[7440-64-4]  · Yb  · Ytterbium(0)  · (MW 173.04)

(reducing agent capable of Birch-type reductions of aromatic and alkenic systems;1-3 promotes the coupling of aldehydes and ketones to generate the corresponding pinacolic product; forms an organoytterbium complex with aromatic substrates which adds nucleophilically to ketones, epoxides, nitriles, and carbon dioxide to generate highly functionalized products5-7)

Physical Data: mp 824 °C; bp 1427 °C; d 6.760 g cm-3; face centered cubic structure at rt.

Form Supplied in: silvery, ductile metal; commercially available as 40 mesh powder, ingot, chip, foil.

Handling, Storage, and Precautions: is somewhat air and moisture sensitive and should be handled under an inert atmosphere.

Dissolving Metal Reductions.

Ytterbium turnings are added piecewise to dry liquid Ammonia, resulting in a deep-blue solution containing the ammoniated electron. Cosolvents such as ether, THF, or t-butanol can be added. The blue Yb-NH3 solution is stable for several hours at -33 °C. Quenching of the solution with water results in replacement of the deep-blue color by a transient green color, characteristic of Yb2+, which changes to a brown suspension with formation of Yb3+ species.

Solutions of ytterbium in ammonia8 have properties similar to those of alkali metal-ammonia solutions9 and are strongly reducing. This is a consequence of the fact that Yb0 (4f146s2), like other lanthanides, is electropositive and dissolves readily in ammonia,10 an excellent supporting medium for electron transfer.11 Dissolution of ytterbium in ammonia yields two electron equivalents (eq 1) and results in Yb2+ which possesses the closed f shell of xenon. However, Yb2+ is readily oxidized in air to Yb3+ (EYb0 - Yb3+ = -2.27 V). The ionic radius of Yb2+ is intermediate between that of Li+ and Na+. Physical characteristics of ytterbium-ammonia solutions have been described.12

Solutions of Yb-NH3 reduce benzenoid13 and other aromatic rings to 1,4-dihydro aromatics and, as in the reduction with alkali metal-ammonia systems,14 substituents exert a profound effect on the rate of the reaction and on the structure of the product. Thus benzoic acid is reduced rapidly to the 1,4-dihydro derivative (eq 2), while anisole and p-cresyl methyl ether are reduced more slowly to 1-methoxy-1,4-cyclohexadienes (eq 3).

Application of this reduction to a derivative of estrone resulted, after hydrolysis of the intermediate enol ether, in dihydrotestosterone (eq 4).15 In this case, no proton donor was added to the medium.

Conjugated dienes are cleanly reduced in a 1,4-manner by Yb-NH3. An application of this reduction in a synthesis of (±)-dehalogenonidifocene resulted in a 9:1 mixture favoring the 1,4-reduction product (eq 5).16 By contrast, Li-NH3 gave a 1:1 mixture of 1,4- and 1,2-reduction products and Ca-NH3 gave a 2:1 ratio of these materials. The greater selectivity of Yb-NH3 is attributed to the weaker basicity of Yb(NH2)2 and to the consequent diminished propensity for alkene isomerization of the product.

Conjugated enones are reduced with Yb-NH3 and, in the presence of a proton source, afford the saturated ketone together with variable quantities of the alcohol resulting from further reduction (eq 6). If a proton donor is absent, a high yield of pinacol dimer is obtained, as exemplified with cholest-4-en-3-one (eq 6).17 Pinacol coupling of aromatic ketones has been accomplished with a slurry of Yb power in THF-HMPA which, under appropriate conditions, can also lead to a high yield of cross coupled product.4 The same reagent effects pinacol-like coupling of diaryl ketones with nitriles to give a-hydroxy ketones, and with epoxides to give 1,3-diols.6 However, with unsymmetrical epoxides the regioselectivity can be low.7 It is not clear that this heterogeneous form of the Yb reagent offers significant advantage over the homogeneous Yb-NH3 system, especially since the metal must be activated (with MeI) before reduction.

