Chromium substituted alcohol oxidations

Although the fate of Cr(IV) is uncertain, (cf. the alcohol oxidation), some characteristics of the intermediate chromium species have been obtained by Wiberg and Richardson from a study of competitions between benzaldehyde and each of several substituted benzaldehydes. The competition between the two aldehydes for Cr(VI) is measured simply by their separate reactivities that for the Cr(V) or Cr(IV) is obtained from estimation of residual aldehyde by a C-labelling technique. If Cr(V) is involved then p values for oxidation by Cr(VI) and Cr(V) are 0.77 and 0.45, respectively. An isotope effect of 4.1 for oxidation of benzaldehyde by Cr(V) was obtained likewise. 

Sheldon and coworkers have developed chromium-substituted molecular sieves (CrAPO-5) as recyclable solid catalysts for several selective oxidations, among them also the allylic" and benzylic ° " ° " ° oxidations using TBHP or O2 as the terminal oxidants (equation 63), which yielded the corresponding benzylic ketones in moderate yield (conv. 13-70%) and moderate to good selectivity (41%, 65-97%). The benzylic alcohols were formed as side products. Allylic oxidation also proceeded with good conversions, while selectivities were lower and both possible products, the allylic ketone (31-77% selectivity) and the allylic alcohol (0-47% selectivity), were formed. Chromium sUicalite showed activity for selective benzylic oxidation in the presence of TBHP as well as giving mainly the allylic ketone (2-cyclohexen-l-one with 74% selectivity) and the allylic alcohol as minor product (2-cyclohexen-l-ol with 26% selectivity) -. ... 

Soluble chromium compounds are known to catalyze the allylic oxidation of olefins [22,23] and benzylic oxidations of alkyl aromatics [22,24] using tert-butyl-hydroperoxide as the primary oxidant. Chromium-substituted aluminophosphates, e. g. CrAPO-5, were shown to catalyze the allylic oxidation of a variety of terpene substrates with TBHP to give the corresponding enones [25,26]. For example, a-pinene afforded verbenone with 77% selectivity (Eq. 6) and 13% of the corresponding alcohol. 

Chromium(VI) catalyzes the oxidation of alcohols with alkyl hydroperoxides . Chromium-incorporated molecular sieves, in particular chromium-substituted aluminophosphate-5 (Cr-APO-5) were shown to be effective for the aerobic oxidation of secondary alcohols to the corresponding ketones (Reaction 19). This, and related catalysts, were first believed to be heterogeneous but more detailed investigations revealed that the observed catalysis is due to small amounts of soluble chromium that are leached from the framework by reaction with hydroperoxides. Reaction 19 may involve initial chromium-catalyzed free radical autoxidation of the alcohol to the a-hydroxyalkyl hydroperoxide followed by chromium-catalyzed oxygen transfer with the latter and/or H202 (formed by its dissociation) via an oxochromium(VI)-chromium(IV) cycle. 

The oxidative conversions described above probably proceed via formation of an N-l-oxymethyl intermediate which undergoes further oxidation to produce 2. If 1 is treated as before with chromium trioxide in the presence of excess methanol, A -methoxymethylvinblastine (56) is isolated in 64% yield (74). Additional N-l-alkyloxymethyl or cycloalkyloxy-methyl compounds have been prepared by substituting other alcohols for methanol in this reaction. 

The stability problems with the tetraalkyls also apply to the tetraalkoxides in addition, the high oxidative power of Cr means that alcoholysis with primary alcohols and most secondary alcohols leads to oxidation to the aldehyde or ketone and the formation of a chromium(III) alkoxide. Relatively stable chromium(IV) alkoxides are obtained only from tertiary alcohols and some heavily substituted secondary alcohols. 

With non-activated tin-carbon bonds such as alkyltins, strong oxidizing agents like chromium trioxide are usually required771. The reactions lead to the corresponding alcohols or ketones, depending on the substitution of the carbon bonded to the tin atom. 

Fig. 17.10. Mechanism of the Cr(VI) oxidation of alcohols to carbonyl compounds. The oxidation proceeds via the chromium(VI) acid ester A ("chromic acid ester") and yields chromium(IV) acid. The chromium(IV) acid may either disproportionate in an "inorganic" reaction or oxidize the alcohol to the hydroxy-substituted radical B. This radical is subsequently oxidized to the carbonyl compound by Cr(VI), which is reduced to Cr(V) acid in the process. This Cr(V) acid also is able to oxidize the alcohol to the carbonyl compound while it is undergoing reduction to a Cr(III) compound.

Although many oxidizing reagents remove the chromium tricarbonyl group, benzylic alcohols can be oxidized to benzaldehydes using dimethyl sulfoxide with acetic anhydride, trifluoroacetic anhydride, or sulfurtrioxide with minimal decomplexation. Asymmetric oxidation of aUcylthio-substituted complexes can be achieved using titanium tetraisopropoxide and an optically active tartrate ester (Scheme 108). Dimethyloxirane can also be used to oxidize sulfides to sulfoxides. 

Schemes have been devised to substitute less toxic metals for more toxic ones. Potassium ferrate on K10 mont-morillonite clay has been used to replace potassium chromate and potassium permanganate in the oxidation of alcohols to aldehydes and ketones in 54-100% yields.153 The potassium ferrate is made by the action of sodium hypochlorite on iron(III) nitrate or by treatment of iron(III) sulfate with potassium peroxymonosulfate.154 After the oxidation, any excess oxidizing agent, and its reduced form, are easy to recover by filtration or centrifugation. In another case, manganese-containing reagents have been substituted for more toxic ones containing chromium and selenium (4.21).155 Selenium dioxide was used formerly in the first step and pyridinium chlorochromate in the second.

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