C O M M U N I C A T I O N S
propargyl alcohols in Table 1 (up to 60% yield) and the enantio-
enrichment of the remaining starting materials.
In summary, we have developed a rhodium-catalyzed asymmetric
synthesis of indanones by isomerization of racemic R-arylpropargyl
alcohols. High enantioselectivity has been achieved by the use of
a newly developed axially chiral bisphosphine ligand ((R,R)-3). This
ligand is unique in the sense that its axial chirality is fixed to a
single configuration upon complexation to a transition metal due
to the chiral axes existing at other positions within the molecule.
Future studies will explore further development and application of
this class of chiral ligands.
Acknowledgment. Support has been provided in part by a
Grant-in-Aid for Scientific Research, the Ministry of Education,
Culture, Sports, Science and Technology, Japan (21 COE on Kyoto
University Alliance for Chemistry).
Figure 1. Catalytic cycle of the rhodium-catalyzed isomerization of
R-arylpropargyl alcohols to indanones.
enantiomeric excess (57% yield, 74% ee; entry 5). We subsequently
identified that the use of a substrate with EtMe2Si or Me3Si group
instead of Et3Si group on the alkyne leads to further enhancement
of enantiomeric excess (92-99% ee (S); entries 8 and 9).16 Under
these conditions, several (()-R-arylpropargyl alcohols bearing
substituents on the aromatic ring are also isomerized to indanones
in high enantiomeric excess (92-96% ee; entries 10-14).
A catalytic cycle of this process is illustrated in Figure 1.5 â-H
elimination of alkoxorhodium intermediate A, followed by conju-
gate hydrorhodation, destroys the stereocenter of the substrate to
give alkenylrhodium species B. After 1,4-rhodium migration (B
f C), a new stereocenter is created at the step of intramolecular
1,4-addition of intermediate C. This sequence indicates that a
racemic substrate could undergo a full conversion to provide the
corresponding indanone, and its stereochemical outcome is con-
trolled at the step of C to D, independent of the original stereo-
chemical information of the substrate. In reality, however, ∼20%
of starting propargyl alcohols always remain under the conditions
we employed, and we found that these remaining substrates are
enantioenriched. For example, 24% of starting material 1b was
recovered in 89% ee (R) in Table 1, entry 8.
Supporting Information Available: Experimental procedures and
compound characterization data (PDF). This material is available free
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(16) The absolute configuration of indanone 2b was determined by converting
it to 1-indanol via cis-selective reduction with LiAlH4 and then desilylation
with TBAF (see Supporting Information for details).
(17) In the reaction of rac-substrates, (R)-isomer must also be isomerized to
indanone to some extent since the yields are typically >50% (Table 1).
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