Stereoelectivity

Aluminum-chromium alloys, 2 312-313 phase transitions, 2 308t Aluminum complexes, stereoelective,... 

Initiating systems, nature of, 14 267—268 Initiating systems, stereoelective, 20 303-306... 

A stereoelective (252, 299, 300) or asymmetric selective (298) or enantioasymmetric (301) polymerization where one of the enantiomers polymerizes in a preferential way in an ideal case 50% of the monomer is converted into a pure optically active polymer while the remainder is recovered as nonreacted compound also in the pure enantiomeric form (e.g., R -t- nS). ... 

The reader should note the difference between the terms stereoselective and stereoelective. The former refers to process 2, the latter to process 3. 

Stereoelective polymerization (type 3) requires the presence of a chiral catalyst with an excess of active centers of a given configuration or with a differential reactivity of the centers that catalyze polymerization of one or the other of the two enantiomers (299). With regard to racemic a-olefins, the best results were obtained with 3,7-dimethyloctene in the presence of TiCl, + Zn[(S)-2-methylbutyl]2 as catalyst (309). The resulting polymer is dextrorotatory, la o = -1-16.1, and the residual monomer is levorotatory, a o = —0.63, (310). These values indicate a rather ihodest degree of stereoelectivity. 

Stereoelective polymerization of enantiomerically unbalanced (i.e., partly resolved) mixtures of oxiranes and thiiranes has given suq)rising results (277, 318, 322). The stereoelectivity ratio, r, is greatly dependent on the enantio-... 

From the point of view of the formation of the optically active 4-methyl-hexanal, it has been defined as a stereoelective (17) hydroformylation (I). 

The stereoelective hydroformylation of (R) (S)-3-methyl-l-pentene also has been achieved with rhodium catalysts the optical purity of the... 

R)] is found for the aldehydes arising from the attack of carbon monoxide at position 3. The above stereochemical relationships are summarized in Table VI. Finally in the stereoelective hydroformylation of racemic 3-methyl-l-pentene the (R) antipode is hydroformylated at a higher rate, yielding the (R)-4-methylhexanal the ratio between the hydroformylation rate of the two antipodes is 1.1. 

Stereoelective Hydroformylation of 3-Methyl-1-pentene. The same apparatus and procedure as for styrene were used. 4.21 grams (0.05 moles) of 3-methyl-l-pentene were hydroformylated in 50 ml of dry, degassed mesitylene in the presence of 115 mg (0.125 mmole) of HRh(CO)-(P< >3)a and 249 mg (0.5 mmole) of ( — )-DIOP. After 70 hrs the conversion was 51.1%. The unreacted olefin and the aldehydes were separated with a 1-m rectification column filled with Fenske rings. The pure olefin (GLC) had [ ]d17 +1.25° (neat). The aldehydes could not be completely separated from the solvent through rectification. A fraction containing 58% of ( —) (R)-4-methylhexanal had < D25 —0.26°. 

Table VII. Stereoelective Hydroformylation of (R) (S)-3-Methyl-1-pentene with HRh(CO) (PPh3)3/( — )-DIOP Catalytic System...

Thus the stereospecific polymerization of a racemic monomer will yield optically active polymers only if it is accomplished stereoelectively, namely in the presence of catalysts capable of polymerizing preferentially one of the two antipodes of the racemic monomer. 

The only case of stereoelective polymerization of vinyl monomers so far known is the polymerization of some racemic a-olefins with the aid of catalysts prepared from TiCl4 or TiCls and bis-[(S)-2-methyl-butyl]-zinc [104,107). 

The remarkable dependence of the polymerization stereoelectivity on the nature of the catalyst (107) and some aspects of the stereoselective polymerization of racemic a-olefins, which will be discussed later, seem to be more consistent with the second or the third hypothesis than with the first one. 

In the field of the stereospecific heterogeneous polymerization of a-olefins, the stereoselective (106,118) and stereoelective (103) polymerization of racemic monomers, having an asymmetric carbon atom in a. 

