Guest exchange

Figure 7. Top The formation of heterodimer 6-7 from softballs 6 6 and 7-7. Bottom Two SN2-like window" mechanisms for guest exchange in the softballs. Attack in an approximate 90°C angle between incoming and leaving guest is favored over backside attack energetically - only ten instead of twelve hydrogen bonds are broken - and entropically - statistically, there are four ways to open two vicinal windows, but only two ways to open windows opposite to each other.

J. Santamaria, T. Martin, G. Hilmersson, S. L Craig, J. Rebek, Jr, Guest exchange in an encapsulation complex A supramolecular substitution reaction , Proc Natl. Acad. Sci. USA 1999, 96, 8344-8347. 

The gas-phase guest-exchange reaction 29 has been employed to probe the enantioselectivity of permethylated /3-CD for pharmacologically important compounds, such as DOPA, amphetamine, ephedrine, and penicillamine (Scheme... 

A variety of alkyl amines B, including 1-propylamine, ethylene diamine, 1,3-diaminopropane, and (7 )-l-amino-2-propanol have been used as reactants. The guest exchange kinetic results are reported in Table 13. The presence of more than one reacting [/3-CD-H-A] structure is observed with A = DOPA and penicillamine. The results have been rationalized in terms of specific interactions in the relevant inclusion complexes which determine their structure and relative stability. 

The molecular asymmetry of 9 is due to the four axial pendants containing the chiral L-vahne group. The efficiency of the gas-phase exchange reaction 30 where A are representative amino acids and B is either (5)-(- -)- or R)- — )-2-butylamine is appreciably affected by the configuration of both A and B. The guest exchange kinetic results are reported in Table 17. The presence of more than one reacting [9-H-A]" structure is observed with A = DOPA. A similar behavior was observed with permethylated /8-CD as the host." " ... 

Pluth, M.D. and Raymond, K.N. (2007) Reversible guest exchange mechanisms in supramolecular host-guest assemblies. Chem. Soc. Rev, 36 (2), 161-171. 

If separate peaks for bound and free guest are not observed at room temperature, sometimes guest exchange can be slowed down sufficiently by lowering the temperature in the NMR probe. Temperatures as low as -90 °C may be reached in conventional deuterated solvents such as CD2C12 or CO (CD3) 2, and in special cases even lower temperatures may be achieved through the use of freons such as CF2C12. 

Given a measurement of guest exchange rate, application of the Eyring equation (3.6) allows the activation free energy AG of the complexation process to be determined. 

The kinetics of guest exchange are fast because the dendrimer branches are held well apart by the large cores. 

Scheme 6.20 Reaction of a resorcarene triol with dihaloalkanes to give a hemicarcerand with a portal for guest exchange.

Permeability should be demonstrated (e.g. by gas sorption measurements, spectroscopic evidence of guest exchange or crystallography). 

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