Ruthenium unsaturated aldehydes

In 1982, the ruthenium-catalyzed isomerization of 2-butyne-l,4-diol to butyrolactone was reported. This reaction was proposed to proceed through initial isomerization of 2-butyne-l,4-diol to a,/3-unsaturated aldehyde followed by cyclization and double bond isomerization (Scheme 52).91... 

It has been shown previously how water-soluble rhodium Rh-TPPTS catalysts allow for efficient aldehyde reduction, although chemoselectivity favors the olefmic bond in the case of unsaturated aldehydes [17]. The analogous ruthenium complex shows selectivity towards the unsaturated alcohol in the case of crotonaldehyde and cinnamaldehyde [31]. 

In 2002, Kiindig et al. [23, 24] developed catalytic DCR between diaryl nitrones and a,(3-unsaturated aldehydes in the presence of Binop-F iron and ruthenium complexes as chiral Lewis-acid catalysts (Scheme 6). The corresponding cycloadducts were obtained in good yields with complete endo selectivity and up to 94% ee. The isoxazolidine products were obtained as a mixture of regioi-somers in molar ratios varying from 96 4 to 15 85. Experimental and computational data show that the regioselectivity correlates directly with the electronic properties of the nitrone. 

The CM of olefins bearing electron-withdrawing functionalities, such as a,/ -unsaturated aldehydes, ketones, amides, and esters, allows for the direct installment of olefin functionality, which can either be retained or utilized as a synthetic handle for further elaboration. The poor nucleophilicity of electron-deficient olefins makes them challenging substrates for olefin CM. As a result, these substrates must generally be paired with more electron-rich crosspartners to proceed. In one of the initial reports in this area, Crowe and Goldberg found that acrylonitrile could participate in CM reactions with various terminal olefins using catalyst 1 (Equation (2))." Acrylonitrile was found not to be active in secondary metathesis isomerization, and no homodimer formation was observed, making it a type III olefin. In addition, as mentioned in Section 11.06.3.2, this reaction represents one of the few examples of Z-selectivity in CM. Subsequent to this report, ruthenium complexes 6 and 7a were also observed to function as competent catalysts for acrylonitrile... 

Ir/tppts catalysts exhibit almost the same selectivity as Ru/tppts in the hydrogenation of a,p-unsaturated aldehydes albeit with approximately 70 times lower rates.485 In sharp contrast to the ruthenium and iridium based tppts catalysts, RhJ tppts complexes catalyse the chemoselective hydrogenation of a,fl-unsaturated aldehydes to the corresponding saturated aldehydes (Figure 14, III).54-485... 

The selective hydrogenation of a,/3-unsaturated aldehydes to give the corresponding unsaturated alcohols [Eq. (9)] was investigated with the ruthenium complex catalysts, initially present as [Ru(H)(Cl)(tppts)3] or [Ru(H)2(tppts)4] (91). 

Dissymmetric ferrocenyldiphosphines have been synthesized from (R)-(+)-N, N -dimethylaminoethylferrocene. The diphosphines have been used as ligands in asymmetric transfer hydrogenation of acetophenone in the presence of ruthenium catalysts.297 Asymmetric transfer hydrogenation of a,/S-unsaturated aldehydes with Hantzsch dihydropyridines and a catalytic amount of MacMillan imidazolidinone salt (12) leads to the saturated carbonyl compounds in high yields and excellent chemo-and enantio-selectivities.298 ... 

A similar mechanism,based on a ruthenacyclopentene, can be proposed for the coupling of alkynes and allylic alcohols to lead to y,<5-unsaturated aldehydes and ketones. When (C5H5)RuC1(COD) was used as a catalyst, the ruthenium-catalyzed coupling between alkynes and substituted allylic alcohols afforded y,<5-unsaturated ketones. The linear isomer was the major product [39] (Eq. 28). Similarly, the linear derivative was also obtained when an allylsi-lylether or an allylic amide was used in place of the allyl alcohol, leading to 1,4-dienes [40]. 

