Abstract:Hemicarcerand 1, whose synthesis from t w o tetrol bowls and four pentamethylene ditosylate molecules was templated b y veratrole, was found to bind in solution (one-to-one) eighteen different guests; the crystal structure of 104MeCN is centrosymmetric; hemicarcerand 2, containing four hexamethylenedioxy groups, is also discussed.
“…As expected, the larger hexamethylene derivative 44a only formed stable, isolable hemicarceplexes with fairly large guests such as 2-adamantanone, 12-crown-4, and 15-crown-5 . Nonetheless, the large range of guests that were both successfully and unsuccessfully complexed with this series of hosts (i.e., 42 − 45 ) nicely demonstrates that as the interhemispheric bridges get longer, the resultant host becomes more adaptable to guest entry and departure.…”
Section: Small Hemicarceplexes Containing Four Slotted Portalssupporting
confidence: 53%
“…The basic synthetic strategy (i.e., a “2 bowl + 4 linker” approach) is the same as that used for carceplex 7 ·guest, as tetrol 6 served as the basic building block for 7 ·guest as well as for a large number of hemicarceplexes. Thus, the versatility of this approach is evident by the large variations in hemicarceplexes and hemicarcerands (i.e., 35 − 48 ) reported to date, ,− as summarized in Chart . 10 1 …”
Section: Small Hemicarceplexes Containing Four Slotted Portalsmentioning
confidence: 99%
“…Likewise, by successively increasing the length of the linker by one and two more methylenes (respectively), the pentamethylene and hexamethylene bridged hemicarcerands 43a and 44a were obtained in ca. 20% yields …”
Section: Small Hemicarceplexes Containing Four Slotted Portalsmentioning
confidence: 99%
“…Not surprisingly, the two penta-atom-bridged hemicarcerands, 43 65 and 45 , display virtually identical complexation properties. These hosts formed stable complexes with larger straight-chain guests such as CHCl 2 CHCl 2 and (±)-MeCHBrCHBrMe, cyclic species such as 12-crown-4, ortho - and para -disubstituted (halo and methoxy) benzenes, trisubstituted benzenes (e.g., 1,2,3-trimethoxybenzene, 1,2,3-tribromobenzene, and 4-bromoveratrole), monohalo- and 1,2-dibromo-substituted cylcohexanes, and various bicylic, tricyclic, and quadracylcic compounds (e.g., naphthalene, 2,5-norbonandiene, norbonene, exo -norborneol, 2-norbornanone, norbornane, exo -2,3-epoxynorbornane, 7-oxabicyclo[2.2.1]heptane, and quadricyclane). , …”
Section: Small Hemicarceplexes Containing Four Slotted Portalsmentioning
confidence: 99%
“…Catalytic hydrogenation over Pd at 10 psi cleanly provided the hexamethylene-bridged hemicarcerand, 44b . It should be noted, however, that the phenethyl-footed analogue ( 44a ) of the latter hemicarcerand has since been directly synthesized by Cram et al in higher overall yield from tetrol 6b 11 12 …”
Section: Hemicarceplexes With Significantly Large Cavitiesmentioning
“…As expected, the larger hexamethylene derivative 44a only formed stable, isolable hemicarceplexes with fairly large guests such as 2-adamantanone, 12-crown-4, and 15-crown-5 . Nonetheless, the large range of guests that were both successfully and unsuccessfully complexed with this series of hosts (i.e., 42 − 45 ) nicely demonstrates that as the interhemispheric bridges get longer, the resultant host becomes more adaptable to guest entry and departure.…”
Section: Small Hemicarceplexes Containing Four Slotted Portalssupporting
confidence: 53%
“…The basic synthetic strategy (i.e., a “2 bowl + 4 linker” approach) is the same as that used for carceplex 7 ·guest, as tetrol 6 served as the basic building block for 7 ·guest as well as for a large number of hemicarceplexes. Thus, the versatility of this approach is evident by the large variations in hemicarceplexes and hemicarcerands (i.e., 35 − 48 ) reported to date, ,− as summarized in Chart . 10 1 …”
Section: Small Hemicarceplexes Containing Four Slotted Portalsmentioning
confidence: 99%
“…Likewise, by successively increasing the length of the linker by one and two more methylenes (respectively), the pentamethylene and hexamethylene bridged hemicarcerands 43a and 44a were obtained in ca. 20% yields …”
Section: Small Hemicarceplexes Containing Four Slotted Portalsmentioning
confidence: 99%
