Bonding benzyne

Zirconium-aryne complexes have also found application in the regiose-lective synthesis of halophenols.62 Reaction of 101 with methyl diethylbori-nate or triethylborate leads to insertion of a boron-oxygen bond into a Zr-benzyne bond with formation of 102 and 104, respectively (Scheme 12). Further reactions of 102 and 104 with bromine or iodine followed by excess NaOH and 30% hydrogen peroxide gives halomethyl(methoxy)phe-nols 103 and 105 in good yields. In most cases only one regioisomer of the halophenol was observed. 

It is difficult to draw this mechanism convincingly. The two flat molecules approach each other in orthogonal planes, so that the oribitals of the localized k bond of benzyne bond with the p orbitals on the central ring of anthracene. 

AUcynylphosphines have also been shown to undergo an interesting double insertion with Ni(0)-benzyne complexes. Thus, two equivalents of Ph2P-CC-R insert into the Ni-benzyne bond in (24) to form a series of novel naphthalene-based bis(phosphine) ligands coordinated to the Ni center (25,... 

Because the nng prevents linearity of the C—C=C—C unit and tt bonding m that unit IS weak benzyne is strained and highly reactive This enhanced reactivity is... 

Its Strained triple bond makes benzyne a relatively good dienophile and when benzyne IS generated in the presence of a conjugated diene Diels-Alder cycloaddition occurs... 

Aryne (Section 23 8) A species that contains a triple bond within an aromatic nng (see benzyne)... 

There have been several representations of the bonding in benzyne. The one most generally used pictures benzyne as being similar to benzene but with an additional weak bond in the plane of the ring, formed by overlap of the two sp orbitals. Comparison of the NMR characteristies with MO ealeulations indieate that the eonjugation is maintained and that benzyne is a strained but aromatie moleeule. ... 

When benzyne is generated in the presence of potential dienes, additions at the highly strained triple bond occur. Among the types of compounds that give Diels-Alder addition products are furans, cyclopentadienones, and anthracene. 

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. 

Figure 16.19 An orbital picture and electrostatic potential map of benzyne. The benzyne carbons are sp2-hybridized, and the "third" bond results from weak overlap of two adjacent sp2 orbitals.

Benzyne (Section 16.8) An unstable compound having a triple bond in a benzene ring. 

This is first illustrated for the two nonbonding -type orbitals n, and n2 of para-benzyne and pyrazine (Fig. 31). These nonbonding orbitals are derived from outer (2s, 2p) sp2 type hybrids which have not been used in any bonding interaction. Although the overlap between n, and n2 is zero each one overlaps with the central CC bond orbitals. All told, there will arise two distinct molecular orbitals in which nj and n2 enter as combinations (symmetric or antisymmetric) and which have different energies, because of selective interactions with the central bonds. 

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