Solutions of Yb-NH3 reduce alkynes to a trans-alkene (eq 7). In certain cases, strained alkenes can be reduced18 as, for example, in the conversion of norbornadiene to bicyclo[2.2.1]heptene (eq 8).

Pinacolic Coupling Reactions.

Ytterbium metal effects the coupling of pinacols (eq 9). Although pinacolic cross-coupling is difficult to achieve, ytterbium metal with diaryl ketones promotes the effective coupling of aldehydes and ketones to afford vicinal diols in good yields (eqs 10 and 11).4-6 The utility of this protocol is limited, however, in that one of the pinacolic coupling partners must contain aromatic functionality.

Nucleophilic Addition of Organoytterbium Complexes to Other Electrophiles.

Treatment of diaryl ketones with 1-2 equiv of Yb metal promotes the nucleophilic addition to ketones, epoxides, nitriles, and carbon dioxide to form the corresponding crossed pinacol, 1,3-diol, a-hydroxy ketone, and a-hydroxy carboxylic acid, respectively (eqs 12 and 13).5-7

Reduction of Ketimines.

Ytterbium also provides a method for the reduction of imines (eq 14); however, considerable amounts of coupling products are observed when there are not two aromatic substituents on the substrate.19

Ketimines are reduced with ytterbium metal and may subsequently be treated with carbon dioxide to generate the ytterbium salt of the a-amino acid, which is converted to the a-amino acid by acidic workup (eq 15).20 This method too is limited to aromatic functionality on the ketimine substrate.

1. White, J. D.; Larson, G. L. JOC 1978, 43, 4555.
2. Hou, Z.; Taniguchi, H.; Fujiwara, Y. CL 1987, 305.
3. Tius, M. A.; Gomez-Galeno, J.; Gu, X.; Zaida, J. H. JACS 1991, 113, 5775.
4. Hou, Z.; Takamine, K.; Fujiwara, Y.; Taniguchi, H. CL 1987, 2061.
5. Hou, Z.; Takamine, K.; Aoki, O.; Shiraishi, H.; Fujiwara, Y.; Taniguchi, H. CC 1988, 668.
6. Hou, Z.; Takamine, K.; Aoki, O.; Shiraishi, H.; Fujiwara, Y,; Taniguchi, H. JOC 1988, 53, 6077.
7. Takaki, K.; Tanaka, S.; Beppu, F.; Tsubaki, Y.; Fujiwara, Y. CL 1990, 1427.
8. Warf, J. C.; Korst, W. L. J. Phys. Chem. 1956, 60, 1590.
9. Birch, A. J.; Subba Rao, G. In Advances in Organic Chemistry, Methods and Results; Taylor, E. C., Ed.; Wiley: New York, 1972.
10. Salot, S.; Warf, J. C. JACS 1968, 90, 1932.
11. Smith, H. Organic Reactions in Liquid Ammonia; Interscience: New York, 1963.
12. Hagedorn, R.; Lelieur, J. P. J. Phys. Chem. 1981, 85, 275.
13. Slaugh, L. H. IC 1967, 6, 851.
14. Rabideau, P. W. T 1989, 45, 1579.
15. Dryden, H. L.; Webber, G. M.; Burtner, R. R.; Cella, J. A. JOC 1961, 26, 3237.
16. Miyashita, K.; Yoneda, K.; Akiyama, T.; Koga, Y.; Tokura, R.; Abe, Y.; Kume, T.; Iwata, C. CPB 1993, 41, 458.
17. Bladon, P.; Cornforth, J. W.; Jaeger, R. H. JCS 1958, 863.
18. Watt, G. W. CRV 1950, 46, 317.
19. Takaki, K.; Tsubaki, Y.; Tanaka, S.; Beppu, F.; Fujiwara, Y. CL 1990, 203.
20. Takaki, K.; Tanaka, S.; Fujiwara, Y. CL 1991, 493.

Gary A. Molander & Christina R. Harris

University of Colorado, Boulder, CO, USA

James D. White

Oregon State University, Corvallis, OR, USA

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