Syn-isomerization

For chromium alkoxycarbene complexes the MLCT and the lowest energy LF bands overlap. Irradiation at A>385 nm led to anti-syn isomerization... 

H NMR of pyrrolophane 30 shows two singlets for SiMe groups that do not coalesce in the temperature range of -80 to +110°C, in contrast to furanophane 28, which shows rapid anti/syn isomerization down to -78°C, and thiophenophane 29, with a coalescence temperature of 15°C. The rigidity of 30 is obviously due to the steric bulk of the 2V-methyl groups. 

The great steric demand of the methyl group on N was further demonstrated by NMR characteristics of Si-O-Si bridged pyrrolophane 32 prepared by oxidation of 30 with excess Me3N-0 in refluxing benzene (74% yield). Even with the considerably longer Si-O-Si bridges, no anti/syn isomerization occurs for 32 between 80 and +110°C. 

NMR chemical shift calculations were performed for the B3LYP/6-31G(d) optimized anti and syn isomeric structures of the analogous l-p-anisyl-2-SiH3 substituted... 

Der Kohlenwasserstoff (21) wird stereospezifisch gebildet. Kein syn-Isomeres von (21) wurde gefunden. 

The mechanism is complicated by the possibility of anti-syn-isomerization and by n - a-rearrangements (it - r 3-allyl Act - r 1 -allyl). In the case of C2-unsubstituted dienes such as BD the syn-form is thermodynamically favored [646,647] whereas the anti-isomer is kinetically favored [648]. If monomer insertion is faster than the anti-syn-rearrangement the formation of the czs- 1,4-polymer is favored. A higher trans- 1,4-content is obtained if monomer insertion is slow compared to anti-syn-isomerization. Thus, the microstructure of the polymer (czs-1,4- and frazzs-1,4-structures) is a result of the ratio of the relative rates of monomer insertion and anti-syn-isomerization. As a consequence of these considerations an influence of monomer concentration on cis/trans-content of BR can be predicted as demonstrated by Sabirov et al. [649]. A reduction of monomer concentration results in a lower rate of monomer insertion and yields a higher trans-1,4-content. On the other hand the czs-1,4-content increases with increasing monomer concentration. These theoretical considerations were experimentally verified by Dolgoplosk et al. and Iovu et al. [133,650,651]. Furthermore, an increase of the polymerization temperature favors the formation of the kinetically controlled product and results in a higher cis- 1,4-content [486]. l,2-poly(butadiene) can be formed from the anti- as well as from the syn-isomer. In both cases 2,1-insertion occurs [486]. By the addition of electron donors the number of vacant coordination sites at the metal center is reduced. The reduction of coordination sites for BD results in the formation of the 1,2-polymer. In summary, the microstructure of poly(diene) depends on steric factors on the metal site, monomer concentration and temperature. 

Ligand exchange studies on Fischer carbenes demonstrate that photolysis results in exclnsive cis substitution. Resonance Raman and matrix studies demonstrate that photolysis into the MLCT band of (CO)5MC(OMe)Ph, where M = Cr, W, initiates an anti syn isomerization of... 

To explain the cis-trans selectivity on the basis of the anti-cis and syn-trans correlation it is necessary to take into account not only the influence of the rate of anti-syn isomerization relative to the rate of the insertion reaction in connection with the different conditions of formation for the anti and the syn structures of the butenyl end group, but also the possible dependence of the cis-trans selectivity on the difference in stability and reactivity of the anti and the syn butenyl groups. 

If the rate of anti-syn isomerization is relatively low, then the cis-trans selectivity can be determined by the formation of the anti- or the 5y/i-butenyl structure, for example from the t] -cis or the if-trans coordinated butadiene, in the catalyst complex. This is the mechanism of stereoregulation which was suggested in the mid-1960s by Cossee and Arlman [34, 35] for titanium-catalyzed butadiene polymerization, and which was reconsidered more recently for the allylne-odymium complex catalysts to explain their cis-trans selectivity [39], But it is also possible that the difference in reactivity between the anti and the syn structure of the catalytically active butenyl complex can determine the cis-trans selec-... 

In accordance with the anti-cis and syn-trans correlation and under the assumption that the insertion step is more rapid than the anti-syn isomerization, the trans units are obtained from 77 -coordinated butadiene in its stable single trans configuration by syn insertion and the " -c/5-coordinated butadiene provides the cis units via anti insertion. 

For each of the butenylnickel(II) complexes, and for the butadiene complexes as well, an anti-syn equilibrium has to be assumed, with the syn configuration having the thermodynamically more stable structure (Xg/s 10 -10 ) [36, 60]. The rate of anti-syn isomerization has proved to be strongly dependent on stmcture. In the bis(ligand)-butenylnickel(II) complexes the isomerization rate is very low = 10 s ) [66], but in the ligand-free Ci2-allylnickel(II) complexes the anti-syn isomerization is accelerated considerably by the coordination of the next double bond, so that it is completed instantaneously even at -70 °C [60]. 

