Benzynes carbon elimination

There are several methods for generation of benzyne in addition to base-catalyzed elimination of hydrogen halide from a halobenzene, and some of these are more generally applicable for preparative work. Probably the most convenient method is diazotization of o-aminobenzoic acid. Concerted loss of nitrogen and carbon dioxide follows diazotization and generates benzyne. Benzyne can be formed in this manner in the presence of a variety of compounds with which it reacts rapidly. 

Elimination, of carbon dioxide and lodobenzene from diphenylio donium 2-carboxylate to generate benzyne, 46,107... 

When benzyne is generated in the absence of another reactive molecule it dimerizes to biphenylene.132 In the presence of dienes, benzyne is a very reactive dienophile and [4+2] cycloaddition products are formed. The adducts with furans can be converted to polycyclic aromatic compounds by elimination of water. Similarly, cyclopentadienones can give a new aromatic ring by loss of carbon monoxide. Pyrones give adducts that can aromatize by loss of C02, as illustrated by Entry 7 in Scheme 11.9. 

Thermolysis of the tantalum-phenyl/methyl complex 114 led to the formation of the Ta-benzyne complex 115 and the elimination of CH4.58 Similar to the reaction of the Ni-benzyne complex 85, one molecule of C02 could be incorporated into the carbon-tantalum bond to form the tantalumcycle 116 as shown in Scheme 40.59... 

Step 1 Elimination stage. Amide ion is a very strong base and brings about the dehydrohalogenation of chlorobenzene by abstracting a proton from the carbon adjacent to the one that bears the leaving group. The product of this step is an unstable intermediate called benzyne. 

The product of the elimination reaction is a highly reactive intermediate 9 called benzyne, or dehydrobenzene, which differs from benzene in having two less hydrogens and an extra bond between two ortho carbons. Benzyne reacts rapidly with any available nucleophile, in this case the solvent, ammonia, to give an addition product ... 

Iodonium salts with a hydroxyl or a carboxyl group are readily converted into their inner salts, i.e. zwitterionic compounds of various types. 2-(Phenyliodonio) benzoate, whose structure may be cyclic, belongs to this category it is prepared from o-iodobenzoic acid upon oxidation and coupling with benzene (or arenes) [51], On strong heating, iodobenzene and carbon dioxide are eliminated, with formation of benzyne ... 

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. 

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. 

In regard to transformation of the benzyne cycloadduct product between benzyne and furan into benzo[c]furan, the direct transformation by elimination of acetylene will not occur because computed activation barriers for this reaction were to high. The AMI computed activation barrier for acetylene elimination was 57.0 kcal/mol, while the activation barriers for the addition of a-pyrone (30.4 kcal/mol) and the elimination of carbon dioxides and benzene (29.9 kcal/mol) were much more energetically favorable (Scheme 1). These computational results are in full agreement with experimental evidence [38]. 

Disubstituted butadienes are good Diels-Alder partners for benzyne. The method affords, after dehydrogenation, a 6+4 carbon assembly route to naphthalenes. Addition of benzyne to 173 affords naphthalene containing 98+2% D, whereas the resulting naphthalene from 174 contained only 2 2% D. Thus not only is the cycloaddition stereoselective, but the elimination from dihydronaphthalene 175 is almost exclusively syn. 

The intermediacy of benzyne in the elimination-addition mechanism for aryl halides accounts for the regioselectivity observed in the substitution reactions of labeled chlorobenzene and 6>-bromotoluene because both can give only a single aryne intermediate. Attack at either of the aryne carbons gives rise to the products. 

We can illustrate this mechanism with the reaction of bromobenzene and amide ion. In the first step (see the following mechanism), the amide ion initiates an elimination by abstracting one of the ortho protons because they are the most acidic. The negative charge that develops on the ortho carbon is stabilized by the inductive effect of the bromine. The anion then loses a bromide ion. This elimination produces the highly unstable, and thus highly reactive, benzyne. Benzyne then reacts with any available nucleophile (in this case, an amide ion) by a two-step addition reaction to produce aniline. 

The reactions of benzyne complexes of zirconium " also occur by electrophilic attack at an M-C bond. The isolated phosphine adduct of a zironocraie-benzyne complex reacts with ketones to imdergo insertion into one of the M-C bonds and with alcohol to make an aryl alkoxo complex, as shown in Equation 12.67. An electron-rich ruthenium-benzyne complex also reacts with electrophiles, such as borzaldehyde or carbon dioxide, to form products from insertion, as shown at the top of Equation 12.68. It also reacts with weak acids, such as aniline, to form products from formal protonation at the Ru-C bond, as shown at the bottom of Equation 12.68. - This reaction with aniline could occur by initial protonation at the metal, followed by C-H bond-forming reductive elimination, or by direct protonation of the M-C bond. Initial protonation of the metal center was proposed. 

A reactive intermediate that is formed by /S-elimination from adjacent carbon atoms of a benzene ring and has a triple bond in the benzene ring. The second irbond of the benzyne triple bond is formed by tveak overlap of coplanar sp orbitals on adjacent carbons. 

---