Arene from aryl alkyl ketones

Another category of reductions involves aryl ketones. The Friedel-Crafts acylation reaction reacts benzene with an acid chloride such as butanoyl chloride (49) to give an aryl ketone, 50. Complete removal of the oxygen from this ketone constitutes a method to make straight-chain arenes, which cannot be prepared via Friedel-Crafts alkylation (see Section 21.3.2). At least two classical methods are used to accomplish this reaction, which is formally a reduction. If 50 is treated with zinc metal in HCl, the product is 1-phenylbutane, 105. This acidic reduction involves a mineral acid such as HCl and an active metal, and it is called the Clemmensen reduction. 

A related approach involves the direct decarbonylation of stable ketones. DauguUs and Brookhart demonstrated that the rhodium-catalyzed decarbonylation of diaryl ketones was feasible [11]. Efficient extrusion of CO from alkyl aryl ketones to form alkylarenes was easily achieved by rhodium(I) catalysis directed by apyridyl ortho to the RCO group (Scheme 22.6) [12]. (CO)2Rh(acac) was found to be the optimal catalyst and the methodology had a broad substrate scope. This method offers an alternative way to synthesize alkyl benzenes through an ARCIS reaction, complementary to the known Friedel-Crafts alkylation reaction of arenes. 

Aryl- and alkyl-carboxylic dihalophosphoric anhydrides acylate activated arenes without the need for a conventional Friedel-Crafts catalyst. The anhydrides are prepared in situ, and the low temperatures required (20 C) allow reasonable yields of ketones to be obtained from mixed anhydrides of low stability, for example those derived from acetic and pivalic acids [equation (8)]. 

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