Phosphines privileged

The hydrogenation of alkenes without privileged functional groups has not been investigated systematically, probably because much more effort is required to achieve good enantioselectivity. Successful examples are Structures 37-39. Of special interest are the Ir phosphine dihydrooxazole (P OXAZ) catalysts [27], even though their functional group tolerance is relatively low. 

Donor sites D. Phosphines have been largely privileged here due to their high affinity for mid and late transition metals with which Lewis acids have been mainly associated so far. Practical reasons (easy synthesis and handling, P NMR probe)... 

Chiral base catalysis is one of the most versatile and broadly applicable types of catalysis. In particular, the potential of tertiary amines to act both as a base and as a nucleophilic catalyst makes chiral tertiary amines like Cinchona alkaloids a privileged catalyst structure in modem synthesis chemistry. In addition, the field of achiral phosphine and carbene catalysis has proven its potential in numerous applications in the past and it is probably only a matter of time until chiral phosphines and carbenes will also be used routinely for other presently demanding natural product total synthesis (Table 7). 

In large-scale processes, a limited number of phosphines and phosphites are used, which can be called, in analogy to ligands in asymmetric catalysis, privileged hgands. Somewhat more variants are available from commercial suppliers. 

The catalytically active metal species must have a vacant coordination site, i.e. NVE = 16 at most or even 14, in order to allow substrate molecules to coordinate. Sometimes, weak ligands or solvent ligands can be present and are easily displaced by substrate molecules. Bulky phosphines such as triphenylphosphine are easily dissociatively displaced and thus constitute a reservoir of vacant coordination sites that can be filled or emptied at will. Noble metals (2" and 3 lines of transition metals of groups 8, 9 and 10) easily forming 16-electron species are privileged catalysts. 

Early results on the preparation and application of hydrophilic phosphines were reviewed in 1986 [3]. Though to date all conceivable approaches to hydrophilization of phosphine ligands had been tried, sulfonated ligands still occupy a privileged place, being not only the most numerous but undoubtedly the best-studied group of phosphines. 

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