Acyclic stereoinduction

While all of the aryl imine substrates examined for this Strecker methodology existed predominantly or exclusively as the E-isomers, this did not appear to be a requirement for high enantioselectivity as demonstrated in the asymmetric 42-cat-alyzed (2 mol% loading) hydrocyanation of the cyclic Z-imine 3,4-dihydroisoquino-line, which was converted to the corresponding adduct (88% yield, 91% cc) with the same sense of stereoinduction with respect to the benzylic stereogenic center as the examined acyclic E-imines (Schemes 6.41 and 6.42) [196]. 

In the case of an acyclic anti homoallyl ether or alcohol, the zirconium-catalyzed ethylmagnesiation reaction leads to the same sense of stereoinduction [93], whereas... 

Stereoselective radical reactions. Giese et al. have found that radicals can show high 1,2-stereoinduction comparable to ionic reactions, and that the selectivity can be reversed when proceeding from cyclic to acyclic radicals. Thus the Bu3SnH radical addition to the dioxanone 1 provides the only one isomer (2) with trans-selectivity. The same radical with a corresponding acyclic system (3) shows c/s-selectivity. 

On the other hand, chiral primary amine-thiourea catalysts 85 and 90 developed by Tsogoeva [125] and Jacobsen [130], respectively, show an opposite sense of relative stereoinduction in the conjugate addition of acyclic ketones to nitroolefins (see Scheme 2.41 for 90). These anti selective catalysts stand in contrast to the usually obtained results which lead to selective formation of the i yn-conflgured diastereoiso-mers. The unexpected situation suggests participation of a Z-enamine intermediate. Moreover, with respect to the electrophile activation by the urea-type catalysts, it is also demonstrated that only one oxygen of the nitro group is bound to the thiourea moiety in an out-of-plane arrangement [125,130]. 

Chiral bidentate bis(oxazoline)-metal complexes (metal = Cu, Ni, Mg, Zn, Sc, La) were also studied, with preference for copper (II) trifiate- and nickel (II) per-chlorate-bis(oxazolines). It was found that as little as 1 mol% of catalyst is adequate for the enantioselective electrophilic fluorination of both cyclic and acyclic (3-ketoest-ers (Scheme 44.22). NFSI was preferred to Selectfiuor or A-fluoropyridinium trifiate to produce higher enantiomeric excesses. Importantly, the use of HFIP as additive greatly improved the ee-values in all the reactions by ca. 15%. An important finding in NFSI mediated fluorination is that the (5,5)-Ph-BOX-Ni(II) complex provides the fiuorinated product with opposite configuration to that obtained with the (5,S)-Ph-BOX-Cu(ll) complex (Scheme 44.22). The origin of the reversed sense of stereoinduction could be a consequence of a change in the metal-center geometry from distorted square-planar (Cu complex) to square-pyramidal (Ni complex) in the reactive intermediates. 

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