Ring-closure dynamics

Cyclization is much more sensitive to excluded volume effects than other properties of polymer chains. These effects operate both to retard ring-closure dynamics and to accelerate the ring-opening step of cyclized chains. 

Wipf et al. <2005OL4483> elaborated a new metathesis method including ring opening in order to generate dynamic combinatorial libraries. The transformation is shown in Scheme 12. The essence of this method is the recognition that an equilibrium takes place with 93 and an aldehyde in aqueous conditions at pH 4 in a phosphate buffer. With the help of this transformation, a series of new R2-substituted products 94 - not available via direct ring closure - have been synthesized. 

The decay of the tetramethylene diradical derived from 2,2,5,5-t/4-cyclopenta-none is much slower than seen for the C4Hg diradical. Both principal decay modes, fragmentation to two ethylenes and ring-closure to cyclobutane, may be dependent dynamically on torsional motions of the terminal methylene groups. 

Both sets of calculations found that ring closure of 8 preferentially occurs by the same mode of coupled methylene rotations as ring opening of 7. Crudely put, the dynamical behavior of 8 can be predicted by, what would be called in classical mechanics, conservation of angular momentum. Chapter 21 in this volume provides examples of other reactions in which dynamical effects cause statistical models, such as TST, to fail to make correct predictions. 

Jiao T, Pang Z, Burkey TJ, Johnston RF, Heimer TA, Kleiman VD, Heil-weil EJ. Ultrafast ring closure energetics and dynamics of cyclopentadienyl manganese tricarbonyl derivatives. J Am Chem Soc 1999 121 4618-4624. 

While complete ring closure is indicated by mass spectral and NMR analysis, characterization of 5 by X-ray and neutron scattering [133], and by dynamic light scattering experiments [134] suggested this polymer has... 

Very revently a paper by Burkey and Heilweil reported photoacoustic measurements providing data concerning the energetics and dynamics of the ring closure of 485 and 486 to 487 and 488, respectively (Scheme 93). 

---