Ligand‐Accelerated Catalysis
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Sharpless K. B., 1986, Chem. Brit., 22, 38
1984, Proc. Robert A. Welch Found. Conf. Chem. Res., 27, 59
1985, CHEMTECH, 15, 692
(a)R. A.Johnson K. B.Sharplessin Ref. [9b] S. 103;
Pfenninger A., 1986, Synthesis, 89
The activation of a stoichiometric reagent by a ligand is considered as “ligand acceleration”. To be aligand‐accelerated catalysis(LAC) a catalytically active species (other than the ligand) must be present.
For a very well studied example of a ligand‐accelerated stereoselective reaction seeW.Klute R.Dress R. W.Hoffmann J. Chem. Soc. Perkin Trans 2 1993 1409.
Brunner H., 1994, Handbook of Enantioselective Catalysis
Noyori R., 1994, Asymmetric Catalysis in Organic Synthesis
Ojima I., 1993, Catalytic Asymmetric Synthesis
Brunner H., 1988, Synthesis, 645
Noyori R., 1985, Chem. Scr., 25, 83
(b)H.Takaya T.Ohta R.Noyori in Ref. [9b] p. 1;
Noyori R., 1992, CHEMTECH, 360
Bradley D. C., 1978, Metal Alkoxides
Clark R. J. H., 1968, The Chemistry of Titanium and Vanadium
Sharpless K. B., 1987, Chem. Scr., 27, 521
If the in situ self‐assembly leads to more than one catalytically active chiral species cooperative or counteracting stereochemical effects may result. Even in the worst cases ligand acceleration can enable the desired transformation to occur in a rapid and highly stereoselective manner.
Sharpless K. B., 1979, Aldrichim. Acta, 12, 63
(a)H. B.Kaganin ref. [9b] p. 203;
Kagan H. B., 1990, Synlett, 643
Zhao S., 1989, Org. Synth., 68, 49
Narasaka K., 1991, Synthesis, 1
(d)K.Marouka H.Yamamotoin ref. [9b] p. 413;
Trost B. M., 1991, Comprehensive Organic Synthesis
The normal catalyst loadings are 8 mol% for the additions of the functionalized organozinc complexes [30a–g].
β‐Naphthyl appears optimal; α‐naphthyl is too hindered and both the rate andeesuffer [29]. A similar effect was reported by Oguni in the enantioselective opening ofmesoepoxides by Me3SiN3catalyzed by titanium/tartrate mixtures. The complex derived from di‐tert‐butyltartrate ester effected enantioselective catalysis whereas the catalyst prepared with the diisopropyl ester was only effective under stoichiometric conditions:M.Hayashi K.Kohmura N.Oguni Synlett1991 774.
I.Paterson D. J.Berrisford University of Cambridge unpublished results.
G. E.Keck University of Utah unpublished results;
K.Mikami Tokyo Institute of Technology unpublished results.
For recent results with a μ‐oxo‐titanium complex seeM.Terada K.Mikami J. Chem. Soc. Chem. Commun.1994 833.
(a) For recent reviews on asymmetric synthesis of cyanohydrins see:M.North Synlett1993 807;
(c)J. Chem. Soc. Chem. Commun.1991 1752;
(e)J. Chem. Soc. Perkin Trans. 11992 3135.
(c)D. W.Nelson K. B.Sharpless The Scripps Research Institute unpublished results.
Keq(quinuclidine) = 80000 L mol−1(toluene):H. C.Kolb K. B.Sharpless The Scripps Research Institute unpublished results. Binding constants for the alkaloids DHQ and DHQD are greater in toluene than intBuOH. The stoichiometric osmylations with bidentate chiral amines [54] are generally run at low temperature in toluene.
(a) The effect of tertiary amine ligands on the catalytic turnover in the original AD process with NMO as the oxidant in a homogeneous acetone/water system is quite different from that seen in the new two‐phase AD process with potassium hexacyanoferrate(III) as the oxidant. In the NMO system quinuclidine strongly inhibits the catalysis at concentrations above 0.01M[5a] and even the alkaloid ligands DHQ and DHQD begin to show inhibition at concentrations greater than 0.5M[5b]. These effects are probably due to “second‐cycle problems” [5b 57b] which will not be discussed here since they are absent in the now favored potassium hexacyanoferrate(III)‐based AD system.
The commercial AD mixes use 0.4 mol% OsO4to ensure reproducible results. With slower reacting olefins the OsO4loading should be increased to 1 mol%.
For example the presence of the methoxy group on the quinoline ring (the difference between the quinine and cinchonine series) imparts an approximately twofold rate increase (kc) for 2‐vinylnaphthalene [55a].
H.‐L.Kwong Dissertation Massachusetts Institute of Technology Boston USA 1993.
The [3 + 2] mechanism is being examined by a theoretical approach:S.Niwayama K. N.Houk University of California Los Angeles unpublished results.
ref. [44]. However we have not observed metallaoxetanes in the osmylation using matrix isolation techniques:D. V.McGrath G. D.Brabson L.Andrews K. B.Sharpless The Scripps Research Institute unpublished results.
Norbornene undergoes ring‐opening metathesis polymerization with OsO4 an observation that can be rationalized by invoking a metallaoxetane:J. G.Hamilton O. N. D.Mackey J. J.Rooney D. G.Gilheany J. Chem. Soc. Chem. Commun.1990 1600.
C.Burns D.Gilheany C. Y.Park K. B.Sharpless The Scripps Research Institute unpublished results.
(b)Y.Orito S.Imai S.Niwa Nippon Kagaku Kaishi1979 118;
(c)Nippon Kagaku Kaishi1980 670;
(d)Nippon Kagaku Kaishi1982 137;
Bartok M., 1985, Stereochemistry in Heterogeneous Metal Catalysis, 511
H.‐U.Blaser Ciba Geigy AG Basel unpublished results.
For a recent study describing enhanced hydrogenation rates with novel alkaloid‐modified Pt catalysts see:S. P.Griffiths P.Johnston W. A. H.Vermeer P. B.Wells J. Chem. Soc. Chem. Commun.1994 2431.