Au-Catalyzed Oxidative Arylation: Chelation-Induced Turnover of <i>ortho</i>-Substituted Arylsilanes

<i>ortho</i>-Substituted aryl silanes have previously been found to undergo much slower Au-catalyzed intermolecular arylation than their <i>m,p</i>-substituted isomers, with many examples failing to undergo turnover at all. A method to indirectly quantify the rates of C–Si auration of <i>o</i>-substituted aryl silanes, under conditions of turnover, has been developed. All examples are found to undergo very efficient C–Si auration, indicative that it is the subsequent C–H auration that is inhibited by the <i>ortho</i> substituent. A simple Ar–Au conformational model suggests that C–H auration can be accelerated by chelation. A series of <i>ortho</i>-functionalized aryl silanes are shown to undergo efficient arylation

Gold-Catalyzed Oxidative Coupling of Arylsilanes and Arenes: Origin of Selectivity and Improved Precatalyst

The mechanism of gold-catalyzed coupling of arenes with aryltrimethylsilanes has been investigated, employing an improved precatalyst (thtAuBr₃) to facilitate kinetic analysis. In combination with linear free-energy relationships, kinetic isotope effects, and stoichiometric experiments, the data support a mechanism involving an Au­(I)/Au­(III) redox cycle in which sequential electrophilic aromatic substitution of the arylsilane and the arene by Au­(III) precedes product-forming reductive elimination and subsequent cycle-closing reoxidation of the metal. Despite the fundamental mechanistic similarities between the two auration events, high selectivity is observed for heterocoupling (C–Si then C–H auration) over homocoupling of either the arylsilane or the arene (C–Si then C–Si, or C–H then C–H auration); this chemoselectivity originates from differences in the product-determining elementary steps of each electrophilic substitution. The turnover-limiting step of the reaction involves associative substitution en route to an arene π-complex. The ramifications of this insight for implementation of the methodology are discussed

Development of a Generic Mechanism for the Dehydrocoupling of Amine-Boranes: A Stoichiometric, Catalytic, and Kinetic Study of H₃B·NMe₂H Using the [Rh(PCy₃)₂]⁺ Fragment

The multistage Rh-catalyzed dehydrocoupling of the secondary amine-borane H₃B·NMe₂H, to give the cyclic amino-borane [H₂BNMe₂]₂, has been explored using catalysts based upon cationic [Rh­(PCy₃)₂]⁺ (Cy = cyclo-C₆H₁₁). These were systematically investigated (NMR/MS), under both stoichiometric and catalytic regimes, with the resulting mechanistic proposals for parallel catalysis and autocatalysis evaluated by kinetic simulation. These studies demonstrate a rich and complex mechanistic landscape that involves dehydrogenation of H₃B·NMe₂H to give the amino-borane H₂BNMe₂, dimerization of this to give the final product, formation of the linear diborazane H₃B·NMe₂BH₂·NMe₂H as an intermediate, and its consumption by both B–N bond cleavage and dehydrocyclization. Subtleties of the system include the following: the product [H₂BNMe₂]₂ is a modifier in catalysis and acts in an autocatalytic role; there is a parallel, neutral catalyst present in low but constant concentration, suggested to be Rh­(PCy₃)₂H₂Cl; the dimerization of H₂BNMe₂ can be accelerated by MeCN; and complementary nonclassical BH···HN interactions are likely to play a role in lowering barriers to many of the processes occurring at the metal center. These observations lead to a generic mechanistic scheme that can be readily tailored for application to many of the transition-metal and main-group systems that catalyze the dehydrocoupling of H₃B·NMe₂H

Gold-Catalyzed Oxidative Coupling of Arylsilanes and Arenes: Origin of Selectivity and Improved Precatalyst

The mechanism of gold-catalyzed coupling of arenes with aryltrimethylsilanes has been investigated, employing an improved precatalyst (thtAuBr₃) to facilitate kinetic analysis. In combination with linear free-energy relationships, kinetic isotope effects, and stoichiometric experiments, the data support a mechanism involving an Au­(I)/Au­(III) redox cycle in which sequential electrophilic aromatic substitution of the arylsilane and the arene by Au­(III) precedes product-forming reductive elimination and subsequent cycle-closing reoxidation of the metal. Despite the fundamental mechanistic similarities between the two auration events, high selectivity is observed for heterocoupling (C–Si then C–H auration) over homocoupling of either the arylsilane or the arene (C–Si then C–Si, or C–H then C–H auration); this chemoselectivity originates from differences in the product-determining elementary steps of each electrophilic substitution. The turnover-limiting step of the reaction involves associative substitution en route to an arene π-complex. The ramifications of this insight for implementation of the methodology are discussed

