**112**

**Figure 44.** Silica-supported catalyst **112**.

Jones and coworkers [144] synthesized a series of heterogeneous polymeric catalysts by immobilizing Co–salen complexes on CAB-O-SIL silica-supported polymer brushes (Figure 45). The HKR of epichlorohydrin in CH2Cl2 was employed as a model reaction to compare the catalytic properties of polymeric catalyst **114** with the less flexible polymeric analogue **113**, the homogenous catalyst **64** (Figure 26), and the monosalen analogue **115**. Amongst all the catalysts, polymer **114** exhibited the highest reactivity and enantioselectivity (close to 50% conversion and > 99% *ee* for both the ring-opened product and the unreacted epoxide). The catalyst with a less flexible linker, polymer **113**, was less reactive than **114**, but still exhibited high enantioselectivity (>99% *ee* for the ring-opened product and 95% *ee* for the unreacted epoxide). Monomeric catalyst **115** performed worst, both in terms of reactivity and enantioselectivity, which was attributed to its low local concentration of Co–salen sites

and thus diminished Co–salen cooperativity. Catalyst **114** was also investigated in recycling studies, where the enantioselectivity could be maintained over five catalytic runs, although the reaction rate decreased after each subsequent run. The FTIR and elemental analysis studies indicated that the loss of reactivity might be caused by cleavage of the salen ligand under the reaction condition.

 **Figure 45.** Silica-immobilized copolymeric Co–salen complexes (**113** and **114**) and silica-immobilized monomeric complex (**115**).

**115**

Jacobsen's group [145] successfully immobilized Co(III)salen complexes on gold colloids through an exchange reaction between thiol-containing Co–salen complexes and *n*-octanethiolates, the latter pre-coordinated to the gold colloid (Figure 46). With low catalyst loading (0.01 mol% Co), the so-immobilized catalyst **116** catalyzed the solvent-free HKR of 1,2-epoxyhexane ten times faster than the non-immobilized analogue **117** (Figure 46), achieving complete resolution within5h(≥50% conversion and >99% *ee* of the unreacted epoxide). The catalyst could be recovered by filtration through centrifugal filter units and recycled several times with maintained reactivity and enantioselectivity, only requiring re-oxidation after six consecutive reaction cycles.

Kim and coworkers [65,128] published several studies on the immobilization of salen catalysts on inorganic materials such as mesoporous silicate or alumosilicate. A series of monomeric and dimeric Co–salen catalysts were grafted onto mesoporous silica MCM-41 (Figure 47). In the solvent-free HKR of epichlorohydrin, styrene oxide, and 1,2-epoxyhexane, each of the silica-supported monosalen catalysts **118**–**121** exhibited decreased reactivities but comparable enantioselectivities compared to homogenous monosalen catalyst **63** (Scheme 40) [65]. In a later study, Kim and coworkers [128] immobilized

dimeric Co–salen complexes **54**–**56** (Scheme 34) on MCM-41 (catalysts **122**–**124** in Figure 47). Similar to what was observed for the immobilized monosalen catalysts **118–121**, the immobilized dimeric catalysts **122**–**124** also exhibited decreased reactivities but comparable enantioselectivities in the HKR of epichlorohydrin compared to homogeneous analogues **54**–**56** (Scheme 34). In both these cases, it is clear that the immobilization hindered efficient catalysis. One explanation could be the relatively short linkers used, as previous studies clearly show that the flexibility and length of the linker is of paramount importance in enabling efficient catalysis. Another possible explanation could be the choice of mesoporous silica as solid support.

**Figure 46.** Au colloid-supported Co–salen catalyst **116** and non-immobilized analogue **117**.

*t*%X

**Figure 47.** MCM-41-supported salen catalysts **118**–**124**.

To this end, Kim and coworkers [146] studied the effect of the pore size and structure of the silica support on the catalytic activity. Dimethylcarbonate (DMC) was used to partially desilylate mesoporous silica, and the resulting material was then used as a solid support for polymeric Co–salen complexes. The so-formed catalysts exhibited increased reactivities and enantioselectivities in the HKR of terminal epoxides compared to catalysts prepared without pretreatment with DMC.

Kim and coworkers [147] also investigated other mesoporous materials such as SBA-15 and SBA-16 with a wider range of pore sizes and higher stability. Co–salen complexes were attached on sulfonate functionalized SBA-16 through electrostatic interaction between the cobalt and the sulfonate (Figure 48). The so-formed catalyst **125** was employed in the HKR of epichlorohydrin in THF, affording the unreacted epoxide in 44% yield and 98% *ee*, although similar results were also obtained with homogeneous analogues.

**Figure 48.** SBA-16-supported Co–salen catalyst **125**.

In a later study, the same group reported another type of non-covalent immobilization based on interactions between fluorine-bearing catalysts and different acidic sites in mesoporous Al-SBA-15 (Figure 49) [148,149]. The so-formed catalyst **126** was investigated in the HKR of terminal epoxides in THF, affording unreacted epoxides in full conversions (50%) and with good-to-excellent enantioselectivities (up to 99% *ee*). However, homogeneous analogues gave comparable results in shorter reaction times.

**Figure 49.** Al-SBA-15-supported Co–salen catalyst **126**, formed by interactions with (**a**) Lewis acidic sites and (**b**) Brønsted acidic sites.
