Selective Carbon–Carbon Bond Activation of Oxirane by a Bisphosphine Pt(0) Complex—A Theoretical Study

Abstract

Platinum(0) complexes with the chelate ligand bis(di-tert-butylphosphino)methane, t Bu 2 P-CH 2 -P t Bu 2 (dtbpm), generated in solution from appropriate precursors, are the only known transition metal species that selectively activate epoxides (oxiranes) by inserting the Pt fragment into their carbon−carbon bond. The mechanism of this unprecedented reaction is studied theoretically using the random phase approximation. We find that the reaction is kinetically controlled and is caused by the formation of a monocoordinate (dtbpm-κ 1 P)Pt(0) fragment rather than a (dtbpm-κ 2 P)Pt(0) chelate complex. Insertion into epoxide C−C bonds occurs without energy barrier. Conceivable competing reactions, oxirane C−O and C−H activation, both proceed via formation of a σ-complex, followed by small but significant barriers for insertion steps. A reversible formation of the σ-complexes would perfectly explain the observed reactivity. For an irreversible formation, we find that intramolecular rearrangement of these σ-complexes toward C− C activation products is faster than both C−O and C−H activation. In principle, the same reactivity should be expected for other monocoordinated platinum(0) phosphine complexes. However, only the specific properties of dtbpm cause the subsequent, rapid, and irreversible closing of the chelate ligand yielding stable, square-planar Pt(II) C−C activation products. ■ INTRODUCTIONPlatinum complexes with the small bite-angle, bulky, and very electron-rich ligand bis(di-tert-butylphosphino)methane t Bu 2 P-CH 2 -P t Bu 2 (dtbpm) show a unique reactivity toward epoxides: they selectively activate the C−C-bond. The stable neopentyl hydride complex (dtbpm-κ 2 P)Pt(Np)H (A2) reductively eliminates neopentane already under ambient conditions and reacts with various epoxides to the corresponding 3-platinaoxetanes F2 (see Scheme 1). 1−3 Kinetic measurements show 1 that the rate of these reactions corresponds precisely to that of a first-order elimination of neopentane from A2 both in absence and in the presence of epoxides and is practically independent of the solvent used (ΔG ‡ ≈ 100 kJ/mol). This suggests a two-step process where neopentane is first eliminated in the rate-determining step and the generated [(dtbpm)Pt(0)] fragment (C1 or C2) reacts with epoxides in a very fast consecutive step.Reactions with substituted epoxides of defined stereochemistry (cis-and rac-trans-2,3-dimethyloxirane) show that the reaction to the corresponding 3-platinaoxetanes F2 occurs under retention of stereochemistry. This excludes free rotation around the C−O bonds on the reaction itinerary, which one would expect for a nonconcerted insertion of the metal fragment into the C−C bond via radical or zwitterionic processes. From experimental evidence, one would therefore deduce a concerted mechanism. 1 The elimination of neopentane from (dtbpm-κ 2 P)Pt(Np)H (A2) in the absence of epoxides or other substrates occurs at approximately the same rate and results in dimerization of [(dtbpm)Pt(0)], giving a dtbpm-ligand bridged dinuclear...

1-Iodoglycal: A Versatile Intermediate for the Synthesis of d-Glyco Amides and Esters Employing Carbonylative Cross-Coupling Reaction

“…Thus, a balance between these two behaviors is necessary for the reaction to take place, explaining why there is no clear trend in the yields observed by changing the aniline substituents. This hypothesis is supported by theoretical calculations carried out at the M06-2x/Def2-SVP level of theory (See Figure S1 in Supporting Information) that show that intermediate III is 4.5 kcal/mol more stable than the isolated reactants when aniline is used, in contrast to benzylamine, which forms intermediate III 4.3 kcal/mol higher in energy than the reactants. This energy difference suggests that the formation of this intermediate III from benzylamine is more difficult than aniline, not allowing the product formation.…”

Organometallic chemistry

“…His research interests combined all three fields, with a focus on homogeneous catalysis with transition metal complexes. Bidentate donor ligands with a small bite angle such as bis(di- tert -butylphosphino)methane (dtbpm) were a unique feature of his work [61,67,70–72 80–81 84,86,93–94 97,100–101 103,107,124,128,148–149] that ultimately enabled C–Si activation of organosilanes and the C–C activation of oxiranes by coordinatively unsaturated platinum species [66,149]. In the mechanism of the Dötz reaction, he found that chromacyclobutene structures were unrealistic intermediates and instead proposed vinylcarbene complexes as much more stable isomers [64,68,88].…”