O-benzynes

Unfortunately, appearance energy measnrements become more complicated with larger substrates, where the cations are more prone to rearrangement during ionization. Eor example, numerous attempts have been made to measnre the energy for formation of o-benzyne cation, CgH4+ by using benzonitrile as a precursor (Eq. 5.6). 

A variation of the halide affinity approach was used by Riveros et al. in the investigation of the enthalpy of formation of o-benzyne. Reaction of bromo- or iodobenzene with base in an ICR leads predominantly to the formation the expected M-1 anion, but also leads to the formation of solvated halide ions (Eq. 5.15). By using substrates with known halide affinities, it was possible to assign limits to the enthalpy of formation of the benzyne product. Ultimately, the experiment is comparable to that outlined in Eq. 5.14, although the acidity and halide affinity measurements are made in a single step. 

In O-benzyne (I) the S-level is calculated by EHT to be 1.52 eV below the A-level while in the 1,8-dehydronaphthalene (II) the ordering of the two levels is reversed. There is experimental evidence that I adds stereospecifically 1,4 to olefins (ethylene structure), while II adds 1,2 (anti-ethylene structure). 

Above 1 mM, C-H activation occurs to form a chemisorbed layer of edge-bonded (2,3-i)2) hydroquinone analogous to o-benzyne organometallic compounds formed with Pt and Os clusters (2) ... 

R. Warmuth, o-Benzyne Strained Alkyne or Cumu-lene - NMR Characterization in a Molecular Container , Angew. Chem. Int. Ed Engl. 1997, 36,1347-1350. 

While the existence of o-benzyne (4) was first suggested in 1930, it was not until 1953 that Roberts reported that treatment of [l- C]chlorobenzene with potassium amide gives a 1 1 mixture of [1- C]- and [2- C]aniline which gave strong support for the intermediacy of o-benzyne in this conversion. Numerous other expCTiments also supported the existence of o-benzyne. It was first... 

Figure 11. Comparison of experimental and theoretical spectra of o-benzyne (4) classified by symmetry.

The observed absorption at 1846 cm (calculated 1961 cm ) was assigned to the triple-bond stretch of 4. This w s supported by its shift to 1793 cm" (a shift of 53 cm", the computed shift was 72 cm" ) in [l,2- C2]-o-benzyne. In fact the computed isotopic shifts were in very good agreement with those observed for all absorptions in both [3,4,5 and [ 1,2- C2]-o-benzyne. The computation of the... 

Both the calculated and observed spectrum are dominated by a very intense band (expt 2085 cm" and calcd 2439 cm" ) which has now been assigned to the ketene double-bond stretch. This particular absorption had been responsible for the earlier confusion over the triple-bond stretch in o-benzyne since the presence of even small amounts of 4a in reaction mixtures containing o-benzyne showed an absorption... 

The distonic radical anions of o-, m-, and p-benzyne were crucial intermediates in an elegant determination of the S,T splitting of the corresponding benzynes. The ions are accessible by well-estabhshed (routine) gas-phase reactions o-benzyne... 

In 1956 Wittig was able to trap o-benzyne in various Diels-Alder reactions. Nonetheless, he remained skeptical about his own hypothesis of the existence of... 

The first direct infrared (IR) spectroscopic detection of o-benzyne was accomplished by Chapman et al., " using matrix isolation spectroscopy at very low temperatures to generate 4 starting from phthaloyl peroxide (5) and benzocyclo-... 

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). 

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