to thiazole the replacement in the 3-position of a carbon atom by a nitrogen induces... 

We will address this issue further in Chapter 10, where the polar effects of the substituents on both the c and n electrons will be considered. For the case of electrophilic aromatic substitution, where the energetics of interaction of an approaching electrophile with the 7t system determines both the rate of reaction and position of substitution, simple resonance arguments are extremely useful. 

The process is exothermic, suggesting that the phenoxy radical is particularly stable. Display the spin density surface for phenoxy radical. Is the unpaired electron localized or delocalized over several centers Is the unpaired electron in the a or 7t system Draw appropriate Lewis structures that account for your data. 

Examine the highest-occupied molecular orbital (HOMO) of singlet methylene. Where is the pair of electrons, inplane or perpendicular to the plane Next, examine the electrostatic potential map. Where is the molecule most electron rich, in the o or the 7t system Where is the most electron poor Next, display the corresponding map for triplet methylene. Which molecule would you expect to be the better nucleophile The better electrophile Explain. Experimentally, one state of methylene shows both electrophilic and nucleophilic chemistry, while the other state exhibits chemistry typical of radicals. Which state does which Elaborate. 

Examine and describe both the highest-occupied and lowest-occupied molecular orbitals (HOMO and LUMO, respectively) oiground state acetone. On which atom(s) is the HOMO primarily concentrated Is it in the o system or in the 7t system Repeat your analysis for the LUMO. 

The mechanism of organolithium addition to naphthyl oxazolines is believed to occur via initial complexation of the alkyllithium reagent to the oxazoline nitrogen atom and the methyl ether to form chelated intermediate 17. Addition of the alkyl group to the arena 7t-system affords azaenolate 18, which undergoes reaction with an electrophile on the opposite face of the alkyl group to provide the observed product 4. The chelating methyl... 

Some additional derivatives containing extended 7t-systems in place of the benzene nucleus are naphthalene, anthracene (2,3-Ac) and phenanthrene (9,10-Phc). They also belong to the phthalocyanine family. For the naphthalene system two types of macrocyclcs, the 1,2-naph-thalocyanine (1,2-Nc) and the 2,3-naphthalocyanine (2,3-Nc), are known. 

In the cyclization of the corresponding cis-epoxides, with the aim of obtaining the corresponding cis-2,3-disubstituted tetrahydropyrans, a similar trend was observed. For these systems, however, the 6-endo pathway was less favored, which was ascribed to difficulties in attaining a TS conformation that would allow for maximum stabilization of the developing p-orbital with the adjacent 7t-system. Alternatively, palladium-catalyzed cyclization of the tetrabutylammonium alkoxide derived from 33b results in the corresponding ris-2,3-disubstituted tetrahydro-pyran in excellent yield and selectivity (90%, dr >99 1), while the ris-epoxide gives stereoisomer 37b (86%, dr 98 2) [112]. 

Surprisingly, the 7t-system geometry in a substrate has a notable influence in the enzymatic aminolysis of esters. The reaction of diethyl fumarate with different amines or ammonia in the presence of CALB led to the corresponding trans-amidoesters with good isolated yields, but in the absence of enzyme, a high percentage of the corresponding Michael adduct is obtained (Scheme 7.9). Enzymatic aminolysis of diethyl maleate led to the recovery of the same a, P-unsaturated amidoester, diethyl fumarate, and diethyl maleate. The explanation of these results can be rationalized via a previous Michael/retro-Michael type isomerization of diethyl maleate to fumarate, before the enzymatic reaction takes place. In conclusion, diethylmaleate is not an adequate substrate for this enzymatic aminolysis reaction [23]. 

It has been contended that here too, as with the benzene ring (Ref 6), the geometry is forced upon allylic systems by the a framework, and not the 7t system Shaik, S.S. Hiberty, P.C. Ohanessian, G. Lefour, J. Nouv. J. Chim., 1985, 9, 385. It has also been suggested, on the basis of ab initio calculations, that while the allyl cation has significant resonance stabilization, the allyl anion has little stabilization Wiberg, K.B. Breneman, C.M. LePage, T.J. J. Am. Chem. Soc., 1990, 112, 61. 

In Chapter 3, it was mentioned that positive ions can form addition complexes with 7T systems. Since the initial step of electrophilic substitution involves attack by a positive ion on an aromatic ring, it has been suggested that such a complex, called a % complex (represented as 10), is formed first and then is converted to the arenium ion 11. Stable solutions of arenium ions or 7t complexes (e.g., with Br2, l2> picric... 

In all of the above discussion, we have assumed that a given molecule forms both the new s bonds from the same face of the 7t system. This manner of bond formation, called suprafacial, is certainly most reasonable and almost always takes place. The subscript s is used to designate this geometry, and a normal Diels-Alder reaction would be called a [7t2s+,i4s] cycloaddition (the subscript n indicates that n electrons are involved in the cycloaddition). However, we can conceive of another approach in which the newly forming bonds of the diene lie on opposite faces of the n system, that is, they point in opposite directions. 

The logical continuation of the stepladder strategy outlined above for minimizing the mutual distorsion of adjacent main chain phenylene units was the incorporation of the complete PPP-parent chromophore into the network of a completely planar ladder polymer. The complete flattening of the conjugated 7T-system by bridging of all the subunits should then lead to maximum conjuga-tive interaction. As with the PTHP 11 systems, alkyl or alkoxy side chains should lead to solubilization of the polymers. 

Plastoquinone, whose line structure appears in the margin, has ten double bonds. Unlike the bonds in the tail of retinal, however, the bonds in the long tail of plastoquinone are not delocalized because s p -hybridized carbon atoms separate them. The delocalized 7t system of plastoquinone is its planar ring of six carbon atoms, with two of the carbon atoms double-bonded to outer oxygen atoms. 

We have only just started to explore empirical access to FMO parameters based on these similarities 52). Recently, others have reported empirical equations for calculating IP s and EA s for a variety of Ji-bonded systems 64). This approach used a large number of parameters for the underlying rc-system, heteroatom substitution, and the substituents on the 7t-system. However, we aim at calculating FMO parameters from fundamental atomic data while taking due account of the bond structure of a molecule. 

While the di- and triphenylmethanide anions are resonance stabilized, there is a tendency for isolating the 7T-system with a systematic localization of the double bonds this discrete arrangement of single and double bonds has been observed and studied in detail in alkali bisdipyridylmethanides.43-45... 

Solomon GC, Andrews DQ, Van Duyne RP, Ratner MA (2009) Electron transport through conjugated molecules when the 7t system only tells part of the story. ChemPhysChem 10(l) 257-264... 

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