1,3,5-Cycloheptatriene bromination

Figure 15.7 Reaction of cycloheptatriene with bromine yields cycloheptatrienylium bromide, an ionic substance containing the cycioheptatrienyl cation. The electrostatic potential map shows that all seven carbon atoms are equally charged and electron-poor blue).

G. Merling had obtained tropylium bromide in 1891 by brominating cycloheptatriene but could not guess its structure tropylium was discovered when prepared again via the same route by W. E. Doering and L. H. Knox in 1957, i.e., 66 years later.23... 

Satake et al. have described the mechanistic aspects of the formation of 2-methoxy-277-azepine derivatives lla-d from 377-azepines lOa-d upon reaction with bromine <2003H(60)2211> (Scheme 1). Unlike the situation observed with cycloheptatrienes, delocalized azatropylium salts were not formed from the reaction of 377-azepines with bromine in the absence of an alcoholic solvent. Reaction of 12 with bromine gave 13 plus the bis-ether 14 and bromomethane. The product 14 was also observed in the reaction of 12 with NBS (0.5 equiv) with 1 equiv of 7V-bromosuccinimide (NBS) 12 afforded the succinimido-substituted derivative 15, which upon elimination of HBr in the presence of base gave the 277-azepine 16 (Scheme 2) <2003H(60)2211>. 

In contrast, it is the resonance-stabili/ed cation derived from cyclo-hep tatriene that possesses the aromatic sextet of Jt-electrons. Tropylium bromide is formed by the addition of bromine to cycloheptatriene and... 

A Bromine adds to one double bond of cycloheptatriene in exactly the same way as it does to ethene (although conjugate 1,4- and 1,6-addition may be the preferred mode of reaction). Dehydrobromination occurs on heating. The driving force for the loss of bromide ion is the extra stability of the resulting cation. The positive charge is delocalized over the whole system (Scheme 1.5). 

Compounds of tropolone type have a duplicitous nature in that the molecule can behave as either an aromatic or an allylic functionality. Tropolones can be brominated according to a method similar to that for alkenes, with tautomerisation restoring the cycloheptatriene backbone. In contrast to this simple halogenation with bromine, other halogenated products form less readily, requiring more advanced organic synthetic pathways. 

Tropylium bromide (cycloheptatrienocarbonium bromide), CrHTBr, required for reaction (2a) above may be prepared by bromination of cycloheptatriene in carbon tetrachloride, followed by removal of the carbon tetrachloride and heating the residue in vacuo (60° at 20 mm.) for several days. The product so obtained is washed with tetrahydrofuran. Recrystallization is unnecessary. Tropylium bromide is deliquescent and must be stored in a dry atmosphere. Allyl chloride or bromide may be substituted for tropylium bromide in reaction (2a). 

The history of carbocations dates back to 1891 when G. Merhng reported that he added bromine to tropylidene (cycloheptatriene) and then heated the product to obtain a crystalline, water-soluble material, C H Br. He did not suggest a structure for it however. Doering and Knox convincingly showed that it was tropylium (cycloheptatrienyhum) bromide (Figure 2.2). This ion is predicted to be aromatic by the Htickel rule. 

An obvious starting material for the preparation of tropylium salts is cycloheptatriene and, as described at the beginning of this chapter, the first method of preparation involved addition of bromine to cycloheptatriene, and the dibromo-adduct decomposed when heated to give tropylium bromide [2,3,16]. The same method has been used to prepare substituted tropylium salts, for exaaple carboxy- [17,18], phenyl- [19] and t-butyl-tropylium [19] salts. Dibromocyclohepta-diene also decomposes to give tropylium bromide when kept in liquid sulphur dioxide [20]. Heptaphenyltropylium bromide has been prepared by the action of bromine on heptaphenylcycloheptatriene [21]. 

This intermediate has also been trapped as a Diels-Alder adduct [137]. The dibromo-adduct does not have a C(l) - C(6) bond but the corresponding bromine adduct from 1,6-oxido[10]annulene is so linked this parallels the relative stabilities of the mono- and bi-cyclic forms of cycloheptatriene and oxepin [136], If another substituent is present at a 2-position of the ring, bromination appears to occur at the 5- and/or 7- positions if the substituent is electron-donating, and at the 10-position in the case of the 2-carboxylic acid [138]. 

When 1,3,5-cycloheptatriene reacts with one molar equivalent of bromine at 0°C, it undergoes 1,6 addition, (a) Write the structure of this product, (b) On heating, this 1,6-addition product loses HBr readily to form a compound with the molecular formula CyHyBr, called tropylium bromide. Tropylium bromide is insoluble in nonpolar solvents but is soluble in water it has an unexpectedly high melting point (mp 203 °C), and when treated with silver nitrate, an aqueous solution of tropylium bromide gives a precipitate of AgBr. What do these experimental results surest about the bonding in tropylium bromide ... 

PROBLEM 13.41 As early as 1891 the German chemist G. Merling showed that the bromination of 1,3,5-cycloheptatriene leads to a liquid dibromide. When the dibromide is heated,... 

This chapter reviews polycyclic arenes that contain seven-membered carbocycles. The parent compound of these seven-membered carbocycles is 1,3,5-cycloheptatriene (1), which occurs in a neutral polycyclic backbone commonly in the form of 1,3,5-cycloheptatrienylidene (2), as shown in Figure 4.1a. As first discovered by Doering and Knox [1], bromination of 1,3,5-cycloheptatriene followed by thermal elimination results in cycloheptatrienylium (or tropylium) ion, a non-benzenoid aromatic carbocycle (Figure 4.1b). 

When 1,3,5-cycloheptatriene is heated with bromine, a stable salt is formed, cycloheptatrienyl bromide. In this molecule, the organic cation contains six delocalized tt electrons, and the positive charge is equally distributed over seven carbons (as shown in the electrostatic potential map in the margin). Even though it is a carbocation, the system is remarkably unreactive, as is expected for an aromatic system. In contrast, the cycloheptatrienyl anion is antiaromatic, as indicated by the much lower acidity of cycloheptatriene (pA"a = 39) compared with that of cyclopentadiene. 

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