Stereoselectivity in complexes with less puckered chelate rings

Abstract

Historical BackgroundWhen ethylenediami6e molecules in [Co(en)3] 3÷ are replaced by an optically active diamine such as R(--)-propylenediamine (R-pn), the resulting two optical isomers or diastereoisomers are no longer in equal amounts. This preference for one optical isomer or one diastereoisomer over the other has been called "ligand stereospecificity" 1). In the same sense, the term "stereoselectivity" has been used by Dunlop and Gillard, who defined it as "the behavior of molecular diastereoisomers" 2). In this chapter, the word "stereoselectivity" is used to mean inclination in abundance of the diastereoisomers of one geometrical isomer. This word differs from either "stereoselective reaction" or "stereospecific reactions". 15Tris(propylenediamine)cobalt(III) salts were first synthesized from racemic propylenediamine 3) Later, the synthesis 7) with the optically active base showed that the action of S(+)-pn on [CoC12(R-pn)z] + gave a mixture of (--)[Co(R-pn)3] 3+ and (+)[Co(S-pn)3] 3+ rather than [Co(R-pn)2(S-pn)] 3 +. After all the salts proved to be ( + ) [ C o ( S -p n ) 3 ] B r 3 • 2 H20 from the S-base and (--)[Co(R-pn)3]Br 3 • 2 H20 from the R-base. 5) However, oxidizing an aqueous solution of Co(II) salt and racemic 1,2-transcyclopentanediamine (cptn) resulted in trans-[CoC12{( +)-cptn}2] + and trans-[CoClz-{(--)-cptn}2] +, without trans-[CoC12{(+)-cptn}{(--)-cptn}] +.When a solution of trans-[CoC12{(+)-eptn}2] + was heated, it was converted to c/s and (--)[CoCI2{(+)-cptn}2] + was preferentially formed. The action of (+)-cptn on the (--)[CoC12{(+)-cptn}2] + complex resulted in (--)[Co{(+)-cptn}3] 3+, whereas the action of (--)-cptn resulted in a 2:1 mixture of (--)[Co{(+)-cptn}3] 3+ and (+)[Co{(--)-cptn}3]3+, Through these and the other studies, early workers thought, in general, that an optically active ligand favored the formation of one isomer to the exclusion of the other.In 1959 Corey and Bailar 7~ dealt with stereo specific effects in complex ions. The results of the conformational analysis for the [Co(en)3] 3+ ion showed that one conformational form of a chelated en, "le/", is more stable than the other, "ob", by ca. 0.6 kcal/mol. For the tris(propylenediamine)cobalt(III) ion, the three most stable pn chelate rings are suggested to have equatorial conformation with is The reactions by which stereoisomers are formed or used up at different rates are called stereoselective reactions, while the reactions by which specific stereochercfical compounds are converted to specific stereochemical products are called stereoSpccific reactions.