32 research outputs found
Dimethylcuprate-Catalyzed Decarboxylative Coupling of Allyl Acetate
Allylic alkylation of organocuprates represents an important
class of C–C bond forming reactions. A drawback of these reactions
is their stoichiometric nature. Although metal-catalyzed decarboxylative
coupling reactions offer new opportunities for formation of C–C
bonds, catalytic allylic alkylation reactions of organocuprates have
not been previously reported. Here, multistage mass spectrometry experiments
performed on ion trap mass spectrometers in conjunction with electronic
structure calculations are used to demonstrate that the dimethylcuprate
anion, [CH<sub>3</sub>CuCH<sub>3</sub>]<sup>−</sup>, can catalyze
decarboxylative coupling of allyl acetate in the gas phase via a simple
two-step catalytic cycle. In step 1, [CH<sub>3</sub>CuCH<sub>3</sub>]<sup>−</sup> undergoes a cross-coupling reaction with allyl
acetate to yield [CH<sub>3</sub>CuO<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup> as the major product ion. Step 2 involves subjecting
the product ion to collision-induced decarboxylation to re-form [CH<sub>3</sub>CuCH<sub>3</sub>]<sup>−</sup>, thereby closing the
catalytic cycle. Since the details of step 2 have been previously
described (Rijs, N.; Khairallah, G. N.; Waters, T.; O’Hair, R. A. J. J. Am. Chem. Soc. 2008, 130, 1069−1079), here the focus is on step 1. Comparison
of the reactivity of dimethylcuprate toward allylic susbtrates reveals
that while allyl acetate reacts around 70 times more slowly than allyl
iodide, it is more selective for cross-coupling. This is rationalized
by DFT calculations, which reveal that an increase in kinetic barrier
is responsible for both reactivity trends. The lowest energy path
was found to involve a stepwise π-oxidative addition proceeding
via an η<sup>2</sup>-C<sub>3</sub>H<sub>5</sub>O<sub>2</sub>CCH<sub>3</sub> intermediate and extrusion of a leaving group (LG)
anion, followed by a reductive elimination where this LG is recomplexed
to the copper center. Finally, DFT calculations were used to shed
light on the role of leaving groups LG = CH<sub>3</sub>CO<sub>2</sub><sup>–</sup>, I<sup>–</sup>, Br<sup>–</sup>,
Cl<sup>–</sup> in the allylic alkylation. While selectivity
is indeed achieved at the cost of reactivity, the LG effects were
more complex than can be accounted for by a simple consideration of
the anion proton affinity (APA) of the LG
Dimethylcuprate-Mediated Transformation of Acetate to Dithioacetate
Dithiocarboxylic
acids, RCS<sub>2</sub>H, and their esters, RCS<sub>2</sub>R′,
are useful reagents that can be synthesized by
the reaction of carbon disulfide with organometallic reagents. Here
the coinage-metal-mediated transformation of acetate to dithioacetate
is explored in the gas phase using multistage mass spectrometry experiments
in a linear ion trap mass spectrometer in conjunction with density
functional theory (DFT) calculations. The ion–molecule reactions
between coinage-metal dimethylmetalate anions [CH<sub>3</sub>MCH<sub>3</sub>]<sup>−</sup> (M = Au, Ag, Cu), formed via double decarboxylation
of the metal acetate anions, [CH<sub>3</sub>CO<sub>2</sub>MO<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup>, and carbon disulfide, were examined.
Only [CH<sub>3</sub>CuCH<sub>3</sub>]<sup>−</sup> reacts with
CS<sub>2</sub> with a reaction efficiency of 0.8% of the collision
rate to yield the adduct [CH<sub>3</sub>CuS<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup> (77.5%) as well as CH<sub>3</sub>CS<sub>2</sub><sup>–</sup> (22.5%). Collision-induced dissociation (CID)
of the adduct [CH<sub>3</sub>CuS<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup> gives CH<sub>3</sub>CS<sub>2</sub><sup>–</sup> as the major
product, with a small amount of [CuS<sub>2</sub>CCH<sub>2</sub>]<sup>−</sup> being formed via loss of methane. DFT calculations
reveal the following. (i) [CH<sub>3</sub>CuCH<sub>3</sub>]<sup>−</sup> reacts via oxidative addition to form a CuÂ(III) intermediate, followed
by reductive elimination of CH<sub>3</sub>CS<sub>2</sub><sup>–</sup>, which is captured by Cu to form [CH<sub>3</sub>CuS<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup>. This energetic adduct can fragment via
loss of CH<sub>3</sub>CS<sub>2</sub><sup>–</sup> or can be
collisionally cooled by the helium bath gas used in the experiments.
