Carbonyl compounds olefin regioselectivity

The proposed reaction course involves the initial Michael-type addition of the olefin to the Lewis acid activated unsaturated carbonyl compound, forming regioselectively a selenocarbenium ion (i). Subsequent 1,2-silyl migration (//), Se-bridging Hi) and 1,3-ring... 

For internal olefins, the Wacker oxidation is sometimes surprisingly regioselective. By using aqueous dioxane or THF, oxidation of P,y-unsaturated esters can be achieved selectively to generate y-keto-esters (Eq. 3.18).86 Under appropriate conditions, Wacker oxidation can be used very efficiently in transforming an olefin to a carbonyl compound. Thus, olefins become masked ketones. An example is its application in the synthesis of (+)-19-nortestosterone (3.11) carried out by Tsuji (Scheme 3.5).87... 

The Peterson olefination reaction involves the addition of an a-silyl substituted anion to an aldehyde or a ketone followed by the elimination of silylcarbinol either under acidic (awP -elimination) or basic (syn-elimination) conditions to furnish olefins178. Thus, Peterson olefination, just like Wittig and related reactions, is a method for regioselective conversion of a carbonyl compound to an olefin. Dienes and polyenes can be generated when the Peterson reaction is conducted using either an ,/l-unsaturated carbonyl compound or unsaturated silyl derivatives as reaction partners (Table 20)179. 

Alkenyliodonium salts can be used as highly reactive substrates for Heck-type olefination and similar palladium-catalyzed cross-coupling reactions [63 -65]. In a recent example, a series of dienes 80 were stereo- and regioselectively prepared by a palladium-catalyzed Heck-type reaction of alkenyliodonium salts 79 with a,/3-unsaturated carbonyl compounds (Scheme 37) [64]. 

The simple diastereoselectivity of the photocycloaddition of electronically excited carbonyl compounds with electron rich olefins was studied as a function of the substituent size—at identical starting conditions ignoring the electronic state involved in the reaction mechanism [123], The [2+2] photocycloaddition of 2,3-dihydrofuran with different aldehydes in the nonpolar solvent benzene resulted in oxetanes 118 with high regioselectivity and suprising simple diastereoselectivites the addition to acetaldehyde resulted in 45 55 mixture of endo and exo diastereoisomer, with increasing the size of the ot-carbonyl substituent (Me, Et, i-Bu, t-Bu), the simple diastereoselectivity increased with preferential formation of the endo stereoisomer (Sch. 37). 

The formation of cyclobutanes starting from a,P-unsaturated carbonyl compounds and olefins is referred to as the de Mayo reaction. The carbonyl component is excited upon irradiation by ti <—ti transition. The regioselectivity is determined by orbital coefficients and polarity effects that depend on the solvent, but the stereochemical information of the olefin is not preserved in intermolecular processes, indicating the non-concerted character of a triplet reaction. In this case 1,4-biradical intermediates are formed and the most stable one determines the stereochemistry of the main product. The cycloadducts of pentanones are most often cis-fused while hexanones preferentially give transcycloadducts. ... 

Cycloadditions of carbonyl compounds to olefins generally involve exci-plexes and biradicals. (Cf. Section 7.4.4.) While normal olefins frequently yield a number of products, photoinduced electron transfer may be utilized in the case of electron-rich olefins to influence the regioselectivity. Thus, irradiation of the ketene acetal 165 and biacetyl (164) yields exclusively the oxetane 167. Since the radical cation 166 could be trapped, electron transfer... 

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). 

Reactions in Ionic Liquids. NBS in an ionic liquid has been used to oxidize benzylic alcohols to carbonyl compounds to convert olefins to v/c-bromohydrins and for the regioselective monobromination of aromatic substrates. ... 

Homer-Wadsworth-Emmons reactions encounter serious problems of reactivity, stereoselectivity, and generality when used to form tetrasubstituted olefins. Most studies on the synthesis of tetrasubstituted olefins exploit alternative routes based on carbometalation of alkynes [17-19]. However, these processes have difficulties associated with the regioselectivity of the initial carbometalation and with reactivity during the final coupling. Here we describe stereoselective s3mtheses of tetrasubstituted olefins, via the olefination of carbonyl compounds with ynolates, as well as some synthetic applications [20,21]. 

The photocycloaddition of triplet benzophenone to norbornene was originally reported by Scharf and Korte. The photoproduct 101 that is formed in high exo-selectivity could be thermally cleaved to the 5,e-unsaturated ketone 102, an appHcation of the carbonyl-olefin metathesis (COM) concept. The 1,4-biradical formed in the interaction of norbornene with o-dibenzoyl-benzene was trapped in an intramolecular fashion by the second carbonyl moiety. A highly regioselective reaction of triplet benzophenone was reported with 5-methylenenorborn-2-ene, with preferential attack toward the exo CC double bond. A number of publications have discussed the photocycloaddition reactions of triplet carbonyl compounds to norbornadiene and quadricyclane, as weU as the competition between the Paterno-Biichi reaction and the sensitized norbornadiene/quadricyclane interconversion. Oxetane formation has also been reported for the photoreaction of biacetyl and para-quinones with benzvalene. ... 

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