A,P-Unsaturated aldehydes and

Aldol condensation offers an effective route to a p unsaturated aldehydes and ketones These compounds have some interesting properties that result from conjugation of the carbon-carbon double bond with the carbonyl group As shown m Figure 18 6 the rr systems of the carbon-carbon and carbon-oxygen double bonds overlap to form an extended rr system that permits increased electron delocalization... 

FIGURE 18 7 Nucleophilic addition to a p unsaturated aldehydes and ketones may take place either in a 1 2 or 1 4 manner Direct addition (1 2) occurs faster than conjugate addition (1 4) but gives a less stable product The product of 1 4 addition retains the carbon-oxygen double bond which is in general stronger than a carbon-carbon double bond... 

The preparation and some synthetic applications of lithium dialkylcuprates were described earlier (Section 14 11) The most prominent feature of these reagents is then-capacity to undergo conjugate addition to a p unsaturated aldehydes and ketones... 

The formation of pyridine derivatives from a, P-unsaturated aldehydes and ammonia involves formation of three bonds during the ring synthesis. For example, with an a, P-unsaturated aldehyde, both 2,5-substituted as well as 3,4-substituted pyridines can be obtained, depending on whether a 1,2- (eq. 17) or 1,4-addition (eq. 18) occurs with ammonia. Reactions are performed in the vapor phase with catalysts. 

Dijbner-von Miller Synthesis. A much less violent synthetic pathway, the Dn bner-von Miller, uses hydrochloric acid or 2inc chloride as the catalyst (43). As in the modified Skraup, a,P-unsaturated aldehydes and ketones make the dehydration of glycerol uimecessary, and allow a wider variety of substitution patterns. No added oxidant is required. With excess aniline the reaction proceeds as follows ... 

Xanthates have been added to activated double bonds, eg, acryhc derivatives and a,P-unsaturated aldehydes and ketones (59—61) ... 

The diminished tt electron density in the double bond makes a,p-unsaturated aldehydes and ketones less reactive than alkenes toward electrophilic addition. Electrophilic reagents—bromine and peroxy acids, for example—react more slowly with the carbon-carbon double bond of a,(3-unsaturated carbonyl compounds than with simple alkenes. 

The product is a P-hydroxy aldehyde (called an aldol) or ketone, which in some cases is dehydrated during the course of the reaction. Even if the dehydration is not spontaneous, it can usually be done easily, since the new double bond is in conjugation with the C=0 bond so that this is a method of preparing a,P-unsaturated aldehydes and ketones as well as P-hydroxy aldehydes and ketones. The entire reaction is an equilibrium (including the dehydration step), and a,P-unsaturated and P-hydroxy aldehydes and ketones can be cleaved by treatment with OH (the retrograde aldol reaction). There is evidence that an SET mechanism can intervene when the substrate is an aromatic ketone. ... 

When unsymmetrical ketones were used in this reaction (with BF3 as catalyst), the less highly substituted carbon preferentially migrated. The reaction can be made regioselective by applying this method to the a-halo ketone, in which case only the other carbon migrates. The ethyl diazoacetate procedure has also been applied to the acetals or ketals of a, P-unsaturated aldehydes and ketones. ... 

W. W. Frenier and F. B. Growcock. Mixtures of a,P-unsaturated aldehydes and surface active agents used as corrosion inhibitors in aqueous fluids. Patent US 4734259, 1988. 

A series of functionalized tetrahydrothiophenes has been prepared by acid induced organocatalytic reactions involving a,p-unsaturated aldehydes and 2-mercapto-l-phenylethanone 24. This procedure led to good yields of products displaying useful enantiomeric excess, as illustrated by construction of the target 25 in the presence of the catalyst 26 <06JA14986>. 

Choppin GR (2003) Actinide speciation in the environment. Radiochim Acta 91 645-649 Claus P (1998) Selective hydrogenation of a,P-unsaturated aldehydes and other C = O and C = C bonds containing compounds. Top Catal 5 51-62 Colon D, Weber EJ, Anderson JL, Winget P, Suarez LA (2006) Reduction of nitrobenzenes and N-hydroxylanilines by Fe(II) species Elucidation of the reaction mechanism. Environ Sci Technol 40 4449-4454... 

