Stereoelectronic effects dictate mechanistic dichotomy between Cu(II)-catalyzed and enzyme-catalyzed reactions of malonic acid half thioesters

Article abstract of DOI:10.1021/ja0673682

Previously we developed a Cu(II)-catalyzed, enantioselective aldol reaction between malonic acid half thioesters (MAHTs) and aldehydes based on the biosynthesis of polyketides and fatty acids in which MAHTs are decarboxylated enzymatically to afford ester

Full text of DOI:10.1021/ja0673682

Published on Web 01/11/2007
Stereoelectronic Effects Dictate Mechanistic Dichotomy between
Cu(II)-Catalyzed and Enzyme-Catalyzed Reactions of Malonic Acid Half
Thioesters
Kevin C. Fortner and Matthew D. Shair*
Department of Chemistry and Chemical Biology, HarVard UniVersity, Cambridge, Massachusetts 02138
Received October 13, 2006; E-mail: shair@chemistry.harvard.edu
In the biosynthesis of polyketides and fatty acids,1 enzymatic
Scheme 2
activation of malonic acid half thioesters (MAHTs) affords ester
enolates that condense with thioesters resulting in a decarboxylative
Claisen condensation (Scheme 1, eq 1). Inspired by this reaction,
we developed a Cu(II)-catalyzed, enantioselective reaction2 between
MeMAHT and aldehydes yielding aldol products resulting from
formal decarboxylation and enolate addition to the aldehydes
(Scheme 1, eq 2). In principle, ester enolate equivalents can be
generated from MAHTs by either decarboxylation or deprotonation,
while the accepted mechanism3 for the enzyme-catalyzed reactions
involves decarboxylation to form thioester enolates followed by
condensation with thioesters (Scheme 1, eq 1).
Scheme 1
tions2 that the reaction rate is highly sensitive to sterics of both the
aldehyde and MAHT and that decarboxylation does not occur in
the absence of aldehyde, suggest that enolization occurs by nonrate-
limiting deprotonation (A f B). This precludes a mechanism
mirroring the enzymatic reaction of MAHTs, involving rate-limiting
decarboxylation.
To further our study of MeMAHT enolization, we synthesized
two isotopically labeled MeMAHTs: MeMAHT deuterated at the
R-position (MeMAHT-D) and a 1:1 mixture of MeMAHT with a
13C label at the carboxylate position and at the thioester position
Below we report evidence based on steric effects, kinetics, kinetic
isotope effects (KIEs), and crossover experiments in support of a
mechanism for the Cu(II)-catalyzed aldol reaction of MAHTs
involving decarboxylation after addition to aldehydes (Scheme 2).
We also provide an explanation, based on stereoelectronic effects,
for the mechanistic dichotomy between Cu(II)-catalyzed and
enzyme-catalyzed reactions of MAHTs.
Kinetics experiments support a transition state composed of Cu-
(II), (R,R)-Phbox, MeMAHT, and aldehyde. The reaction between
dihydrocinnamaldehyde and a mixture of Cu(OTf)2, (R,R)-Phbox,
and MeMAHT in an optimized, fixed ratio of 10:13:100 was
kinetically first-order in the Cu(OTf)2, (R,R)-Phbox, and MeMAHT
mixture taken as a whole. No reaction occurred when the (R,R)-
Phbox/Cu(OTf)2 ratio was equal to one, suggesting that (R,R)-Phbox
serves as both a ligand and a catalytic base (Scheme 2, A f B).
The reaction was first-order in aldehyde at [aldehyde] e 0.2 M,
but the rate decreased from that predicted by first-order kinetics at
higher concentrations. These results, combined with our observa-
but containing no deuterium (MeMAHT-13C) (see Scheme 3). We
measured an enolization rate of (5.2 ( 0.2) × 10-5 M/s (t1/2 ) 5.5
( 0.2 min) by observing the rate of deuterium exchange between
MeMAHT-D and MeMAHT-13C under the aldol conditions. Eno-
lization is therefore too fast to be rate-limiting under standard aldol
conditions but is slow enough to be partially rate-limiting when
[aldehyde] > 0.2 M. A 1H NMR spectrum of all the reaction
components exhibited two broad methyl peaks for MeMAHT when
(R,R)-Phbox was used and only one peak when (()-Phbox was
used. We assign the two peaks when using (R,R)-Phbox to
diastereomeric complexes formed between (R,R)-Phbox/Cu(II) and
(()-MeMAHT in rapid equilibrium with free MeMAHT4a indicating
that the coordinated MeMAHT is not enolized in the catalyst resting
state5 (Scheme 2, A).
We measured an isotope effect of kH/kD ) 1.06 ( 0.02 on the
rate of an aldol reaction using unlabeled MeMAHT compared to
MeMAHT-D; however, a 1:1 mixture of these MAHTs produced
an aldol product (Scheme 2, E) with a 6:1 ratio of hydrogen to
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J. AM. CHEM. SOC. 2007, 129, 1032-1033
10.1021/ja0673682 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