Stereoelectivity strongly depends on the type of both the transition metal compound (see Table 3) and the metal alkyl used to prepare the catalyst (107). 

On the above experimental basis, it appears that the stereoselectivity and the stereoelectivity of the polymerization of racemic a-olefins mainly depend, though not exclusively, on the type of catalytic system used. 

Further investigations are necessary to clarify the mechanism of stereoregulation in the polymerization of olefins and diolefins probably the stereoelective polymerization of vinyl monomers and the asymmetric polymerization of diolefinic compounds will give further interesting contributions to the future progresses in this field. 

Furthermore, the stereoselectivity and stereoelectivity of some particular types of organometallic catalysts for the racemic a-olefin polymerization were proved. 

This line of thinking prompted the treatment of 86 with zinc in refluxing methanol for the purpose of generating 88. When this ketone was reduced with diisobutylaluminum hydride in tetrahydrofuran at low temperature, the cis diol 89 was produced with a stereoelectivity in excess of 10 1. Monomesylation of the... 

The purpose of most of the investigations of the hydroformylation of racemic olefins has been to contribute to the understanding of the origin of asymmetric induction its significance for synthetic purposes is small. In fact, with few exceptions, this type of reaction never leads to even a fair optical purity of the reaction products (stereoelective synthesis) or of the recovered substrate (kinetic resolution) (Scheme 1, reaction 3). 

Pyrylium behaves as the synthetic equivalent of the pentadienal cation in its reactions with organometallic reagents, allowing the stereoelective formation of 2Z,4 -dienals. This approach to the synthesis of retinoids using a variety of pyrylium salts has been most successful (95JCS(P1)2385). 

As in stereoselectivity, the degree of stereoelection is reduced as the chiral centre is positioned further away from the C = C bond only racemic a-olefins with a chiral carbon atom in the a- or -position to the double bond have been stereoselectively polymerised. Note that, in general, stereoelectivity is lower than stereoselectivity. 

From the scientific point of view, however, polymerisation of higher a-olefins is of considerable interest, since the relationship between the structure of the catalyst and that of the resulting polymers may be explained by considering the influence of the size of the alkyl group in the monomer. Moreover, polymerisation of chiral a-olefins allows study of stereoselection or stereoelection phenomena during polymerisation. 

The first enantioasymmetric polymerisation of (R, propylene oxide, reported in 1962, was carried out in the presence of the diethylzinc (+)-borneol catalyst [23,24]. Other optically active catalysts for propylene oxide stereoelective polymerisation, e.g. diethylzinc (R)-( )-3,3-dimethyl-1,2-butanediol were described later on [25]. 

Polymerisations of tiiranes in the presence of coordination catalysts containing multinuclear species have been extensively studied in terms of their stereoselective and stereoelective behaviour. For monosubstituted tiiranes, the polymerisation can proceed enantiosymmetrically and lead to a mixture of isotactic chains of opposite configurations. By using optically active catalysts, the polymerisation may occur enantioasymmetrically, with the enchainment of only one of the two enantiomers. 

Some cadmium compounds, including simple salts, were revealed to be excellent catalysts for the enantiosymmetric polymerisation of propylene sulphide [156,157], For instance, the proportion of isotactic diads in the polypropylene sulphide) sample obtained in polymerisation with the cadmium (7 )-tartrate catalyst was more than 95%, higher than the 69% which was characteristic of a polymer sample prepared using the zinc (i )-tartrate catalyst [158]. The superior stereoselectivity of the cadmium (i )-tartrate catalyst is also borne out by the more effective separation into fractions having opposite optical rotations of the polypropylene sulphide) yielded by cadmium tartrate, compared with that yielded by zinc (i )-tartrate. Note the quite different behaviour of these two catalysts in terms of their stereoelectivity in the polymerisation of propylene sulphide only very slight optical activity was found for the polypropylene sulphide) sample prepared using cadmium tartrate, whereas that associated with the polymer sample obtained with zinc tartrate was found to have a much higher value [158]. 

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