27 research outputs found
A step towards hydroformylation under sustainable conditions: platinum-catalysed enantioselective hydroformylation of styrene in gamma-valerolactone
Platinum-catalysed enantioselective hydroformylation of styrene was performed in Îł-valerolactone (GVL) as a proposed environmentally benign reaction medium. Optically active bidentate ligands, possessing various types of chirality elements e.g. central (BDPP), axial (BINAP, SEGPHOS, DM-SEGPHOS, DTBM-SEGPHOS) and planar/central (JOSIPHOS) elements, were applied in in situ generated Pt-diphosphine-tin(II)chloride catalyst systems. In general, slightly higher activities and regioselectivities towards a branched aldehyde (2-phenylpropanal) were obtained in toluene as a reference conventional solvent. However, higher chemoselectivities towards aldehydes (up to 98%) in GVL were obtained at lower temperatures. The application of GVL proved to be also advantageous regarding enantioselectivity: although moderate enantioselectivities were obtained in both solvents, in most cases higher ee values were detected in GVL. From the mechanistic point of view, the formation of different catalytic intermediates and/or different kinetics can be envisaged from the different temperature dependences of ee in GVL and toluene. The 31P-NMR characterization of catalyst species in GVL was also provided
Immobilized Bisdiazaphospholane Catalysts for Asymmetric Hydroformylation
Condensation reactions of enantiopure
bis-3,4-diazaphospholanes
(BDPs) that are functionalized with carboxylic acids enable covalent
attachment to bead and silica supports. Exposure of tethered BDPs
to the hydroformylation catalyst precursor, RhÂ(acac)Â(CO)<sub>2</sub>, yields catalysts for immobilized asymmetric hydroformylation (iAHF)
of prochiral alkenes. Compared with homogeneous catalysts, catalysts
immobilized on Tentagel resins exhibit similarly high regioselectivity
and enantioselectivity. When corrected for apparent catalyst loading,
the activity of the immobilized catalysts approaches that of the homogeneous
analogues. Excellent recyclability with trace levels of rhodium leaching
are observed in batch and flow reactor conditions. Silica-bound catalysts
exhibit poorer enantioselectivities
Libraries of Bisdiazaphospholanes and Optimization of Rhodium-Catalyzed Enantioselective Hydroformylation
Twelve chiral bis-3,4-diazaphospholane
ligands and six alkene substrates
(styrene, vinyl acetate, allyloxy-<i>tert</i>-butyldimethylsilane,
(<i>E</i>)-1-phenyl-1,3-butadiene, 2,3-dihydrofuran, and
2,5-dihydrofuran) probe the influence of steric bulk on the activity
and selectivity of asymmetric hydroformylation (AHF) catalysts. Reaction
of an enantiopure bisdiazaphospholane tetraacyl fluoride with primary
or secondary amines yields a small library of tetracarboxamides. For
all six substrates, manipulation of reaction conditions and bisdiazaphospholane
ligands enables state-of-the-art performance (90% or higher ee, good
regioselectivity, and high turnover rates). For the nondihydrofuran
substrates, the previously reported ligand, (<i>S</i>,<i>S</i>)-<b>2</b>, is generally most effective. However,
optimal regio- and enantioselective hydroformylation of 2,3-dihydrofuran
(up to 3.8:1 α-isomer/β-isomer ratio and 90% ee for the
α-isomer) and 2,5-dihydrofuran (up to <1:30 α-isomer/β-isomer
ratio and 95% ee for the β-isomer) arises from bisdiazaphospholanes
containing tertiary carboxamides. Hydroformylation of either 2,3-
or 2,5-dihydrofuran yields some of the β-formyl product. However,
the absolute sense of stereochemistry is inverted. A stereoelectronic
map rationalizes the opposing enantiopreference
Libraries of Bisdiazaphospholanes and Optimization of Rhodium-Catalyzed Enantioselective Hydroformylation
Twelve chiral bis-3,4-diazaphospholane
ligands and six alkene substrates
(styrene, vinyl acetate, allyloxy-<i>tert</i>-butyldimethylsilane,
(<i>E</i>)-1-phenyl-1,3-butadiene, 2,3-dihydrofuran, and
2,5-dihydrofuran) probe the influence of steric bulk on the activity
and selectivity of asymmetric hydroformylation (AHF) catalysts. Reaction
of an enantiopure bisdiazaphospholane tetraacyl fluoride with primary
or secondary amines yields a small library of tetracarboxamides. For
all six substrates, manipulation of reaction conditions and bisdiazaphospholane
ligands enables state-of-the-art performance (90% or higher ee, good
regioselectivity, and high turnover rates). For the nondihydrofuran
substrates, the previously reported ligand, (<i>S</i>,<i>S</i>)-<b>2</b>, is generally most effective. However,
optimal regio- and enantioselective hydroformylation of 2,3-dihydrofuran
(up to 3.8:1 α-isomer/β-isomer ratio and 90% ee for the
α-isomer) and 2,5-dihydrofuran (up to <1:30 α-isomer/β-isomer
ratio and 95% ee for the β-isomer) arises from bisdiazaphospholanes
containing tertiary carboxamides. Hydroformylation of either 2,3-
or 2,5-dihydrofuran yields some of the β-formyl product. However,
the absolute sense of stereochemistry is inverted. A stereoelectronic
map rationalizes the opposing enantiopreference