Threo units

O.I4 at 0 °C to 0.60% at 70 °C) than that of C2-I-I. It is worth noting here that there is no detectable 2,1 — 3,1 isomerization with C2-I-9/MAO, as only 2,1-erythro and 2,1-threo units were observed (see section VILA for details). As the 2,1 — 3,1 isomerization reaction would be faster (relative to the following primary insertion) than epimerization, its absence (or very low extent, as in C2-I-I/MAO) is an indication of the absence of epimerization in liquid monomer. 

For stereoregular insertion there are two modes to consider—cis insertion and trans insertion. For both isotactic and syndiotactic production, the cis mechanism has been determined to be in operation. This was established by polymerizing with cis-, and trans-l-deuteriopropylene or related monomers. The expected stereochemistry was demonstrated when deuteriopropylene was polymerized. The cis monomers produce erythro monomer imits whereas the trans monomer yields the threo units when cis- and tra/is-l-d-propylene is polymerized. In some cases the nomenclature appearing in the literature can be confusing and contradictory, but all indicate cis insertion. To be specific, as defined below, stereochemical structures from cis and trans addition to the double bond of cis-(l-di) and trans-(l-di)-propylene to isotactic polypropylene are as follows (229) ... 

When a monomeric unit contains more than one tetrahedral stereoisomeric center, the relative configuration of the centers has to be defined. In the case of two adjacent stereoisomeric centers, for instance, -CHA-CHB-, with A B, two configurational signs can be assigned to the bonds connecting the centers (Figure 2.3a). The pairs (+, +) or (—, —) define a relative threo configuration,... 

Double bonds present along a polymer chain are stereoisomeric centers, which may have a cis or trans configuration. Polymers of 1,3-dienes with 1,4 additions of the monomeric units contain double bonds along the chains and may contain up to two stereoisomeric tetrahedral centers. Stereoregular polymers can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndio-tactic if two stereoisomeric tetrahedral centers are present. In the latter case erythro and threo structures are defined depending on the relative configurations of two chiral carbon atoms.1... 

On the basis of the assignments given in Table II, one may conclude that the threo structure is slightly predominant in the hydrogenated 1,4-polydimethylbutadiene sample. This result taken in conjunction with the microstructure of the starting material (74% trans-1,4, 23% cis-1,4, and 3% 1,2) indicates a nonstereospecific addition of hydrogen to the unsaturated 1,4 units of polydimethylbutadi ene. 

G. aabbitt In the same spectrum, you showed pairyof methylene signals and attributed the doubling to erythro and threo structures of the H-Cl units. Three units were pictured an ethylene unit in the center with H-Cl containing units on either end. It is possible that both ends can be erythro, or both ends can be threo, or be mixed. Yet, only doublets are obtained. It seems to me there should be more multiplicity in the methylene signals. 

These monomers produce a single disyndiotactic polymer characterized by alternating erythro and threo relationships between adjacent substituents, 18 or 19. In such a polymer it is not possible—unless one turns to selective isotopic labeling—to determine whether the erythro and threo relationship refers to the substituents A and B of the same monomer unit or to those of two successive monomeric units. 

Cyclic olefin monomers such as indene, benzofuran, and so on, can give rise to two diisotactic polymers eiythro and threo, illustrated in the Fischer projection in 26 and 27, and to two different disyndiotactic polymers 28 and 29 (Scheme 5), as each monomeric unit is clearly defined and quite distinct from its neighbors (58, 61). For polymers of this type the terminology eiythro and threo is used also. 

The combination of cis-trans isomerism with iso-syndio and erythro-threo dispositions gives complex stractures as exemplified by the 1,4 polymers of 1-or 4-monosubstituted butadienes, such as 1,3-pentadiene (72, 73), and 2,4-pentadienoic acid (74, 75) and of 1,4-disubstituted butadienes, for example, sorbic acid (76). This last example is described in 32-35 (Scheme 6, rotated Fischer projection). Due to the presence of three elements of stereoisomerism for each monomer unit (two tertiary carbons and the double bond) these polymers have been classed as tritactic. Ignoring optical antipodes, eight stereoregular 1,4 structures are possible, four cis-tactic and four trans-tactic. In each series (cis, trans) we have two diisotactic and two disyndiotactic polymers characterized by the terms erythro and threo in accordance with the preceding explanation. It should be noted that here the erythro-threo relationship refers to adjacent substituents that belong to two successive monomer units. 

If, during ring opening polymerization a tertiary carbon is involved in the reaction, a configurational inversion may occur in which case a change in the stereochemistry of the monomer unit with respect to the monomer will be observed. Thus, the cf5-2,3-epoxybutane gives rise to a polymer with threo monomer units, 39, whereas the trans isomer is transformed into erythro or meso units, 40 (Scheme 8) (81, 82). 

By the presence of cyclic monomer units obtained, for example, from cyclic olefins (benzofuran, indene, etc.) (58, 254). The erythro- and threo-diisotactic stractures (26, 27 or in a different representation 77, 78) are chiral. If B is equal to A (cyclobutene or analogous monomers) only the threo-diisotactic structure 27 is chiral. 

