Heterobimetallic
bismuth–rhodium paddlewheel complexes with
phenylglycine ligands carrying TIPS-groups at the
meta
-positions of the aromatic ring exhibit outstanding levels of selectivity
in reactions of donor/acceptor and donor/donor carbenes; at the same
time, the reaction rates are much faster and the substrate scope is
considerably wider than those of previous generations of chiral [BiRh]
catalysts. As shown by a combined experimental, crystallographic,
and computational study, the new catalysts draw their excellent application
profile largely from the stabilization of the chiral ligand sphere
by London dispersion (LD) interactions of the peripheral silyl substituents.
Bioinspired complexes employing the ligands 6‐tert‐butylpyridazine‐3‐thione (SPn) and pyridine‐2‐thione (SPy) were synthesized and fully characterized to mimic the tungstoenzyme acetylene hydratase (AH). The complexes [W(CO)(C2H2)(CHCH‐SPy)(SPy)] (4) and [W(CO)(C2H2)(CHCH‐SPn)(SPn)] (5) were formed by intramolecular nucleophilic attack of the nitrogen donors of the ligand on the coordinated C2H2 molecule. Labelling experiments using C2D2 with the SPy system revealed the insertion reaction proceeding via a bis‐acetylene intermediate. The starting complex [W(CO)(C2H2)(SPy)2] (6) for these studies was accessed by the new acetylene precursor mixture [W(CO)(C2H2)n(MeCN)3−nBr2] (n=1 and 2; 7). All complexes represent rare examples in the field of W−C2H2 chemistry with 4 and 5 being the first of their kind. In the ongoing debate on the enzymatic mechanism, the findings support activation of acetylene by the tungsten center.
The Cu‐catalyzed reaction of substituted α‐diazoesters with fluoride gives α‐fluoroesters with ee values of up to 95 %, provided that chiral indane‐derived bis(oxazoline) ligands are used that carry bulky benzyl substituents at the bridge and moderately bulky isopropyl groups on their core. The apparently homogeneous solution of CsF in C6F6/hexafluoroisopropanol (HFIP) is the best reaction medium, but CsF in the biphasic mixture CH2Cl2/HFIP also provides good results. DFT studies suggest that fluoride initially attacks the Cu‐ rather than the C‐atom of the transient donor/acceptor carbene intermediate. This unusual step is followed by 1,2‐fluoride shift; for this migratory insertion to occur, the carbene must rotate about the Cu−C bond to ensure orbital overlap. The directionality of this rotatory movement within the C2‐symmetric binding site determines the sense of induction. This model is in excellent accord with the absolute configuration of the resulting product as determined by X‐ray diffraction using single crystals of this a priori wax‐like material grown by capillary crystallization.
Heterobimetallic [BiRh] tetracarboxylate catalysts endowed with 1,3‐disilylated phenylglycine paddlewheels benefit from interligand London dispersion. They were originally designed for asymmetric cyclopropanation but are now shown to perform very well in asymmetric C−H functionalization reactions too. Because of the confined ligand sphere about the derived donor/acceptor carbenes, insertions into unhindered methyl groups are kinetically favored, although methylene units also react with excellent levels of asymmetric induction; even gaseous ethane is a suitable substrate. Moreover, many functional groups in both partners are tolerated. The resulting products are synthetically equivalent to the outcome of traditional asymmetric ester alkylation, allylation, benzylation, propargylation and aldol reactions and therefore constitute a valuable nexus to more conventional chemical logic.
Intending to deepen
our understanding of tungsten acetylene (C
2
H
2
) chemistry, with regard to the tungstoenzyme
acetylene hydratase, here we explore the structure and reactivity
of a series of tungsten acetylene complexes, stabilized with pyridine-2-thiolate
ligands featuring tungsten in both +II and +IV oxidation states. By
varying the substitution of the pyridine-2-thiolate moiety with respect
to steric and electronic properties, we examined the details and limits
of the previously reported intramolecular nucleophilic attack on acetylene
followed by the formation of acetylene inserted complexes. Here, we
demonstrate that only the combination of high steric demand and electron-withdrawing
features prevents acetylene insertion. Nevertheless, although variable
synthetic approaches are necessary for their synthesis, tungsten acetylene
complexes can be stabilized predictably with a variety of pyridine-2-thiolate
ligands.
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