5 research outputs found
Application Scope and Limitations of TADDOL-Derived Chiral Ammonium Salt Phase-Transfer Catalysts
We have recently introduced a new class of chiral ammonium salt catalysts derived from easily available TADDOLs. To get a full picture of the scope of application and limitations of our catalysts we tested them in a variety of different important transformations. We found that, although these compounds have recently shown their good potential in the asymmetric α-alkylation of glycine Schiff bases, they clearly failed when we attempted to control more reactive nucleophiles like b-keto esters. On the other hand, when using them to catalyse the addition of glycine Schiff bases to different Michael acceptors it was found necessary to carefully optimize the reaction conditions for every single substrate class, as seemingly small structural changes sometimes required the use of totally different reaction conditions. Under carefully optimized conditions enantiomeric ratios up to 91:9 could be achieved in the addition of glycine Schiff bases to acrylates, whereas acrylamides and methyl vinyl ketone gave slightly lower selectivities (up to e.r. 77:23 in these cases). Thus, together with additional studies towards the syntheses of these catalysts we have now a very detailed understanding about the scope and limitations of the synthesis sequence to access our PTCs and about the application scope of these catalysts in asymmetric transformations
Facile Synthesis of β-Tribromomethyl and Dibromomethylenated Nitroalkanes via Conjugate Addition of Bromoform to Nitroalkenes
Generation and Trapping of a Cage Annulated Vinylidenecarbene and Approaches to Its Cycloalkyne Isomer
A novel cage-annulated (bis-homocubyl) vinylidenecarbene
has been
generated and successfully trapped without any intermediacy of its
cycloalkyne isomer. The greater kinetic and thermodynamic stability
of the vinylidenecarbene vis-à-vis its cycloalkyne isomer has
been predicted by DFT B3LYP/6-31G* calculations. The calculated results
suggest the prospects of the cycloalkyne becoming amenable for trapping,
if generated under suitable experimental conditions, owing to the
substantial kinetic energy barrier associated with its possible ring
contraction via 1,2-shift to the vinylidenecarbene isomer and marginal
ground state energy difference. However, all of our attempts to directly
generate and trap the cycloalkyne yielded unsatisfactory results.
Attempted generation and trapping of a C2-symmetric bis-vinylidenecarbene
from a bis-vinylidenedibromide met with unexpected failure
Generation and Trapping of a Cage Annulated Vinylidenecarbene and Approaches to Its Cycloalkyne Isomer
A novel cage-annulated (bis-homocubyl) vinylidenecarbene
has been
generated and successfully trapped without any intermediacy of its
cycloalkyne isomer. The greater kinetic and thermodynamic stability
of the vinylidenecarbene vis-à-vis its cycloalkyne isomer has
been predicted by DFT B3LYP/6-31G* calculations. The calculated results
suggest the prospects of the cycloalkyne becoming amenable for trapping,
if generated under suitable experimental conditions, owing to the
substantial kinetic energy barrier associated with its possible ring
contraction via 1,2-shift to the vinylidenecarbene isomer and marginal
ground state energy difference. However, all of our attempts to directly
generate and trap the cycloalkyne yielded unsatisfactory results.
Attempted generation and trapping of a C2-symmetric bis-vinylidenecarbene
from a bis-vinylidenedibromide met with unexpected failure