Energetics and mechanism of organolanthanide-mediated phosphinoalkene hydrophosphination/cyclization. A density functional theory analysis

This contribution focuses on organolanthanide-mediated hydrophosphination processes and analyzes the hydrophosphination/cyclization of a prototypical phosphinoalkene, H₂P(CH₂)₃-CH=CH₂, catalyzed by Cp₂LaCH(TMS)₂, using density functional theory methods, and compares/contrasts the results to analogous hydroamination/cyclization processes. The reaction is found to proceed in two discrete steps: namely, cyclization via C=C insertion into the Cp ₂La-P(phosphido) linkage to form La-C and C-P bonds and subsequent La-C protonolysis. Calculations have been carried out for: (i) insertion of the olefinic moiety into the La-P bond and (ii) La-C protonolysis by a second substrate molecule. The cyclized phosphinoalkane is released in the latter process and the active catalyst regenerated. DFT energetic profiles have been generated for the entire catalytic cycle. DFT-derived geometries and stabilities of reactants, intermediates, and products have also been analyzed. The picture that emerges is one of approximately thermoneutral insertion of the alkene fragment into the Cp₂La-P(phosphido) bond via a highly organized, seven-membered chairlike cyclic transition state. The resulting cyclopentylmethyl complex then undergoes turnover-limiting but exothermic protonolysis to yield a phosphine-phosphide complex, the likely resting state of the catalyst. Interestingly, this energetic ordering of barriers is exactly reversed from that in the formally analogous hydroamination/cyclization process, where C=C insertion is turnover-limiting and protonolysis rapid. The calculations provide evidence that, in the hydrophosphination/cyclization process, there is competition at the lanthanide center between cyclized product and incoming unconverted substrate and, therefore, some inhibition of the protonolysis step, in accord with experiment. Parallel calculations demonstrate that such effects are not generally important in the analogous hydroamination/cyclization process. Derived thermodynamic and kinetic parameters are in excellent agreement with experiment.