Catalysis of Organic..

Riermeier et al. 457It is interesting to note, that in these diphosphines, with the exception ofButiPHANE, both chiral units are linked by an 1,2-phenylene and 1,2-ethylenebridge, respectively, or in case of BPF/FerroTane by an 1,1-ferrocenyl moiety.Based on the original procedure of Burk such ligands are prepared by the doublenucleophilic substitution of chiral cyclic sulfates with corresponding primarydiphosphines in the presence of strong bases. This tedious approach is limited to thesynthesis of bisphospholanes (bisphosphetanes) with those bridging units where thecorresponding primary phosphines are synthetically accessible. Due to this obstacleto date DuPHOS-type Rh-catalysts with definite P-Rh-P bite angles cannot beconstructed. Hence, the ligand synthesis does not allow flexibility and theperformance can not be tuned for a given challenging substrate.Results and DiscussionAs a part of the collaboration between Degussa AG and the Leibniz-Institut ofOrganic Catalysis in Rostock we envisaged the modular synthesis of diverse arraysof chiral vicinal bisphospholanes. These ligands, now commercially available inmulti-kg scale under the trademark catASium fi M, can be characterized by varying P-C=C angles and electronic properties in the bridge (Figure 3). For the constructionof catASium fi M ligands, we searched for a general structural principle, which allowsin the corresponding Rh-catalysts the stepwise variation of the bite angle (α) over abroad range. We anticipated that due to their large variety 1,2-functionalizedheterocycles derived from maleic acid could be a promising scaffold for this goal. Inparticular, we targeted larger bite angles (14) than usually found in DuPHOS-Rhcomplexes.We speculated that an increase of the bite angle could reinforce thediastereo-discriminating interactions between catalyst and coordinated prochiralsubstrate. Furthermore, small changes, eg. O vs. NMe, have incremental effects andshould allow for fine tuning of these new ligands.RRORAORPRPRhRαPRRhRPα'DuPHOSα < α'catASium ® MA=O, NR, (CH 2 ) 0 etc.Figure 3. Influence of the shape of the backbone on the bite angle αThe synthesis of the new ligands should be based on a convergent methodology, thatis easy to perform and convenient to scale up.As mentioned above, for several reasons the commonly used method for thepreparation of bisphospholanes and bisphosphetanes tracing back to the seminal workof Burk does not match the requirements for the synthesis of catASium fi M ligands.