18 research outputs found
Regioselectivity of the CâMetalation of 6âFurylpurine: Importance of Directing Effects
We report the C-metalation of 6-furylpurine
with Pt<sup>2+</sup>, Pd<sup>2+</sup>, and Hg<sup>2+</sup>. 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. Pt<sup>2+</sup> and Pd<sup>2+</sup> coordinate the
furyl moiety in its β position and Hg<sup>2+</sup> in its electronically
preferred Îą position
The âCatalytic Nitrosyl Effectâ: NO Bending Boosting the Efficiency of Rhenium Based Alkene Hydrogenations
Diiodo
ReÂ(I) complexes [ReI<sub>2</sub>(NO)Â(PR<sub>3</sub>)<sub>2</sub>(L)]
(<b>3</b>, L = H<sub>2</sub>O; <b>4</b> ,
L = H<sub>2</sub>; R = <i>i</i>Pr <b>a</b>, Cy <b>b</b>) were prepared and found to exhibit in the presence of âhydrosilane/BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>â 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 H<sub>2</sub>/D<sub>2</sub> scrambling and slow
alkene isomerizations pointing to an Osborn type hydrogenation cycle
with rate determining reductive elimination of the alkane. In the
catalystsâ activation stage phosphonium borates [R<sub>3</sub>PH]Â[HBÂ(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>] (<b>6</b>,
R = <i>i</i>Pr <b>a</b>, Cy <b>b</b>) are formed.
VT <sup>29</sup>Si- and <sup>15</sup>N NMR experiments, and dispersion
corrected DFT calculations verified the following facts: (1) Coordination
of the silylium cation to the O<sub>NO</sub> atom facilitates nitrosyl
bending; (2) The bent nitrosyl promotes the heterolytic cleavage of
the HâH bond and protonation of a phosphine ligand; (3) H<sub>2</sub> adds in a bifunctional manner across the ReâN bond.
Nitrosyl bending and phosphine loss help to create two vacant sites,
thus triggering the high hydrogenation activities of the formed âsuperelectrophilicâ
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-bisÂ(pentafluorophenyl)Âboryl-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)Ânorbornene
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 Î<i>H</i><sup>⧧</sup> of 13 kcal mol<sup>â1</sup>, 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 isomerizationâring
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) Mes<sub>2</sub>PCH<sub>2</sub>CH<sub>2</sub>BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> <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-BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> Lewis acid and an <i>endo</i>-2-PMes<sub>2</sub> Lewis base functionality coordinated at the norbornane
framework. This noninteracting FLP adds carbon monoxide in solution
at â35 °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 FLPH<sub>2</sub> 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 H<sub>2</sub> 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) mes<sub>2</sub>PâCH<sub>2</sub>CH<sub>2</sub>âBÂ(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> (<b>3</b>) reacts with phenylÂ(trimethylsilyl)Âacetylene
by
1,1-carboboration to give the extended C<sub>3</sub>-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 diphenylphosphinoÂ(trimethylsilyl)Âacetylene
to give the P/B/P FLP <b>11</b> that features a central unsaturated
four-membered heterocyclic P/B FLP and a pendant CH<sub>2</sub>CH<sub>2</sub>âPmes<sub>2</sub> functional group. Compound <b>11</b> reacts with nitric oxide (NO) by oxidation of the pendant
Pmes<sub>2</sub> unit to the PÂ(O)Âmes<sub>2</sub> 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 PÂ(O)Âmes<sub>2</sub> formation and opening of the central Ph<sub>2</sub>P¡¡¡BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub> linkage triggered by the pendant CH<sub>2</sub>CH<sub>2</sub>âPÂ(O)Âmes<sub>2</sub> group
Intermolecular On-Surface ĎâBond 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 Ď-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 hexaethynylÂbenzene
at Ag(111). Furthermore, it is also shown that on the Au(111) surface
this Ď-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
[Mes<sub>2</sub>Pî¸C<sub>6</sub>H<sub>10</sub>î¸BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>] <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-NOî¸CH<sub>2</sub>Ph <b>4b</b>, both
characterized by X-ray diffraction. Structurally related FLPs [Mes<sub>2</sub>Pî¸CHR<sup>1</sup>î¸CHR<sup>2</sup>î¸BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>] <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-NOî¸CHMePh <b>6b</b> derived from 1-bromoethylbenzene
undergoes NOî¸C bond cleavage at 120 °C with an activation
energy of <i>E</i><sub>a</sub> = 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 C<sub>2</sub>-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
[Mes<sub>2</sub>Pî¸C<sub>6</sub>H<sub>10</sub>î¸BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>] <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-NOî¸CH<sub>2</sub>Ph <b>4b</b>, both
characterized by X-ray diffraction. Structurally related FLPs [Mes<sub>2</sub>Pî¸CHR<sup>1</sup>î¸CHR<sup>2</sup>î¸BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>] <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-NOî¸CHMePh <b>6b</b> derived from 1-bromoethylbenzene
undergoes NOî¸C bond cleavage at 120 °C with an activation
energy of <i>E</i><sub>a</sub> = 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 C<sub>2</sub>-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
[Mes<sub>2</sub>Pî¸C<sub>6</sub>H<sub>10</sub>î¸BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>] <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-NOî¸CH<sub>2</sub>Ph <b>4b</b>, both
characterized by X-ray diffraction. Structurally related FLPs [Mes<sub>2</sub>Pî¸CHR<sup>1</sup>î¸CHR<sup>2</sup>î¸BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>] <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-NOî¸CHMePh <b>6b</b> derived from 1-bromoethylbenzene
undergoes NOî¸C bond cleavage at 120 °C with an activation
energy of <i>E</i><sub>a</sub> = 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 C<sub>2</sub>-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
[Mes<sub>2</sub>Pî¸C<sub>6</sub>H<sub>10</sub>î¸BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>] <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-NOî¸CH<sub>2</sub>Ph <b>4b</b>, both
characterized by X-ray diffraction. Structurally related FLPs [Mes<sub>2</sub>Pî¸CHR<sup>1</sup>î¸CHR<sup>2</sup>î¸BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>2</sub>] <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-NOî¸CHMePh <b>6b</b> derived from 1-bromoethylbenzene
undergoes NOî¸C bond cleavage at 120 °C with an activation
energy of <i>E</i><sub>a</sub> = 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 C<sub>2</sub>-bridged intramolecular P/B frustrated
Lewis pairs to nitric oxide