The construction of a novel class of aminophosphite/phosphinite/phosphine ligands containing a protected pyrrolidine-3,4-diol moiety is presented. These ligands are obtained from readily available sugars. They thus contain the advantages of carbohydrates in terms of selection of the stereogenic carbons, polyfunctional groups able to modulate the electronic and steric properties, and the general good stability of carbohydrate derivatives. They constitute a novel class of P,Nligands that have been used in the enantioselective allylic substitutions of acyclic and cyclic substrates with varied electronic and steric requirements, using different C-and N-nucleophiles, with high enantioselectivities. Among the three groups of P,Nligands (amino-P; P = phosphite, phosphinite, and phosphine groups) the new amino-phosphite ligands give the widest substrate and nucleophile scope, including the more challenging hindered linear and cyclic substrates. In particular, for carbohydratederived amino-phosphite ligands and linear substrates, high enantioselectivity in the reactions requires an R configuration of the binaphthyl moiety. However, for cyclic substrates both product enantiomers can be reached by setting out the chirality of the binaphthyl phosphite moiety. A detailed investigation of the appropriate Pd intermediates is also presented.
We
report a reduced but structurally valuable phosphite/phosphinite-thioether
ligand library for the Ir-hydrogenation of 40 minimally functionalized
alkenes, including relevant examples with poorly coordinative groups.
We found that enantiomeric excesses are mainly dependent on the substrate
structure and on some ligand parameters (i.e., the type of thioether/phosphorus
moieties and the configuration of the phosphite group), whereas the
substituents of the biaryl phosphite moiety had little impact. By
tuning the ligand parameters we were able to find highly selective
catalysts for a range of substrates (ee’s up to 99%). These
phosphite/phosphinite-thioether ligands have a simple backbone and
thus yield simple NMR spectra that reduce signal overlap and facilitate
the identification of relevant intermediates. Therefore, by combining
HP-NMR spectroscopy and theoretical studies, we were also able to
identify the catalytically competent Ir-dihydride alkene species,
which made it possible to explain the enantioselectivity obtained.
A large family of phosphite‐thioether/selenoether ligands has been easily prepared from accessible L‐(+)‐tartaric acid and D‐(+)‐mannitol and applied in the M‐catalyzed (M=Ir, Rh) asymmetric hydrogenation of a broad number of substrates (46 in total). Its highly modular architecture has been crucial to maximize the catalytic performance. Improving most of the reported approaches, this ligand family presents a broad substrate scope. By selecting the ligand parameters high enantioselectivities (ee's up to 99 %) have therefore been achieved in a broad range of both, functionalized and unfunctionalized substrates. Interestingly, both enantiomers of the hydrogenation products can be usually achieved by changing the ligand parameters.
We
studied for the first time the potential of novel and simple
Ir/thioether-NHC complexes in the asymmetric hydrogenation of unfunctionalized
olefins and cyclic β-enamides. For comparison, we prepared and
applied the analogues thioether–phosphinite/phosphite complexes.
We found that the efficiency of the new Ir/thioether-NHC catalyst
precursors varies with the type of olefin. Thus, while the Ir/thioether-NHC
catalyst precursors provided lower catalytic performance than their
related Ir/thioether-P complexes in the hydrogenation of olefins lacking
a coordinating group, the catalysts had similar good performance for
the reduction of functionalized olefins (e.g., tri- and disubstituted
enol phosphonate derivatives). Catalytic results together with the
studies of the reactivity toward H2 indicated that the
thioether-carbene design favors the formation of inactive trinuclear
species, which are responsible for the low activities obtained with
these carbene-type catalysts. Nevertheless, this catalyst deactivation
can be avoided by using functionalized olefins such as enol phosphonates.
We also report the discovery of simple-to-synthesize Ir/thioether-P
catalysts containing a simple backbone that gave high enantioselectivities
for some trisubstituted olefins, some challenging 1,1′-disubstituted
olefins, and cyclic β-enamides.
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