Flexible steric bulk

Flexible steric bulk is a characteristic of a new class of imidazolium salts derived from bisoxazolines (IBiox) (Glorius et al. 2002 Altenhoff et al. 2004), which were tested for the organocatalytic synthesis of bu-tyrolactones (Burstein and Glorius 2004). 

Until now, only two families of ligands have been realized where both P groups are attached to side chains, probably because the resulting metal complexes have relatively large chelate rings which usually are not suitable for enantioselective catalysis. A cursory inspection of the ligands depicted in Fig. 25.14 shows that, due to steric bulk of the ferrocene backbone, both diphosphines probably have sufficiently restricted flexibility so that good stereocontrol is still possible. 

The substituent effect of 10,10 -positions of bis[2-(9-anthryl)ethyl] glutarates 353a-e, which have more flexible chains, was investigated by irradiation with 350-nm light in dilute (10-4 M) deoxyganated benzene [349], The quantum yields for the dimerization (dimer) falls with the steric bulk of the substituents. 

At this point the author would like to emphasize the importance and flexibility of the silylamide route. Ln(tritox)3 complexes are readily available from Ln[N(SiMe3)2]3 only for the larger lanthanide elements. However, the decreased steric bulk in Ln[N(SiHMe2)2]3(THF)2 amide precursors also allows the isolation of Ln(tritox)3 complexes of the smaller lanthanide elements [16, 40]. 

In search for a model giving a concise description of all these observed sterical effects, the concept of the accessible molecular surface (AMS) of the rhodium center within the flexible [(P2)Rh] fragment has been developed for the catalyst system of type 13. It has been demonstrated how the shape and the size of the open cavity is determined not only by the P-Rh-P angle and the steric bulk of the groups R at phosphorus, but also by the flexibility of R and by possible internal movements within the chelate ring. All relevant parameters for the description of the intrinsic steric properties of the [(P2)Rh] fragment is reflected by the cavity. 

---