Tropylium cation synthesis

The first reaction provides a route for the reduction of alkyl halides since the carbo-cation (isopropyl, in Rl) may be prepared from action of AICI3 on the corresponding alkyl halide. Reactions of the type Rl are also important in the process, catalytic cracking, in the manufacture of gasoline. They have also been studied in mass spectro-metric experiments [235]. Reaction R2 is one route to the preparation of carbocations under stable ion conditions. Reaction R3 is employed in the laboratory synthesis of the tropylium cation. Reaction R4, the (crossed) Cannizzaro reaction, is unusual in that it takes place under strongly basic conditions. The oxy dianion is an intermediate in the reaction of concentrated hydroxide with the aldehyde, R HO. None of R1, R2, or R3 may have hydrogen atoms a to the carbonyl groups. Formaldehyde (R1 = H) is readily... 

Apart from the di- and oligoolefm iron tricarbonyl complexes, which nowadays are frequently used in organic synthesis [71, 72], the chemistry of the readily accessible cyclohepatriene chromium and molybdenum tricarbonyls 2 and 3 was the focus of intense research efforts as well. Only a few months after the synthesis of 2 and 3 was published [58,59], both Hyp Dauben and Peter Pauson reported that these compounds react with triphenylmethyl tetrafluoro-borate in methylene chloride to give the tropylium complexes 4 and 5 in excellent yield (Scheme 7.1) [73, 74]. Later this method of hydride abstraction was also used for the preparation of the tropylium cation itself and subsequently led to the generation of several cationic rc-complexes of iron, manganese and cobalt [71, 72], The reactions of the cations of 4 and 5 with nucleophilic... 

Synthesis of Azulenes. Reaction of tropylium cations with allenylsilanes produces substituted azulenes. Typically, commercially available tropylium tetrafluoroborate (2 equiv) is ert5)loyed. The second equivalent dehydrogenates the dihydroazu-lene intermediate to produce the aromatic product. Poly(4-vinyl-P3Tidine) (poly (4-VP)) or methyltrimethoxysilane is used to scavenge the HBF4 produced in the reaction. 

Komatsu et al. have reported the first cyclic 7t-conjugated silatropylium ion by hydride removal from 184 on reaction with triphenylmethyl tetrakis(pentafluorophenyl)borate in dichloromethane at -50 °C for 0.5 hours. The cation was characterized by H, 13C and 29Si NMR spectroscopy, with the observed chemical shifts suggestive of a degree of aromaticity not greatly less than that of the tropylium ion. The precursor 184 was made from 180 via the lithiated derivative 181, the dichloride 182, and the mesityl derivative 183 [01T3645], An extensive report on the synthesis and chemistry of enantiomerically pure dihydrodibenzo[i/]phosphine 5-oxides (e.g. 185) has been published by Wyatt et al. [01JCS(P 1)279],... 

Preparation of Aromatic Compounds via Dehydrogenation. Dihydroaromatic compounds are easily converted into the corresponding aromatic compound by treatment with triphenylcarbenium tetrafluoroborate followed by base. Certain a,a-disubstituted dihydroaromatics are converted to the 1,4-dialkylaromatic compounds with rearrangement (eq 1). Nonbenzenoid aromatic systems, e.g. benzazulene or dibenzosesquifulvalene, are readily prepared from their dihydro counterparts. Aromatic cations are also easily prepared by hydride abstraction, for example, tropylium ion (e.g. in the synthesis of heptalene (eq 2)), cyclopropenyl cation, and others, including heterocycUc systems. Some benzylic cations, especially ferrocenyl cations, can also be formed by either hydride abstraction or trityl addition. 

---