“…Not surprisingly, the two penta-atom-bridged hemicarcerands, 43 65 and 45 , display virtually identical complexation properties. These hosts formed stable complexes with larger straight-chain guests such as CHCl 2 CHCl 2 and (±)-MeCHBrCHBrMe, cyclic species such as 12-crown-4, ortho - and para -disubstituted (halo and methoxy) benzenes, trisubstituted benzenes (e.g., 1,2,3-trimethoxybenzene, 1,2,3-tribromobenzene, and 4-bromoveratrole), monohalo- and 1,2-dibromo-substituted cylcohexanes, and various bicylic, tricyclic, and quadracylcic compounds (e.g., naphthalene, 2,5-norbonandiene, norbonene, exo -norborneol, 2-norbornanone, norbornane, exo -2,3-epoxynorbornane, 7-oxabicyclo[2.2.1]heptane, and quadricyclane). , …”
Section: Small Hemicarceplexes Containing Four Slotted Portalsmentioning
confidence: 99%
“…Catalytic hydrogenation over Pd at 10 psi cleanly provided the hexamethylene-bridged hemicarcerand, 44b . It should be noted, however, that the phenethyl-footed analogue ( 44a ) of the latter hemicarcerand has since been directly synthesized by Cram et al in higher overall yield from tetrol 6b 11 12 …”
Section: Hemicarceplexes With Significantly Large Cavitiesmentioning
The role of templating has been found in a myriad of chemical processes and is especially important in biological systems. The most remarkable example, perhaps, is DNA replication, where a single strand of DNA templates the formation of a complementary strand through the assembly of nucleotide bases. Outside biological systems, templates play integral roles in the formation of crown ethers [ 11, catenanes and rotaxanes [2, 31 (Chapter 3), molecularly imprinted polymers [4] (Chapter 2), zeolites [5], polyporphyrins [6] (Chapter I), and other large-ring systems [7]. Thus, the role of templating is invaluable to the field of supramolecular chemistry [ 1 b, 81, where chemical synthesis of complex structures is not limited by classical approaches that strictly involve covalent bond formation. Templates aid the efficient assembly of multiple components into a particular geometry through the use of non-covalent interactions such as hydrogen bonding, w-stacking, and van der Waals attractions. The word template has been described by Busch as a chemical entity that "organizes an assembly of atoms, with respect to one or more geometric loci, in order to achieve a particular linking of the atoms" [9]. In this chapter, we discuss recent research that fits this definition nicely, and focuses on the use of templating in the synthesis of the host-guest systems known as carceplexes [lo], hemicarceplexes [lo], and molecular capsules [ 111.Donald J. Cram created the field of carceplexes and hemicarceplexes [ 101. Carceplexes are defined as globe-shaped molecules that permanently entrap smaller guest molecules within their confines [ lo]. Consequently, the incarcerated guest cannot escape without breaking covalent bonds [ 101. Hemicarceplexes are closely related to carceplexes as both can contain molecules within their interiors. However, hemicarceplexes contain portals through which the entrapped guest can exit, given the appropriate conditions [lo]. Carcerands and hemicarcerands are analogous to the above, consisting of the corresponding host shell containing no entrapped guest [ 101. Capsules are reversible assemblies formed in solution consisting of closed-off spheroid cavities capable of encapsulating guests while excluding the solvent. The presence of a suitable template appears to be required for the formation of carceplexes, and sometimes for the formation of hemicarceplexes and molecular capsules.
The thermodynamically controlled reactions of water-soluble tetraformylcavitand 2 with two equivalents of H(2)N(CH(2))(n) NH(2) (n=2-4) in the presence of a suitable templating guest give hemicarceplexes 1 a-c⋅guest, the yield of which depends on the match between size and shape of the guest and that of the inner phase. These hemicarceplexes are dynamic and dissociate upon addition of acid and reform upon basification. In water, they exchange guests through temporary hydrolysis of imine bonds. To test 1 b as molecular reaction flask, 3-noradamantyldiazirine 6 was encapsulated and photolyzed at 350 nm to produce Bredt olefin protoadamantene 5 and 1-noradamantyldiazomethane 8 in a 4:1 ratio. Encapsulated protoadamantene is stable for days at room temperature in (CD(3))(2)SO/CD(3)CN (t(1/2) = 5.5 days) and has a lifetime of several minutes in D(2)O.
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