Copper-based catalysts enantioselective allylic

On the other hand, the exceptional capabilities of these catalysts for enantiocon-trol are evident in results obtained in intramolecular cyclopropanations, which usually occur with greater enantioselectivity than they do with copper catalysts. The example shown in eq. (8) illustrates the synthesis of a strained bicyclic lactone from a readily available allyl diazoacetate [24]. Similarly, high enantioselec-tivities for intramolecular cyclopropanations of homoallylic diazoacetates and homoallylic diazoacetamides have been reported [24 b]. A comparative evaluation of enantiocontrol for cyclopropanation of allylic diazoacetates with chiral Cu, Rh", and Ru" catalysts showed the superiority of Rh-based catalysts in these intramolecular reactions [24 c], an observation that cannot however be extrapolated to different substrates [24 d]. 

The allylic acyloxylation of alkenes, the Kharasch-Sosnovsky reaction, Eq. 81, would be an effective route to nonracemic allylic alcohol derivatives, if efficient, enantioselective catalysts were available. The reaction is mediated by a variety of copper salts, and as such, has been the target of considerable research in an attempt to render the process enantioselective. The original reaction conditions described by Kharasch require high temperatures when CuBr is used as the catalyst (93). However, the use of CuOTf (PhH)0 5 allows the reaction to proceed at temperatures as low as -20°C. Unfortunately, long reaction times are endemic in these processes and the use of excess alkene (2-100 equiv) is conventional. Most yields reported in this field are based on the oxidant. 

In 2013, Riant et al. developed a novel enantioselective domino conjugate reduction-allylic allylation reaction based on a cooperative dual catalysis involving a chiral copper catalyst and an achiral palladium catalyst. The reaction occurred between cyclic a-substituted a,(3-unsaturated ketones and ally methylcarbonate, providing the corresponding chiral cyclic a-allylic... 

During the course of their pioneering work on the copper-catalyzed cyclopropanation reaction of alkenes [25], the copper-catalyzed aziridination of alkenes using PhI=NTs was also discovered by Evans et al. in 1991 [26], A wide range of alkenes can be smoothly converted to the corresponding aziridines in the presence of Cu(I) and Cu(II) salts, such as Cu(MeCN)4C104 and Cu (acac)2, respectively (Scheme 2.16). Notably, aliphatic alkenes afforded the desired aziridines without allylic C-H amidation products. In the hterature, only one example of the enantioselective aziridination of styrene using chiral bis (oxazoline)-based copper catalyst was demonstrated. At the... 

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