Phobane ligands

Alkyldiphosphines turned out to be very useful in a different reaction, namely the carbonylation/hydrogenation of ethylene oxide to give 1,3-propanediol also using cobalt catalysts. Interestingly, the ligand contains two phobane units bridged by 1,2-ethenediyl. The process was commercialised by Shell [18]. 

For the further discussion of the catalytic applications it is important to note the acronym system which we apply to designate each ligand easily and unequivocally. According to the general label R/PR 2/PR", R denotes the substituent in the paro-position to the chiral a-chain (when R = H, it is omitted), PR 2 stands for the group in the ortho-position and finally PR" for the phosphino group in the a-chain itself (see Fig. 1.4.4). The abbreviations PbA and PbB refer to the regioisomeric phobane skeletons (PbA = phosphabicyclo[3.3.1]nonane, PbB = phosphabicyclo[4.2.1]nonane), Ind stands for indane and the (R,R)-2,5-dimethylphospholane unit is abbreviated to (R,R)-DMP . 

Eicosyl phobanes, HP-NMR and HP-IR studies, 1, 490-491 EIEs, see Equilibrium isotope effects Eighteen-electron complexes, ligand substitution in, 1, 96 Eighteen-electron rule... 

Secondary phosphines can also be prepared by addition of P—H bonds across olefmic double bonds.40 For example, the industrially important bicyclic secondary phosphine phobane (43) can be synthesized from phosphine gas and cycloocta-1,5-diene, albeit as a mixture of two isomers (43a) and (43b) (Scheme 3). These isomers have recently been separated by an elegant sequence of hydrophosphination/dehydrophosphination steps, affording pure isomers which have been used in subsequent tertiary and ditertiary phosphine ligand syntheses.57... 

In a second report, the Bergman and Ellman groups improved on their original procedure for the arylation of heterocycles with aryl halides [103]. The key improvements over the previous method were three-fold (1) the use of the hindered tertiary amine base Pr.NBu in replacement of Et3N (2) microwave irradiation for rapid (<1 h), high temperature (250 °C) reactions in 1,2-dichlorobenzene (3) the use of bulky bicyclic trialkylphosphine ligands of the phobane family in replacement of PCy3 (Scheme 15). Under typical reaction conditions, the heterocycle... 

The scope of the reaction is broadened by the use of the tetrahydrophosphepine ligand. In addition to heterocycles that were reactive with PCy3 or cyclohexyl-phobane as ligands, 4,5-dimethylthiazole can be arylated. The scope with respect to the bromoarene partner is also broadened to include electron-rich bromoarenes and heteroaryl bromides such as 3-bromothiophene, 5-bromo-iV-methylindole, 5-bromobenzofuran, and 5-bromobenzothiophene. Functional groups including sulfoxides, chlorides, fluorides, ketones, esters, primary and secondary amides, phenols, anilines, and pyridines can be tolerated under these conditions. Sterically hindered and/or ortho substituted substrates are unreactive, but meta and para substituted substrates are well-tolerated. 

Shell was the first company to utilize cyclic trialkylphosphines for optimization of Co catalysts [13]. Meanwhile, these ligands are commercially available as a mixture under the collective chemical name phobanes. Bungu and Otto [14] evaluated various venues to phobanes by taking yields and safety issues into consideration. Differences in the methods consisted mainly in the employment of... 

Besides the symmetric phobane s-PhobPR, nonsymmetric phobanes exist as two diastereomers, g-PhobPR and fly-PhobPR (Figure 2.5), which can be separated via their BHg adducts [19]. The donor strength decreases in the sequence Oy>s>a, but steric bulk follows the reverse order [20]. Highest reaction rates in the Co-catalyzed hydroformylation have been achieved with aj-PhobPR as ligand, but on the technical scale a mixture of all isomers is used. 

A typical benchmark system is the Shell catalyst for reductive hydroformylation of 1-dodecene based on the mixture of Phobane-C2o ligands, which operates under a syngas pressure of 85 bar with an excess of H2 in comparison to CO to give 86.9% yield of isomeric tridecanols (Scheme 5.44) [46a]. 1-Tridecanol is used as a lubricant or as an ingredient of surfactants, ink solvents, and pesticides. 

---