Enantiomer-selective carbamoylation of racemic alpha-hydroxy gamma-lactones with chiral Cu(II) Catalysts: an example of a highly active Lewis acid catalyzed reaction.
Enantiomer-selective carbamoylation of racemic alpha-hydroxy gamma-lactones with half equivalents of isocyanates in the presence of chiral Cu(II) catalysts was studied. Among a series of catalyst bearing chiral bis(oxazoline) (box) and pyridine(bisoxazoline) ligands, [Cu(tBu-box)]X(2) [X=OSO(2)CF(3) (3 a), SbF(6) (3 b)] showed the highest enantioselectivity in the reaction of pantolactone (1 a). Use of n-C(3)H(7)NCO, a small alkyl isocyanate, in CH(2)Cl(2) solution was important to achieve a high level of enantiomer selection. The tBu-box-Cu(II) catalyst efficiently differentiated two enantiomers of beta-substituted alpha-hydroxy gamma-lactones under the optimized reaction conditions, resulting in a stereoselectivity factor (s=k(fast)/k(slow)) of up to 209. Furthermore, this catalyst is highly active, so that the carbamoylation can be conducted with a substrate-to-catalyst molar ratio of 2000-3000. A catalytic cycle of this reaction is also proposed.
Kurono
Nobuhito
N
Division of Chemical Process Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, Japan.
Kondo
Tadahiro
T
Wakabayashi
Masanori
M
Ooka
Hirohito
H
Inoue
Tsutomu
T
Tachikawa
Hiroto
H
Ohkuma
Takeshi
T