The
reaction of readily available E-allyl difluorides
(R1CH = CHCF2COR2) containing a CF2COR2 moiety with arylboronic acids catalyzed by
a chiral diene-rhodium complex gives high yields of chiral monofluoroalkenes
(R1ArC*HCH = CFCOR2) with perfect enantioselectivity
(enantiomeric excess (ee) of > 99% in all cases) and high level
of Z/E selectivity (up to 20:1).
The reaction
is proposed to proceed through enantioselective arylrhodation of the E-allyl difluorides forming a β,β-difluoroalkylrhodium
intermediate, followed by stereoselective β-fluoride elimination.
The CF2COR2 moiety plays a key role in activating
the E-allyl difluorides toward arylrhodation/defluorination
in executing the catalytic cycle, and it serves as a handle for further
functionalization.
Rhodium-catalyzed stereospecific cyanation of olefinic C−H bond using environmetally benign N-cyano-N-phenyl-p-methylbenzenesulfonamide (NCTS) as a cyanating reagent for the synthesis of β,β-disubstituted acrylonitriles has been developed. The mechanism involves electrophilic cyanation...
A rhodium-catalyzed highly enantio- and diastereoselective
alkenylation
of β,γ-unsaturated butenolides is reported. The use of
a chiral diene ligand, (S,S)-Ph-bod,
enables the facile synthesis of chiral butyrolactones in high yields
with extremely high enantioselectivities (>99% ee in all cases)
and
high diastereoselectivities (up to >20:1 dr). The key process of
the
reaction involves the isomerization of β,γ-unsaturated
butenolides to racemic α,β-unsaturated butenolides and
the subsequent dynamic kinetic resolution through ligand-controlled,
enantioselective alkenylation with an alkenylrhodium species that
is generated in situ via 1,4-rhodium migration.
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