Scheme 4
deuterium at the R-position at <3% conversion. This result indicates
that the elementary step responsible for the H:D ratio in the product
occurs after the rate-limiting step and is likely a protonation step
late in the reaction pathway (Scheme 2, D f E).
Holden8 proposed that enzymes activate MAHTs by polarizing the
thioester group while orienting the carboxylate orthogonally to
enforce overlap between the σ orbital of the scissile C-C bond
and π* of the thioester carbonyl, which is stereoelectronically
required for decarboxylation (Scheme 4, eq 1).9 Deprotonation is
prevented because if the MAHT is stereoelectronically aligned for
decarboxylation, it cannot be aligned for deprotonation. We believe
that in our aldol reaction, bidentate coordination of MAHT by Cu-
(II) orients the C-2 proton orthogonal to the π system, allowing
deprotonation but not decarboxylation (Scheme 4, eq 2). The aldol
reaction mechanism (Scheme 2) also explains why this reaction is
compatible with protic functional groups: because the only strongly
basic intermediates (i.e., Scheme 2, D) are generated in small
concentrations late in the catalytic cycle.
To determine whether deprotonative enolization is essential rather
than incidental to the aldol reaction, we carried out an aldol reaction
with 50% MeMAHT-D and 50% MeMAHT-13C. We removed 90%
of the reaction mixture from the reaction vessel and isolated the
product after a reaction time of 5 min (<3% conversion). The
MeMAHT remaining in the reaction vessel was reisolated 1 min
later (Scheme 3). If the mechanism involved decarboxylation
followed by addition to the aldehyde, then the original isotope
pattern in the starting materials would be largely retained in the
1
products; however, H NMR analysis4b showed that the isotopic
labeling is completely scrambled in the product. The scrambling
cannot occur after the reaction because we have observed that the
product is configurationally stable to the reaction conditions, nor
does complete scrambling occur prior to the reaction because the
reisolated MeMAHT is not yet completely scrambled. The only
possibility is that scrambling by deprotonation and reprotonation
occurs during the aldol reaction itself. Moreover, the equal H/D
ratios in the products indicate that the labeled MeMAHTs proceed
through essentially identical intermediates (i.e., Scheme 2, B, C,
and D) each having lost its isotopic distinction partway through
the reaction. This is consistent with our proposed mechanism
(Scheme 2) of deprotonative enolization, addition to the aldehyde,
decarboxylation, and protonation of the â-hydroxy enolate.
Additional information about the reversibility of the elementary
steps was obtained by measuring the 13C KIE at the carboxylate
carbon of MeMAHT-13C in an aldol reaction with dihydrocin-
namaldehyde. We measured a KIE of 1.020 ( 0.002 using a
modification of the Singleton method5 which is consistent with a
scenario in which the MeMAHT enolate (Scheme 2, B) adds
reversibly to the aldehyde and then decarboxylates.6 Furthermore,
MeMAHT-13C reisolated at high conversion was found not to have
formed a statistical mixture of isotopic isomers indicating that
decarboxylation is irreversible.
Acknowledgment. Gifts from Merck Research Laboratories and
Novartis are acknowledged for support of this work. K.C.F.
acknowledges NSF for a predoctoral fellowship.
Supporting Information Available: Representative experimental
procedures and characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
References
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(4) See Supporting Information for further details: (a) S30-S35; (b) S44-
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(5) We believe that the resting state catalyst binds reversibly to a second
MeMAHT since the aldol reaction is inverse first order in [MeMAHT].
(6) Singleton, D. A.; Thomas, A. A. J. Am. Chem. Soc. 1995, 117, 9357-
9358. See Supporting Information (S48-S53) for the modification details.
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(9) The reactant in Scheme 4, eq. 1 is based on a study of an MAHT modeled
into the active site of KAS I from the following reference: Olsen, J. G.;
Kadziola, A.; von Wettstein-Knowles, P.; Siggaard-Andersen, M.; Larsen,
S. Structure 2001, 9, 233-243.
Our results demonstrate that Cu(II)-catalyzed aldol reactions of
MAHTs occur by a different mechanism from enzyme-catalyzed
decarboxylative Claisen condensations of MAHTs. Gerlt and
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