Aggregated species

Another chiral titanium reagent, 11, was developed by Corey et al. [17] (Scheme 1.24). The catalyst was prepared from chiral ris-N-sulfonyl-2-amino-l-indanol and titanium tetraisopropoxide with removal of 2-propanol, followed by treatment with one equivalent of SiCl4, to give the catalytically-active yellow solid. This catalyst is thought not to be a simple monomer, but rather an aggregated species, as suggested by NMR study. Catalyst 11 promotes the Diels-Alder reaction of a-bro-moacrolein with cyclopentadiene or isoprene. 

The position of the equilibrium depends upon the solvent and the identity of the specific organic group, but in ether lies well to the left for simple aryl-, alkyl-, and alkenylmagnesium halides.21 Solutions of organomagnesium compounds in diethyl ether contain aggregated species.22 Dimers predominate in ether solutions of alkylmagnesium chlorides. 

E. Watson and W. C. Kenney, High performance size exclusion chromotography of recombinant derived proteins and aggregated species, J. Chromotogr., 436, 289 (1988). 

Further studies have shown that the degree of aggregation, and hence the number of active sites, is a function of the alkoxide substituent.813 For example, in (270) one of the i.vo-propoxide groups bridges two aluminum centers the other three are terminal ligands and all three initiate the ROP of CL. Less aggregated species such as (270) generally exhibit simple first order kinetics. 

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