Regioselectivity of the C‑Metalation of 6‑Furylpurine: Importance of Directing Effects

We report the C-metalation of 6-furylpurine with Pt²⁺, Pd²⁺, and Hg²⁺. The ligand binds the metal ions in a bidentate fashion, involving the N7 purine atom and one of the furyl carbon atoms. The regioselectivity is determined by the metal ion. Pt²⁺ and Pd²⁺ coordinate the furyl moiety in its β position and Hg²⁺ in its electronically preferred α position

The “Catalytic Nitrosyl Effect”: NO Bending Boosting the Efficiency of Rhenium Based Alkene Hydrogenations

Diiodo Re­(I) complexes [ReI₂(NO)­(PR₃)₂(L)] (<b>3</b>, L = H₂O; <b>4</b> , L = H₂; R = <i>i</i>Pr <b>a</b>, Cy <b>b</b>) were prepared and found to exhibit in the presence of “hydrosilane/B­(C₆F₅)₃” co-catalytic systems excellent activities and longevities in the hydrogenation of terminal and internal alkenes. Comprehensive mechanistic studies showed an inverse kinetic isotope effect, fast H₂/D₂ scrambling and slow alkene isomerizations pointing to an Osborn type hydrogenation cycle with rate determining reductive elimination of the alkane. In the catalysts’ activation stage phosphonium borates [R₃PH]­[HB­(C₆F₅)₃] (<b>6</b>, R = <i>i</i>Pr <b>a</b>, Cy <b>b</b>) are formed. VT ²⁹Si- and ¹⁵N NMR experiments, and dispersion corrected DFT calculations verified the following facts: (1) Coordination of the silylium cation to the O_NO atom facilitates nitrosyl bending; (2) The bent nitrosyl promotes the heterolytic cleavage of the H–H bond and protonation of a phosphine ligand; (3) H₂ adds in a bifunctional manner across the Re–N bond. Nitrosyl bending and phosphine loss help to create two vacant sites, thus triggering the high hydrogenation activities of the formed “superelectrophilic” rhenium centers

Reaction of a Bridged Frustrated Lewis Pair with Nitric Oxide: A Kinetics Study

Described is a kinetics and computational study of the reaction of NO with the intramolecular bridged P/B frustrated Lewis pair (FLP) <i>endo</i>-2-(dimesitylphosphino)-<i>exo</i>-3-bis­(pentafluorophenyl)­boryl-norbornane to give a persistent FLP-NO aminoxyl radical. This reaction follows a second-order rate law, first-order in [FLP] and first-order in [NO], and is markedly faster in toluene than in dichloromethane. By contrast, the NO oxidation of the phosphine base 2-(dimesitylphosphino)­norbornene to the corresponding phosphine oxide follows a third-order rate law, first-order in [phosphine] and second-order in [NO]. Formation of the FLP-NO radical in toluene occurs with a Δ<i>H</i>^⧧ of 13 kcal mol^–1, a feature that conflicts with the computation-based conclusion that NO addition to a properly oriented B/P pair should be nearly barrierless. Since the calculations show the B/P pair in the most stable solution structure of this FLP to have an unfavorable orientation for concerted reaction, the observed barrier is rationalized in terms of the reversible formation of a [B]-NO complex intermediate followed by a slower isomerization–ring closure step to the cyclic aminoxyl radical. This combined kinetics/theoretical study for the first time provides insight into mechanistic details for the activation of a diatomic molecule by a prototypical FLP

Carbonylation Reactions of Intramolecular Vicinal Frustrated Phosphane/Borane Lewis Pairs

The intramolecular frustrated Lewis pair (FLP) Mes₂PCH₂CH₂B­(C₆F₅)₂ <b>4</b> adds cooperatively to carbon monoxide to form the five-membered heterocyclic carbonyl compound <b>5</b>. The intramolecular FLP <b>7</b> contains an <i>exo</i>-3-B­(C₆F₅)₂ Lewis acid and an <i>endo</i>-2-PMes₂ Lewis base functionality coordinated at the norbornane framework. This noninteracting FLP adds carbon monoxide in solution at −35 °C cooperatively to yield a five-membered heterocyclic FLP-carbonyl compound <b>8</b>. In contrast, FLP <b>7</b> is carbonylated in a CO-doped argon matrix at 25 K to selectively form a borane carbonyl <b>9</b> without involvement of the adjacent phosphanyl moiety. The free FLP <b>7</b> was generated in the gas phase from its FLPH₂ product <b>10</b>. A DFT study has shown that the phosphonium hydrido borate zwitterion <b>10</b> is formed exergonically in solution but tends to lose H₂ when brought into the gas phase

Frustrated Lewis Pair Modification by 1,1-Carboboration: Disclosure of a Phosphine Oxide Triggered Nitrogen Monoxide Addition to an Intramolecular P/B Frustrated Lewis Pair

