Carbonium fluoroborates

Amide acetals. Carboxamides (1) are alkylated in high yield (95-100%) by triethyloxonium fluoroborate to give the corresponding carbonium fluoroborates (2). As a rule, the reaction of (2) with sodium ethoxide at 0° affords an amide acetal (3). 

From the above discussion it follows that the probability of carbonium ion formation during decomposition of RTIX2 compounds by a Type 5 process is low when X is carboxylate, but significantly higher when X is nitrate, sulfate, perchlorate, or fluoroborate. The important role played by the anion of the metal salt in oxymetallation has in fact been recognized only very recently for both oxymercuration 11, 12) and oxythallation (92). The... 

Fyfe et al. (355) were able to produce a very informative 13C CP/MAS NMR spectrum of the triphenylmethyl carbonium ion by using the tetra-fluoroborate counterion and by employing simultaneous 19F and H decoupling during spectral acquisition (see Fig. 81). The nonequivalence of the ortho and meta carbons is readily seen in the spectrum. 

Oxidmion of ketone acetals and ethers. Ketones can be regenerated from the ethylene acetal derivatives by treatment with trityl fluoroborate in dry dichloro-methanc (Nj) at room temperature. Thus the reaction of trityl fluoroborate with cyclohexanone ethylene acetal results in cyclohexanone (80%) and triphenylmethane. The reaction thus involves hydride transfer to the triphenyl carbonium ions. Triethyl-oxonium fluoroborate can also be used but is somewhat less effective than trityl fluoroborate. 

Completely analogous to the protonation of azide is the attack by carbonium ions. This could be accomplished only by using very powerful carbonium ion donors. The reaction of ethyl azide and triethyloxonium fluoroborate thus yielded the imine 48 . Other examples of this reaction have been reported . ... 

N-Benzylideneaniline treated with an equivalent amount of nitrosyl fluoroborate in acetonitrile -> benzenediazonium fluoroborate. Y almost 100%. Also formation of carbonium ions by reaction of nitrosyl fluoroborate with azides s. M. P. Doyle and W. Wierenga, Am. Soc. 92, 4999 (1970). 

The presumed argento-aryl carbonium ion intermediate in the silver-catalysed ring-opening of benzocyclopropene has been trapped by addition to butadiene, which, in contrast to the non-silver-catalysed reaction, gives the adducts (409) and (410). With silver fluoroborate in benzene or chloroform, 9,10-dihydroanthracene was obtained, in contrast to the thermal reaction which affords 9,10-dihydrophenanthrene and in conflict with an earlier report that 9,10-dihydrophenanthrene is the product of silver-catalysed dimerization of benzocyclopropene. ... 

It is often difficult to induce selective addition to double bonds in the presence of triple bonds because of the reactivity of the latter. Dicobalt octa-carbonyl selectively adds to a triple bond in the presence of a double bond and allows selective transformation of the non-co-ordinated olefinic bond. Removal of the protecting metal is simple. Thus vinylacetylenes when treated with strong acids usually form the products of hydration of the triple bond, and the ene-yne-ol (140) reacts with fluoroboric-acetic acid at 25 C for 24 h to form an intractable mixture. Its complex (141) reacted at 0 °C for 15 min to give 91 % of (142) on work-up, which implies a metal-stabilized carbonium ion intermediate. Oxidative degradation with Fe(N03)3 generates the acetylene (143) in excellent yield. 

The reaction of nitronium fluoroborate with olefins in acetonitrile leads, via a Ritter reaction of the intermediate carbonium ion with solvent, to 1-nitro-2-acetamidoalkanes. The inherent instability of DMF towards aqueous base has been noted. ... 

The trityl carbonium ion is well documented as a hydride-ion acceptor indeed it can be reduced by aldehyde acetals in an intramolecular hydride-transfer process. This is pivotal in a new method for the deacetaliza-tion of ketone acetals by oxidative hydride transfer trityl fluoroborate is used as reagent, and the glycol moiety is oxidized to an a-ketol, as is shown in Scheme 79. This novel procedure is successful also with hemithioacetals, but fails with thioacetals. 

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