Oxidation-reduction reactions, carbonyl compounds

Perhaps the most valuable reaction of alcohols is their oxidation to yield car-bony compounds—the opposite of the reduction of carbonyl compounds to yield alcohols. Primary alcohols yield aldehydes or carboxylic acids, secondary alcohols yield ketones, but tertiary alcohols don t normally react with most oxidizing agents. 

The catalytic effect of metal ions such as Mg2+ and Zn2+ on the reduction of carbonyl compounds has extensively been studied in connection with the involvement of metal ions in the oxidation-reduction reactions of nicotinamide coenzymes [144-149]. Acceleration effects of Mg2+ on hydride transfer from NADH model compounds to carbonyl compounds have been shown to be ascribed to the catalysis on the initial electron transfer process, which is the rate-determining step of the overall hydride transfer reactions [16,87,149]. The Mg2+ ion has also been shown to accelerate electron transfer from cis-dialkylcobalt(III) complexes to p-ben-zoquinone derivatives [150,151]. In this context, a remarkable catalytic effect of Mg2+ was also found on photoinduced electron transfer reactions from various electron donors to flavin analogs in 1984 [152], The Mg2+ (or Zn2+) ion forms complexes with a flavin analog la and 5-deazaflavins 2a-c with a 1 1 stoichiometry in dry MeCN at 298 K [153] ... 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

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.

The reaction differs from the Ritter reaction by the two types of electrophilic activation of the reagents and by the two types of rearrangement of nitrilium 285 and carboxonium ions 288 (equation 94). Besides, this interaction proceeds at an oxidation level of two, while the Ritter reaction occurs at an oxidation level of one17. While it may be shown that A-acyliminium ions 365 can be obtained from a carbonyl compound and a nitrile via the Ritter reaction, then it is only the second step b) in a three-step process where the first step (a) is the reduction of carbonyl compound 364 to alcohol 366 and the third step (c) is an oxidative dehydrogenation of amide 369 obtained3 (equation 105). 

In fact, a variation of this reaction has been utilized in the well-known Meerwein-Ponndorf-Verley reduction of carbonyl compounds (reverse of Oppenauer oxidation of alcohols) by aluminum isopropoxide The reaction involves a six-centered transition state, wherein the P-hydride is delivered into an incoming carbonyl group [Eq. (6.86)]. The stereochemistry of this reaction has been studied in detail. ... 

The Meerwein-Ponndorf-Verley reduction of carbonyl compounds and the Oppenauer oxidation of alcohols, together denoted as MPVO reactions, are considered to be highly selective reactions. For instance, C=C double bonds are not attacked. In MPV reductions a secondary alcohol is the reductant whereas in Oppenauer oxidations a ketone is the oxidant. It is generally accepted that MPVO reactions proceed via a complex in which both the carbonyl and the alcohol are coordinated to a Lewis acid metal ion after which a hydride transfer from the alcohol to the carbonyl group occurs (Fig. 1) [1]. Usually, metal ec-alkoxides are used as homogeneous catalysts in reductions and metal t-butoxides in oxidations [1]. 

This reaction was first reported concurrently by Meerwein and Schmidt and Verley in 1925, and by Ponndorf in 1926, respectively. It is an aluminum alkoxide-catalyzed reduction of carbonyl compounds (ketones and aldehydes) to corresponding alcohols using another alcohol (e.g isopropanol) as the reducing agent or hydride source. Therefore, it is generally known as the Meerwein-Ponndorf-Verley reduction (MPV) or Meerwein-Ponndorf-Verley reaction. Occasionally, it is also referred to as the Meerwein-Ponndorf reduction, Meerwein-Ponndorf reaction, or Meerwein-Schmidt-Ponndorf-Verley reaction. About 12 years later, Oppenauer reported the reversion of this reaction in which alcohols were reversely oxidized into carbonyl compounds. Since then, the interchanges between carbonyl compounds and alcohols in the presence of aluminum alkoxide are generally called the Meerwein-Ponndorf-Oppenauer-Verley reduction or Meerwein-Ponndorf-Verley-Oppenauer reaction." ... 

Many biological processes involve oxidation of alcohols to carbonyl compounds or the reverse process, reduction of carbonyl compounds to alcohols. Ethanol, for example, is metabolized in the liver to acetaldehyde in a reaction catalyzed by the enzyme alcohol... 

CHAPTER 12 ALCOHOLS FROM CARBONYL COMPOUNDS Oxidation-Reduction and Organometaiiic Compounds 12.20 Predict the major organic product from each of the following reactions. 

The radicals are often themselves electroactive and can therefore undergo further reductitMi or oxidation to form anions or cations, respectively. Radicals can also be produced in other ways. One well-known example of this is alkyl halides. They are reduced to radicals at a cathode by a process in which electron transfer and bond breaking are concerted. The electrogenerated radical is often then reduced to a carbanimi (Fig. 2). Reduction of carbonyl compounds in acidic media proceeds via the conjugate acid of the carbonyl compound (Fig. 2), which can then undergo reduction to a ketyl radical. This entry will address the variety of ion radicals and radicals that can be readily generated electrochemically, as well as typical chemical reactions. 

The addition of ozone (O3) to alkenes to give a primary ozonide (molozonide), which rearranges to an ozonide and eventually leads, on reduction, to carbonyl compounds (aldehydes and/or ketones), has already been mentioned and the reaction itself is shown in Scheme 6.11. However, it is important to recognize that this is only one example of a 4th- 2n electrocyclic addition and that orbital overlap for many sets of these reactions dictates their courses as well. Thus, to show the similarity of some of these dipolar 3 -f 2 addition reactions Equations 6.53-6.56 are provided. Although any alkene might be used as an example, (Z)-2-butene is used in each to emphasize that aU of them occur with retention of stereochemistry and, in the first (Equation 6.53), the reaction with ozone to form the primary ozonide (molozonide) is presented again (i.e., see Scheme 6.11). In a similar way, with a suitable azide, R-N3, readily prepared from an alkyl halide (Chapter 7), the same alkene forms a triazoline (Equation 6.54) and with nitrous oxide (N2O) the heterocycle (Chapter 13) cis -4,5-dimethyl-A -l,2,3-oxadiazoline (ds-4,5-dihydro-4,5-dimethyl-l,2,3-oxadiazole) (Equation 6.55). Finally, with a nitrile oxide, such as the oxide derived from ethanenitrile (acetonitrile [CH3ON]), the same alkene yields a different heterocycle, the dihydroisoxazole, 3,4,5-trimethyl-4,5-dihydroisoxazole (Equation 6.56). 

FIGURE 16.65 Alcohols can be oxidized to carbonyl compounds. This reaction is the reverse of what you have just learned— the reduction of carbonyl compounds to alcohols through reactions with metal hydride reagents. 

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