Epoxide ring-opening reactions

Jacobsen further tested catalyst 13 in solvent-free conditions to generate an asymmetric catalytic reaction with maximum volumetric productivity. Also, recyclability of the catalyst would yield no waste and, therefore a highly efficient catalyst. Upon reaction with cyclohexene oxide the chromium complex was successfully recycled three times to increase the isolated yield (86 to 91%) and ee (84 to 88%). When reacted with cyclopentene oxide for 4 h reaction time, 81% yield and 94% ee was observed. 

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.  

Chromium terephthalate (MIL-101) has been used for synthesis in MOF development. It exhibits good thermal stability and large pore channels, which promotes mass transport. MIL-101 can be functionalised with palladium or gold nanoparticles, for example, due to the presence of acidic ccxjrdinatively unsaturated sites present on the MOF. This leads to applications of chromium MOFs in hydrogenation reactions and oxidative catalysis. 

Juliao et al. recently disclosed an update on the use of chromium-based MOF catalysts in desulfurisation catalysts. In this work, a composite 

Chromium has a large contribution to the MOF arena due to the fact that it has created the largest pore-sized metal catalysts yet known. Ferey created a crystal structure for porous chromium terephthalate, MIL-101, with very large pore sizes and surface area. Its zeotype cubic structure has an enormous cell volume ( 702 000 A ), a hierarchy of extra-large pore sizes (30 to 34 A), and a Langmuir surface area for N2 of 5900 300 m This 

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Scheme 8.37 Use of the lithium enolate of acetaldehyde DMH in an epoxide ring-opening reaction.

Thioacetals eliminate to vinylsulfides in the presence of CuOTf (Scheme 46).192 Cu1 and Cu11 triflates are mild Lewis acids for Friedel-Crafts acylation and alkylation reactions. CuOTf effectively catalyzes the reaction of anisole with selenoesters.193,194 Copper(II) sulfate promotes epoxide ring opening reactions in the presence of pyridine,195 with retention of configuration being observed. Cu(OTf)2 is a catalyst for the ring opening of aziridine by aniline.196... 

Complexes of other metals such as gallium, indium, lead, and antimony have also been used as Lewis acids. Catalytic enantioselective meso-epoxide ring-opening reactions using a chiral gallium(III) catalyst (Ga-Li-linked-BINOL) have been reported (Scheme 84).348 The chemical yields are much improved by linking two BINOL units. 

Pedragosa-Moreau, S., Archelas, A. and Furstoss, R. Tetrahedron, 1996, 52, 4593 Pedragosa-Moreau, S., Morisseau, C., Zylber, J., Archelas, A., Baratti, J. and Furstoss, R. J. Org. Chem., 1996, 61, 7402. A theoretical analysis of such epoxide ring-opening reactions has been published, Moussou, P., Archelas, A., Baratti, J. and Furstoss, R. Tetrahedron Asymmetry, 1998, 9, 1539. 

One can easily envisage the formation of stereochemically enriched materials by an asymmetric version if the methodology developed by Jacobsen et al. <2000ACR421> of epoxide ring-opening reactions with azides is applied in the reaction sequence. 

Model computational studies aimed at understanding structure-reactivity relationships and substituent effects on carbocation stability for aza-PAHs derivatives were performed by density functional theory (DFT). Comparisons were made with the biological activity data when available. Protonation of the epoxides and diol epoxides, and subsequent epoxide ring opening reactions were analyzed for several families of compounds. Bay-region carbocations were formed via the O-protonated epoxides in barrierless processes. Relative carbocation stabilities were determined in the gas phase and in water as solvent (by the PCM method). 

Figure 8. Epoxide ring opening reactions of bay-region diol epoxides and epoxides from dibenzo[a,h] acridine, and epoxides from benzo[a]acridine and benzo[c]acridine. Figure adaptedfrom reference 27. 

Figure 10. Epoxide ring opening reactions of the fluor mated derivatives of dibenzo [a,h] acridine-1,2-epoxide. Figure adapted from reference 27.

Figure 15. Epoxide ring opening reactions of epoxides and diol epoxides from benzo[a]pyrene, 4-azabenzo[a]pyrene and 10-azabenzo [a]pyrene.

Scheme 2 Various strategies for enantioselective epoxide ring opening reactions.

Jacobsen, E. N. (2000) Asymmetric catalysis of epoxide ring-opening reactions, Acc. Chem. Res., 33 421-431. 

Ready, J. M. Jacobsen, E. N. (2001) Highly active oligomeric (salen)Co catalysts for asymmetric epoxide ring opening Reaction, J. Am. Chem. Soc., 123 2687-2688. 

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