Reduction to alkanes

Reduction to Alkanes. Carbonyl groups can be reductively deoxygenated to methylene functions if both of the two steps represented by Eqs. 1 and 2 proceed to completion. With aldehydes, this process leads to the transformation of the CHO group into a CH3 group. 

Benzaldehyde itself forms no toluene only dibenzyl ether and benzyl trifluo-roacetate are formed. Triethylsilane (2.2 equivalents) causes the transformation of /7-anisaldehyde into /7-methylanisole in 76% yield after only 30 minutes. Use of a three-fold excess of dimethylphenylsilane in place of the triethylsilane results in a slight improvement in yield to 83% after 45 minutes.69 

Similar treatment of a trifluoroacetic acid solution of p-tolualdehyde with triethylsilane gives only a 20% yield of /7-xylene after 11 hours reaction time followed by basic workup. Use of 2.5 equivalents of dimethylphenylsilane enhances the yield to 52% after only 15 minutes. This reaction proceeds stepwise through the formation of a mixture of the trifluoroacetate and the symmetrical ether. These intermediates slowly form the desired /7-xylene product along with Friedel-Crafts side products under the reaction conditions (Eq. 192).73 Addition of co-solvents such as carbon tetrachloride or nitromethane helps reduce the amount of the Friedel-Crafts side products.73 

Treatment of a polyfunctional chromium-tricarbonyl-complexed hydroxy aldehyde with an excess of Et3SiFFTFA for 4.5 hours gives an 82% yield of fully reduced product with both the formyl and hydroxy groups completely and selectively reduced (Eq. 193).352 

The sequence of reagent and substrate addition can be quite important in these reactions. For example, a trifluoroacetic acid solution of 2,4,6-trimethylbenzalde-hyde forms isodurene in 98% yield within 15 minutes when 2.2 equivalents of triethylsilane are added to the reaction mixture at room temperature 69 In 

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S ilylation-intramolecular reduction, ketone-alcohol reduction, 78-79 Single-electron transfer (SET) process, alkyl halides and triflate reduction to alkanes, 28-31... 

Group VIA -Cr, Mo, W. The stable, water-soluble metal alkyl [(H20)5-Cr( y -<7-CH2C6H5)] is obtained by one-electron oxidative additions to [Cr-(H20)J with reactive alkyl halides. The rate-determining, halogen-transfer step generates an alkyl free radical, which then rapidly reacts with a second molecule of Cr(ll) ion or undergoes radical coupling and reduction to alkane ... 

NaH in the presence of r-butoxide and Ni acetate reduces mono- and di-substituted alkenes. Trisubstituted alkenes do not react. 1-Alkynes are reduced to mixtures of cis- and rranr-alkenes, which undergo competitive, further reduction to alkanes. MgH2, in the presence of Cu iodide or f-butoxide in THF at -78 C, reduces 1-alkynes to 1-alkenes, which are stable toward further reduction. Disubstituted acetylenes are cleanly reduced to ds-alkenes. ... 

Reaction (d) occurs for terminal alkynes (R = H), 1-hexyne, 1-octyne and phenyl acetylene (80-98%), with no further reduction to alkanes . The internal alkynes, 2-hexyne and diphenylacetylene, give exclusively cis-alkene (80-95%) ". However, the organometallic intermediates of Eq. (d) may contain Cu—C rather than Mg—C bonds. 

Catalytic hydrogenation of carbonyl compounds to alkanes is a difficult proposition under normal conditions, although limited success is attainable with aromatic ketones. However, certain enolates derived from ketones have been shown to undergo catalytic reduction to alkanes quite efficiently. For example, enol triflates of ketones are reduced over platinum oxide catalyst to alkanes (equation 56) . Similarly, enol phosphates, conveniently prepared from ketones, can be quantitatively hydrogenated to alkanes (equation 57) . ... 

Second, despite the generalization above, specihc conditions required for reduction may well vary from one alkene to the next. It is particularly worthy to note that most arenes (in accord with their extraordinary stability and as thus expected) can only be reduced with difficulty. In contrast to simple alkenes, which can often be reduced at or near 1 atm of hydrogen in the presence of a Pt or Pd catalyst, benzene requires high pressures and temperatures (e.g., powdered Ni, 40 atm H2, 200°C) (Equation 3.3). Further, the reduction of alkynes to (Z)-alkenes (addition of hydrogen [H2] being suprafacial) frequently cannot be stopped at the alkene stage and complete reduction to alkane occurs. Special poisoned catalysts have been developed, which frequently succeed in stopping the reaction. (Equation 3.4). 

Finally, a Pd complex 67 was applied to TH of alkynes (Equation (13.2)). Internal substrates were selectively hydrogenated with excellent yields and over-reduction to alkane was almost negligible. ... 

As shown in the Scheme, B-acyloxy- o(,B-unsaturated acid derivatives 3 may enter alternative reactions they either rearrange to 4 or undergo reduction to alkan-2-ol ester 7 proceeded by decarboxylation to 5 the latter vinylester may transform into hentriacontan-14,16-dione 6. 

Conversion of Ketones to Alcohols and Subsequent Reduction to Alkanes—Reduction of Benzophenone to 1,1-Diphenylethane... 

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