Electrophilic Si−H Activation by Acetonitrilo Benzo[h]quinoline Iridacycles: Influence of Electronic Effects in Catalysis

Abstract

The performance of six newly synthesized benzo[h]quinoline-derived acetonitrilo pentamethylcyclopentadienyl iridium(III) tetrakis(3,5-bis-trifluoromethylphenyl)borate salts bearing different substituents À X (À OMe, À H, À Cl, À Br, À NO 2 and À (NO 2 ) 2 ) on the heterochelating ligand were evaluated in the dehydro-O-silylation of benzyl alcohol and the monohydrosilylation of 4-methoxybenzonitrile by Et 3 SiH, two reactions involving the electrophilic activation of the SiÀ H bond. The benchmark shows a direct dependence of the catalytic efficiency with the electronic effect of À X, which is confirmed by theoretical assessment of the intrinsic silylicities Π of hydridoiridium(III)-silylium adducts and by the theoretical evaluation of the propensity of hydridospecies to transfer the hydrido ligand to the activated substrate. The revisited analysis of the IrÀ SiÀ H interactions shows that the most cohesive bond in hydridoiridium(III)-silylium adducts is the IrÀ H one, while the IrÀ Si is a weak donor-acceptor dative bond. The Si…H interaction in all the cases is noncovalent in nature and dominated by electrostatics confirming the heterolytic cleavage of the hydrosilane's SiÀ H bond in this key catalytically relevant species.

Joint Experimental/Theoretical Investigation of the Chemoselective Iridium(III) Metallacycle-Catalyzed Reduction of Substituted γ-Lactams by Et3SiH

“…Kinetic studies carried out with 1a as a substrate show that the first step of the reduction of the intracyclic amide function is the formation of the product of hydrosilylation of amide’s carbonyl, that is, a N -substituted silyl ether produced by a pseudo-zero-order rate law that matches a Michaelis–Menten catalysis under the catalyst’s saturation regime. In all the cases treated here, the productiveness of the catalyzed reduction by Et 3 SiH is remarkable and confirms the potential of the used benzo[ h ]quinoline-derived cationic iridacyclic catalyst. , On the issue of selectivity of the reductive reactions, the main conclusions drawn are the following: (1) the catalysis is mostly governed by the silylium’s reactivity, that is, chemoselectivity is driven by the substrate’s site affinity for silylium, (2) the transfer of hydride to the silylium-activated substrate is not necessarily charge controlled, (3) substitution of the amide’s intracyclic N center with an ethylenic moiety reduces the charge density at the amide carbonyl oxygen center which results in a significant decrease of the affinity for the silylium cation and overall chemoselectivity, (4) a protic function such as hydroxy in 1k is readily silylated without compromising the main amide’s reduction, and, finally, (5) even though their affinity for the silylium transient might be significant, catalytic runs showed that methoxy and dioxomethylene groups (in 2c , 2e , and 2d and in 7 ) are left untouched. Further investigations are underway to attempt the capture of the spectroscopic signature of the key [ 1a / 4a -SiEt 3 · IrH ] + adducts.…”

“…In other previously reported cases implying the same type of iridacyclic catalyst such catalyst–substrate adducts escaped DFT localization of the potential surface and were not isolated as local mimima: rather, the hydrido ligand transfer used to occur in an apparent “barrier-less” fashion. ,, …”

“…Any steric cluttering would result in an increase of the concentration of "idle" silylium species hanging at Lewis bases, leading either to its decomposition or to its migration to any alternative Lewis base, thus reducing not only the yield of the desired reaction but also its chemo-and regioselectivity. 15 The iridium catalyst [Ir-NCMe]BArF 24 (5 mol %) promotes the reduction of a variety of pyrrolidin-2-ones (cyclic amides) 1a−k (Scheme 2) by Et 3 SiH at 60 °C of DCE (Table 2). The reduction was shown to be compatible with aliphatic, aromatic, and heteroaromatic substituents to produce the corresponding pyrrolidines 2a−j (cyclic amine) in excellent yields.…”

“…It must be pointed out that the successful reduction of the amide function of the lactam into a cyclic amine such as 2a requires the crucial electrophilic centers at the heterocycle to be sterically accessible by the hydrido-iridium [ IrH ] species. Any steric cluttering would result in an increase of the concentration of “idle” silylium species hanging at Lewis bases, leading either to its decomposition or to its migration to any alternative Lewis base, thus reducing not only the yield of the desired reaction but also its chemo- and regioselectivity …”

“…The Si–H bond may either undergo oxidative-addition at the catalyst’s metal center or the electrophilic cleavage of the bond resulting in the transfer of the hydride to the metal and the capture of the formed silylium cation by any Lewis base, including one or several at the substrate. While the oxidative pathway requires the organic unsaturated substrate to directly interact with the metal center to undergo further reaction, the electrophilic activation pathway reduces the whole question of the selectivity of the reductive hydrosilylation reaction to the competition between various Lewis basic sites present at the substrate for capturing the generated highly electrophilic [SiR 3 ] + silylium cation. , 2-Phenylpyridine and benzo[ h ]quinoline-derived iridium(III)-based metallacycles are efficient hydrosilylation catalysts that operate by the electrophilic Si–H bond activation pathway; both the neutral and cationic variants of these organoiridium complexes have shown outstanding efficiency in the hydrosilylation of a great variety of organic functions such as alkenes, alkynes, amides, ketones, aldehydes, and nitriles. ,,,,,,,, In the present study a cationic acetonitrilo iridium complex [ Ir-NCMe ]BArF 24 (Scheme ) is used that requires no activating cocatalyst and possesses an outstanding reactivity with organic unsaturated functions such as in nitrile compounds R–CN. ,, …”