Oxidative-Addition of Organic Monochloro Derivatives to Dinuclear Rhodium Complexes:  Mechanistic Considerations

Abstract

The addition of RCH2Cl to the complex [{Rh(μ-Pz)(CNBut)2}2] (Pz = pyrazolate, 1) occurs under mild conditions to yield the bis(alkyl)rhodium(III) complexes [{Rh(μ-Pz)(η1-CH2R)(CNBut)2}2(μ-Cl)]Cl (R = Ph, CHCH2, CO2Me, COMe). These reactions proceed through two separate steps, as evidenced by the observation of the intermediate mixed-valence complex [(CNBut)2RhI(μ-Pz)2RhIII(η1-CH2Ph)(Cl)(CNBut)2] resulting from the trans-addition of PhCH2Cl at a single metal center in 1. Similar Rh(I)−Rh(III) complexes [(cod)Rh(μ-Pz)2Rh(η1-CH2R)(Cl)(CNBut)2] (R = Ph (7), CHCH2, CO2Me) result from the addition of RCH2Cl to the mixed-ligand complex [(cod)Rh(μ-Pz)2Rh(CNBut)2] (6). These exist in benzene and toluene as two interconverting conformers, differentiated by the location of the RCH2 group either between the two metals in the pocket of the complexes (endo conformer) or at the trans position (exo conformer). An intramolecular boat-to-boat inversion accounts for this exchange. The thermodynamic parameters for complex 7 were ΔH ⧧ = 16.2 kcal·mol-1, ΔS ⧧ = 4.9 eu, and ΔG 293 ⧧ = 14.9 kcal·mol-1. Primary alkyl bromides RCH2Br react with 1 and with 6 to give the Rh(III) complexes [{Rh(μ-Pz)(η1-CH2R)(CNBut)2}2(μ-Br)]Br (R = Ph, CO2Me) and the mixed-valence complex [(cod)Rh(μ-Pz)2Rh(η1-CH2Ph)(Br)(CNBut)2]. However, the secondary alkyl bromide PhCH(Me)Br reacts with 1 and with 6 to give the dirhodium(II) complexes [{Rh(μ-Pz)(Br)(CNBut)2}2] and [(cod)(Br)Rh(μ-Pz)2Rh(Br)(CNBut)2], respectively, along with 2,3-diphenylbutane and traces of styrene and ethylbenzene, suggesting free-radical pathways for these reactions. The thermal decomposition of the bis(alkyl)chloro complexes gives free isobutene and the cyanide complexes [(CNBut)2(η1-CH2R)Rh(μ-Pz)2(μ-Cl)Rh(η1-CH2R)(CN)(CNBut)] (R = Ph, COMe, CO2Me). Decomposition of the bis(alkyl)bromo derivatives occurs under sunlight irradiation and anaerobic conditions to give bibenzyl, cis- and trans-stilbene, and toluene.

Oxidative Additions to Ti(IV) in [(dadi)4–]TiIV(THF) Involve Carbon–Carbon Bond Formation and Redox-Noninnocent Behavior

“…Several factors implicated a radical process. − Attempts to measure rates of oxidative addition were hampered by the sporadic nature of the reaction. It was generally the case that the benzyl halides reacted faster than the methyl halides and that the bromide substrates were faster than the chlorides, but statistically significant rate data could not be obtained.…”

“…Given the results in Scheme , concerted pathways − for the addition of RX to (dadi)­Ti­(THF) ( 1 -THF) were considered. Since experiments with these highly sensitive and reactive titanium species have intrinsic limitations, and no intermediates were detected, calculations were utilized to assess certain mechanisms with BnBr as the prototypical substrate.…”

“…While similar to the previous mechanism with regard to the alkylation, it is not a chain process. Initial C–C bond formation is propsed to generate an ion-pair analogous to related RX additions of square planar d 8 complexes. − Calculations on the 1,2-H shift suggest it again has too high a barrier (∼30 kcal/mol), but in this instance, activity of the counteranion may play a role, a factor that was difficult to test with high veracity. It is also feasible that alkylation and 1,2-H shift occur concomitantly.…”

Erste Röntgenstrukturanalysen von Rhodium(III)‐η1‐Allyl‐Komplexen und ein plausibler Mechanismus für allylische Isomerisierungsreaktionen

“…[7a] Allerdings wurde bislang kein struktureller Beweis für einen Rh(h 1 -allyl)-Komplex veröf-fentlicht, obwohl ein derartiger Koordinationsmodus für Rhodium-Komplexe bereits seit längerem NMR-spektroskopisch identifiziert ist. [8] Kürzlich haben wir über den stark nucleophilen Rh ICarbonylkomplex 1 berichtet, der einen auf Carbazol basierenden Pinzetten-Liganden mit zwei N-heterocyclischen Carbeneinheiten (bimca) trägt. [9] Wegen der ausgeprägten Elektronendonoreigenschaften des bimca-Liganden verläuft die oxidative Addition (S N 2-Mechanismus) von Methyliodid extrem schnell.…”

Steric Effects in the Oxidative Addition of MeI to a Sulfido-Bridged ZrRh2Early–Late Heterobimetallic Compound

“…This mechanism can be also operative in dinuclear complexes, as has been recently shown by DFT calculations on a molecular model of Freixa’s [Rh 2 (μ-Cl) 2 (SPAN-PPh 2 )(CO) 2 ] methanol carbonylation catalyst, where a linear arrangement of the I–CH 3 –Rh moiety was identified . An alternative mechanism involving free alkyl radical intermediates seems to be less likely, as the reaction also takes place in the dark and no side reactions were observed …”