Diphenylprolinol catalyst preparation

The catalyst was prepared by reaction of (iS)-diphenylprolinol with dimethylphosphinite and triethylamine in the presence of carbon tetrachloride. The N-(0,0-dimethylphosphoryl) derivative obtained was treated with an excess of p-anisylmagnesium bromide to give the oxazaphosphinamide catalyst[13]. 

Two different approaches can be followed to prepare and use the catalyst. The first is to prepare it in situ by mixing (R)- or (S)-diphenylprolinol (DPP) (4) and a borane complex (Scheme 16.2). This route is advantageous because there is no need to use boronic acids (or boroxines) and to remove water to form the catalyst. Another possible way is to use preformed catalysts, some of which are commercially available from suppliers such as Callery. 

In the case of H-CBS, this catalyst could be prepared by mixing diphenylprolinol with borane-THF (tetrahydrofuran) or borane dimethylsulfide. Despite numerous efforts in many groups to isolate or even characterize H-CBS by nuclear magnetic resonance spectroscopy, all attempts have been unsuccessful. H-CBS is used in situ, and good results can be obtained in many cases. 

Problems with the preparation and stability of oxazaborolidine (6) led to the development of a series of B-substituted ox-azaborolidines derived from diphenylprolinol. The B-methyl substituted oxazaborolidine (9a) was first prepared (eq 5) by reaction of diphenylprolinol (1) with methylboronic acid under dehydrating conditions (toluene at 23 °C in the presence of 4 X molecular sieves or toluene at reflux using a Dean-Stark trap) followed by vacuum distillation (0.1 mmHg, 170°C). Based on NMR evidence, the product (mp 74-87 °C) was reported to be a mixture of monomer and dimer. The corresponding B-butyloxazaborolidine (9c), prepared in a similar manner from n-butylboronic acid, was also reported to be a mixture of monomer and dimer. Subsequent investigations demonstrated that the reported dimers were in fact the intermediate (8) and the more stable disproportionation product (10) (eq 6). Furthermore, the presence of (8) or (10) was demonstrated to be deleterious to the enantioselectivity of the catalyst. ... 

A series of bicyclic P-cbiral Ugands for tbe Rh metal has been prepared from (S)-a,a-diphenylprolinol and RPCI2. Various degrees of success are seen with the derived catalysts for asymmetric hydrogenation. ... 

---