Unsaturated aldehydes modifier effect

Selective Reactions on Modified Metal Surfaces the Ligand Effect Hydrogenation of a- Unsaturated Aldehydes... 

Under relatively mild conditions the Ru/C catalyst poisoned with Sn (lines 1 and 2), the Ir/C catalyst (lines 14 and 15), and the Raney-cobalt catalyst modified with CoCl2 (line 19) seem likely systems to try when initiating a search for an effective method for selectively hydrogenating the C=0 bond in an a, 3-unsaturated aldehyde. 

Another a, i-unsaturated aldehyde analyzed is cinnamaldehyde. Its liquid-phase hydrogenation has been studied in our research group [20, 51, 94], using Pt, Ni and Cu-based tin-modified hi- and organobimetaUic catalysts (in all cases with Si02 as support). The catalytic results obtained showed that in aU cases there was a marked promoting effect of Sn on the selectivity to cinnamic alcohol (UOL). The specific modification of the monometallic systems due to Sn addition from the application of SOMC/M markedly increases the selectivity to UOL, especially in the case of Ni, where it goes from zero selectivity for the monometallic to 25% for the NiSn catalyst. Pt-based systems modified by Sn yield the best Suol values. 

Novel bidentate chiral Lewis acids derived from 1.8-naphthalenediylbis(dichloroborane) and modified amino acids as chiral auxiliary have been successfully utilized as effective catalysts for the asymmetric Diels-Alder reaction of a,[ -unsaturated aldehydes. The enantioselectivity is highly sensitive to the kind of chiral amino acids. Moderate enantioselectivity was obtained with the tryptophan-derived ligand for the endo adduct, but amino acids without aromatic groups... 

Nef s mechanism was recently modified by Mednick and by Moye and Krzeminski. Instead of the formation of (57) from (56), they proposed, in effect, a /3-hydroxy-carbonyl elimination to yield the a, 8-unsaturated aldehyde (59). Although, with this alteration, the steps from (56) to (59) and to 5-(hydroxymethyl)-2-furaldehyde are quite plausible, no evidence has been presented for the formation of (56), nor have any of the above... 

Also, palladium-catalyzed hydroformylation of alkynes is known [98]. Particularly, Beller s group showed that a wide range of symmetrically and unsymmet-rically substituted alkynes can be converted into the unsaturated aldehydes by the effect of a palladium complex modified with a bidentate 2-pyrrolyl phosphine (Scheme 4.20) [103]. 

Ru/tppms catalysts exhibited excellent yields (98%) in the transfer hydrogenation of unsaturated aldehydes such as cinnamaldehyde or crotonaldehyde, to the corresponding unsaturated alcohols under mild reaction conditions (30-80°C), with HCOONa in an aqueous/organic two phase system. Similarly, ruthenium modified with the water soluble, air stable phosphine 100 (pta Table 5) is an effective catalyst for the chemoselective transfer hydrogenation of a,P-unsaturated aldehydes to unsaturated alcohols using formate in an aqueous/ organic two phase system. In contrast, Rh/pta afforded the cor-... 

Nevertheless, it must be pointed out that the formation of such transient species has never been spectroscopically observed. Native CDs are effective inverse phase-transfer catalysts for the deoxygenation of allylic alcohols, epoxydation,or oxidation " of olefins, reduction of a,/ -unsaturated acids,a-keto ester,conjugated dienes,or aryl alkyl ketones.Interestingly, chemically modified CDs like the partially 0-methylated CDs show a better catalytic activity than native CDs in numerous reactions such as the Wacker oxidation,hydrogenation of aldehydes,Suzuki cross-coupling reaction, hydroformylation, " or hydrocarboxylation of olefins. Methylated /3-CDs were also used successfully to perform substrate-selective reactions in a two-phase system. 

Enamines and metalloenamines provide a valuable alternative to the use of eno-lates for the selective alkylation of aldehydes and ketones. Enamines are a,p-unsaturated amines and are obtained simply by reaction of an aldehyde or ketone with a secondary amine in the presence of a dehydrating agent, or by heating in benzene or toluene solution in the presence of toluene-/7-sulfonic acid (TsOH) as a catalyst, with azeotropic removal of water (1.31). Pyrrolidine and morpholine are common secondary amines useful for forming enamines. All of the steps of the reaction are reversible and enamines are readily hydrolysed by water to reform the carbonyl compound. All reactions of enamines must therefore be conducted under anhydrous conditions, but once the reaction has been effected, the modified carbonyl compound is liberated easily from the product by addition of dilute aqueous acid to the reaction mixture. 

The effect on selectivity and activity for the hydrogenation of 3-methyl crotonaldehyde using Pt/SiOj modified with cations from successive groups in the periodic table was studied by Ponec et The most effective promoter was found to be Fe and Sn both of which raised the selectivity to the unsaturated alcohol from 21% to about 80%. Interestingly, this increase in selectivity was also accompanied by a substantial rate enhancement from 2.0 to 12.7 pmol s" (g cat) in the case of Pt Sn prepared with a 4 1 ratio. Similar selectivity and activity enhancement were reported by Vannice and Sen for crotonaldehyde hydrogenation over Pt/TiOj in the vapour phase using a Hg aldehyde ratio of 22.7 Under both low tem-... 

The first example of enantioselective trimethylsilylcyanation of aldehydes catalyzed by chiral binaphthol and binaphthol-modified lanthanum alkoxides shows an obvious effect of substituents at the 3,3 -positions of BINOL. The 3,3 -bis(methoxyethyl)-BINOL had the advantage over simple BINOL to give (S)-products in excellent yields with 73% ee (119). Chiral lanthanum and samarium complexes catalyze the Michael addition of thiols to a, )3-unsaturated carbonyl compounds (120). 

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