Alkene Insertions into a Ru–PR₂ Bond

Abstract

An unusually broad series of discrete alkene insertion reactions has provided the opportunity to examine the mechanism(s) of this fundamental carbon−heteroatom bondforming process. Ethylene, electron-rich and electron-poor (activated) alkenes all react with the Ru−P double bond in Ru(η 5 -indenyl)(PCy 2 )(PPh 3 ) to form κ 2 -ruthenaphosphacyclobutanes. Thermal decomposition of these metallacycles in solution, via alkene deinsertion and β-hydride elimination, is particularly favored for electron-rich alkenes, and hydride-containing decomposition products are implicit intermediates in the observed isomerization of 1-hexene. Kinetic studies, including a Hammett analysis of the insertion reactions of para-substituted styrenes, suggest that two distinct inner-sphere pathways operate for the insertion of electron-rich versus activated alkenes. DFT analyses have identified one pathway involving simple cycloaddition via a four-centered transition state and another that proceeds through an η 2 -alkene intermediate. Such an intermediate was observed spectroscopically during formation of the ethylene metallacycle, but not for substituted alkenes. We propose that "pre-polarized", activated alkenes participate in direct cycloaddition, while ratedetermining η 2 -adduct formation is necessary for the activation of electron-rich alkenes toward migratory insertion into the Ru−P bond.

A Commercially Available Ruthenium Compound for Catalytic Hydrophosphination

“…Rosenberg's systems, commonly (indenyl)(PPh 3 )Ru=PRR’, possess significantly different ligands, with respect to electronic effects, than the Cp and CO ligands of 2 . Moreover, Rosenberg's compounds are coordinately unsaturated and exhibit substantial phosphorus‐to‐metal π bonding . The relationship between these compounds is significant nevertheless, and an insertion‐based mechanism is the working hypothesis here.…”

“…The literature provides some useful indications of a likely mechanism. Rosenberg and coworkers provide excellent evidence in support for a concerted, inner‐sphere (e. g., insertion) reaction of alkenes at the Ru−P bond of related indenyl ruthenium phosphido compounds . That precedent provides a strong basis for a mechanistic hypothesis that involves insertion into a Ru−P bond in this system.…”

Terminal Phosphido Complexes of the Ru(η5-Cp*) Fragment

“…RX or Michael-type alkenes) or inner-sphere migratory insertion processes . Our studies of phosphido complexes of a ruthenium indenyl half-sandwich fragment have demonstrated both inner- and outer-sphere reactivities, and we have reported the activity of this system for the catalytic hydrophosphination of activated alkenes, which apparently relies on outer-sphere P–C bond formation …”

Formation of an α-Diimine from Isocyanide Coupling Using Thorium(IV) and Uranium(IV) Phosphido–Methyl Complexes

“…Metal–phosphido bonds can be exploited for a variety of uses including hydrophosphination catalysts, small-molecule activation, − and advancing our knowledge of their interactions with electropositive metals . A key observation in hydrophosphination is that the reactivity of metal–phosphido complexes involves the basicity of the phosphido ligand. − For example, (η 5 -indenyl)­Ru­(PPh 3 )­(Cl)­(HPR 2 ), R = Cy, i Pr has the ability to catalyze the hydrophosphination of substituted alkenes, initially forming a reactive ruthenium phosphido complex upon deprotonation of the phosphine …”