Therefore the cis-lrans selectivity can be determined neither solely by the mode of butadiene coordination nor by the rate of anii-syn isomerization as has been suggested in the literature [50, 68-70] rather, it is regulated kinetically by the different reactivity of the anti- and the iyn-butenylnickel(II) complex, depending on the mode of butadiene coordination, and thermodynamically by the concentration of the stmcturally different butadiene complexes. 

Since the rate of anti-syn isomerization has been proven to be very rapid in the ligand-free Ci2-allyInickel(II) cation [Ni( / , 7 -Ci2Hi9)], in this case the cis-trans selectivity depends solely on the difference in the free enthalpies of the transition states for the insertion reaction via the anti- or the syn-butenyl complex, independently of the concentrations of the two complexes in the anti-syn equilibrium, according to the Curtin-Hammett principle [26, 61]. 

During a very short initiation period the cation [Ni(Ci2H 9)] , which is present mainly in the thermodynamically more stable syn form b, reacts via the less stable but more reactive anti form a with insertion of butadiene into the anti--polybutadienyl complex c. As a result of the very rapid anti-syn isomerization this complex also exists in equilibrium (cf. Kf) with the more stable syn complex d, which must be regarded as the stable storage complex under the conditions of polymerization. With butadiene the polybutadienyl-butadiene complexes e and f are formed as the actual catalysts. By the much higher reactivity of the less stable anti complex e, the formation of cis units is catalyzed. Since all the equilibria can be assumed to be rapid, the insertion reaction of butadiene (/ 2c) has to be taken as the rate-determining step in the catalytic cycle. Thus, the catalytic activity is dertermined thermodynamically by the concentration of the -c/5-butadiene complex in the anti form e and kinetically by its reactivity k2c-... 

They also performed MNDO calculations in order to understand the bonding of aldehyde-BF3 adducts, employing acetaldehyde as a model substrate [8]. Minimum energy geometries revealed that the anti form (1), in which the C-C-O-B skeleton lies in a common plane, turned out to be more stable than the syn isomer (3) by 1.8 kcal/mol. Although the linear form (2) represents the least-energy transition state for internal anti/syn isomerization, it is not a minimum on the energy surface (Scheme 1-1). 

Insertion of the monomer, bonded to the metal in rf-cis fashion, into the metal-polymer bond forms a new 7t-allyl polymer end with the substituent at the anti-position. Successive insertion of the new monomer with if-cis coordination would produce cis- 1,4-polybutadiene. Insertion of if-trans-co-ordinated monomer into the metal-polymer bond leads to trans-1,4-polybu-tadiene via syn zr-allyl intermediates. The above anti Tt-allyl polymer end is often equilibrated with the thermodynamically more favorable syn zr-allyl structure via n-a-n rearrangement. Thus, the ratio of cis-1,4 and trans-1,4 repeating units of the polymer produced depends on the relative rates of the two reactions C-C bond formation between the monomer and the polymer end, and anti to syn isomerization of the zr-allyl end of the growing polymer. If the anti-syn isomerization of the anti zr-allyl polymer end occurs more rapidly than the insertion of a new monomer, the polymer with trans-1,4 units is formed even from 7j4-ds-coordinated monomer. The polymerization catalyzed by Ti, Co, or Ni complexes shows high cis-1,4 selectivity, while that with low monomer concentration results in increase of the trans content of... 

The alternative trans-DT generating pathway with the r -syn,r[ A-trans isomer of 5 as the precursor, however, is not reachable. This can be concluded from the comparison of the moderate barriers for the preferred trans-DT pathway (see above) with the kinetically difficult (which has a 7.0 kcal mol" higher free-energy barrier, see above) but necessary anti-syn isomerization of the r -anti,r, h-trans precursor 5. 

Table 7. Relative Energies and Associated Torsion Angles < ) for the Iso and Syn Isomeric Forms of the [-SiPhMe-] Model Compound...

Ann-complex is responsible for c/s-1,4-polydienes formation. As a consequence cis- and rrans-units content in chain is determined by ratio of chain propagation rate to the rate of anti-syn-(cis-rran5)-isomerization of end unit in AS. Authors of [175] explain the formation of l,2-(3,4-)-units by coordinating polymerization mechanism when anri-syn-isomerization proceeds over a-state (a) in which o-bond is conjugated with double bond. 

Transition state for syn isomerization of cyclohexene oxide to allylic cyclohexenol. (Adapted from reference 58.)... 

During the past decades, iridium-catalyzed allylation chemistry developed dramatically, and the improvements made have been summarized in several reviews [143]. In contrast to the paUadiumallyl complex derived from a branched or linear ( )linear products with conserved double-bond geometry are formed, indicating a slow anti-syn isomerization (Scheme 12.63) [144]. 

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