Epoxidations, MOFs

Figure 4.6 An enantioselec-live MOF epoxidation catalyst. Zn-(l,4 -biphenyldicarboxylate) sheets in the ab direction are connected by functionalized Mn-salen ligands to form a doubly...

The attractive (80) features of MOFs and similar materials noted above for catalytic applications have led to a few reports of catalysis by these systems (81-89), but to date the great majority of MOF applications have addressed selective sorption and separation of gases (54-57,59,80,90-94). Most of the MOF catalytic applications have involved hydrolytic processes and several have involved enantioselec-tive processes. Prior to our work, there were only two or three reports of selective oxidation processes catalyzed by MOFs. Nguyen and Hupp reported an MOF with chiral covalently incorporated (salen)Mn units that catalyzes asymmetric epoxidation by iodosylarenes (95), and in a very recent study, Corma and co-workers reported aerobic alcohol oxidation, but no mechanistic studies or discussion was provided (89). 

As already discussed in this chapter, aluminum, in addition to its well-known high oxygenophilicity (Al-O = 511 3 kJ mol ), has exceedingly high affinity toward fluorine this is evident from the bond strengths in several metal-fluorine diatomic molecules Al-F, 663.6 6.3 kJ moFh Li-F, 577 + 21 kJ mol" Ti-F, 569 + 34 kJ moF Si-F, 552.7 + 2.1 kJ moF Sn-F, 466.5 + 13 kJ moF and Mg-F, 461.9 + 5.0 kJ moF [76]. Organoaluminum reagents seem, therefore, quite suitable for fluorine-assisted selective alkylation of fluoro epoxides, which also represents the experimental demonstration of the intervention of pentacoordinate chelate complexes of trialkyl-aluminums as plausible intermediates [63]. 

Thermodynamic parameters have been obtained from kinetic studies of the thermal decomposition of thiepins <74X2431>. The sulfur extrusion reaction of (S3) was examined kinetically and showed a large negative value (-24 J moF ) consistent with a decrease in entropy in the transition state which would be anticipated to be close to a thianorcaradiene structure. Thus while the latter valence tautomer of any thiepin has yet to be detected directly, the kinetic data, allied to the other evidence discussed, support the view that thiepins are analogous to oxepins in undergoing spontaneous tautomerization to their episulflde or epoxide forms. 

Two different Cu-based bipyridine MOFs were also reported to be active epoxidation catalysts. [Cu(H2btec)(bipy)] was used for the epoxidation of cyclohexene and styrene with tert-butyl hydroperoxide (TBHP) as the oxidant, with yields of 65% and 24%, respectively, after 24h of reaction at 75 °C and TOFs as high as 79 h for cyclohexene [115]. [Cu(bipy)(H20)2(BF4)2(bipy)] was also used for the solvent-free... 

When appHed in the asymmetric epoxidation of 2,2-dimethyl-2H-chromene, this MOF was less active than the homogeneous salen complex, but more stable over time, as the immobihzation of the Mn(salen) complexes precludes oxidation of one metal center by another. Immobilization decreases the flexibility of the hgands, explaining the slight decrease in enantioselectivity compared to the free complex. While a small loss of catalytically inactive Mn was observed after each ran, the enantioselectivity remained constant however, upon prolonged use of this catalyst, the outer layers of the MOF crystals were gradually damaged by oxidation. This... 

Magnetically separable catalysts synthesised by Saikia that show effective utilisation of Fe304 impregnated chromium-based MOF have been used for the solvent-free oxidation of benzyl alcohol in the presence of TBHP. Sulfonic acid functionalised chromium MILlOl can be used as an efficient catalyst for the vapour-phase dehydration of butanol by oleic acid. Also, bimetallic MOF catalysts of iron and chromium (Fe(Cr)-MIL-lOl) have received attention for epoxidation of styrene to styrene oxide with excellent selectivity and catalytic recyclability. ... 

Figure 8.25 The plausible reaction mechanism for the cycloaddition of CO with epoxides catalyzed by MOF-5 and TBABr (adapted from Ref. [96]).

Scheme 17.9 Heterogeneous cataiysis by Cd(ii) MOFs 23 and 24, during (a) epoxidation reactions of oiefins (b) intermoiecuiar aidol reactions of aide-hydes and ketones.

Suslick et al found that the porphyrinic MOF, PIZA-3, is capable to heterogeneously oxidize a range of linear and cyclic alkanes and epoxidize cyclic alkenes. The oxidation performance is moderate, which is similar to the homogeneous or heterogeneous Mffi Lporphyrin catalysts. Since PIZA-3 did not have shape-selective property in the reaction, the authors concluded that the catalysis occurred on the exterior surface instead of inside the pores. Their hypothesis was proved by subsequent control experiment. They added the bases that are too peripherally bulky to... 

To incorporate the metallosalen complex into the framework. Hupp and coworkers reported a micro-porous homochiral pillared-layer MOF as an highly effective enantioselective catalyst for olefin epoxidation. A solvothermal reaction between a chiral (salen)Mn complex (R,R)- -)-1,2-cyclohexanediamino-A,M -bis(3-... 

On top of the oxidation reactions, the same team demonstrated that the silver-containing MOF-5 turned out to be active in propene epoxidation with molecular oxygen [126]. 

Recently, Cui and coworkers [101] reported that Co(Salen) (L25 in Scheme 10.6) incorporated in chiral MOFs also could go through bimolecular reaction pathways for the HKR of racemic epoxides with up to 99.5% ee. Crystal structure analysis suggests that the MOP structure brought Co(Salen) units into a highly dense arrangement and close proximity which could enhance the bimetallic cooperative interactions. The same bimolecular activation process in Co(Salen)-based MOP has also been found by lin and coworkers [102]. 

The same group also realized the tandem epoxidation and ring-opening reaction on Mn(Salen)-based MOF prepared using Lj2 as the organic linker and Zn as the inorganic node [118]. 

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