Subject unsaturated carbonyl compounds

Dimerizations of oc,/ -unsaturated carbonyl compounds are perhaps the most interesting reactions, and certainly are the subjects of the most wide-spread investigations. Many are photosensitized, including that of coumarin, Eq. 50. 123> Other reactions of simple enones also involve... 

The epoxidation of electron-deficient alkenes, particularly a,P-unsaturated carbonyl compounds, continues to generate much activity in the literature, and this has been the subject of a recent concise review <00CC1215>. Additional current contributions in this area include a novel epoxidation of enones via direct oxygen atom transfer from hypervalent oxido-).3-iodanes (38), a process which proceeds in fair to good yields and with complete retention of... 

Whilst reactions of a, -unsaturated carbonyl compounds with 2 have been the subject of a number of studies, the corresponding reactions of their enolic tautomers have received little attention. Reaction of the /S-hydroxy-a, /i-unsaturated ketones... 

Recall that the LUMO shows which regions of a molecule are most electron deficient, and hence most subject to nucleophilic attack. One such region is over the carbonyl carbon, consistent with the observation that carbonyl compounds undergo nucleophilic addition at the carbonyl carbon. Another region is over the P carbon, again consistent with the known chemistry of a,P-unsaturated carbonyl compounds, in this case conjugate or Michael addition. 

A wide range of alkenes may be used as substrates. The reaction is most commonly performed with alkenes of normal electronic nature, but electron deficient alkenes, such as a, 3-unsaturated carbonyl compounds, and very electron rich alkenes, such as enol ethers and enamines, have also been used successfully. Not surprisingly, the cyclopropylcarbinyl ethers and amines that are formed in the latter reactions (see Table S) are subject to facile rearrangements. 

Substituted allylsilanes are subject to ene reaction with aldehydes and a,/3-unsaturated carbonyl compounds in the presence of a Lewis acid. The Et2AlCl-promoted reaction of /3-siloxymethyl-substituted allylsilane 27 with aldehydes gives more functionalized allylsilanes (Equation (37)).148 The use of TiCU instead of Et2AlCl leads to the Hosomi-Sakurai allylation. Catalytic enantioselective carbonyl-ene reactions of methallylsilanes have been achieved by using chiral Ti and A1 complexes.149,150... 

Since the disclosures that the thermal dimerizations of acrolein and methyl vinyl ketone provide the 3,4-dihydro-2//-pyrans (1, 2) derived from 4ir and 2Tt participation of the a,3-unsaturated carbonyl compound in a Diels-Alder reaction, an extensive series of related observations have been detailed. This work has been the subject of several comprehensive reviews - - including the Desimoni and Tacco-ni extensive tabular compilation of work through 1974. Consequently, the prior reviews should be consulted for thorough treatments of the mechanism, scope, and applications of the [4 + 2] cycloaddition reactions of a,3-unsaturated carbonyl compounds. The [4 + 2] cycloaddition reactions of 1-oxa-1,3-butadienes with their 4-it participation in the Diels-Alder reaction exhibit predictable regioselectivity with the preferential or exclusive formation of 2-substituted 3,4-dihydro-2W-pyrans (equation 1). The exceptions to the predicted regioselectivity that have been observed involve the poorly matched [4 + 2] cycloaddition reaction of an electron-deficient l-oxa-l,3-butadiene with an electron-deficient dienophile, e.g. methyl crotonate or methacrolein. - Rigorous or simplified theoretical treatments of the [4 + 2] cycloaddition reaction of 1-oxa-1,3-butadienes predict the preferential formation of 2-substituted 3,4-dihy-dro-2f/-pyrans and accommodate the preferred endo approach of the reactants in which the carbon-carbon bond formation is more advanced than carbon-oxygen bond formation, i.e. a concerted but nonsynchronous [4 + 2] cycloaddition reaction. ... 

Several ketene silyl acetals add in a conjugate fashion to a,/3-unsaturated carbonyl compounds (Mukaiyama-Michael reaction) in the presence of LiC104. The more sterically demanding the ketene silyl acetals used, the greater is the concentration of LPDE required. For example, ketene silyl acetal Si-2 subjected to an LPDE solution (1.0 m) of 2-cyclohexen-l-one (12) gave 87 % of the 1,4 adduct whereas Si-3 required... 

Protic-acid-catalyzed Michael additions (59) are subject to most of the limitations of base-catalyzed Michael additions (regioselectivity and stereoselectivity of enol generation, polyaddition, etc.), and hence, the stereochemistry has been little studied (60). At low temperatures silyl and stannyl enol ethers,+ ketene acetals, and allyl species are unreactive to all but the most reactive activated olefins. However, it was discovered by Mukaiyama and co-workers that enol ethers and ketene acetals react with a,/f-unsaturated carbonyl compounds in the presence of certain Lewis acids (4,61,62). Sakurai, Hosomi, and co-workers found that allylsilanes behave similarly (5,63,64). 

