The well-defined aluminum monohydride compound [{(2,4,6-Me3-C6H2)NC(Me)}2(Me)(H)]AlH·(NMe2Et) (1) catalyzes hydroboration of a wide range of aldehydes and ketones under mild reaction conditions. Moreover, compound 1 displayed chemoselective hydroboration of aldehydes over ketones at rt.
This report describes an approach
for preparing tethered tungsten-imido
alkylidene complexes featuring a tetra-anionic pincer ligand. Treating
the tungsten alkylidyne [
t
BuOCO]WC
t
Bu(THF)2 (1) with
isocyanates (RNCO; R =
t
Bu, Cy, and Ph)
leads to cycloaddition occurring exclusively at the CN bond
to generate the tethered tungsten-imido alkylidenes (6-NR). Unanticipated intermediates reveal themselves, including the discovery
of [(O2C
t
BuC)W(η2-(N,C)-RNCO)(THF)] (11-R) and an unprecedented decarbonylation product [(
t
BuOCO)W(NR)(
t
BuCCO)] (14-R), on the pathway to the formation
of 6-NR. Complex 11-R is kinetically stable
for sterically bulky isocyanate R =
t
Bu
(11-
t
Bu) and is isolated
and characterized by single-crystal X-ray diffraction. Finally, adding
to the short list of catalysts capable of ring expansion metathesis
polymerization (REMP), complexes 6-NR and 11-
t
Bu are active for the stereoselective
synthesis of cyclic polynorbornene.
This report details the preparation
of the trianionic pincer ligand
supported Mo–alkylidyne complex [
t
BuOCO]MoCMes(THF)2 (5). The catalytic
activity of [
t
BuOCO]MoCMes(THF)2 (5) for alkyne polymerization is limited in
contrast to its W-congener. Solution and crystallographic evidence
for the conversion of the trianionic pincer complex [
t
BuOCO]MoCMes(THF)2 (5) to a tetraanionic pincer complex [O2C(MesC)Mo(η2-CHC
t
Bu)(THF)] (6) upon exposure to tert-butylacetylene is
provided.
An easier, safer, and scalable approach to the synthesis
of a trianionic
pincer ligand and its use in the preparation of the Mo cyclic polymer
catalyst [O2C((p-OMe-C6H4)C)Mo(η2-CHC
t
Bu)(THF)] (7) are reported. The synthesis
of the [
t
BuOCO]Me2 (1) ligand is simplified to a reaction that allows scaling up, thus
reducing the main barrier to accessing cyclic polymer catalysts. The
synthesis allows for the derivatization of the C
ipso
carbon of the central ring of the pincer. Taking advantage
of this, a deuterium atom was substituted for kinetic isotope measurements.
Complex 7 was tested for activity in the polymerization
of phenylacetylene. Confirmation of the cyclic polymer topology comes
from gel permeation chromatography (GPC), dynamic light scattering
(DLS), and intrinsic viscosity (η) measurements.
The reactivity of phosphaalkynes, the isolobal and isoelectronic congeners to alkynes, with metal alkylidyne complexes is explored in this work. Treating the tungsten alkylidyne [ t BuOCO]W�C t Bu(THF) 2 (1) with phosphaalkyne (10) results in the formation of [O 2 C( t BuC=)W{η 2 -(P,C)À P�CÀ Ad}(THF)] (13-t Bu THF ) and [O 2 C(AdC=)W{η 2 -(P,C)À P�CÀ t Bu}(THF)] (13-Ad THF ); derived from the formal reductive migratory insertion of the alkylidyne moiety into a WÀ C arene bond. Analogous to alkyne metathesis, a stable phosphametallacyclobutadiene complex [ t BuOCO]W[k 2 -C( t Bu)PC-(Ad)] ( 14) forms upon loss of THF from the coordination sphere of either 13-t Bu THF or 13-Ad THF . Remarkably, the CÀ C bonds reversibly form/cleave with the addition or removal of THF from the coordination sphere of the formal tungsten(VI) metal center, permitting unprecedented control over the transformation of a tetraanionic pincer to a trianionic pincer and back. Computational analysis offers thermodynamic and electronic reasoning for the reversible equilibrium between 13-t Bu/Ad THF and 14.
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