Copper II -catalyzed Claisen Rearrangement

The [Cu (box)](OTf)2 (13) atalyzed Claisen rearrangement of allyl vinyl ethers, in which only one center of chirahty is formed, has been investigated (Table 2.4). 

The enantioselectivity of [Cu(S,S)-tBu-box](OTf)2 (13b) atalyzed Claisen rearrangement is explained as follows (Fig. 2.4). The alkoxycarbonyl and ether oxygens coordinate in a bidentate fashion to the Cu (box) complexes. The square planer geometry around the copper(II) cation has been proposed and a chair-hke transition-state model is suggested. The aUyUc ether moiety should approach the vinyl ether moiety from the opposite direction of the t-Bu substituents on the box-hgand. The Cu (box) catalyst differentiates between two enantiomeric chair-hke transition state by selective coordination of enantiotopic lone pairs on oxygen to form (S,S,pro-S)-14a. 

The 2-oxazoline-substituted substrate 15 has been examined but the selectivity is moderate (Table 2.6) [15]. The diastereoselectivity depends on the configuration of the vinyl ether double bond (entry 1, 3). In this case, the diastereoselectivity is mainly dictated by substrate configuration (entry 1, 2). 

In this reaction, two enantiomerically pure diastereomers 18 out of eight possible stereoisomers are obtained (Table2.7). 

Late transition-metal-catalyzed asymmetric Claisen rearrangement takes place in a different mode from that of Lewis-acid-catalyzed Claisen rearrangement Late transition metal catalysis is based on affinity for the Claisen diene system. Among late transition metals, palladium complexes are the most useful and effective for the Claisen rearrangement. 

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