Département de chimie moléculaire

The authors report a d. functional study (B97-D2 level) of the mechanism(s) operating in the alc. decarbonylation that occurs as an important side-reaction during dehydrogenation catalyzed by [RuH2(H2)(PPh3)3]. By using MeOH as the substrate, three distinct pathways were fully characterized involving either neutral tris- or bis-phosphines or anionic bis-phosphine complexes after deprotonation. $\alpha$-Agostic formaldehyde and formyl complexes are key intermediates, and the computed rate-limiting barriers are similar between the various decarbonylation and dehydrogenation paths. The key steps also were studied for reactions involving EtOH and iPrOH as substrates, rationalizing the known resistance of the latter towards decarbonylation. Kinetic isotope effects (KIEs) were predicted computationally for all pathways and studied exptl. for one specific decarbonylation path designed to start from [RuH(OCH3)(PPh3)3]. From the good agreement between computed and exptl. KIEs (obsd. kH/kD=4), the rate-limiting step for methanol decarbonylation was ascribed to the formation of the first agostic intermediate from a transient formaldehyde complex. [on SciFinder(R)]