Isomerization of Internal Alkynes to Iridium(III) Allene Complexes via C–H Bond Activation: Expanded Substrate Scope, and Progress towards a Catalytic Methodology

Abstract

Abstract:The synthesis of a series of allene complexes (POCOP)Ir(η 2 -RC==CR') 1b-4b (POCOP = 2,6-bis(di-tert-butylphosphonito)benzene) via isomerization of internal alkynes is reported. We have demonstrated that the application of this methodology is viable for the isomerization of a wide variety of alkyne substrates. Deuterium labeling experiments support our proposed mechanism. The structures of the allene complexes 1b-4b were determined using spectroscopic data analysis. Additionally, the solid-state molecular structure of complex 2b was determined using single crystal X-ray diffraction studies and it confirmed the assignment of an iridium-bound allene isomerization product. The rates of isomerization were measured using NMR techniques over a range of temperatures to allow determination of thermodynamic parameters. Finally, we report a preliminary step towards developing a catalytic methodology; the allene may be liberated from the metal center by exposure of the complex to an atmosphere of carbon monoxide.

Direct Observation of a Photochemical Alkyne–Allene Reaction and of a Twisted and Rehybridized Intramolecular Charge-Transfer State in a Donor–Acceptor Dyad

“…This reaction is fully photoinduced and does not require elaborate and expensive catalysts, as routinely employed in organic chemistry. 18,19 Another important finding of this study concerns the structural transformation undergone by D-III during this photochemical process. As one of the sp alkyne carbon atoms transforms into an sp 2 carbon, the initially linear molecule adopts a kink at that atom.…”

“…We will show that, upon optical excitation, this molecule undergoes an efficient alkyne–allene reaction, a process that has only been demonstrated before with the simplest acetylene and allene hydrocarbons. , Although this reaction is found here to be fully reversible, possible extensions can be foreseen, even for preparative organic chemistry, including structural modifications, trapping and scavenging of the photoproduct, and one-pot domino reactions. This reaction is fully photoinduced and does not require elaborate and expensive catalysts, as routinely employed in organic chemistry. , …”

Excited-state symmetry breaking: from fundamental photophysics to asymmetrical photochemistry

“…Since the reaction is fully photoinduced, it does not require elaborate and expensive catalysts, as routinely employed in organic chemistry. 321,322 The interest in the chemistry of allenes has recently undergone a veritable boom, 323,324 because this class of compounds is known to serve as valuable building blocks for the synthesis of complex molecular targets, including natural products and drugs, and for material science. In general, these compounds are prepared using classical reactions, such as addition, elimination, substitution and rearrangement.…”

Rhodium-Catalyzed Intermolecular Carbonylative [2 + 2 + 1] Cycloaddition of Alkynes Using Alcohol as the Carbon Monoxide Source for the Formation of Cyclopentenones

“…With this in mind, we designed a new, one-pot synthesis including alkyne isomerization followed by intermolecular [2 + 2 + 1] carbonylative cycloaddition using an alcohol as a CO surrogate. We hypothesized that the alkyne could act as an allene replacement due to the feasibility of its isomerization with a transition-metal hydride, such as rhodium or iridium . Thus, 1-aryl-1-propynes were expected to serve both as an alkene and alkyne-like moiety in the construction of a cyclopentenone skeleton (Scheme c).…”

“…On the basis of these experimental results, together with observations from previous studies, ,− a plausible reaction mechanism is proposed for the carbonylative [2 + 2 + 1] cycloaddition of methylphenylethyne in the presence of a rhodium catalyst with an alcohol as a CO surrogate (Scheme ). The reaction begins with the formation of an Rh–H intermediate via a sequential reaction of the rhodium compound with an alkyne and alcohol in path I .…”