Mechanistic Versatility at Ir(PSiP) Pincer Catalysts: Triflate Proton Shuttling from 2-Butyne to Diene and [3]Dendralene Motifs

Abstract

The five-coordinate hydrido complex [IrH(OTf)(PSiP)] ( 1 ) catalytically transforms 2-butyne into a mixture of its isomer 1,3-butadiene, and [3]dendralene and linear hexatriene dimerization products: ( E )-4-methyl-3-methylene-1,4-hexadiene and (3 Z )-3,4-dimethyl-1,3,5-hexatriene, respectively. Under the conditions of the catalytic reaction, benzene, and 363 K, the hexatriene further undergoes thermal electrocyclization into 2,3-dimethyl-1,3-cyclohexadiene. The reactions between 1 and the alkyne substrate allow isolation or nuclear magnetic resonance (NMR) observation of catalyst resting states and possible reaction intermediates, including complexes with the former PSiP pincer ligands disassembled into PSi and PC chelates, and species coordinating allyl or carbene fragments en route to products. The density functional theory (DFT) calculations guided by these experimental observations disclose competing mechanisms for C–H bond elaboration that move H atoms either classically, as hydrides, or as protons transported by the triflate. This latter role of triflate, previously recognized only for more basic anions such as carboxylates, is discussed to result from combining the unfavorable charge separation in the nonpolar solvent and the low electronic demand from the metal to the anion at coordination positions trans to silicon. Triflate deprotonation of methyl groups is key to release highly coordinating diene products from stable allyl intermediates, thus enabling catalytic cycling.

Insights into the Palladium(II)-Catalyzed Wacker-Type Oxidation of Styrene with Hydrogen Peroxide and tert-Butyl Hydroperoxide

“…Overall, these results highlight the noninnocent role of the triflate counteranion by enabling a competitive pathway that explains both the experimental observation of a Pd-enolate species and the low D incorporation in the acetophenone product. We also note that the role of triflate in facilitating proton transfer steps has also been reported in other reactions. , …”

“…We also note that the role of triflate in facilitating proton transfer steps has also been reported in other reactions. 28,29 To understand the remarkable stability of Pd-enolate CR, we next performed a natural bond orbital (NBO) analysis, the results of which are summarized in the inset of S2. These donor−acceptor interactions were further quantified via second-order perturbation theory (SOPT) to be −61.9 and −23.3 kcal/mol, respectively.…”

Tetrelanes versus Tetrylenes as Precursors to Transition Metal Complexes Featuring Tridentate PEP Tetryl Ligands (E=Si, Ge, Sn)

“…This simple synthetic procedure and the easy availability of the starting materials have allowed a wide development of the transition metal chemistry of silanes of type C (Table 2). [9–53] The formation of the metal complex involves the coordination of both P atoms and the oxidative addition of the Si−H bond to a low valent transition metal complex, the most common metal precursors being [Fe(PMe 3 ) 4 ], [RuCl 2 (PPh 3 ) 3 ], [Co 2 (CO) 8 ], [M 2 Cl 2 (cod) 2 ] (M=Rh, Ir), [Ni(cod) 2 ], [NiCl 2 (dme)] (dme=1,2‐dimethoxyethane), [PdCl 2 (MeCN) 2 ] and [PtCl 2 (cod)]. A Si−F bond cleavage was observed in the reaction of Si{( o ‐C 6 H 4 )PPh 2 } 2 F 2 with [IrH(CO)(PPh 3 ) 3 ], which led to [Ir{Si{( o ‐C 6 H 4 )PPh 2 } 2 F}(CO)(PPh 3 )] [39b] .…”

“…[40a,42,43-45,47] Curiously, the dihydridosilane Si{(o-C 6 H 4 )PPh 2 } 2 H 2 reacted with [Rh 2 Cl 2 (cod) 2 ] to give the chloridosilyl complex [Rh{SiCl{(o-C 6 H 4 )PPh 2 } 2 }(cod)]. [44b] Many complexes derived from silanes of type C are efficient catalyst precursors for alkene hydrosilylation, [15,38] carbonyl group hydrosilylation, [33,40b] alkene hydrogenation, [20,24,44a] alkyne hydrogenation, [20] dinitrogen silylation, [9,25,38,52] silane deuteriation, [50] organic azide carbonylation, [13] allene carboxylation, [31,36d,f] CO 2 hydroboration, [31,32] alkene isomerization, [32] alkene dehydrogenative borylation, [36a,b] sila-Negishi coupling, [39a] ketone transfer hydrogenation, [21,34,40a] 2butyne isomerization and dimerization [53] and arene borylation. [27] PSiP silyl complexes of Rh, [54] Ir, [54] Ni, [55] Pd [55] and Pt [55] have been recently prepared from bis(indolyl)silane D (Scheme 2), reacting it with [M 2 Cl 2 (cod) 2 ] (M=Rh, Ir), [NiCl 2 (dme)], [Pd 2 Cl 2 (allyl) 2 ] and [PtCl 2 (SEt 2 ) 2 ], respectively (Scheme 2).…”

“…Me iPr Fe [9,40b] , Ru [10][11][12] ,Co [13][14][15] , Ir [16,27,53] , Ni [18] , Pd [19] Me Cy Fe [20,25,52] , Ru [22,23] , Co [24,25,52] , Rh [17,26] , Ir [17,26,27] , Ni [28][29][30]32] , Pd [19,28,29,31] , Pt [19,37] Me tBu Ir [21,27] Me Ph Fe [25,33,40,52] , Ru [34] , Co [15,33] , Rh [34,35] , Ir [35] , Ni [32,33,36c] , Pd [28,34,36] , Pt [34,36c,37 Ph Rh…”