Arynes stability

An additional effect of benzannelation on aryne stability can be envisioned. Although the implications of other data may be contradictory, ... 

If the competition data are compared with electronegativity values for the halogens 85>, then tetrafluorobenzyne is clearly in an anomalous position. The only reasonable explanation available at present is that tetrafluorobenzyne is so destabilized by the inductive effect of the fluorine atoms that it has lost a considerable amount of the selectivity which arynes normally show. Estimates for the heats of formation of the isomeric dichlorobenzynes and for tetrachlorobenzyne have recently been made from mass spectrometric studies and these do indicate a low stability for tetrachlorobenzyne 86>. Evidently more data are required for the tetrahalogenobenzynes. 

Arynes present structural features of some interest. They clearly cannot be acetylenic in the usual sense as this would require enormous deformation of the benzene ring in order to accommodate the 180° bond angle required by the sp1 hybridised carbons in an alkyne (p. 9). It seems more likely that the delocalised 7i orbitals of the aromatic system are left largely untouched (aromatic stability thereby being conserved), and that the two available electrons are accommodated in the original sp2 hybrid orbitals (101) ... 

Preparation of aryne derivatives in the presence of tertiary amines leads to betaines which are stabilized via various rearrangement reactions.214-217... 

In connection with the substituent effects, the kinetic stability of benzyne is suggested to be increased by electron withdrawal (-/) and decreased by electron release (+/).73 However, the inference cannot be extrapolated to selectivity of substituted arynes in general. For example, in additions involving competition between phenyllithium and lithium piperidide, the methyl substituents (+/) on benzyne increase its selectivity, whereas methoxy groups (-/) decrease it (Scheme 6). On the other hand, in reactions of car-banions derived from acetonitrile in alkylamine solvents both +/ and -/ benzyne substituents lower selectivity and cause predominant amination. Thus, the method was found unsuitable for preparation of many substituted benzyl nitriles.74 In symmetrically disubstituted arynes there is partial cancellation of polarization, and in fact acceptable yields of acetonitrile adducts could be obtained from 3,6-dimethoxy-benzyne.75 The selectivity of substituted arynes varies with the set of nucleophiles in the competition and no comprehensive theory or simple generalization is available on this point. 

Aromatic fused anhydrides are well known to act as a source of arynes by pyrolytic loss of CO2 and CO. In some cases the cyclopropenone resulting from loss of only CO2 can be trapped and this is the case for 294, where FVP at 500 °C allows trapping of both the thienocyclopropenone 295 stabilized as the dipolar form shown, and the thiophyne 296147. 

Theoretical studies have indicated that m-bcnzync is monocyclic with a C(l)-C(3) distance of 2.0 A whereas in tetrafluoro-w-benzyne the increased eclipsing strain between fluorine atoms stabilizes the bicyclo[3.1.0]hexatriene form with a C(l)-C(3) distance of 1.75 A.56 Computational studies coupled with gas-phase experimental studies show that appropriate substituents can be used to tune the reactivity of 1,3-arynes. Thus the presence of NH+ at C(5) makes (13) mildly carbocationic whereas the addition of OH at C(4) in (14) gives a highly reactive (bi)radical.57... 

Examples for frequently encountered intermediates in organic reactions are carbocations (carbenium ions, carbonium ions), carbanions, C-centered radicals, carbenes, O-centered radicals (hydroxyl, alkoxyl, peroxyl, superoxide anion radical etc.), nitrenes, N-centered radicals (aminium, iminium), arynes, to name but a few. Generally, with the exception of so-called persistent radicals which are stabilized by special steric or resonance effects, most radicals belong to the class of reactive intermediates. 

Tetrachlorophthalic anhydride gave a relatively low yield of products derived from tetrachlorobenzyne. The pyrolysis tube was badly carbonized evidently extensive decomposition of the anhydride, the aryne, or the chlorinated products had occurred. Tetrabromophthalic anhydride gave a still lower yield of products. These are not listed in the table because none of them retained all four bromine atoms and could be definitely ascribed to reactions of tetrabromobenzyne, although tri-bromo- and dibromonaphthalene were present in appreciable amounts. Tetraphenylphthalic anhydride also gave low yields of products of the reaction of tetraphenylbenzyne with pyridine. This was not because of the stability of the anhydride and its reluctance to form the aryne, but rather because the aryne preferred to stabilize itself intramolecularly. The behavior of tetraphenylphthalic anhydride is discussed in another Section. 

This new benzyne precursor combines practical facility in its preparation with relative stability and safety in handling in addition, its conversion into benzyne does not require a strong base or high temperature. Another useful feature of this methodology is that, as with substituted diphenyliodonium 2-carboxylates, only one aryne can be formed from substituted precursors. Indeed, two methyl analogues of 2-trimethylsilyl-diphenyliodonium triflate, in the presence of furan, afforded quantitatively the corresponding adducts high yields of benzyne-diene adducts were also obtained with other 1,3-dienes. 

The first step in the reaction is therefore addition of the nitrene to the aromatic double bond forming an aziridine 26 71>. This intermediate can now collapse to the sulphonamide 27 and an aryne which produces tars 68>. Ring opening to form an aniline derivative 28 — a formal insertion product — is another possibility of stabilization. 

The structural characterization of (775-C5H5)2Zr(772-C6H4)(PMe3) 255 in 1986 established the utility of formally divalent zirconocene complexes to stabilize otherwise reactive and transient organic molecules.131 The synthesis and reactivity of these species has been the subject of two recent reviews.74,132 Building on these seminal discoveries, formally low-valent zirconocene fragments have been used to stabilize other aryne species. Typically, these syntheses are achieved by... 

Aryne formation by elimination of lithium fluoride limits the stability of 113 a polymer is formed, but attempts to trap the aryne (114) were unsuccessful.352 Decarboxylation of metal salts of tetrafluoroisonicotinic add occurs,357 giving the corresponding metal derivative, but pyrolysis of the disilver salt (115) appears to produce the aryne (114), since small amounts of diazabiphenylenes may be isolated from the reaction product.358 359... 

The practice of using an insoluble polymer to isolate and kinetic-ally stabilize a reactive intermediate has been addressed in several reports, most commonly using DVB cross-linked polystyrene as a support. In these cases, the three dimensional structure of the polymer and rigidity of the polymer backbone diminish intramolecular reactivity between two sites on the same polymer bead. Physical constraints preclude any significant reaction between two different polymer beads. Similar, less dramatic reduced intramolecular reactivity has also been noted for reactive intermediates bound to linear polystyrene. For example, o-benzyne bound to linear polystyrene has been shown by Mazur to have enhanced stability relative to non-polymer-bound -benzyne (35). In this case, o-benzyne was generated by lead tetraacetate oxidation of a 2-aminobenzotriazole precursor, 1. Analysis of the reaction products after cleaving the benzyne derived products from the polymer by hydrolysis showed a 60% yield of aryl acetates was obtained (Equation 11). In contrast, the monomeric aryne forms only coupled products under similar conditions. Further comparisons of the reactivity of -benzyne bound to insoluble 2% or 20%... 

---