The vicinal frustrated Lewis pair (FLP) mes₂P–CH₂CH₂–B­(C₆F₅)₂ (<b>3</b>) reacts with phenyl­(trimethylsilyl)­acetylene by 1,1-carboboration to give the extended C₃-bridged FLP <b>6</b> featuring a substituted vinylborane subunit. The FLP <b>6</b> actively cleaves dihydrogen. The FLP <b>3</b> also undergoes a 1,1-carboboration reaction with diphenylphosphino­(trimethylsilyl)­acetylene to give the P/B/P FLP <b>11</b> that features a central unsaturated four-membered heterocyclic P/B FLP and a pendant CH₂CH₂–Pmes₂ functional group. Compound <b>11</b> reacts with nitric oxide (NO) by oxidation of the pendant Pmes₂ unit to the P­(O)­mes₂ phosphine oxide and N,N-addition of the P/B FLP unit to NO to yield the persistent P/B/PO FLPNO aminoxyl radical <b>14</b>. This reaction is initiated by P­(O)­mes₂ formation and opening of the central Ph₂P···B­(C₆F₅)₂ linkage triggered by the pendant CH₂CH₂–P­(O)­mes₂ group

Intermolecular On-Surface σ‑Bond Metathesis

Silylation and desilylation are important functional group manipulations in solution-phase organic chemistry that are heavily used to protect/deprotect different functionalities. Herein, we disclose the first examples of the σ-bond metathesis of silylated alkynes with aromatic carboxylic acids on the Ag(111) and Au(111) surfaces to give the corresponding terminal alkynes and silyl esters, which is supported by density functional theory calculations and further confirmed by X-ray photoelectron spectroscopy analysis. Such a protecting group strategy applied to on-surface chemistry allows self-assembly structures to be generated from molecules that are inherently unstable in solution and in the solid state. This is shown by the successful formation of self-assembled hexaethynyl­benzene at Ag(111). Furthermore, it is also shown that on the Au(111) surface this σ-bond metathesis can be combined with Glaser coupling to fabricate covalent polymers via a cascade process

<i>N</i>,<i>N</i>-Addition of Frustrated Lewis Pairs to Nitric Oxide: An Easy Entry to a Unique Family of Aminoxyl Radicals

The intramolecular cyclohexylene-bridged P/B frustrated Lewis pair [Mes₂PC₆H₁₀B­(C₆F₅)₂] <b>1b</b> reacts rapidly with NO to give the persistent FLP-NO aminoxyl radical <b>2b</b> formed by P/B addition to the nitrogen atom of NO. This species was fully characterized by X-ray diffraction, EPR and UV/vis spectroscopies, C,H,N elemental analysis, and DFT calculations. The reactive oxygen-centered radical <b>2b</b> undergoes a H-atom abstraction (HAA) reaction with 1,4-cyclohexadiene to give the diamagnetic FLP-NOH product <b>3b</b>. FLP-NO <b>2b</b> reacts with toluene at 70 °C in an HAA/radical capture sequence to give a 1:1 mixture of FLP-NOH <b>3b</b> and FLP-NOCH₂Ph <b>4b</b>, both characterized by X-ray diffraction. Structurally related FLPs [Mes₂PCHR¹CHR²B­(C₆F₅)₂] <b>1c</b>, <b>1d</b>, and <b>1e</b> react analogously with NO to give the respective persistent FLP-NO radicals <b>2c</b>, <b>2d</b>, and <b>2e</b>, respectively, which show similar HAA and O-functionalization reactions. The FLP-NOCHMePh <b>6b</b> derived from 1-bromoethylbenzene undergoes NOC bond cleavage at 120 °C with an activation energy of <i>E</i>_a = 35(2) kcal/mol. Species <b>6b</b> induces the controlled nitroxide-mediated radical polymerization (NMP) of styrene at 130 °C to give polystyrene with a polydispersity index of 1.3. The FLP-NO systems represent a new family of aminoxyl radicals that are easily available by <i>N</i>,<i>N</i>-cycloaddition of C₂-bridged intramolecular P/B frustrated Lewis pairs to nitric oxide

<i>N</i>,<i>N</i>-Addition of Frustrated Lewis Pairs to Nitric Oxide: An Easy Entry to a Unique Family of Aminoxyl Radicals

