Product-induced distortion of a metalloporphyrin host: Implications for acceleration of Diels-Alder reactions

Journal of the American Chemical Society

Volumn 122, Issue 22, 2000, Pages 5286-5293

  Nakash, Moshe ^a   Clyde Watson, Zöe ^a   Feeder, Neil ^a   Davies, John E ^a   Teat, Simon J ^b   Sanders, Jeremy K M ^a  

^a UNIVERSITY OF CAMBRIDGE   (United Kingdom)

^b DARESBURY LABORATORY   (United Kingdom)

Author keywords

[No Author keywords available]

Indexed keywords

METALLOPORPHYRIN;

ARTICLE; CHEMICAL REACTION; CHEMICAL STRUCTURE; MOLECULAR INTERACTION; SYNTHESIS; UNITED KINGDOM;

EID: 0034616805     PISSN: 00027863     EISSN: None     Source Type: Journal    
DOI: 10.1021/ja9922227     Document Type: Article

References (59)

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21. The X-ray crystal structures for complexes between an antibody and a transition state analogue and an inhibitor have been recently reported. (a) Romesberg, F. E.; Spiller, B.; Schultz, P. G.; Stevens, R. C. Science 1998, 279, 1929. (b) Heine, A.; Stura, E. A.; Yli-Kauhaluoma, J. T.; Gao, C.; Deng, Q.; Beno, B. R.; Houk, K. N.; Janda, K. D.; Wilson, I. A. Science 1998, 279, 1934.
22. The general route is described in: Twyman, L. J.; Sanders, J. K. M. Tetrahedron Lett. 1999, 40, 6681.
23. 5 suggest that the accelerated reaction in the presence of such cyclic-metalloporphyrin hosts occurs inside the cavity of the host.
24. 2+ cations, and could account for the low acceleration rate (of only 2-fold) observed with monomer 8.
25. 2+ cations, and could account for the low acceleration rate (of only 2-fold) observed with monomer 8.
26. 2+ cations, and could account for the low acceleration rate (of only 2-fold) observed with monomer 8.
27. (c) As each of the porphyrin units in hosts 4-7 can bind the reactants inside or outside the cavity, only a small fraction of bound host is the reactive complex (host + the two reactants inside the cavity) which leads to accelerated product formation. Therefore, the acceleration rates shown in Table 1 underestimate the true ability of such hosts to accelerate the Diels-Alder reaction inside the cavity of the host. The kinetic and stoichiometric aspects of the Diels-Alder reaction in the presence of cyclic-metalloporphyrin hosts have been thoroughly discussed elsewhere: Wylie, R. S.; Sanders, J. K. M. Tetrahedron 1995, 51, 513.
28. (a) Because of the poor quality of the diffraction data, it cannot be proved unequivocally that disordered solvent is not present in the crystal structure of the 6-3 complex.
29. (b) However, as 3 occupies a substantial part of the cavity of 6. clearly seen in Figure 3c and more so in the space-filling model of the 6-3 complex (not shown), the Diels-Alder product 3 is poorly solvated (if at all) inside the cavity of 6.
30. (c) In addition, it is likely that even if solvent is trapped in the space left in this cavity there will still be fewer molecules of solvent + ligand 3 in the cavity in the case of the 6-3 complex (Figure 3c) than in the cavity of the free host 6 (see Figures 2 and 3a).
31. In some systems, displacement of solvent molecules has been suggested as the driving force for binding and acceleration: (a) Kang, J.; Rebek, J., Jr. Nature 1996, 382, 239. (b) See also ref 6c.
32. In some systems, displacement of solvent molecules has been suggested as the driving force for binding and acceleration: (a) Kang, J.; Rebek, J., Jr. Nature 1996, 382, 239. (b) See also ref 6c.
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39. (a) As the two porphyrin units in 6 and in the 6·3 complex are qualitatively not offset relative to each other (see Figure 3b,d), this angle is truly describing the degree of leaning toward or away from the cavity of the two porphyrin units in the host.
40. (b) The same is also true for 7 (Figure 4).
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42. (a) This could be seen by examining molecular models.
43. (b) Due to the lack of high-quality Zn⋯N parameters in force field packages that we know of, we do not present here a calculated structure for the 6·12 complex. Obviously, these type of complexes are too large for high-level molecular orbital calculations with current computational resources available to us.
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46. As the precision in the location of hydrogen atoms is less than that for oxygen atoms, we present the O⋯O distances.
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