(ii) Loss of CH<sub>4</sub> from [CH<sub>3</sub>CuS<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup> also involves a CuÂ(III) intermediate
and results in formation of the metalladithiolactone [CuÂ(CH<sub>2</sub>CS<sub>2</sub>)]<sup>−</sup>
Mass Spectrometric and Computational Studies on the Reaction of Aromatic Peroxyl Radicals with Phenylacetylene Using the Distonic Radical Ion Approach
Product
and mechanistic studies were performed for the reaction
of aromatic distonic peroxyl radical cations 4-PyrOO<sup>•+</sup> and 3-PyrOO<sup>•+</sup> with phenylacetylene (<b>7</b>) in the gas phase using mass spectrometric and computational techniques.
PyrOO<sup>•+</sup> was generated through reaction of the respective
distonic aryl radical cation Pyr<sup>•+</sup> with O<sub>2</sub> in the ion source of the mass spectrometer. For the reaction involving
the more electrophilic 4-PyrOO<sup>•+</sup>, a rate coefficient
of <i>k</i><sub>1</sub> = (2.2 ± 0.6) × 10<sup>–10</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> was determined at 298 K, while a value of <i>k</i><sub>2</sub> = (8.2 ± 2.1) × 10<sup>–11</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup> was obtained for the reaction involving the less electrophilic 3-PyrOO<sup>•+</sup>. This highlights the role of polar effects in these
reactions, which are likely of high relevance for processes in combustions
and atmospheric transformations. The mechanism was studied by computational
methods, which showed that radical addition occurs exclusively at
the less substituted alkyne site to give the distonic vinyl radical
cation <b>8</b>. The latter undergoes a series of subsequent
rearrangements/fragmentations that are similar for both isomeric PyrOO<sup>•+</sup>. γ-Fragmentation in <b>8</b> leads to
the distonic aryloxyl radical cation PyrO<sup>•+</sup> and
a singlet carbene <b>10</b>. The product association complex
[PyrO<sup>•+</sup> – <b>10</b>] is the starting
point for two important subsequent reactions, e.g., (i) rapid hydrogen
transfer to form ketenyl radical <b>11</b> and the closed-shell
species PyrOH<sup>+</sup>, and (ii) oxygen transfer from PyrO<sup>•+</sup> to <b>10</b> that leads to α-keto aldehyde <b>13</b> and Pyr<sup>•+</sup>, followed by hydrogen abstraction
to give acyl radical <b>14</b> and PyrH<sup>+</sup>. Additional
major products are the closed-shell aromatic carbonyl compounds <b>20</b> and <b>30</b> that result from multistep rearrangements
in vinyl radical <b>8</b>, which are terminated by homolytic
bond scission and release of neutral acyl radicals
Dimethylcuprate-Mediated Transformation of Acetate to Dithioacetate
Dithiocarboxylic
acids, RCS<sub>2</sub>H, and their esters, RCS<sub>2</sub>R′,
are useful reagents that can be synthesized by
the reaction of carbon disulfide with organometallic reagents. Here
the coinage-metal-mediated transformation of acetate to dithioacetate
is explored in the gas phase using multistage mass spectrometry experiments
in a linear ion trap mass spectrometer in conjunction with density
functional theory (DFT) calculations. The ion–molecule reactions
between coinage-metal dimethylmetalate anions [CH<sub>3</sub>MCH<sub>3</sub>]<sup>−</sup> (M = Au, Ag, Cu), formed via double decarboxylation
of the metal acetate anions, [CH<sub>3</sub>CO<sub>2</sub>MO<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup>, and carbon disulfide, were examined.
