Lanthanide carbonyls

A particularly useful reaction has been the selective 1,2-reduction of a, P-unsaturated carbonyl compounds to aHyUc alcohols, accompHshed by NaBH ia the presence of lanthanide haUdes, especially cerium chloride. Initially appHed to ketones (33), it has been broadened to aldehydes (34) and acid chlorides (35). NaBH by itself gives mixtures of the saturated and unsaturated alcohols. 

Carbon monoxide [630-08-0] (qv), CO, the most important 7T-acceptor ligand, forms a host of neutral, anionic, and cationic transition-metal complexes. There is at least one known type of carbonyl derivative for every transition metal, as well as evidence supporting the existence of the carbonyls of some lanthanides (qv) and actinides (1) (see AcTINIDES AND THANSACTINIDES COORDINATION COMPOUNDS). 

The reaction is general and has been applied to many transition metals as well as lanthanides and actinides. Variants use metal carbonyls and other complexes to supply the capping unit, e.g. 

There have been few mechanistic studies of Lewis acid-catalyzed cycloaddition reactions with carbonyl compounds. Danishefsky et ah, for example, concluded that the reaction of benzaldehyde 1 with trans-l-methoxy-3-(trimethylsilyloxy)-l,3-di-methyl-1,3-butadiene (Danishefsky s diene) 2 in the presence of BF3 as the catalyst proceeds via a stepwise mechanism, whereas a concerted reaction occurs when ZnCl2 or lanthanides are used as catalysts (Scheme 4.3) [7]. The evidence of a change in the diastereochemistry of the reaction is that trans-3 is the major cycloaddition product in the Bp3-catalyzed reaction, whereas cis-3 is the major product in, for example, the ZnCl2-catalyzed reaction - the latter resulting from exo addition (Scheme 4.3). 

A closely related method does not require conversion of enantiomers to diastereomers but relies on the fact that (in principle, at least) enantiomers have different NMR spectra in a chiral solvent, or when mixed with a chiral molecule (in which case transient diastereomeric species may form). In such cases, the peaks may be separated enough to permit the proportions of enantiomers to be determined from their intensities. Another variation, which gives better results in many cases, is to use an achiral solvent but with the addition of a chiral lanthanide shift reagent such as tris[3-trifiuoroacetyl-Lanthanide shift reagents have the property of spreading NMR peaks of compounds with which they can form coordination compounds, for examples, alcohols, carbonyl compounds, amines, and so on. Chiral lanthanide shift reagents shift the peaks of the two enantiomers of many such compounds to different extents. 

The lanthanide salts are unique among Lewis acids in that they can be effective as catalysts in aqueous solution.61 Silyl enol ethers react with formaldehyde and benzaldehyde in water-THF mixtures using lanthanide triflates such as Yb(03SCF3)3. The catalysis reflects the strong affinity of lanthanides for carbonyl oxygen, even in aqueous solution. 

The lanthanides are congeners of the Group IIIA metals scandium and yttrium, with the +3 oxidation state usually being the most stable. These ions are strong oxyphilic Lewis acids and catalyze carbonyl addition reactions by a number of nucleophiles. Recent years have seen the development of synthetic procedures involving lanthanide metals, especially cerium.195 In the synthetic context, organocerium... 

Jenner investigated the kinetic pressure effect on some specific Michael and Henry reactions and found that the observed activation volumes of the Michael reaction between nitromethane and methyl vinyl ketone are largely dependent on the magnitude of the electrostriction effect, which is highest in the lanthanide-catalyzed reaction and lowest in the base-catalyzed version. In the latter case, the reverse reaction is insensitive to pressure.52 Recently, Kobayashi and co-workers reported a highly efficient Lewis-acid-catalyzed asymmetric Michael addition in water.53 A variety of unsaturated carbonyl derivatives gave selective Michael additions with a-nitrocycloalkanones in water, at room temperature without any added catalyst or in a very dilute aqueous solution of potassium carbonate (Eq. 10.24).54... 

