198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

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Functional Organic Zeolite Analogues 155 In the absence of any catalyst, the reaction is ~ 90% endo-selective and is very slow with a half-life of t = 500 h at 25 °C. The reaction is catalyzed by the solid state of the host (29) almost equally when the latter is used as pulverized powders (with a turnover rate constant of 0.33 h –1 ) or as nonpulverized pieces of an approximate size of 1 ¥ 1 ¥ 1 mm (0.20 h –1 ). The catalyzed reaction gives rise to the endo product (68 endo) with a 96% selectivity. In reference to the criteria shown above, the present heterogeneous catalysis is approximately an order of magnitude more efficient than the corresponding homogeneous catalysis exhibited by soluble resorcinol and shows a remarkably small size effect for the solid catalyst. When immersed in a mixture of reactants, the host readily forms a 1:2:2 adduct 29 ◊ 2(acrolein) ◊ 2(diene) in seconds. The isolated adduct undergoes a gradual intracavity Diels-Alder reaction with an approximate half-life of 2 h. The resulting product adduct 29 ◊ 2(68 endo), when dipped in the reaction mixture, undergoes a facile product/reactant exchange to regenerate the reactant adduct 29 ◊ 2(acrolein) ◊ 2(diene) in minutes. These results, coupled with the negligibly small size effect for the catalyst, suggest that the principal catalytic sites are the internal cavities and not the surface of the solid catalyst. The plausible mechanism (Fig. 17) involves (1) simultaneous binding of two reactants in a cavity, (2) facilitated (~250-fold as compared with the spontaneous process) but still rate-determining intracavity reaction which is preorganized and hence stereoselective, and (3) product/reactant exchange resulting in turnover of the catalyst. In light of the crystal structure for an analogous adduct 29 ◊ 2(ethyl acrylate)◊(diene), there is no doubt that the proximity of dienophile and diene is responsible for the acceleration of the present bimolecular process in the cavity.However,it should not be overlooked that the dienophile hydrogenbonded to the host (O-H◊◊◊O-H◊◊◊O=C-C=C) must be activated electronically, although separation of an overall effect into steric and electronic factors is not easy. The effect of hydrogen bonding becomes clearer when one examines a unimolecular process which should be free from proximity effects. Host 29 also catalyzes the intramolecular ene reaction of an unsaturated terpenoid aldehyde cytronellal (69) to isopulegol (70) (Eq. 3) [79]. 69 70 In the initially formed 1:2 adduct 29 ◊ 2(69), substrate 69 is hydrogen-bonded to the host with its alkenyl group in an extended conformation. Nevertheless, the present cyclization reaction in the cavity is ~ 100-fold faster than the spontaneous process. The acceleration should be primarily ascribed to the hostsubstrate hydrogen bonding. The Diels-Alder and related ene reactions provide a mechanistic prototype of the zeolitic catalyses played by microporous organic solids. From a practical (3)
156 Y. Aoyama Fig. 17. Suggested mechanism for the Diels-Alder reaction catalyzed by host 29 as a microporous solid. Each cavity actually binds two molecules of both reactants. Only one molecule of each is shown here for clarity point of view, there are a couple of problems. One is efficiency. The activity of the present hydrogen-bonded network host 29 is much smaller than those of soluble Lewis acid catalysts conventionally used in organic syntheses. The other is generality. Although the acrolein reaction goes in a catalytic manner, this is not the case for otherwise closely related alkyl acrylates. For example, as noted above, ethyl acrylate forms a similar ternary adduct 29 ◊ 2(dienophile)◊(diene), which
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198 Topics in Current Chemistry Edi
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Design of Organic Solids Volume Edi
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Volume Editors Prof. Dr. Edwin Webe
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VIII preface tion at an amazing lev
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Contents of Volume 196 Carbon Rich
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2 J.P. Glusker 5 Interactions of Pl
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4 J.P. Glusker Perturbations to the
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6 J.P. Glusker 2.1 Sources of Cryst
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8 J.P. Glusker other induced, will
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10 J.P. Glusker The forces here hav
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12 J.P. Glusker lographic structure
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14 J.P. Glusker a c Fig. 6 a - c. D
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16 J.P. Glusker hydrogen bond H◊
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18 J.P. Glusker Fig. 9. Citric acid
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20 J.P. Glusker peptide plane and a
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22 J.P. Glusker 4.5 F◊◊◊H Int
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24 J.P. Glusker rine-containing com
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26 J.P. Glusker Fig. 17. The packin
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28 J.P. Glusker bind to a metal ion
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30 J.P. Glusker As for carboxylate
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32 J.P. Glusker Fig. 24. The magnes
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34 J.P. Glusker structures of magne
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36 J.P. Glusker b a Fig. 28 a, b. A
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38 J.P. Glusker Fig. 30. Complexati
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40 J.P. Glusker 7.1 Descriptions of
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42 J.P. Glusker Fig. 34. Glycine cr
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44 J.P. Glusker bonding capabilitie
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46 J.P. Glusker 2. Non-intercalativ
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48 J.P. Glusker d Fig. 37 d. The ch
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50 J.P. Glusker Fig. 39. Hydrogen-b
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52 J.P. Glusker 72, 104], but other
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54 J.P. Glusker 23. Cody V, Murray-
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56 J.P. Glusker: Directional Aspect
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58 A. Nangia · G.R. Desiraju 7 Sup
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60 A. Nangia · G.R. Desiraju elect
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62 A. Nangia · G.R. Desiraju 14 15
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64 A. Nangia · G.R. Desiraju [23]
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66 A. Nangia · G.R. Desiraju struc
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68 A. Nangia · G.R. Desiraju Fig.
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70 A. Nangia · G.R. Desiraju -NH 2
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72 A. Nangia · G.R. Desiraju ingfu
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74 A. Nangia · G.R. Desiraju and r
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76 A. Nangia · G.R. Desiraju bonds
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78 A. Nangia · G.R. Desiraju 2.86-
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80 A. Nangia · G.R. Desiraju from
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82 A. Nangia · G.R. Desiraju ted a
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84 A. Nangia · G.R. Desiraju envir
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86 A. Nangia · G.R. Desiraju PYRAZ
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88 A. Nangia · G.R. Desiraju napht
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90 A. Nangia · G.R. Desiraju b c F
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92 A. Nangia · G.R. Desiraju all a
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94 A. Nangia · G.R. Desiraju 42. J
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Hydrogen-Bonded Ribbons, Tapes and
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Page 153 and 154: 146 Y. Aoyama clusion compounds (se
Page 155 and 156: 148 Y. Aoyama are intriguing guests
Page 157 and 158: 150 Y. Aoyama The tuning or enginee
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Page 165 and 166: 158 Y. Aoyama 5 Concluding Remarks
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Crystalline Polymorphism of Organic
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Author Index Volumes 151-198 Author
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Author Index Volumes 151 - 198 2 1
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Author Index Volumes 151 - 198 213
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Author Index Volumes 151 - 198 2 ]
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Springer and the environment At Spr
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