Arene trapping

Intramolecular electrophilic aromatic substitution to give tricyclic products 142 is also a viable process, with trapping efficiency related to the electron density of the arene trap (equation 3)67. With a simple phenyl group pendant, rearrangement to the 2-pyrone was... 

Figure 13.68. Arene trapping correlates with 7t complexation.

Enantioselective cycloisomerizations of enynoic acids are possible in the presence of BIPHEP-gold catalysts. Under these conditions, 1,6-enynes bearing an intramolecular amide, phenol, or electron-rich arene trap also provide complex polycyclic products with high ee (Scheme 4-34). 

Grant, T. N., West, F. G. (2007). Interrupted Nazarov reactions using dichlorocyclo-propanes A novel mode of arene trapping. Organic Letters, 9,3789-3792. 

West FG, WiUoughhy CA. Intramolecular arene trapping of pyran-4-one derived zwitterions a two-step synthesis of tet-rahydrobenz[e]inden-l-ones. J. Org. Chem. 1993 58 3796 ... 

Mechanistically there is ample evidence that the Balz-Schiemann reaction is heterolytic. This is shown by arylation trapping experiments. The added arene substrates are found to be arylated in isomer ratios which are typical for an electrophilic aromatic substitution by the aryl cation and not for a homolytic substitution by the aryl radical (Makarova et al., 1958). Swain and Rogers (1975) showed that the reaction takes place in the ion pair with the tetrafluoroborate, and not, as one might imagine, with a fluoride ion originating from the dissociation of the tetrafluoroborate into boron trifluoride and fluoride ions. This is demonstrated by the insensitivity of the ratio of products ArF/ArCl in methylene chloride solution at 25 °C to excess BF3 concentration. 

The network structures to be discussed will all involved hydrogen bonding as the supramolecular synthon. It should be noted however that other interactions such as coordinate bonds and host-guest interactions may also organise host molecules into network structures. Coordination polymers constructed from molecular hosts may involve functionalised calixarenes [8-11], cyclotriveratrylene [12], or cucurbituril [13]. Calixarenes have also been used to build up network structures via host-guest interactions [14,15]. It is also notable that volatile species may be trapped within the solid state lattice of calix[4] arene with a structure entirely composed of van der Waals interactions [16]. 

Relatively simple spectra are obtained from spin adducts of the hindered nitroso-arenes, and these may be further refined by deuteration of the spin trap (Terabe et al., 1973). In spite of being substantially dimerized, even in dilute solution,6 nitrosodurene (ND) has two considerable advantages over MNP. Firstly, it is more reactive towards radical addition (Table 5, p. 33). Secondly, it is not sensitive to visible light, and even on ultraviolet irradiation any photodecomposition is apparently not a major source of nitroxides. 

The reaction looks like a simple Friedel-Crafts alkylation, but there is a twist — the leaving group is not on the C which becomes attached to the ring. After formation of the C7 carbocation, a 1,2-hydride shift occurs to give a C6 carbocation. The 1,2-hydride shift is energetically uphill, but the 2° carbocation is then trapped rapidly by the arene to give a 6-6 ring system. 

Detailed kinetic studies comparing the chemical reactivity ofK-region vs. non-K-region arene oxides have yielded important information. In aqueous solution, the non-K-region epoxides of phenanthrene (the 1,2-oxide and 3,4-oxides) yielded exclusively phenols (the 1-phenol and 4-phenol, respectively, as major products) in an acid-catalyzed reaction, as do epoxides of lower arenes (Fig. 10.1). In contrast, the K-region epoxide (i.e., phenanthrene 9,10-oxide 10.29) gave at pH < 7 the 9-phenol and the 9,10-dihydro-9,10-diol (predominantly trans) in a ratio of ca. 3 1. Under these conditions, the formation of this dihydrodiol was found to result from trapping of the carbonium ion by H20 (Fig. 10.11, Pathway a). At pH > 9, the product formed was nearly ex-... 

