Conjugated carbonyl compounds, formation

The formation of an oxirane from the reaction of dimethylsulfonium methylide with conjugated carbonyl compounds was applied to... 

Although definitive evidence for the intermediacy of oxabicyclobutanes in the peracid oxidation of cyclopropenes is lacking, the production of conjugated carbonyl compounds from such reactions is most readily rationalized from the formation and spontaneous rearrangement of such species (equation Calculations suggest... 

Baylis-Flillman reaction Formation of a C-C single bond between the a-position of conjugated carbonyl compounds or conjugated carboxylic acid derivatives and aldehydes or ketones. 48... 

Where, as illustrated in the above described example, the Diels-Alder reaction provides avenues into the formation of C-glycosides from furan-derived dienes, the hetero Diels-Alder reaction allows for the direct formation of sugar rings from carbonyl groups. As shown in Scheme 7.5.2, Schmidt, et al.,25 effected reactions between conjugated carbonyl compounds and olefins. The illustrated reaction proceeded in 81% yield giving an adduct which, after further manipulations, was converted to a C-aryl glycoside. 

Transfer hydrogenation. - Mediated by Pd/C, alkenes readily accept hydrogen from ammonium formate. Conjugated carbonyl compounds are more readily reduced. ... 

DFT calculations have been used to obtain mechanistic insights into the reaction of sulfur ylides PhHC (S+Me2) with dienals and enones by identifying all key transition states and intermediates along the reaction pathway for the 1,2-, 1,4-, and 1,6-nucleophilic attacks at PhCH=CHCH=CHCH=0 and for the 1,2- and 1,4-attacks at MeCH=CHCOMe. The final outcome of the reaction with both substrates has been found to be decided by the interplay between kinetic and thermodynamic factors. Thus, addition of a semi-stabilized ylide to conjugated carbonyl compounds prefers the 1,4-pathway under thermodynamic conditions, in consonance with the experimental reports. However, the formation of epoxides via a 1,2-addition pathway is equally competitive and could be favoured under kinetic conditions. The 2,3-trans cyclo-propanecarbaldehyde is the major product of the 1,4-addition pathway. The enone also prefers the 1,4-addition. ... 

In the reaction with a Br0nsted-Lowry acid or a Lewis acid, a carbonyl reacts similarly to alkenes. The major difference is formation of the oxocarbenium ion products, which are resonance stabilized, whereas a simple noncon-jugated alkene forms a carbocation (a carbenium ion) that is not resonance stabilized. Some alkenes are exceptions to this statement. When the C=C unit is conjugated to another x-bond, as in styrene (phenylethene, 25), reaction with an acid generates a resonance-stabilized carbocation, 26. A conjugated carbonyl compound such as benzaldehyde (27) will also form a conjugated oxocarbenium ion (28) that is resonance stabilized as shown in the illustration. 

In a well-known reaction of conjugated carbonyl compounds, a nucleophile adds to the C=C unit. The formation of 38 previously used the C=C unit of the conjugated system as a Brpnsted-Lowry base, donating electrons to the acidic proton. It is known from Chapter 16 that the acyl carbon will accept electrons in a reaction with a nucleophile. Imagine that the 7i-system effectively extends the reactivity of the carbonyl to the C=C unit then, the C-C should also react with a nucleophile. The extension of points of reactivity by conjugating n-bonds is called vinylogy. 

Why do the nucleophiles listed in Table 19.1 react with conjugated carbonyl compounds by 1,4-addition rather than 1,2-addition The answer has to do with kinetic control versus thermodynamic control of product formation. It has been shown that 1,2-addition of nucleophiles to the carbonyl carbon of a, 8-unsaturated carbonyl compounds is faster than conjugate addition. If formation of the 1,2-addition product is irreversible, then that is the product observed. If, however, formation of the 1,2-addition product is reversible, then an equilibrium is established between the more rapidly formed but less stable 1,2-addition product and the more slowly formed but more stable 1,4-addition product. As mentioned at the beginning of the chapter, a carbon-oxygen double bond is stronger than a carbon-carbon double bond. Thus, under conditions of thermodynamic (equilibrium) control, the more stable 1,4-Michael addition product is formed. 

A. Hydrogenation of Alkenes (and Aryls) B. Formation of Aryls C. Alkylations and Arylations of Alkenes D. Conjugate Reduction of Conjugated Carbonyl Compounds and Niuiles E. Conjugate Alkylations F. Cyclopropanations, including balocyclopropaiiations... 

Oda, K., Nakagami, R., Nishizono, N., and Machida, M., Pyridine ring formation through the photoreaction of arenecarbothioamides with diene-conjugated carbonyl compounds,/. Chem. Soc., Chem. Commun., 2371,1999. 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

The 1,4-addition of an enolate anion 1 to an o ,/3-unsaturated carbonyl compound 2, to yield a 1,5-dicarbonyl compound 3, is a powerful method for the formation of carbon-carbon bonds, and is called the Michael reaction or Michael addition The 1,4-addition to an o ,/3-unsaturated carbonyl substrate is also called a conjugate addition. Various other 1,4-additions are known, and sometimes referred to as Michael-like additions. 

The reaction conditions needed for aldol dehydration are often only a bit more vigorous (slightly higher temperature, for instance) than the conditions needed for the aldol formation itself. As a result, conjugated enones are usually obtained directly from aldol reactions without isolating the intermediate jS-hydroxy carbonyl compounds. 

A number of methods are available for following the oxidative behaviour of food samples. The consumption of oxygen and the ESR detection of radicals, either directly or indirectly by spin trapping, can be used to follow the initial steps during oxidation (Andersen and Skibsted, 2002). The formation of primary oxidation products, such as hydroperoxides and conjugated dienes, and secondary oxidation products (carbohydrides, carbonyl compounds and acids) in the case of lipid oxidation, can be quantified by several standard chemical and physical analytical methods (Armstrong, 1998 Horwitz, 2000). 

Dimethylsulfonium methylide is both more reactive and less stable than dimethylsulfoxonium methylide, so it is generated and used at a lower temperature. A sharp distinction between the two ylides emerges in their reactions with a, ( -unsaturated carbonyl compounds. Dimethylsulfonium methylide yields epoxides, whereas dimethylsulfoxonium methylide reacts by conjugate addition and gives cyclopropanes (compare Entries 5 and 6 in Scheme 2.21). It appears that the reason for the difference lies in the relative rates of the two reactions available to the betaine intermediate (a) reversal to starting materials, or (b) intramolecular nucleophilic displacement.284 Presumably both reagents react most rapidly at the carbonyl group. In the case of dimethylsulfonium methylide the intramolecular displacement step is faster than the reverse of the addition, and epoxide formation takes place. 

Besides direct nucleophilic attack onto the acceptor group, an activated diene may also undergo 1,4- or 1,6-addition in the latter case, capture of the ambident enolate with a soft electrophile can take place at two different positions. Hence, the nucleophilic addition can result in the formation of three regioisomeric alkenes, which may in addition be formed as E/Z isomers. Moreover, depending on the nature of nucleophile and electrophile, the addition products may contain one or two stereogenic centers, and, as a further complication, basic conditions may give rise to the isomerization of the initially formed 8,y-unsaturated carbonyl compounds (and other acceptor-substituted alkenes of this type) to the thermodynamically more stable conjugated isomer (Eq. 4.1). 

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