Antisteric

It may be noted that all of the homosteric catalysts were chiral zinc systems, the composition of which was expressed as [RZnOR ]x[R,OZnOR,]J,(v/y < 1). Antisteric catalysts were chiral zinc- or cadmium-based systems of a common compositional feature expressed as [RZnOR ]x[R OZnOR ]>, or [RCdOR ]x [R OCdOR ]) (x/y > 2). The nature of homosteric and antisteric stereoelection has not yet been elucidated fully at the molecular level because the structure of the operating species and the polymerisation mechanism with these catalysts are not clearly established [52]. 

We were able to establish in the case of methylthiirane that when the initiator is prepared in such conditions that alkylalco-holate species predominate over dialcoholate species, the initiator system elects the antipode the configuration of which is opposite to that of its chiral ligand. Such type of election was called "antisteric". The results were established in the case of 1,2 diols of serie (I) and for several alcohols which were reacted with three different organometallic derivatives (18). 

I = 0.44).When the same reagents are reacted 1n (1 0,5) amounts tne choice is opposite (1=4 antisteric process). 

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 . ... 

It was foimd however, that in some other cases the choice corresponds to opposite configuration, that is to an antisteric type process. 

We have demonstrated in the case o few thiiranes, oxiranes and recently for a,a disubstituted 3 propiolactones (36) that the type of the choice (homosteric or antisteric) is depending only on the x/y ratio i.e. the ratio of alkylalkoxide species over dialkoxide species. 

In some cases both type of species could be isolated under a soluble form. For example, when reacting diethylzinc with (+)3,3 dimethy1-2-butanol, antisteric species with a composition close to Et Zn (OR) were found (40) which are similar in composition to those reported for the methanol derivatives (13). The homosteric species corresponded to Zn(0R)2.EtZn0R co osition. 

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. 

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. 

The polymerization of cis dimethyl thiirane (Y CH ) and cyclohexene sulfide (YOy cyclohexyl) was studied for this purpose using the standard ZnEt -RC") DMBD (1 1) initiator (50). Polymers of high optical activity were obtained, as shown in table 4, demonstrating the preferential attack of one of the asymmetric carbons. This is obviously supported by the opposite sign found for polymers obtained with CdMe2-R(") DMBD an antisteric initiator (table 4). 

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...

Homosteric and antisteric polymerization of methylthiirane using dialkyl metal-(-)-3,3-dimethyl-l,2>... 

When the monomer chosen corresponds in configuration to the chiral agent used, the process is called homosteric when it is of opposite configuration the process is called antisteric . 

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. 

The apparent solubility or the insolubility is not a crucial factor, both homosteric and antisteric species being available in soluble or insoluble forms. 

An antisteric initiator had a stoichiometric composition very close to Et6Zn7(OR)g or Zn(OR)2 -eEtZnOR (—OR being the 3,3-dimethyl-2-butoxy group). Such a structure was previously described in the literature for methoxy groups. 

Varying the substitution of the P-Iactone, by using racemic a-ethyl-a-methyl-P-propiolactone or a-propyl-a-methyl-P-propiolactone, led to the production of an optically active polymer which obeyed stereoselection in the presence of a diethylzinc/(R)-(-)-3,3-dimethyl-l,2-butanediol catalytic system, whereas the diethylzinc/methanol system led only to atactic polymers [56-58]. These findings indicated that the enantiomorphic sites of the zinc coordination catalyst were unable to recognize the chirality of the lactone monomer used for the polymerization. It should also be noted here that an antisteric type of steroselection in the polymerization of racemic a-propyl-a-methyl-P-propiolactone was reported when dimethylcadminium/(R)-(-)-3,3-dimethyl-l,2-butanediol was used as catalyst [56]. 

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