Daniphos

The general synthesis of the Daniphos ligands starting from enantiomerically pure [(R)-l-(phenylethyl)dimethylamine]chromiumtricarbonyl 1, is depicted in Scheme 1.4.1 [15]. A directed ortho-metallation (DOM) and subsequent quench with a chlorophosphine leads to an enantiomerically pure planar-chiral complex, which after chlorination using ACE chloride (1-chloroethyl chloroformate) is transformed into the desired diphosphine by a nucleophilic substitution without any loss of optical purity (Scheme 1.4.1) [6, 10]. 

As mentioned above, this synthetic route enables a modular approach to ligand design. The concept of modularity, as introduced for the Josiphos ligands by Togni [17], is sketched schematically in Fig. 1.4.2 for the Daniphos ligands. 

I 1.4 The "Daniphos Ligands Synthesis and Catalytic Applications Table 1.4.2 Results of the hydrogenation of the MEA imine. 

Enantiomerically pure citronellal in both of its antipodal forms has outstanding importance as a key intermediate for the production of fine chemicals, especially for the production of fragrances and flavors. In this respect the isomerization of diethylgeranylamine to R) -citronellal enamine in the presence of Rh /(S) -BINAP is an exceptional industrial process, for instance as one of the key steps of the Takasago process for the production of (-) -menthol [22]. In the search for alternatives for this process, both Josiphos and Daniphos derivatives were evaluated (Scheme 1.4.5) [23]. 

Both classes of ligands gave very good results, the Daniphos ligands having higher enantioselectivities but slower kinetics (see Table 1.4.4). 

Scheme 1.4.9 Allylic sulfonation with the Daniphos ligands.

Structural data confirm that the Daniphos ligands are readily adaptable to most ligand environments. A major advantage of the chromium system is the commercial availability of phenylethylamine and its derivatives in both enantiomeric forms. In contrast to the cyclopentadienyl ring, arenes have an almost unlimited potential for controlled substitutional variation, accessible through standard reaction protocols. 

Recently it has become known from work at Firmenich, that BINAP can be effectively replaced by the Josiphos and Daniphos ligands. [129] From diethyl-geranylamine one obtains after hydrolysis (S>citronellal, and from diethyl-nerylamine (R)-citronellal (analogous to (R)-BINAP). By attaching a linker to the unsubstituted cyclopentadiene ring, the catalyst can be immobilised on various carriers. In some cases this reduces the activity, but the stereoselectivity remains comparable. 

The "Josiphos ligands" were developed in the 1990s by Antonio Togni at the ETH and termed after Josi Puleo, the technician who prepared the first one. In analogy to "Josiphos", Albrecht Salzer at the RWTH Aachen named in 2003 the "Daniphos ligands" after his coworker Daniela Vasen.[129]... 

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