Ketones ionic hydrogenation

Stoichiometric ionic hydrogenation of the C=C bond of a,/ -unsaturated ketones by HOTf and [Cp(CO)3WH] results in the formation of -ketone complexes of tungsten [32]. As exemplified in Eq. (17), hydrogenation of methyl vinyl ketone gives a 2-butanone complex of tungsten. The bound ketone is displaced by the triflate counterion, giving the free ketone. Similar reactions were reported for hydrogenation of the C=C bond of a,/ -unsaturated aldehydes. 

Several systems have been reported involving stoichiometric hydrogenation of ketones or aldehydes by metal hydrides in the presence of acids. An ionic hydrogenation mechanism accounts for most of these hydrogenations, though in some examples alternative mechanisms involving the insertion of a ketone into a M-H bond are also plausible. 

A competition between stoichiometric hydrogenation of acetone and acetophenone resulted in hydrogenation of the acetone [42]. Competitions of this type could be influenced by both the basicity of the ketone, as well as by the kinetics of hydride transfer to the protonated ketone. An intramolecular competition between an aliphatic and aromatic ketone resulted in preferential hydrogenation of the aliphatic ketone, with the product shown in Eq. (24) being isolated and fully characterized by spectroscopy and crystallography. Selective ionic hydrogenation of an aldehyde over a ketone was also found with HOTf and [Cp(CO)3WH],... 

Stoichiometric Ionic Hydrogenation of Ketones with Metal Dihydrides... 

Catalytic Ionic Hydrogenation of Ketones by Anionic Cr, Mo, and W Complexes... 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

Molybdenum and tungsten carbonyl hydride complexes were shown (Eqs. (16), (17), (22), (23), (24) see Schemes 7.5 and 7.7) to function as hydride donors in the presence of acids. Tungsten dihydrides are capable of carrying out stoichiometric ionic hydrogenation of aldehydes and ketones (Eq. (28)). These stoichiometric reactions provided evidence that the proton and hydride transfer steps necessary for a catalytic cycle were viable, but closing of the cycle requires that the metal hydride bonds be regenerated from reaction with H2. 

Scheme 17.8 Concerted ionic hydrogenation mechanisms for the hydrogenation of (a) ketones and (b) C02. The acidic ligand is shown as an alcohol in Scheme 17.7 b, but could equally well be water, a secondary amine, or a carboxylic acid.

Stoichiometric Ionic Hydrogenation of Ketones and Aldehydes using Metal Hydrides as Hydride Donors and Added Acids... 

Scheme 11. Proposed mechanism for catalytic ionic hydrogenation of ketones.

Alkoxy)diborons, alkyne additions, 10, 727-728 Alkoxy-hydro carbonylation, in ionic liquids, 1, 863 Alkoxy ketones, asymmetric hydrogenation, 10, 47 Alkoxylation... 

Synthesis of standards. Friedel-Crafts acylation of 2,3-dimethylthiophene with hexadecanoic acid and a subsequent reduction of the ketone with LiAlD4 yielded the 2,3-dimethyl-5-(l ,l -d2-hexadecyl)thiophene (II, Table II). Experimental details of these reactions have been reported elsewhere (10). Ionic hydrogenation of the 2,3-dimethyl-5-(l ,l -d2-hexadecyl)thiophene in trifluoroacetic acid using triethylsilane and BF3.etherate as catalyst yielded 2,3-dimethyl-5-(l ,l -d2-hexadecyl)thiolane (IV, Table II). For... 

Aromatic aldehydes and aromatic ketones also can be reduced to hydrocarbons in a completely different manner, namely via the so-called ionic hydrogenation followed by an ionic hydrogenolysis. This kind of reduction is possible only if it can proceed via resonance-stabilized cationic intermediates. This resonance stabilization is readily achieved in a benzylic position, and it is therefore advantageous to employ aromatic carbonyl compounds in this kind of reduction. The carboxonium ion A, formed... 

In the classical hydrogenation catalysis the substrate must be coordinated to the metal prior to its insertion into a M-H bond. However, very recently the possibility of ionic hydrogenation of unsaturated substrates (such as ketones or imines) in which the coordination of the substrate to the metal is not required, has been recognized. Equation Boxl shows the proposed cycle for catalytic ionic hydrogenation of ketones. 

Although hydrosilanes reduce ketones, in trifluoroacetic acid, to the corresponding methylene compounds or dimeric ethers via ionic hydrogenation, the reduction of a-amino and a-oxy ketones and p-keto acid derivatives with hydrosilanes, particularly PhMe2SiH, under these conditions proceeded with high anti selectivity to the alcohols. No racemization was observed at the carbon a to the carbonyl group. Intramolecular hydrosilylation catalyzed by Lewis acids provided a highly stereoselective route to anti-1,3-diols from p-hydroxy ketones (Section 1.1.3. ). ... 

Ionic hydrogenation is the noncatalytic addition of hydrogen to carbon-carbon or carbon-heteroatom double bonds, and the hydrogenolysis of carbon-oxygen or carbon-halogen single bonds. Thus, the conversion of aldehydes and ketones to the corresponding hydrocarbon is one of its many manifesta-... 

---