Diorganozinc reagents preparation

Scheme 11.23. Preparation of various functionally substituted aminocyclopro-panes from diorganozinc reagents 80 and N,N-dialkylcarboxamides [119],...

The remaining unreacted (RJ-20 can be trapped with an aldehyde such as pivalaldehyde giving the chiral homopro-pargylic alcohol in 87.5% ee.74 This kinetic resolution has been used to prepare anti/inti-vicinal amino diols in > 95% ee and dr > 40 l.75 In general, diorganozincs are more easily prepared in optically pure form. On another hand, secondary zinc reagents prepared by the direct zinc insertion to secondary alkyl iodides are obtained without stereoselectivity (Scheme 4).76... 

Immobilization of chiral ligands to effect asymmetric induction in alkylation of aromatic aldehydes by diorganozinc reagents promoted by PEG-im-mobilized ligands 54-57 can also be promoted by soluble polystyrene-bound species. A recent example of this is work where a polystyrene-bound BINOL was prepared [ 105]. This polymer 69 was used to form titanium-BINOLate and AlLibis(binaphthoxide) catalysts for Et2Zn reaction with benzaldehyde and for asymmetric Michael additions of stabilized carbanions to cyclohexenone. While good stereoselectivities were obtained with these catalysts, the synthetic yields were modest. 

Diorganozinc reagents 55 require different methods of preparation, snch as iodine-zinc and boron-zinc exchange to form 56 (Langer et al. 1993,1996). These methods are applicable to the formation of primary and secondary diorganozinc species 55 (Scheme 7.20). 

Figure 30 - Conjugate additions of sonochemically prepared diorganozinc reagents...

Scheme 2-104. Preparation and reactivity of the mixed diorganozinc reagent 273.

The preparation of salt-free diorganozinc reagents is quite important in many syntheses, especially for catalytic asymmetric addition reactions. Such diorganozinc reagents can be obtained by usual transmetalation from Grignard reagents followed by the addition of dioxane, which causes the precipitation of all the Mg-salts from the solution due to the formation of dioxane complexes. ... 

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