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note
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It should be noted that, because of LAE, the chiral ligand concentration also affects the reaction rate. In fact, in the runs with variable amounts of 5 the expected trends in the catalytic activity were generally observed for both styrene and 1-phenyl-cyclohexene, but they are not discussed in detail here due to the difficulty of obtaining good-quality kinetic data in the biphasic system under study. In any case, >95% conversion values could be obtained in 16 h, regardless of the alkaloid concentration.
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Ph. D. Dissertation, Univ. of Pisa
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A. Mandoli, Ph. D. Dissertation, Univ. of Pisa, 1998.
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(1998)
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Mandoli, A.1
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75
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0026868216
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a) K. B. Chung, M. Tomoi, J. Polym. Sci., Part A: Polym. Chem. 1992, 30, 1089-1097;
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(1992)
J. Polym. Sci., Part A: Polym. Chem.
, vol.30
, pp. 1089-1097
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Chung, K.B.1
Tomoi, M.2
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76
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0001685528
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b) M. E. Wilson, K. Paech, W.-J. Zhou, M. J. Kurth, J. Org. Chem. 1998, 63, 5094-5099.
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(1998)
J. Org. Chem.
, vol.63
, pp. 5094-5099
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Wilson, M.E.1
Paech, K.2
Zhou, W.-J.3
Kurth, M.J.4
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77
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16444382624
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note
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For instance, the last step of a hydroboration/oxidation/Williamson alkylation sequence afforded an intractable mixture of products, possibly due to a competitive nitrogen quaternization followed by base-promoted cleavage of the quinuclidine structure. An alternative route, based on the initial olefin cross-metathesis with 10-undecene-1-ol followed by hydrogenation, resulted in low conversions in both steps under a range of conditions.
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78
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0026340731
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Even if it has been shown that substrates containing thioether groups can be dihydroxylated without affecting the sulfur moiety, previous work in this group indicated that the omission of the oxidation step causes a retardation of AD when the supported ligands are employed in catalysis. The presence of sulfone groups in 12 and 13 is assumed by analogy with solution-phase behavior: W. Priebe, G. Grynkiewicz, Tetrahedron Lett. 1991, 32, 7353-7356.
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(1991)
Tetrahedron Lett.
, vol.32
, pp. 7353-7356
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Priebe, W.1
Grynkiewicz, G.2
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79
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n. See, for example: J. E. Hofmann, T. J. Wallace, P. A. Argabright, A. Schriesheim, Chem. Ind. (London) 1963, 1243-1244.
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(1963)
Chem. Ind. (London)
, pp. 1243-1244
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Hofmann, J.E.1
Wallace, T.J.2
Argabright, P.A.3
Schriesheim, A.4
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80
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16444378930
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note
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The glass frit employed had an average porosity of 10-16 μm, but further filtration of the solution containing the leached alkaloid through a 0.2 μm membrane did not cause the absorbance to change appreciably. Therefore, the size of the dissolved species is, at most, in the sub-micron range.
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81
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16444365648
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note
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2O solvent system was also observed in the case of the modified architecture of 12 and 13a,b. At present, however, the complex nature of the triphasic system under investigation makes it difficult to ascertain if this behavior is related to a slow base-induced fragmentation of the covalent linker or to the intrinsic heterogeneity of the polymeric materials, that could lead to a medium-dependent release of minor amounts of trapped monomers and soluble oligomers into solution.
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82
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note
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4 volatilization, the procedure discussed in the text was preferentially used in this work.
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note
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In response to a referee's comment, it must be noted that because the focus of this study was on the recycling of the chiral ligand, no detailed investigation was carried out to asses the extent of metal leaching or to optimize the recovery of osmium compounds. Nonetheless, after extensive washing with water (required to remove the abundant precipitate of inorganic salts) a very low catalytic activity in the AD reaction was obtained for the recovered polymeric material, thereby suggesting a substantial metal loss. This conclusion is in agreement with previous observations.
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84
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16444376605
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note
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Effectively, because the achievement of a certain level of enantioselectivity is dependent on both the ligand and the osmium concentration, it could be speculated that the higher ee values obtained in the supposed heterogeneous runs are a consequence of the trapping of most of the osmium by the polymeric ligand in a catalytically inactive form. This would leave a small residual amount of active metal species in solution, for which the tiny amount of leached alkaloid could prove sufficient in promoting a highly stereoselective homogeneous process. This hypothesis can, however, be discarded in view of the kinetic data presented. In particular, the dihydroxylation rate in the presence of 12 appears to be not lower than in the control experiments, thus demonstrating that the concentration of catalytically active osmium species attained in the former situation is at least comparable with that employed in the latter case.
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85
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0010324782
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The possibility of running the AD, for preparative purposes, with reduced amounts of both the chiral ligand and the osmium compound has been noted before: J. M. McIntosh, E. J. Kiser, Synlett 1997, 1283-1284.
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(1997)
Synlett
, pp. 1283-1284
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McIntosh, J.M.1
Kiser, E.J.2
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86
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0000656358
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R. A. Sheldon, M. Wallau, I. W. C. E. Arenas, U. Schuchardt, Acc. Chem. Res. 1998, 31, 485-493.
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(1998)
Acc. Chem. Res.
, vol.31
, pp. 485-493
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Sheldon, R.A.1
Wallau, M.2
Arenas, I.W.C.E.3
Schuchardt, U.4
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87
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note
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It must be noted that in the preparation of 8 and 11 prolonged heating above 50 °C, in the presence of NaH, should be avoided to prevent the formation of by-products, presumably having epimeric structures (by NMR spectroscopy).
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D. C. Sherrington, A. Lanver, H.-G. Schmalz, B. Wilson, X.-W. Ni, S. Yuan, Angew. Chem. Int. Ed. 2002, 41, 3656-3659, Angew. Chem. 2002, 114, 3808-3811.
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(2002)
Angew. Chem. Int. Ed.
, vol.41
, pp. 3656-3659
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Sherrington, D.C.1
Lanver, A.2
Schmalz, H.-G.3
Wilson, B.4
Ni, X.-W.5
Yuan, S.6
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89
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0037020377
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D. C. Sherrington, A. Lanver, H.-G. Schmalz, B. Wilson, X.-W. Ni, S. Yuan, Angew. Chem. Int. Ed. 2002, 41, 3656-3659, Angew. Chem. 2002, 114, 3808-3811.
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(2002)
Angew. Chem.
, vol.114
, pp. 3808-3811
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