Alcohols reactivity with alkenes

The use of sofid supports in conjunction with permanganate reactions leads to modification of the reactivity and selectivity of the oxidant. The use of an inert support, such as bentonite (see Clays), copper sulfate pentahydrate, molecular sieves (qv) (151), or sifica, results in an oxidant that does not react with alkenes, but can be used, for example, to convert alcohols to ketones (152). A sofid supported permanganate reagent, composed of copper sulfate pentahydrate and potassium permanganate (153), has been shown to readily convert secondary alcohols into ketones under mild conditions, and in contrast to traditional permanganate reactivity, the reagent does not react with double bonds (154). 

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

Primary nitro compounds are good precursors for preparing nitriles and nitrile oxides (Eq. 6.31). The conversion of nitro compounds into nitrile oxides affords an important tool for the synthesis of complex natural products. Nitrile oxides are reactive 1,3-dipoles that form isoxazolines or isoxazoles by the reaction with alkenes or alky nes, respectively. The products are also important precursors for various substrates such as P-amino alcohols, P-hydroxy ketones, P-hydroxy nitriles, and P-hydroxy acids (Scheme 6.3). Many good reviews concerning nitrile oxides in organic synthesis exist some of them are listed here.50-56 Applications of organic synthesis using nitrile oxides are discussed in Section 8.2.2. 

The reactivities of carbenes toward alkenes have been correlated with the inductive and resonance effects of the carbene substituents, log k — a Eat + fcEaR+ + c.m Analogous correlations cannot be obtained for the reaction rates of carbenes with alcohols, neither with the substituent parameters used by Moss,109 nor with related sets.110 In particular, the substituent parameters do not describe the strong, rate-enhancing effect of aryl groups. For a detailed analysis, see the discussion of proton affinities (Section V.A). 

The reactivities of [Ru "(0)(14-TMC)(X)]"+ and its related 15-TMC, 16-TMC, and CRMes coi lexes with organic substrates have also been examined. " " In contrast to polypyridyl Ru =0 species, these macrocyclic Ru =0 complexes are weak oxidants. They oxidize benzyl alcohol to benzaldehyde but do not react with alkenes at room temperature. The lower oxidizing ability of these systems than the polypyridyl systems is due to their lower values. However, [Ru (0)(H20)(N202)](C104)2, which has a higher H value, is able to catalyze the oxidation of norbornylene, styrene, and cyclooctene by PhlO. " ... 

Since reactivity of alkenes increases with increasing alkyl substitution, hydration is best applied in the synthesis of tertiary alcohols. Of the isomeric alkenes, cis compounds are usually more reactive than the corresponding trans isomers, but strained cyclic isomeric olefins may exhibit opposite behavior. Thus, for example, frans-cyclooctene is hydrated 2500 times faster than cw-cyclooctene.6 Similar large reactivity differences were observed in the addition of alcohols to strained trans cycloalkenes compared with the cis isomers. frans-Cycloheptene, an extremely unstable compound, for instance, reacts with methanol 109 faster at —78°C than does the cis compound.7... 

Ozone reacts slowly with saturated hydrocarbons usually to give alcohols.92 93 The reactivity of alkanes toward ozone is several orders of magnitude less than that of alkenes. Oxidation of saturated hydrocarbons takes place preferentially at the tertiary carbon. In liquid-phase ozonation94 the order of reactivity of the primary, secondary and tertiary C—H bonds is 1 13 110. The formation of tertiary alcohols occurs with high degree (60-94%) of stereoselectivity.94-96... 

The initial coordination of reactants has indeed been proposed to explain the selective oxidation of alkenes in the presence of saturated hydrocarbons. It was argued that, owing to the hydrophobic nature of titanium silicates, the concentration of both hydrocarbons inside the catalyst pores is relatively high and hence the alkenes must coordinate to TiIv. Consequently, the titanium peroxo complex will be formed almost exclusively on Tilv centers that already have an alkene in their coordination sphere, and will therefore oxidize this alkene rather than an alkane which may be present in the catalyst (Huybrechts et al., 1992). Objections to this proposal are based on the fact that the intrinsically higher reactivity of alkenes with respect to saturated hydrocarbons is sufficient to account for the selectivity observed (Clerici et al., 1992). But coordination around the titanium center of an alcohol molecule, particularly methanol, is nevertheless proposed to explain the formation of acidic species, as was previously discussed. In summary, coordination around Tiiv could play a more important role than it does in solution chemistry as a consequence of the hydrophobicity of the environment where the reactions take place. 