Intermolecular Alkyne Hydroacylation. Mechanistic Insight from the Isolation of the Vinyl Intermediate That Precedes Reductive Elimination

The isolation of the branched alkenyl intermediate that directly precedes reductive elimination of the final α,β-unsaturated ketone product is reported for the hydroacylation reaction between the alkyne HCCAr^F (Ar^F = 3,5-(CF₃)₂C₆H₃) and the β-S-substituted aldehyde 2-(methylthio)­benzaldehyde: [Rh­(<i>fac</i>-κ³-DPEphos)­(C­(CH₂)­Ar^F)­(C­(O)­C₆H₄SMe)₂]­[CB₁₁H₁₂]. The structure of this intermediate shows that, in this system at least, hydride migration rather than acyl migration occurs. Kinetic studies on the subsequent reductive elimination to form the crystallographically characterized ketone-bound product [Rh­(<i>cis</i>-κ²-DPEphos)­(η²:η²,κ¹-H₂CC­(Ar^F)­C­(O)­(C₆H₄SMe)]­[CB₁₁H₁₂] yield the following activation parameters for reductive elimination, which follows first-order kinetics (<i>k</i>_obs = (6.14 ± 0.04) × 10^–5 s^–1, 324 K): Δ<i>H</i>^⧧ = 95 ± 2 kJ mol^–1, Δ<i>S</i>^⧧ = −32 ± 7 J K^–1 mol^–1, Δ<i>G</i>^⧧(298 K) = 105 ± 4 kJ mol^–1. Mechanistic studies, including selective deuteration experiments, show that hydride insertion is not reversible and also reveal that an interesting isomerization process is occurring between the two branched alkenyl protons that is suggested to occur via a metallocyclopropene intermediate. During catalysis, the consumption of substrates and evolution of products follow pseudo zero-order kinetics. The observation of both linear and branched products under stoichiometric and catalytic regimes, in combination with kinetic modeling, allows for an overall mechanistic scheme to be presented. Partitioning of linear and branched pathways at the hydride insertion step occurs with an approximate 2:1 selectivity, while reductive elimination of the linear product is at least 3 orders of magnitude faster than that from the branched. An explanation for the large difference in rate of reductive elimination in this system, as recently outlined by Goldman, Krogh-Jespersen, and Brookhart, is that steric crowding in branched intermediates can slow C–C reductive elimination even though such species are higher in energy than their linear analogues, if the rotation of the vinyl group to the appropriate orientation is inhibited by steric crowding in the branched isomers

Intermolecular Alkyne Hydroacylation. Mechanistic Insight from the Isolation of the Vinyl Intermediate That Precedes Reductive Elimination

The isolation of the branched alkenyl intermediate that directly precedes reductive elimination of the final α,β-unsaturated ketone product is reported for the hydroacylation reaction between the alkyne HCCAr^F (Ar^F = 3,5-(CF₃)₂C₆H₃) and the β-S-substituted aldehyde 2-(methylthio)­benzaldehyde: [Rh­(<i>fac</i>-κ³-DPEphos)­(C­(CH₂)­Ar^F)­(C­(O)­C₆H₄SMe)₂]­[CB₁₁H₁₂]. The structure of this intermediate shows that, in this system at least, hydride migration rather than acyl migration occurs. Kinetic studies on the subsequent reductive elimination to form the crystallographically characterized ketone-bound product [Rh­(<i>cis</i>-κ²-DPEphos)­(η²:η²,κ¹-H₂CC­(Ar^F)­C­(O)­(C₆H₄SMe)]­[CB₁₁H₁₂] yield the following activation parameters for reductive elimination, which follows first-order kinetics (<i>k</i>_obs = (6.14 ± 0.04) × 10^–5 s^–1, 324 K): Δ<i>H</i>^⧧ = 95 ± 2 kJ mol^–1, Δ<i>S</i>^⧧ = −32 ± 7 J K^–1 mol^–1, Δ<i>G</i>^⧧(298 K) = 105 ± 4 kJ mol^–1. Mechanistic studies, including selective deuteration experiments, show that hydride insertion is not reversible and also reveal that an interesting isomerization process is occurring between the two branched alkenyl protons that is suggested to occur via a metallocyclopropene intermediate. During catalysis, the consumption of substrates and evolution of products follow pseudo zero-order kinetics. The observation of both linear and branched products under stoichiometric and catalytic regimes, in combination with kinetic modeling, allows for an overall mechanistic scheme to be presented. Partitioning of linear and branched pathways at the hydride insertion step occurs with an approximate 2:1 selectivity, while reductive elimination of the linear product is at least 3 orders of magnitude faster than that from the branched. An explanation for the large difference in rate of reductive elimination in this system, as recently outlined by Goldman, Krogh-Jespersen, and Brookhart, is that steric crowding in branched intermediates can slow C–C reductive elimination even though such species are higher in energy than their linear analogues, if the rotation of the vinyl group to the appropriate orientation is inhibited by steric crowding in the branched isomers

