Theoretical Study of POCOP-Pincer Iridium(III)/Iron(II) Hydride Catalyzed Hydrosilylation of Carbonyl Compounds: Hydride Not Involved in the Iridium(III) System but Involved in the Iron(II) System

The catalytic hydrosilylation of carbonyl compounds by two POCOP-pincer transition-metal hydrides, (POCOP)­Ir­(H)­(acetone)⁺ (<b>1A-acetone</b>) and (POCOP)­Fe­(H)­(PMe₃)₂ (<b>1B</b>) (POCOP = 2,6-bis­(dibutyl-/diisopropylphosphinito)­phenyl), was theoretically investigated to determine the underlying reaction mechanism. Several plausible mechanisms were analyzed using density functional theory calculations. The <b>1A-acetone</b>-catalyzed hydrosilylation of carbonyl compounds proceeds via the ionic hydrosilylation pathway, which is initiated by the nucleophilic attack of the η¹-silane metal adduct by carbonyl substrate. This attack results in the heterolytic cleavage of the Si–H bond and the generation of a siloxy carbenium ion paired with a neutral iridium dihydride, [(POCOP)­Ir­(H)₂]­[R₃SiOCHR′]⁺, followed by transfer of hydride from the metal center to the siloxy carbenium ion to yield the silyl ether product. The activation energy of the turnover-limiting step was calculated as ∼15.2 kcal/mol. This value is energetically more favorable than those of other pathways by as much as 22.6 kcal/mol. The most energetically favorable process for the hydrosilylation of carbonyl compound catalyzed by POCOP-pincer iron hydride <b>1B</b> was determined as the carbonyl precoordination pathway, which involves the initial coordination of the carbonyl substrate to the metal center and subsequent migratory insertion into the M–H bond to give the alkoxide intermediate. This intermediate then undergoes M–O/Si–H σ-bond metathesis to yield the silyl ether product. The ionic hydrosilylation pathway requires an activation energy that is ∼30.0 kcal/mol higher than that of the carbonyl precoordination pathway. Our calculation results indicate that the hydride moiety is not involved in the POCOP-pincer iridium­(III) hydride <b>1A-acetone</b>-catalyzed hydrosilylation of carbonyl compounds but is involved in the POCOP-pincer iron­(II) hydride <b>1B-</b>catalyzed process

Hydrosilylation of Carbonyls Catalyzed by the Rhenium(V) Oxo Complex [Re(O)(hoz)₂]⁺A Non-Hydride Pathway

Catalytic conversion of silane and carbonyls by the cationic rhenium oxo complex [Re­(O)­(hoz)₂]⁺ (<b>1</b>; hoz = 2-(2′-hydroxyphenyl)-2-oxazoline(1−)) was examined using density functional theory. It is shown that complex <b>1</b> catalyzed the carbonyl hydrosilylation via a non-hydride pathwaythe ionic hydrogenation mechanism. The complete catalytic cycle is proposed to involve three steps: the formation of <i>cis</i> η¹-silane Re­(V) adduct, the heterolytic cleavage of a Si–H bond through <i>anti</i> attack of carbonyls at the <i>cis</i> η¹-silane Re­(V) adduct, and transfers between the rhenium and activated silylcarbonium ion to produce the silyl ether product and regenerate catalyst <b>1.</b> The σ-bond metathesis like transition state suggested by Abu-Omar, although not located, can be inferred from the ionic hydrogenation transition states (<b>TS_3</b><i><b>syn</b></i> and <b>TS_5</b><i><b>syn</b></i>, in which the carbonyls <i>syn</i> attack the η¹-silane Re­(V) adduct) associated with the higher energy barrier

Hydrosilylation of Carbonyls Catalyzed by the Rhenium(V) Oxo Complex [Re(O)(hoz)₂]⁺A Non-Hydride Pathway