Propylene oxide-has two asymmetric carbon atoms, and exists as cis- and trans-isomers. If the polymerization of cis- or trans-propylene oxide- -dj proceeds exclusively by /3-opening and by either retention or inversion of the attacked /3-carbon atom, the polymer composed of the monomeric unit having exclusively an erythro or a threo configuration is expected to be formed. 

Whether the ring opening is accompanied by retention or inversion of configuration, i.e., whether the polymer is composed of threo or erythro monomeric unit, could not be determined unambiguously by 1H—NMR analysis. This problem is expected to be answered unequivocally by the detailed analysis of the IR spectra, which is now in progress by Professor Tadokoro and his coworkers. 

Analogous remarks can be generally made upon polymers of monomers (VIII) (7, 33) even supposing that the main chains are of infinite length, monomeric units existing in two enantiomeric forms can be foreseen in the case both of the erythro and the threo diisotactic macromolecules when R and R" are different. On the contrary when R = R", only the monomeric units of the threo diisotactic macromolecules can exist in two enantiomeric forms. Some stereoisomers of the polymers of monomers containing one double bond in a ring (5, 127), such as for... 

Whatever the explanation for the stereocontrol, these processes are quite useful synthetically, enabling one to prepare nearly pure erythro or threo diastereomers in high yield. A number of groups have shown that by using a chiral auxiliary in the enamine, ester or amide unit, products of very high enantiomeric excess can be obtained.69... 

The Lewis acid catalyzed conjugate addition of allylsilanes (140) to (142) and allylstannanes (154) and (155) to ot,0-enones, described by Sakurai,68a,68b is highly efficient and experimentally simple in contrast to the allylcuprate additions. Various substituents can be incorporated into the allylsilanes (allylstannanes), e.g. alkoxy, alkoxycarbonyl and halogen, some of which are incompatible with cuprate reagents 69 In addition, Heathcock and Yamamoto report that diastereoselectivity is correlated to the alkene geometry of both the allylmetals and the acceptor units for example, allylation of ( )-enones (143) and (146) affords predominantly the syn adducts (144) and (147), while (Z)-enone (149) gives predominantly the anti adduct (150 Scheme 25).680 On the other hand, with cyclohexen-2-one the (Z)-silane (141) affords predominantly the threo adduct (152), while (142) affords erythro adduct (ISS).686 The more reactive allylstannanes (154) and (155) also afford similar diastereoselectivity.68e,f... 

The signal a of methyl protons consists of two doublet resonances. The weak doublet resonance (d, 8 1.98, JH H = 6.8 Hz) at lower field is related to the erythro (isotactic) placement of the last propylene unit (4-e), whereas the strong doublet resonance (d, 8 1.95, JH H = 6.8 Hz) at higher field is related to the threo (syndiotactic) placement of the same unit (4-t),... 

It must be noted that the approach of the monomer to the reactive metal-carbon bond should occur from the less hindered side. The greater stability of the trans complex should arise because the substituted carbon atom of the last unit is involved in the formation of the four-membered ring, so that reaction is hindered. The structure of copolymers of deuteropropylene from cis- and trans-(U,)-propylene proved that, even in syndiotactic polymerization, cis-addition to the double bond occurs, because with 1,2-disubstituted monomers, structures of ery-thro and threo di-syndiotactic polymers degenerate (see Fig. 4). 

A acrylonitrile unit, B butadiene unit, V 1,2-butadiene unit, C cis-1,4 butadiene unit, primed units reversed arrangement, T trans-1,4 butadiene unit, CyCA 2-cyclohexene-1-carbonitrile and I 4,cyanopentanoic acid end group a Erythro or threo defined by a Newman projection of this carbon and the other nearest asymmetric centre ... 

The relative configurations at two contiguous carbon atoms in these monomer units are designated by the prefix erythro or threo. 

Stereoregular polymers that can be afforded by 2,4-hexadiene and other symmetric terminally disubstituted butadienes (of the CHR CH CH CHR type) exhibit still more complex stereoisomerism, since each monomeric unit in these polymers possesses three sites of isomerism. The formation of these polymers involves 1,2- and 1,4-polymerisation. The 1,2-polymers derived from the CHR=CH—CH=CHR monomers exhibit the same type of stereoisomerism as polymers with a 3,4 structure obtained from monomers of the CH2 CH CH=CHR type. However, owing to the presence of the R substituent at the double bond in the side group of the polymer derived from a monomer of the CHR=CH—CH=CHR type, two types of eryt/zro-diisotactic, t/zraz-diisotactic and disyndiotactic polymer are foreseeable, each type with either cis or trans configuration of the double bond, as in the 1,2-polymer derived from a monomer of the CH2 CH CH CHR type. Thus, six stereo-isomeric forms of 1,2-polymer are possible for the CHR CH CH CHR monomer. The 1,4 monomeric units in the polymers formed by the polymerisation of CHR CH CH CHR monomers contain one double bond (in either cis or trans configuration) and two tertiary carbon atoms and therefore can exist as two sets of enantiomers, erythro and threo ... 

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