96 research outputs found
Yttrium-Catalyzed Amine–Silane Dehydrocoupling: Extended Reaction Scope with a Phosphorus-Based Ligand
The
scope of the catalytic dehydrocoupling of primary and secondary
amines with phenylsilanes has been investigated using [YÂ{NÂ(SiMe<sub>3</sub>)<sub>2</sub>}<sub>3</sub>] and a four-coordinate analogue
bearing a cyclometalated phosphonium methylide ligand. Inclusion of
the phosphorus-based ligand on yttrium results in increased substrate
scope in comparison to the trisÂ(amide) analogue. While reversible
C–H bond activation of the cyclometalated ligand was observed
in stoichiometric experiments, D-labeling experiments and DFT calculations
suggest that reversible ligand activation is not involved in silazane
formation under catalytic conditions. We suggest that the extended
reaction scope with the four-coordinate yttrium phosphonium methylide
complex relative to the three-coordinate yttrium (tris)Âamide complex
is a result of differences in the ease of amine inhibition of catalysis
Activation of N-heterocyclic carbenes by {BeH<sub>2</sub>} and {Be(H)(Me)} fragments
A stable three-coordinate dimethylberyllium
species coordinated
by the 1,3-bisÂ(2,4,6-trimethylphenyl)Âimidazol-2-ylidene (IMes) ligand
is readily converted to the corresponding methylhydrido derivative
through metathetical reaction with phenylsilane. Attempts to synthesize
the corresponding molecular dihydrides are, however, unsuccessful
and result in ring opening of an IMes ligand through hydride transfer
to the donor carbon atom and the consequent formation of a heterocyclic
beryllium organoamide. In agreement with previous calculations, we
suggest that this process occurs via a Schlenk-type equilibration
process and formation of a four-coordinate bis-NHC beryllium dihydride.
These species are not observed, however, as the steric pressure exerted
by coordination of the two sterically demanding IMes ligands is sufficient
to induce hydride transfer. The latter deduction is supported by the
observation that a similar ring-opened product, but derived from methyl
and hydride transfer, is available through the introduction of a further
equivalent of IMes to the isolated beryllium methyl hydride species.
In the latter case the ring-opening process is more facile, which
we ascribe to the increased steric pressure achieved upon the formation
of four-coordinate beryllium. In a further striking reaction under
more forcing thermal conditions, the carbene carbon center of an IMes
ligand is observed to be completely eliminated with selective formation
of a three-coordinate diamidoberyllium species
Mononuclear three-coordinate magnesium complexes of a highly sterically encumbered β-diketiminate ligand
Alkyne Hydroamination and Trimerization with Titanium Bis(phenolate)pyridine Complexes: Evidence for Low-Valent Titanium Intermediates and Synthesis of an Ethylene Adduct of Titanium(II)
Heavier Group 2 Metals and Intermolecular Hydroamination: A Computational and Synthetic Assessment
Heterobimetallic Rebound: A Mechanism for Diene-to-Alkyne Isomerization with M--Zr Hydride Complexes (M = Al, Zn, and Mg)
Heterobimetallic Rebound: A Mechanism for Diene-to-Alkyne Isomerization with M‑--Zr Hydride Complexes (M = Al, Zn, and Mg)
The reaction of a series of <b>M<b>·</b>Zr</b> heterobimetallic hydride complexes
with dienes and alkynes has been
investigated (M = Al, Zn, and Mg). Reaction of <b>M<b>·</b>Zr</b> with 1,5-cyclooctadiene led to diene isomerization to 1,3-cyclooctadiene,
but for M = Zn also result in an on-metal diene-to-alkyne isomerization.
The resulting cyclooctyne fragment is trapped between Zr and Zn metals
in a heterobimetallic species that does not form for M = Mg or Al.
The scope of diene isomerization and alkyne trapping has been explored
leading to the isolation of three new heterobimetallic slipped metallocyclopropene
complexes. The mechanism of diene-to-alkyne isomerization was investigated
through kinetics. While the reaction is first-order in <b>Zn·Zr</b> at high diene concentration and proceeds with Δ<i>H</i><sup>‡</sup> = +33.6 ± 0.7 kcal mol<sup>–1</sup>, Δ<i>S</i><sup>‡</sup> = +23.2 ± 1.7
cal mol<sup>–1</sup> K<sup>–1</sup>, and Δ<i>G</i><sup>⧧</sup><sub>298 K</sub> = +26.7 ±
1.2 kcal mol<sup>–1</sup>, the rate is dependent on the nature
of the diene. The positive activation entropy is suggestive of involvement
of a dissociative step. On the basis of DFT calculations, a heterobimetallic
rebound mechanism for diene-to-alkyne isomerization has been proposed.
This mechanism explains the origin of heterobimetallic control over
selectivity: Mg---Zr complexes are too strongly bound to generate
reactive fragments, while Al---Zr complexes are too weakly bound to
compensate for the contrathermodynamic isomerization process. Zn---Zr
complexes have favorable energetics for both dissociation and trapping
steps
Reversibility in the protonolysis of a β-diketiminate stabilised calcium bis(trimethylsilyl)amide with benzylamine
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