Anti and syn forms

In CCI4, the. -concentration is ca. 63%, whereas in CH3CN and DMSO, both anti- and syn-form concentrations are approximately equal <2001BOC57>. 

The chemoselectivity of the polymerisation, i.e. the formation of 1,4 as against 1,2 monomeric units in resulting polymers of conjugated dienes, depends on whether enchainment of the incoming monomer occurs at the Ci or at the C3 atom of the last inserted monomeric unit. Enchainment via the Mt-Ci bond (in both the anti and syn forms) gives rise to the formation of a 1,4 monomeric unit (cis or trans respectively), whereas enchainment via the Mt-C3 bond (in both the anti and syn forms) gives rise to a 1,2 unit. Both cases of the reaction in the butadiene polymerisation system are illustrated below ... 

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-... 

Mt ( -butenyl) active species can be presented for anti and syn forms as follows ... 

The methylated complex of cobyric acid is in isomer equilibrium under photolytic or thermal condition to give anti- and syn-forms at a 92 8 molar ratio (Figure 7). The anti- to syn-form ratio for the ethyl-... 

The free energy difference between the anti- and syn-forms of the methylated cobyric acid RTIn (92/8) — 1.4 kcal, CH3—CH3 repulsion energy. 

Molecular mechanics calculations and temperature-dependent CD spectra may reveal details of the conformational map not attainable by DNMR. Thus, double minima within each of the anti and syn forms were inferred from molecular mechanics calculations and were also supported by the temperature dependence of the CD spectra. Rotational strength calculations for the stable conformations predicted by the molecular mechanics calculations (or for any other arrangement) are in fair agreement with respect to both the sign and the dissymmetry factor for many of the studied compounds and may also allow for tentative assignments of absolute configuration in cases where this is not known (87JA492). 

The process of reduction of 2-PAM was then studied. The preferred (3) 1,6- and 1,4-dihydro derivatives were calculated both in the case of the anti and syn forms. The results are given in Table III. 

It is interesting to see that the approximate 4.5 kcal/mol difference between the anti and syn forms disappears as either dihydro derivative is formed. Thus, the 1,4-dihydro forms XV and XVI have essentially the same heat of formation and so do the corresponding 1,6-dihydroderivatives XVI and XVII. 

The rotational isomers exist in cis and trans, anti and syn, or endo and exo forms. The cis and trans forms arise from slow rotation of the side-chain carbonyl groups, hindered by adjacent ortho-substituted iodine atoms in the ring, resulting in the two carbonyl groups aligned either parallel or antiparallel to each other (268,269). The anti and syn forms refer to the position of N-methyl or N-alkyl group relative to that of adjacent carbonyl oxygen (266). The endo and exo forms refer to the po-... 

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... 

CNMR studies show that in CD3NO2 solution, however, this complex exists as an approximately 50 50 equilibrium mixture of the anti and syn forms... 

Relatively stable conformations may exist when is about —30 , and when it is about +150 these have been defined as the anti and syn forms, respectively (Fig. 2-7). Recently (d), the suggestion has been made that torsion angles between —62° and +118 should be designated anti, and all others, syn (Fig. 2-8). The barrier between syn and anti conformations is determined by steric interactions between the N-3 of purine bases or the 0-2 of pyrimidine bases, and the oxygen of the pentose ring. In almost every case studied, the anti conformation (/, 7-11) is preferred in solution, although both forms are found in crystals (e.g., 3). 

Furthermore, the two isomeric butenylnickel(II) forms of the active catalyst complex, the anti and syn forms, can differ in their reactivity depending on the catalyst structure. The key to understanding the cis-trans regulation lies in the different reactivities of the anti- and syn-butenylnickel(II) forms of the active catalyst complex, both in relation to their interconversion and together with the associated anti-syn equilibrium. 

The example of case (b) is a double-armed tertiary terephthalamide derivative 4 with an N-aryl [C6H4CCC6H5] and N-alkyl [nBu] group on each amide nitrogen, which was isolated as a mixture of anti and syn forms that interconvert to each other due to free rotation about the Cc=o Ccentrai single bonds (Figure 14.2)P The anti form adopts a non-helical conformation with a center of symmetry in which the dipoles... 

A secondary terephthalamide derivative (R,R)-9 with four aryl blades on the central benzene ring and N-alkyl substitution on each nitrogen [(R)-C HMe(cHex)], which is the synthetic precursor of the above-mentioned propeller-shaped tertiary derivative (R,R)-syn-7 (Figure 14.4), exists as a mixture of anti and syn forms due... 

Similarly, these molecular structures can readily invert in solution, forming an equilibrium mixture of anti and syn isomers as observed independently in the NMR spectra at room temperature (Figure 5.8). The isomerization barriers of the anti and syn forms are in the range 16.5-21.3kcalmol , depending on the bulkiness of the COT substituents. 

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