Catalytic conversion of silane and carbonyls by the cationic rhenium oxo complex [Re­(O)­(hoz)₂]⁺ (<b>1</b>; hoz = 2-(2′-hydroxyphenyl)-2-oxazoline(1−)) was examined using density functional theory. It is shown that complex <b>1</b> catalyzed the carbonyl hydrosilylation via a non-hydride pathwaythe ionic hydrogenation mechanism. The complete catalytic cycle is proposed to involve three steps: the formation of <i>cis</i> η¹-silane Re­(V) adduct, the heterolytic cleavage of a Si–H bond through <i>anti</i> attack of carbonyls at the <i>cis</i> η¹-silane Re­(V) adduct, and transfers between the rhenium and activated silylcarbonium ion to produce the silyl ether product and regenerate catalyst <b>1.</b> The σ-bond metathesis like transition state suggested by Abu-Omar, although not located, can be inferred from the ionic hydrogenation transition states (<b>TS_3</b><i><b>syn</b></i> and <b>TS_5</b><i><b>syn</b></i>, in which the carbonyls <i>syn</i> attack the η¹-silane Re­(V) adduct) associated with the higher energy barrier

New Insights into Hydrosilylation of Unsaturated Carbon–Heteroatom (CO, CN) Bonds by Rhenium(V)–Dioxo Complexes

The hydrosilylation of unsaturated carbon–heteroatom (CO, CN) bonds catalyzed by high-valent rhenium­(V)–dioxo complex ReO₂I­(PPh₃)₂ (<b>1</b>) were studied computationally to determine the underlying mechanism. Our calculations revealed that the ionic outer-sphere pathway in which the organic substrate attacks the Si center in an η¹-silane rhenium adduct to prompt the heterolytic cleavage of the Si–H bond is the most energetically favorable process for rhenium­(V)–dioxo complex <b>1</b> catalyzed hydrosilylation of imines. The activation energy of the turnover-limiting step was calculated to be 22.8 kcal/mol with phenylmethanimine. This value is energetically more favorable than the [2 + 2] addition pathway by as much as 10.0 kcal/mol. Moreover, the ionic outer-sphere pathway competes with the [2 + 2] addition mechanism for rhenium­(V)–dioxo complex <b>1</b> catalyzing the hydrosilylation of carbonyl compounds. Furthermore, the electron-donating group on the organic substrates would induce a better activity favoring the ionic outer-sphere mechanistic pathway. These findings highlight the unique features of high-valent transition-metal complexes as Lewis acids in activating the Si–H bond and catalyzing the reduction reactions

Halogen-Bond-Promoted Double Radical Isocyanide Insertion under Visible-Light Irradiation: Synthesis of 2‑Fluoroalkylated Quinoxalines

A halogen-bond-promoted double radical isocyanide insertion with perfluoroalkyl iodides is reported. With perfluoroalkyl iodides as halogen-bond donors and organic bases as halogen-bond acceptors, fluoroalkyl radicals can be generated by a visible-light-induced single electron transfer (SET) process. The fluoroalkyl radicals are trapped by <i>o</i>-diisocyanoarenes to give quinoxaline derivatives. This mechanistically novel strategy allows the construction of 2-fluoroalkylated 3-iodoquinoxalines in high yields under visible-light irradiation at room temperature

Halogen-Bond-Promoted Double Radical Isocyanide Insertion under Visible-Light Irradiation: Synthesis of 2‑Fluoroalkylated Quinoxalines

A halogen-bond-promoted double radical isocyanide insertion with perfluoroalkyl iodides is reported. With perfluoroalkyl iodides as halogen-bond donors and organic bases as halogen-bond acceptors, fluoroalkyl radicals can be generated by a visible-light-induced single electron transfer (SET) process. The fluoroalkyl radicals are trapped by <i>o</i>-diisocyanoarenes to give quinoxaline derivatives. This mechanistically novel strategy allows the construction of 2-fluoroalkylated 3-iodoquinoxalines in high yields under visible-light irradiation at room temperature