C O M M U N I C A T I O N S
nucleophilic attack at C2/C4 is believed to be due to charge control,
while attack at C1/C5 is attributed to frontier orbital control.9 In
the present case, for the sodium salts (and Li+/12-crown-4), the
malonate anion is expected to be completely dissociated, and thus
nucleophilic attack is anticipated to occur under charge control. In
comparison, for the lithium salts (and Na+/ZnCl2), there should be
greater association between the malonate anion and the counterion;
this decreased polarization in electron density would lead to frontier
orbital controlled nucleophilic attack. It should be noted that the
steric bulk of the nucleophile may play an additional role on the
regioselectivity of nucleophilic attack (entry 6). In the case of
lithium methyl cyclohexanecarboxylate (entry 8), attack at C2 may
be due to the greater strength of this nucleophile, compared to
malonate (i.e., charge control).
Previous efforts at the desymmetrization of achiral (cyclohexa-
dienyl)- and (cycloheptadienyl)iron(1+) cations with chiral sulfoxi-
minyl acetates13a or chiral N-acetyl- or N-propionyl oxazolidinones13b
resulted in enantioselectivities ranging from 11 to 60% ee. With
these precedents in mind, we therefore sought the desymmetrization
of achiral cation 1a. To this end, reaction 1a with sodium
bis[(-)-8-phenylmenthyl]malonate14 gave the cyclohexenone (-)-
13 as a single diastereomer in excellent yield (Scheme 3). Luche
reaction of the dissociated malonate-counterion pairs proceed at
the C2 internal carbon of the pentadienyl ligand. Reaction of 1a
with the sodium salt of bis(8-phenylmenthyl)malonate proceeds with
excellent diastereocontrol to afford a single diastereomeric cyclo-
hexenone.
Acknowledgment. This material is based upon work supported
by the National Science Foundation (CHE-0415771). Mass spec-
trometry was provided by the Washington University Mass
Spectrometry Resource, an NIH Research Resource (Grant No.
P41RR0954).
Supporting Information Available: Details of experimental
procedures, characterization, and analytical data for the products (18
pages, PDF). This material is available fee of charge via the Internet
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Scheme 3
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1
based on the H NMR chemical shifts of the alkenyl proton (H2)
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assigned as (S).
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Figure 1. Diastereomeric transition states for attack on 1a. [Fe(CO)3], which
points away from view, is not shown for clarity.
In conclusion, the malonate-cation association controls the
regioselectivity for nucleophilic attack on 1a. Use of strongly
associated salts of malonate gave C1 nucleophilic attack, while
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