On the other hand, unsaturated aldehydes and ketones were obtained using allylic alcohols as alkene components [68]. Similarly, allyl f-butyldimethylsilyl ether and N-allylamides gave silyl enol ethers [69] and enamides [70], respectively. The ruthenium-catalyzed alkene-alkyne coupling was successfully combined with the palladium-catalyzed intramolecular asymmetric allylic alkylation [71] to provide a novel one-pot heterocyclization method [72]. 

A similar reaction was reported by Ma et al. in preference to Trost s work [15]. In the isomerization of 2-ynols to a,/3-unsaturated aldehydes, the combination of a ruthenium catalyst, RuCl2(PPh3)3, and 2 equiv. of an aliphatic phosphine ligand, such as P Bu3 or P Pr3, is effective. 

Catalytic tandem isomerization/Claisen reaction of bis allyl ether was reported by Dixneuf et al. [23]. A cationic bis-oxazoline-ruthenium-arene complex S3 in the presence of both l,3-bis(2,6-diisopylphenyl)imidazolinium chloride and CS2CO3 catalyzes the selective transformation of bis-allyl ether 51 into ) c5-unsaturated aldehyde 52 via successive alkene isomerization and Claisen rearrangement (Eq. 12.21). 

With the catalysts derived from (S,S)-l,2-bis(diphenylphosphinomethyl)cyclobutane and [RhH(CO)(PPh3)3] or rhodium carbonyls, the a,3-unsaturated aldehydes, neral and geranial, are hydrogenated to (E)- and (S)-citronellal in 79% and 60% ee, respectively. Cyclic a,3-unsaturated ketones such as isophorone and 2-methyl-2-cyclohexenone have been hydrogenated using ruthenium hydrides coordinated with chiral diphosphines in up to 62% ee to give chiral ketones, though conversions are not satisfactory. ... 

Although ruthenium is the most active known catalyst for hydrogenation of the carbonyl group, it is possible to reduce a,/3-unsaturated aldehydes to saturated aldehydes quantitatively. The Engelhard workers, however, regard palladium as the catalyst of choice for this conversion. For hydrogenation to the saturated alcohol, they prefer a two-step process Pd-reduction of the double bond, followed by Ru-reduction of the aldehyde group. 

Eilbracht et al. have developed rhodium- or ruthenium-catalyzed one-pot synthesis of cyclopentanones from allyl vinyl ether via tandem Claisen rearrangement and hydroacylation [109-111]. This protocol requires elevated temperature (140-220°C) and also requires alkyl or aryl substituents at the terminal position of the allylic double bond to prevent undesirable double bond migration in the intermediary formed, unsaturated aldehyde. 

One of the most interesting applications of these catalytic systems is the regioselective reduction of a, -unsaturated aldehydes to unsaturated alcohols or saturated aldehydes [18,19]. For example, 3-methyl-2-buten-l-al or prenal was selectively reduced to prenol with a selectivity up to 97% using ruthenium complexes associated with tppts in a mixture of water/toluene at 35 °C and 20 bars hydrogen [Eq. (1)] conversely, the saturated aldehyde was obtained with a selectivity up to 90% using RhCl(tppts)3 as the catalyst at 80 °C and 20 bars hydrogen. The same selectivities were observed for (Ej-cinnamaldehyde,2-buta-nal and citral. 

Reduction of unsaturated substrates can also be performed by hydrogen transfer, usually from formate, catalyzed by rhodium and ruthenium complexes. Joo and co-workers have shown that RuCl2(tppms)2 [44] and RuCl2(PTA)4 [45,46] transforms aromatic as well as a, -unsaturated aldehydes to the corresponding aromatic or unsaturated alcohols, with a selectivity up to 98% in the latter case,... 

Water and supercritical carbon dioxide form an excellent medium for hydrogenation of unsaturated aldehydes to allylic alcohols with ruthenium(lll) chloride and a water-soluble triarylphosphine, because the limitation pertaining to gas-liquid-liquid mass transfer is eliminated due to the very high. solubility of reactant gas. 

B. Unsaturaied compounds Table I also includes examples of the reduction of some unsaturated aldehydes and ketones (No. 11-14). Ruthenium... 

---