Arynes structure-reactivity

For discussions concerning their structure and further information on the generation and reactivity of arynes the reader is referred to a series of published reviews. ... 

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... 

Now analysis of the literature on palladium-catalyzed cyclotrimerizations suggests that arynes and alkynes have very different reactivities while arynes, which are strongly electrophilic, are efficiently cyclotrimerized by Pd(0) catalysts [ 39 ], there are few examples of efficient cyclotrimerization of alkynes using these complexes [61,62]. This difference in reactivity between arynes and alkynes in Pd-catalyzed cyclotrimerizations, rather than being a disadvantage, provides the chemoselectivity required for their cocyclization to be synthetically useful. Interestingly, cocyclizations of arynes and alkynes frequently show marked dependence on factors such as the electronic nature of the alkyne, the catalyst structure, or the reaction conditions. 

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%... 

There are many other kinds of reactive intermediates, which do not fit into the previous classifications. Some are simply compounds that are unstable for various possible reasons, such as structural strain or an unusual oxidation state, and are discussed in Chapter 7. This book is concerned with the chemistry of carbocations, carbanions, radicals, carbenes, nitrenes (the nitrogen analogs of carbenes), and miscellaneous intermediates such as arynes, ortho-quinone methides, zwitterions and dipoles, anti-aromatic systems, and tetrahedral intermediates. This is not the place to describe in detail the experimental basis on which the involvement of reactive intermediates in specific reactions has been estabhshed but it is appropriate to mention briefly the sort of evidence that has been found useful in this respect. Transition states have no real hfetime, and there are no physical techniques by which they can be directly characterized. Probably one of the most direct ways in which reactive intermediates can be inferred in a particular reaction is by a kinetic study. Trapping the intermediate with an appropriate reagent can also be very valuable, particularly if it can be shown that the same products are produced in the same ratios when the same postulated intermediate is formed from different precursors. 

For an introduction to the structures and reactions of carbenes, see (a) Liebman, J. F. Simons, J. in Liebman, J. F. Greenberg, A., Eds. Molecular Structure and Energetics, Volume 1 Chemical Bonding Models VCH Publishers Deerfield Beach, FL, 1986 p. 51 (b) Moss, R. A. Jones, M., Jr., in Jones, M., Jr. Moss, R. A., Eds. Reactive Intermediates, Vol. 2 John Wiley Sons New York, 1981 (c) Kirmse, W. Carbene Chemistry Academic Press New York, 1964 Gilchrist, T. L. Rees, C. W. Carbenes, Nitrenes and Arynes Appleton-Century-Crofts New York, 1969 (d) Mine, J. Divalent Carbon Ronald Press New York, 1964 Bertrand, G. in Moss, R. A. Platz, M. S. Jones, M., Jr., Eds. Reactive Intermediate Chemistry John Wiley Sons Hoboken, NJ, 2004 chapter 8 (e) Jones, M., Jr. Moss, R. A. in Moss, R. A. Platz, M. S. Jones, M., Jr., Eds. Reactive Intermediate Chemistry John Wiley Sons Hoboken, NJ, 2004 chapter 7. 

Although not formally arynes those species whose reactive orbitals are on nonadjacent atoms are usually considered in the same category of reactive intermediates. For such species as 2 or 3, a diradical notation, 2a or 3a, will be used rather than the bonded structures, 2b or 3b, since the latter have been shown to be distinguishably different entities. 

Several reviews of the chemistry of strained cyclic alkynes and arynes and otiho-ztene cyclynes, " which include synthesis, structure, and reactivity of platinum complexes, have also appeared. The behavior of 1,4-enynes in the presence of transition metals has been reviewed. ... 

A variety of catalytic transformations for constructing aromatic skeletons by use of arynes and o-QDMs have been developed based on the dearomatization/aromatization sequence, leading to the direct formation of benzo-annulated structures and multisub-stituted arenes of structural diversity. Because precise control of the high reactivity... 

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