Guest complexes aldehydes

Small changes in the shape of the aldehydes have dramatic effects on reactivity with the encapsulated host-guest complex. For example, the host-guest complex reacts with isobutyraldehyde (entry 5) with a lower diastereoselectivity than with n-butyraldehyde (entry 3). This may be because the isobutyraldehyde complex is more spherical than the n-butyraldehyde complex. When comparing the five-carbon aldehydes (entries 6-8), only isovaleraldehyde undergoes C-H bond activation in 1. 

Likewise, bifunctional thiourea catalysts have been appUed in MCRs in a host-guest complex, largely increasing the reaction stereocontrol [36]. Thus, aliphatic aldehydes 9, acetone... 

With the knowledge that 14 can activate aldehydes in 1, the role of 1 in the reaction was explored further. Specifically, the relative rates of C—H bond activation and guest ejection, and the possibility of ion association with 1, were investigated. The hydrophobic nature of 14 could allow for ion association on the exterior of 1, which would be both cn t h al pi cal I y favorable due to the cation-it interaction, and entropically favorable due to the partial desolvation of 14. To explore these questions, 14 was irreversibly trapped in solution by a large phosphine, which coordinates to the iridium complex and thereby inhibits encapsulation. Two different trapping phosphines were used. The first, triphenylphosphine tris-sulfonate sodium salt (TPPTS), is a trianionic water-soluble phosphine and should not be able to approach the highly anionic 1, thereby only trapping the iridium complex that has diffused away from 1. The second phosphine, l,3,5-triaza-7-phosphaadamantane (PTA), is a water-soluble neutral phosphine that should be able to intercept an ion-associated iridium complex. 

Although, at that time, the term supramolecular chemistry had not yet been coined, the practical potential for inclusion complexation for acetylene alcohol guests 1 and 2 was recognized back in 1968 [12], Spectroscopic studies showed that 1 and 2 formed molecular complexes with numerous hydrogen-bond donors and acceptors, i.e. ketones, aldehydes, esters, ethers, amides, amines nitriles, sulfoxides and sulfides. Additionally, 1 formed 1 1 complexes with several n-donors, such as derivatives of cyclohexene, phenylacetylene, benzene, toluene, etc. The complexation process investigated by IR spectrometry revealed the presence of OH absorption bands at lower frequencies than those for uncomplexed 1 and 2 [12], These data, followed by X-ray studies, confirmed that the formation of intermolecular hydrogen bonds is the driving force for the creation of complexes [13],... 

A direct solution to the problem of enantioselective furan-carbonyl photoproduct synthesis would require a photoaddition that proceeded with enantiofacial selectivity in the aldehyde or furan component and maintained the relative face selectivity that is intrinsic to the reaction. Preliminary work used host-guest chemistry to achieve this objective.In aqueous dioxane, a 1 1 1 inclusion complex of unmodified 3-cyclodextrin, furan and benzaldehyde is formed. Upon irradiation (Hanovia 450 W lamp, Vycor filter), a rapid photoaddition occurs to afford a photoproduct of 10-20% ee. 

The use of cyclodextrins14 has provided the ability to conduct the Strecker reaction with TMSCN in water via supramolecular catalysis involving reversible guest-host interactions. Activation of imine 16 by complexation with the hydroxyl groups present in cyclodextrins was found to work best with p-cyclodextrin. This chemically green reaction could be applied to ketones as well as aldehydes. 

In receptors 23a-e, and 24 the uranyl center contains only one "vacant" position. We have also synthesized so-called "naked salophenes" 25a-c which have two vacant positions for complexation with guests [ref 28]. "Naked" U02-salophenes 25a-c have been prepared by reaction of the corresponding aldehydes, o-aminophenol and U02(0Ac)2 2H20 in refluxing MeOH in yields of 91-98%.Oi ange single crystals of the complex of "naked" ligand 25a with... 

The use of chiral diols as co-catalyst in aldol reaction led to an improvanent of the achieved results [41]. Thus, when acetone (3a, 8.18 equiv.) was reacted with benzaldehyde (2 h) in DMSO at 0°C catalyzed by (5)-proline (30 mol%) the expected product 4 was obtained in 72% ee, while a 96% ee was achieved in the presence of (R)-BINOL (0.5 mol%). A hypothetical explanation from the authors for this effect is the possible template effect of the chiral diol which may activate and ordered the aldehyde and enamine nucleophile. The same reason was claimed for the beneficial effect achieved by addition of a 10 mol% of (3,5-bistrifluoromethylphenyl)thiourea in the aldol reaction between cyclohexanone (3b) and several aromatic aldehydes catalyzed by proline (1,10 mol%) in hexane a 25°C [42], In this case, reaction times, yields as well as diastereo- and enantioselectivities were improved (75-98%, 76-88% de, 98-99% ee), with these results being also attributed to the enhancement of the proline solubility by the formation of a host-guest proline-thiourea complex. 

Kikuchi, Y., Kobayashi, K., and Aoyama, Y. (1992) Complexation of chiral glycols, steroidal polyols, and sugars with a multibenzenoid, achiral host as studied by induced circular-dichroism spectroscopy exciton chirality induction in resorcinol-aldehyde cyclotetramer and its use as a supramolecular probe for the assignments of stereochemistry of chiral guests, J. Am. Chem. Soc. 114, in press. 

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