Unsaturated aldehydes with active hydrogen

Bianchini and coworkers [126] found a difference in the chemoselectivity between the metals Fe, Ru, and Os in the complexes [M(H2)H(P(CH2CH2PPh2)3)]-BPh4 in the hydrogenation of benzylideneacetone by transfer from iso-propanol. The Fe and Ru catalysts reduced the 0=0 bond to give the allyl alcohol, with Ru more active than iron (TOF 79 IT1 at 60°C for Ru versus 13 IT1 at 80°C for Fe), while the Os catalyst first reduced the 0=0 bond but then catalyzed isomerization of the allyl alcohol to give the saturated ketone (TOF 55 IT1 at 80°C). The difference in reactivity was attributed to the weak binding of the alkene of the allyl alcohol to Fe and Ru relative to Os in these complexes. A variety of selec-tivities was noted for other unsaturated ketones, whereas unsaturated aldehydes were not hydrogenated. 

Iminium-Activated Epoxidations Similar to ylide 153 and bromomalo-nates 157, it was proposed that hydrogen peroxide could also be used as an amphiphilic reactant for [2-1-1] reactions of a,P-unsaturated aldehydes to furnish epoxidation products. Inspired by this hypothesis, Jprgensen s group developed an organocatalytic asymmetric epoxidation system of a,P-unsaturated aldehydes with HjOj as the oxidant (Scheme 1.63) [ 106]. The reactions take place under mild conditions in good to high yields and enantio- and diastereoselectivities. 

Ir catalysts supported on binary oxides of Ti/Si and Nb/Si were prepared and essayed for the hydrogenation of a,P-unsaturated aldehydes reactions. The results of characterization revealed that monolayers of Ti/Si and Nb/Si allow a high metal distribution with a small size crystallite of Ir. The activity test indicates that the catalytic activity of these solids is dependent on the dispersion obtained and acidity of the solids. For molecules with a ring plane such as furfural and ciimamaldehyde, the adsorption mode can iirfluence the obtained products. SMSI effect (evidenced for H2 chemisorption) favors the formation of unsaturated alcohol. 

As for some of the monodentate phosphine-based catalysts, ds-[Ru(6,6 -Cl2bpy)2(0H2)2][CF3S03]2 was found to require water for the best catalytic activity in the reduction of aldehydes and ketones [57]. Aldehydes and ketones were found to be hydrogenated, with reasonable yields. Unsaturated aldehydes were reduced with selectivity towards the unsaturated alcohol, whereas unsaturated ketones showed selectivity towards the saturated ketones. 

Further investigations on the selective hydrogenation of o, 6-unsaturated aldehydes showed the general use of the applied concept. The hydrogenation of four different unsaturated aldehydes to their corresponding allylic alcohols could be conducted with high conversions and selectivity. As a result of these research activities, one is now able to set up the best reaction system for the biphasic selective conversion of unsaturated aldehydes. 

It is to be mentioned that water-soluble phosphine complexes of rhodium(I), such as [RhCl(TPPMS)3], [RhCl(TPPTS)3], [RhCl(PTA)3], either preformed, or prepared in situ, catalyze the hydrogenation of unsaturated aldehydes at the C=C bond [187, 204, 205]. As an example, at 80 °C and 20 bar H2, in 0.3-3 h cinnamaldehyde and crotonaldehyde were hydrogenated to the corresponding saturated aldehydes with 93 % and 90 % conversion, accompanied with 95.7 % and 95 % selectivity, respectively. Using a water/toluene mixture as reaction medium allowed recycling of the catalyst in the aqueous phase with no loss of activity. 

Monoacetals of substituted succinaldehydes (162), readily prepared by hydroformylation of optically active a,(3-unsaturated aldehyde acetals, were used to synthesize 3-substituted thiophenes having an optically active substituent (163). In these cases, while the use of hydrogen sulfide and HC1 in methanol at room temperature was more convenient, comparison with formation of (163) by the Paal synthesis from an appropriately substituted succinic acid salt gave products having about the same amount of racemization (73JOC2361). 

Hypothesizing that primary amine catalysts, due to their reduced steric requirements, might be suitable for the activation of ketones, we studied various salts of a-amino acid esters. (For pioneering use of primary amine salts in asymmetric iminium catalysis involving aldehyde substrates, see Ishihara and Nakano 2005 Sakakura et al. 2006 for the use of preformed imines of a, 3-unsaturated aldehydes and amino acid esters in diastereoselective Michael additions, see Hashimot et al. 1977.) We have developed a new class of catalytic salts, in which both the cation and the anion are chiral. In particular, valine ester phosphate salt 35 proved to be an active catalyst for the transfer hydrogenation of a variety of a, 3-unsaturated ketones 36 with commercially available Hantzsch ester 11 to give saturated ketones 37 in excellent enantiose-lectivities (Scheme 28 Martin and List 2006). 

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