Intermolecular Alkyne Hydroacylation. Mechanistic Insight from the Isolation of the Vinyl Intermediate That Precedes Reductive Elimination

The isolation of the branched alkenyl intermediate that directly precedes reductive elimination of the final α,β-unsaturated ketone product is reported for the hydroacylation reaction between the alkyne HCCAr^F (Ar^F = 3,5-(CF₃)₂C₆H₃) and the β-S-substituted aldehyde 2-(methylthio)­benzaldehyde: [Rh­(<i>fac</i>-κ³-DPEphos)­(C­(CH₂)­Ar^F)­(C­(O)­C₆H₄SMe)₂]­[CB₁₁H₁₂]. The structure of this intermediate shows that, in this system at least, hydride migration rather than acyl migration occurs. Kinetic studies on the subsequent reductive elimination to form the crystallographically characterized ketone-bound product [Rh­(<i>cis</i>-κ²-DPEphos)­(η²:η²,κ¹-H₂CC­(Ar^F)­C­(O)­(C₆H₄SMe)]­[CB₁₁H₁₂] yield the following activation parameters for reductive elimination, which follows first-order kinetics (<i>k</i>_obs = (6.14 ± 0.04) × 10^–5 s^–1, 324 K): Δ<i>H</i>^⧧ = 95 ± 2 kJ mol^–1, Δ<i>S</i>^⧧ = −32 ± 7 J K^–1 mol^–1, Δ<i>G</i>^⧧(298 K) = 105 ± 4 kJ mol^–1. Mechanistic studies, including selective deuteration experiments, show that hydride insertion is not reversible and also reveal that an interesting isomerization process is occurring between the two branched alkenyl protons that is suggested to occur via a metallocyclopropene intermediate. During catalysis, the consumption of substrates and evolution of products follow pseudo zero-order kinetics. The observation of both linear and branched products under stoichiometric and catalytic regimes, in combination with kinetic modeling, allows for an overall mechanistic scheme to be presented. Partitioning of linear and branched pathways at the hydride insertion step occurs with an approximate 2:1 selectivity, while reductive elimination of the linear product is at least 3 orders of magnitude faster than that from the branched. An explanation for the large difference in rate of reductive elimination in this system, as recently outlined by Goldman, Krogh-Jespersen, and Brookhart, is that steric crowding in branched intermediates can slow C–C reductive elimination even though such species are higher in energy than their linear analogues, if the rotation of the vinyl group to the appropriate orientation is inhibited by steric crowding in the branched isomers

Dehydrocoupling of Dimethylamine Borane Catalyzed by Rh(PCy₃)₂H₂Cl

The Rh­(III) species Rh­(PCy₃)₂H₂Cl is an effective catalyst (2 mol %, 298 K) for the dehydrogenation of H₃B·NMe₂H (0.072 M in 1,2-F₂C₆H₄ solvent) to ultimately afford the dimeric aminoborane [H₂BNMe₂]₂. Mechanistic studies on the early stages in the consumption of H₃B·NMe₂H, using initial rate and H/D exchange experiments, indicate possible dehydrogenation mechanisms that invoke turnover-limiting N–H activation, which either precedes or follows B–H activation, to form H₂BNMe₂, which then dimerizes to give [H₂BNMe₂]₂. An additional detail is that the active catalyst Rh­(PCy₃)₂H₂Cl is in rapid equilibrium with an inactive dimeric species, [Rh­(PCy₃)­H₂Cl]₂. The reaction of Rh­(PCy₃)₂H₂Cl with [Rh­(PCy₃)­H₂(H₂)₂]­[BAr^F₄] forms the halide-bridged adduct [Rh­(PCy₃)₂H₂(μ-Cl)­H₂(PCy₃)₂Rh]­[BAr^F₄] (Ar^F = 3,5-(CF₃)₂C₆H₃), which has been crystallographically characterized. This dinuclear cation dissociates on addition of H₃B·NMe₂H to re-form Rh­(PCy₃)₂H₂Cl and generate [Rh­(PCy₃)₂H₂(η²-H₃B·NMe₂H)]­[BAr^F₄]. The fate of the catalyst at low catalyst loadings (0.5 mol %) is also addressed, with the formation of an inactive borohydride species, Rh­(PCy₃)₂H₂(η²-H₂BH₂), observed. On addition of H₃B·NMe₂H to Ir­(PCy₃)₂H₂Cl, the Ir congener Ir­(PCy₃)₂H₂(η²-H₂BH₂) is formed, with concomitant generation of the salt [H₂B­(NMe₂H)₂]­Cl

