14 research outputs found
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 (thtAuBr<sub>3</sub>) to facilitate kinetic analysis. In combination with linear
free-energy relationships, kinetic isotope effects, and stoichiometric
experiments, the data support a mechanism involving an AuÂ(I)/AuÂ(III)
redox cycle in which sequential electrophilic aromatic substitution
of the arylsilane and the arene by AuÂ(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 H<sub>3</sub>B·NMe<sub>2</sub>H Using the [Rh(PCy<sub>3</sub>)<sub>2</sub>]<sup>+</sup> Fragment
The multistage Rh-catalyzed dehydrocoupling of the secondary
amine-borane
H<sub>3</sub>B·NMe<sub>2</sub>H, to give the cyclic amino-borane
[H<sub>2</sub>BNMe<sub>2</sub>]<sub>2</sub>, has been explored using
catalysts based upon cationic [RhÂ(PCy<sub>3</sub>)<sub>2</sub>]<sup>+</sup> (Cy = cyclo-C<sub>6</sub>H<sub>11</sub>). 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 H<sub>3</sub>B·NMe<sub>2</sub>H to give the amino-borane H<sub>2</sub>BNMe<sub>2</sub>, dimerization of this to give the
final product, formation of the linear diborazane H<sub>3</sub>B·NMe<sub>2</sub>BH<sub>2</sub>·NMe<sub>2</sub>H as an intermediate, and
its consumption by both B–N bond cleavage and dehydrocyclization.
Subtleties of the system include the following: the product [H<sub>2</sub>BNMe<sub>2</sub>]<sub>2</sub> 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 RhÂ(PCy<sub>3</sub>)<sub>2</sub>H<sub>2</sub>Cl; the dimerization of H<sub>2</sub>Bî—»NMe<sub>2</sub> 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 H<sub>3</sub>B·NMe<sub>2</sub>H
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 (thtAuBr<sub>3</sub>) to facilitate kinetic analysis. In combination with linear
free-energy relationships, kinetic isotope effects, and stoichiometric
experiments, the data support a mechanism involving an AuÂ(I)/AuÂ(III)
redox cycle in which sequential electrophilic aromatic substitution
of the arylsilane and the arene by AuÂ(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<sup>F</sup> (Ar<sup>F</sup> = 3,5-(CF<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>) and the β-S-substituted
aldehyde 2-(methylthio)Âbenzaldehyde: [RhÂ(<i>fac</i>-κ<sup>3</sup>-DPEphos)Â(CÂ(î—»CH<sub>2</sub>)ÂAr<sup>F</sup>)Â(CÂ(O)ÂC<sub>6</sub>H<sub>4</sub>SMe)<sub>2</sub>]Â[CB<sub>11</sub>H<sub>12</sub>]. 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 [RhÂ(<i>cis</i>-κ<sup>2</sup>-DPEphos)Â(η<sup>2</sup>:η<sup>2</sup>,κ<sup>1</sup>-H<sub>2</sub>Cî—»CÂ(Ar<sup>F</sup>)ÂCÂ(î—»O)Â(C<sub>6</sub>H<sub>4</sub>SMe)]Â[CB<sub>11</sub>H<sub>12</sub>] yield the
following activation parameters for reductive elimination, which follows
first-order kinetics (<i>k</i><sub>obs</sub> = (6.14 ±
0.04) × 10<sup>–5</sup> s<sup>–1</sup>, 324 K):
Δ<i>H</i><sup></sup><sup>⧧</sup> = 95 ±
2 kJ mol<sup>–1</sup>, Δ<i>S</i><sup></sup><sup>⧧</sup> = −32 ± 7 J K<sup>–1</sup> mol<sup>–1</sup>, Δ<i>G</i><sup></sup><sup>⧧</sup>(298 K) = 105 ± 4 kJ mol<sup>–1</sup>.
