Rearrangement of aldehydes and ketones

Acid-Catalyzed Rearrangements of Aldehydes and Ketones 1 /Alkyl,2/alkyl-interchange, etc. 

The pinacol rearrangement reaction is of limited synthetic importance although it can be a useful alternative to the standard methods for synthesis of aldehydes and ketones." Especially in the synthesis of ketones with special substitution pattern—e.g. a spiro ketone like 5—the pinacol rearrangement demonstrates its synthetic potential ... 

Sulfur ylides can also transfer substituted methylene units, such as isopropylidene (Entries 10 and 11) or cyclopropylidene (Entries 12 and 13). The oxaspiropentanes formed by reaction of aldehydes and ketones with diphenylsulfonium cyclopropylide are useful intermediates in a number of transformations such as acid-catalyzed rearrangement to cyclobutanones.285... 

Alkyl- and aryl-hydrazones of aldehydes and ketones readily peroxidise in solution and rearrange to azo hydroperoxides [1], some of which are explosively unstable [2], Dry samples of the p-bromo- and p-fluoro-hydroperoxybenzylazobenzenes, prepared by oxygenation of benzene solutions of the phenylhydrazones, exploded while on filter paper in the dark, initiated by vibration of the table or tapping the paper. Samples were later stored moist with benzene at —60°C to prevent explosion [3], A series of a-phenylazo hydroperoxides derived from the phenyl-or p-bromophcnyl-hydrazones of acetone, acetophenone or cyclohexanone, and useful for epoxidation of alkenes, are all explosive [4], The stability of several substituted phenylazo hydroperoxides was found to be strongly controlled by novel substituent effects [5],... 

Amines are prepared by aminolysis of alkyl halides, and also reductive amination (reduction in the presence of ammonia) of aldehydes and ketones (see Section 5.7.19). They are obtained conveniently from Hofmann rearrangement of amides. 

Diphenylsulfonium cyciopropanide undergoes addition to the C — C double bond of oc,/ -unsat-urated carbonyl compounds to produce spiropentanes,57,58 and to the C —O double bond of aldehydes and ketones to produce oxaspiropentanes. These can be isolated59,60 or rearranged in situ 57,61,62 to produce cyclobutanones. Dimethylaminocyclopropylphenyloxosulfonium cyciopropanide reacts analogously.63,64... 

Also, hydrogen transfer through ring intermediates has been suggested as a mechanism for intramolecular rearrangements in the photolysis of aldehydes and ketones 84... 

Lanthanide Lewis acids catalyze many of the reactions catalyzed by other Lewis acids, for example, the Mukaiyama-aldol reaction [14], Diels-Alder reactions [15], epoxide opening by TMSCN and thiols [14,10], and the cyanosilylation of aldehydes and ketones [17]. For most of these reactions, however, lanthanide Lewis acids have no advantages over other Lewis acids. The enantioselective hetero Diels-Alder reactions reported by Danishefsky et al. exploited one of the characteristic properties of lanthanides—mild Lewis acidity. This mildness enables the use of substrates unstable to common Lewis acids, for example Danishefsky s diene. It was recently reported by Shull and Koreeda that Eu(fod)3 catalyzed the allylic 1,3-transposition of methoxyace-tates (Table 7) [18]. This rearrangement did not proceed with acetates or benzoates, and seemed selective to a-alkoxyacetates. This suggested that the methoxy group could act as an additional coordination site for the Eu catalyst, and that this stabilized the complex of the Eu catalyst and the ester. The reaction proceeded even when the substrate contained an alkynyl group (entry 7), or when proximal alkenyl carbons of the allylic acetate were fully substituted (entries 10, 11 and 13). In these cases, the Pd(II) catalyzed allylic 1,3-transposition of allylic acetates was not efficient. 

Due emphasis will be laid on the discussion of photochemical rearrangements. Greater attention has been paid to these important and interesting reactions only during the last decade in spite of the early pioneering work of Norrish. However, today they constitute the most rapidly expanding part of the photochemistry of aldehydes and ketones. 

Several theoretical studies have considered LiH addition as a model for the more computationally difficult L1A1H4 and NaBH4 [45]. However, reaction of aldehydes and ketones with LiH seldom if ever leads to reduction [46]. AIH3, while less commonly used than the complex boron and aluminium hydrides, is useful for reducing carbonyls [47] and therefore is a suitable model for computational study. Calculations [2, 5] show that gas-phase reduction of formaldehyde by AIH3 occurs by formation of complex 1, which rearranges via a four-centre transition state to form an aluminium methoxide product. Two conformational isomers of... 

Although not of much real preparative interest, radical decarbonylation has been observed with a large number of aldehydes and ketones at high temperatures and/or under ultraviolet irradiation or in the presence of peroxides. For example, carbon monoxide is eliminated in 90% yield when 3-methyl-3-phenylbutanal is heated for five hours at 130° in the presence of di-tert-butyl peroxide fractionation of the residue then affords 70% of a 1 1 mixture of terr-butyl- and isobutyl-benzene, so that the / ,/ -dimethyl-phenethyl radical formed on elimination of carbon monoxide from the C6H5C(CH3)2CH2CO— radical must be assumed to have partially rearranged.66... 

Fluorination of alcohols. Primary, secondary, and tertiary alcohols are converted by (1) into the corresponding fluorides the reaction occurs readily at 10° or below. One advantage of this reagent is that products of dehydration and carbonium ion rearrangement are formed in smaller amounts than with SF4, SeF4 pyridine (5, 576-577), and (C2Hs)2NCF2CHClF (5,214-216). It is also useful for fluorination of aldehydes and ketones that are sensitive to acid. 

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