Abstract
A series of thiophene- and benzothiophene-oxazoline containing ligands, were applied
in the intermolecular asymmetric phenylation and cyclohexenylation of 2,3-dihydrofuran
1 . Phenylation proceeded in moderate to high chemical yields, with good regioselectivities
and in up to 95% ee. Cyclohexenylations gave similarly high chemical yields and regioselectivities
with the optimal result being a 96% yield of the major product in 97% ee. 2,2-Dialkyl-2,3-dihydrofurans
were also tested as substrates and the phenylation and cyclohexenylation of 2,2-dimethyl-2,3-dihydrofuran
proceeded in high yields and ee’s up to 91% and 89%, respectively. The phenylation
and cyclohexenylation of the 2,2-diethyl analogue proceeded in excellent yields and
ee’s up to 99% and 87%, respectively. For each substrate, palladium complexes formed
from the t -butyl-substituted ligand 10 gave the highest yields, regioselectivities, and enantioselectivities over the broad
range of reaction conditions studied. 2,2-Diisopropyl-2,3-dihydrofuran was prepared
but was found to be unreactive in the intermolecular Heck reaction thus providing
insight into to the steric limits for 2,3-dihydrofuran substrates.
Key words
P,N ligands - oxazoline - asymmetric catalysis - intermolecular Heck reaction - palladium
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