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<sup>F</sup> (Ar<sup>F</sup> = 3,5-(CF<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>) and the β-S-substituted
aldehyde 2-(methylthio)Âbenzaldehyde: [RhÂ(<i>fac</i>-κ<sup>3</sup>-DPEphos)Â(CÂ(î—»CH<sub>2</sub>)ÂAr<sup>F</sup>)Â(CÂ(O)ÂC<sub>6</sub>H<sub>4</sub>SMe)<sub>2</sub>]Â[CB<sub>11</sub>H<sub>12</sub>]. 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 [RhÂ(<i>cis</i>-κ<sup>2</sup>-DPEphos)Â(η<sup>2</sup>:η<sup>2</sup>,κ<sup>1</sup>-H<sub>2</sub>Cî—»CÂ(Ar<sup>F</sup>)ÂCÂ(î—»O)Â(C<sub>6</sub>H<sub>4</sub>SMe)]Â[CB<sub>11</sub>H<sub>12</sub>] yield the
following activation parameters for reductive elimination, which follows
first-order kinetics (<i>k</i><sub>obs</sub> = (6.14 ±
0.04) × 10<sup>–5</sup> s<sup>–1</sup>, 324 K):
Δ<i>H</i><sup></sup><sup>⧧</sup> = 95 ±
2 kJ mol<sup>–1</sup>, Δ<i>S</i><sup></sup><sup>⧧</sup> = −32 ± 7 J K<sup>–1</sup> mol<sup>–1</sup>, Δ<i>G</i><sup></sup><sup>⧧</sup>(298 K) = 105 ± 4 kJ mol<sup>–1</sup>.
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<sup>F</sup> (Ar<sup>F</sup> = 3,5-(CF<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>) and the β-S-substituted
aldehyde 2-(methylthio)Âbenzaldehyde: [RhÂ(<i>fac</i>-κ<sup>3</sup>-DPEphos)Â(CÂ(î—»CH<sub>2</sub>)ÂAr<sup>F</sup>)Â(CÂ(O)ÂC<sub>6</sub>H<sub>4</sub>SMe)<sub>2</sub>]Â[CB<sub>11</sub>H<sub>12</sub>]. 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 [RhÂ(<i>cis</i>-κ<sup>2</sup>-DPEphos)Â(η<sup>2</sup>:η<sup>2</sup>,κ<sup>1</sup>-H<sub>2</sub>Cî—»CÂ(Ar<sup>F</sup>)ÂCÂ(î—»O)Â(C<sub>6</sub>H<sub>4</sub>SMe)]Â[CB<sub>11</sub>H<sub>12</sub>] yield the
following activation parameters for reductive elimination, which follows
first-order kinetics (<i>k</i><sub>obs</sub> = (6.14 ±
0.04) × 10<sup>–5</sup> s<sup>–1</sup>, 324 K):
Δ<i>H</i><sup></sup><sup>⧧</sup> = 95 ±
2 kJ mol<sup>–1</sup>, Δ<i>S</i><sup></sup><sup>⧧</sup> = −32 ± 7 J K<sup>–1</sup> mol<sup>–1</sup>, Δ<i>G</i><sup></sup><sup>⧧</sup>(298 K) = 105 ± 4 kJ mol<sup>–1</sup>.
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(PCy<sub>3</sub>)<sub>2</sub>H<sub>2</sub>Cl
The RhÂ(III) species
RhÂ(PCy<sub>3</sub>)<sub>2</sub>H<sub>2</sub>Cl is an effective catalyst
(2 mol %, 298 K) for the dehydrogenation
of H<sub>3</sub>B·NMe<sub>2</sub>H (0.072 M in 1,2-F<sub>2</sub>C<sub>6</sub>H<sub>4</sub> solvent) to ultimately afford the dimeric
aminoborane [H<sub>2</sub>BNMe<sub>2</sub>]<sub>2</sub>. Mechanistic
studies on the early stages in the consumption of H<sub>3</sub>B·NMe<sub>2</sub>H, 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 H<sub>2</sub>Bî—»NMe<sub>2</sub>, which then
dimerizes to give [H<sub>2</sub>BNMe<sub>2</sub>]<sub>2</sub>. An
additional detail is that the active catalyst RhÂ(PCy<sub>3</sub>)<sub>2</sub>H<sub>2</sub>Cl is in rapid equilibrium with an inactive dimeric
species, [RhÂ(PCy<sub>3</sub>)ÂH<sub>2</sub>Cl]<sub>2</sub>. The reaction
of RhÂ(PCy<sub>3</sub>)<sub>2</sub>H<sub>2</sub>Cl with [RhÂ(PCy<sub>3</sub>)ÂH<sub>2</sub>(H<sub>2</sub>)<sub>2</sub>]Â[BAr<sup>F</sup><sub>4</sub>] forms the halide-bridged adduct [RhÂ(PCy<sub>3</sub>)<sub>2</sub>H<sub>2</sub>(μ-Cl)ÂH<sub>2</sub>(PCy<sub>3</sub>)<sub>2</sub>Rh]Â[BAr<sup>F</sup><sub>4</sub>] (Ar<sup>F</sup> = 3,5-(CF<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>), which has been crystallographically
