Phosphines basicity

For a recent discussion about phosphine basicity see Woska, D. Prock, A. Giering,... 

The basicity of halogenophosphines has attracted some attention this year. A careful study of the protonation of several halogenophosphines (6) has led to the characterization by 31P n.m.r. of phosphonium species, such as (7).12 These salts show 31P shifts to relatively high fields, and large Vph values e.g., (7) has 1190 Hz.12 Attempts have been made to correlate phosphine basicity with Vpb values in borane complexes,13 and, in turn, to correlate Vpb with electrostatic interactions in the phosphorus-boron bond.13 From such correlations, the authors deduce that this bond is dominated by electrostatic contributions, and not by other factors, such as -bonding. 

Hydroformylation results in Table 8.3 show that, with the exception of ligands 28 and 30, the rate of the reaction increases with decreasing phosphine basicity. An explanation for the deviant behaviour of 28 and 29 can be that catalyst formation is incomplete or deactivation of the catalyst occurs. Decreasing phosphine basicity facilitates CO dissociation from the (diphosphine)Rh(CO)2H complex and enhances alkene coordination to form the (diphosphine)Rh(CO)H(alkene) complex, and therefore, the reaction rate increases. 

While the l b ratio increases with the %-vaIuc. no electronic effect of the ligands on the selectivity for linear aldehyde is observed. The selectivities for linear aldehyde are all between 92 and 93%. The increase in l b ratio with decreasing phosphine basicity can be attributed completely to an increased tendency of the branched alkyl rhodium species to form 2-octene instead of... 

The beneficial effect of the o-methoxy groups seems to be both steric and electronic in nature these groups do not seem to influence the phosphine basicity, rather... 

Some recent quantum mechanical calculations show that all P-donor ligands, PX3, are good <7 donors, including PF3. These calculations estimate that the a donor contribution to the proton affinity is only 10% larger in PMes than in PF3. Thus, the controversy about the significance of phosphine basicities is far from settled. 

As for silanes, decreasing the number of electron-donating substituents on Ge increases the Ge-H activation and favors A over formation of the a complex. The OA product MoH(GePhH2)(CO)(depe)2 then results on addition of GePhH3, which has only one substituent [Eq. (11.29)]. As expected, decreasing phosphine basicity (dppe instead of depe) on M gives back the a complex, which is in solution... 

The rate of substitution of a given amine has been found to reflect the phosphine basicity PBus PCy3 > P(OCH2)3CEt > P(SCH2)3CMe > P(OBu)3 > PPhj > P(OPh)3. 

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