Isomerisation anti-syn

The anti and syn forms of the rc-allylic ligand are in equilibrium. If no bulky substituent is present at the C2 atom of the butenyl group, the equilibrium at ambient temperature is completely shifted towards the syn form which is thermodynamically much more stable than the anti form [148,189]. Therefore, trans-1,4 monomeric units can be generated, either involving a coordinated transoid monomer [pathways (a)-(b) and (a )-(b), scheme (10)] or involving a coordinated cisoid monomer [pathway (c)-(e)-(b), scheme (10)], if the rate of anti —> syn isomerisation [pathway (e), scheme (10)] is greater than that of insertion. When the rate of this isomerisation is lower that that of insertion, cis-... 

The anti > syn isomerisation is also responsible for a decrease in the content of cis-1,4 monomeric units and the simultaneous increase in the trans-1,4 content of polybutadienes derived from polymerisation with several Ti-, Co- and Ni-based... 

It should be noted that trans- 1,4-polymers formed by anti —> syn isomerisation, i.e. via the pathway (c)-(e)-(b) [scheme (10)] when the monomer is coordinated to a metal atom as an s-cis-rf ligand, are produced by various catalysts, especially by soluble ones. It is also worth noting that polymers with a trans-1,4 structure of monomeric units are formed by heterogeneous catalysts via the pathway (a)-(b) or (a )-(b) [scheme (10)] from the (Z)-isomers of conjugated dienes such as CH2=CH-CH=CHR, for which the s-trans-rf or s-trans-rf coordination is preferred and the s-cis-rf coordination is unfavoured [13,193],... 

Catalyst complexation with a Lewis base or other electron donor may affect the polymer microstructure in different ways. If the added component occupies one coordination site, a monomer coordinates to another site of the active species with one double bond, i.e. as an s-trans-rf ligand, which gives rise to the formation of trans-1,4 monomeric units via the pathway (a)-(b) [scheme (10)]. Depending on the lifetimes of metal species complexed with the monomer and with the Lewis base or the other donor [scheme (11)], mixed cis-1,4/trans- 1,4-polybutadienes or an eb-czs-1, 1 A trans-1,4-polymer can be formed. One should mention in this connection that equibinary cis-l,A/trans- 1,4-butadiene polymers can also be formed in systems without the addition of a Lewis base or other electron donor in such cases, the equilibrium of the anti-syn isomerisation is not shifted and there are equal probabilities for the reaction pathways involving coordination of a transoid monomer and a cisoid monomer [7]. 

Let us recall that a syn form of the (/ -butenyl group can also be obtained by anti —> syn isomerisation [scheme (10)]. The stereochemistry of this isomerisation is presented in Figure 5.9 [7],... 

Figure 5.10 shows schematically [41] the formation of trans-1,4-threo-duso-tactic poly(2,4-hexadiene) by the anti —> syn isomerisation of the last polymerised unit in a polymerisation rim with the Co(Acac)2—AlEt2Cl or Nd (OCOR)j—A1 Et2Cl—Al(i-Bu)3 catalyst [18,19],... 

Figure 5.9 Stereochemistry of the anti —> syn isomerisation of an //3-butenyl group in the polymerisation of terminally symmetrically disubstituted conjugated dienes...

Figure 5.10 Schematic presentation of the formation of lrans-, 4-ihreo-1,4-diisotactic poly[( , ,)-2,4-hexadiene] involving an anti —> syn isomerisation...

Also note that, in accordance with the calculated data, the energies of the o-form of the piperylene active centre, in which the Nd atom is linked to the C atom, and the o-form of the centre in which the metal atom is bonded to the C, atom, differ slightly. The corresponding value of AE is maximal, when the terminal units are in the trans-cisoid conformation and is as low as 4 kj/mol. In the models of the butadiene centres, this difference is much greater (12.7 kJ/mol) [76], that is, in polymerisation of piperylene, the o-structure of the active centre with the Nd-C bond is realised more often than in the butadiene polymerisation. This accounts for why the total amount of 1,2- and trans-, A-units must be greater in the polymerisation of piperylene than in the polymerisation of butadiene, due to insertion via the Nd-C bond and the anti-syn isomerisation of the terminal unit. This was shown experimentally [82]. 

The Role of Anti-Syn Isomerisation of the Terminal Unit of the Growing Polymer Chain... 

The possibility for the occurrence of anti-syn isomerisation was qualitatively estimated from the experimental dependence of the relative content of cis- and trans-units in the... 

