Homosteric initiators

We were able to isolate species of both types in the case of diethylzinc-(+) 3,3 dimethyl 2 butanol initiator system. The antisteric species has a composition close to Etf.Zn(0R)j, or Zn(0R) . (EtZnOR)g (-OR being the 3,3 dimethyl 2 butoxy group), while an homosteric initiator had a Zn(0R)j>. EtZnOR composition. Both species were soluble in benzene and were studied by H-NMR (18). It was possible to transform one specie into the other by ailing ZnEtp or by drying or heating (loss of ZnEt, and disproportionation . ... 

Influence of the nature of the monomer Oxiranes and thiiranes could be polymerized by the same type of initiators which makes easy a way of comparison of their behaviour. We shall now use our standard homosteric initiator ZnEt2 (-)DMBD (1 1) and study the influence of the nature of the monomer on the stereoselectivity and the stereoelectivity of the process. 

In practice we have prepared the homosteric initiator by reacting at room temperature diethylzinc with (-) 3,3 dimethyl 1,2 butanediol and the antisteric initiator by reacting the latter diol with dimethylcadmium in the same conditions. 

Homosteric initiator prepared by reacting ZnEt with chiral hydroxy reagent in —Zn—Et/—OH =1/1 ratio.)... 

A typical homosteric initiator had a Zn(OR)2. EtZnOR composition. Both species were soluble in benzene and were studied by H—NMR [16, 19]. 

Polymerization of chiral monomers was reviewed recently by Sigwalt430). Two situations may arise. The non-chiral initiator may interact randomly with either enantiomer, but its parity introduces a bias in the subsequent addition. For a homosteric bias, i.e. when the addition of one enantiomer favors the subsequent addition of the same kind, the resulting polymer is composed of sequences of blocks of one enantiomer followed by a block of the other. This is a typical example of stereoselectivity. For an antistericbias, i.e. the addition of, say R enantiomer is favored by the presence of terminal S enantiomer, or vice versa, the resulting polymer shows a bias for a simple alteration. 

Conflguratlonarrelatlons could be established In several cases. If one considers the absolute configuration of the chiral ligand and that of the cyclic monomer, the choice of the initiator would correspond to an "homosteric" type process if the chosen enantiomer has the same configuration as the chiral ligand used in the initiator (18). Homosteric configurational relations are illustrated In the next scheme. 

We shall now consider only homosteric type initiators for simplicity. 

Effect on stereoelectivity If one considers initiators prepared in homosteric cond itibhs (I < 1) it is possible to compare the efficiency of resolution depending on the chiral hydroxy ligand associated with the organometallic compound. 

The stereoelective choice of monomers is however in agreement with configuration rules as seen from the sign of optical activity of unreacted monomer. Homosteric and antisteric processes are observed with considerable amount of a-scission (39). According to the chemical composition of these initiators a cationic character of the latter seems to be excluded and therefore this particular behaviour could be due to some steric reasons which are not yet completely understood. New studies are now in progress. 

Fig. 6. Polymerization of methyl thiirane using various chiral initiators. Variation of the optical activ-ity ajj (neat, dm) of residual monomer with conversions. homosteric type process using ZnEtj — (-) -tBu—CHOH—CHjOH (1 1) as initiator system antisteric type process using CdMej —(-) -tBu—CHOH—CH OH (1 1) as initiator system...

By extension one may call a homosteric type initiator an initiator which gives an homosteric stereoelection with a given monomer. Antisteric type process were observed in the case of methyl thiirane [19, 23, 24] and more recently in the case of methoxy methyl thiirane [25] and ethyl thiirane [26]. However, f-butyl thiirane did not give inversion with conventional antisteric initiators. This could be due to its high sterical hindrance. 

It is rather difficult to give a defined structure to homosteric and antisteric species for the reason that most of these initiators are in aggregate or polymeric form, some of them being soluble, others insoluble. 

In the following sections we shall consider mainly initiators corresponding to homosteric type composition with / < 1. 

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