Regio- and stereo-selectivity in the hydrogenation of aryl phosphines by niobium aryloxide compounds

Abstract

The sequential hydrogenation of the three aryl rings in PPh3 b y the catalyst system [Nb(OC6HPh4-2,3,5,6)~C121/3Bu"Li occurs with relative rates of 39:28: 1; NMR analysis of the PhPCy2 obtained from [P(C6D5)3fH2] and [P(c6H&/D2] shows a predominately all cis hydrogenation of both aryl rings has occurred.

Homogeneous metal catalysis for conversion between aromatic and saturated compounds

“…In addition to polycyclic arenes, 84 was also found to be remarkably effective for hydrogenation of aryl phosphines to their cycloalkyl counterparts (Scheme 98) [132]. In particular, the aryl groups of monodentate phosphines were reduced sequentially at successively slower rates; in contrast, those of bidentate substrates appeared to be jointly reduced to the final product through a single interaction with the catalyst [133].…”

A new generation of homogeneous arene hydrogenation catalysts

“…of Bu n Li followed by exposure to H 2 . 26 This procedure allows the hydrogenation to be carried out under milder conditions where hydrogenolysis of Nb-CH 2 SiMe 3 and Nb-CH 2 Ph bonds is slow. The niobium catalysts obtained by these methods demonstrate almost identical regio-and stereo-selectivity compared to the tantalum hydride systems.…”

“…This reaction profile can be simulated accurately using a simple kinetic model involving sequential first-order rate constants. 26 The success of this kinetic model for the reaction places constraints on the rate constants within a more complex model in which rates of phosphine coordination and dissociation are included. Although many regimes can exist for this model, the restraint that the rates of phosphine association/ dissociation are faster than the rate of saturation of the arene ring means that the first-order constants from the simple model are proportional to the product of the formation constants of the metal phosphine and the rate of arene ring hydrogenation.…”

“…By exploiting this reaction profile we have been able to synthesize the deuteriated compounds [P(C 6 H 5 )(C 6 H 5 D 6 ) 2 ] and [P(C 6 D 5 )(C 6 D 5 H 6 ) 2 ] and show that the reaction occurs in a predominantly all-cis fashion for attack at both phenyl rings. 26 The detection of non-all-cis stereoisomers is almost certainly a consequence of the intermediate regiochemistry of the reaction. 19 Due to the much documented lack of inversion of configuration at phosphorus under ambient conditions, substrates of stoichiometry RPPh 2 are prochiral.…”

“…In the case of [Pr i PPh(C 6 H 5 D 6 )], labelling studies again show a predominance of all-cis hydrogenation. 26 The use of phosphines with chiral alkyl groups leads to diastereomeric pairs. Analysis by 31 P NMR indicates a slight diastereomeric excess for the alkyls so far examined (Fig.…”