This scheme takes into account that propagation is a two-stage reaction, coordination and insertion and that anti-syn isomerisation of the terminal unit of the growing chain... 

It is generally accepted that, in the polymerisation of dienes on lanthanide catalysts, the growing chain is attached to the transition metal by an 7t-allyl bond and that the chain growth occurs by incorporation of the monomer via the metal-carbon o-bond. In the case of neodymium catalysts, the delocalised 7t-allyl type structure of the terminal unit has been observed by spectroscopic methods [8, 26, 28, 58-60]. The results reported in these papers show that the relative contents of cis-l,A- and tri2ns-1,4-units in polydienes depend on the type of solvent used, the polymerisation temperature, structure of diene monomer, and the composition of lanthanide-based catalysts. These data can be interpreted in terms of the concept of isomerisation equilibrium between anti- and syn-forms of n-allyl terminal unit. One of the arguments in favour of the existence of this isomerisation... 

Scheme 14 A syn to anti isomerisation sequence within a 7-phosphanorbornene derivative...

These findings can be rationalised by consideration of a common intermediate. An intermediate carbinolamine could undergo either a nucleophilic substitution reaction, probably through an ionisation step facilitated by the high temperature and by assistance from the nitrogen lone pair to form the spiro compound in dimethylacetamide, or the intermediate may suffer dehydration in apolar solvents to form the corresponding isatin-3-imine. This imine can undergo facile syn-anti isomerisation upon protonation in acetic acid and thus yields the indoloquinoxaline derivative... 

Nitrites exist as a mixture of the syn and anti forms, in dynamic equilibrium. The rate of configurational isomerisation in methyl nitrite has been determined by NMR complete line-shape analysis , and the following parameters were derived Ea = 11.1 kcal.mole S A = 2x10 sec (at 237° K) and AS = —3.5 eu (237 °K). The isomerization mechanism is believed to be intramolecular, involving partial delocalization of the N=0 double bond. 

Methanol and propan-2-ol add non-stereospecifically to cholest-4- and -5-ene as do ethylene and tetrafluoroethylene to 3 -acetoxypregna-5,16-dien-20-one although only 16a,17a-addition products were isolated from allene, acetylene, and dichloroethylene the derived 20-anti-oxime isomerises to the syn-isomer on irradiation in THF and cyclises internally in benzene solution. Irradiation of the trans-acid (561) in methanol resulted only in a cis-trans equilibration, although the trans-acid (562) produced the unsaturated lactone (563). 

Often there are stereochemical alternatives, for instance in several Diels-Alder reactions both endo and exo (or syn and anti) adducts can be expected. Quantitative differentiation of such configurational isomers has been made possible by various methods specific chemical treatment (iodolactonisation, to which only do-acids respond) and gravimetry ir or nmr spectroscopic analysis , gas chromatography . Kinetic control was checked in these cases, i.e. it has been experimentally verified that under the reaction conditions there is no isomerisation of the products, as could happen if conditions were severe enough to allow reversion to reactants or intramolecular rearrangement. 

Using the aldehyde in excess led to optimal conversions (5 equivalents). Due to the precedence of imidazole to catalyse the syn-anti isomerisation of aldol and Mannich products, it was selected as a base, which led to an increase in both reaction rate and yield (with 1 equivalent), however, excess imidazole led to byproduct formation and decomposition of the product (5 equivalents). 

The isomerisation between syn and anti rotamers of a large number of M(NR)(CHR XOR )2 (M = Mo (H W) complexes has been studied. Rate constants and acdvaticHi energies for this process are dependent on the nature of R, R, R2 and the solvent. The result of bulk polymerisations with 2,3-b/s(trifluoromethyl)norbom Kliene suggests a relationship between alkylidene rotamer isomerisation rates and the polymer cis/trans content.33 The molybctenum complex, Mo OC(CF3)2Me 2 N(2,6-P r 2C6H3) (CHCMe2Ph), catalyses the cross-metathesis of aryl- and alkyl-substituted terminal alkenes to unsymmetrical alkenes widi trans selectivities of >95%.335 The molybdenum alkylidene... 

Pure enantiomers of protected anti methyl isoserinates 34 and 35 could be prepared in good yields through isomerisation of the syn p-lactams 26 and 27, under basic conditions, to tmns P-lactams 32 and 33, followed by acidic methanolysis, and reductive cleavage (Scheme 14). 

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