Only [CH<sub>3</sub>CuCH<sub>3</sub>]<sup>−</sup> reacts with
CS<sub>2</sub> with a reaction efficiency of 0.8% of the collision
rate to yield the adduct [CH<sub>3</sub>CuS<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup> (77.5%) as well as CH<sub>3</sub>CS<sub>2</sub><sup>–</sup> (22.5%). Collision-induced dissociation (CID)
of the adduct [CH<sub>3</sub>CuS<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup> gives CH<sub>3</sub>CS<sub>2</sub><sup>–</sup> as the major
product, with a small amount of [CuS<sub>2</sub>CCH<sub>2</sub>]<sup>−</sup> being formed via loss of methane. DFT calculations
reveal the following. (i) [CH<sub>3</sub>CuCH<sub>3</sub>]<sup>−</sup> reacts via oxidative addition to form a CuÂ(III) intermediate, followed
by reductive elimination of CH<sub>3</sub>CS<sub>2</sub><sup>–</sup>, which is captured by Cu to form [CH<sub>3</sub>CuS<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup>. This energetic adduct can fragment via
loss of CH<sub>3</sub>CS<sub>2</sub><sup>–</sup> or can be
collisionally cooled by the helium bath gas used in the experiments.
(ii) Loss of CH<sub>4</sub> from [CH<sub>3</sub>CuS<sub>2</sub>CCH<sub>3</sub>]<sup>−</sup> also involves a CuÂ(III) intermediate
and results in formation of the metalladithiolactone [CuÂ(CH<sub>2</sub>CS<sub>2</sub>)]<sup>−</sup>
Solution and Gas-Phase Investigations of Trimethylsilylpropyl-Substituted Pyridinium Ions. Manifestation of the Silicon δ Effect
Computational studies on the <i>N</i>-methyl-2-trimethyl-M-propylpyridinium ions <b>15a</b> (M = Si), <b>15b</b> (M = Ge), <b>15c</b> (M = Sn), and <b>15d</b> (M = Pb) and <i>N</i>-methyl-4-trimethyl-M-propylpyridinium ions <b>16a</b> (M = Si), <b>16b</b> (M = Ge), <b>16c</b> (M = Sn), and <b>16d</b> (M = Pb) provide evidence for a significant through-bond (double hyperconjugative) interaction between the M–CH<sub>2</sub> bond and the low-lying π* orbital of the pyridinium ion. The strength of this interaction increases in the order Si < Ge < Sn < Pb, in line with the σ-donor abilities of the C–M bond. The through-bond interaction for M = Si has been studied in solution using <sup>13</sup>C and <sup>29</sup>Si NMR studies; however, the effect is small. The collision-induced dissociation fragmentation reactions of <b>15a</b> and <b>16a</b> are strongly influenced by the through-bond interaction, with the major fragmentation pathway proceeding via extrusion of ethylene to yield the trimethylsilylmethyl-substituted pyridinium ions <b>1a</b> and <b>2a</b>
Gas-Phase Unimolecular Reactions of Pallada- and Nickelalactone Anions
Electrospray ionization in combination with multistage
mass spectrometry experiments in a linear ion trap mass spectrometer
was used to generate and study the gas-phase ion chemistry of the
metallalactones [(CH<sub>3</sub>CO<sub>2</sub>)ÂNiÂ(CH<sub>2</sub>CO<sub>2</sub>)]<sup>−</sup> (<i>m</i>/<i>z</i> 175, <b>5a</b>) and [(CH<sub>3</sub>CO<sub>2</sub>)ÂPdÂ(CH<sub>2</sub>CO<sub>2</sub>)]<sup>−</sup> (<i>m</i>/<i>z</i> 223, <b>5b</b>). Low-energy collision-induced dissociation
(CID) resulted in decarboxylation to produce novel organometallic
ions at <i>m</i>/<i>z</i> 131 (Ni) and <i>m</i>/<i>z</i> 179 (Pd). Isotope labeling experiments,
bimolecular gas-phase reactions with allyl iodide, and DFT calculations
reveal that decarboxylation primarily occurs from the acetato ligand
to yield ions of the form [(CH<sub>3</sub>)ÂMÂ(CH<sub>2</sub>CO<sub>2</sub>)]<sup>−</sup> (M = Pd, Ni). Further CID experiments
on [(CH<sub>3</sub>)ÂMÂ(CH<sub>2</sub>CO<sub>2</sub>)]<sup>−</sup> together with DFT calculations highlight the following. (1) Both
palladium and nickel can facilitate C–C bond formation, with
elimination of ethylene being observed. (2) The mechanism for formation
of ethylene from [(CH<sub>3</sub>)ÂMÂ(CH<sub>2</sub>CO<sub>2</sub>)]<sup>−</sup> is inherently different for M = Pd versus M = Ni.
