Retro-Friedel-Craft alkylation

Mohanty et al. were the first to introduce pendent r-butyl groups in die polymer backbones. The resulting material was quite soluble in aprotic dipolar solvents.83 The PEEK precursors were prepared under a mild reaction condition at 170°C. The polymer precursor can be converted to PEEK in die presence of Lewis acid catalyst A1C13 via a retro Friedel-Crafts alkylation. Approximately 50% of die rerr-butyl substitutes were removed due to die insolubility of the product in die solvent used. Later, Risse et al. showed diat complete cleavage of f< rf-butyl substitutes could be achieved using a strong Lewis acid CF3SO3H as both die catalyst and the reaction medium (Scheme 6.15).84... 

Resole resin syntheses, 398-406 Retro Friedel-Crafts alkylation, 342 Riffle, J. S 375 Rigid block copolymers, 53 Rigid foams... 

Macknight, E., McClelland, R. A. A photochemical retro-Friedel-Crafts alkylation. Rapid rearrangement of cyclohexadienyl cations. Can. J. Chem. 1996, 74, 2518-2527. 

The only way to functionalize the upper rim of 4-f-butylcalix[4]arene is to remove the f-butyl group and replace it with something more useful. Two methods have been reported by which this de-ferf-butylation, sometimes referred to by the more evocative term of neutering , may be achieved. The first method is probably the more conventional of the two and involves the use of aluminium trichloride and phenol. It is in essence a Friedel-Crafts reaction where a f-butyl group is attached to either benzene or phenol the innovation is that the source of the f-butyl substituent comes from the calixarene [1]. This retro-Friedel-Crafts reaction is thus an effective method for transalkylation that removes an unwanted alkyl group from calix[4]arenes as shown in Figures 3.10 and 3.11. Moreover, it is possible to subtly alter the reaction conditions and partially de-ferf-butylate... 

As mentioned in the introduction to this chapter, retro-Friedel-Crafts is the basic disconnection of the Ar-CO bond in alkyl-aryl ketones. Hereafter, we will discuss two examples where problems of regioselectivity in acylation and reactivity of the aromatic ring are tackled. 

The retro-synthetic analysis ouflined in Scheme 6.1 shows that in addition to the Pictet-Spengler route previously used by discovery, several others are also possible. The Bischler-Napieralski condensation of 5-methyltryptamine and 2-cyclohexylacetic acid chloride as well as the Friedel-Crafts reaction of the N-protected 5-methyltryptamine allows preparation of the prochiral imine derivative 7. The 2-cyclohexylmethyl side chain may also be introduced via alkylating an unsubstituted THpC by 2-cyclohexylmethyl chloride. As shown in Scheme 6.1, all of these routes involve 5-methyltryptamine as a common building block. 

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