Bridge conformation

Fig. 57 Schematic comparison of chain conformations of the midblock for ABC and ABA triblocks. ABC triblock terpolymers (a) have bridge conformations only, whereas ABA triblock copolymers (b) have bridge and loop conformations. From [159]. Copyright 2002 Wiley...

For noncrowded complexes, the influence of the ethylene bridge conformation (and hence of the chirality of the tertiary carbon atom of the bridge) on the stereospecific behavior is poor (compare calculations for ligands 38 and 39, in Table 1.1). This is in qualitative agreement with measured stereospecificities of corresponding catalytic systems (39% vs. 34% of mmmm pentads, respectively).63... 

Cyclophanes are naturally suited for MMPI (15b) calculations. The results ofsuch calculations regarding the structures and electronic spectra of the [m] paracyclophanes (n = 5-10) agreed well with the experimental data (169). Attempted X-ray analyses of [2.4]- and [2.5](9,10)-anthracenophanes (46) encountered serious disorder in the ahphatic bridges. MMPI calculations of all possible conformers of these molecules revealed four and six energy minima for 46a and 46b, respectively. Comparison of the calculated C10 C10 distances and bridge conformations with X-ray information unambiguously identified two conformations each for 46a and 46b as the final solutions. These and the calculated structures of photoisomer 47 were highly useful in the interpretation of fluorescence spectra and photoisomerization processes of 46 (170). 

Over the last twenty years biophysical work on this preparation has concentrated mainly on the elucidation of the filament structure and cross-bridge conformations (Reedy et al., 1965 Squire et al., 1977 Wray, 1979 Clarke et al., 1986 Reedy et al., 1987), and on the mechanical characterisation of various equilibrium states and of the kinetics of the cross-bridge cycle (Jewell Ruegg, 1%6 White, 1970 Tregear, 1977 Gtith et al., 1981 White Thorson, 1983). The biochemistry of ATP hydrolysis by the insect proteins has received less attention than that of vertebrate muscle proteins, primarily because of shortage of tissue, but recently aspects of the biochemical kinetics have been investigated (White et al., 1986). 

The stereostructures of the N,N-bridged 1,3-diazacyclohexanes 105-110 (cf. Scheme 41) were studied both in the gas phase and in solution (93JA6580). The borderline for change over from the axial,axial-bridged conformer to the axial,equatorial-bridged conformer was defined for n = 4... 

The association of flower-like micelles through bridged conformations in an ABA triblock copolymer can be avoided by replacing the ABA triblock by a cyclic diblock copolymer. In this case, sunflower micelles have been observed for example by Borsali and coworkers from the assembly of cyclic and asymmetric polystyrene- >-polvisoprene diblock copolymer (PS-h-PI) and compared to the corresponding linear diblock [46, 47]. 

Paracyclophane, which exists as the minor component of an equilibrium with its valence-isomeric Dewar benzene, is the smallest member of this series to exhibit sufficient stability to be studied directly [3]. In order to further stabilize the [5]paracyclophane unit, Bickelhaupt [4] recently synthesized a benzannulated version, namely [5](l,4)naphthalenophane (2, Scheme 1). Even though this compound also exists in equilibrium with the Dewar naphthalene 1 from which it was produced, the proportion of the cy-clophane (35 % of the mixture) is higher than that in any previously reported case. A particularly interesting feature of this system is that there are two observable bridge conformers of 2 in solution in a ratio of 95 5. Unfortunately, despite considerable effort, assignment of the two conformers was not possible. 

Rieger, B. Jany, G. Fawzi, R. Steimann, M. Unsymmetric ani a-zirconocene complexes with chiral ethylene bridges influence of bridge conformation and monomer concentration on the stereoselectivity of the propene polymerization reaction. Organometallics 1994, 13, 647. 

With flexible bridges, conformational parameters become. 126-129... 

Fig. 7. Transition from a two-domain structure to a three-domain structure in an ABC triblock copolymer lamellar phase. Although state ii is thermodynamically the most stable state, transition to this state from i is hindered because of the high free energy cost in switching the orientation of the bridges and in turning the loops into bridges. Thus, a kinetically more likely process is for the A and C blocks from the bridge conformation to separate laterally, with the loops straddling the interfaces between the A and the C domains.

The combination of dimethylphenylsilane and trifluoroacetic acid has been used to reduce protected a-amino ketones of the general form 1 with excellent selectivity for the a /z -isomer (H-isomer)1 2. The stereoselectivity may be explained by assuming that the reaction occurs under chelation control via the proton-bridged conformation 2. 

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