Epoxide ring opening catalysts

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. 

In summary, the formation of optically active compounds through hydrolysis reactions is dominated by biocatalysis mainly due to the availability and ease of use of a wide variety of esterases, lipases and (to a lesser extent) acylases. Epoxide ring-opening (and related reactions) is likely to be dominated by salen-metal catalysts while enzyme-catalysed nitrile hydrolysis seems destined to remain under-exploited until nitrilases or nitrile hydratases become commercially available. 

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. 

Consecutive CO2 insertion has not been observed (and is assumed to be thermodynamically unfavored). However, consecutive epoxide ring opening is common, particularly for Lewis acidic catalysts like zinc derivatives (Scheme 4). 

From the NMR spectrum of copolymers produced from cyclohexene oxide and carbon dioxide it is difficult to assess low levels of asymmetric induction, i.e., low degrees of desymmetrization in the epoxide ring-opening step. In order to determine the extent of asymmetric induction it is necessary to hydrolyze the copolymer leading to the tra s-cyclohexane-l,2,-diol and examine the enantiomeric excess (4) [22]. Figure 4 shows the NMR spectrum in the carbonate region of atactic copolymer produced from cyclohexene oxide and CO2 using an achiral (salen)CrX catalyst. 

Polyaniline-supported Co(II) catalyst 41 was used to catalyze the epoxidation of various alkenes under oxygen atmosphere at ambient temperature.61 63 One report61 described a synthesis of a-hydroxy- 3-aminopyrrolidine amides as potential HIV protease inhibitors (Scheme 1). In this synthesis, catalyst 41 also mediated epoxide ring opening of the epoxide intermediate by an aniline to afford the desired product. 

Figure 8.3 Transition state of the epoxide ring-opening step (P = growing polymer chain) for (a) dizinc catalyst (b) dimagnesium catalyst.

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