Elimination of ethylene is competitive with further decarboxylation
in the case of nickel, and nickel remains in the 2+ oxidation state
throughout. In contrast, the palladium complex is reduced to palladium(0)
upon C–C bond formation and undergoes a second decarboxylation
before ethylene is released. Finally, the products of the ion–molecule
reactions of [(CH<sub>3</sub>)ÂPdÂ(CH<sub>2</sub>CO<sub>2</sub>)]<sup>−</sup> (<b>7b</b>) with allyl iodide provide evidence
for the formation of the PdÂ(IV) intermediate [(CH<sub>3</sub>)Â(I)Â(CH<sub>2</sub>CHCH<sub>2</sub>)ÂPdÂ(CH<sub>2</sub>CO<sub>2</sub>)]<sup>−</sup>, which decomposes via a range of processes, including losses of
iodide, propionate, allyl, and methyl radicals and reductive elimination
of butane and methyl iodide
Gas-Phase Reactivity of Group 11 Dimethylmetallates with Allyl Iodide
Copper-mediated allylic substitution reactions are widely
used
in organic synthesis, whereas the analogous reactions for silver and
gold are essentially unknown. To unravel why this is the case, the
gas-phase reactions of allyl iodide with the coinage metal dimethylmetallates,
[CH<sub>3</sub>MCH<sub>3</sub>]<sup>−</sup> (M = Cu, Ag and
Au), were examined under the near thermal conditions of an ion trap
mass spectrometer and via electronic structure calculations. [CH<sub>3</sub>CuCH<sub>3</sub>]<sup>−</sup> reacted with allyl iodide
with a reaction efficiency of 6.6% of the collision rate to yield:
I<sup>–</sup> (75%); the cross-coupling product, [CH<sub>3</sub>CuI]<sup>−</sup> (24%); and the homo-coupling product, [C<sub>3</sub>H<sub>5</sub>CuI]<sup>−</sup> (1%). [CH<sub>3</sub>AgCH<sub>3</sub>]<sup>−</sup> and [CH<sub>3</sub>AuCH<sub>3</sub>]<sup>−</sup> reacted substantially slower (reaction
efficiencies of 0.028% and 0.072%). [CH<sub>3</sub>AgCH<sub>3</sub>]<sup>−</sup> forms I<sup>–</sup> (19%) and [CH<sub>3</sub>AgI]<sup>−</sup> (81%), while only I<sup>–</sup> is formed from [CH<sub>3</sub>AuCH<sub>3</sub>]<sup>−</sup>. Because the experiments do not detect the neutral productÂ(s) formed,
which might otherwise help identify the mechanisms of reaction, and
to rationalize the observed ionic products and reactivity order, calculations
at the B3LYP/def2-QZVP//B3LYP/SDD6-31+GÂ(d) level were conducted on
four different mechanisms: (i) S<sub>N</sub>2; (ii) S<sub>N</sub>2′;
(iii) oxidative-addition/reductive elimination (OA/RE) via an MÂ(III)
η<sup>3</sup>-allyl intermediate; and (iv) OA/RE via an MÂ(III)
η<sup>1</sup>-allyl intermediate. For copper, mechanisms (iii)
and (iv) are predicted to be competitive. Only the CuÂ(III) η<sup>3</sup>-allyl intermediate undergoes reductive elimination via two
different transition states to yield either the cross-coupling or
the homo-coupling products. Their relative barriers are consistent
with homo-coupling being a minor pathway. For silver, the kinetically
most probable pathway is the S<sub>N</sub>2 reaction, consistent with
no homo-coupling product, [C<sub>3</sub>H<sub>5</sub>AgI]<sup>−</sup>, being observed. For gold, no C–C coupling reaction is kinetically
viable. Instead, I<sup>–</sup> is predicted to be formed along
with a stable AuÂ(III) η<sup>3</sup>-allyl complex. These results
clearly highlight the superiority of organocuprates in allylic substitution
reactions
Unraveling Organocuprate Complexity: Fundamental Insights into Intrinsic Group Transfer Selectivity in Alkylation Reactions
The
near thermal conditions of an ion-trap mass spectrometer were
used to examine the intrinsic gas-phase reactivity and selectivity
