Faster than believed: Transmetalation from aryl boronic acids and triaryl boroxines to diethylzinc takes place within minutes following a two‐step, low‐energy pathway (see picture), according to ...density functional calculations and reaction microcalorimetry measurements. The experimental conditions for a practical, atom‐economical, and highly enantioselective catalytic arylation of a variety of aldehydes have been developed from these data.
The structural optimization of a family of modular, enantiopure β-amino alcohol ligands with a common 2-amino-2-aryl-1,1-diphenylethanol skeleton, whose stereogenicity was introduced through the ...Jacobsen epoxidation of 1,1-diphenyl-2-arylethylenes, has led to the identification of a small set of optimal catalysts with enhanced activity and enantioselectivity in the addition of alkylzinc and arylzinc reagents to aldehydes. Criteria for the discrimination between apparently analogous, highly enantioselective ligands are proposed.
The kinetics of the citral–acetone condensation over activated Mg–Al hydrotalcite has been systematically studied in an automated 16 parallel-batch reactor. Three days of experimentation generated ...150 data points with high accuracy and reproducibility in a broad range of conditions. The acetone-to-citral ratio is a key reaction descriptor. The experiments have been adequately described by means of a Langmuir–Hinshelwood scheme.
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The kinetics of the citral–acetone condensation to pseudoionone (PS) over activated Mg–Al hydrotalcite (meixnerite) was studied in a 16 parallel-batch reactor set-up varying the temperature, catalyst amount, and molar acetone-to-citral (A/C) ratio. Tests with different stirring speeds and particle sizes were conducted in order to exclude the presence of mass transport limitations. The A/C ratio is a key reaction descriptor. The initial rate of pseudoionone formation exhibits a maximum at A/C
=
3, leading to a PS selectivity of 100%. At higher or lower A/C ratios, diacetone alcohol is the main by-product and at A/C
<
2 the product of citral self-condensation is additionally formed. Due to this, the pseudoionone selectivity at extreme A/C ratios decreases to 70–80%. The amount of carbonaceous deposits was determined by thermogravimetry and increased as the A/C ratio decreased. A Langmuir–Hinshelwood model involving acidic Al
3+ and basic OH
− sites and the surface reaction as rate-determining step described well the experimental data. The position of the optimal A/C ratio is determined by the relative ratio of adsorption constants of the reactants on the catalyst, being one order of magnitude larger in citral than in acetone. This feature can be generalized to aldol reactions involving other substrates.
Two new highly pyramidalized tricyclo3.3.0.0
3,7oct-1(5)-ene derivatives containing ether and acetal functionalities have been generated, trapped as Diels–Alder adducts and dimerized. The initially ...obtained diene dimers were photochemically converted into cyclobutane derivatives. The thermal reversion of several cyclobutane derivatives to the corresponding dienes has been studied by
1H NMR, ab initio calculations and DSC. For the first time, transannular additions of bromine and iodine to a diene dimer of this series have been observed.
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A 3-step, regio- and stereo-selective, high-yielding synthesis of
t-2-(diphenylphosphinoyl)methyl-
c-3-(disubstitutedphosphinoyl)-
r-1-cyclopentanols (
8a–
d
) is described. Reaction of epoxides
2a–
...d
and/or
5a–
d
with the lithium derivative of methyldiphenylphosphine oxide (
9
) gave cyclopentanols
8a–
d
. Base-catalyzed rearrangement of epoxides
2a–
d
and/or
5a–
d
led to 3-(disubstitutedphosphinoyl)cyclopent-2-en-1-ols (
7a–
d
) which on reaction with the lithium derivative of
9
gave also
8a–
d
. The relative configurations of
2a
and
8b–
d
were obtained by single crystal X-ray diffraction analysis.
Graphic
Schneller als angenommen: Dichtefunktionalrechnungen und mikrokalorimetrischen Messungen zufolge ist die Transmetallierung von Arylboronsäuren und Triarylboroxinen mit Diethylzink binnen Minuten ...beendet und verläuft über einen zweistufigen Pfad niedriger Energie (siehe Bild). Die experimentellen Bedingungen für eine atomökonomische und hoch enantioselektive katalytische Arylierung von Aldehyden beruhen auf diesen Daten.
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