W-Benzyne

The matrix isolation and spectroscopic characterization of m- and p-benzynes and their derivatives have been reported. Fourier transform ion cyclotron resonance mass spectrometry has been employed to investigate the reactivity of w-benzyne biradical with a pyridinium charge site in the 5-position. The chemical properties of w-benzyne in the gas phase differ from those of the monoradical and... 

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... 

Since the nitrogen in pyridine is electron attracting it seemed reasonable to predict that the trihalopyridynes would also show the increased electrophilic character necessary to form adducts with aromatic hydrocarbons under similar conditions to those employed with the tetra-halogeno-benzynes. The availability of pentachloropyridine suggested to us and others that the reaction with w-butyl-lithium should lead to the formation of tetrachloro-4-pyridyl-lithium 82 84>. This has been achieved and adducts obtained, although this system is complicated by the ease with which pentachloropyridine undergoes nucleophilic substitution by tetrachloro-4-pyridyl lithium. Adducts of the type (45) have been isolated in modest yield both in the trichloro- and tribromo- 58) series. 

The formation of anthracene in reactions of 185 and 186 with benzyne, which was unexplained by Wittig et aZ., possibly is due to an alternative reaction of the intermediate zwitterion (202) with another molecule of benzjme or with a benzyne precursor. Benzyne reacted with the isoindole (206) to give the tetramethyltriptycene (208) and, in a separate run using excess of the benzyne precursor, W-benzylcarbazole. The latter product would appear to be made up of the iV-benzyl group from an intermediate anthracen-9,10-imine (207) and two molecules of benzyne. Mass spectral evidence also implicated the adduct 207, and the formation of 208 was attributed to benzyne-induced deamination of 207 to 1,4,9,10-tetramethylanthracene, which was trapped by further addition of benzyne across the 9- and 10-positions. 

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... 

Till about 1978 not much was known about the reactive intermediates in organosilicon chemistry but after that an almost explosion has taken place and now the vast amount of literature has shown their importance in the synthetic organic chemistry and in a w they are more important than purely organic reactive intermediates like carbenes, benzynes or nitrenes. 

Winkler, M. Sander, W. The structure of /neto-benzyne revisited-a close look into a-bond formation, J. Phys. Chem, A 2001,105,10422-10432. 

The electronic structure of benzyne, shown in Fipire 16.2C. is that sf a highly distorted alkyno. Although a typical alkyne triple bond uses sp hybridized carbon atoms, the bt rtiByne triple bond uses sp hyhi idized tar-honSs Furthermore, a typical alKyne triple bnnd has two mutually perpendicular n bonds formed by p-p overlap, but the benzyne triple bond Ims one w bond foTinwl by p-p uv( >rUp and o fte ir bond I ormed by. p -4ip overlap The latter w bond is in the plane of the- ring and U very weak. 

Problem 11.25 How do the following observations support the benzyne mechanism (a) Compounds lacking ortho H s, such as 2,6-dimethylchlorobenzene, do not react, b) 2,6-Dideuterobromobenzene reacts more slowly than bromobenzene, (c) o-Bromoanisole, o-CH30C6H4Br, reacts with NaNH2/NH3 to form W-CH30C6H4NH2. d) Chlorobenzene with ( 11 bonded to C gives almost 50% aniline having NH2 bonded to NH2 bonded to an ortho C. 

The electrophilicity of benzyne (90) is 1.95 eV, a value that falls within the range of strong electrophiles in the w scale.102 This value, which is larger than that evaluated for acetylene (32), to = 0.54 eV, allows to explain the reactivity of the benzyne derivatives towards nucleophilic additions. The electrophilicity of the fused four-membered... 

Dihydroacridines are formed along with 5,6-diphenyl-5,6-dihydrophenanthridines in the reaction of benzyne with imines such as PhCH NPh, which confirms the intermediacy of henzazetidines (C.W.6. Fishwick, R. C. Gupta and R. C. Storr, J. chem. Soc. Perkin I, 1984, 2827). 

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