The intramolecular cyclohexylene-bridged P/B frustrated Lewis pair [Mes₂PC₆H₁₀B­(C₆F₅)₂] <b>1b</b> reacts rapidly with NO to give the persistent FLP-NO aminoxyl radical <b>2b</b> formed by P/B addition to the nitrogen atom of NO. This species was fully characterized by X-ray diffraction, EPR and UV/vis spectroscopies, C,H,N elemental analysis, and DFT calculations. The reactive oxygen-centered radical <b>2b</b> undergoes a H-atom abstraction (HAA) reaction with 1,4-cyclohexadiene to give the diamagnetic FLP-NOH product <b>3b</b>. FLP-NO <b>2b</b> reacts with toluene at 70 °C in an HAA/radical capture sequence to give a 1:1 mixture of FLP-NOH <b>3b</b> and FLP-NOCH₂Ph <b>4b</b>, both characterized by X-ray diffraction. Structurally related FLPs [Mes₂PCHR¹CHR²B­(C₆F₅)₂] <b>1c</b>, <b>1d</b>, and <b>1e</b> react analogously with NO to give the respective persistent FLP-NO radicals <b>2c</b>, <b>2d</b>, and <b>2e</b>, respectively, which show similar HAA and O-functionalization reactions. The FLP-NOCHMePh <b>6b</b> derived from 1-bromoethylbenzene undergoes NOC bond cleavage at 120 °C with an activation energy of <i>E</i>_a = 35(2) kcal/mol. Species <b>6b</b> induces the controlled nitroxide-mediated radical polymerization (NMP) of styrene at 130 °C to give polystyrene with a polydispersity index of 1.3. The FLP-NO systems represent a new family of aminoxyl radicals that are easily available by <i>N</i>,<i>N</i>-cycloaddition of C₂-bridged intramolecular P/B frustrated Lewis pairs to nitric oxide

<i>N</i>,<i>N</i>-Addition of Frustrated Lewis Pairs to Nitric Oxide: An Easy Entry to a Unique Family of Aminoxyl Radicals

The intramolecular cyclohexylene-bridged P/B frustrated Lewis pair [Mes₂PC₆H₁₀B­(C₆F₅)₂] <b>1b</b> reacts rapidly with NO to give the persistent FLP-NO aminoxyl radical <b>2b</b> formed by P/B addition to the nitrogen atom of NO. This species was fully characterized by X-ray diffraction, EPR and UV/vis spectroscopies, C,H,N elemental analysis, and DFT calculations. The reactive oxygen-centered radical <b>2b</b> undergoes a H-atom abstraction (HAA) reaction with 1,4-cyclohexadiene to give the diamagnetic FLP-NOH product <b>3b</b>. FLP-NO <b>2b</b> reacts with toluene at 70 °C in an HAA/radical capture sequence to give a 1:1 mixture of FLP-NOH <b>3b</b> and FLP-NOCH₂Ph <b>4b</b>, both characterized by X-ray diffraction. Structurally related FLPs [Mes₂PCHR¹CHR²B­(C₆F₅)₂] <b>1c</b>, <b>1d</b>, and <b>1e</b> react analogously with NO to give the respective persistent FLP-NO radicals <b>2c</b>, <b>2d</b>, and <b>2e</b>, respectively, which show similar HAA and O-functionalization reactions. The FLP-NOCHMePh <b>6b</b> derived from 1-bromoethylbenzene undergoes NOC bond cleavage at 120 °C with an activation energy of <i>E</i>_a = 35(2) kcal/mol. Species <b>6b</b> induces the controlled nitroxide-mediated radical polymerization (NMP) of styrene at 130 °C to give polystyrene with a polydispersity index of 1.3. The FLP-NO systems represent a new family of aminoxyl radicals that are easily available by <i>N</i>,<i>N</i>-cycloaddition of C₂-bridged intramolecular P/B frustrated Lewis pairs to nitric oxide

<i>N</i>,<i>N</i>-Addition of Frustrated Lewis Pairs to Nitric Oxide: An Easy Entry to a Unique Family of Aminoxyl Radicals

The intramolecular cyclohexylene-bridged P/B frustrated Lewis pair [Mes₂PC₆H₁₀B­(C₆F₅)₂] <b>1b</b> reacts rapidly with NO to give the persistent FLP-NO aminoxyl radical <b>2b</b> formed by P/B addition to the nitrogen atom of NO. This species was fully characterized by X-ray diffraction, EPR and UV/vis spectroscopies, C,H,N elemental analysis, and DFT calculations. The reactive oxygen-centered radical <b>2b</b> undergoes a H-atom abstraction (HAA) reaction with 1,4-cyclohexadiene to give the diamagnetic FLP-NOH product <b>3b</b>. FLP-NO <b>2b</b> reacts with toluene at 70 °C in an HAA/radical capture sequence to give a 1:1 mixture of FLP-NOH <b>3b</b> and FLP-NOCH₂Ph <b>4b</b>, both characterized by X-ray diffraction. Structurally related FLPs [Mes₂PCHR¹CHR²B­(C₆F₅)₂] <b>1c</b>, <b>1d</b>, and <b>1e</b> react analogously with NO to give the respective persistent FLP-NO radicals <b>2c</b>, <b>2d</b>, and <b>2e</b>, respectively, which show similar HAA and O-functionalization reactions. The FLP-NOCHMePh <b>6b</b> derived from 1-bromoethylbenzene undergoes NOC bond cleavage at 120 °C with an activation energy of <i>E</i>_a = 35(2) kcal/mol. Species <b>6b</b> induces the controlled nitroxide-mediated radical polymerization (NMP) of styrene at 130 °C to give polystyrene with a polydispersity index of 1.3. The FLP-NO systems represent a new family of aminoxyl radicals that are easily available by <i>N</i>,<i>N</i>-cycloaddition of C₂-bridged intramolecular P/B frustrated Lewis pairs to nitric oxide