3
©
DAUESEN etal. 1957.
= O
D.D.Q.
=0
VANTAMELEN , 1956.
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B.III♦ Formation of Tropylium Salts from Cyclpheptatrienes using quinones in the presence of acid,
1• Introduction
In 1931? showed theoretically that a seven-membered ring possessing a conjugated system of
ir electrons and bearing a positive charge could be aromatic and energetically stable*
Twenty-three years passed before this conclusion received experimental confirmation with the formation of
27a
tropylium bromide by the thermal elimination of hydrogen bromide from dibromocycloheptadiene. It is somewhat
paradoxical that, forty years before Huckel’s prediction, O*7K
Merling had apparently prepared tropylium bromide although he himself was unable to identify it as such*
Since 1954 several routes to the tropylium ion have been 6
reported* Of these Daub on* s hydride-transfer method involving triphenylmethyl perchlorate is the most useful, yields of
6O-9OJo being recorded. This reaction was later adapted as a general method for the preparation of organic cations2**^.
High potential quinones in the presence of strong acid have been shown to readily dehydrogenate dihydroheterocycles
to heterocyclic salts by a similar mechanism and thereby present an attractive route to the tropylium, phenalenium
and cyclopropenylium systems* The method has proved successful and the resultant carbonium ions have been isolated as the
salts of various acids*
2* Dehydrogenation of Cycloheptatriene
There existed in the literature one or two reports of quinone dehydrogenation of seven-membered rings* Using .
van Tamelen had succeeded in obtaining tropone from cyoloheptadienone in 10% yield* Seto and Ogura^* \ had found that £-chloranil or p-benzoquinone dibenzene-
sulphonamide would not themselves dehydrogenate cyclo heptatriene but that in the presence of boron trichloride these reagents would given an almost quantitative yield of tropylium salt* The preparative potential of this reaction does not appears to have, been realised then but has since been developed as a general method for preparing organic cations*
Tropylium perchlorate, a typical salt, has been prepared from cycloheptatriene using any one of a variety of quinones* It appears that in general the dehydrogenative efficiency of
the quinonesin this reaction increases with increasing redox potential (see Table 3)*
Table 3
. *• 68 :
Effect of quinone redox potential on the yield of tropylium perchlorate
Quinone (Solvent) E°(v) Yield (pure material)
D.D.Q. (CHgClg) - 1# 90$
D.D.Q. (CH^.COgH) - 1.0 “ 95$
o-Chloranil (CHj.COgfi) 0.37 95$
p-Chloranil (CH,.COgH) 0.71 . 70%
p~bemo quinone (CH^.GOpH) 0.70 . 30$
D.D.Q.(with NaClO^ in CH^.CQpH) ~ 1.0 75$ As in the dehydrogenation of dihydroheterocycles , chioranil was again the most convenient reagent and in reactions involving D.D.Q. the removal of 2/Wiehloro~5,6- dicyano-1quinol presented minor difficulties as before, With jo-chloranil some decomposition occurred and this was
even more marked with £~b emo quinone where the product required considerable purification* Xn the last reaction with D.D.Q„; replacement of perchloric acid by sodium perchlorate resulted
X
+ QH,XXV1I1
□<
p\ - Me,Ph.
i)N.B.S.,
XXIX
XXX
— 69
T-
in a lowering of the yield. This may reflect hi$ier
efficiency in dehydrogenations involving the protonated quinone althou^i some losses did occur during the Reparation from the sodium quinolate.
Tropylium picrate, prepared in methylene chloride, was obtained in two different, coloured forms both of which had the same melting-point. These two forms were produced by different rates of crystallisation and v/ere interconvertible. The yield (51$) was not improved by replacing the methylene
chloride with acetic acid or by using a larger excess of picric acid.
With jq-chloranil and oxalic acid dihydrate as dehydro genating agent, tropylium tetroxalate was obtained in good yield.
Xn the absence of acid,D.D.Q.. and Cycloheptatriene react to form a dark-coloured product (discussed later under section B. 17.1.) . This product, formulated as
tropylium 2,3-dicIiloro-5,6-dicyano-l ,4-semxquinone (XXTXIl), yielded tropylium salts on treatment with acid or alkali metal
salts. Lithium bromide and the above tropylium semiquinone (XXTIIl) gave a small quantity of tropylium bromide. factors Contributing to the low yield were the difficulty in isolation
70
(xt separated as an oil) and the hygroscopic nature of the salt which caused it to decompose slowly in the atmosphere*
Addition of boiling acetic acid to a mixture of the tropylium semiquinone (XXVTIl) and sodium iodide yielded a stable crystalline solid, whose composition, corresponded to Treatment of the compound with acetone gave scarlet crystals of tropylium iodide and subsequent addition
of
ether to the acetone mother liquor afforded the garnet* coloured tri-iodide. This information suggests that thecompound would he better formulated as <Wv a lattice complex containing both the iodide and tri*iodide ions.
The product formed from the tropylium semiquinone (XXVTEl) and p~toluene*<5ulphonic aoid had a comparatively low
melting-point and was unstable in moist air being slowly converted to a brownish oil. These properties suggest that it is probably more covalent than ionic in character.
dehydrogenation of Allyl* and Arylcycloheptatrienes._T-W '"R1 rw*i*w**«te«*9*>h-.- • - ■.“-rrrTnrrtrr-T:"tr tj:.;..:,irixrTJfrr. •
Only a
f
ew substituted ti'opylium salts are known and in fact no really satisfactory method has yet been devised for their preparation. A number4 of unsubstituted and alkyl*- f ( .
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6 cycloheptatriene using the triphenylmethyl carbonium ion
» ■
hut in the reaction with methojcycycloheptatriene, methoxy 1
transfer occurred and tropylium bromide was produced. Other known substituted tropylium salts include the carboxyiropylium salts'^ and halotropylium salts. With the successful
preparation of unsubstituted tropylium salts using quinones and acid,it was hoped that substituted salts might be obtained from 7-substituted cycloheptatrienes in a similar manner.
Such a reaction might also provide more information about the mechanism of the process and, by means of a suitable derivative, afford a new route to tropone.
Vdien treated with oychloranil and perchloric acid y 7-methylcycloheptatriene yielded methyl tropylium perchlorate. The product, initially an oily solid, required considerable purification and consequently the yield was low. Sven the purified salt decomposed slowly in atmospheric moisture.
7-Phenylcycloheptatriene gave a more satisfactory yield of perohlorate when treated with the same reagent and
the product displayed a greater stability. The presence of two aromatic rings contributes largely to this stability and the molecule may be compared to biphenyl of which it is the ^-electron analogue-5**. p-Phen.ylene-bis (tropylium)