Meta-benzynes

Table 8 Geometry of singlet and triplet state of meta-benzyne ...

From Ref. [58], all calculations use a modified cc-pVTZ in which the f functions on carbons and the d functions on hydrogens are omitted. All geometries come from reference 72, which used unrestricted CCSD(T)/cc-pVDZ geometries and harmonic vibrational frequencies for para-benzyne, and restricted CCSD(T)/cc-pVDZ for the meta-benzyne s geometries and harmonic vibrational frequencies. The ZPE differences are —0.3, 0.7 and 0.3 kcal/mol respectively Ref. [79]... 

For meta-benzyne, given that methods in Table 9 have not performed a basis set extrapolation, one may reasonably beheve that all of the proposed methods may change by 2 kcal/mol. The DEA-CCSD value is basis set... 

Coordination to strongly orf/zo-directing groups is responsible for the regiochemistry of some other reactions which do not involve ortholithiation. For example, while the electron-withdrawing nature of the oxazoline would be expected to direct the addition of the organolithium nucleophile to benzyne 11 towards the meta position, the major product that arises is the result of addition at the ortho position to give 12 (Scheme 1). ... 

The addition-elimination mechanism involves two intermediates, a chlorophenyl anion and benzyne. A simple displacement mechanism can be ruled out because reaction of ort/io-chlorotoluene gives not only orf/to-methyl phenol but also meta-methylphenol. 

Carbanions derived from side chain tertiary amides have also been cyclized to provide isoquinolones and isoindoles (equation 36).125 126 While benzyne intermediacy in the formation of the former is likely, the latter seems to arise through a SrnI reaction pathway. Synthesis of indole from the meta bromo compound (87), on the other hand, clearly involves an aryne cyclization. 27 A more versatile route to indoles is based on intramolecular addition of aminyl anions to arynes (equation 38).128 A somewhat similar dihydroindole preparation constitutes the first step in a synthesis of lycoranes (equation 39).129 The synthesis of (88) also falls in the same category of reactions, but it is noteworthy because only a few examples of ring closure of heteroarynes are mentioned in literature.27 28... 

One of the characteristics of reactions involving benzyne intermediates is that the nucleophile can bond to the same carbon to which the leaving group was bonded, or it can bond to the carbon adjacent to the one to which the leaving group was bonded. This often results in the formation of isomeric products when substituted aromatic halides are used. For example, the reaction of yo-bromotoluene with sodium dimethyl-amide in dimethylamine as the solvent gives a 50 50 mixture of the meta and para... 

This reaction proceeds by a benzyne mechanism but produces only the meta-isomer of the product. Explain this observation. 

Three papers have appeared in the past two years on catalysts that are either supported on polymers or are heterogeneous. Djakovitch first reported animation reactions catalyzed by palladium particles immobilized on metal oxide supports, as well as by palladium complexes contained in NaY zeolites [172]. In most cases, these reactions were conducted at high temperatures, generally 135 °C. When NaOtBu was used as the base, competing amination through a benzyne intermediate was observed. Thus, para meta regioselectivity was not high, and reaction yields were modest. 

Amide ion is a strong nucleophile, attacking at either end of the weak, reactive benzyne triple bond. Subsequent protonation gives toluidine. About half of the product results from attack by the amide ion at the meta carbon, and about half from attack at the para carbon. 

There is no mistake in this scheme. The amine is really at the meta position even though the chlorine was at the ortho position. It would be very difficult to explain this by any other mechanism but very easy to explain using a benzyne mechanism. Using the same two steps that we have used before, we can write this, the elimination step... 

Kraka, E. Cremer, D. Ortho-, meta-, and para-benzyne. A comparative CCSD (T) investigation, Chem. Phys. Lett. 1993,216, 333-340. 

Wenthold, P. G. Squires, R. R. Biradical thermochemistry from collision-induced dissociation threshold energy measurements. Absolute heats of formation of ortho-, meta-, and para-benzyne, /. Am. Chem Soc. 1994, 776, 6401-6412. 

N-AlkylaniUmes. Treatment ofbromobenzene substituted in the meta position by electron-attracting groups [OCH3, Cl, N(CH,)2] with sodium amide generates a benzyne derivative which reacts with a primary aliphatic amine to give the corresponding... 

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