A(, -Dimethyl(methylene)iminium salts have been the most widely used class of preformed iminium salts, mainly due to their applications in the synthesis of a,p-unsaturated carbonyl compounds, normally accomplished by subjecting the /V, -dimethyl Mannich base to quatemarization followed by base-induced elimination. Table 3 outlines various counterion forms of /V, -dimethyl(methylene)iminium salts that have been used in Mannich reactions as well as their synthetic precursors. The crystalline iodide (30), known also as Eschenmoser s salt , has seen the most widespread use and is prepared by thermal fragmentation of (iodomethyl)trimethylammonium iodide or, more conveniently, by a variant of the... 

Since the thermal dimerizations of acrolein and methyl vinyl ketone were shown to provide the 3,4-dihydro-2//-pyrans l,12 an extensive range of related observations have been disclosed. This work has been the subject of several reviews.3 14 Only the work reported since the extensive Desimoni and Tacconi account5 of the Diels-Alder reaction of a,/3-unsatu-rated carbonyl compounds, 1-oxabutadienes bearing an oxygen atom at the diene terminus, has been detailed herein. The prior reviews should be consulted for an excellent discussion of the mechanism, scope, and application of the [4 + 2] cycloaddition reactions of a,/3-unsaturated carbonyl compounds as well as for extensive tabular compilations of the work through 1974.3-6... 

The Michael addition of secondary amines to a,)S-unsaturated carbonyl compounds under MWI essentially required the presence of water to drive the amine addition to completion and the presence of at least 10 mole equivalents of the amine. Besides increasing the polarity and possibly some micellar effect, water facilitates protonation of the resulted enolate from amine addition. In a typical procedure, benzalacetophenone was mixed with morpholine (469) and water in a teflon flask and subjected to MWI for 2 min to yield 93% of the 1,4-adduct 471 (Scheme 92). The method was also extended to other morpholine derivatives 472 and 473 in 30% and 100% yields, respectively (00SC643). 

Imines of a-jd-unsaturated carbonyl compounds (1-azadienes) 239 can be subjected to [4 + 2] cycloadditions with ketenes to give rise to dihydropyr-idones 240. When the imine moiety is part of a conjugated system, both 1,2- and 1,4-cycloadducts can be obtained. Although challenging [2 + 2] cycloaddition to the imine moiety of azadienes infrequendy leads to product mixtures containing jd-lactams 241 (Scheme 75), the [4 + 2] cycloaddition commonly proceeds smoothly, giving the products in satisfactory yield at room temperature or sl hdy elevated temperatures via an uncatalyzed reaction (1970TL245). 

Fluorescence characteristics of some common polymers have been listed [505]. Fluorescence analysis of polyolefins has been the subject of much controversy but is generally now considered to be associated with the presence of low levels of cyclic a, -unsaturated carbonyl compounds of the enone or enal type [514]. Fluorescence and phosphorescence excitation and emission of LLDPE, HDPE and mPE were reported [515]. 

The reaction of a-bromo-a,p-unsaturated ketones with diethyl phosphonate and triethylamine affords p,y-unsaturated ketones selectively (Scheme 2.5) [15]. This process comprises reduction of the bromide and deconjugation of an olehnic moiety. a-Bromo-a,p-unsaturated carbonyl compounds are readily prepared by treatment of l,l-dibromo-2-siloxycyclopropanes with diethyl phosphonate. This finding makes it possible to prepare p.y-unsaturated ketones from l,l-dibromo-2-siloxycy-clopropanes in one pot. As one of the plausible reaction paths, the a-bromo-p,y-unsaturated ketone, arising from the intervention of the dienol, is subjected to debromination with diethyl phosphonate and triethylamine to give the p,y-unsaturated ketone. 

We begin by introducing the chemistry of enolates and enols. Especially important is a reaction between enolate ions and carbonyl compounds, called the aldol condensation. This process is widely used to form carbon-carbon bonds both in the laboratory and in nature. Among the possible products of aldol condensation are a,/3-unsaturated aldehydes and ketones, which contain conjugated carbon-carbon and carbon-oxygen tt bonds. As expected, electrophilic additions may take place at either bond. However, more significantly, a, -unsaturated carbonyl compounds are also subject to nucleophilic attack, a reaction that may involve the entire conjugated system. 

The preparation of a,p-unsaturated carbonyl compounds is a subject of great interest among synthetic organic chemists. The aldol method is perhaps the most elegant way to generate such molecules, but there are other ways. Design a synthesis of but-2-enal from butanal. 

Ti -Allyl(dicarbonyl)(nitrosyl)iron complexes can also be reacted with electrophiles. Subjection to a,P-unsaturated carbonyl compounds leads to 5,E-unsaturated carbonyl compounds (Scheme 4-91). For this transformation, the allyliron complexes can even be generated in situ from haloalkenes and TBAFe. ... 

Johnson et al have used the availability of the above reaction to study further the overall mechanism of the reaction of sulphonium and oxysul-phonium ylides with carbonyl compounds and a 3-unsaturated carbonyl compounds. The contrasts between these two types of ylides are well known and can be exemplified as follows (i) in reactions with cyclic carbonyl compounds, sulphonium ylides attack from the axial position while sulphoxonium ylides attack from the equatorial position (ii) in reactions with a/3-unsaturated ketones, sulphonium ylides react at the carbonyl group to form oxirans while sulphoxonium ylides react at the olefinic group to form cyclopropanes. These results can be explained by considering the reversibility of the initial step of each ylide reaction and whether the reaction is subject to kinetic or thermodynamic control. 

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