Macroscopic timescale

II of the actual atoms (or at least the non-hydrogen atoms) in the core system are lented explicitly. Atomistic simulations can provide very detailed information about haviour of the system, but as we have discussed this typically limits a simulation to nosecond timescale. Many processes of interest occur over a longer timescale. In the if processes which occur on a macroscopic timescale (i.e. of the order of seconds) rather simple models may often be applicable. Between these two extremes are imena that occur on an intermediate scale (of the order of microseconds). This is the... 

The third timescale is the laboratory, or macroscopic, timescale of minutes or more if an organolithium can be formed as a single enantiomer and subsequently reacted with an electrophile to produce a product with at least some enantiomeric excess, then it has configurational stability on the macroscopic timescale. 

On the other hand, a-alkoxyorganolithiums are not configurationally stable on a macroscopic timescale when they are secondary and allylic or benzylic. For example, despite the known (see section 5.2.1) stereospecificity of the tin-lithium and lithium-tin exchanges of similar compounds, tin-lithium exchange of 110 with rc-BuLi/TMEDA at -78 °C gives an organolithium 111 which has completely racemised after 10 min stannylation returns racemic stannane 112.55 Similarly, 111 racemises rapidly at -70 °C in pentane/cyclohexane in the... 

McDougal showed that 219 epimerises rapidly on a macroscopic timescale, and that both diastereoisomers of 218 give the same 98 2 ratio of diastereoisomers after transmetallation and silylation.105... 

Lithiated sulfones are configurationally stable even on the macroscopic timescale. The enantiomerically pure sulfones 247 and 251 may be deprotonated and react with electrophiles after 3 min at -105 °C to return highly enantiomerically enriched products.118 Polarimetry indicates a barrier to racemisation of 71 mol-1 at 239 K for 248 (a half-life of 30 days at -78 °C) and of 53.5 kJ mol"1 at 298 K for 252 (a half-life of 3 h at -105 °C). 

No organolithium a to phosphorus has been shown unequivocally to be configurationally stable. The phosphonamide 279 is configurationally unstable on a macroscopic timescale,128 the phosphine oxide 280 gives racemic products on lithiation even in the presence of an internal quench,129 and in a Hoffmann test the phosphine oxide 281 gave the same ratio of diastereoisomers with either racemic or enantiomerically pure 6.129... 

Exceptions are the few secondary benzyllithiums which show configurational stability on the microscopic (but not macroscopic) timescale - in other words, they racemise slower than they react with electrophiles, though they still cannot be maintained in stereoisomerically pure form for periods of minutes or more. These are the carbamates 288 and the sulfones 289, discussed in sections 5.1.4 and 5.I.7.6 It is significant that both of these compound classes contain powerful lithium-coordinating oxygen atoms, which may hold the lithium counterion close to one face of the benzylic system.134-120... 

Three types of secondary benzyllithium are known which are configurationally stable on the macroscopic timescale, and they too each contain powerful lithium-coordinating oxygen atoms. One is the N-Boc amino substituted 180 discussed above.80 Partial configurational stability is evident in a close analogue of 180.81... 

It can be checked that Eqs. (B.12 and B.15) are identical. One recognizes in Eq. (B.15) the three terms of the Langevin equation (see Ref. [21]). However, we emphasize that expression (B.12) has many advantages since it relates directly the complex poles of P/(z—L " (z)) to the various relevant microscopic and macroscopic timescales. 

Here, the number of molecular collisions is so large that the flow reaches the equilibrium state in a time short compared to the macroscopic timescale. 

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