Unsaturated aldehydes catalyst effect

Chiral aluminum catalyst 2, prepared from Et2AlCl and a Vaulted biaryl ligand, is reported to be an effective Lewis acid catalyst of the Diels-AIder reaction between methacrolein and cyclopentadiene, affording the adduct in 97.7% ee [4] (Scheme 1.2). Although the Diels-AIder reaction with other a,/ -unsaturated aldehydes has not been described, that only 0.5 mol% loading is sufficient to promote the reaction is a great advantage of this catalyst. 

In 1989 Yamamoto et al. reported that the chiral (acyloxy)borane (CAB) complex 3 is effective in catalyzing the Diels-AIder reaction of a number of a,/ -unsaturated aldehydes [5]. The catalyst was prepared from monoacylated tartaric acid and bo-... 

To overcome these problems with the first generation Brmsted acid-assisted chiral Lewis acid 7, Yamamoto and coworkers developed in 1996 a second-generation catalyst 8 containing the 3,5-bis-(trifluoromethyl)phenylboronic acid moiety [10b,d] (Scheme 1.15, 1.16, Table 1.4, 1.5). The catalyst was prepared from a chiral triol containing a chiral binaphthol moiety and 3,5-bis-(trifluoromethyl)phenylboronic acid, with removal of water. This is a practical Diels-Alder catalyst, effective in catalyzing the reaction not only of a-substituted a,/ -unsaturated aldehydes, but also of a-unsubstituted a,/ -unsaturated aldehydes. In each reaction, the adducts were formed in high yields and with excellent enantioselectivity. It also promotes the reaction with less reactive dienophiles such as crotonaldehyde. Less reactive dienes such as isoprene and cyclohexadiene can, moreover, also be successfully employed in reactions with bromoacrolein, methacrolein, and acrolein dienophiles. The chiral ligand was readily recovered (>90%). 

Reduction of unsaturated carbonyl compounds to the saturated carbonyl is achieved readily and in high yield. Over palladium the reduction will come to a near halt except under vigorous conditions (73). If an aryl carbonyl compound, or a vinylogous aryl carbonyl, such as in cinnamaldehyde is employed, some reduction of the carbonyl may occur as well. Carbonyl reduction can be diminished or stopped completely by addition of small amounts of potassium acetate (i5) to palladium catalysts. Other effective inhibitors are ferrous salts, such asferroussulfate, at a level of about one atom of iron per atom of palladium. The ferrous salt can be simply added to the hydrogenation solution (94). Homogeneous catalysts are not very effective in hydrogenation of unsaturated aldehydes because of the tendencies of these catalysts to promote decarbonylation. 

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. 

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. 

Miscellaneous Iminium Catalyzed Transformations The enantioselective construction of three-membered hetero- or carbocyclic ring systems is an important objective for practitioners of chemical synthesis in academic and industrial settings. To date, important advances have been made in the iminium activation realm, which enable asymmetric entry to a-formyl cyclopropanes and epoxides. In terms of cyclopropane synthesis, a new class of iminium catalyst has been introduced, providing the enantioselective stepwise [2 + 1] union of sulfonium ylides and ot,p-unsaturated aldehydes.As shown in Scheme 11.6a, the zwitterionic hydro-indoline-derived catalyst (19) enables both iminium geometry control and directed electrostatic activation of sulfonium ylides in proximity to the incipient iminium reaction partner. This combination of geometric and stereoelectronic effects has been proposed as being essential for enantio- and diastereocontrol in forming two of the three cyclopropyl bonds. 

The [3+2] cycloaddition has also been shown to be effective in the reaction of azomethine imines 32 with a,P-unsaturated aldehydes by Chen and co-workers [70], A survey of seven catalysts revealed some interesting trends, with the diarylprolinol derivative 31 giving the highest yields and selectivities (40-95% yield endo. exo 1 4.3-1 49 77-96% ee for exo) with short reaction times (5-24 h) and low catalyst loading (10 mol%) (Scheme 11). The reaction was particularly sensitive to the amount of water present in the reaction medium and the choice of co-acid. This phenomenon is a reoccurring theme in many of the publications in the area of iminium ion catalysis and, as yet, no general explanation has been proposed to account for these observations. 

Suitable conditions for the highly selective conjugate addition of nitromethane to a,P-unsaturated aldehydes were developed by Ye and co-workers [103]. Catalyst loadings as low as 2 mol% were found to be effective with a range of substrates (65-80% yield 88-97% ee) (Scheme 25). 

J0rgensen [111] and Vicario [112] independently described the conjugate addition of both triazole and tetrazole based nucleophiles to a,P-unsaturated aldehyde substrates as an alternative method for C-N bond formation. These reactions were catalysed by the diarylprolinol and imidazolidinone scaffolds with equal efficiency showing the complementarity and efficacy of both these catalyst architectures. In addition, Jprgensen has also shown succinimide to be an effective Michael donor (see Sect. 2.3.5 Scheme 49 for further details) [113]. 

The development of the first alkyne silylformylation reaction was reported in 1989 by Matsuda [27]. Alkynes were treated with Me2PhSiH and Et3N with 1 mol% Rh4(CO)i2 under CO pressure to produce yS-silyl-a,/ -unsaturated aldehydes (Scheme 5.20). A second report from Ojima detailed the development of rhodium-cobalt mixed metal clusters as effective catalysts for alkyne silylformylation [28]. Shortly thereafter, Doyle reported that rhodium(II) perfluorobutyrate was a highly efficient and selective catalyst for alkyne silylformylation under remarkably mild reaction conditions (0°C, 1 atm CO) [29]. In all these reports, terminal alkynes react regiospedfically with attachment of the silane to the unsubstituted end of the alkyne. The reaction is often (but not always) stereospecific, producing the cis-product preferentially. 

RuCl2(TDCPP) as RuClj(TDCPP)/(CljpyNO)/CDCl3 oxidised terminal alkenes RCH3=CH to the aldehydes RCH CHO 1,3-dienes gave the unsaturated aldehydes RuCljtTMP) was also effective as a catalyst [584]. 

Unsaturated aldehydes.4 A nickel boride catalyst similar to P-2 nickel boride is obtained by reaction of NiCl2 and excess NaBH4 in C2H5OH. It effects selective hydrogenation of a,P-alkynal acetals to the (Z)-a,p-alkenal acetals. 

Preliminary results for NaY- and KY-supported Ru catalysts demonstrate significant effects of the nature of the zeolite cation for the selectivity of 3-methyl crotonaldehyde hydrogenation. It was suggested that increased basicity of the zeolite resulted in increased selectivity toward the unsaturated alcohol product. These results agree with earlier suggestions that the nature of the support can have significant influence on the product distribution in the hydrogenation of < ,/3-unsaturated aldehydes. 

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... 

A chiral [2.2]paracyclophane bearing a /3-hydroxyamino side-chain catalyses enan-tioselective reaction with aromatic and a -unsaturated aldehydes.220 Comparison with simpler catalysts suggests that the new one exhibits cooperative effects between planar and central chiralities. 

Inanaga et al. have reported the preparation of the new chiral Yb(III) phosphate complex 180 (Figure 43), which is an efficient catalyst for asymmetric hetero-Diels-Alder reactions of 181 with aromatic aldehydes and/or a,P-unsaturated aldehydes. 2,6-Lutidine is found to be an effective additive to obtain high enantioselectivities.98 Another asymmetric catalysis for hetero-Diels-Alder reaction was reported by Mikami et al.99 An illustrative example is shown in Figure 44, wherein the interesting effects of water as an additive were observed to increase not only the enantioselectivity but also the chemical yield. 

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