C02Et Br KOf-Bu/THF/-20°C aliphatic and aromatic aldehydes aliphatic, cycloaliphatic and aromatic ketone, a,p-unsaturated aldehydes and ketones 52-90... 

The Catalysis Concept of Iminium Activation In 2000, the MacMillan laboratory disclosed a new strategy for asymmetric synthesis based on the capacity of chiral amines to function as enantioselective catalysts for a range of transformations that traditionally use Lewis acids. This catalytic concept was founded on the mechanistic postulate that the reversible formation of iminium ions from a,p-unsaturated aldehydes and amines [Eq. (11.10)] might emulate the equilibrium dynamics and 7i-orbital electronics that are inherent to Lewis acid catalysis [i.e., lowest unoccupied molecular orbital (LUMO)-lowering activation] [Eq. (11.9)] ... 

Another type of Cinchona alkaloid catalyzed reactions that employs azodicarbo-xylates includes enantioselective allylic amination. Jprgensen [51-53] investigated the enantioselective electrophilic addition to aUyhc C-H bonds activated by a chiral Brpnsted base. Using Cinchona alkaloids, the first enantioselective, metal-free aUyhc amination was reported using alkylidene cyanoacetates with dialkyl azodi-carboxylates (Scheme 12). The product was further functionalized and used in subsequent tandem reactions to generate useful chiral building blocks (52, 53). Subsequent work was applied to other types of allylic nitriles in the addition to a,P-unsaturated aldehydes and P-substituted nitro-olefins (Scheme 13). 

Jprgensen has also developed a one-pot three component coupling of 1,3-dicarbonyl compounds, a,P-unsaturated aldehydes and primary amines to give a series of Hantzsch ester analogues [99],... 

Additionally, investigations into imidazolidinone catalysed Diels-Alder reactions (Schemes 2 and 6) [234] have shown that iminium ions of a,P-unsaturated aldehydes and ketones have lower activation barriers for the Diels-Alder reaction with cyclopentadiene than the parent compound (13 and 11 kCal mol", respectively). It was also noted that transition structures show the formation of the bonds is concerted but highly asynchronous. 

Substrate scope - can a universal catalyst be developed for the activation of any a,P-unsaturated aldehyde and ketone with multiple substitutions ... 

Enantiopure sugar-derived tetrahydroquinolines can be synthesized through a Bi(OTf)3-catalyzed reaction of 8-hydroxy-a, p-unsaturated aldehydes and aryl amines [130] (Fig. 31). 

For a review, sec Nigh, in Trahanovsky Oxidation in Organic Chemistry, pt. B Academic Press New York, 1973. pp. 67-81. Cupric chloride has been used to chlorinate a.p-unsaturated aldehydes and ketones in the y position Dietl Normark Payne Thweatl Young Tetrahedron Lett. 1973, 1719. 

Marnett, L.J. (1994) DNA adducts of a,P-unsaturated aldehydes and dicarbonyl compounds. In Hemminki, K., Dipple, A., Shuker, D.E.G, Kadlubar, F.F., Segerback, D. Bartsch, H., eds, DNA Adducts Identification and Biological Significance (lARC Scientific Publications No. 125), Lyon, lARC, pp. 151-163... 

As we have already seen, delocalization of electrons by conjugation decreases the energy difference between the HOMO and LUMO energy levels, and this leads to a red shift. Alkyl substitution on a conjugated system also leads to a (smaller) red shift, due to the small interaction between the cr-bonded electrons of the alkyl group with the K-bond system. These effects are additive, and the empirical Woodward-Fieser rules were developed to predict the 2max values for dienes (and trienes). Similar sets of rules can be used to predict the A ax values for a,P-unsaturated aldehydes and ketones (enones) and the Amax values for aromatic carbonyl compounds. These rules are summarized in Table 2.4. 

Using the Woodward-Fieser rules, the of dienes and trienes, a,p-unsaturated aldehydes and ketones (enones), and for aromatic carbonyl compounds can be reliably calculated and predicted. 

We chose the methyl-3, butenal or prenal as a-p unsaturated aldehyde and we carried out the reaction in gaseous phase on a polycrystalline sample of pure platinum in order to avoid any support effect. The objectives of this study were the following ... 

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