Quantum Yields for Photochemical Production of NO₂ from Organic Nitrates at Tropospherically Relevant Wavelengths

Absorption cross-sections and quantum yields for NO₂ production (Φ_NO₂) are reported for gaseous methyl, ethyl, <i>n</i>-propyl, and isopropyl nitrate at 294 K. Absorption cross-sections in the wavelength range of 240–320 nm agree well with prior determinations. NO₂ quantum yields at photoexcitation wavelengths of 290, 295, and 315 nm are unity within experimental uncertainties for all of the alkyl nitrates studied and are independent of bath gas (N₂) pressure for total sample pressures in the range of 250–700 Torr. When averaged over all wavelengths and sample pressures, values of Φ_NO₂ are 1.03 ± 0.05 (methyl nitrate), 0.98 ± 0.09 (ethyl nitrate), 1.01 ± 0.04 (<i>n</i>-propyl nitrate), and 1.00 ± 0.05 (isopropyl nitrate), with uncertainties corresponding to 1 standard deviation. Absorption cross-sections for ethyl nitrate, isopropyl nitrate, and two unsaturated dinitrate compounds, but-3-ene-1,2-diyl dinitrate and (<i>Z</i>)-but-2-ene-1,4-diyl dinitrate in acetonitrile solution, are compared to gas-phase values, and over the wavelength range of 260–315 nm, the gas-phase values are well-reproduced by dividing the liquid-phase cross-sections by 2.0, 1.6, 1.7, and 2.2, respectively. Reasonable estimates of the gas-phase absorption cross-sections for low-volatility organic nitrates can therefore be obtained by halving the values for acetonitrile solutions. The quantum yield for NO₂ formation from photoexcitation of but-3-ene-1,2-diyl dinitrate at 290 nm is significantly lower than those for the alkyl (mono) nitrates: a best estimate of Φ_NO₂ ≤ 0.25 is obtained from the experimental measurements

Mechanisms of the Thermal and Catalytic Redistributions, Oligomerizations, and Polymerizations of Linear Diborazanes

Linear diborazanes R₃N–BH₂–NR₂–BH₃ (R = alkyl or H) are often implicated as key intermediates in the dehydrocoupling/dehydrogenation of amine-boranes to form oligo- and polyaminoboranes. Here we report detailed studies of the reactivity of three related examples: Me₃N–BH₂–NMe₂–BH₃ (<b>1</b>), Me₃N–BH₂–NHMe–BH₃ (<b>2</b>), and MeNH₂–BH₂–NHMe–BH₃ (<b>3</b>). The mechanisms of the thermal and catalytic redistributions of <b>1</b> were investigated in depth using temporal-concentration studies, deuterium labeling, and DFT calculations. The results indicated that, although the products formed under both thermal and catalytic regimes are identical (Me₃N·BH₃ (<b>8</b>) and [Me₂N–BH₂]₂ (<b>9a</b>)), the mechanisms of their formation differ significantly. The thermal pathway was found to involve the dissociation of the terminal amine to form [H₂B­(μ-H)­(μ-NMe₂)­BH₂] (<b>5</b>) and NMe₃ as intermediates, with the former operating as a catalyst and accelerating the redistribution of <b>1</b>. Intermediate <b>5</b> was then transformed to amine-borane <b>8</b> and the cyclic diborazane <b>9a</b> by two different mechanisms. In contrast, under catalytic conditions (0.3–2 mol % IrH₂POCOP (POCOP = κ³-1,3-(OP<i>t</i>Bu₂)₂C₆H₃)), <b>8</b> was found to inhibit the redistribution of <b>1</b> by coordination to the Ir-center. Furthermore, the catalytic pathway involved direct formation of <b>8</b> and Me₂NBH₂ (<b>9b</b>), which spontaneously dimerizes to give <b>9a</b>, with the absence of <b>5</b> and BH₃ as intermediates. The mechanisms elucidated for <b>1</b> are also likely to be applicable to other diborazanes, for example, <b>2</b> and <b>3</b>, for which detailed mechanistic studies are impaired by complex post-redistribution chemistry. This includes both metal-free and metal-mediated oligomerization of MeNHBH₂ (<b>10</b>) to form oligoaminoborane [MeNH–BH₂]_<i>x</i> (<b>11</b>) or polyaminoborane [MeNH–BH₂]_<i>n</i> (<b>16</b>) following the initial redistribution reaction