A somewhat similar oxidation of terminal alkenes to methyl ketone and alcohol by 02 in the presence of Co(salMDPT) [salMDPT = bis(salicylideneiminopropyl)methylamine] and in ethanol solvent has recently been reported by Drago and coworkers (equation 244).560 Only terminal alkenes were found to be reactive with this catalytic system. The reaction is alcohol dependent and occurs in ethanol and methanol but not in t-butyl or isopropyl alcohols. The alcohol is concomitantly oxidized during the reaction, and may act as a coreducing agent and/or favor the formation of cobalt hydride. This oxidation might occur according to the mechanism of equation (243). 

Surprisingly, the kinetic measurements now available for the nucleophilic trapping of carbocations with water are not always matched by measurements of rate constants for formation of the carbocation from the corresponding alcohol required to evaluate the equilibrium constant AR. Although carbocations are reactive intermediates in the acid-catalyzed dehydration of alcohols to form alkenes,85,86 the equilibrium in this reaction usually favors the alcohol and the carbocation forming step is not rate-determining. Rate constants may... 

The conventional resinsulfonic acids such as sulfonated polystyrenes (Dowex-50, Amberlite IR-112, and Permutit Q) are of moderate acidity with limited thermal stability. Therefore, they can be used only to catalyze alkylation of relatively reactive aromatic compounds (like phenol) with alkenes, alcohols, and alkyl halides. Nafion-H, however, has been found to be a suitable superacid catalyst in the 110-190°C temperature range to alkylate benzene with ethylene (vide infra) 16 Furthermore, various solid acid catalysts (ZSM-5, zeolite /3, MCM-22) are applied in industrial ethylbenzene technologies in the vapor phase.177... 

When olefins are used as alkylating agents, the catalytic activity of Nafion-H slowly decreases, most probably due to some polymerization on the surface, which deactivates the catalytic sites. The activity decreases faster when more reactive branched alkenes are used. The use of alcohols instead of olefins as the alkylating agents improves the lifetime of the catalyst. With alcohols, no ready polymerization takes place, since water formed as byproduct inhibits polymerization of any olefin formed (by dehydration) but does not affect the acidity of the catalyst at the reaction temperatures. 

In trimethylenemethane complexes, the metal stabilizes an unusual and highly reactive ligand which cannot be obtained in free form. Trimethylenemethanetricar-bonyliron (R=H) was the first complex of this kind described in 1966 by Emerson and coworkers (Figure 1.2) [38]. It can be obtained by reaction of bromomethallyl alcohol with Fe(CO)5. Trimethylenemethaneiron complexes have been applied for [3+2]-cycloaddition reactions with alkenes [39]. 

The rate constants of the reaction of 2,6-dimethyloct-7-en-2-ol separately with ozone and hydroxyl radical, in the gas phase, have been determined. The OH radical can either abstract hydrogen or add to the double bond. Ozone adds to the double bond. The formation of acetone, 2-methylpropanal, 2-methylbutanal, ethanedial, and 2-oxopropanal was discussed.191 The rate laws and activation parameters for the ozone oxidation of alcohols in aqueous solution have been determined and explained on the basis of formation of an ozone-alcohol complex.192 The reactivity of alkenes towards ozone, in aqueous solution, correlates well with Taft s equation.193... 

Recall that alkyl substituents on the double bond increase the reactivity of alkenes toward electrophilic addition. Propene therefore reacts faster than ethylene with sulfuric acid, and the mixture of alkyl hydrogen sulfates is mainly isopropyl hydrogen sulfate, and the alcohol obtained on hydrolysis is isopropyl alcohol. 

Like the radicals in Chapter 39, carbenes are extremely reactive species. As you have just seen, they are trapped by alcohols to make etheVs, but more importantly they will react with alkenes to make cyclopropanes, and they will also insert into C-H bonds. 

The hydrides are reactive and readily react with halogenocarbons, alcohols, and ketones alkenes afford alkyls. 

Asyimnetric hydrogenation of prochiral ketones is an important method for the preparation of chiral secondary alcohols. Until recently, however, such reactions were limited to substrates with pendent metal binding sites, like /3-keto esters. Many of the catalysts that efficiently hydrogenate C-C double bonds exhibit little or no reactivity with isolated ketones. This discrepancy may be ascribed to the different binding modes of alkenes and ketones, and the chemoselectivity of catalysts for these groups. While substrates with C-C double bonds can form metal... 

Hydroboration. When these two reagents are mixed in THF, hydrogen is evolved and a solution of a violet titanium-boron complex forms. This complex catalyzes the hydroboration of alkenes with LiBH4 to form lithium alkylboro-hydrides, which are converted to alcohols by NaOCHj and H2O2. The relative reactivity of alkenes is terminal > cyclic > internal. The reaction involves anti-Markownikoff addition. 

---