Sc(OTf)3 is stable in water, and effectively activates carbonyl and related compounds as a Lewis acid in water. This is remarkable, because most Lewis acids react immediately with water rather than the substrates, and are decomposed or deactivated. It has already been found that lanthanide trifiates Ln(OTf)3 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) and yttrium trifiate Y(OTf)3 are stable in water, and can act as Lewis-acid catalysts in aqueous media.46-48 They are used catalytically in many reactions and can often be recovered and reused, because they are stable under the usual water-quenching conditions. 

Computer simulation of lanthanide-induced shifts in the 2-formyl and 2-acetyl derivatives of furan, thiophene, selenophene, and tellurophene108 indicate a nearly equipopulated mixture of trans and cis conformers of the furan, and a preponderance of the cis for the thiophene, selenophene, and tellurophene derivatives. This difference is due to an interaction between the ring heteroatom and the carbonyl oxygen lone pair electrons. 

At the first step, the insertion of MMA to the lanthanide-alkyl bond gave the enolate complex. The Michael addition of MMA to the enolate complex via the 8-membered transition state results in stereoselective C-C bond formation, giving a new chelating enolate complex with two MMA units one of them is enolate and the other is coordinated to Sm via its carbonyl group. The successive insertion of MMA afforded a syndiotactic polymer. The activity of the polymerization increased with an increase in the ionic radius of the metal (Sm > Y > Yb > Lu). Furthermore, these complexes become precursors for the block co-polymerization of ethylene with polar monomers such as MMA and lactones [215, 217]. 

Shibasaki s lanthanide-alkaline metal-BINOL system, discussed in Chapters 2 and 3, can also effect the asymmetric conjugate addition reaction. As shown in Scheme 8-35, enantioselective conjugate addition of thiols to a,/ -unsaturated carbonyl compounds proceeds smoothly, leading to the corresponding products with high yield and high ee.76... 

An interesting novel coupling reaction of allenes with carbonyl compounds mediated by a lanthanide metal species was reported recently [80], The samarium(II) iodide-mediated reaction of various ketones or aldehydes 153 with methoxyallene (56) afforded exclusively y-addition products 4-hydroxy-l-enol ethers 154 in moderate to good yields with low cis/trans selectivity (Scheme 14.39). 

The types of dienophiles which have been studied most are acrylic aldehydes, acrylates and 3-acryloyl-l,3-oxazolidines. The latter have been used predominantly in copper, magnesium, zinc and lanthanide catalyzed reactions in which the chiral Lewis acid binds in an rj2 fashion to the dienophile (complexation to both carbonyls). 

Another example of the use of Lewis acids in organic reactions in water is the lan-thanide(III) triflate catalysed aza-Diels-Alder reaction, exemplified in Scheme 14. In this reaction the hetero-dienophile is formed in situ from a primary ammonium hydrochloride and a carbonyl compound followed by the actual Diels-Alder reaction288,289. This type of reaction proceeds readily in aqueous media290-296, and a dramatic increase in the yield upon addition of lanthanide triflates was observed288,289. The exact role of the catalyst, however, is not entirely clear. Although it was suggested that the catalyst binds to the dienophile, other mechanisms, such as simple proton catalysis, are also plausible. Moreover, these reactions are further complicated since they are often heterogeneous. 

A number of 2-thiopyran derivatives with one carbonyl group as acceptor (66,67 Table 13) has been studied by Kretschmer et al. (80). The conformations with respect to the C,—C2 and C2=C3 bonds (ZZ, ZE, EZ, or EE) were analyzed with lanthanide shift reagents. With compounds 66 (R] = H, R2 = Ph, and... 

The aim of this Specialist Periodical Report is to cover in a comprehensive way the chemistry of the transition metals including the lanthanides and actinides. It includes published data on the metal carbonyls but does not necessarily include results concerned with organometallic complexes or spectroscopic data which are published elsewhere. 

---