Allylic from simple alkenes

The hydroxymercuration-demercuration of conjugated dienes generally does not afford monohydration products selectivity, but diols can sometimes be obtained in reasonable yield. The direction of addition of H—OH is that expected by extrapolation from simple alkenes and allylic alcohols. 

A related reaction of selenium in its +4 oxidation state (as selenium dioxide, Se02) allows us to make allylic alcohols and enals from simple alkenes. The overall reaction is the simple oxidation shown in the margin, but the route by which the compound gets there involves two successive pericyclic reactions. 

Heterogeneous palladium catalysts proved to be active in the conversion of simple alkenes to the corresponding allylic acetates, carbonyl compounds, and carboxylic acids.694 704 Allyl acetate or acrylic acid from propylene was selectively produced on a palladium on charcoal catalyst depending on catalyst pretreatment and reaction conditions.694 Allylic oxidation with singlet oxygen to yield allylic hydroperoxides is discussed in Section 9.2.2. 

When simple alkenes were employed as reaction partners for silenes of the type (Me3Si)2Si=C(OSiMe3)R1, silacyclobutanes were obtained, provided that no allylic hydrogen is present in the alkene. In the reaction with alkenes with allylic hydrogens the ene reaction becomes predominant (see Table 11). Thus, while the reaction with styrene exclusively gives the four-membered ring compound 454, with 1-methylstyrene the ene products 455 were obtained (equation 144). Similarly, from the reaction of 150 with 1-octene only the ene product 456 was isolated (equation 145). 

The reaction was also successful for substituted salicylaldehydes. When Jacobsen came to develop his asymmetric epoxidation, which, unlike the Sharpless asymmetric epoxidation, works for simple alkenes and not just for allylic alcohols, he chose salens as his catalysts, partly because they could be made so easily from salicylaldehydes. For example ... 

Similar conclusions are drawn by Cvetanovic et al. from their results of ozonization of alkenes in the gas phase (9) and in CC14 solution (10). The rate constants for the ozonolysis of chloroethylenes and allyl chloride, in CC14 solution, indicate (11) that the rate of ozone attack decreases rapidly as the number of chlorine atoms in the olefin molecules is increased. However, to explain the departures from simple correlations, in some cases steric effects and the dipolar character of ozone had to be invoked (10). The relevance of the dipolar character of ozone in its reactions has also been stressed by Huisgen (12), who provided evidence that the ozone—olefin reaction is usually a 1,3-dipolar cycloaddition. 

In general, the anodic oxidation of simple alkenes in nucleophilic solvents yields products resulting from both allylic substitution and oxidative addition of nucleophiles. Cyclohexene has been studied extensively as starting compound. The anodic oxidation of cyclohexene in methanol or acetic acid... 

Cp)2Cr(NO)2]2 dehalogenates vicinal dibromoalkanes to form alkenes this complex does not abstract halogen from simple alkyl or allyl halides. 

In several instances, Mannich-type cyclizations can be carried out expeditiously under photochemical conditions. The photochemistry of iminium ions is dominated by pathways in which the excited state im-inium ion serves as a one-electron acceptor. The photophysical and photochemical ramifications of such single-electron transfer (SET) processes as applied to excited state iminium ions have been expertly reviewed. In short, one-electron transfer to excited state iminium ions occurs rapidly from one of several electron donors electron rich alkenes, aromatic hydrocarbons, alcohols and ethers. Alternatively, an excited state donor, usually aromatic, can transfer an electron to a ground state iminium ion to afford the same reactive intermediates. Scheme 46 adumbrates the two pathways that have found most application in intramolecular cyclizations. Simple alkenes and aromatic hydrocarbons will typically suffer addition processes (pathway A). However, alkenic and aromatic systems with allylic or benzylic groups more electrofugal than hydrogen e.g. silicon, tin) commonly undergo elimination reactions (pathway B) to generate the reactive radical pair. 

The previous section described metal catalyzed epoxidation of allylic alcohols by alkyl hydroperoxides, and 193 was proposed as a model to predict the diastereoselectivity of these reactions,. In the cases presented, the reaction was diastereoselective but not enantioselective (sec. 1.4.F) and those epoxidation reactions generated racemic epoxides. To achieve asymmetric induction one must control both the relative orientation of the alkene relative to the peroxide and also the face of the substrate from which the electrophilic oxygen is delivered. Control of this type can be accomplished by providing a chiral ligand that will also coordinate to the metal catalyst, along with the peroxide and the alkene unit. There are two major asymmetric epoxidation reactions, one that can be applied only to allylic alcohols and is the prototype for asymmetric induction in these systems. The other is a procedure that can be applied to simple alkenes. Both procedures use a metal-catalyzed epoxidation that employs alkyl hydroperoxides, introduced in section 3.4.B.ii. 

The Sharpless asymmetric epoxidation is reliable, but it works only for allylic alcohols. There is an alternative, however, which works with simple alkenes. The method was developed by Eric Jacobsen and employs a manganese catalyst with a chiral ligand built from a simple diamine. The diamine is not a natural compound and has to be made in enantiomeric form by resolution, but at least that means that both enantiomers are readily available. The diamine is condensed with a derivative of salicylaldehyde to make a bis-imine known as a salen. ... 

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