Triple bonds in benzyne

As expected, the formal C—C triple bond in benzyne is significantly weaker than in unstrained alkynes, the C=C stretching vibrations of which usually fall in the region 2150 cm. Nevertheless, o-benzyne is better described as a strained alkyne rather than a biradical, which is evident from the large singlet-triplet splitting of 37.5 0.3 kcal/mol as well as the alkyne-like reactivity (e.g., in Diels-Alder reactions). The enthalpy of formation of 4 was determined to be 106.6 3.0 kcal/mol by Wenthold and Squires. For the C=C bond length a value of 124 2 pm was found experimentally, " which comes closer to a typical C C triple bond (120.3 pm in acetylene) rather than a C C double bond (133.9 pm in ethylene). 

The structure of benzyne, the parent member of the aryne group of intermediates, has been the focus of many experimental and theoretical studies. For instance, its IR spectrum has been recorded by low temperature solid matrix photolysis of precursors like benzocyclobutanedione (4) and diazalactone (S).6-9 Force field calculations based on these observations indicate an extended triple bond in benzyne (6a).10 Semiempirical and ab initio calculations also predict the arynic bond to be longer than a normal triple... 

Orbital pictures of the bond formed (a) by overlapping p orbitals in a normal triple bond and (b) by overlapping sp orbitals in the distorted "triple bond" in benzyne. 

In the presence of a strong base, an aryl halide undergoes a nucleophilic substitution reaction via a benzyne intermediate. After a hydrogen halide is eliminated, the nucleophile can attack either of the carbons of the distorted triple bond in benzyne. Direct substitution is substitution at the carbon that was attached to the leaving group cine substitution is substitution at the adjacent carbon. 

Attack by the NH3 on the benzyne intermediate is about equally probable in forming a C—N bond at either end of the triple bond in benzyne. Therefore, the product distribution is approximately 50% of both 0—N and N bonds. Employing in the reactant 1— C-chlorobenzene produces results that provide a way to explain which carbon atom the NH2 group attaches to. As illustrated earlier, the use of a tracer gives information about the type of intermediate formed that is not easily obtained in any other way. 

The triple bond in benzyne is different from the usual triple bond of an alkyne. In benzyne, one of the 7T components of the triple bond results from p-p overlap and is part of the delocalized tt system of the aromatic ring. The second it component, which results from overlapping adjacent -hybridized orbitals, lies in the plane of the ring and is not part of the aromatic tt system, as shown in Figure 12.8 and the electrostatic potential map. 

The triple bond in benzyne is strained and is a dienophile in Diels-Alder reactions. Alternative methods exist for the generation of benzyne in cycloadditions and other synthetic applications. In the following example, 6>-bromofluorobenzene is treated with magnesium in tetrahydrofuran (THF). When carried out in the presence of cyclohexadiene, a Diels-Alder reaction occurs. 

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... 

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

The triple bond in the benzyne intermediate is very reactive toward a wide range of nucleophiles. The reaction of acetaldehyde with methanol to produce a hemiacetal is also a base-catalyzed reaction. In this reaction, the methoxide ion, CH30, is the base ... 

A reactive intermediate in some nucleophilic aromatic substitutions, benzyne is benzene with two hydrogen atoms removed. It can be drawn with a highly strained triple bond in the six-membered ring. (p. 789)... 

The intermediate is called benzyne as it is an alkyne with a triple bond in a benzene ring. But what does this triple bond mean It certainly isn t a normal alkyne as these are linear. In fact one tc bond is normal—it is just part of the aromatic system. One tc bond—the new one—is abnormal and is formed by overlap of two sp2 orbitals outside the ring. This external tc bond is very weak and benzyne is a very unstable intermediate. Indeed, when the structure was proposed few chemists believed it and some pretty solid evidence was needed before they did. We shall come to that shortly, but let us first finish the mechanism. Unlike normal alkynes, benzyne is electrophilic as the weak third bond can be attacked by nucleophiles. 

Notice the symmetry in this mechanism. Benzyne is formed from an ortho carbanion and it gives an ortho carbanion when it reacts with nucleophiles. The whole mechanism from bromobenzene to aniline involves an elimination to give benzyne followed by an addition of the nucleophile to the triple bond of benzyne. In many ways, this mechanism is the reverse of the normal addition-elimination mechanism for nucleophilic aromatic substitution and it is sometimes called the elimination-addition mechanism, the elimination step... 

Benzyne may not look like a good dienophile but it is an unstable electrophilic molecule so it must have a low-energy LUMO (71 of the triple bond). If benzyne is generated in the presence of a diene, efficient Diels-Alder reactions take place. Anthracene gives a specially interesting product... 

A Phenyllithium is polarized such that the phenyl group is car-banionic in character and behaves as a strong base, abstracting the 2-H of fluorobenzene. The resulting ortho lithiation is followed by loss of lithium fluoride to form the benzyne intermediate. The second molecule of phenyllithium adds across the triple bond in a non-selective manner to generate biphenyl with 50% of the product with the aryl-aryl bond at the original labelled carbon atom, according to the mechanism in Scheme 9.8. 

The incoming nucleophile can attack either of the carbons of the triple bond of benzyne. Protonation of the resulting anion forms the substitution product. The overall reaction is an elimination-addition reaction Benzyne is formed in an elimination reaction and immediately undergoes an addition reaction. 

The triple bond in ortHo-benzyne can be stabilized by complexation with transition metals. Aryne-metal complexes were originally proposed as intermediates in the decomposition of various aryl derivatives of early transition metals, and the first fuUy characterized mononuclear ortho-benzyne complex, TaMe2(q -C5Me5) (q -CjH4), was prepared. Although this method does not appear general for all transition metals, various complexes of zirconium, rhenium, and niobium have been characterized. More recently, complexes of nickel and platinum have also been... 

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