Alkane from hydrazones

Fig. 1.47. Myers synthesis of atkanes from aldehydes via the sulfonylated hydrazones B and their silyl derivatives C. This procedure provides a skeletongenerating three-step synthesis of alkanes from aldehydes. In terms of Figure 1.2, the chain reaction F —> I involved is a "substitution reaction by fragmentation." (The sulfonylated hydrazide anion, which here results from the addition of R3-Li to the sulfonyl hydrazone C, does—at this reaction s temperature of-78 °C—not release toluene sulfinate. Compare the different behavior of the sulfonylated hydrazide anion C in the reaction of Figure 17.69, where a mesitylene sulfinate residue is eliminated at a temperature that is higher by 160 °C).

A method of almost universal applicability for the deoxygenation of carbonyl compounds is the Wolff-Kishner reduction While the earlier reductions were carried out in two steps on the derived hydrazone or semicarbazone derivatives, the Huang-Minlon modification is a single-pot operation. In this procedure, the carbonyl compound and hydrazine (hydrate or anhydrous) are heated (180-220 °C) in the presence of a base and a proton source. Sodium or potassium hydroxide, potassium-t-butoxide and other alkoxides are the frequently used bases and ethylene glycol or its oligomers are used as the solvent and proton source. Over the years, several modifications of this procedure have been used to cater to the specific needs of a given substrate. The Wolff-Kishner reaction works well with both aldehydes and ketones and remains the most routinely used procedure for the preparation of alkanes from carbonyl compounds (Table 9). This method is equally suitable for the synthesis of polycyclic and hindered alkanes. 

Oxidation of the isohumulones with oxygen gas at 40°C produces a complex mixture, from which carbonyl derivatives have been isolated as 2,4-dinltrophenyl-hydrazones. To these belong alkanones from C3 to C- 1, alkanals from C2 to Ciq. 

Hydroperoxides have been obtained from the autoxidation of alkanes, aralkanes, alkenes, ketones, enols, hydrazones, aromatic amines, amides, ethers, acetals, alcohols, and organomineral compounds, eg, Grignard reagents (10,45). In autoxidations involving hydrazones, double-bond migration occurs with the formation of hydroperoxy—azo compounds via free-radical chain processes (10,59) (eq. 20). 

Mechanism. The aqueous base deprotonates the hydrazone, and the anion produced is resonance stabilized. The carbanion picks up a proton from water, and another deprotonation by the aqueous base generates an intermediate, which is set up to eliminate a molecule of nitrogen (N2), and produce a new carbanion. This carbanion is quickly protonated by water, giving the final reduced product as alkane. 

In the meanwhile, a new method for preparing various sec-alkane-phosphonates (103) from ketones by way of the hydrazone 101 and then the substituted tolylhydrazines 102, had been developed.57 By using this... 

Tosylhydrazones can be silylated with (fert-bufyldimethylsilyl)trifluoromethanesulfonate (Me2teriBuSi03SCF3) on the sulfonamide nitrogen. This is how the starting material for a broadly applicable alkane synthesis is produced ( Figure 1.49). Due to their ability to undergo reductive cyanization (Figure 17.69) mesitylene sulfonyl hydrazones allow for a two-step synthesis of nitriles from ketones. 

Starting from isolated hydrazones, reduction to the corresponding hydrocarbons by treatment with base in an aprotic solvent takes place at temperatures significantly below the 200 °C of the Huang-Minlon modification of the Wolff-Kishner reduction. However, hydra-zones cannot be prepared in a one-step reaction between a ketone and hydrazine, since usually azines (R1R2C=N=N=CR1R2) are formed instead. However, semicarbazones are hydrazone derivatives that are easily accessible by the reaction of a ketone with semicarbazide (for the mechanism, see Table 9.2). Semicarbazones can be converted into alkanes with KO/Bu in toluene at temperatures as low as 100 °C. This method provides an alternative to the Wolff-Kishner reduction when much lower than usual reduction temperatures are desirable. 

Cathodic reduction consists of electron supply from the cathode, followed by acquisition of protons by anionically charged substrates. Thus, ready accessibility of the reductive electron transfer processes leads to a variety of reactions, including reductive abstraction of halogens [13, 14], reduction of alcohols to alkanes [15], reduction of imines, oximes, and hydrazones to the corresponding amines [16], and conversion of nitro groups to the corresponding nitroso, hydroxy amino, and amino groups [17]. In... 

Scheme 9.12. In the top half, the formation of a hydrazone derivative from ethyl phenyl ketone and hydrazine is shown. In the lower half, the decomposition of the hydrazone in basic diethylene glycol to alkane and nitrogen is presented.

The third example in Table 9.4, that is, 3a-c, where G = NH2, describes the addition of hydrazine (H2NNH2) and substituted hydrazines (ArNHNH2) to the carbonyl (C=0) to produce the corresponding hydrazones ( hydrazine derivatives ) of the aldehyde or ketone with which the reaction began. It will be remembered that the hydrazone derived from hydrazine (H2NNH2) itself has already been encountered during the discussion of the reduction of ketones to alkanes (Scheme 9.12) as part of the Wolff-Kishner reaction sequence vide supra). 

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