characterized. This dinuclear cation dissociates on addition of H<sub>3</sub>B·NMe<sub>2</sub>H to re-form RhÂ(PCy<sub>3</sub>)<sub>2</sub>H<sub>2</sub>Cl and generate [RhÂ(PCy<sub>3</sub>)<sub>2</sub>H<sub>2</sub>(η<sup>2</sup>-H<sub>3</sub>B·NMe<sub>2</sub>H)]Â[BAr<sup>F</sup><sub>4</sub>]. The fate of the catalyst at low
catalyst loadings (0.5 mol %) is also addressed, with the formation
of an inactive borohydride species, RhÂ(PCy<sub>3</sub>)<sub>2</sub>H<sub>2</sub>(η<sup>2</sup>-H<sub>2</sub>BH<sub>2</sub>), observed.
On addition of H<sub>3</sub>B·NMe<sub>2</sub>H to IrÂ(PCy<sub>3</sub>)<sub>2</sub>H<sub>2</sub>Cl, the Ir congener IrÂ(PCy<sub>3</sub>)<sub>2</sub>H<sub>2</sub>(η<sup>2</sup>-H<sub>2</sub>BH<sub>2</sub>) is formed, with concomitant generation of the salt [H<sub>2</sub>BÂ(NMe<sub>2</sub>H)<sub>2</sub>]ÂCl
Quantum Yields for Photochemical Production of NO<sub>2</sub> from Organic Nitrates at Tropospherically Relevant Wavelengths
Absorption
cross-sections and quantum yields for NO<sub>2</sub> production (Φ<sub>NO<sub>2</sub></sub>) 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. NO<sub>2</sub> 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 (N<sub>2</sub>) pressure for total sample
pressures in the range of 250–700 Torr. When averaged over
all wavelengths and sample pressures, values of Φ<sub>NO<sub>2</sub></sub> 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 NO<sub>2</sub> 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<sub>NO<sub>2</sub></sub> ≤ 0.25 is obtained from the experimental
measurements
Mechanisms of the Thermal and Catalytic Redistributions, Oligomerizations, and Polymerizations of Linear Diborazanes
Linear
diborazanes R<sub>3</sub>N–BH<sub>2</sub>–NR<sub>2</sub>–BH<sub>3</sub> (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: Me<sub>3</sub>N–BH<sub>2</sub>–NMe<sub>2</sub>–BH<sub>3</sub> (<b>1</b>), Me<sub>3</sub>N–BH<sub>2</sub>–NHMe–BH<sub>3</sub> (<b>2</b>), and MeNH<sub>2</sub>–BH<sub>2</sub>–NHMe–BH<sub>3</sub> (<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
(Me<sub>3</sub>N·BH<sub>3</sub> (<b>8</b>) and [Me<sub>2</sub>N–BH<sub>2</sub>]<sub>2</sub> (<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
[H<sub>2</sub>BÂ(μ-H)Â(μ-NMe<sub>2</sub>)ÂBH<sub>2</sub>]
(<b>5</b>) and NMe<sub>3</sub> 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 % IrH<sub>2</sub>POCOP (POCOP = κ<sup>3</sup>-1,3-(OP<i>t</i>Bu<sub>2</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)), <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 Me<sub>2</sub>Nî—»BH<sub>2</sub> (<b>9b</b>), which spontaneously dimerizes to give <b>9a</b>, with the absence of <b>5</b> and BH<sub>3</sub> 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BH<sub>2</sub> (<b>10</b>) to form oligoaminoborane [MeNH–BH<sub>2</sub>]<sub><i>x</i></sub> (<b>11</b>) or polyaminoborane [MeNH–BH<sub>2</sub>]<sub><i>n</i></sub> (<b>16</b>) following
the initial redistribution reaction