Tropylium bromide, isolation

On the academic side, W. H. Perkin made mention of unsuccessful experiments directed towards the preparation of benzocyclobutene in 1888. This compound was eventually prepared by H. Finkelstein, a student of J. Thiele, in 1909, as reported in his long-unknown Ph.D. thesis. Benzocyclobutene and biphenylene were also reported by Lothrop in 1941. Tropylium bromide was synthesized in 1891 by Merling, but its structure was only established in 1954 by Doering and Knox. The isolation and structure determination of the aromatic steroids estrone, estriol, and estradiol arrived only in 1929 (E. A. Doissy, A. Bute-nandt), 1931 (G. F. Marian), and 1935 (E. A. Doissy), respectively, with the advent of X-ray crystallography. 

In certain cases, the carbocations are so stable that their solid salts have been isolated. For example, triphenylmethyl perchlorate (2.1) exists as a red crystalline solid and tropylium bromide (2.2) has been isolated as a yellow solid. Tropylium bromide (2.2) is stabilized by aromatization as the tropylium cation is planar and has 6tt electrons like benzene. 

As another example, the tropylium ion [3 ], which is stabilized by virtue of the 67t electrons spread over a heptagonal sp hybridized carbon framework [Hiickel s (4n 4- 2)v rule with = 1], is also unstable in the gas phase. Its formation from toluene or the benzyl cation has been a long-standing problem in organic mass spectrometry, and the reaction mechanism and energetics have recently been exhaustively discussed (Lif-shitz, 1994). It was, however, isolated as the bromide salt by Doering and Knox (1954, 1957), and was the first non-benzenoid aromatic carbocation. 

In an improved process for the synthesis of tropilidene (5) by E. Muller, a solution of diazomethane in benzene is added gradually to refluxing benzene containing cuprous bromide as catalyst. Benzene is used in large excess, and the product is isolated most easily by filtering the solution from the catalyst and adding it to a solution of phosphorus pentachloride in carbon tetrachloride. The tropylium chloride which separates is dissolved in water and treated with perchloric acid to afford tropylium perchlorate in 85% yield. The success of the method is attributed to formation of the intermediate (3), a deactivated electrophilic carbon metal complex. Tropilidene... 

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