Slippage mechanism

D. H. Macartney, The Self-Assembly of a [2]Pseudoro-taxane of a-Cydodextrin by the Slippage Mechanism , J. Chem. Soc Perkin Trans. 21996, 2775-2778. 

A ring slippage mechanism was first proposed tn 1966. Fora discussion of this week, see Basdo. F. Potyhcdnm 1990, 9. 505- 535. 

It is worth noticing that this chain slippage mechanism applies both to CSCs and CDCs. The only difference concerns the MW of the fibril polymer chains. Indeed, in the case of CDCs, this is the bulk polymer MW since only disentanglement has occurred. In contrast, for CSCs, the fibril polymer MW is lower than that of the bulk polymer, nevertheless, the average MW of chain fragments in the fibril is higher when the bulk polymer MW is larger. 

Many of the other chapters in this volume deal with evolutionary analyses of specific genes and unique DNA sequences.24-28 There are, however, some evolutionary aspects unique to repeated DNA sequences. The most important of these factors is the amplification dynamics. Sequences become repetitive because there are amplification processes that make extra copies of them. These include retroposition and transposition mechanisms that would explain the majority of interspersed repeated DNA sequences, as well as recombination or replication slippage mechanisms that would probably explain most tandem replications. For any given repeated sequence, various factors may combine to increase or decrease the amplification rate of that sequence at various times in the evolutionary process. Thus, the dynamics of the amplification process could greatly affect the observed evolution of the family. This is particularly important in cross-species comparisons, because the amplification dynamics of a specific repeated DNA family may be altered in one species, relative to another. 

Kinetic studies at different phosphine concentrations indicated that the substitution reactions occur totally by a dissociative mechanism, while the ring expansion reaction is by an associative mechanism. The associative reaction could proceed by an 18-electron transition state involving either a bent NO or an rj to rj ring slippage mechanism. The bent NO mechanism seems more likely, because the ring slippage mechanism is known to result in the formation of oxocyclobutenyl product with ring expansion and because the isoelec-tronic cobalt complex does not react by a parallel associative pathway. 

In other studies, matched complementarity between the cavity size of the macro-cyclic component and the size of the bulky stopper groups has been exploited to allow preparation of rotaxanes by means of the slippage mechanism mentioned previously. 

A ring slippage mechanism was first proposed in 1966. For a discussion of this work, see Basolo, F. Polyhedron 1990. 9. 1503-1535. 

The slow (ri/2 = 68.8h) reaction, which occurs at 90°C under CO pressure, can be accelerated by the presence of free trimethylphosphine the reaction has been proposed to occur via a ring slippage mechanism rj to to t] ). 

Figure 10.15. Schematic illustrations of theories of stress softening, (a) Mullins and Tobin (1956) considered the filled rubber as a heterogeneous system comprised of hard and soft phases. Deformation breaks down the hard phase, but the degree of breakdown depends on the maximum extension of the sample, (b) F. Bueche (1965) attributed stress-softening to the breakage of network chains attached to adjacent filler particles (A molecule breaks first), (c) Dannenberg (1966) and Boonstra (1965) suggested that reinforcement can be understood through chain slippage mechanisms. The slippage is shown by the chain marks. (Smith and Rinde, 1969.)...

Marx and Lees studied quantum yields, O, for the photochemical reaction of (Cp)It(CO)2 with in hexafluorobenzene at 20°C and found that O shows a saturation effect with increasing and is independent of [CO]. They concluded that the rate-controlling process is not CO dissociation and suggest that it is an T to T slippage, which then allows complexation of benzene prior to oxidative addition. Drolet and Lees proposed a similar slippage mechanism for the photolysis of (Cp)Rh(CO)2 in the presence of PPhj in decalin at lOX. Their conclusion is based on the observation that the quantum yield for CO replacement is first-order in [PPhj] and unaffected by 9xl(T M CO. 

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