Hydride acceptors carbonium ions

It is known that tropylium may be prepared from tropylidene via hydride abstraction by PhgC or MegC carbonium ions therefore, it is very likely that here too the dehydrogenation is a hydride transfer from the 1,5-dione to an acceptor. A similar dehydrogenation of chromanones to chromones, with triphenylmethyl perchlorate was reported. A study of the electrooxidation of 1,5-diones on a rotating platinum electrode showed that 1,5-diaryl-substituted diones afford pyrylium salts in these conditions and that the half-wave potentials correlate with yields in chemical dehydrogenations. 

Cycloheptatriene and derivatives thereof donate hydride readily to a variety of carbonium ion acceptors. The position of the end equilibrium depends on the thermodynamics of the exchange. " These reactions are prototypes of a broad area of carbonium ion chemistry wherein carbonium ions equilibrate via intra- and inter-molecular hydride shifts between a donor C—H bond, usually jp hybridized, and a carbonium ion acceptor. This chemistry is often achieved with heterogeneous catalysts and is of great industrial significance it lies outside the emphasis of this review, however. Excellent treatises are available, and a review has appeared on the use of carriers like adamantane to promote hydride transfer in hydrocarbons under strongly acidic conditions. ... 

As can be seen, there are two possible ways in which the chain-initiating carbonium ion is formed. If the first way, involving the formation of the catalyst-carbon linkage by loss of hydride ion from a paraffin is very rapid, then we should expect that paraffins should crack as readily as olefins, since the same intermediate structure is formed. Yet we do know that the catalytic cracking of olefins occurs at considerably lower temperatures than that of paraffins. Thus, it appears that the explanation given by Thomas (4) involving the formation of a small amount of olefin to serve as the proton acceptor is the more likely one. 

Carbonium ion rearrangements. At low temperatures, strong acids (- Ho about 10) induce methyl shifts in branched alkanes (a hydride acceptor must be present to form the carbonium ion). 

The isoparaffin is reacted with a carbonium ion as hydride ion acceptor. In this reaction a tertiary H-atom of the isoparaffin is transferred to the prior formed carbonium ion. The new carbonium ion obtained by transfer from the isoparaffin reacts with CO to give carboxylic acids [640]. 

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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