One of the first notions of EGA-catalyzed reactions was the rationalization [8, 14] of the unexpected outcome of anodic oxidation of methyl arenes, (1), in MeGN containing various amounts of water. Preferentially A-benzyl acetamides, (3), rather than the benzyl alcohols, (2), were formed [15, 16] (with increasing amounts of water, increasing amounts of aldehyde was formed as a side product [16]). Since water is a more powerful nucleophile than MeCN, it is reasonable to believe that the carbocation formed by overall two-electron oxidation and deprotonation is initially trapped by water. However, the process is reversible in the presence of a strong EGA (protons liberated from the oxidized substrate), and the carbocation is eventually trapped by the excess MeCN, Scheme 1. 

Materials may be added to prevent a build-up of undesired products. For example, a base may be present to trap released acid. These compounds aren t part of the mechanism. 

TABLE 8.27 LITHIATION OE OXAZOLINYLFERROCENE 439 AND TRAPPING WITH TMSCl, 457 TABLE 8.28 LITHIATION OF ARENES BY EVL-HMPA, 459... 

Arylnitrenium ions are likewise capable of adding to n nucleophiles. With substituted aromatics (e.g., toluene) there exists the possibility of three reactive sites on the nitrenium ion (the nitrogen, ortho- and para-ring positions), along with up to three possible sites on the arene (ortho, para, and meta in the case of a monosub-stituted trap). Thus in a typical case there is the possibility of nine distinct regio-isomers. Obviously, any synthetic utility of such chemistry relies on the ability of the reagents to react in a selective manner. 

The LFP studies of the reaction of the A-methyl-A-4-biphenylylnitrenium ion with a series of arenes showed that no detectable intermediate formed in these reactions. The rate constants of these reactions correlated neither with the oxidation potentials of the traps (as would be expected were the initial step electron transfer) nor with the basicity of these traps (a proxy for their susceptibility toward direct formation of the sigma complex). Instead, a good correlation of these rate constants was found with the ability of the traps to form n complexes with picric acid (Fig. 13.68). On this basis, it was concluded the initial step in these reactions was the rapid formation of a ti complex (140) between the nitrenium ion (138) and the arene (139). This was followed by a-complex formation and tautomerization to give adducts, or a relatively slow homolytic dissociation to give (ultimately) the parent amine. 

Takeuchi and co-workers showed that the ratio of N- to C-substitution on the apparent intermediate 75a by a series of aromatics in arene/TFA mixtures was sensitive to the structure of the precursor to 75a (phenyl azide, N-phenylhydroxylamine or O-trifluoroacetyl-N-phenylhydroxylamine) even though the decomposition rates of these precursors were unaffected by the identity of the arene. This result is consistent with the characterization of 75a from N3 and Br trapping studies as a short-lived intermediate that will usually react through ion pair or preassociation processes that may be leaving-group dependent. ... 

Typically, the cumulative electron-withdrawing effect of metallating more than half of the arenes of the macrocycle is required in order to observe anion inclusion. Thus, we have focused on tetrametallated derivatives of calix[4]arene and di- or trimetalated derivatives of the related orr/io-cyclophane, cyclotriveratrylene (CTV). The X-ray crystal structure of the BF salt of the water soluble bear trap 51 demonstrates the suitability of the narrow calix[4]arene cavity for small tetra-... 

A similar solvent trapping mechanism has been proposed to account for formation of 5-indanol from acid-catalyzed isomerization of the arene oxide tautomer of oxepin (22) (73JA60641). 

Substituted l,2,4-triazoline-3,5-diones are excellent dienophiles which react rapidly at room temperature with oxepins, but particularly with the arene oxide valence tautomer. A similar [4+2] cycloaddition reaction between the episulfide tautomer of thiepin (44) and 4-phenyl-l,2,4-triazoline-3,5-dione has been reported (74AG(E)736>. Benzene episulfide (the valence tautomer of thiepin 44) was generated in situ by thermal decomposition of the diepisulfide (151) at 20 °C and trapped as a cycloadduct at the same temperature (equation 34). A 1,3-dipolar cycloaddition reaction between thiepin (152) and diazomethane has been reported (56CB2608). Two possible cycloadduct products are shown since the final structure has not been unequivocally established (equation 35). 

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