Diastereoselectivity, in metal-catalysed

Stereosectivity is a broad term. The stereoselectivity in cyclopropanation which has been discussed in the above subsection, in fact, can also be referred to as diastereoselectivity. In this section, for convenience, the description of diastereoselectivity will be reserved for selectivity in cyclopropanation of diazo compounds or alkenes that are bound to a chiral auxiliary. Chiral diazoesters or chiral Ar-(diazoacetyl)oxazolidinone have been applied in metal catalysed cyclopropanation. However, these chiral diazo precursors and styrene yield cyclopropane products whose diastereomeric excess are less than 15% (equation 129)183,184. The use of several a-hydroxy esters as chiral auxiliaries for asymmetric inter-molecular cyclopropanation with rhodium(II)-stabilized vinylcarbenoids have been reported by Davies and coworkers. With (R)-pantolactone as the chiral auxiliary, cyclopropanation of diazoester 144 with a range of alkenes provided c yield with diastereomeric excess at levels of 90% (equation 130)1... 

A review has appeared on the synthesis of enantiomerically enriched aziridines by the addition of nitrenes to alkenes and of carbenes to imines.45 A study of the metal-catalysed aziridination of imines by ethyl diazoacetate found that mam group complexes, early and late transition metal complexes, and rare-earth metal complexes can catalyse the reaction.46 The proposed mechanism did not involve carbene intermediates, the role of the metal being as a Lewis acid to complex the imine lone pah. Ruthenium porphyrins were found to be efficient catalysts for the cyclopropana-tion of styrenes 47 High diastereoselectivities in favour of the //-product were seen but the use of chiral porphyrins gave only low ees. 

One important field of application of the NHC in transition metal-catalysed reactions is obviously the olefin metathesis. Among the various applications described, one can notice the nice use of complex (15) to affect the desymmetrization of oxabicyclic alkenes by a Z-diastereoselective and enantioselective ring opening/cross metathesis with enol... 

Metallic lanthanum effects the stereoselective reductive dimerisation of 3-iodoflavanones to 3,3 -biflavanones as exemplified by the first synthesis of cf/-chamaejasmine <05OL271> and a Pd-catalysed diastereoselective cyclisation features in a route to the marine metabolite (-)-15-oxopuupehenol <05OL1477>. 

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