Ketone to alkyl benzenes

Oemmensen reduction (Section 18.14B) A method to reduce aryl ketones to alkyl benzenes using Zn(Hg) in the presence of a strong acid. 

This method is used for the reduction of acyl benzenes to alkyl benzenes, but it also reduces aldehydes and ketones to alkanes. 

Ketones formed as products in Friedel-Crafts acylation can be reduced to alkyl benzenes by two different methods. 

Xanthenyliumsodium (from sodamide and xanthene) reacts with aziridines to give a mixture of 9-mono- (271) or 9,9-di-substituted xanthenes (272).Addition of perchloric acid to unsymmetric allenes such as the 9-xanthylidene derivative (273) (prepared by a new route from 9,9-dichloroxanthene and an alkene) gave a red xanthylium salt (274), which was converted into a colourless spiro-indene (275) on heating.Full details have now been published of the properties and of the reactions of 9-diazoxanthene (276) and 9-xanthylidene (278) with methyl acrylate, substituted styrenes, several ketones, and alkyl-benzenes. The kinetics of the reaction with styrenes were studied and the conversion of (276) into (278) was achieved by photolysis of the tosylhydrazone (277) at —25 °C. 

Note that the Wolff-Kishner reduction accomplishes the same overall trans-fonnation as the catalytic hydrogenation of an acylbenzene to yield an alkyl-benzene (Section 16.10). The Wolff-Kishner reduction is more general and more useful than catalytic hydrogenation, however, because it works well with both alkyl and atyl ketones. 

Rhodium(III) complexes [e.g. (i-Pr,P)2Rh(H)Cl2] in the presence of quaternary ammonium salts are excellent catalysts for the hydrogenolysis of chloroarenes under mild conditions [5] other labile substituents are unaffected. Hydrodehalogenation of haloaryl ketones over a palladium catalyst to give acylbenzenes is also aided by the addition of Aliquat [6]. In the absence of the phase-transfer catalyst, or when the hydrogenation is conducted in ethanol, the major product is the corresponding alkyl-benzene, which is also produced by hydrodehalogenation of the halobenzyl alcohols. 

The photocyclization of o-alkoxy phenyl ketones to yield benzofuranols (57 and 58) represents one of the earliest example of 8-H-abstraction from the lowest n, n triplet Wagner et al. [18] have provided detailed photokinetic data studying the photocyclization of a variety of o-alkoxyphenyl ketones 56, and have revealed that quantum efficiency for cyclization for 56d was the lowest (0.023) and that for 56f the highest (1.00). The diastereoselectivity for cyclization of 56 was found to be higher in benzene and lower in polar solvents. From the estimated kH values (0.6-25 x 106 s 1), it was inferred that the low rate constant for 56e (8 x 106 s ) compared to that for 56g (25 x 106s 1) i s due to the alkyl chain in the alkoxy groups that points away from the o-carbonyl moiety in the most populated equilibrium conformations (Table 8.1). 

Hydrogen abstraction from alkyl benzenes occurs efficiently by using aromatic ketones The mechanism of the reaction has been extensively studied, with ketones having both a mi and a jiji state as the lowest triplet, and found to involve some degree of electron transfer, which grows with more easily reduced ketones [32,33]. The same reaction occurs intramolecularly, e.g., in the photoinduced hydrogen transfer in 2-methyl-benzophenone to give the (trappable) enol [34-36]. 

This two-step sequence can synthesize many alkylbenzenes that are impossible to make by direct alkylation. For example, we saw earlier that n-propylbenzene cannot be made by Friedel-Crafts alkylation. Benzene reacts with n-propyl chloride and AICI3 to give isopropylbenzene, together with some diisopropylbenzene. In the acylation, however, benzene reacts with propanoyl chloride and A1C13 to give ethyl phenyl ketone (propiophenone), which is easily reduced to n-propylbenzene. 

Hammond and Cole reported the first asymmetric photosensitized geometri-r cal isomerization with 1,2-diphenylcyclopropane (Scheme 2) [29]. The irradiation of racemic trans-1,2-diphenylcylcopropane 2 in the presence of the chiral sensitizer (R)-N-acetyl-1 -naphthylethylamine 4 led to the induction of optical activity in the irradiated solution, along with the simultaneous formation of the cis isomer 3. The enantiomeric excess of the trans-cyclopropane was about 1% in this reaction. Since then, several reports have appeared on this enantiodifferentiating photosensitization using several optically active aromatic ketones as shown in Scheme 2 [30-36]. The enantiomeric excesses obtained in all these reactions have been low. Another example of a photosensitized geometrical isomerization is the Z-E photoisomerization of cyclooctene 5, sensitized by optically active (poly)alkyl-benzene(poly)carboxylates (Scheme 3) [37-52]. Further examples and more detailed discussion are to be found in Chap. 4. 

Three common procedures are available for the transformation of aldehydes and ketones to hydrocarbons (1) reduction by zinc and hydrochloric acid (Clemmensen), (2) reduction by hydrazine in the presence of a base (Wolff-Kishner), and (3) catalytic hydrogenation. In view of the complicated mixtures obtained by the polyalkylation of benzene by the Friedel-Crafts reaction (method 1), reduction of alkyl aryl ketones is the most reliable method for the preparation of di- and poly-alkylbenzenes. 

Aldehydes and ketones can be reduced to hydrocarbons by the action (a) of amalgamated zinc and concentrated hydrochloric acid, the Clemmensen reduction or (b) of hydrazine, NH2NH2, and a strong base like KCm or potassium teh-butoxide, the Wolff-Kishner reduction. These are particularly important when applied to the al faTryl ketones obtailTed from Friedel-Crafts acylation, since this reaction sequence permits, indirectly, the attachment of straight alkyl chains to the benzene ring. For example ... 

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