Alkenes, cyclic => allylic alcohols

Numerous chiral cyclic allyl alcohol derivatives have been used as the chiral alkene part in 1,3-dipolar cycloadditions. In general, the more rigid conformational... 

When the alkene is neither terminal nor conjugated, addition appears to occur the other way round - to give a (3-alkoxylithium such as 34 - which eliminates Li20 under the conditions of the reaction.26 Cyclic allylic alcohols similarly give alkenes by elimination, and geminally disubstituted allylic alcohols such as 35 are simply unreactive.25... 

Many epoxidising agents, notably mCPBA and f-BuOOH/VO(acac)2, are capable of bonding to the OH group of an allylic alcohol. Epoxidation then occurs more readily on allylic alcohols and on the same side of the alkene as the OH groups, rather in the style of the Simmons-Smith reaction. The stereochemical outcome is easiest to see in cyclic allylic alcohols. Thus the cyclohexenol 120 gives the syn epoxide 121 cleanly with mCPBA via the conformation 122 in which the axial OH delivers the reagent to the same face of the alkene. 

The Simmons-Smith reaction is influenced by a suitably situated hydroxy group in the alkene substrate. With allylic and homoallylic alcohols or ethers, the rate of the reaction is greatly increased and, in five- and six-membered cyclic allylic alcohols, the product in which the cyclopropane ring is cis to the hydroxy group is formed stereoselectively (4.89). These effects are ascribed to co-ordination of the oxygen atom to the zinc, followed by transfer of methylene to the same face of the adjacent double bond. 

If an alkene has a hydroxyl (or other functional group capable of donating a proton in a hydrogen bonding interaction) in the allylic position, that group can affect the stereochemistry of the epoxide product. For example, reaction of cyclic allylic alcohols produces a 10 1 ratio of product with the epoxide cis to the alcohol fimction (equation 9.55) relative to the trans product. This diastereoselectivity has been attributed to hydrc en bonding in the transition structure for the epoxidation (Figure... 

Wilkinson s catalyst has also been utilized for the hydroboration of other alkenes. Sulfone derivatives of allyl alcohol can be hydroborated with HBcat and subsequently oxidized to give the secondary rather than primary alcohol. This reactivity proves to be independent of substituents on the sulfur atom.36 Similarly, thioalkenes undergo anti-Markovnikoff addition to afford a-thioboronate esters.37 The benefits of metal-catalyzed reactions come to the fore in the hydroboration of bromoalkenes (higher yields, shorter reaction times), although the benefits were less clear for the corresponding chloroalkenes (Table 3).38,39 Dienes can be hydroborated using both rhodium and palladium catalysts [Pd(PPh3)4] reacts readily with 1,3-dienes, but cyclic dienes are more active towards [Rh4(CO)i2].40... 

The observation of stable alkyl cations in antimony pentafluoride solutions also opened up the possibility of investigating the electronic spectra of these solutions. It has been reported73 that solutions of alkyl cations in HSC F-SbFs solution at — 60°C showed no absorption maxima above 210 nm. In view of this observation, it was resolved that previous claims relating to a 290-nm absorption of alcohols and alkenes in sulfuric acid solutions were due to condensation products or cyclic allylic ions and not to simple alkyl cations.74... 

Benzylic and allylic positions are hydroxylated by CPO in halide-dependent catalytic transformations. Toluene and p-xylene are oxidized to the respective aldehydes and carboxylic acids [247, 248]. Ethylbenzene and other substrates with longer alkyl chains form the respective benzylic/allylic alcohols with high enantio-selectivity. Straight-chain aliphatic and cyclic (Z)-alkenes are hydroxylated, favoring small unsubstituted substrates in which the double bond is not more than two carbon atoms from the terminus. Steric control is observed for benzylic hydroxylations. 

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