Alkane from carbonyls

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. 

Pd-catalyzed cross-coupling reactions were studied in one-pot multicatalytic processes to synthesize disubstituted alkenes and alkanes from carbonyl derivatives [68]. The use of Cu-catalyzed methylenation reactions was the key starting reaction to produce terminal alkenes that are not isolated but submitted to further structure elongation (hydroboration followed by Suzuki cross-coupling) (Figure 1.13). These processes have been used to synthesize methoxylated ( )-stilbenoids (i.e., ( )-1,3-dimethoxy-5-(4-methoxystyryl)benzene). 

Ruthenium catalysts prepared from Ru3(CO)i2 and other related carbonyl-derived complexes have been widely used in reactions of hydrogenolysis, homologation and dimerization of alkanes [114—116]. Catalysts derived from carbonyl precursors usually show higher catalytic activities than conventionally prepared supported mthenium catalysts. This correlates well with the smaller crystallite size achieved by using carbonyl precursors. 

Transition-metal-catalyzed carbonylations are of great importance in organic synthesis as a powerful tool to prepare a variety of carbony compounds. Among them, hydroformylation has been most extensively studied not only in the laboratory but also in industry. Industrial production of alkanals from 1-alkenes... 

From the three direct [2 + l]-cycIoaddition routes, path a employing electronrich olefins and acceptor-substituted carbenes is the most efficient one, since the alkenes can be synthesized from carbonyl compounds or other precursors and the carbenes are produced from easily available diazo alkanes. Therefore this very flexible mode to construct donor-acceptor substituted cyclopropanes is by far the most frequently used route. 

Beginning with reductions, we must remember that this class of reactions results in a decrease in the molecules oxidation state. In other words, there is a net loss of functionality on the molecule being reduced. This loss of functionality may be manifested in the conversion of a double bond to a single bond (olefins to alkanes and carbonyls to alcohols), or in the complete removal of heteroatom-based functionalities (deoxygenation, dehalogenation, deamination). In all cases, the net result of a reduction reaction is the formation of a simpler and less reactive structure from one... 

Fig. 1.1. Shapes of molecules represented by envelopes of constant electronic charge density. The envelope shown has the value of 0.001 au. The molecules are (a)-(f) the normal alkanes from methane to hexane (g) isobutane (h) neopentane (i) cyclopropane (j) cyclobutane (k) formaldehyde, H2OK) (/) acetone, (CH3)2C=0. The intersections of the zero-flux interatomic surfaces with the envelope are shown in some cases. They define the methyl, methylene, hydrogen, and carbonyl groups. The isobutane molecule (g), for example, exhibits three methyl groups topped...

The C atom on the carbonyl group is included in the count of C atoms when determining the alkane from which the aldehyde is derived. ... 

The copolymerization of ethene and CO produces an interesting polar material with alternating C2 alkane and carbonyl groups. If a monosub-stituted alkene is used, isotactic and syndiotactic polymers can result. The usual catalyst for such a process is a Pd-phosphine complex. If the phosphine ligand is chiral, stereoregularity with reference to the pendant group from the alkene can result. 

Synthesis of alkanes from a-silyl carbanions and carbonyl compounds. In cases where separation of p-sllyl alcohol diastereomers (e.g. 6) can be achieved, pure Z or E olefins can be isolated (see 1st edition). 

Here is a clever way to alkylate carbonyl compounds at the carbonyl carbon atom. Thioacetals, also called didiianes, can be made from carbonyl compounds and the sulfur counterparts of diols (Rg. 19.114). Dithianes are more acidic than typical alkanes and can be deprotonated with alkyllithium reagents (Rg. 19.115). 

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. 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

Carbonyl compounds are more acidic than alkanes for the same reason that carboxylic acids are more acidic than alcohols (Section 20.2). In both cases, the anions are stabilized by resonance. Enolate ions differ from carboxylate ions, however, in that their two resonance forms are not equivalent—the form with the negative charge on oxygen is lower in energy than the form with the charge on carbon. Nevertheless, the principle behind resonance stabilization is the same in both cases. 

A range of transformation products has been identified from simulated reactions of alkanes. These include alkyl nitrates by reactions that have already been given, but also include a range of hydroxy-carbonyls that are summarized in Table 1.2 (Reisen et al. 2005). 

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