Phosphoramidate-Assisted Alkyne Activation: Probing the Mechanism of Proton Shuttling in a N,O-Chelated Cp*Ir(III) Complex

Abstract

Ligand lability offers a unique opportunity for access to metal-ligand cooperativity (MLC), helping to direct new organometallic and catalytic reactions. In recent years, ligand-assisted C-H bond activation, and more generally, proton migration, have been of particular interest. This contribution describes a detailed computational study into the mechanism, regio-, and stereoselectivity observed in a recently reported transformation where MLC in a 16-electron iridium(III) phosphoramidate complex plays a critical role in directing the activation of terminal alkynes toward the generation of novel fivemembered (E)-vinyloxyirida(III)cycles. Five possible pathways for the formation of such products were investigated. Based on our findings, it is proposed that the reaction proceeds via a ligand-assisted proton shuttle (LAPS) mechanism, where the phosphoramidate phosphoryl (P=O) group assists in both alkyne C-H bond activation and C-H bond formation to form a vinylidene intermediate. Next, CO bond formation occurs via nucleophilic attack at the -carbon of the vinylidene giving the observed product. Although C-N (and not CO) bond formation is thermodynamically favored in this model system, this trend is not observed experimentally and the computational study suggests that the observed regioisomer is simply the kinetic reaction product. In terms of stereoselectivity, formation of the (E)irida(III)cycle is explained by its thermodynamic stability, when compared to the (Z)-isomer, and the relatively low barrier to interconversion between them.

Five‐Membered Ruthenacycles: Ligand‐Assisted Alkyne Insertion into 1,3‐N,S‐Chelated Ruthenium Borate Species

“…The reactions might proceed through the ligand‐assisted proton shuttle (LAPS) mechanism in which the nucleophilic sites, N or S, of the benzothiazolyl moiety in 2 a’ would act as the internal base for proton shuttling thereby assisting both in the C−H bond activation as well as in the formation steps. The generation of five‐membered metallacycles ( 4 , 5 , or 5’ ) is very similar to the formation of five membered ( E )‐vinyloxy iridium(III) complex, which also occurred through the LAPS mechanism, as reported by Love and Schafer . This led us to believe that the insertion of terminal alkynes into Ru−N and Ru−S bonds of 2 a’ follows the LAPS mechanism, which facilitates the formation of C−N and C−S bonds (Scheme ) …”

“…The generation of five‐membered metallacycles ( 4 , 5 , or 5’ ) is very similar to the formation of five membered ( E )‐vinyloxy iridium(III) complex, which also occurred through the LAPS mechanism, as reported by Love and Schafer . This led us to believe that the insertion of terminal alkynes into Ru−N and Ru−S bonds of 2 a’ follows the LAPS mechanism, which facilitates the formation of C−N and C−S bonds (Scheme ) …”

Recent Advances in the Synthesis and Reactivity of Transition Metal σ-Borane/Borate Complexes

“…The formation of five-membered metallacycles is very similar to the formation of an (E)vinyloxyiridium(III) complex that was found to occur via a ligand-assisted proton-shuttle (LAPS) mechanism. 78…”

“…Subsequently, the nitrogen or sulfur atom of the benzothiazolyl ring coordinates to the alkyne via anti-Markovnikov addition. The formation of five-membered metallacycles is very similar to the formation of an ( E )-vinyloxyiridium­(III) complex that was found to occur via a ligand-assisted proton-shuttle (LAPS) mechanism …”

Metal–Ligand–Anion Cooperation in C–H Bond Formation at Platinum(II)

“…Through studies of model transition metal systems, several distinct mechanisms have been established for this bond cleavage step that generates a metal–alkyl. Mechanisms that require a change in oxidation state, e.g., oxidative addition (OA, Chart a), are often contrasted with those that maintain the metal oxidation state, e.g., concerted metalation deprotonation (CMD, Chart b). ,− Metal–ligand cooperative (MLC, Chart c) mechanisms have been recently invoked. − The heterolytic H 2 activation step in Noyori’s hydrogenation reactions is a classic example of MLC bond activation . In C–H activation via MLC, a basic site on the ligand formally accepts the proton, with the anionic carbon fragment binding to the Lewis acidic metal.…”