A,S-Unsaturated ketones

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

It is possible to monitor the reaction and determine the end point by the absence of an a,/S-unsaturated ketone absorption in the UV or by the determination of the consumption of ca. one molar equivalent of hydrogen peroxide by permanganate titration. 

A solution of 0.1 mole of the a,/S-unsaturated ketone dissolved in 15 ml of anhydrous ether is added dropwise, whereupon the solution is rapidly decolorized. The stirring is continued for 4 hours during which time it slowly becomes blue or green. 

The effectiveness of LP-NM with respect to LP-DE has also been proven by the cycloaddition of ketals of a,)S-unsaturated ketones with open-chain and cycloaliphatic dienes [46]. In 4.0m LP-NM the Diels-Alder reaction occurs with good yields and selectivities without using CSA, which is absolutely necessary when the reaction is performed in LP-DE (Section 6.2.1). Some examples are illustrated in Scheme 6.25. 

Thus, like a, )S-unsaturated ketones and sulfones, both thiirene dioxides and thiirene oxides are preferentially attacked by the less basic nucleophiles on the vinylic carbon atom. This would lead to formally 1,4 Michael-type adducts and/or other products resulting from further transformations following the initial formation of the a-sulfonyl and a-sulfoxy carbanions. 

A third important group of reactions that may be discussed with the first two groups is not necessarily photosensitized. The dimerizations and additions of cyclic a,/S-unsaturated ketones can be initiated by direct n-n excitation of the ketone, followed by addition reactions. However, the reactions are efficiently photosensitized by triplet sensitizers, and it is reasonable to propose that the unsensitized cycloaddition reactions also proceed via triplet states. 8>63>94> Examples are given in Eqs. 28—... 

Supplemental References for Table 31. Asymmetric Organosilane Reduction of a,/S-Unsaturated Ketones... 

R)-product. Although there is still room for further improvement of the enantioselectivity, this first example of an enantioselective reaction with a,(S-unsaturated ketones reveals the potential of acylzirconocene chlorides as unmasked acyl anion donors. 

The ligand chosen to complex n-BuLi affects not only the configuration of the carban-ion but also the regioselectivity of the addition to a,/S-unsaturated ketones (Scheme 65). Depending on the chelating diamine, the ratio of 1,2- V5. 1,4-addition can range from 95 5 to 6 94. With (—)-sparteine, 1,4-addition to enones predominates when TMSCl is present. Diastereoselectivity in the formation of the two new stereocenters is >99 1, and enantioselectivity is >96%. Subsequent alkylation, in one pot, is also possible. ... 

Funabiki prepared a-fluoro-/ -phenylacrylaladehyde (92), a particularly useful synthetic intermediate, by an addition elimination strategy (Scheme 34) [81], Electrochemical fluorination of //-phenylsulfenyl-a, S-unsaturated ketones can form a-fluoro-a, S-unsaturated carbonyl compounds (Scheme 35) [82], a-Fluoro- S-thio-a, S-unsaturated ketone (93) was synthesized by the selective nucleophilic... 

Extending their earlier work on Vilsmeier formylation (c/. Vol. 7, p. 36), Traas et al. have formylated the a/S-unsaturated ketone isophorone almost exclusively at the more favoured exocyclic position to yield (190), which may be selectively... 

Another general method for the preparation of 19,19,19- and 20,20,20-trifluor-oretinals involves the aldol condensation of trifluoroacetone with an ethylenic aldehyde. When performed in the presence of acetic acid and piperidine, it affords an a, S-unsaturated ketone. Applied to cyclocitral and its homologues, the method leads to polyenic trifluoromethyl ketones. From these ketones, the various tri-fluorinated retinoids are prepared by known methods (Figure 4.24). ... 

The behavior of 2-methyl-2-octen-4-one demonstrates the speed with which a,/S-unsaturated ketones relax to unreactive excited states. The compound is essentially photoinert.273 No matter what kind of twisting occurs around the enone system, the free butyl group is readily accessible to the carbonyl oxygen. Nevertheless, no intramolecular hydrogen transfer takes place, indicating that any k,tt states formed decay very rapidly to a relatively unreactive state, presumably the T,r,7t triplet. 

As an extension of this research, Maruoka and coworkers succeeded in the catalytic asymmetric conjugate addition of nitroalkanes to cyclic a,[S-unsaturated ketones under phase-transfer conditions (Scheme 5.40) [39]. Here, the use of 3,5-bis(3,4,5-trifluorophenyl)phenyl-substituted catalyst (S,S)-lj is crucial for obtaining the high enantioselectivity. 

Chiral BINOL-indium(ni) complexes have been employed in several enantioselective allylations (i) in the ionic liquid, hexylmethylimidazolium-PF6, for aldehydes,190 (ii) a moisture tolerant version, for a wide variety of aldehyde types,191 and (iii) a recyclable example, useful for aromatic, aliphatic, and a,/S-unsaturated ketones.192... 

In theory, three isoxazolines are capable of existence 2-isoxazoline (2), 3-isoxazoline and 4-isoxazoline. The position of the double bond may also be designated by the use of the prefix A with an appropriate numerical superscript. Of these only the 2-isoxazolines have been investigated in any detail. The preparation of the first isoxazoline, 3,5-diphenyl-2 isoxazoline, from the reaction of /3-chloro-/3-phenylpropiophenone with hydroxylamine was reported in 1895 (1895CB957). Two major syntheses of 2-isoxazolines are the cycloaddition of nitrile JV-oxides to alkenes and the reaction of a,/S-unsaturated ketones with hydroxylamine. Since 2-isoxazolines are readily oxidized to isoxazoles and possess some of the unique properties of isoxazoles, they also serve as key intermediates for the synthesis of other heterocycles and natural products. 

The reaction was applied to the formation of arylcopper used for homocoupling and coupling reactions, which have already been described in Section IV.B.2. In addition, it was established that the simple use of copper(I) salts in polar solvents permitted the transmetallation from tin to copper. The transient vinylcopper reagent was subjected to various intramolecular reactions such as coupling with vinyl halides , addition to a, /S-unsaturated ketones , to a, /3-unsaturated esters and addition to a, /3-alkynic esters . In addition to copper(I) halides, the reaction can be mediated by copper(I) cyanide and... 

Metal catalyzed cross-coupling reactions with acylzirconocene chlorides and various organic halides or acetates can give rise to unsymmetrical ketones or a mixture of /3,y-and a ,/3-unsaturated ketones (Scheme 22). The advantage of the use of a Cu catalyst instead of a Pd catalyst is to avoid the undesirable formation of a ,/S-unsaturated ketone. ... 

Recently, Shchepin and coworkers described Michael addition reactions with zinc ketone enolates (144) for the preparation of chroman-2-one derivatives 146. The enolates were generated in situ from a-bromo ketones 143 by addition of excess zinc (equation 40), and subsequent conjugate addition to a, S-unsaturated ketones (145) (equation 40 ) provided chroman-2-ones (146a, b) in yields up to 82% . 

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