Cyclic ethers, catalytic effectiveness

It was noted early by Smid and his coworkers that open-chained polyethylene glycol type compounds bind alkali metals much as the crowns do, but with considerably lower binding constants. This suggested that such materials could be substituted for crown ethers in phase transfer catalytic reactions where a larger amount of the more economical material could effect the transformation just as effectively as more expensive cyclic ethers. Knbchel and coworkers demonstrated the application of open-chained crown ether equivalents in 1975 . Recently, a number of applications have been published in which simple polyethylene glycols are substituted for crowns . These include nucleophilic substitution reactions, as well as solubilization of arenediazonium cations . Glymes have also been bound into polymer backbones for use as catalysts " " . 

The reaction described is of considerable general utility for the preparation of benzoyloxy derivatives of unsaturated hydrocarbons.2"8 Reactions of 2-butyl perbenzoate with various other classes of compounds in the presence of catalytic amounts of copper ions produce benzoyloxy derivatives. Thus this reaction can also be used to effect one-step oxidation of saturated hydrocarbons,9, 10 esters,6,11 dialkyl and aryl alkyl ethers,12 14 benzylic ethers,11,15 cyclic ethers,13,16 straight-chain and benzylic sulfides,12, 17-19 cyclic sulfides,11,19 amides,11 and certain organo-silicon compounds.20... 

As depicted in Eqs (3)-(5), kinetic resolution of a variety of cyclic allylic ethers is effected by asymmetric Zr-catalyzed carbomagnesation. Importantly, in addition to six-membered ethers, seven- and eight-membered ring systems can be readily resolved by the Zr-catalyzed protocol. It is worthy of note that the powerful Ti-catalyzed asymmetric epoxidation procedure of Sharpless [15] is often used in the preparation of optically pure acyclic allylic alcohols through the catalytic kinetic resolution of easily accessible racemic mixtures [16]. When the catalytic epoxidation is applied to cyclic allylic substrates, reaction rates are retarded and lower levels of enantioselectivity are observed. Ru-catalyzed asymmetric hydrogenation has been employed by Noyori to effect resolution of five-... 

Use of PEG has been limited until now to classical phase-transfer catalysis and also in this case only a few applications have been reportedi Also for their catalytic effect PEG have been compared to crown ethers and the examples are restricted to substitution reactions, In fact, PEG can be regarded as non cyclic crown ethers and opening of the ring is accompanied by decrease of the complexation constant, due to conformational entropy factors as well as to solvation effects on the ligand. However, from the results described in this report PEG have also proven to be valuable solvents for classical substitution reactionsJ oxidations and reductions. 

Catalytic RCM and another Zr-catalyzed process, the kinetic resolution of cyclic allylic ethers, joined forces in our laboratories in 1995 to constitute a fully-cata-lytic two-step synthesis of optically pure 2-substituted chromenes. These structural units comprise a critical component of a range of medicinally important agents (see below). Our studies arose from unsuccessful attempts to effect the catalytic kinetic resolution of the corresponding chromenes [13] a representative example is illustrated in Eq. 3. 

Subsequent mechanistic studies suggested that the abovementioned effect of ethylene on reaction efficiency is connected to a mechanistic divergence that exists for reactions of terminal styrenyl ethers versus those of disubstituted styrene systems [13b]. Whereas with monosubstituted styrenyl substrates the initial site of reaction is the terminal alkene, with disubstituted styrene systems the cyclic ji-systems react first. This mechanistic scenario suggests two critical roles for ethylene in the catalytic reactions of disubstituted styrenes ... 

As with acyclic dienes, methods have been developed for enantioselective and diastereoselective complexation of prochiral and chiral cyclic dienes. An approach has been developed for the asymmetric catalytic complexation of prochiral eyelohexa-1,3-dienes nsing (1) in the presence of catalytic amounts of l-azabuta-l,3-dienes such as (232) or (233) an enantiomeric excess as high as 86% has been reported. By contrast, attempts to effect diastereoselective complexations using cyclic diene systems eqnipped with chiral auxiliaries have met with limited success. On the other hand, direct complexation of chiral cyclic dienes snch as (234) and (235) proceed with a high degree of diastereoselectivity, where the iron tricarbonyl fragment is directed syn to alcohols or ethers by transient coordination ( heteroatom dehvery ) (Scheme 66). ... 

On the other hand, there is a need to use inhibitors of the platinum catalysts temporarily to reduce their catalytic activity in the presence of hydro- and vinyl-polysiloxanes in order to stop the curing process at room temperature, but to allow the platinum catalyst to be activated at elevated temperature. Among the principal types of compounds reported are alkenyl derivatives, esters of unsaturated acids, crown ethers, organic nitrogen compounds, phosphines, linear and cyclic vinyl-siloxanes, and poly(vinyl)siloxanes [2], and recently fumarate [44] and maleinate [33]. New co-activators of the catalysts (precursors) have been revealed in the 1990s to reduce to ppm the levels of platinum required to effect hydrosilylation curing [45, 46]. 

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