Benzynes structure

Dft calculations indicate that the benzo-metallaoxetane structure V is the lowest energy for [CrC6H40]+ but the benzyne structure VIH has the lowest energy for [CrC6Hg02]+ ions (182). 

A triple bond doesn t exist in the true benzyne structure. The extra bond results from the overlap of sp2 orbitals on adjacent carbon atoms of the ring. The axes of these sp2 orbitals are in the same plane as the ring, and therefore, they don t overlap with the n orbitals of the aromatic system. Consequently, there s little to no interference with the aromatic system. The additional bond is weak and benzyne is thus highly unstable and highly reactive. Figure 4-1 shows the true structure of benzyne ... 

When the C—H bonds ortho to the halogen are nonequivalent, two isomeric benzyne structures form. In general, the formation of the benzynes is not regioselective. However, there may be a small preference for formation of the benzyne that results from deprotonation of the more acidic C—H bond. Note that the acidity of these bonds is affected only by the inductive effect of substituents. The electron pair of the anion occupies an sp hybrid orbital. The pair cannot be delocalized in the 71 system because it is perpendicular to the 2p orbitals that form the aromatic system. 

The reaction proceeds by formation of the Grignard reagent from o bromofluorobenzene Because the order of reactivity of magnesium with aryl halides is Arl > ArBr > ArCl > ArF the Gngnard reagent has the structure shown and forms benzyne by loss of the salt FMgBr... 

Give the structure of the cycloaddition product formed when benzyne is generated in the presence of furan (See Section 11 22 if necessary to remind yourself of the structure of furan )... 

X-ray crystal structure, 6, 516 Benzophenanthridines synthesis from anils, 2, 416 Benzophenanthridines, tetrahydro-synthesis, 2, 469 Benzo[c]phenanthridines synthesis, 2, 414 from aryl isocyanides, 2, 411 from benzynes, 2, 432 Benzo[i]phenanthridines synthesis, 2, 414 Benzophenanthridinones... 

In the reactions of benzyne with enamines, arylated enamines or amino-benzocyclobutenes can be obtained, depending on reaction conditions and the structure of the enamine. Thus the presence of a proton source such as a secondary amine will favor the enamine product through capture of the zwitterionic intermediate, whereas in the absence of protons one sees... 

It should be noted that CASSCF methods inherently tend to give an unbalanced description, since all the electron correlation recovered is in die active space, but none in the inactive space, or between the active and inactive electrons. This is not a problem if all the valence electrons are included in the active space, but this is only possible for small systems. If only part of die valence electrons are included in the active space, the CASSCF methods tend to overestimate the importance of biradical structures. Consider for example acetylene where the hydrogens have been bent 60° away from hnearity (this may be considered a model for ort/zo-benzyne). The in-plane jt-orbital now acquires significant biradical character. The true structure may be described as a hnear combination of the three configurations shown in Figure 4.11. 

Although neither benzyne nor any other aryne has been isolated, it appears from the foregoing that we are justifled in considering these intermediates to be short-lived substances, the properties of which can be described by the series of resonance structures 17 and 21-23. 

The electronic structure of benzyne, shown in Figure 16.19, is that of a highly distorted alkyne. Although a typical alkyne triple bond uses sp-hybridized carbon atoms, the benzyne triple bond uses sp2-hybridized carbons. Furthermore, a typical alkyne triple bond has two mutually perpendicular it bonds formed bv p-p overlap, but the benzyne triple bond has one tt bond formed by p-p overlap and one tt bond formed by sp2 sp2 overlap. The latter tt bond is in the plane of the ring and is very weak. 

Sjsj2 reaction and, 377-378 Benzylic radical, resonance in, 578 spin-density surface of, 578 Benzylpenicillin, discovery of, 824 structure of, 1 Benzyne, 575... 

The elimination-addition mechanism involves a highly unstable intermediate called dehydrobenzene or benzyne.123 (See Section 10.6 of Part A for a discussion of the structure of benzyne.)... 

The reactions of benzyne with enol-ethers and enol-acetates have been much studied very recently 115-118). We were not surprised therefore to isolate a product derived from the attack of tetrafluorobenzyne on the bis-enol-ether (75). This product is derived from a (2 + 2) n cycloaddition and the available evidence suggests that this product has the structure (78). 

In O-benzyne (I) the S-level is calculated by EHT to be 1.52 eV below the A-level while in the 1,8-dehydronaphthalene (II) the ordering of the two levels is reversed. There is experimental evidence that I adds stereospecifically 1,4 to olefins (ethylene structure), while II adds 1,2 (anti-ethylene structure). 

There are several different types of chemical species (molecules, ions, radicals, car-benes nitrenes, benzynes, etc.) for which SPQR can be determined. Three kinds of structure are possible ... 

Three new spirans, including 131 and 132, incorporating a 1,5-benzodithiepin system have been synthesised from methyl benzynes. The structure of 131 was confirmed by X-ray crystallography, and the seven-membered ring was shown to have a chair conformation and the five-membered ring an envelope conformation . 

The benzene molecule is attached as a phenyl group, between two metal centers (structure I), and as a "benzene four-electron donor, three-center coordinating group in structures II, VI, and X. The C-C distance in all these molecules is 1.42 A, and the dihedral angle between the planes containing the osmium atoms and the mean plane of the benzyne in the three complexes is remarkably constant at 69 3°. 

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