Isomerization of Gomberg's Trityl by Protonic Acids in Benzene

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Gomberg's trityl (1-diphenylmethylene-4-trityl-2,5-cyclohexadiene) (I) was isomerized by protonic acids such as HCl and phenol in benzene to form p-benzhydryltetraphenylmethane(II). p-(Benzhydryl-α-d)-tetraphenylmethane (III) was formed by the treatment of I with phenol-d. Ph3CH and Ph3CCl were treated under the same conditions, but II was not obtained. It was not formed either by the reaction of phenylazotriphenylmethane with phenol. This observation indicates that H+ adds to the terminal methylene of I to form the cyclohexadienyl cation followed by deprotonation to give II. Similarly, treatment of 1-(phenyl-p-tolymethylene)-4-(diphenyl-p-tolylmethyl)-2,5-cyclohexadiene (IV) with HCl in benzene afforded α,α,α′-triphenyl-α,α′-(di-p-tolyl)-p-xylene (V). The results support the view that Gomberg's trityl is not hexaphenylethane but 1-diphenylmethylene-4-trityl-2,5-cyclohexadiene as suggested by Lankamp et al. (Tetrahedron Lett., 1968, 249). 1,1,2,2-Tetraphenylethane and 1,1,2,2-tetra-o-tolylethane not having the cyclohexadiene structure could not be isomerized by protonic acids (HCl and PhOH) in benzene. Thus, isomerization of this type is characteristic of the cyclohexadiene structure such as that of Gomberg's trityl.
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