of nucleophilic substitution reactions. The well-defined organocuprate
anions [CH<sub>3</sub>CuR]<sup><b>–</b></sup> (R = CH<sub>3</sub>CH<sub>2</sub>, CH<sub>3</sub>CH<sub>2</sub>CH<sub>2</sub>, (CH<sub>3</sub>)<sub>2</sub>CH, PhCH<sub>2</sub>CH<sub>2</sub>,
PhCH<sub>2</sub>, Ph, C<sub>3</sub>H<sub>5</sub>, and H) were reacted
with CH<sub>3</sub>I. The rates (reaction efficiencies, Ï•) and
selectivities (the product ion branching ratios) were compared with
those of [CH<sub>3</sub>CuCH<sub>3</sub>]<sup><b>–</b></sup> reacting with CH<sub>3</sub>I. Alkyl R groups yielded similar
efficiencies, with selectivity for C–C bond formation at the
coordinated R group. Inclusion of unsaturated R groups curbed the
overall reactivity (Ï• = 1 to 2 orders of magnitude lower). With
the exception of R = PhCH<sub>2</sub>CH<sub>2</sub>, these switched
their selectivity to C–C bond formation at the CH<sub>3</sub> group. Replacing an organyl ligand with R = H significantly enhanced
the reactivity (8-fold), resulting in the selective formation of methane.
Unique decomposition and side-reactions observed include: (1) spontaneous
β-hydride elimination from [RCuI]<sup><b>–</b></sup> byproducts; and (2) homocoupling of the pre-existing organocuprate
ligands in [CH<sub>3</sub>CuC<sub>3</sub>H<sub>5</sub>]<sup><b>–</b></sup>, as shown by deuterium labeling. DFT (B3LYP-D/Def2-QZVP//B3LYP/SDD:6-31+GÂ(d))
predicts that the alkylation mechanism for all species is via oxidative
addition/reductive elimination (OA/RE). OA is the rate-limiting step,
while RE determines selectivity: the effects of R on each were examined
Theoretical Approaches To Estimating Homolytic Bond Dissociation Energies of Organocopper and Organosilver Compounds
Although organocopper and organosilver compounds are
known to decompose
by homolytic pathways among others, surprisingly little is known about
their bond dissociation energies (BDEs). In order to address this
deficiency, the performance of the DFT functionals BLYP, B3LYP, BP86,
TPSSTPSS, BHandHLYP, M06L, M06, M06-2X, B97D, and PBEPBE, along with
the double hybrids, mPW2-PLYP, B2-PLYP, and the ab initio methods,
MP2 and CCSDÂ(T), have been benchmarked against the thermochemistry
for the M–C homolytic BDEs (<i>D</i><sub>0</sub>)
of Cu–CH<sub>3</sub> and Ag–CH<sub>3</sub>, derived
from guided ion beam experiments and CBS limit calculations (<i>D</i><sub>0</sub>(Cu–CH<sub>3</sub>) = 223 kJ·mol<sup>–1</sup>; <i>D</i><sub>0</sub>(Ag–CH<sub>3</sub>) = 169 kJ·mol<sup>–1</sup>). Of the tested methods,
in terms of chemical accuracy, error margin, and computational expense,
M06 and BLYP were found to perform best for homolytic dissociation
of methylcopper and methylsilver, compared with the CBS limit gold
standard. Thus the M06 functional was used to evaluate the M–C
homolytic bond dissociation energies of Cu–R and Ag–R,
R = Et, Pr, <i>i</i>Pr, <i>t</i>Bu, allyl, CH<sub>2</sub>Ph, and Ph. It was found that <i>D</i><sub>0</sub>(Ag–R) was always lower (∼50 kJ·mol<sup>–1</sup>) than that of <i>D</i><sub>0</sub>(Cu–R). The trends
in BDE when changing the R ligand reflected the H–R bond energy
trends for the alkyl ligands, while for R = allyl, CH<sub>2</sub>Ph,
and Ph, some differences in bond energy trends arose. These trends
in homolytic bond dissociation energy help rationalize the previously
reported (Rijs, N. J.; O’Hair, R. A. J. <i>Organometallics</i> <b>2010</b>, <i>29</i>, 2282–2291) fragmentation
pathways of the organometallate anions, [CH<sub>3</sub>MR]<sup>−</sup>