Diastereoselectivity

When more than one diastereomer is formed in an organic process than any other, this is known as diastereoselectivity. Cis/trans descriptors lose their validity when the only bond connecting the two centres is free to spin. Depending on the particular metal, the following changes in the diastereoselectivity of the enantioselective addition of various allylic organometallic reagents to carbonyl compounds, catalysed by either chiral Lewis acids or bases, or chelating ligands:

Independent of the initial allylic geometry,
(a) Si, Sn, and B allylic reagents mostly produce syn diastereoselectivity;
(b) Regardless of the initial allylic geometry, allylic reagents including Cr, Zn, and In produce mostly anti diastereoselectivity;
(c) The E/Z ratio of the beginning allylic geometry is reflected in the syn/anti diastereoselectivity of allylic trichlorosilanes (SiCl3).