JT systems

The overall hydroxylation or epoxidation reaction catalyzed by cytochrome P450s involves the insertion of one oxygen atom, derived from molecular oxygen, into a C-H bond or into the Jt-system of an olefin, with the concomitant reduction of the... 

The coordinated quinone methide Jt-system of complex 24 can also undergo cycloaddition (Scheme 3.17). When 24 was reacted with /V-methylmaleimide, a [3+2] cycloaddition took place to give the tricyclic iridium complex 29. The closest example to this unprecedented reactivity pattern is a formal [3 + 2] cycloaddition of /)-quinone methides with alkenes catalyzed by Lewis acids, although in that reaction the QMs serve as electron-poor reagents. 36... 

The benzene molecule in its equilibrium configuration is planar. Its symmetry is described by the point group as shown in Fig. 8-l(c). The delocalized n system is represented there by dotted lines. The six pz orbitals contribute to tbe jt system, as simply described by the Htickel approximation. The reduction r, = B2g Hjg E2u can be found as in the previous examples. 

Scheme 6.276 An intramolecular [3+2] cycloaddition reaction of a push-pull dipole across a heteroaromatic JT-system.

Mejla-Oneto and Padwa have explored intramolecular [3+2] cycloaddition reactions of push-pull dipoles across heteroaromatic jr-systems induced by microwave irradiation [465]. The push-pull dipoles were generated from the rhodium(II)-cata-lyzed reaction of a diazo imide precursor containing a tethered heteroaromatic ring. In the example shown in Scheme 6.276, microwave heating of a solution of the diazo imide precursor in dry benzene in the presence of a catalytic amount of rhodium I) pivalate and 4 A molecular sieves for 2 h at 70 °C produced a transient cyclic carbonyl ylide dipole, which spontaneously underwent cydoaddition across the tethered benzofuran Jt-system to form a pentacyclic structure related to alkaloids of the vindoline type. 

However, the introduction of sterically hindered substituents at the p-car bon atom of nitroalkene (42) completely changes the ring-chain tautomerism of conjugated nitroalkenes. Apparently, steric hindrance caused by two bulky Bu groups in product (42a) (Scheme 3.47) prevents effective conjugation of the jt systems of the C,C double bond and the nitro group, thus causing its deviation from the plane of the C=C bond as a result of which isomer (47a) becomes thermodynamically more favorable. 

There is also evidence that conjugation of the nitronate fragment with jt systems can stabilize acyl nitronates. For example, Perekalin and coworkers (224) isolated a very stable nitronate (63a), which was prepared by acetylation of l,4-dinitrobut-2-ene salts (65) in a mixture with the corresponding C-acylation product (66) (Scheme 3.66), by column chromatography. 

Despite its 10-electron aromatic Jt-system, indolizine apparently participates as an eight-electron system in its reaction with acetylene dicarboxylate, although the process may be stepwise and not concerted. By carrying out the reaction in the presence of a noble metal as catalyst, the initial adduct is converted into an aromatic cyclazine. 

A theoretical study (144) and several experimental studies using selectively deuterated mono-substituted benzenes (145), chlorobenzene (146), and warfarin (147) (Fig. 4.79) provided strong evidence for the stepwise mechanism. Most recently, a theoretical study using density functional calculations reported the same basic conclusion (148). The reaction proceeds by a stepwise mechanism involving initial attack of FeO3"1" on the jt system to form a tetrahedral intermediate (pathway 2). The tetrahedral intermediate then goes... 

The cumulated Jt-system of allenes has been described as consisting of two comparatively unperturbed double bonds with regard to its reactivity towards nucleophiles or electrophiles [10]. Early reports on radical additions to 1,2-dienes, however, already pointed to peculiarities of the allene system concerning its reactivity towards intermediates with unpaired electrons [11-14], It was soon realized that no such correlation between polar and steric substituent effects existed, similar to what had been uncovered for the reaction of radicals with olefins, in order to predict selectivities in radical additions to cumulated dienes [4, 15],... 

The preferred site for radical addition to a cumulated Jt-system of a 1,2-diene depends on (i) the degree of substitution of the allene, (ii) the nature of the attacking radical and (iii) reaction parameters such as temperature and initial reactant concentrations. 

Various predictive methods based on molecular graphs of Jt-systems as described in Section 3 have been critically compared by Klein (Klein et al., 1989) and can be extended to more quantitative treatments. In principle, the effective exchange integrals /ab in the Heisenberg Hamiltonian (4) for the interaction of localized electron spins at sites a and b are calculated as the difference in energies of the high-spin and low-spin states. It was Hoffmann who first tried to calculate the dependence of the M—L—M bond... 

Carbocations have similar electronic structures to carbenes. The P-protonated derivative of phosphinine should also be similar to 23. Indeed, while investigating the proton affinity of 3. the most preferred protonation site was phosphorus and not carbon, whereby the cyclic jt system would be interrupted. ... 

Cation-TT interactions are another important motif, dominated by electrostatic interactions [47] within mainly hydrophobic subsites. Such a stabilizing interaction takes place between cations like ammonium ions and electron-rich jt-systems of neighboring aromatic rings. Both interactions are often not explicitly contained in common force fields or scoring functions. 

In a series of important papers, MacMillan described the alkylation of electron rich aromatic and heteroaromatic nucleophiles with a,P-unsaturated aldehydes, using catalysts based upon the imidazoUdinone scaffold, further establishing the concept and utility of iminium ion activation. In line with the cycloaddition processes described above, the sense of asymmetric induction of these reactions can be rationalised through selective (F)-iminium ion formation between the catalyst and the a,P-unsaturated aldehyde substrate, with the benzyl arm of the catalyst blocking one diastereoface of the reactive Jt-system towards nucleophilic attack (Fig. 3). 

Cleavage of benzyl bonds is faster than cleavage of bonds to the aromatic ring because of overlap, in the transition-state, between the jt-system containing the unpaired electron and the stretching o-bond. Compare the reactivity of 4-... 

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